Se eee ISA, eSATA THE LIBRARY COLLEGE OF AGRICULTURE \ / NEW YORK STATE COLLEGE OF AGRICULTURE, DEPARTMENT OF HORTICULTURE, CORNELL UNIVERSITY, ITHACA, N. Y. text-book of botany, A TEXT-BOOK OF BOTANY A TEXT-BOOK OF BOTANY BY DR. E. STRASBURGER DR. FRITZ NOLL PROFESSOR IN THE UNIVERSITY PRIVAT DOCENT IN THE UNIVERSITY OF BONN OF BONN DR. H. SCHENCK DR. A. F. W. SCHIMPER PRIVAT DOCENT IN THE UNIVERSITY PROFESSOR IN THE UNIVERSITY OF BONN OF BONN TRANSLATED FROM THE GERMAN BY H. C. PORTER, PuD. ASSISTANT INSTRUCTOR OF BOTANY, UNIVERSITY OF PENNSYLVANIA WITH 594 ILLUSTRATIONS, IN PART COLOURED London MACMILLAN AND CoO., Liutrep NEW YORK: THE MACMILLAN COMPANY 1898 TRANSLATORS PREFACE TO THE ENGLISH EDITION IN presenting this translation of the “ Strasburger ” Botany, no words from the translator are needed in commendation of the original. The names of its authors and the distinguished position they occupy in the world of botanical science testify to the high character of the book, while the necessity of issuing a second edition within a year after its first appearance, evidences the speedy recognition of its merits awarded in Germany. Embodying the well-considered con- clusions of a lifetime devoted to botanical work on the part of its chief editor, Strasburger, and the investigations of his able collaborators, Noll, Schenck and Schimper, it will also be found to include all the latest results of botanical study and research. The translation has been undertaken with the consent and approval of both authors and publishers, and is of the second revised German edition. It has been my aim, as translator, to adhere closely to the German, making neither alterations nor omissions. Only in this way it seemed to me possible to ensure a fair repre- sentation of the author’s views, not only on questions of botanical significance, but also on the methods to be pursued in teaching the different branches of Botany. It has also been my effort to avoid any unnecessary introduction of new terms, and I have adopted, as far as consistent with the German, the existing terminology. Wherever possible, in translating technical words of a purely German signification, I have conformed to the usage of previous translations. In seeking for an appropriate translation of the German word ‘ Anlage,” I have reverted to the earlier rendering, rudiment,. which vi BOTANY in its common meaning of “first, unshapen beginning, or “the first or embryotic origin of anvthing,” conveys more accurately than any word yet proposed the true significance of the term Aniage as used in a morphological sense. I have also followed the German custom in using, where consistent with brevity and concizeness, ordinary rather than technical, descriptive word: whose comprehension requires a constant reference, on the part of the student, to a glossary or botanical dictionary. The expression “‘ Hochblatter” I have tran:- lated az bracteal leaves, in conformity with the express statement of the German author, by whom they are also designated a: bractex. In finding satisfactory English equivalents tor German terms lo eretotore untranslated, considerable difficulty has been experienced. It gives me great pleasure to acknowledge the helpful suggestion: and advice received on such points from Professor Mactarlane and Doctor Harshburger, and to expres: my indebtedness to them and to Doctor: Osterhout and Lungershauzer for the kind agsistance rendered in other detail: of the work of translation. H. C. PORTER. TSIVERSITY Or PESSSYLVAéI4, PHILADELPHIA, February 1496. CONTENTS PAGEL EA THODUCTION ‘ : ' ‘ ‘ | PART I GENERAL BOTANY RECTION — MORPHOLOGY WXTUNAL, MOMPHOLOGY The developiment of form Di the plank bingdous 4 i i . LO Meluhions of symimnebry F ‘ i ih Virinely wysbenis ' , ‘ : ; ' vi Nhe shoal i ; , ' » 14 Ths wher or nis of diss Hhool, ; : i ' rs) Whe dont r A és : é ‘ . YH The rool, ‘i i é 5 i i 3 AQ) Whe onhogeny off plaate : : i » Ad INTMIINAL MOMPHOLOGY (Wistology ond Anmbonmy) Whe coll, i ' ' » AT Cel fusion ; ; ‘ ‘ 4 Waa, ‘ ; ; ; ‘ , » BG “Vinstie syste ' ‘ ; ' ' ' ‘ » 0 Sho priney binnnes ; ' ; : ' : » Who iste bation of ble primacy Gsstes ; ; i i » 10K Whe necoudiry Gated é ' , , ‘ » 120 The phylogeny of thie Titeraned wii : ' ' ' Th Nhe ontogeny of bho tribe abe tiire , : ‘ yd Atrnehirad deviabbon i i 3 » bd SHOTION Th PIPYSTOLOCY The pliysdoal and vite abbeiba hon of plaka ‘| ’ » WO TH SY Aery oe Tit Poa ter Bos, i , ‘ » Td Weg : : : , : : , » 106 Terion of (Lawes ' , ' : ‘ ' 16 Moohaniond Gast Gibarconti) i i : , 19 vill BOTANY Nuvrrition The constituents of the plant ‘body The essential constituents of plant food The process of absorption Water and mineral substances The absorption of carbon (assimilation) The utilisation of the products of assimilation Transfer of the products of assimilation The storage of reserve material Special processes of nutrition RESPIRATION Intramolecular r aepietion Heat produced by respiration The movement of gases within the plant Phosphorescence GROWTH . The embryonal development of the organs The phase of elongation The internal development of the oigans Periodicity in development, duration of life, and continnity of the embryonic substance THE PHENOMENA OF Movement Movements of naked protoplasm The movements of protoplasm within walled cells Movements producing curvature Hygroscopic curvatures Growth curvatures . Movements due to changes of turgor REPRODUCTION Vegetative neproducbion Sexual reproduction Alternation of generations ‘i The dissemination and germination of seeds . THALLOPHYTA Myxomycetes Schizophyta Diatomeae Peridineae Conjugatae Chlorophyceae Phaeophyceae Rhodophyceae Characeae PART II, SPECIAL BOTANY SECTION I. CRYPTOGAMS PAGE 171 171 172 176 178 195 201 202 204 206 216 219 220 221 293 223 224 229 237 237 241 242 244 247 247 248 269 274 277 280 289 291 301 302 305 312 315 315 318 329 334 337 CONTENTS 1x PAGE Hy phomycetes 340 Lichenes 375 BryoPpHyTta 381 Hepaticae 385 Musci . , 390 PreripoPpHyTa . 397 Filicinae 400 Equisetinae 412 Lycopodinae 415 SECTION II. PHANEROGAMIA GYMNOSPERMAE . 434 Cycadinae 437 Coniferae 438 Gnetineae 443 ANGIOSPERMAE . 444 Monocotyledones 462 Dicotyledones . 490 List oF OFFICINAL PLANTS 601 List or Poisonous PLANTS 603 INDEX 605 ERRATA Page 285, line 5 from foot, for protogymous read protogynous. Page 355, line 2 from foot, for paraphyses read periphyses.: INTRODUCTION It is customary to divide all living organisms into two great kingdoms, animal and vegetable. A sharp boundary line between animal and vegetable life can, however, be drawn only in the case of the more highly developed organisms ; while in those of more simple organisa- tion all distinctions disappear, and it becomes difficult to define the exact limits of Botany and Zoology. This, in fact, could scarcely be otherwise, as all the processes of life, in both the animal and vegetable kingdoms, are dependent on the same substance, protoplasm. The more elementary the organism, the more apparent the general quali- ties of this protoplasm become, and hence the correspondence between the lower organisms is specially striking. With more compli- cated organisation, the specific differences increase, and the character- istics distinguishing animal from vegetable life become more obvious. For the present, it must be confessed, the recognition of an organism, as an animal or a plant, is dependent upon its supposed correspondence with an abstract idea of what a plant or animal should be, based on certain fancied points of agreement between the members of each class. A satisfactory basis for the separation of all living organisms into the categories of animals or plants can only be obtained when it is shown that all organisms distinguished as animals are in reality genetically connected, and that a similar connection exists between all plants. The method by which such evidence may be arrived at has been indicated in the THEoRY oF EVOLUTION. From the paleontological study of the imprints of fossil animals and plants, it has been established that in former epochs forms of life differing from those of the present age existed on the earth. It is also generally assumed that all living animals and plants have been derived from previously existing forms. The conclusion is a natural one, that those organisms possess- ing almost exactly similar structures which have been united as species under the same genera are in reality related to one another. Indeed, it is permissible to take a further step, and assume that the B 2 BOTANY union of corresponding genera into one family serves to give expression to a real relationship existing between them. The evolution of a living organism from others previously existing and different in form has been distinguished by HaECKEL as its phylogenetic development or PHyLocENy. Every organism arising from a like organism must, before attaining its mature state, com- plete its own individual development, or, as it has been termed by HAECKEL, its ontogenetic development or ONTOGENY. The supposi- tion that the successive steps in the ontogenetic development of an organism correspond to those of its phylogenetic development, and that the ontogeny of an organism is accordingly a more or less complete repetition of its phylogeny, was first asserted by FRITZ MULLER, who based his conclusions on the results of comparative research. The idea of the gradual evolution of higher organisms from lower was familiar to the Greek philosophers, but a scientific basis was first given to this hypothesis in the present century. Through the work of CHARLES DARWIN in particular, the belief in the immutability of species has been overturned. DARWIN is also the author of the so-called THEORY oF SELECTION. In drawing his conclusions, he proceeds from the variability of living organisms, as shown by the fact that the offspring neither exactly resemble their parents nor each other. To establish this theory, he also called attention to the coristant over-production of embryonic germs, by which the destruction of the greater part must inevitably result. If this were not so, and all the embryos produced by a single pair attained their full development, they would alone, in a few generations, completely cover the whole surface of the earth. The actual condition of the floras and faunas is thus maintained by the restricted development of the embryos. On account of insufficient space for all, the different claimants are engaged in an uninterrupted struggle, in which the victory is gained by those that, for any reason, have an advantage. Through this “struggle for existence,” as only those organisms possessing some advantage live and mature, a process of enforced selection between the more fortunate survivors must result. In this manner Darwin arrived at the supposition of a process of NatTuraL SELECTION, and confirmed his position by analogy with known results obtained by experimental cross-breeding and cultivation. Newly-developed peculiarities arising from individual variability must be inherited in order to become permanent characteristics of a later generation. Just as in artificial selection, natural selection, although unconsciously, accomplishes this result. As individual peculiarities may be developed by careful breeding and rendered permanent, so by natural selection those qualities which are advantageous in the struggle for existence become more pronounced and are finally con- firmed by heredity. By the continued operation of natural selection, INTRODUCTION 3 organisms must result which are, in the highest degree, fitted and adapted to their environment. Thus, by the survival of the fittest, through natural selection, that adaptability to the environment is gradually evolved which is such a striking characteristic of organic life. That the transitional forms in this process of phylogenetic de- velopment no longer exist, is accounted for in the theory of natural selection by the assumption that the struggle for existence must necessarily have been most severe between similar organisms. For similar organisms must have similar necessities, and the new and better-equipped forms must ultimately prevail over the original less specialised organisms, which, thus deprived of the essential requisites for their existence, finally disappear. Although the great importance of natural selection in the develop- ment of the organic world has been fully recognised by most naturalists, the objection has been raised that it alone is not a sufficient explanation of all the different processes in the phylogeny of an organism. Attention has been called to such organs as would be incapable of exercising their function until in an advanced stage of development, and so could not originally have been of any advantage in a struggle for existence. How could natural selection tend to develop an organ which would be useless so long as it was still in a rudimentary condition? This objection has led to the supposition of an internal force residing in the substance of the organisms themselves, and controlling their continuous development in certain definite directions. Many naturalists, indeed, have gone so far as to affirm that only less advantageous qualities have been affected by the struggle for existence, while the more advantageous have been uninfluenced by it. The phylogenetic changes in the species have been so gradually accomplished as to have escaped observation, and indirect evidence of their existence is all that can be obtained. If the higher organisms have been evolved from the lower, there must, at one time, have been no sharp distinction between plants and animals. The simplest organisms which now exist are in all proba- bility similar to those which formed the starting-point in the phylo- genetic development of animal and vegetable life; and it is still impossible to draw a sharp distinction between the lower forms of plants and animals. The walls which surround the elementary organs of the plant body, and the green colouring matter formed within them, have been cited as decisive indications of the vegetable character of an organism. Surrounded by firm walls, the living substance becomes more isolated, and, consequently, independence of action in plants, as compared with animals, is diminished. By means of the green colour- ing matter, plants have the power of producing their own nutritive substances from certain constituents of the air and water, and from the salts contained in the soil, and are thus able to exist independently ; 4 BOTANY while animals are dependent for their nourishment, and so for their very existence, on plants. Almost all the other differences which distinguish plants from animals may be traced to the structure of plants, characterised by the firm walls of the simple organs, or to the manner of obtaining food. Another characteristic of plants is the un- limited duration of their ontogenetic development, which is continuous, at certain points at least, during their whole life. That none of these criteria are alone sufficient for distinguishing plants from animals is evident from the fact that all the Fungi are devoid of green pigment, and, like animals, are dependent on green plants for their nourish- ment. On the borderland of the two kingdoms, where all other dis- tinctions are wanting, phylogenetic resemblances, according as they may indicate a probable relationship with plants or animals, serve as a guide in determining the position of an organism. While it is thus ditficult to sharply distinguish the two great groups of living organisms from one another, a distinction between them and lifeless bodies is readily recognised. Living organisms are endowed with the quality of IRRITABILITY, in which all lifeless bodies are deficient. External or internal stimuli influence living organisms to an activity, which is manifested in accordance with the requirements and conditions of their internal structure. Even in the smallest known organisms all manifestations of life are occasioned by a similar sensitiveness to external or internal stimuli. The question, however, continually arises whether, in the smallest and simplest organisms at present discernible with the highest magnifying power of the microscope, the ultimate limit of possible life is actually represented. As this limit has always been extended with the increased capabilities of optical instruments, it would seem arbitrary to assert that it would now be impossible to extend it still further. NAGELI accordingly assumed that beyond what is now made visible by the microscope there exists a world of still more and more simple organisms. These he conceived of as showing such a degradation of the vital processes that they finally resemble mere albuminous bodies, which, he supposed, under certain conditions might be produced by purely synthetic processes. In order that a living organism may develop out of such albuminous bodies it must originally have inherent in it the capability of development, that is, the capability of variation and the ability to retain the results of this variability as new qualifications. It must, in addi- tion, have the capability of growth, or of enlarging the mass of its body at the cost of foreign substances, and finally, the power of reproduction, that is, of multiplication by a separation into distinct parts. For the substance itself which serves as a basis for all development, the supposition of an inorganic origin would not be incredible; it would even be possible to imagine that, under certain conditions, this substance is continually in process of formation. On the other INTRODUCTION 5 hand, it must not be forgotten that, so far as is actually known, all living organisms have arisen only from similar organisms. So far as experience has shown, spontaneous generation is unknown. In olden times it was a common supposition that all nature itself was endowed with universal life. According to Aristotle, frogs and snakes sprang from mud and slime. In the same degree that knowledge of the actual development of living organisms was extended, the previously accepted cases of spontaneous generation became more and more restricted, and were finally limited to intestinal worms which could not otherwise, it was thought, be accounted for, and to microscopic organisms the origin of which was also not understood. Now, for such organisms the possibility of a spontaneous generation has been disproved by more modern investigations; the history of the development of intestinal worms is known, and the germs of organic life have been found to exist everywhere. SCHWANN and PasTEuR have been pioneers in this work, and have shown that it is possible to hinder the development of the lower organisms, in places where it is customary to find them, by destroying all existing germs and at the same time preventing the entrance of new ones. It is due to the results obtained by these men in their investigations on spontaneous generation that we are now able to preserve food in a scientific manner. The germs previously existing in the substance to be conserved are destroyed by heat, while, by a proper mode of sealing, the entrance of new germs is rendered impossible, and the decomposition which their presence would occasion is accordingly prevented. All known living organisms have been derived from other living organisms. The attempt to relegate spontaneous generation to an un- known field, and to admit the origin of living from dead substances, has on the other hand derived support from the progress of chemical research. In the early decades of the present century it was customary to draw a distinct line of separation between organic and inorganic chemistry, and to assume that the substances dealt with by organic chemistry could only be produced by the vital action of organisms. The laws governing inorganic chemistry appeared to have no refer- ence to organic chemistry, the formation of organic substance being due to a special force, the “life force.” In 1828 WOHLER obtained urea from ammonium cyanate, and thus for the first time produced an organic compound from an inorganic substance. In 1845 KoLBE completely synthesised trichloracetic acid, and in 1850 BERTHELOT synthesised alcohol and formic acid. By these results the former distinction between organic and inorganic chemistry was destroyed. Organic chemistry has become the chemistry of carbon compounds. Botany, or the science of plants, may be divided into a general and a special part. In the general part, the structure and functions of plants as such will be considered ; in the special part, the particular 6 BOTANY structure and functions of the separate orders of plants will be discussed. The study of the structure of plants is called MorrHouocy ; that of their functions PHystioLocy. In the general part, morphology and physiology will be treated separately ; in the special part, con- jointly. PART I GENERAL BOTANY SECTION I MORPHOLOGY GENERAL BOTANY SECTION I MORPHOLOGY THE object of vegetable morphology is the scientific study of the forms of plants. It does not attempt to discover the causes of the variation in the forms, but rather has accomplished its purpose when it succeeds in showing how one form may be derived from another. The basis of morphological study is, accordingly, phylogeny (p 2). As phylogenetic development can only be inferred, and cannot be directly followed, the methods of morphology must also be indirect. They are dependent for their successful application upon ontogenetic comparison ; for, in the ontogenetic development (p. 2) of a plant, its phylogeny is, to a certain extent, repeated, so that, by a comparison of transitional forms, it is often possible to discover a connection between plants which are apparently most dissimilar. As, however, the ontogeny of a plant is neither an exact nor invariable repetition of its phylogeny, and as connecting links between extreme forms are often wanting, the results of morphological study are frequently imperfect and incomplete. Such parts or mem- bers of plants which it is reasonable to presume have had a common origin are distinguished as HomoLoGous ; those which, while probably having different origins, yet exercise the same functions, are termed ANALOGOUS. Through the adaptation of different parts to the same function, a similarity in both external form and internal structure often results ; and in this way the correct determination of morphological relationships is rendered extremely difficult. Only homologous parts have the same morphological value. This homology is determined by the facts of phylogeny and origin, and not by any correspondence in function. On account, however, of the intimate relation existing between the form and function, and the modifying influence of the one upon the other, it will be necessary in the morphological 10 BOTANY PARI L study of the different members of plants to take into consideration their physiological signification, as organs. When, for phylogenetic reasons, it seems possible to attribute to a number of different mem- bers a common origin, such a hypothetical original form is termed the fundamental or primitive form (“Grundform”). The various modifica- tions which the primitive form has passed through constitute its META- MORPHOSIS. In this way the theory of the metamorphosis of plants, which was once but an ideal conception, attains its true significance. Slightly differentiated structures, which are found at the beginning of a series of progressively differentiating forms, are termed RUDI- MENTARY ; imperfect structures, which have arisen as the result of the deterioration of some perfect forms, are termed REDUCED. Vegetable morphology includes the study of the external form and the internal structure of plants. The descriptive study of the external form of plants has been incorrectly termed ORGANOGRAPHY, for, by the use of the term “organ,” it would seem to have a physiological signi- fication. Morphology takes no recognition of the parts of a plant as organs, but treats of them merely as members of the plant body. The study of the internal structure of plants is often designated ANATOMY or PHytoromy ; but as it usually includes also the study of the more minute internal structure, it resembles rather histology, in the sense in which that term is used by zoologists, and concerns itself to a much less degree with anatomy, properly speaking. In any case, it is the simplest plan to designate the study of the outer forms EXTERNAL MorpPHoLoey, and that of the inner structure INTERNAL MoRPHOLOGY. I. EXTERNAL MORPHOLOGY Plants show a great diversity in the form and arrangement of their members ; it is the task of morphology to determine the points of agreement existing between them. To do this, it is necessary to discover a common origin for their similar but variously developed members. The Development of Form in the Plant Kingdom The Thallus.—The simplest form that we can imagine for an organism is that of a sphere, and this is actually the form of some of the lower plants. The green growth often seen on damp walls consists of an aggregation of the small see bodies of Gloeocapsa polydermatica (Fig. 1), an Alga belonging to the lowest division of the vegetable kingdom. The single plants of the Beer-yeast (Saccharomyces cerevisiae) are ellipsoidal ; but, from their peculiar manner of growth, by budding, they form lateral outgrowths, and thus often appear SECT. I MORPHOLOGY 11 constricted (Fig. 2). Cylindrical and also disc-shaped forms are common to various Algae. The Diatomeae (Fig. 3), in particular, furnish a great variety 2g9 of spindle, canoe, helmet, a & and fan-like shapes ; but z they may all be derived : from the more simple Fia. 2.— Saccharomyces : a e cerevisiae. 1, Cells spherical, discoidal, or without bas: 2 and cylindrical forms. The 3, budding cells. (x Bacteria, which, as the °4°) cause of contagious diseases and of de- ligase een Gaesaal composition, have been the object of so Alootnnen ereae ce Sia: much recent investigation, also exhibit a B, shortly after division; C,a great diversity of form. A small quantity MURDERED, nee of the white deposit on teeth will furnish examples of spherical, rod-like, fibrous, and spiral bacteria (Fig. 4). In the course of the development of a single species several of A Fia. 4.—Bacteria from deposits on teeth. «, Leptothrix bucealis ; a*, the same after treat- ment with iodine; b, Micrococcus ; ¢, Spir- illum dentium after treatment with iodine; d, comma bacilli of the mucous membrane of the mouth. (x 800.) Fic. 3.—Pinnularia viridis. A, Surface view ; B, lateral view. (x 540.) these different forms frequently occur. The next stage in the pro- gressive development of external form in the vegetable kingdom is exhibited by such plants as show a DIFFERENTIATION INTO APEX 12 BOTANY PART I AND BASE. The base serves as a point of attachment, while growth is localised at the apex. In this way a growing point is developed at the apex. As an example of such a form, a young plant of the green Alga, Ulva Lactuca (Fig. 5), may be taken. The development of a more complicated external form is represented by the branched, filamentous, or band-shaped Algae, in which the origin of new formations is more and more restricted to the apex. An ACRO- PETAL order of development, in which the youngest Fic. 5.—Ulva Laetuca, young stage, show- ing apex and _ base. (x 220.) Fia. 6.—Portion of Cladophora glomerata. Fic. 7.—Cladostephus verticillatus. (After (x 48.) PRINGSHEIM, X30.) lateral members are always nearest the growing apex, is clearly demonstrated by the branched filaments of the common green Alga, Cladophora glomerata (Fig. 6). Still more pronounced is the apical SECT. I MORPHOLOGY 13 growth in the brown sea-weed Cladostephus verticillatus (Fig. 7). The great variety in the form of the larger Fungi and Lichens, by which they are distinguished as club-, umbrella-, salver-, or bowl-shaped, or as bearded or shrub-like, is due to the union or intertwining of apically growing filaments. This manner of develop- ment is limited to Fungi and Lichens. In _ other cases, the more complete segmentation exhibited by the lower plants results from the differentiation of independently branching filaments and bands. As the apex itself may undergo successive modifications through continuous bifurcation, as in the case of Dictyota dichotoma (Fig. 8), it does not always necessarily follow that the for- mation of new members must proceed directly from the ori- ginal apex. The highest de- gree of external differentiation among the lower plants is met with in the group of the red sea-weeds (thodophyceae). Many representatives of this class re- semble the higher plants in the formation and arrangement of their members ; Hydrolapathwm sanguineum (Fig. 9), for ex- ample, as is indicated by its name, has a strong resemblance to a species of umex, and affords a remarkable illustration of the analogy of form existing be- tween plants phylogenetically unconnected. On account of a supposed phylogenetic connec- tion between the lower plants, they have been collectively de- Fic. 9.—Hydrolupathum sanguineum. (4 nat. size.) Signated THALLOPHYTES, while the body of the individual organisms, having neither true leaves nor stem, is referred to asa THALLUS. In contrast to the thallus, the body of the higher plants, Fic. 8.—Dictyota dichotoma, (3 nat. size.) 14 BOTANY PART I with its segmentation into stem and leaves, is frequently termed a coRMUS, and the plants themselves CorMopHYTES. To the Cormo- phytes belong all plants from the Mosses upwards. 3 Transition from the Thallus to the Cormus.—The lowest division of the Bryophytes, the Liverworts (Hepaticae), although in many cases Fic. 10.—Riccia fluitans. (Nat. size.) Fic. 11.—Blasia pusilla. s, Sporogonium ; r, Thizoids. (x2.) possessing thalloid bodies without any segmentation into members, contain also forms with the same differentiation into stem and leaves as the higher plants. As between these two extremes there may be found transi- tional forms, this class of plants, accord- ingly, affords valuable assistance in the phylogenetic study of the development of higher plants. A few examples will best illustrate these stages of differentiation exhibited by the Hepaticae. The bifur- cately branching thallus of Riccia fluitans (Fig. 10) is flat and ribbon-like, and in its general appearance resembles the thallus of the previously mentioned brown Alga, Dictyota dichotoma (Fig. 8). A more advanced development is shown by Blasia pusilla (Fig. 11), which has incisions in the sides of its ribbon-like body. The lobes thus formed by the lateral incisions, as is shown by comparison with other Tei ee more highly differentiated Hepaticae, and é also by the study of their development, are properly to be regarded as rudimental leaves. Finally, in Plagio- chila asplenioides (Fig. 12), with alternating ovate leaves and elongated fibrous stems, the segmentation into stem and leaf is complete. The Cormus.—With the segmentation into stem and leaf, the SECT. I MORPHOLOGY 15 distinctive differentiation of the Cormophyte is completed. This, in all probability, has occurred twice in the phylogenetic development of the vegetable kingdom ; once in the Bryophytes, and a second time in the evolution of the Pteridophytes, presumably from ancestral forms resembling the Liverworts. All Bryophytes are attached to the surface on which they grow, by means of root-like hairs or RHIZOIDS (Fig. 11, 7). It is in the next higher group of plants, which, as Vascular Cryptogams, are united in one class, that true roots, in a morphological sense, first make their appearance. They are for the most part cylindrical bodies with apical growing points. Disregarding the distinctions perceptible in its internal structure, a root may always be distinguished from a stem by the ROOT-CAP or CALYPTRA sheathing its apex, and also by the absence of leaves. The Metamorphosis of the Primitive Forms.—After the completion of its differentiation into stem and leaf, and the appearance of roots, there occur only such modifications of the primitive form of the plant body of a Cormophyte as are embraced under its metamorphosis (p. 9), occasionally including a more or less complete fusion of parts originally separate and distinct. The relationships between homologous members, which are often very striking, did not escape the notice of earlier observers. They suggested comparisons, although no real phylogenetic basis for such comparisons existed. Thus, an ideal conception of the form of external members was developed, and finally reached its highest elaboration in GorTun’s Theory of Metamorphosis ; and its abstract scientific conclusion in the writings of ALEXANDER Braun. As the great variety exhibited in the external appearance of the lower plants precluded any possibility of assigning to them hypothetical primitive forms, the whole terminology of the external morphology of plants has been derived from conceptions applicable only to the Cormophytes. Even to-day, the same terms used in reference to the Cormophytes are applied to parts of the Thallophytes, which are evidently cnly analogous. In this sense it is customary to distinguish between stem and leaf in such Algae as Hydrolapathwm (Fig. 9). Such a use of terms is only permissible where reference is made to the manner of segmentation, with the intention of emphasising the analogy with the somewhat similar members of the Cormophytes. The question whether, in the different groups of the Cormophytes, all the members designated by the same names are really homologous, cannot properly be discussed here. It would seem almost impossible to derive from the Bryophytes all the forms of cormophytic segmentation shown by the Pteridophytes. However this may be, from the Pteridophytes upwards, the segmentation of the members appears to have had a similar origin, and the similarity of terminology is based, therefore, upon an actual homology of the parts. Relations of Symmetry Every section through an organ or member of a plant, made in the direction of its longitudinal axis, is distinguished as a longitudinal 16 ‘BOTANY PART I section ; those at right angles to it being termed cross or transverse sections. Such parts of plants as may be divided by each of three or more longitudinal planes into like halves are termed either PoLysyM- Fic. 13.—Diagram showing the so-called de- Fic. 14.—Diagram showing two-ranked cussate arrangement of leaves. alternate arrangement of leaves. METRICAL, RADIAL, or ACTINOMORPHIC. The degree of symmetry peculiar to any leafy shoot will be more Ss apparent from a diagram, that is if the leaves which it bears be projected on a plane at right angles to its axis. The radial symmetry of a shoot with opposite leaves is clearly shown in the adjoining diagram (Fig. 13). A shoot with its leaves arranged alternately in two rows shows quite different relations of sym- metry. The diagram of such a shoot (Fig. 14) can only be divided into similar halves by two planes. When such a condition exists, a member or plant is said to be BISYMMETRICAL. When, however, a division into two similar halves is only possible in one plane, the degree of sym- metry is indicated by the terms sym- METRICAL, MONOSYMMETRICAL, or ZYGO- MORPHIC. When the halves are equal, but have a different structure and are spoken of as ventral and dorsal sides, the body Fro. 15-—Diagram of a foliage-leaf, is termed DORSIVENTRAL. Ordinary foli- A, Surface view; B, transverse sy + : : sce Ulun) a -Ulane of eAUHIGLiN: age-leaves exhibit this dorsiventral struc- ture, and their upper and lower surfaces are not only different in appearance but they also react differently to external influences. In the accompanying figure (Fig. 15) such a SECT. I MORPHOLOGY 17 monosymmetrical, dorsiventral foliage leaf is diagrammatically repre- sented. From the surface view (4) and from the cross-section (B), in which the distinction between the dorsal and ventral sides is in- dicated by shading, it is obvious that but one plane of symmetry (s) can be drawn. As the zoologists often term this degree of symmetry BILATERAL, the same term is frequently employed with reference to plants. Branch Systems Thallophytes as well as Cormophytes exhibit systems of branching, resulting either from the formation of new growing points by the bifurcation of a previously existing growing point, or from the develop- ment of new growing points in addition to those already present. In this way there are produced two systems of branching, the DICHOTO- MOUS and the MONOPODIAL. By the uniform development of a continu- ously bifureating stem, a typical dichotomous system of branching is produced, such as is shown in Dictyota dichotoma (Fig. 8). In a typically developed example of the monopodial system there may always be distinguished a main axis, the MONOPODIUM, which gives rise to lateral branches from which, in turn, other lateral branches are developed. A good example of this form of branching is afforded by a Fir-tree. Where one of the two branches is regularly developed at the expense of the other, the dichotomous system assumes an appearance quite different from its typical form. The more vigorous branches may then, apparently, form a main axis, from which the weaker branches seem to spring, just as if they were lateral branches. This mode of branching is illustrated by the Selaginellae (Fig. 351). Such an apparent main axis is termed, in accordance with its origin, a SYM- popium. On the other hand, in the monopodial system two or even several lateral branches may develop more strongly than the main axis, and so simulate true DICHOTOMY or POLY- Tomy. Such monopodial forms of branching are referred to as FALSE DICHOTOMY or FALSE POLYTOMY, as the case may be. A good example of false dichotomy may be seen in the Pirin eae a beatae Mistletoe (Viscum album, Fig. 16). "* “Gaceaicnotomy. (nat.sve) If, however, a lateral branch so ex- ceeds the main axis in development that it seems ultimately to become a prolongation of the axis itself, a sympodium is again formed. This is exactly what occurs in the Lime and Beech; in both of these trees the terminal buds of each year’s growth die, and the prolonga- Cc 18 BOTANY PART I tion of the stem, in the following spring, is continued by a strong lateral bud, so that in a short time its sympodial origin is no longer recognisable. In most rhizomes, on the other hand, the sympodial nature of the axis can be easily distinguished ; as, for example, in the rhizome of Polygonatum multiflorum (Fig. 21), in which, every year, the terminal bud gives rise to an aerial shoot, while an axillary bud pro- vides for the continuance of the axis of the rhizome. In the flower- producing shoots or inflorescences of Phanerogams the different systems of branching assume very numerous forms. These will be more fully-described in their proper place. To such inflorescences belong the ventrally coiled dorsiventral shoots, which produce new shoots from their convex dorsal surfaces, instead of in their leaf-axils. The Shoot The Development of the Shoot.—Under the term shoot a stem and its leaves are collectively included. A stem possesses an apical mode of growth (Fig. 17), and its unprotected growing point is described as naked, in contrast to that of the root with its sheathing root- cap. The apex of the shoot gener- ally terminates in a conical pro- tuberance, designated the VEGE- TATIVE CONE. As it is always too small to be visible to the unaided eye, it is best seen in magnified median longitudinal sections. So long as the apex of the shoot is still internally un- differentiated, it continues in em- bryonic, condition, and it is from the still embryonal vegetative cone that the leaves take their origin. They first appear in acropetal succession as small, Fic. 17.—Apex of a shoot of a phanerogamic conical protuberances, and attain plant. v, Vegetative cone; f, leaf rudiment ; : g, rudiment of an axillary bud. (x 10.) a larger size the further removed they are from the apex of the stem. As the leaves usually grow more rapidly than the stem which produces them, they envelop the more rudimentary leaves, and over- arching the vegetative cone, form, in this manner, a BUD. Buds are therefore merely undeveloped shoots. If they are to remain for a long time undeveloped, as for example is the case with winter buds, they are protected in a special manner during their period of rest. SECT, I MORPHOLOGY 19 The Origin of New Shoots.—The formation of new growing points by the bifurcation of older points of growth, in a manner similar to that already described for Dictyota dichotoma (Fig. 8), occurs also, in almost typical form, in the lower thalloid Hepaticae (Miccia jluitans, Fig. 10). Among the Cormophytes this method of producing new shoots is of less frequent occurrence, and is then mainly limited to the Pteridophytes, for one division of which, the Lycopodiuceae, it is characteristic. In this case, whenever a shoot is in process of bifurcation, two new vegetative cones are formed by the division of the growing point (Fig. 18). In most of the Lycopodiaceae the new shoots thus formed develop unequally ; the weaker Fic. 18.—Longitudinal section of a becomes pushed to one side and ultimately _ bifurcating shoot (p) of Lyco- appears as a lateral branch (Fig. 19). podium alpinum, showing me . a a i‘ equal development of the rudi- Although a relationship as regards posi- jnentary shoots, p', »”; b, leat tion is generally apparent between the rudiments; ¢, cortex; /, vascular origin of leaves and the lateral shoots, in aa (Alter HEGELMAIER, the system of branching resulting from such : a bifurcation of the vegetative cone this connection does not exist. In the more highly developed Bryophytes, particularly in the true Mosses, new shoots arise obliquely below the still rudimentary leaves at some distance from the growing point. In the Phanerogams new shoots generally arise in the axils of the leaves. In the accompanying illustration of a longi- tudinal section of a phanerogamic shoot (Fig. 17) the rudiment of a shoot (g) is just appear- Fic, 19.—Bifureating shoot ing in the axil of the third uppermost leaf ; in (p) of Lycopodivan inun- the axils of the next older leaves the conical datum, showing unequal a development of the rudi- protuberances of the embryonic leaves are mentary shoots, y’, p”; already beginning to appear on the still rudi- b, leaf rudiments. (After : Heorvaanan S40) mentary shoot. These rudimentary shoots may either continue to develop, or they may remain for a time in an embryonic condition, as buds. Shoots thus pro- duced in the axils of leaves are termed AXILLARY SHOOTS. The leaf in the axil of which a shoot develops is called its SUBTENDING LEAF. An axillary shoot is usually situated in a line with the middle of its subtending leaf, although it sometimes becomes pushed to one side. As a rule, only one shoot develops in the axil of a leaf, yet there are instances where it is followed by additional or ACCESSORY SHOOTS, which either stand over one another (serial buds), as in Lonicera, Gleditschia, Gymnocladus, or side by side (collateral buds), as in many Liltaceae. 20 BOTANY PART I Although in the vegetative regions, iz. the regions in which merely vegetative organs are produced, the rudiments of the new shoots of phanerogamic plants make their appearance much later than those of the leaves, in the generative or flower-producing regions the forma- tion of the shoots follows directly upon that of their subtending leaves, or it may even precede them. In this last case the subtending leaves are usually either poorly developed or completely suppressed, as in the inflorescence of the Cruciferae, in which a series of phylogenetic changes has probably led to this result. Shoots developing in definite succession from the growing points of other shoots are designated NORMAL, in contrast to ADVENTITIOUS SHOOTS, which are produced irregularly from the older portions of a plant. Such adventitious shoots show no definite arrangement, and frequently spring from old stems, also from the roots of herbaceous plants (Brassica oleracea, Anemone sylvestris, Convolvulus arvensis, Rumex Acetosella), or of bushes (Rubus, Rosa, Corylus), or of trees (Populus, Ulmus, Robinia), or they may develop even from leaves, particularly from the fronds of Ferns. An injury to a plant will frequently induce the formation of adventitious shoots, and for this reason gardeners often make use of pieces of stems, rhizomes, or even leaves as cuttings from which to produce new plants. A leaf of a Begonia merely placed upon damp soil will soon give rise adventitiously to new plants. Leaves and also normal shoots, which make their appearance as outgrowths from the portions of the parent shoot still in embryonic condition, have an external or EXOGENOUS origin. Adventitious shoots, on the other hand, which arise from the older parts of stems or roots, are almost always ENDOGENOUS. They must penetrate the outer portions of their parent shoot before becoming visible. Adventi- tious shoots formed on leaves, however, arise, like normal shoots, exogenously. The further Development of the Shoot.—All normal shoots are dependent for their origination upon the embryonic substance of the growing point of the parent shoot ; even when they make their appearance at some distance from the growing apex (Fig. 17), embryonic substance has been reserved at that point for their forma- tion. The growing points of adventitious shoots are also, for the most part, produced from tissue which has retained its embryonic condi- tion in the older portions of the plant. In some cases, however, they arise from newly-developed growing points, and afford evidence of the power inherent in plants to return to an embryonic state and produce new growing points. The processes of development which result in the production of new segments at the apex of a shoot are followed by an increase in size and by the further growth of the segments. This growth is usually introduced by the vigorous elonga- tion of the segments, by means of which their rapid unfolding from SECT. I MORPHOLOGY 21 the bud is brought about. The region of strongest growth in a shoot is always at some distance from its growing point. The growth in length and consequent elongation of the shoot is in some cases so slight that the leaves remain close together, and leave no free spaces on the stem, thus forming so called DWARF SHOOTS. As examples of such dwarf shoots may be mentioned the thickly-clustered needles or fascicled leaves of the Larch, the rosettes formed by the fleshy leaves of the House-leek (Sempervivum), and also the flowers of Phanerogams with their thickly-crowded floral leaves. In the ordinary or ELONGATED SHOOTS, such as are formed in the spring by most deciduous trees, the portions of the stem between the insertions of the leaves become elongated by the stretching of the shoot. The stem of a shoot, as contrasted with the leaves, is often spoken of as the axis ; while the portions of the stem axis between the insertions of the leaves are termed the INTERNODES, and the parts of the axis from which the leaves arise the NoDES. When the base of the leaves encircles the stem, or when several leaves take their origin at the same node, the nodes become strongly marked (Labiatue). In some cases the growth in length of a shoot continues for a longer time at certain intermediate points by means of INTERCALARY GRowtTH. Such points of intercalary growth are generally situated at the base of the internodes, as in the case of the Grasses. A displace- ment from the position originally occupied by the members of a shoot frequently results from intercalary growth. A bud may thus, for example, become pushed out of the axil of its subtending leaf, and so apparently have its origin much higher on the stem ; or a subtending leaf, in the course of its growth, may carry its axillary bud along with it, so that the shoot which afterwards develops seems to spring directly from its sub- tending leaf; or, finally, the subtending leaf may become attached to its axillary shoot, and growing out with it, may thus appear to spring from it (Fig. 20). Resting Buds.—As a means of protection, buds may become invested, in winter, with scale-like leaves or BUD-SCALES, which are rendered still more effective as protective struc- tures by hairy outgrowths and excretions of resin and gum, and also by the occurrence of ; ats Fic. 20.—Samolus Valerandi, air-spaces. Not infrequently the subtending Q,ch axinary shoot leaf takes part in the protection of its axillary bearing its subtending bud, and the base of the leaf-stalk, after the oe ee im leaf itself has fallen, remains on the shoot and oats forms a cap-like covering for the winter bud. The buds of tropical plants, which have to withstand a dry period, are similarly protected ; 22 BOTANY PART I but where the rainfall is evenly distributed throughout the year buds develop no such means of protection. Many of the deciduous trees in Temperate regions are inclined to unfold their winter buds in the same vegetative period in which they are produced. This tendency is particularly marked in the Oak, and results in the development of a MIDSUMMER GROWTH. All the buds of a plant do not develop; there are numerous deciduous trees— such as the Willow, in which the terminal buds of the year’s growth regularly die. Sometimes buds, usually the first-formed buds of each year’s shoot, seem able to remain dormant during many years without losing their vitality ; these are termed DORMANT BuDS. In the case of the Oak or Beech such latent buds can endure for hundreds of years ; in the meantime, by the elongation of their connection with the stem, they continue on its surface. Often it is these, rather than adventitious buds, which give rise to the new growths formed on older parts of stems. It may sometimes happen that the latent buds lose their connection with the woody parts of their parent stem, but nevertheless grow in thickness, and develop their own wood ; they then form remarkable spherical growths within the bark, which may attain the size of a hen’s egg and can be easily separated from the surrounding bark. Such globular shoots are frequently found in Beech and Olive trees. The Metamorphosis of the Shoot.—The BULBILS and GEMMA, which become separated from their parent plant and serve as a means of reproduction, are special forms of modified buds. They are always well supplied with nutritive substances, and are of a corresponding size. Many plants owe their specific name to the fact that they produce such bulbils, as, for example, Lilium bulbiferum and Dentaria bulbifera. Shoots that live underground undergo characteristic modifications, and are then termed ROOT-STOCKS or RHIZOMES. By means of such sub- terranean shoots many perennial plants are enabled to persist through the winter. A rhizome develops only modified leaves in the form of larger or smaller, sometimes scarcely visible, scales. By the presence of such scale leaves and by its naked vegetative cone, as well as by its internal structure, a rhizome may be distinguished from a root. Rhizomes usually produce numerous roots; but when this is not the case, the rhizome itself functions as a root. Rhi- Fic. 21.—Rhizome of Polygonatum multiflorum. «, Bud of zomes often attain a con- next year’s aerial growth; b, scar of this year’s, and siderable thickness and c, d, e, scars of three preceding years’ aerial growth; wv, store up nutritive material roots. (} nat. size.) for the formation ak serial shoots. In the accompanying illustration (Fig. 21) is shown the root-stock of the so-called Solomon’s Seal (Polygonatum nulti- florum). At d and c are seen the scars of the aerial shoots of the SECT. I MORPHOLOGY 23 two preceding years; and at b may be seen the base of the stem growing at the time the rhizome was taken from the ground, while at a is shown the bud of the next year’s aerial growth. The rhizome of Coralliorrhiza innata, a saprophytie Orchid, affords a good example of a root-stock functioning as a root (Fig. 22). Buss, also, belong to the class of metamorphosed shoots. They represent a shortened shoot with a flattened, discoid stem (Fig. 23, zk), the fleshy thickened scale Fic. 22.—Rhizome of Coralliorrhiza innata, Fic. 23. — Longitudinal section of tulip a, Floral shoot; b, rudiments of new bulb, Tulip Gesneriana. zk, Modified rhizome branches. (After ScHAcuT, stem; zs, scale leaves; v, terminal nat. size.) bud; k, rudiment of a young bulb; w, roots. (Nat. size.) leaves (zs) of which are filled with reserve food material. The aerial growth of a bulb develops from its axis, while new bulbs are formed from buds () in the axils of the scale leaves. Another form of underground shoot, allied to bulbs and connected with them by transitional forms, is distinguished as a TUBER. The axis of a typical tuber, in contrast to that of a bulb, is fleshy and swollen, functioning as a reservoir of reserve material, while the leaves are thin and scaly. Of such tubers those of the Meadow Saffron (Colchicum autumnale) or of Crocus sativus are good examples. In the Meadow Saffron new tubers arise from axillary buds near the base of the modified shoot, but in the Crocus from buds near the apex. In consequence of this, in the one case the new tubers appear to grow out of the side, and in the other to spring from the top of the old tubers. The tubers of the Potato 24 BOTANY PART I (Fig. 24) or of the Jerusalem Artichoke (Helianthus tuberosus) are also subterranean shoots with swollen axes and reduced leaves. They are formed from the ends of branched, underground shoots or runners (STOLONS) and thus develop at a little distance from the parent plant. The so-called eyes on the outside of a potato, from which the next year’s growth arises, are in reality axillary buds, but the scales which represent their subtending leaves can only be distinguished on very young tubers. The parent plant dies after the formation of the tubers, and the reserve food stored in the tubers nourishes the young plants which afterwards develop from the eyes. As, in their uncultivated Fic. 24.—Part of a growing Potato plant, Solanum tuberosum. The whole plant has been de- veloped from the dark-coloured tuber in the centre. (From Nature, copied from one of Bai.on’s illustrations, 4 nat. size.) state, the tubers of the Potato plant remain in the ground and give rise to a large number of new plants, it is of great advantage to the new generation that the tubers are produced at the ends of runners, and are thus separated from one another. Similar advantages accrue from surface runners, such as are produced on Strawberry plants. Surface runners also bear scale-like leaves with axillary buds, while roots are developed from the nodes. The new plantlets, which arise from the axillary buds, ultimately form independent plants by the death of the intervening portions of the runners. Still more marked is the modification experienced by shoots which only develop reduced leaves, but the axes of which become flat and leaf-like, and assume the functions of leaves. Such leaf-like shoots are called CLADODES or PHYLLOCLADES. Instructive examples of such forma- SECT. I MORPHOLOGY 25 tions are furnished by Luscus aculeatus (Fig. 25), a small shrub whose stems bear in the axils of their scale-like leaves (/) broad, sharp-pointed cladodes (cl), which have altogether the appearance of leaves. The flowers arise from the upper surface of these cladodes, in the axils of scale leaves. In like manner the stems of the Opuntias (Fig. 26) are considerably fattened, while the leaves are reduced to small thorny protuberances. In this case the juicy flat shoots perform not only the functions of assimilatory organs, but also serve as water-reservoirs in time of drought. It is possible that all the leaves of a plant may become more or less completely reduced, without any marked change Fic. 25.—Twig of Ruscus aculeatus. f, Leat ; Fie. 26.—Opuntia monacantha Haw., showing flower cl, cladode ; bl, flower. (Nat. size.) and fruit. (After Scuumany, } nat. size.) occurring in the appearance of the stems, except that they then take on a green colour; this, for example, is the case in the Scotch Broom (Spartium scoparium), which develops only a few quickly-falling leaves at the end of its long, naked twigs; or, as in many species of rushes (Juncus, Scirpus), whose erect, slender, wand-like stems are entirely leafless and at the same time unbranched. As a rule, however, all leafless green Phanerogams will be found to have swollen stems, as in the variously shaped Huphorbiae and Cacti. A great reduction in the leaves, and also in the stems, often occurs in phanerogamic parasites, in consequence of their parasitic mode of life. The leaves of the Dodder (Cuscuta, Fig. 185, 6) are only represented by very small, yellowish scales, and the stem is similarly yellow instead of green. The green colour would, in fact, 26 BOTANY PART I be superfluous, as the Dodder does not. produce its own nourishment, but derives it from its host plant. Cuscuta Trifolii, one of the most fre- quent of these parasites, is often the cause of the large yellow areas frequently observable in the midst of clover fields. In certain tropl- cal parasites belonging to the families Rafflesi- aceae and Balanophoraceae, the process of re- duction has advanced so far that the flowers alone are left to represent the whole plant. Rafflesia Arnoldi, a plant growing in Sumatra, is a remarkable example of this ; its flowers, although they are a metre wide, the largest flowers in existence, spring directly from the roots of another plant (species of Cissus). A peculiar form of metamorphosis is ex- hibited by some climbing plants through the transformation of certain of their shoots into TENDRILS. Such tendrils assist the parent plant in climbing, either by twining about a support or otherwise holding fast to it. The twining bifurcated tendrils of the Grape-vine, for example, are modified shoots, and so are also the more profusely branched, hold-fast tendrils of Ampelopsis Veitchit Fic. 27.— Ampelopsis l’eitchii. (Fig. 27). R, R, Stem-tendrils. (j nat. Shoots may undergo a still greater re- ne duction by their modification into THORNS, as a defence against the depredations of animals. Of shoots modi- fied in this manner, the Black Thorn (Prunus spinosa), the White Thorn (Crataegus), and the Honey Locust (Gleditschiw) afford instructive examples. The thorns are simple or branched, hard, pointed bodies. In Gleditschia (Fig. 28) the thorns are developed primarily from the uppermost of several serial buds; while secondary thorns may develop on older por- tions of the stem from the lower buds of the series, and thus give rise to clusters of thorns. The most marked change in the form of the shoot, in addition to the displacement and union of its different members, takes place in phanerogamic flowers. The shoots from which py, 28.—Stem-thorn of Gledit- flowers are developed are termed FLORAL _ schiv trivcunthos. (4 nat. SHOOTS, in contrast to the FOLIAGE sHoots, ‘”°) the functions of which are merely vegetative. The axis of the floral SECT. I MORPHOLOGY 27 shoot remains short and becomes flattened or even depressed at the tip. The vegetative cone of the rudimentary flower-bud also undergoes corresponding modifications. The floral leaves, which spring from the floral axis, often grow together, and in many cases become so united with the axis, that it is only possible to discover the different steps of this process by means of thorough phylogenetic and comparative morphological investigation. In most instances the rule seems to hold that axillary buds are not formed within a flower except in cases of abnormal development. Shoots and their Order of Sequence.—lIf the vegetative cone of the primary axis of a plant, after reaching maturity, is capable of reproduction, a plant with but one axis will result, and the plant is designated UNIAXIAL or haplocaulescent. Usually, however, it is not until a plant has acquired axes of the second or third order, when it is said to be DIPLOCAULESCENT or TRIPLOCAULESCENT, or of the mth order, that the capacity for reproduction is attained. A good illustration of a plant with a single axis is afforded by the Poppy, in which the first shoot produced from the embryo terminates in a flower, that is, in that organ of Phanerogams which gives rise to the embryonic germs. As an example of a plant with a triple axis may be cited the common Plantain, Plantago major, whose primary axis produces only foliage and scale leaves; while the secondary axes give rise solely to bracteal leaves, from the axils of which finally spring the axes of the third order, which terminate in the flowers. In the case of trees, only shoots of the nth order can produce flowers. The Habit or General Aspect of Plants is dependent upon their origin, mode of growth, and duration, and upon the peculiar develop- ment of their branch systems. Cormophytes which develop herbace- ous aerial shoots, and persist only so long as is requisite for the development and ripening of their fruit, be it one or several vegetative periods, are called HERBS. Herbaceous plants, however, which, although annually dying down to the ground, renew their existence each year by means of new shoots produced from underground shoots, rhizomes, or roots, are further distinguished as PERENNIALS or perennial herbs. SHRUBS or TREES, on the other hand, have woody, persistent shoots, which bear fruit repeatedly. Shrubs retain their lateral shoots, so that their branches are formed near the ground; trees, on the contrary, soon lose their lower lateral branches, and have a main stem or trunk, which bears a crown of branches and twigs. In catalogues and descriptions of plants the duration of the period of growth is usually expressed by special symbols: thus © indicates an annual ; © a biennial, and 1 4 perennial herb ; his employed to designate both trees and shrubs, and for trees the sign fj is also in use. a 28 BOTANY Fant The Stem or Axis of the Shoot According as the axis of a shoot remains herbaceous or becomes hard and lignified, a distinction is drawn between an herbaceous and a woody stem. A long leafless shoot arising from a rosette of radical leaves and producing only flowers is called a scape. The hollow jointed stems of the Gramineae are termed GRASS-HAULMS, and should be distinguished from the similar stems or haulms of the Tuncaceae and Cyperaceac, which are unjointed and filled with light porous pith. Plants with short swollen stems, being apparently stem- less, are described as ACAULESCENT. The actual stem of such acaul- escent plants may be thickly clothed with leaves throughout its entire length, as in the case of the Agave, or it may bear leaves only at its apex, as in the Cyclamen. Stems are also distinguished as round, elliptical, angular, etc., according to their appearance in cross-section. The Leaf Development of the Leaf.—tThe first appearance of the leaf as a lateral protuberance (Fig. 17, f) on the vegetative cone of the shoot has already been referred to (p. 18). In a transverse section through the apex of a shoot (Fig. 29), the origin of leaves as lateral protuberances is more evident than in a longi- tudinal section. The embryonic leaf rudiment generally occupies but a small portion of the periphery of the vegetative cone; it may, how- ever, completely invest it. In like manner, when the mature leaves are arranged in whorls, the developing protuberances of the rudimentary leaves may, although this is not usually the case, form at first a continuous wall-like ring around the growing point; and only give rise Jater to the separate leaf rudiments. Leaves take their origin only from such parts of a plant as have remained in an embryonic condition. To this Fic. 29.—Apical view of the y vegetative cone of a shoot rule there are no exceptions. A leaf never of Evonymus japonicus. arises directly from the older parts of a plant. (x 12.) In cases where it apparently does so its develop- ment has been preceded by the formation of a growing point of a new shoot. When it first appears on the vegetative cone a rudimentary leaf resembles an embryonic shoot, but a difference soon manifests itself, and the shoot rudiment develops a vegetative cone and lateral protuberances for the formation of leaves. The growing point of a shoot has usually an UNLIMITED GROWTH, while the growth of a leaf is LrMITED. A leaf usually continues to grow at its apex for a SECT. I MORPHOLOGY 29 short time only, and then completes its segmentation and develop- ment by intercalary growth. It is true that some leaves, as those of Ferns, not only continue growing for a long time, but also retain « continuous apical growth and complete their whole segmentation in acropetal succession. On the other hand, the leaf-like cladodes, although they are in reality metamorphosed shoots, exhibit a limited apical growth like that of ordinary leaves. Leaving out of consideration the Ferns and a few related plants, the following observations in regard to the development of the leaf hold good for the majority of Cormophytes. The unsegmented protuber- ance of the still rudimentary leaf, termed by EICHLER the PRIMORDIAL LEAF (Fig. 30, 4, )), first projects from the vegetative cone of the shoot (4, 1). This is usually followed by a separation of the primor- dial leaf into the LEAF- BASE (g in 4 and 8B) and the rudimentary lamina or UPPER LEAF (0 in 4 and 8). The leaf-base, or the part of the rudimentary leaf which F!6- 30.—Apex of an Elm shoot, Ulmus campestris. A, C : I dani hi Showing the vegetative cone v, with the rudiments immediate Y adjoins the of a young leaf, b, still unsegmented, and of the next vegetative cone, either takes older leaf, exhibiting segmentation into the Jamiuar no farther part in the eue- fatima 9 sud lath g: 2 shoving te oe ceeding differentiation of the leaf, or it develops into a LEAF-SHEATH (vagina) or into STIPULES. The upper leaf, on the other hand, gives rise to the leaf-blade or LAMINA. If the fully-developed leaf possesses a LEAF-STALK (petiole), it becomes afterwards interposed by intercalary growth between the upper leaf and the leaf-base. The metamorphosis of the leaf is exhibited in its greatest diversity by the leaves of Phanerogams, in which the various homologous leaf structures have been distinguished as SCALE LEAVES, FOLIAGE LEAVES, BRACTEAL LEAVES, and FLORAL LEAVES. Foliage Leaves, generally referred to simply as leaves, are the leaf structures on which devolves the task of providing nourishment for their parent plants. As the exercise of this function is dependent upon the presence of a green pigment, foliage leaves have, accordingly, a green colour. In certain cases, where their form is extremely simple, as in the needles of Conifers, the primordial leaf simply increases in length without any further differentiation into parts. In other un- divided leaves, however, whether lanceolate, elliptical, ovate, or other- wise shaped, the flat leaf lamina is distinct from the leaf-base, while a leaf-staik may also be interpolated between them. If no leaf-stalk is de- veloped the leaf is said to be SESSILE, otherwise it is described as STALKED. 30 BOTANY PART I The sessile leaves usually clasp the stem by a broad base. Where, as in the case of the Poppy (Papaver somniferum) and of the different species of Bupleurum, the leaf-base surrounds or clasps the stem, the leaves are described as PERFOLIATE. If the bases of two opposite leaves have grown together, as in the Honeysuckle (Lonicera Cauprifolium), they are said tobe CONNATE, Where the blade of the leaf continues downwards along the stem, as in the winged stems of the common Mullein (Ver- bascum thapsiforme), the leaves are distinguished as DECURRENT. The petiole of a leaf merges either directly into the leaf-base, or it swells at its lower end into a LEAF-CUSHION or PULVINUS, and is thus articulated with the leaf-base. This is the case, for instance, with many of the Leguminosae (Fig. 213). The leaf-blade, in turn, may be either sharply marked off from the petiole, or it may be prolonged so that the petiole appears winged, or again it may expand at its junction with the petiole into ear-like lobes, A leaf is said to be ENTIRE if the margin of the leaf- blade is wholly free from indentations; otherwise, if only slightly indented, it is usually described as SERRATE, DENTATE, CRENATE, UN- DULATE, SINUATE, or INCISED, as the case may be. When the inci- sions are deeper, but do not extend more than half-way to the middle of the leaf-blade, a leaf is distinguished as LOBED or CLEFT according to the character of the incisions, whether more or less rounded or sharp; if the incisions are still deeper the leaf is said to be PARTITE, and if they penetrate to the midrib or base of the leaf-blade it is termed DIVIDED. The divisions of the leaf-blade are said to be PINNATE or PALMATE, according as the incisions run towards the midrib or towards the base of the leaf-blade. Where the divisions of the leaf-blade are distinct and have a separate insertion on the common leaf-stalk or on the midrib, then termed the SPINDLE or RHACHIS, a leaf is spoken of as COMPOUND; in all other cases it is said to be stmpLe. The single, separate divisions of a compound leaf are called leaflets. These leaflets, in turn, may be entire, or may be divided and undergo the same segmentation as single leaves. In this way double and triple compound leaves may be formed. The leaflets are either sessile or stalked ; and sometimes also, as in Lobinia and Alimosa, their stalklets articulate with the spindle by means of swollen pulvini. The term PEDATE is applied to leaves on which segments are further divided on one side only, and the new segments are similarly divided. Variations in the outline of leaves, whether they are entire, serrate, dentate, crenate, incised, etc, as well as peculiarities in their shape and segmentation, are of use in the determination of plants. The VENATION or NERVATURE of leaves is also taken into consideration, and leaves are in this respect described according to the direction of their so-called veins or nerves, as PARALLEL VEINED or NETTED VEINED. In parallel venation the veins or nerves run either approximately parallel with each other or in curves, con- verging at the base and apex of the leaf (Fig. 31, s); in netted veined SECT, I MORPHOLOGY 31 leaves (Fig. 178) the veins branch off from one another, and gradually decrease in size until they form a fine anastomosing network. In leaves with parallel venation the parallel main nerves are usually united by weaker cross veins. Netted or reticulately veined leaves in which the side veins run from the median main nerve or MIDRIB are further distin- guished as PINNATELY VEINED, or as PAL- MATELY VEINED when several equally strong ribs separate at the base of the leaf-blade, and give rise in turn to a network of weaker veins. Parallel venation is characteristic, in general, of the Monocotyledons; reticulate venation, of Dicotyledons. Monocotyledons | have usually simple leaves, while the leaves | of Dicotyledons are often compound, and are | also more frequently provided with stalks. j Many plants are characterised by the de- | ; velopment of different forms of foliage leaves. Such a condition is known as heterophylly. Thus the earlier leaves of Hucalyptus globulus are sessile and oval, while those subsequently formed are stalked and sickle-shaped. In other cases the heterophyllous character of the leaves may represent an adaptation to the surrounding environment, as in the Water Crowfoot (Ranunculus aquatilis), in which the floating leaves are lobed, while those entirely Fis. 31.—Part of stem and leaf 0 és stats a grass. h, Haulm; v, leaf- submerged ane finely divided. sheath ; k, swelling of the leaf- i . 4 sheath above the node; s, part The nerves or veins give to a leaf its necessary of jeaf-nlade; 1, ligule. (Nat. mechanical rigidity and render possible its flattened size.) form. The branches of the veins parallel to the margin of most leaves prevent their tearing: when there are no such marginal nerves in large thin leaves, the lamina is easily torn into strips by the wind and rain. This frequently happens to the leaves of the Banana (Musa), which, consequently, when growing under natural conditions in the open air, presents quite a different appear- ance than when grown under glass. The leaves of the Banana, after becoming thus divided, offer less resistance to the wind. Ina similar manner the leaves of Palms, although undivided in their bud state, become torn even during the process of their unfolding. A similar protection from injury is afforded to the Aroid (Jon- stera) by the holes with which its large leaf-blades become perforated. Equally advantageous results are secured by many plants whose leaves are, from their very inception, divided or dissected. The submerged leaves of aquatic plants, on the other hand, are generally finely divided or dissected, not only for mechanical purposes, but also to afford a more complete exposure of the leaf surface to the water. Accordingly, in such water-plants as Ranunculus aquatilis (Fig. 197), which possess both floating and submerged leaves, it is generally the latter only 32 BOTANY PART I that are dissected and filiform in character. The pointed extremity of the foliage leaves of many land plants, according to Sraut, facilitates the removal of water from the leaf surface. Fleshy so-called succulent leaves, like fleshy stems, serve as reservoirs for storing water. In Monocotyledons the leaf-base very often forms a SHEATH about the stem ; in Dicotyledons this happens much less frequently. In the case of the Gramineae, the sheath is open on the side of the stem opposite the leaf-blade (Fig. 31, v), while in the Cyperaceae it is com- pletely grown together. The sheath of the grasses is prolonged at the base of the lamina into a scaly outgrowth, the ligule. Such a sheath, while protecting the lower part of the internodes which remain soft and in a state of growth, gives them at.the same time rigidity. Stipules.—These are lateral appendages sometimes found at the base of leaves. When present they may be either small and incon- spicuous, or may attain a considerable size. When their function is merely to protect the young growth in the bud, they are usually of a brown or yellow colour, and are not persistent ; whereas, if destined to become assimilatory organs, and to assist in providing nourishment, they are green, and may assume the structure and form of the leaf-blade, which sometimes becomes modified and adapted to other purposes (Figs. 35, 36). Normally, the stipules are two in number, that is, one on each side of the petiole. In many species of Galium, where the stipules resemble leaf-blades, the leaf-whorls appear to be composed of six members, but consist actually of but two leaves with their four stipules, which may be easily distinguished by the absence of any buds in their axils. In other species of the same genus (Galium cruciatum and palustre) there are only four members in the whorls, as each two adjoining stipules become united. In many cases the stipules have the form of appendages to the enlarged leaf-base. Sometimes both stipules are united into a single one, which then appears to have an axillary origin; or the stipules may completely encircle the stem, and thus form a sheath about the younger undeveloped leaves. This sheath-like fusion of the stipules may be easily observed on the India- rubber tree (Ficus elastica), now so commonly grown as a decorative plant. In this case the stipular sheath is burst by the unfolding of each new leaf and pushed upwards on the stem. In the Polygonaceae the stipular covering is similarly torn apart by the developing leaves, but then remains on the stem in the form of a membranous sheath (ochrea). Scale Leaves possess a simpler structure than foliage leaves, and are attached directly to the stem, without a leaf-stalk. They exercise no assimilatory functions, and are more especially of service as organs of protection. Scale leaves exercise their most important function as bud-scales; they are then hard and thick, and usually of a brown colour. They most frequently take their origin from the enlarged leaf-base; in that case the upper leaf either does uot SECT. £ MORPHOLOGY 33 develop, or exists only in a reduced condition at the apex of the scale. The true morphological value of scale leaves of this nature is very evident in the bud scales of the winter buds of the Horse- chestnut (desceulus Hipporastanum) ; for, while the outer scales show no perceptible indications of an upper leaf, small leaf-blades can be distinctly distinguished at the apices of the inner scales. In other cases the scale leaves are modified stipules, and are then also derived from the leaf-base; while, in other instances, they themselves form the enlarged, but still undifferentiated, primordial leaves. The bud scales of the Oak are the stipules of leaves in which the laminz are only represented by minute scales. Scale leaves, usually colourless and in various stages of reduction, are found on rhizomes (Fig. 21), bulbs (Fig. 23), and tubers (Fig. 24). On the aerial stems arising from such subterranean shoots the formation of similar scale leaves generally precedes the development of the foliage leaves, with which they are connected by a series of transitiorfal forms. — Bracteal Leaves resemble scale leaves in form, and have a similar development. They act as subtending leaves for the floral shoots, and are termed BRAcTs. They are connected with foliage leaves by intermediate forms. Though they are not infrequently green they may be otherwise coloured, or even altogether colourless. Floral Leaves.—The modified leaves which form the flowers of Phanerogams are termed floral leaves. In the highest development attained by a phanerogamic flower (Fig. 32), the successive floral leaves are distinguished as_ sepals (k), petals (¢), stamens (a), and carpels (7). In most cases the sepals are green and of a firm structure; the petals, on the other hand, are more delicate and variously coloured. The stamens are generally filament- ous, and produce the pollen in special receptacles. The carpels more closely resemble scale leaves, and by closing to- gether form receptacles within which the ovules are DEO: Fic. 82.—Flower of Paueonia peregrina. k, Sepals; e, duced. The stamens and petals ; a, stamens; y, carpels. Part of the sepals, carpels of Phanerogams corre- petals, and stamens have been removed to show : the pistil, consisting of two separate carpels. (Half spond to the spore-bearing jat, size.) leaves of the Vascular Crypto- gams. Such spore-bearing leaves are termed SPOROPHYLLS, and even in the Vascular Cryptogams exhibit a greater or less departure from the form of other foliage leaves. It is evident that the scale and bracteal leaves are to be considered as rudimental foliage leaves, not D 34 BOTANY PART I only from the mode of their development but also from the possibility of transforming them into foliage leaves. GOEBEL, by removing the growing tip and foliage leaves of a shoot, succeeded in forcing it to develop other foliage leaves from its scale leaves. Rhizomes, grown in the light, develop foliage leaves in place of the usual scale leaves, and even on a potato it is possible to induce the formation of small foliage leaves instead of the customary scale leaves. Leaf-Sears.—After a leaf has fallen, its previous point of insertion on the stem is marked by the cicatrix or scar left by the fallen leaf. In winter, accordingly, when the trees are denuded of their leaves, the axillary buds are plainly perceptible above the leaf-scars. The Metamorphosis of Foliage Leaves.—A form of slightly modi- fied foliage leaves is seen in peltate leaves, or those of which the petioles are attached to their lower surfaces somewhat within the margin, as in the leaves of the Indian Cress (Zvo- pacolum majus, Fig. 180). In the process of their de- velopment the young leaf- blades, in this case, grow not only in the same direc- tion as the petioles, as a prolongation of them, but also horizontally in front ofthem. The tubular leaves of many insectivorous plants may have commenced their development in much the same way. The leaves of Nepenthes robusta (Fig. 33), for example, in the course of adaptation to the per- formance of their special function, have acquired the form of a pitcher with a lid which is closed in young leaves, but eventu- ally opens. The pitcher, as GOEBEL has shown, arises as a modification of the leaf-blade. At the same time the leaf- base becomes expanded into a leaf-like body, while the petiole between the two parts sometimes fulfils the office of a tendril. By a similar metamorphosis of its leaflets, bladder-like cavities are Fic. 33.—Nepenthes robusta. (4 nat. size.) SECT. I MORPHOLOGY 85 developed on the submerged leaves of Utriculuria (Fig. 34). The entrance to each bladder is fitted with a small valve which permits the ingress but not the egress of small water-animals. While such leaves display a progressive metamorphosis, in other instances the modifica- tions are of the nature of a reduction. A metamorphosis of the whole leaf lamina, or a part of it, into tendrils (LEAF-TENDRILS) is a compara- tively frequent occurrence, especially among the Puapilionaceae. In the adjoining figure of a Pea leaf (Fig. 35), the upper pair of leaflets have become transformed into delicate tendrils which have the power of Fic. 34.—Utricularia vulgaris. A, Part of leaf with several bladders (x 2). B, Single pinnule of leaf with bladder (x 6). C (after Gorpex), Longitudinal section of a bladder (x 28); v, valve; a, wall of bladder ; J, cavity of bladder. twining about a support. In the case of the yellow Vetchling, Lathyrus Aphaca (Fig. 36), the whole leaf is reduced to a tendril and the function of leaf-blade is assumed by the stipules (~). A comparison between these two forms is phylogenetically instructive, as it indicates the steps of the gradually modifying processes which have resulted in the complete reduction of the leaf lamina of Lathyrus. But, for still other reasons, the last case deserves attention, as it shows clearly the morphological distinction between leaf and stem tendrils, and emphasises the value of comparative morphological investigation. In Lathyrus Aphaca the stipules assume the function of the metamorphosed leaf laminz ; in other instances, as in the case of the Australian Acacias (Fig. 48, 7, 8, 9), it is the leaf petioles which, becoming flattened and leaf-like in appearance, supply the place of the undeveloped leaf-blades. Such a metamorphosed petiole is called a 36 BOTANY PART I PHYLLODE, and, except that it is expanded perpendicularly, exactly resembles a cladode. From the latter, however, it is morphologically different, for the one represents a metamorphosed petiole, the other a metamorphosed shoot. In accordance with this distinction phyllodes OD Fic. 35.—Portion of stem and leaf of the common Pea, Pisum Fic. 36.—Lathyrus Aphaca. sativum. s, Stem; n, stipules ; b, leaflets of the compound s, Stem; 2, stipules; leaf; 7, leaflets modified as tendrils; u, floral shoot. b, leaf-tendril. (4 nat. ($ nat. size.) size.) do not, like cladodes, spring from the axils of leaves. Just as stems become modified into thorns (Fig. 28), by a similar metamorphosis leaves may be converted into leaf thorns. Whole leaves on the main axis of the Barberry (Berberis vulgaris) become thus transformed into thorns, usually three, but in their character of leaves still give rise to axillary shoots pro- vided with foliage leaves. By a similar meta- morphosis, the two stipules of the leaves of the common Locust (Robinia Pseudacacia) become modified into thorns, while the leaf lamina persists as a foliage leaf (Fig. 37). In addition to stem and leaf thorns, many plants are provided with other outgrowths of similar appearance, which are often wrongly called thorns; but as they have, in reality, an alto- gether different morphological origin, they are more correctly termed prickles. The Fie, 37.--Part of stem and com. Prickles so characteristic of the Rose and pound leaf of Robinia Pseuda- Blackberry belong to the same category as cain, mM, andsae te Ht hairs, and in no way represent metamorphosed ea OO" segments of the plant body. Like hairs, ie they are also superficial outgrowths (EMER- GENCES). They have no definite fixed relation to the external seg- mentation of a plant, but arise from any part of its surface. SECT, I MORPHOLOGY ww =F Prickles vary considerably in number, they are not arranged in any definite manner, and in some cases are entirely absent. Vernation and Astivation.—A section through a winter bud shows a wonderful adaptation of the rudimentary leaves to the narrow space in which they are confined (Fig. 38). They may be so disposed that the separate leaves are spread out flat, but more frequently they are folded, either cross-wise or length-wise on the midrib (conduplicate), or in longitudinal plaits, like a fan (plaited, plicate) ; or they may be crumpled with no definite arrangement of the folds ; or each leaf may be rolled, either from the tip downwards (circinate) or longitudinally, from one margin to the other (convolute), or from both margins towards the midrib, either outwards (revolute) or inwards (in- volute, Fig. 38,2). The manner in which each separate leaf is disposed Fia. 38.—Transverse section of a bud of Populus Fic. 39.—Transverse section of a leaf-bud nigra. k, Bud-seales showing imbricated of Tsuga canadensis, just below the estivation ; 1, foliage leaves with involute apex of the shoot, showing a yy, diver- vernation; s, each leaf has two stipules. gence. (After HormMEISTER.) (x 15.) in the bud is termed VERNATION. On the other hand, the arrangement of the leaves in the bud with respect to one another is designated AESTIVATION. In this respect the leaves are distinguished as FREE when they do not touch, or VALVATE when merely touching, or IMBRICATED, in which case some of the leaves are overlapped by others (Fig. 38, &). If, as frequently occurs in flower-buds, the margins of the floral leaves successively overlap each other in one direction, obliquely or otherwise, the estivation is said to be CONTORTED. The Arrangement of Leaves.—In all erect elongated shoots, and still more so in dwarf shoots, it is apparent that there is a marked regularity in the arrangement of leaves. This regularity may be most easily recognised in cross-sections of buds (Fig. 39), particularly in sections showing the apex of the vegetative cone (Fig. 29). From such an apical section it is easily seen that the regularity in the 38 BOTANY PART I order of arrangement of the rudimentary leaves is determined by their conformity with the position of the older leaves on the vegetative cone, and the consequent necessity of utilising the remaining free space. Thus, the position of newly developing leaves is influenced by those already existing, while their formation is the result of in- ternal causes. After the rudiments of the- new leaves have become protruded from the vegetative cone, they come in direct contact with the older leaves, and may then, as SCHWENDENER has shown, become displaced through the consequent mutual pressure, by which correspond- ing changes in their ultimate position may be effected. If the axis does not grow in length, but only in thickness, as the rudimentary leaves increase in size, their points of insertion will be displaced later- ally by longitudinal pressure ; if the axis increases in length, and not in thickness, the insertion of the leaves will be displaced by a trans- verse pressure. The arrangement of the leaves would also be affected by any increase or decrease in the size of the vegetative cone, un- accompanied by a corresponding increase or cessation of the growth of the rudimentary leaves. Abrupt changes in the usual position of the leaves may also be occasioned by the torsion of their parent stem. Thus, the leaves of Pandanus first appear in three straight rows on the vegetative cone, and their subsequent spiral arrangement, according to SCHWENDENER, results from the torsion of the stem. An irregular arrangement of the leaves, such as occurs, for example, on the flower-stalk of the Crown Imperial (Fritillaria imperialis), may result from the unequal size of the leaves at the time of their in- ception on the vegetative cone. A frequent mode of arrangement of foliage leaves is the decussate, in which two-leaved whorls alternate with each other (Fig. 29). A whorled arrangement is characteristic of floral leaves. When the number of leaves in each whorl is the same the ® whorls usually alternate. On the other hand, the Jom number of members in the different whorls of (A =) floral leaves will often be found to vary greatly ; or a whorl, the existence of which would be expected from the position of other whorls and from a comparison with allied plants, may be Nea”, altogether wanting. In this connection a com- parison of the flowers of the Liliaceae and Iridaceae will be instructive. The flowers of the Liliaceae Fic. 40.—Diagram of a . Be ae aise (Fig. 40) are composed of five regularly alter- main axis is indicated nating, three-leaved whorls or cycles, viz. a calyx by a black dot, oppo- and a corolla (each consisting of three leaves, and site to which’ is the s Ds ? ieiek: on account of their similar appearance usually referred to conjointly as the PERIANTH), an outer and an inner cycle of stamens, and finally, in the centre of the flower, an ovary of three carpels. In the flowers of the Jridaceae (Fig. 41) SECT, MORPHOLOGY 39 the arrangement is exactly similar, except that one whorl, that of the inner cycle of stamens, is lacking, but the three carpels are situated exactly as if the missing cycle of stamens were present. From this similarity of arrangement, despite the absence of the one cycle of stamens, the conclusion has been drawn that, at one time, the inner row of stamens was actually present, but has now disappeared. In constructing a THEORETICAL DIAGRAM of the Jridaceae the missing cycle of stamens is indicated by some special sign (by crosses in Fig. 41); a diagram in which theoretical sup- positions are not taken into consideration is called ; an EMPIRICAL DIAGRAM. Diagrams showing the Peet alternate arrangement of leaves, in cases where of the Iris. ‘The ab- only a single leaf arises from each node, may Cae ar ete tea be constructed by projecting the successive nodes Oo of a stem upon a plane by means of a series of concentric circles, on which the position of the leaves may be indicated (Fig. 42). The angle made by the intersection of the median planes of any two successive leaves is called their DIVERGENCE, Se Za 'f Fic. 42.—Diagram showing # position of Fic. '43.—The 7 position on the ontspread leaves. The leaves numbered according surface of the axis. v, Orthostichies ; p, to their genetic sequence. parastichies. The leaves are numbered according to their genetic sequence. and is expressed in fractions of the circumference; for example, in case the angular divergence between two successive leaves is 120°, their divergence is expressed by the fraction 4. In the adjoining diagram (Fig. 42) a 2 divergence is indicated. Where the lateral distance between two successive leaves is 2 of the circumference of the stem, the sixth leaf is above the first, the seventh above the second, 40 BOTANY PART I and so on. The leaves form on the axis five vertical rows, which are spoken of as ORTHOSTICHIES. Where the leaves are very much crowded, as in dwarf-shoots, a set of spiral rows called PARASTICHIES, due to the contact of the nearest laterally adjacent members, becomes much more noticeable than the orthostichies. If the surface of such an axis be regarded as spread out horizontally, the parastichies become at once distinguishable (Fig. 43), and it will be evident that the sum of the parastichies cut by every cross-section through such an axis must equal the number of the orthostichies. On objects like pine cones, in which the parastichies are easily recognised, they may be used to determine the leaf arrangement. The most common divergences are the following, 4, 1, 2, 4, +5, 3, 34, etc. In this series it will be observed that in each fraction the numerator and denominator are the sum of those of the two preceding fractions. The value of the different fractions varies, accordingly, between } and 4, while always approaching a divergence angle of 137° 30’ 28”. The frequent recurrence of the divergence angles, expressed by the fractions of this series, is, no doubt, due to the fact that by such arrangements of the leaves, the space available is utilised to the best advantage, and with the least possibility of mutual hindrance in the performance of the assimilatory functions. If a line be drawn on the surface of a stem, so as to pass in the shortest way successively through the points of insertion of every leaf, a spiral called the GENETIC SPIRAL will be constructed. That portion of the genetic spiral between any two leaves directly over each other on the same orthostichy is termed a cycLe. Where the divergence is 2, a cycle will accordingly include five leaves, and will in such a case have made two turns about the stem. An attempt has been made to trace spirals even where the leaves are arranged in whorls, but now that the genetic causes controlling such leaf arrangements are understood, such a procedure seems rather superfluous. It is, moreover, no longer attempted to extend the spiral theory to dorsiventral shoots ; since it is now known that this arrangement of the leaves is due, not to an ideal spiral law, but to mechanical causes regulating their development. The tips of dorsiventral shoots are frequently coiled ventrally inwards, bearing their leaves either dorsally or on the sides, but, in the latter case, more on the dorsal than ventral surface. The creeping stems of many Ferns or the flower-bearing shoots of Forget-me-not (Jfyosotis) are good examples of such dorsiventral shoots. The line joining successive leaves in such cases is, at the best, but a zigzag. The Root The third member of the plant body of Cormophytes, in its typical development as an UNDERGROUND ROOT, shows but little varia- SECT. I MORPHOLOGY 41 tion. This regularity of form is due to the uniformity of the conditions to which roots are exposed in the ground, for AERIAL Roots, which are for the most part restricted to the moist climate of the tropics, exhibit a much greater tendency to modification. The covered vegetative cone and the inability to develop leaves are characteristic of roots, and furnish an easy means of distinguishing them from underground shoots. A ROOT-CAP or CALYPTRA affords the vegetative cone of a root the protection that is provided to the apex of a stem by the rudimentary leaves. Although, generally, the existence of a root-cap is only dis- closed by a median, longitudinal section through the root-tip, in some roots it is plainly distinguishable as a cap-like covering. The very noticeable caps on the water roots of Duckweed (Lem) are not, in reality, root-caps, as they are not derived from the root, but from a sheath which envelops the rudimentary root at the time of its origin. They are accordingly termed RooT-pocKETS (Fig. 415, wi), As a general rule, however, roots without root-caps are of rare occurrence, and in the case of the Duckweed the root-pockets perform all the functions of a root-cap. The short-lived roots of the Dodder (p. 25) afford another example of roots devoid of root-caps. Characteristic of roots are also the ROoT-HAIRS (Fig. 47, 7), which are found at a short distance from their apices. As the older root-hairs die at the same rate that the new ones are developed, only a small portion of a root is provided with root-hairs at the same time. In other respects, root- hairs, like prickles, show no regularity in their individual position or number. In some few instances roots develop no root-hairs; this is true of the roots of many Conifers, and of most aerial roots. Branching of the Root.—Just as a shoot may become bifurcated by the division of its growing point (Fig. 18), so a root may become similarly branched. For the most part, this mode of branching takes place only in the roots of Lycopodiaceae, the shoots of which are also dichotomously branched (p. 19). The branching of roots usually occurs in acropetal succession, but the lateral roots (Fig. 47, sw) make their appearance at a much greater distance from the growing point of the main root, than lateral shoots from the apex of their parent stem. By reason of the internal structure of their parent root, lateral roots always develop in longitudinal rows (Fig. 47). They are of endogenous origin, and before reaching the surface must break through the surrounding and overlying tissue of the parent root, by the ruptured portions of which they are often invested, as with a collar. ADVENTITIOUS ROOTS, just as adventitious shoots, may arise from any part of a plant. They are especially numerous on the underside of rhizomes (Fig. 21, w), and also, when the external conditions are at all favourable, they seem to develop very readily from the stem nodes. A young shoot, or a cutting planted in moist soil, quickly forms adventitious roots, and roots may also arise in a similar manner from leaves, especially from Begonia leaves. The origin of adventitious roots, 42 BOTANY PART I as of all roots, is endogenous. Dormant root rudiments occur in the same manner as dormant buds of shoots. The ease with which willows are propagated from shoots is well known, and is due to the prompt- ness with which they develop adventitious roots from apparently latent embryonic tissue, when the requisite conditions of moisture and darkness are ful- filled. The Metamorphosis of the Root.—The customary nomen- clature for the various root forms is based on their shape, size, and mode of branching. A root which is a prolongation downwards of the main stem is called the main root or TAP- ROOT; the other roots are termed, with reference to the tap-root, LATERAL ROOTS of different orders, according to the order of their develop- ment. The roots may enlarge Fia. 44.—Root-tubers of Dahlia is arias, s, The and become turnip-shaped or lower portions of cut stems. (} nat. size.) tuberous (Fig. 44), Such tuberous growths often greatly resemble stem tubers, but may be dis- tinguished from them by their root-caps, by the absence of any indica- tions of leaf development, and by their internal structure. The tubers of the Orchidaceae exhibit, morphologically, a peculiar mode of for- mation. They are, to a great extent, made up of fleshy, swollen roots, fused to- gether and terminating above in a shoot-bud. At their lower extremity the tubers are either simple or palmately segmented. In the adjoining figure (Fig. 45) both an old (¢’) and a young tuber (¢") are represented still united together. The older tuber has produced its flowering shoot (4), and has begun to shrivel and dry up; a bud, formed at the base of the shoot, in the axil of a scale leaf (s), has already developed the adventitious roots, which, swollen and fused together, have given rise to the younger tuber. The aerial roots of tropical Epiphytes differ considerably in their structure from underground roots. The aerial roots of the Orchidaceae and of many Aroideue are provided with a spongy sheath, the VELAMEN, by means of which they are enabled to absorb moisture from the atmosphere. Aerial roots, in some cases, grow straight downwards, and upon reaching the ground, branch and function as nutritive roots for the absorption of nourishment ; in other instances, they turn from the light, and, remaining comparatively short and unbranched, fasten them- selves as CLIMBING ROOTS to any support with which they come in SECT. I MORPHOLOGY 43 contact. The climbing roots of many Orchids, Aroids, and Ferns branch and form lodgment places for humus ; and into this the nutritive root branches penetrate as special outgrowths of the climbing roots. Pendent aerial roots generally contain chlorophyll. In the Orchid Angraecum globulosum the task of nourishing the plant is left entirely to the aerial roots, which are then devoid of a velamen, and very much flattened. They are distinctly green-coloured, and supply the place of the leaves which lose their green colour and are reduced to scales. The aerial roots of the epiphytic Bromeli- aceae are developed exclusively as climbing roots, while the leaves function not only as assimilating organs, but also assume the whole task of water-absorption. All the fio. 45.—orchis latifolia. ¢', The aerial roots of Epiphytes are, so far as their oe. ce toe origin is concerned, adventitious. scale leaf with axillary bud, hk, from which the new tuber The numerous adventitious roots which form has arisen ; r, OTANAEY, adven- a thickly-matted covering on the trunks of Tree- POETS Lee ey ferns become hard after death, and serve as organs of protection. In some Palms (Acanthorrhiza, Iriartea) the adventitious roots on the lower part of the stem become modified into thorns, RooT-THORNS. The roots of certain tropical plants, such as Pandanus and the swamp-inhabiting Mangrove trees, are specially modified. These plants develop on their stems adventitious roots, which grow obliquely downwards into the ground, so that the stems finally appear as if growing on stilts. The Banyan trees of India (Ficus Indica) produce wonder- ful root-supports from the under side of their branches, upon which they rest as upon columns. The lateral roots of certain Mangrove trees become modified as peculiar breathing organs, and for this purpose grow upwards into the air out of the swampy soil or water in which the trees grow; they then become greatly swollen or flattened, and provided with special aerating passages. Such RESPIRA- TORY or AERATING RooTs surround the Mangrove trees like vigorous Asparagus stalks, and enable the roots growing below in the mud to carry on the necessary exchange of gases with the atmosphere. The roots of parasites usually undergo a far-reaching reduction. The roots of the Dodder (Cuscuta) form wart-like excrescences (Fig. 185, H) at the point of contact with their nourishing host, which they finally penetrate. They draw nourishment from the host plant, and are consequently termed SUCTION ROOTS or HAUSTORIA; such haustoria divide within their host into single threads, and from each thread a new parasitic plant may be formed. The immense flowers of Rafflesia Arnoldi, which spring directly from the roots of Cissus, owe their origin to similar haustoria. The reduction of the roots may extend to such a degree that, in many plants, no roots are formed. It has been already mentioned (p. 23) that in the case of Corallior- rhiza innata (Fig. 22) the rhizome assumes all the functions of the 44 BOTANY PART I roots, which are entirely absent. Also in many aquatics, Salvinia, Wolfia arrhiza, Utricularia, Ceratophyllum, roots are altogether absent. The Ontogeny of Plants Just as in the phylogenetic development of the vegetable kingdom there is an evolution from simpler to more complex forms, so each plant in its ontogeny passes through a similar process of evolution. The study of the ontogenetic development of a plant is termed EMBRYOLOGY. A young plant, in its rudimentary, still unformed condition, is called an EMBRYO or GERM ; and the early stages of its development are spoken of as GERMINATION. Asa rule, the embryo, in the beginning of its de- velopment, is microscopic and of a spherical form. In a lower organism this condition may continue from the beginning to the end of its development, as is the case in Gloeocapsa polydermatica (Fig. 1, p. 11); or the development may proceed further to the formation of filament- ous, ribbon-like or cylindrical bodies. If the future plant is to have a growing point, a part of the germ substance is retained in its embryonic condition, and further development proceeds from this embryonic substance. In the more highly-organised plants the different members arising from the growing point only gradually attain that degree of development characteristic of the particular plant. The plant must develop and attain maturity, and it is not until it has accomplished this that certain portions of the embryonic substance of the growing point are appropriated to the production of new embryos. The different generations arising from an embryo of a plant may exactly resemble each other, or an ALTERNATION OF GENERATIONS may occur, in which case each succeeding generation is unlike its immediate predecessor. As a general rule, the alternate generations are equiva- lent, although this is not necessarily the case. One of the alternating generations is usually sexually differentiated, that is, its reproductive cells are only capable of development after a fusion with other repro- ductive cells. This process of the fusion of two sexually differentiated cells is called FERTILISATION, and its product a fertilised egg. The asexual generation, on the contrary, produces reproductive cells, termed SPORES, which require no fertilisation before germinating. In the case of the Thallophytes, the alternation of generations is often extremely complicated by the irregularity of the recurrence of the different generations, and by the interposition of other modes of reproduction, not in line with the regular succession of generations. In the Cormophytes, however, asexual and sexual generations regularly alternate, and consequently, whenever an alternation of generation occurs, more than one generation is requisite to complete a cycle in the development of a species. Accordingly, in the conception of a species, two or more individuals are included. These individuals may exist separately and distinct from each other, or they may be so SECT. I MORPHOLOGY 45 united as to appear but a single organism; as, for example, in the Mosses, where the spore-producing generation lives upon the sexual plant, or as in Phanerogams, where, conversely, the sexual generation completes its development within the asexual plant. In Phanerogams, owing to the formation of the embryo within seeds, that stage of the development of a plant which is termed germination is clearly defined; for not until the seed is completely Fic. 46.—Thuja occidentalis. A, Median longitudinal section through the ripe seed (x5); B, C(x2); D, E (nat. size), different stages of germina- tion ; h, hypocotyl; c, cotyledons ; 7, radicle ; v, vegetative cone of stem. Fic. 47.—Seedling of Carpinus Betulus. h, Hypocotyl; c, cotyledons; hw, main root ; sw, lateral roots ; r, root-hairs ; e, epicotyl ; 1,1’, foliage leaves. (Nat. size.) formed does the newly-formed plantlet begin its independent exist- ence. The embryo, while still enclosed within the seed, generally exhibits the segmentations characteristic of Cormophytes. Protected by the hard seed-coats, it is enabled to sustain a long period of rest. Abundant deposits of nutritive material in the embryo itself, or surrounding it, are provided for its nourishment during germination. The different segments of a phanerogamic embryo have received distinctive names ; thus, asin the embryo of the American Arbor Vitae (Thuja occidentalis, Fig. 46), the stem portion (h) is termed the HYPO- 46 BOTANY PART I COTYL, the first leaves (c) are the SEED LEAVES or COTYLEDONS, while the root (r) is distinguished as the RADICLE. The tap-root of the fully- developed plant is formed by the prolongation of the radicle. In Fig. 47 a germinating plantlet of the Hornbeam (Carpinus Betulus) is shown with its hypocotyl (h) and both cotyledons (c); but its radicle has already developed into a tap-root (hw) with a number of lateral roots (sw). An internode and foliage leaf (/) have been produced from the vegetative cone of the stem; while the next higher internode is also distinguishable, but has not yet elongated, and a second foliage leaf (I’) is unfolding. A highly organised plant, which begins its development with the simplest stages and gradually advances to a higher state of differentiation, repeats in its ontogeny its phylogenetic develop- ment. In the process of its ontogenetic development much has been altered, and much omitted, so that it presents but an imperfect picture of its past history ; nevertheless, this representation is valuable, and, next to comparative methods, furnishes the most important source of morphological knowledge. Whatever is true of the development of a plant from the embryo is also, as a rule, applicable to its further growth from the growing point, and, con- sequently, a knowledge of the mode of development at the growing point is of great importance in detecting homologies. The earlier a characteristic makes itself apparent in the embryo, or the nearer it is to the growing point of the old plant, so much the greater is its value in determining the general relationships existing be- tween the different plants ; the later it is exhibited in the embryo, or the farther removed it is from the . 48,—Seedling of Acaci ‘ i i Pte i ete, Tare Laas QFOWINE point ofthe plant, pinnate, the following leaves bipinnate. The petioles Ss ue, of leaves 5 and 6 are vertically expanded; and in the but the greater, in propor- following leaves, 7 8, 9, modified as phyHodia, with tion, its importance in de- nectaries, n. (x circa 3.) fini ning the character of a genus or species. From the fossil remains of former geological periods, it is safe to conclude that such Conifers as Thauju, Biota, and the various SECT. I MORPHOLOGY 47 Junipers, that now have scale-like compressed leaves, have been derived from Conifers with needle-shaped leaves. This conclusion is further confirmed by the fact, that on the young plants of the scaly-leaved Conifers typical needle-shaped leaves are at first developed. The modified leaf forms do not make their appearance until the young plant has attained a certain age, while in some Junipers needle-shaped leaves are retained throughout their whole existence. Even still more instructive are the Australian Acacias, whose leaf-stalks become modified, as phyllodia (p. 35), to perform the functions of the reduced ' leaf-blades. The proof for such an assertion is furnished by a germinating plantlet of Acacia pycnantha (Fig. 48), in which the first leaves are simply pinnate, and the succeeding leaves bipinnate. In the next leaves, although still compound, the leaf-blades are noticeably reduced, while the leaf-stalks have become somewhat expanded in a perpendicular direction. At length, leaves are produced which possess only broad, flattened leaf-stalks. As many other species of this genus are provided only with bipinnate leaves, it is permissible on such phylogenetic grounds to conclude that the Australian Acacias have lost their leaf-blades in comparatively recent times, and have, in their stead, developed the much more resistant phyllodes as being better adapted to withstand the Australian climate. The appearance, accordingly, of the phyllodes at so late a stage in the ontogenetic development of these Acacias is in conformity with their recent origin. It may, in like manner, be shown that in the case of plants with similarly modified leaf forms, the metamorphosis of the leaves does not take place until after the cotyledons and the first foliage leaves have been developed, and it is then usually effected by degrees. Il. INTERNAL MORPHOLOGY (Histology and Anatomy) The Cell All plants, as all animals, are composed of elementary organs called cells. In contrast to animal cells, typical vegetable cells are surrounded by firm walls, and are thus sharply marked off from one another. In fact, it was due to the investigation of the cell walls that the cell was first recognised in plants. An English micrographer, Ropertr Hooks, was the first to notice vegetable cells). He gave them this name in his Micrographia in the year 1667, because of their resemblance to the cells of a honeycomb, and published an illustration of a piece of bottle-cork having the appearance shown in the adjoining figure (Fig. 49). Rospert Hooke, however, was only desirous of ex- 48 BOTANY PART I hibiting by means of different objects the capabilities of his microscope ; consequently, the Italian, MARCELLO MALPIGHI, and the Englishman, NEHEMIAH GREW, whose works appeared almost simultaneously a few years after Hooke’s Micro- graphia, have been regarded as the founders of vegetable Histology. The living contents of the cell, the real body or substance, was not re- cognised in its full significance until the middle of the present century. Only then was attention turned more earnestly to this study, which has since been so especially advanced by MEYEN, SCHLEIDEN, Huco v. Mout, NAGELI, FERDI- Fic. 49.—Copy of a part of Hooxe’s illustration of NAND CoHN, PRINGSHEIM, and Max SCHULTZE. bottle-cork, which he en- If an examination be made of a thin longi- Pied Sehematisim or tex tudinal section of the apex of a stem of a phanero- gamic plant, with a higher magnifying power than that used in the previous investigation (Fig. 17) of the vegetative cone, it will be seen that it consists of nearly rectangular cells (Fig. 50), which are full of protoplasm and separated from one another by delicate walls. In each of the cells there will be clearly dis- tinguishable a round body (k), which fills up the greater part of the cell cavity. This body is the cell NucLEUS. If sections, made in different directions through the vegetative cone, be compared with one another, it will be seen that its component cells are nearly cubical or tabular, while yy. 50.—Kmbryonic cell from the the nuclei are more or less spherical or vegetative cone ofa phanerogamic disc-shaped. The finely granular substance = Plant. Nucleus; Tw, nuclear a: " nembrane ; 7, nucleolus ; c, cen- (cy) filling in the space between the nucleus —trospheres; cy, cytoplasm; ch, (&) and the cell wall (m) is the CELL PLASM — chromatophores; m, cell wall. or CYTOPLASM. Recent investigations have ees Mavala remy neha ates shown that two extremely small, colourless bodies le in the cytoplasm, near the nucleus. These are the CENTROSPHERES or ATTRACTION SPHERES (cs). In addition to these there are to be found, about the nucleus, an indefinite number of somewhat larger bodies, which are also colourless and highly refrac- tive; these are the pigment-bearers or CHROMATOPHORES (ch). NUCLEUS, CENTROSPHERES, CYTOPLASM, and CHROMATOPHORES, CON- STITUTE THE ELEMENTS OF THE LIVING BODY OF A TYPICAL VEGETABLE ceLL. ‘To designate all these collectively, it is customary to use the term PROTOPLASM, which is then to be understood as including all the living constituents of a cell. Protoplasm does not show the same degree of internal differentia- tion in all vegetable organisms. The protoplasm of the Fungi has no SECT. I MORPHOLOGY 49 chromatophores. In the protoplasm of the lowest plants, the Fission plants or the Schizophytes, the internal differentiation does not seem to have progressed to the same extent as in the more highly organised plants. The protoplasm of animal cells, on the other hand, is devoid of chromatophores. While animal cells usually remain continuously filled with protoplasm, vegetable cells soon form large sAP CAVITIES. It is only the embryonic cells of plants that are entirely filled with protoplasm, as the cells, for ex- ample, of an ovule or of a growing point; they afterwards become larger and contain proportionally less protoplasm. This can be seen in any longitudinal section through a stem apex. At a short distance from the growing point the enlarged cells have already begun to show cavities or VACUOLES (v in 4, Fig. 51) in their cytoplasm. These are filled with a watery fluid, the CELL sap. The cells continue to increase in size, and usually soon reach a condition in which their whole central portion is filled by a single, large sap cavity (vin B, Fig. 51). This is almost always the case when the increase in the size of the cell is considerable. The cytoplasm then forms only a thin layer lining the cell wall, while the nucleus takes a parietal position in the peripheral cytoplasmic layer. At other times, however, the sap cavity of a fully-developed cell may be traversed by bands and threads of cytoplasm; and in that case the nucleus is suspended in the centre of the cell. But whatever position the nucleus may occupy, it is always embedded in cytoplasm ; and there is always an unbroken peripheral layer of cytoplasm lining the cell wall. This cytoplasmic peripheral layer is in con- tact with the cell wall at all points, and, so long as the cell remains living, it continues in that condition. In old cells, however, this cyto- plasmic layer frequently becomes so thin as to escape direct observation, and is not perceptible Fic. 51.—Two cells taken at different distances from the growing point of a phanero- gamie shoot. k, Nucleus; cy, cytoplasm; v, vacuoles, represented in B by the sap cavity. (Somewhat diagram- matic, x cirea 500.) until some dehydrating reagent, which causes it to recede from the wall, has been employed. Such a thin cytoplasmic peripheral layer has been described by Huco v. Mout under the name of PRIMORDIAL UTRICLE. E 50 BOTANY PART I As a rule, every living vegetable cell has a nucleus. Dead cells lose their living protoplasmic contents, and, strictly speaking, should no longer be termed cells, although the name was first applied to them when in that condition. In reality they represent only cell cavities. With their death, however, cells do not lose their importance to a plant. Without such cell cavities a plant could not exist, as they perform for it the office of water-carriers, while at the same time exercising other functions. The necessary rigidity of a plant is also dependent, to a great extent, on the mechanical support afforded by a framework composed of dead cells. Thus the heart of a tree consists exclusively of the walls of dead cells. The Protoplasm.—We naturally begin with that substance which constitutes the living plant body, the Protoplasm, also more shortly designated the Plasma. In order to facilitate an insight into the real character of protoplasm, attention will first be directed to the Strue Funel or fungus animals (Myxomycetes), a group of organisms which stand on the border between the animal and vegetable king- doms. These Myxomycetes are characterised at one stage of their development by the formation of a PLASMODIUM, a large naked mass of protoplasm. The plasmodium is formed from the protoplasm of the spores. These spores are unicellular bodies (Fig. 52, a, 0), filled with cytoplasm, in which lies a central nucleus, and are surrounded by tenacious cell walls. The spores germinate in water, their contents, breaking through the spore walls, come out (c, d) and round themselves off. A change of form soon takes place; the protoplasmic mass elongates and assumes somewhat the shape of a pear, with the forward end prolonged into a fine whip-like process or flagellum (¢, f, g). Thus the contents of the spore have become transformed into a SWARM-SPORE, which now swims away by means of whip-like movements of its flagellum. In addition to the nucleus, which is visible in the front end of every swarm-spore, a vesicle may be seen at the other end, which, after gradually increasing in size, suddenly vanishes, only to swell again into view. This vesicle is a CONTRACTILE VACUOLE. The presence of such a contractile vacuole in an organism was formerly considered a certain indication of its animal nature. Now, however, contractile vacuoles have been observed in the swarm-spores of many green Algae, of whose vegetable nature there can be no doubt. The swarm-spores of the Myxomycetes soon lose this characteristic swarm-movement, draw in their flagella, and pass into the ameba stage of their development, in which, like animal amcebe, they assume irregular, constantly changing shapes, and are capable of performing only amceboid creeping movements. In the case of Chondrioderma difforme, » Myxomycete of frequent occurrence in rotting parts of plants (Fig. 52), a number of the amcebe eventually collect together (/) and coalesce. In this way, as is also the case with SECT. | MORPHOLOGY 51 most other Myxomycetes, the amcebe ultimately give rise to a plas- modinm (i). Although each one of the amcebe is so small that it can only be seen with the aid of a microscope, the plasmodium into which they become united may attain a size large enough to be measured in centimetres. SOWA Pic. 52.—Chondrioderma difforme. u, Dry, shvivelled spore ; 0, swollen spore; ¢ and d, spores showing escaping contents ; ¢, f, g, swarm-spores ; h, swarm-spore changing to a myxoameeba ; i,syounger, k, older myxoamceba ; J, myxoameebee about to fuse; m, small plasmodium; x, portion of fully-developed plasmodium. (a-m, X 540: ”, x 90.) In a single amceba of the Myxomycetes, and still better in a plasmodium, it can be seen that the fundamental substance of the cytoplasm is hyaline and viscid. This fundamental substance is called HYALOPLASM. The hyaloplasm is denser on the surface of the plas- modium, entirely free from granules, and forms a homogeneous superficial layer, sometimes referred to as the PROTOPLASMIC MEMBRANE. In the 52 BOTANY PART I interior, on the other hand, the hyaloplasm is thin and fluid-like, it contains numerous granules, and is then designated GRANULAR PLASM. In the granular plasm will be found the nuclei of the various amoebee from which the plasmodium has been formed. The granular plasm of plasmodia exhibits streaming movements, as of different commingling currents, and affords a good example of the internal movements commonly shown by living protoplasmic masses. Thus, in addition to the FLAGELLAR or CILIARY MOVEMENTS, by means of which, as was observed in the swarm-spores of Chondrioderinu, & change of position is effected through the whip-like motion of fine cytoplasmic threads, and the creeping AM@BOID MOVEMENTS, such as were also exhibited by Chrondrioderma in the amceba stage of its development, there may also be recognised, as in the case of the plasmodium, INTERNAL PROTOPLASMIC MOVEMENTS. A plasmodium is also capable of creeping movements. It sends out new protrusions, and draws in others previously formed. If two protrusions meet, they unite to add a new mesh to the network of the plasmodium (1, Fig. 52). The viscous structureless superficial pellicle of hyaloplasm exhibits only creeping movements, while internal protoplasmic move- ments also take place in the more fluid granular plasm. Thus the granular plasm is continually flowing in irregular currents, alternately towards or away from the surface of the plasmodium. The plasmodium is able to surround and take within itself foreign bodies. These are then enclosed in vacuoles and, as far as possible, digested. The granular plasm seems to be separated from the vacuoles by a pellicle of hyaloplasm, similar to that on the surface of the plasmodium. Protoplasmic bodies, or PROTOPLASTS, enclosed by cell walls, likewise separate themselves by a similar hyaloplasmic pellicle from the cell walls and sap cavities, and all other vacuoles. The granular plasm is accordingly enclosed on all sides by hyaloplasm, while the cell nucleus, with its centrospheres and chromatophores, always lies embedded in the granular plasm. Within the walled protoplasts, the granular protoplasm often exhibits internal flowing movements. Such movements are especially noticeable when, by a wound, such as might result from a cut in preparing a section, a stimulus is given to the protoplasm. In cells in which the protoplasm forms only a peripheral layer, there may frequently be observed a movement in a continuously circling direction ; this is known as RoTation. If, however, the sap cavity is penetrated by bands or threads of cytoplasm, the motion will generally be of that kind known as CIRCULATION, in which case the currents of proto- plasm move in separate courses with different and frequently chang- ing directions. Rotation is the more frequent form of protoplastic movement in the cells of water-plants, while in land plants circulation is generally the rule. SECT. I MORPHOLOGY 53 A particularly favourable object for the study of protoplasin in circulation is afforded by the staminal hairs of 7'radcseantia virginica. In each cell (Fig. 53) small, fine currents of protoplasm flow in different directions in the peripheral cytoplasmic layer, as well as in the cytoplasmic threads, which penetrate the sap cavity. These cytoplasmic threads gradually change their form and structure, and thus alter the position of the cell nucleus. The layers of hyaloplasm separating the granular plasm from the cell walls and the internal sap cavities do not, in all cases, take part in any of these circulatory movements. When the protoplasm is in rotation, the cell nucleus and chromatophores are usually carried along by the current, yet there may be an outer layer of granular plasm which remains motionless and retains the chromatophores. This is the case with the Stoneworts (Characcae), whose long internodal cells, especially in the genus .Vitella, afford good examples of well-marked rotation. Active cytoplasm is a viscous substance. Deprived of its com- ponent water it becomes hard and tenacious, and, without losing its vitality, it ceases to perform any of its vital functions until again awakened into activity by a fresh supply of water. In case of a scarcity of water the plasmodia of the Myxomy- cetes may form SCLEROTIA, that is, masses of rest- ing protoplasm of an almost wax-like consistency. Months and indeed sometimes years afterwards, it is possible from such sclerotia, if water be properly supplied, to again produce motile plasmodia, Simi- larly, in, seeds kept for a long time, the proto- plasm consolidates into a hard mass, which may be easily cut with a knife, while the nuclei will be found to have shrunk and lost their original shape. Nevertheless the protoplasts, after ab- sorbing water, may return again to a condition of activity. Protoplasm is not a simple substance chemically ; it consists rather of different components, which are subject to continual change and in a state of mutual reaction. Treated as a uniform mass, © 5%—Cell from a . = 3 staminal hair of 7'ra- protoplasm always gives a proteid reaction; when — deseantia virginica, incinerated, fumes of ammonia are given off. showing nucleus sus- pended by protoplas- mic strands. (x 240.) Active protoplasm generally gives an alkaline, and, under certain conditions, a neutral reaction, but never an acid one. The protoplasm of the higher plants coagulates at a tempera- ture not much over 50° C., in the Schizophytes, however, usually not below 75° C. In a state of inactivity, as in spores and seeds, it can endure a still higher temperature without coagulating; when coagulation has once taken place, death ensues. The spores of many Bacteria can withstand a tempera- ture as high as 105°C. Treated with alcoho] or ether, or with acids of definite concentrations, with bichromates of the alkali metals, or with corrosive sub- limate, protoplasm quickly coagulates, while at the same time insoluble proteid 54 BOTANY PART I compounds ave formed. Coagulating reagents, accordingly, play an important part in microscopic technique; of especial value are such which, while fixing and hardening the protoplasm, change its structure in the least degree. As a fixing and hardening reagent for vegetable tissues, alcohol is particularly serviceable ; under certain conditions, sublimate alcohol, or 1 to 2 per cent formaldehyde. For animal cells and for the lower plants, 1 per cent chromic acid, 1 per cent acetic acid, 0°5 to 1 per cent osmic acid, concentrated picric acid, or corresponding mixtures of these acids, and also formaldehyde, are used for the same purpose. Iodine stains protoplasm brownish yellow ; nitric acid, followed by caustic potash, yellowish brown ; sulphuric acid, if sugar be present, rose red. Acid nitrate of mercury (MILLON’s reagent) gives to protoplasm a brick-red colour. Treated with copper sulphate, followed by caustic potash, protoplasm is coloured violet ; with an aqueous or alcoholic solution of alloxan, red. Aromatic aldehydes in the presence of a reagent for effecting condensation, such as sulphurig or hydrochloric acid, and an oxidising substance or a‘higher chloride, also produce in protopiasm characteristic colour reactions; thus, benzaldehyde gives a blue-green to blue ; piperonal, a violet-blue ; vanillin, a violet or violet-blue reaction. Protoplasm is soluble in dilute caustic potash and also in eau de Javelle (potassium-hypo- chlorite), and accordingly both of these reagents may be recommended for clearing specimens, when the cell contents is not to be investigated. All of the above- mentioned reagents kill protoplasm ; until they have done so, their characteristic reactions are not manifested. In their greater or less resistance to the action of solvents, in the degree of their sensitiveness to reagents, and in the intensity of the reactions, the various constituents of protoplasm, cytoplasm, nucleus, centrospheres, and chromatophores differ from one another, and thus a means of determining their component substances is afforded. Accordingly a large number of albuminous bodies or albuminates have been named which are said to enter into the composi- tion of living protoplasm. It is worthy of note that these compounds, although still for the most part not fully determined, all contain phosphorus. Such as are peculiar to the nucleus have been comprehended under the term NUCLEIN. Stain- ing reagents have also become an important help to microscopic investigations for determining the composition of protoplasm. This is due to the fact that the different constituents of protoplasm take up and retain the stain with different degrees of intensity and energy. Asa general rule, only coagulated protoplasm can absorb colouring matter, although some few aniline stains can, to a limited extent, permeate living protoplasts. For staining vegetable protoplasts, which have been previously hardened, the various carmines, hematoxylin, iodine green, acid fuchsin, eosin, methylene blue, and aniline blue, have been found particularly convenient. The different components of the protoplasm absorb the stains with different intensities, and, when reagents are employed to remove the colouring matters, they exhibit differences in their power to retain them. The nucleus generally becomes more intensely coloured than the rest of the protoplasm, especially a part of its substance, which is therefore called CHromaTIN. The chromatin, moreover, is not affected by gastric juices nor by solutions of pepsin containing hydrochloric acid, although both cytoplasm and chromatophores are at once digested by them. On the other hand, with a trypsin solution, chromatin is quickly dissolved. In addition to those substances, which are to be regarded as integral parts of active protoplasm, it always includes derivative products of albuminates, particularly amides, as asparagin, glutamin; also ferments, as diastase, pepsin, invertin ; at times alkaloids, and always carbohydrates and fats. The ash left after incineration also shows that protoplasm always contains mineral matter, even if only in small quantities. All substances which, as such, do not SECT. I MORPHOLOGY 55 enter directly into the composition of protoplasm, but are only included within it, are designated by the term METAPLAsM. The Cytoplasm.—In describing the cytoplasm of the plasmodia of the Myxomycetes and of the walled protoplasts of vegetable cells, mention has been made of a hyaline fundamental substance, the hyaloplasm, which forms a superficial layer on the surface of the cyto- plasm entirely free from granules, while in the interior, as granular plasm, it includes granular matter. The cytoplasm was likewise shown to be a viscous substance, in which internal streaming move- ments of the particles take place, while at the same time its superficial layer of hyaloplasm remains unchanged. In accordance with its viscous fluid character, cytoplasm possesses certain physical peculi- arities. If cells full of protoplasm be opened under water, the out- flowing cytoplasm assumes the form of a drop. The cytoplasm in the cells of many Algae has a structure resembling that of foam, while in the higher plants it is no less distinctly fibrillar in structure, and composed of protoplasmic threads. In both cases the chambers or spaces enclosed by the foam-like or thread-like cytoplasm are filled with solutions of various substances. All the granular inclusions lie in the cytoplasm, either in the walls of the cytoplasmic chambers or in the cytoplasmic threads. The small granules which are never absent from the granular plasm, and give to it its name, are called MIcROSOMES. As they show different chemical reactions, it is inferred that they have also different chemical organisations. Sometimes they appear to be vesicles filled with liquid, and are then termed PHysopEs. In the cells of many Algae such vesicles attain a considerable size, and undergo modifica- tions of their shapes. Large vesicles or vacuoles filled with watery solutions are found in the cytoplasm. The cytoplasm separates itself from such vacuoles by means of a protoplasmic membrane or pellicle of hyaloplasm. The sap cavities in the cells of the more highly organised plants are, in this sense, merely large vacuoles. The protoplasmic membranes which surround the vacuoles are particularly tenacious of life; thus after the other cytoplasm of a cell has been killed with a 10 per cent solution of saltpetre, the walls of the vacuoles will still continue living. As the pressure of the cell sap is controlled by these living vacuolar membranes, H. pe Vries has given them the name TONO- PLASTS. Through the division of the cytoplasm its tonoplasts. may become bisected, and in this way multiply. On the other hand, a single large vacuole may result from the fusion of several smaller ones. It has also been demonstrated by PFEFFER that new pellicular membranes may be formed around liquid substances in the cytoplasm. The Cell Nucleus.—The nucleus is in all cases fibrillar in struc- ture. It appears to be made up Sf threads twisted together and forming an anastomosing network (Fig. 54), which, however, in 56° BOTANY PART I living objects can only be distinguished by the punctated appearance It gives to the nucleus. Streaming movements do not take place within the nucleus. An insight into the nuclear structure is only to be attained with the help of properly fixed and stained preparations. It is then possible to determine that the greater part of this nuclear network is composed of delicate and, for the most part, unstained threads, in which lie deeply stained granules. The substance of the threads has been distin- guished as LININ (J), that of the granules as CHROMATIN (ch). One or more large nuclear Fic. 54.—Quiescent nucleus from bodies, OF nucleoli (n), DOCuE ay the inter- the developing endosperm of Sections of some of the linin threads which, Fritilaria imperiatis, hard- although deeply stained, have not taken the ened with aleoholand stained same tint as the chromatin granules. The with safranin, 1, Linin; ch d sha chromatin; n, nucleolus; w, network of the nucleus lies within the nuclear membrane; ¢ een- NUCLEAR CAVITY, which is filled with nuclear ae oe “sap and surrounded by a membrane (vw). Although this is generally spoken of as the nuclear membrane, strictly speaking it is a part of the surrounding cytoplasm, and is the protoplasmic layer or pellicle with which the cytoplasm separates itself from the nuclear cavity. The Centrospheres.—The existence of these bodies, now universally acknowledged in animal cells, is generally ad- mitted in the case of all vegetable cells, although their demonstration has not, in all cases, been successful. They form, as GUIGNARD in parti- cular has shown, two small homogeneous spheres lying near the nucleus and embedded in the cytoplasm. Each centrosphere has in its centre a body termed the CENTROSOME (¢, Figs. 50, 54), composed of one or more small granules. As the successful fixing and staining of the centro- spheres in vegetable cells require extreme care, their detection in the granular cytoplasm is ~ rendered difficult. The Chromatophores.—In the embryonic cells of growing points, where the chromatophores (Fig. 50, ch) are principally located around the nucleus, they first appear as small, colourless, Fis. 55.—Two cells from a highly refractive bodies; and in the embryonic i. a ad cells of ovules they have a similar appear- », amcieus, Gay ance. They may retain the same appearance in older cells (Fig. 104, 4, 2), but in them they also attain a further development. CHLOROPLASTS, LEUCOPLASTS, or CHROMOPLASTS may SECT. I MORPHOLOGY ‘BT be developed from a similar original substance ; they are all included in the one term, CHROMATOPHORES. In parts of plants which are exposed to the light the chromato- phores usually develop into chlorophyll bodies or CHLOROPLASTS. These are generally green granules of a somewhat flattened ellipsoidal shape (Fig. 55), and are scattered, in great numbers, in the parietal cytoplasm of the cells. All the chloroplasts in the Cormophytes and, for the most part also, in the green Thallophytes present this same granular form. In the lower Algae, however, the chlorophyll bodies may assume a band-like (Fig. 235), stellate or tabular shape. The fundamental substance of the chlorophyll bodies is itself colourless, but contains numerous coloured drops, which are termed GRANA. These consist of an oleaginous substance, which holds various pigments in solution ; a green, known as chlorophyll or chlorophyll-green ; a yellow, called xanthophyll ; and a reddish orange, termed carotin. These colouring substances may be extracted by means of alcohol, leaving only the colourless plasmic substance of the chlorophyll body remaining. The easiest way in which a solution of chlorophyll can be prepared, is to extract the chlorophyll by means of alcohol from green leaves that have been previously boiled in water. The green chlorophyll pigment is also soluble in ether, fatty and ethereal oils, paraffine, petroleum, and carbon disulphide. The alcoholic solutions appear green in transmitted light; blood red in reflected light, on account of fluorescence. If a ray of sunlight be made to pass through a tolerably thick layer of an alcoholic solution of chlorophyll, and then decomposed by a prism, the resulting £6 Fr a Fia. 56.—Spectrum of an alcoholic solution of chlorophyll extracted from foliage leaves. (After Kraus.) The absorption bands in the less refractive part of the spectrum (B-£) are given by a concentrated solution, those in the more highly refractive part of the spectrum bya dilute solution. spectrum will show seven absorption bands (Fig. 56). The darkest band extends from FRAUNHOFER’s line, B, to some distance beyond the line C. The other bands are not so intense: one lies between Cand D, another near D, and one near £, while the other three bands are broader and cover almost the whole blue half of the spectrum. If benzole be added to an alcoholic solution of chlorophyll, prepared as directed above, and the mixture, after being well shaken, is allowed to settle, the benzole will be found to have taken up the chlorophyll pigment and the carotin, while the xanthophyll will be left in the alcohol, and will collect, as a yellow solution, in a layer below the green benzole. The amount of chlorophyll in a green plant is 58 BOTANY PART I very small. Tscurncy has calculated that out of a square metre of green foliage leaves only from 0°1 to 0°2 grams of chlorophyll can be obtained. Acids decompose chlorophyll ; contact even with the acid cell sap is sufficient to change the colour of the chlorophyll bodies to a brownish green. It is due to this fact that a plant turns brown when dried. The green colour of the chlorophyll in some groups of Algae 1s more or less masked by other pigments. In addition to the chlorophyll green, with its accompanying yellow and orange-red pigments, many of the blue-green Schizophyceae contain a blue colouring matter, phyco- cyanin ; the brown Algae, a brown pigment called phycophein ; while the red Algae possess a red pigment termed phycoerythrin. These Fic. 57.—Cell from the upper surface of the ealyx of Tropaeolum majus, showing chroma- tophores. (x 540.) Fic. 59.—Chromoplasts of the Carrot, some with starch grains. (x 540.) Fic. 58.—Cell from the red pericarp of the fruit of Crataegus coccinea. n, Nucleus. (x 540.) pigments, which are peculiar to Algae, are soluble in water, and are characterised by a beautiful fluorescence. The phycocyanin may often be found as a blue border surrounding a blue-green Fission- Alga which has been dried in a press. Red seaweeds washed up by the ocean soon become green, as, owing to the rapid decomposition of the phycoerythrin, the chlorophyll is no longer concealed. Before leaves fall in the autumn, their cells lose almost all of their cytoplasmic contents, and at the same time the chloroplasts undergo disorganisation. There remains only a watery substance in the cell cavity, in which a few oil globules and crystals, together with a few yellow, strongly refractive bodies, can be seen. Sometimes this liquid in the cell cavities becomes red, and thus imparts to the SECT. I MORPHOLOGY 59 foliage its autumnal brilliancy. In the leaves of coniferous trees, which only indicate the approaching winter by assuming a somewhat brownish tint, the case is different. The chlorophyll-green of their chloroplasts changes to a brownish green, but in the following spring regains its characteristic colour. In such phanerogamic parasites or humus- plants as are devoid of green colour, the chloroplasts either do not develop, or they are white, or have only a brownish or green- ish colour. No chromatophores are found in the Fungi. In the interior of plants, where light cannot penetrate, LEUCOPLASTS are developed instead of chloroplasts from the rudiments of the chromatophores. They are of a denser consistency than the chloroplasts, and re- sembling a flattened ellipsoid in shape, are often somewhat elongated in consequence of enclosed albuminous crystals. If the leuco- plasts become at any time exposed to the light, they not infrequently change into chloroplasts. This frequently occurs, for ex- ample, in potatoes. The CHROMOPLASTS of most flowers and fruits arise either directly from the rudiments of colourless chromatophores, or from pre- viously formed chloroplasts. In shape the chromoplasts resemble the ellipsoidal granules of the chloroplasts, except that they are usually smaller; or, in consequence of the crystallisation of their colouring pigment, they pig, ¢0.—A cell of Cladophora assume a triangular, tabular, needle, or fan- — glomerata, fixed with 1 per cent shaped form (Figs.57, 58,59). Thecolourol Seniesa ee the chromoplasts varies from yellow to red, — jnatophores: », pyrenoids; according to the predominance of xanthophyll __ stareh grains. (x 540.) or carotin. The name carotin has been derived from the Carrot (Daucus Carota), in the roots of which it is particularly abundant (Fig. 59). The frequent crystalline form of the chromoplasts is, in a great part, due to the tendency of carotin to crystallisation, although it may be also occasioned by needle-like crystals of albumen. Xanthophyll, however, is never present in the chromoplasts except in an amorphous condition. Multinuclear Cells. — While the cells of the Cormophytes are almost exclusively uninuclear, in the Thallophytes, on the contrary, ti 60 BOTANY PART I multinuclear cells are by no means infrequent. Jn the Fungi, and in the Siphoneae among the Algae, they are the rule. The whole plant 1s thus composed either of but one single multi- nuclear cell, which may be extensively branched (Fig. 250), or it may consist of a large number of multinuclear cells, forming together one organism. Thus, after suitable treatment, several nuclei may be detected in the peri- pheral cytoplasm in the cells of the common filamentous fresh-water Alga Cladophora glome- rata (Fig. 6, p. 12) (Fig. 60). : The nuclei of the long, multinuclear cells (Fig. 61, 2) of fungoid filaments, or HYPHA, and also of many Siphoneae, are characterised by their diminutive size. The Origin of the Living Elements of Protoplasm.—Every nucleus in an organism owes its origin to the nucleus of the germ cell (egg or spore); the nuclei of the germ ta : cells are descended from the nuclei of previous 1G. 61.—Portions of two ad- L jacent cells ina hypha from generations. The spontaneous formation of a the stalk of a Mushroom, nucleus never takes place. In the same ease Nv- manner, the cytoplasm of ‘every organism is derived from the cytoplasm of the germ cell, and, so far as is yet known, both centrospheres and chromatophores take their origin, each only from its own kind. Nuclear Division.— Except in a few limited cases, nuclei reproduce themselves hy MITOTIC or INDIRECT DIVISION. This process, often referred to as KARYOKINESIS, is somewhat complicated, but seems necessary in order to effect an equal division of the substance of the mother nucleus between the two new daughter nuclei. In its principal features the process is similar in plants and animals. In vegetable cells, the threads composing the nuclear network (p. 56) first become thicker and correspondingly shorter (Fig. 62, 1), the anastomosing connections forming the meshes are drawn in, while the thread itself straightens out and becomes less entangled, and in consequence more easily distinguished. At the same time the amount of the chromatin increases, and this increases its capacity of absorbing stains. Finally, the chromatin substance in the thread becomes arranged in parallel discs (4) united by linin. The thread itself then divides transversely into a definite number of segments, the CHROMOSOMES (2, 3), which thereupon range themselves in a plane in a special manner, and form the so-called NUCLEAR PLATE (3). ‘Then, or sometimes before, the segments divide longitudinally (4, B, C), and the halves thus produced separate (s) from each other in opposite directions to form the daughter nuclei. SECT. I MORPHOLOGY 61 In the meantime other definite processes have been taking place ; while the thread of the nuclear network has been shortening and dis- entangling, the two centrospheres (1, ¢c), previously lying together close to the nuclear membrane, have separated and taken up a position opposite each other (2, ¢). They constitute the poles of the division figure. Beginning at these two points, the nuclear membrane dis- appears, and the nucleoli also become more or less completely dissolved, influenced in all probability by the centrospheres. SpiNDLE Fic. 62.—Successive stages in nuclear and cell division. v, Ceutrospheres ; 7, nucleolus ; s, chro- mosomes ; sp, spindle fibres; 4, B, C, chromosomes, showing longitudinal division and the arrangement of the chromatin. (x circa 600.) FIBRES then arise from protoplasmic threads found within the nuclear cavity, presumably with the co-operative activity of the nucleolar substance. The spindle fibres converge towards both poles of the division figure, and, viewed as a whole, they have the form of a spindle. While some of the spindle fibres extend uninterruptedly from pole to pole, others become connected with the chromosomes. Through this arrangement of the spindle fibres, the position of the nuclear plate in the equatorial plane of the spindle figure is determined ; while by the contraction of the spindle fibres in connection with the chromosomes, 62 BOTANY PART I the longitudinal halves of the chromosomes are drawn in opposite directions towards either pole of the division figure (5, 6, 7). In the process of this movement of the chromosomes towards the poles, the other continuous spindle fibres seem to serve as supports. Before the chromosomes, however, reach the poles, a division of the centrospheres (5), commencing with their centrosomes, takes place, so that two centrospheres are previously provided for each new daughter nucleus. In the nuclei of vegetable cells, the primary ‘spindle fibres connected with the chromosomes unite with the spindle fibres extending from pole to pole. The number of these secondary spindle fibres corresponds with the number of the chromosomes. In forming the daughter nuclei, the free ends of the chromosomes first become drawn in (8), and the surrounding cytoplasm separates itself by means of a protoplasmic membrane (9) from the developing nuclei. Within the nuclear cavities which are thus produced the chromosomes elongate (10), and joining together, end to end, become again intertangled. The chromatin substance is diminished in quantity, nucleoli at length appear in the enlarging nuclei, and finally a con- dition of rest is again reached. The changes occurring in a mother nucleus preparatory to division are termed the propHAsgEs of the karyokinesis. These changes extend to the formation of the nuclear plate, and include also the process of the longitudinal division of the chromosomes. The separation of the daughter chromosomes is accomplished in the METAPHASES, and the formation of the daughter nuclei in the ANAPHASES of the karyokinesis. The real purpose of the whole process is consummated in the quantitative and qualitative division of the chromosomes, resulting from their longitudinal segmentation (4, B, C). The anaphases of the karyokinesis are but a reverse repetition of the prophases. Exceptions to the process as here described, are not of special importance, and need not be discussed. In, addition to the mitotic or indirect nuclear division there is also a DIRECT or AMITOTIC division, sometimes called FRAGMENTA- TION (Fig. 63). It usually occurs in old cells, or in cells in which the cell contents become disorganised shortly after the nuclear division. Instructive examples of direct nuclear division are afforded by the long internodal cells of the Stoneworts (Characeac), and also by the old internodal cells of Z'radescantia (Fig. 63). The direct nuclear division is chiefly a process of constriction which, however, need not result in new nuclei of equal size. In the case of the Stoneworts, after a remarkable increase in the size of the nucleus, several successive rapid divisions take place, so that a con- tinuous row of bead-like nuclei results. The old internodal cells of Tradescantia (Fig. 63) very frequently show half-constricted nuclei of irregular shape. While in uninuclear cells indirect nuclear division is as a rule, followed by cell division, this is not the case after direct nuclear division. SECT. I MORPHOLOGY 63 Cell Division.—In the uninuclear cells of the Cormophytes, cell division and nuclear division are, generally, closely associated as parts of one and the same act. The spindle fibres extending from pole to pole persist as CONNECTING FIBRES, be- tween the developing daughter nuclei A (Fig. 62, 6, 7). The number of the |, connecting fibres is increased by the in- }}. terposition of others in the equatorial ||” plane. In consequence of this a barrel- a shaped figure is formed, which either |: separates entirely from the developing daughter nuclei, or remains in connection with them by means of a peripheral sheath, the CONNECTING UTRICLE. The gil first is the casé in cells rich in cyto- ||| Weel. plasm, the latter when the cells are more || « Se ORO é abundantly supplied with cell sap. At ||; oy the same time the connecting fibres become granularly thickened (8, 9) at the equatorial plane, and form what is known as the CELL PLATE. In the case of cells rich in protoplasm or small in diameter, the connecting fibres become F'¢. 63.—Old cells from the stem of Tradescantia virginieu, showing more and more extended, and touch the nuclei in process of direct division. cell wall at all points of the equatorial (x 540.) plane (10). The granular elements of the _ cell plate then unite and form a partition wall, which thus sIMUL- TANEOUSLY divides the mother cell into’ two daughter cells (10). If, however, the mother cell has a large sap cavity, the connecting utricle cannot at once become so extended, and the partition wall is then formed SUCCESSIVELY (Fig. 64). In’ that case, the partition wall first com- mences to form at the point where the utricle is in contact with the side walls of the mother cell (Fig. 64, 4). The protoplasm then de- 4 ee ot Thred: stages inthe division of ® 4 aches iteall from the pattol the new iving cell of Epipactis palustris. (After e ns TREUB, X 365.) wall in contact with the wall of the mother cell, and moves gradually across until the septum is completed (Fig. 64, B and C); the new wall is thus built up by successive additions from the protoplasm. In the Thallophytes, even in the case of uninuclear cells, the parti- tion wall is not formed within connecting fibres, but arises either simultaneously from a previously formed cytoplasmic plate, or suc- 64 BOTANY PART I cessively, by means of diaphragm-like projections from the wall of the mother cell. It was a division process of this kind (Figs. 65, 66), first investigated in fresh-water Algae, that gave rise to the conception of cell division, which for a long time prevailed in both animal and vegetable Histology. In this form of cell division the new wall com- mences as a ring-like projection from the inside of the wall of the mother cell, and gradually pushing further into the cell, finally extends completely across it-(Figs. 65,. 66). In a division of this sort, in uninuclear cells, nuclear division precedes cell division, and the new wall is formed midway between the daughter nuclei (Fig. 65), In the multinuclear cells of the Thallophytes, on the other hand, although the nuclear division does not differ from that of uninuclear cells, cell division (Fig. 66) is altogether independent of nuclear division. And in multinuclear, unicellular Thallophytes, nuclear division 1s not fol- Fic. 65.—Cell of Spirogyra in division. , Fic. 66.— Portion of a dividing cell of One of the daughter nuclei; w, developing Cladophora fracta. w, Newly - forming partition wall; ch, chlorophyll band, partition wall; ch, intercepted chromato- pushed inward by the newly-forming wall. phore ; x, nuclei. (x 600.) (x 280.) lowed by a cell division. The interdependence of nuclear and cell division in uninuclear cells is necessary to ensure a nucleus to each daughter cell. In multinuclear cells it is not essential that cell division should always be accompanied by nuclear division, as in any case sufficient nuclei will be left to each daughter cell. Free Nuclear Division and Multicellular Formation.—The nuclear division in the multinuclear cells of the Thallophytes may serve as an example of free nuclear division, that is, of nuclear division unaccom- panied by cell division. In plants with typical uninuclear cells, examples of free nuclear division also occur; although; in that case, the nuclear division is customarily followed by cell division. This is often the case in the formation of germ cells, and is due to the fact that while the nuclei increase in number this process is not accom- panied by a corresponding cell division. When, however, the number of nuclei is completed, then the cytoplasm between the nuclei SECT. I MORPHOLOGY 65 divides simultaneously into as many portions as there are nuclei. In this process we have anexample of multicellular formation. This method of development is especially instructive in the embryo-sac of Phanerogams, a cell, often of remarkable size and rapid growth, in which the future embryo is developed. The nucleus of the embryo-sac divides, the two daughter nuclei again divide, their successors repeat the pro- cess, and so on, until at last thousands of nuclei are often formed. No cell division accompanies these repeated nuclear divisions, but the nuclei lie scattered through- out the peripheral, cyto- plasmic lining of the em- bryo-sac. When the embryo- sac ceases to enlarge, the nuclei surround themselves with connecting strands, Fic. 67. —Portion of the peripheral protoplasm of the : g 3 embryo-sac of Reseda odorata, showing the com- which then : radiate from mencement of multicellular formation. (x 240.) them in all directions (Fig. 67). Cell-plates make their appearance in these connecting strands, and from them cell walls arise. In this manner the peripheral proto- plasm of the embryo-sac divides, simultaneously, into as many cells as there are nuclei. Various intermediate stages between simultaneous, multicellular formation and successive cell division can often be observed in an embryo-sac. Where the embryo-sac is small and of slow growth, successive cell division takes place, so that multicellular formation may be regarded as but an accelerated form of successive cell division, induced by an extremely rapid increase in the size of the sap cavity. Free Cell Formation.—Cells produced by this process differ con- spicuously from those formed by the usual mode of cell division, in that the free nuclear division is followed by the formation of cells which have no contact with each other. This process can be seen in the developing embryo of the Gymnosperms, in Ephedra, for example, and also in the formation of the spores of the Ascomycetes. In the case of Ephedra there first occurs a free division of the nucleus of the fertilised ege ; each daughter nucleus then divides once or twice, so that four or eight nuclei are ultimately produced. A rounded, cytoplasmic mass collects about each nucleus and surrounds itself with a cell wall F 66 BOTANY PART I (Fig. 68); but the four or eight cells thus formed have no contact, with each other, and the cytoplasm of the mother cell is not totally consumed by their formation. . Cell-Budding.—This is simply a special variety of ordinary cell division, in which the cell is not divided in the middle, but, instead, pushes out a protuberance which, by constriction, becomes separ- ated from the mother cell. This mode of cell multiplication is characteristic of the Yeast plant (Fig. 2, p. 11); and the spores, known as conidia, which are produced by numerous Fungi, have a similar origin (Fig. 286). Cell-Formation by Conjugation.—A sexual cell is only able to continue its development after fusion with another sexual cell. The two cells so uniting are either alike, and in that case are called : GAMETES, or unlike, and are then distinguished as Fic. 68.—Freecellforma- EGG and SPERMATOZOID. The spermatozoid is the tion in the fertilised male, the egg the female sexual cell. The gametes ege-cell of Ephedra : - cS altissima. (x100.) may be motile or non-motile (Fig. 69, B). The motile gametes frequently resemble the swarm- spores (Fig. 69, 4) generated by the same parent for the purpose of asexual reproduction. As a rule, however, they are smaller than the swarm-spores, and have usually only half as many cilia. In the more highly specialised sexual cells the egg usually retains the structure of an embryonic cell, but the spermatozoid under- goes various changes. A cytoplasmic cell body, a nucleus, and the rudiments of chromatophores are always present in the egg. The male sexual cell (Fig. 70), on the other hand, becomes transformed, in the more extreme cases, into a spirally twisted body, provided with cilia, and exhibiting an apparently homogeneous structure. Only a knowledge of the history of its development, and the greatest care in hardening and staining, have rendered it possible to recognise the homology of the structure of such a spermatozoid with that of an embryonic cell. It has been shown that one part of its spiral body corresponds to the cell nucleys (), another, together with the cilia, to the cytoplasm (c), and the vesicle (8), at the other extremity, to the sap cavity of a cell. After the spermatozoids are set free from the sexual organs, they require water for their dispersal. They are motile, and are thus enabled to seek out the egg-cells, which, in most cases, await fertilisation within the organ in which they have been formed. Motile, male sexual cells occur only in the Cryptogams. In the Phanerogams (Fig. 71) the non-motile male cell (gz) is carried to the egg by thé growth of the POLLEN TUBE (Fig. 71, 4), in which it is enclosed. In the union of the two sexual cells in the act of fertilisa- tion, the egg nucleus (ek) and the sperm nucleus (sk) fuse and form SECT. I MORPHOLOGY 67 the nucleus of the fertilised egg-cell. The cytoplasm of the male cell also commingles with that of the female cell) but the chroma- tophores of the embryo are derived from the egg-cell alone. It is Fic. 69.—1A, An asexual swarm-spore of Ulothriz zonata; B,1,a gamete ; 2and 8, conjugating gametes; 4, zygote, formed by the fusion of two gametes. (x 500.) Fic. 71.-—Fertilisation of a phanerogamic Angiosperm, somewhat diagrammatic. A, End of pollen tube; in it the genera- tive cells gz, each of which contains a sperm nucleus; vk, the vegetative cell in process of dissolution. B-D, Egg in™ successive stages of fertilisation,—B, show- ing the generative cell with its sperm Fic. 70.—A, Spermatozoid of Chara nucleus, sk, penetrating the egg; C, the Sragilis ; B, spermatozoid of the union of sperm nucleus, sk, and ege Fern Phegopteris Giesbrechtii. The nucleus, ek; v, centrospheres; D, the darker portion, k, corresponds to germ nucleus, kk, resulting from the the cell nucleus; the lighter, fusion of the sperm and egg nuclei; ch, e, to the cell cytoplasin ; el, cilia ; rudiments of chromatophores. (x circa b, vesicle. (x 540.) 500.) still uncertain whether a similar fusion of the centrospheres of the sexual cells also takes place. It is regarded as more probable that the centrospheres of the egg nucleus—more rarely those of the sperm nucleus—become functionless, so that the centrospheres of the fertilised egg are derived only from the sperm nucleus, or from the nucleus of the female cell. 68 BOTANY PART I The egg becomes capable of development as the result of fertilisa- tion, although there are exceptional cases in the organic kingdom, especially among the Arthropods, where an unfertilised egg may produce an embryo. This is called PARTHENOGENESIS. In the vegetable kingdom the existence of parthenogenesis in plants with advanced sexual differentiation has only been proved in the case of Chara crinita, one of the Characeue. Multiplication of the Chromatophores.—This is accomplished by a direct division, as a result of which, by a pro- 2) 2 cess of constriction, a chromatophore becomes QO divided into two nearly equal halves. The = OR € stages of this division may best be observed in C85 the chloroplasts (Fig. 72). Sh os Inclusions of the Protoplasm—StTARcH. — ey The chloroplasts in plants exposed to the light ie, 32)—entoneenytt almost always contain starch grains. These grains from the leaf of grains of starch found in the chloroplasts are the Funaria hygrometrica, first, visible products of the assimilation of in- resting, and in process ° si of division, (x 540.) Organic matter. They are formed in large numbers, but as they are continually dissolving, always remain small. Large starch grains are found only in the reservoirs of reserve material, where starch is formed from the de- posited products of previous assimilation. Such starch is termed RESERVE STARCH, in contrast to the ASSIMILATION sTaRCcH formed in the chloro- plasts. All starch used for economic purposes is reserve starch. The starch grains stored as reserve material in potatoes are comparatively large, attain- ing an average size of 0°09 mm. As shown in the adjoining figure (Fig. 73), they are plainly stratified. Their stratification is due to the varying densi- ties of the successive layers. They are eccentric im structure, Fic. 73.—Starch grains from a potato. A simple ; as the organic centre, about 2, haltcompound; ¢ and Dp, compound: starch which the different layers are oneke S ae Conure of the starch grains, or laid down, dees: ace. -cauume nucleus of their formation, (x 540.) spond with the centre of the grain. The starch grains of the legumes and cereals, on the other hand, are concentric, and the nucleus of their formation is in the centre of the grain. The starch grains of the Bean, Phuseolus vulgaris (Fig. 74), have the shape SECT. I MORPHOLOGY 69 of a flattened sphere or ellipsoid ; they show a distinct stratification, and are crossed by fissures radiating from the centre. The disc-shaped starch grains of wheat are of unequal size, and only indistinctly strati- fied (Fig. 65). A comparison of the accompanying figures (Figs. 72- 75), all equally magnified, will give an idea of the varying size of the starch grains of different plants. The size of starch grains varies, in fact, from 0°002 mm. to 0°170 mm. Starch grains 0°170 mm. large, such as those from the rhizome of Canna, may be seen even with the naked eye, and have the appearance of brilliant points. In addition to the simple’starch grains so far described, half-compound Fic. 75.—Starch grains of wheat, Triticwm durum. 5, Large; B, small grains. (x 540.) Fic. 74.—Starch Fic. 76.—Starch grains of oats, Fic. 77.—Leucoplasts from an aerial tuber grains from the Avena sativa. A, Compound of Phajus grandifolius, A, C, D, F, cotyledons of grain; B, isolated component Viewed from the side; B, viewed from Phaseolus vul- grains of a compound grain. above; EZ, leucoplast becoming green garis. (xX 540.) (x 540.) and changing toachloroplast. (x 540.) and compound starch grains are often found. Grains of the former kind are made up of two or more individual grains, surrounded by a zone of peripheral layers enveloping them in common. The com- pound grains consist merely of an aggregate of individual grains un- provided with any common enveloping layers. Both half-compound (Fig. 73, B) and compound starch grains (Fig. 73, C, D) occur in potatoes, together with simple grains. In oats (Fig. 76) and rice all the starch grains are compound. According to NAGELI, the compound starch grains of rice consist of from 4 to 100 single grains; those of Spinacia glabra sometimes of over 30,000. Starch thus formed from previously assimilated organic substances also requires chromatophores for its production. Itis produced by means of leucoplasts, which are, in consequence, often termed STARCH-BUILDERS. If the formation of a starch grain should begin near the periphery of a leucoplast, the grain would eventually, by its continued enlargement, protrude from the leucoplast. As new layers of starchy matter are then deposited only 70 BOTANY PART I on the side remaining in contact with the plastid, the starch grain thus becomes eccentric (Fig. 77). Should, however, several starch grains commence to form at the sarhe time in one leucoplast, they would become crowded together and form a compound starch grain, which, if additional starchy layers are laid down, gives rise to a half- compound grain. It has recently been asserted that starch grains are crystalline bodies, so-called spherites, and are composed of fine, radially arranged, needle-shaped crystals (trichites). Their stratification, according to this view, is due to variations in the form and number of the crystal needles in the successive layers. In a few individual cases, ARTHUR MEYER has succeeded in showing that the stratification of the starch grains corresponds to the alternation of the periods of day and night, i.c. to the interference which is thus caused in the nutritive processes. The growth of starch grains is also affected by the solvent action of surrounding substances, whereby the peripheral layers may be partially removed, and then no longer com- pletely envelop the entire grain. Starch grains are composed of a carbohydrate, the formula of which is (CgsHyOs5)n. Most starch grains only contain amyloid, one variety of which becomes liquid in the presence of water at a temperature of 100° C., and another, which, under the same conditions, does not become liquid. In addition to this amyloid many starch grains contain also amylodextrin. In certain cases, as in Oryza sativa var. glutinosa and Sorghum vulgare var. glutinosnm, the starch grains consist principally of amylodextrin. Although starch rich in amyloid gives a blue reaction with a solution of iodine, the starch rich in amylo- dextrin takes a red wine colour. Starch grains become swollen in water at a tem- perature of 60° to 70° C., according to ARTHUR MEyER, because of the conversion into tenacious globules of the more readily soluble of the two amyloids ; at 138° C. starch grains become completely dissolved. Starch swells very readily at ordinary temperatures in solutions of potassium, or sodium hydrate. Heated without addi- tion of water, t.e. roasted, starch becomes transformed into dextrin, and is then soluble in water and correspondingly more digestible. That starch grains give a dark cross in polarised light is due to the double refraction of the component crystalline elements. The amount of starch contained in reservoirs of reserve material is often con- siderable ; in the case of potatoes 25 per cent of their whole weight is reserve starch, and in wheat the proportion of starch is as high as 70 per cent. The starch flour of economic use is derived by washing out the starch from such reservoirs of reserve starch. In the preparation of ordinary flour, on the contrary, the tissues containing the starch are retained in the process of milling. ALEURONE.—Aleurone or protéin grains (gluten) are produced in the seeds of numerous plants, especially in those containing oil. They are formed from vacuoles, the contents of which are rich in albumen, and harden into round grains or, sometimes, into irregular bodies of indefinite shape. A portion of the albumen often crystal- lises, so that frequently one and occasionally several crystals are formed within one aleurone grain. In aleurone grains containing albumen crystals there may often be found globular bodies, termed GLOBOIDS, which, according to PrErrsr, consist of a double phosphate of magnesium and calcium in combination with some organic substances, SECT. I MORPHOLOGY ea Crystals of calcium oxalate are also found enclosed in the aleurone grains. The seeds of Ricinus (Fig 78) furnish good examples of aleurone grains with enclosed albumen crystals and glo- boids. The aleurone grains them- selves lie embedded in a cytoplasm B that is rich in oil. In the cereals ee a the aleurone grains which lie only in | Sake y the outer cell layer of the seeds (Fig. 79, a2) are small, and free from all inclusions ; they contain neither crystals nor globoids. As the outer cells of wheat grains contain only aleurone, and the inner almost ex- clusively starch, it follows that flour Fic. 78.—A, Cell from the endosperm of Ricinus is the richer or poorer in albumen, communis, in water; B, isolated aleurone grains the more or less completely this outer in olive oil; x, albumen crystals; g, globoid. (x 540.) layer has been removed before the wheat is ground. From the inner layers finer and whiter flour can be made ; while more nourishing flour is obtained from the outer layers. Reactions for aleurone are the same as those already mentioned for the albuminous substance of protoplasm. Treat- 2 ae ea ee eee ment of a cross-section of a grain > eS ans = of wheat (Fig. 79) with a edict = ee of iodine would give the aleurone layer a yellow-brown colour, while the starch layers would be coloured blue. ALBUMEN CrysTALs.—Crystals of this nature are especially fre- quent in aleurone grains (Fig. 78). They have previously been men- tioned as occurring in the chroma- tophores. In the illustration of the leucoplasts of Phajus grandi- folius (Fig. 77), the rod-shaped crystals are represented as light stripes (in B and £). In the . . Oe green Algae, the angular, strongly Fic. 79.—Part of a section of a grain of wheat, Triticum j K : 2 vulgare. p, Pericarp; t, seed coat, internal to which refractive bodies lying in the is the endosperm; ai, aleurone grains; am, starch chloroplasts and surrounded by a grains; n, cell nucleus. (x 240.) ring of starch granules are albu- men crystals. A good example of these bodies, known as PYRENOIDS or AMYLUM CENTRES, may be seen in the green bands of Spirogyra (Fig. 235). Albumen crystals may also occur di- rectly in the cytoplasm ; as, for instance, in the cells poor in starch, in the peri- pheral layers of potatoes. Albumen crystals are sometimes found even in the cell nucleus. This is particularly the case in the Toothwort (Lathraea squamaria). Albumen crystals usually belong either to the regular or to the hexagonal crystal system. They differ from other crystals in that, like dead — =e st Ww BOTANY PART I albuminous substances, they may be stained, and also in that they are capable of swelling by imbibition. Subjected to the action of water or a dilute solution of caustic potash, they at first increase in size without losing their crystalline outline. CRYSTALS OF CaLcIuM OXALATE.—Few plants are devoid of such crystals. They are formed in the cytoplasm, within vacuoles which afterwards enlarge and sometimes almost fill the whole cell. In such cases the other components of the cell become greatly reduced; the cell walls at the same time are often converted into cork, and the whole cell becomes merely a repository for the crystal. The crystals may be developed singly in a cell, in which case they belong either to the tetragonal or monosymmetrical crystal system ; or, as is more frequently the case, they form CRYSTAL AGGREGATES, clusters of crystals radiat- ing in all directions from a common centre. In the Liliaceae, Orchidaceae, and other Monocotyledons, compact bundles of needle-shaped crystals of calcium oxalate, the so-called RAPHIDES, are especially frequent (Fig. 80). Such crystal bundles are always enclosed in a large vacuole filled with a mucilaginous sub- stance. The degree of concentration of the mother liquor from which the crystals have separated, determines, according to Kwny, their erystal form, whether tetragonal or monoclinic. SILICEOUs BODIES, which are only soluble in hydro- fluoric acid, are often found in the cytoplasm of many cells, especially of Palms and Orchids, and often com- pletely fill the whole cellular space. TANNIN.—Highly refractive vacuoles filled with a concentrated solution of tannin are of frequent occurrence in the cytoplasm of cortical cells, and may often grow to a con- siderable size. The dark-blue or green colour reaction obtained on treatment with a solution of ferric chloride or ferric sulphate, and the reddish-brown precipitate formed with an aqueous solution of potassium bichromate, are usually accepted as tests for the recognition of tannin, although equally applicable for a whole group of similar sub- stances. eis eae Call Teen Teor Fats and OILs in plants are mixtures of of Dracaena rubra, led fatty acid esters. Frequently, as in species of ae pres dae rele Allium and Aloe, a fatty oil appears in the an containing a uncle . of raphides, 7. (x 160.) old chlorophyll grains. The occurrence of castor oil in the form of highly refractive drops in the cytoplasm of the aleurone-containing cells in the endosperm of the castor-oil seeds, has already been referred to. Oil usually occurs in SECT, T MORPHOLOGY 73 this form. But fatty substances may also appear in the cytoplasm as irregularly-shaped, more or less soft grains, as for example in the vegetable butters and in the wax of various seeds; they may even be crystalline, as in the needle-like crystals of Para-nuts (Bertholletea excelsa) and of Nutmeg (J/yristica fragrans). Guiycocrn.—This substance, related to sugar and starch, and of frequent occur- rence in animal tissues, fulfils, according to Errera, the same functions in the Fungi as sugar and starch in the higher plants. Cytoplasm containing glycogen is coloured a reddish-brown with a solution of iodine. This colour almost wholly disappears if the preparation be warmed, but reappears on cooling. ETHEREAL OILS AND Restns.—In most cases the strongly refractive drops found dispersed throughout cytoplasm are globules of some ethereal oil. It is the presence of such oils in the petals of many flowers that give to them their agreeable perfume. Under certain conditions the oil globules may become crystallised. This occurs, for example, in Rose petals. Secretions from surrounding cells are often deposited in special receptacles in which, through oxidisation, camphor or resin is formed. Special cells of this kind, with corky walls and filled with resin or ethereal oils, are found in the rhizomes of certain plants, as for instance in those of Calamus (Acorus Calamus) and of Ginger (Zingiber officinale) ; also in the bark, as, for example, of Cinnamon trees (Cinnamomwm) ; in the leaves, as in the Sweet Bay (Laurus nobilis) ; in the pericarp and seed of the Pepper (Piper nigrum) ; in the pericarp of Anise seeds (ddiciwin anisatum). Mucriacinous MATTER is often found as a part of the cell contents in the cells of bulbs, as in Allium Cepa and Scilla maritima, in the tubers of Orchids, also in aerial organs, especially in the leaves of Succulents, which, living in dry places, are thus enabled to maintain their water-supply by means of their mucilaginous cells. CAOUTCHOUC AND GUTTA-PERCHA.—These substances are found in a number of plants belonging to different groups, in particular in the Urticaceae, Euphorbiaceae, and Sapotaceae. They occur in the so-called milk sap of special cells in the form of small, dense globules, which, suspended in the watery cytoplasm, give it its milky appearance. SutpHur.—As being of unusual occurrence, mention should be made of the presence of sulphur in the form of small refractive grains in the protoplasm of certain Bacteria, the Beggiatoae. These Bacteria live in water containing much organic matter, and, according to WrinoGkapsky, obtain their sulphur from sulphuretted hydrogen. In fulfilling its function in the Bacteria the sulphur becomes oxidised into sulphuric acid. The Cell Sap.—Under this term is included especially the fluid which in old cells fills the inner sap cavity. It is generally watery and clearer than the fluid contained in the smaller vacuoles of the 74: BOTANY PART I cytoplasm. No sharp distinction can, however, be drawn between the sap cavity and vacuoles, and, moreover, a number of such vacuoles may take the place of the sap cavity itself. The cell sap usually gives an acid reaction, though in water-plants, according to TSCHIRCH, this reaction is often uncertain. The substances held in solution by the cell sap are very various. The soluble carbohydrates, in parti- cular the sugars, cane sugar, the glucoses, and especially grape sugar, frequently occur in the cell sap. The glucoses may be recognised by their reducing properties. If preparations containing glucose be placed in a solution of ‘copper sulphate, and, after being washed out, are transferred to a solution of caustic potash and heated to boiling, they will give a brick-red precipitate of cuprous oxide. If cane sugar or saccharose be present, this same treatment gives only a blue colour to the cell sap. Carbohydrates are transported in a plant principally in the form of glucose ; cane sugar, on the contrary, is stored up as reserve material ; as for example, in the sugar-beet, in the stems of sugar-cane, and in other plants from which the sugar of economic use is derived. INULIN, a carbohydrate in solution in cell sap, takes the place of starch in many orders of plants, as, for example, in the Compositae. Treated with alcohol, inulin is precipitated in the form of small granules, which may be redissolved in hot water. When portions of, plants containing much inulin, such as the root tubers of Dahlia variabilis, are placed in alcohol or dilute glycerine, the inulin crystallises out and forms spherites, spheroidal bodies com- posed of radiating crystal needles arranged in concentric layers. . ASPARAGIN is also generally present in the cell sap. There are frequently found dissolved in the cell sap TANNINS, ALKALOIDS, and GLUCOSIDES, such as coniferin, hesperidin, amygdalin, solanin, exsculin, saponin, and also bitter principles related to the glucosides. It is also often possible to detect in the cell.sap one of the benzole group, phloroglucin, which, in the presence of hydrochloric acid, stains lignified cell walls a violet colour. Organic acids are also of frequent occurrence in the cell sap; thus, malic acid is usually present in the leaves of the succulents. For the most part, these organic acids unite with bases, and the salts which are formed often crystallise. Of acid salts, which are less frequent than free acids, the binoxalate of potassium found in Field Sorrel (Rwmea) and Wood Sorrel (Owalis) deserves special mention. The cell sap is often coloured, principally by the so-called ANTHO- CYANIN. This is blue in an alkaline, and red in an acid reacting cell sap, and, under certain conditions, also dark red, violet, dark-blue, and even black. Blood-coloured leaves, such as those of the Purple Beech, owe their characteristic appearance to the united presence of green chlorophyll and anthocyanin. The different colours of flowers are due to the varying colour of the cell sap, to the different distribution of the cells containing the coloured cell sap, and also to the different combina- tions of dissolved colouring matter with the yellow, yellowish red, or SECT. I MORPHOLOGY 75 red chromoplasts and the green chloroplasts. There is occasionally found in the cell sap a yellow colouring matter known as xanthin ; it is nearly related to xanthophyll, but soluble in water. The cell sap also contains inorganic salts in solution, particularly nitrates, sulphates, and phosphates. The Cell Wall.—At the growing points of plants the cells are separated from one another only by extremely thin membranes or cell walls. The rapid growth in length which sets in a short distance from the growing point, as a result of the increase in the size of the cells, must be accompanied by a corresponding GROWTH IN SURFACE of the Fic. 82.—Part of a sclerenchy- matous fibre from Vince Fic. 81.—Strongly thickened cell major, The striations of from the pith of Clemutis vitalba. the outer layers are more m, Middle lamella; %, intercel- apparent than those of lular space; ¢, pit; w, pitted the inner layers. The transverse cell wall. (x300.) walls, as seen in optical section, are also shown. (x 500.) cell walls. So long as this growth in surface continues, the cell walls remain thin. After the cells have attained their ultimate size, the GROWTH IN THICKNESS of the cell walls then begins. Such thickened cell walls are not, in most cases, homogeneous, but exhibit a stratified appearance (Fig. 81), owing to the different refractive power of the thickening layers. Treated with caustic potash, these different layers appear as if composed of still thinner lamella. In many cases the thick- ening layers exhibit delicate striations in surface view. The striations extend through the whole thickness of the layers, usually running obliquely to the long axis of the cell, and often crossing one another in the different thickening layers (Fig. 82). In a much-thickened cell wall, owing to chemical and optical 76 BOTANY PART I differences, there can frequently be distinguished three distinct layers —a primary, a secondary, and a tertiary thickening layer. These layers are deposited on the primary cell wall, which, in the case of cells arising from cell division, is represented by the newly-formed partition wall. The secondary thickening layer is usually the most strongly developed, and forms the chief part of the cell wall. The tertiary or inner layer is thinner and more highly refractive. In special cases, but only in the formation of reproductive cells, an inner thickening layer, completely detached from the others, is produced, as in the formation of pollen grains and spores, which, enclosed only Fic. §3,—From the wood of the Pine, Pinus sylvestris. Fic. 84.—Cells from the endo- A, Bordered pit, in surface view ; LZ, bordered pit in sperm of Ornithogalum wm- tangential section; ¢, torus; C, transverse section of bellatum. m, Pits in surface a tracheid; m, middle lamella, with gusset, m*; view ; p, closing membrane ; i, inner peripheral layer. (x 540.) n, nucleus. (x 240.) by this inner membrane, finally become freed from the older thickening layer. This process is often alluded to as REJUVENESCENCE ; in such cases, it should be noted, there are, in reality, no new cells formed. The thickening of the cell wall seldom takes place uniformly over the whole surface; but some portions are thickened, while, at other points, the original or primary cell wall remains unchanged. In this way pores are formed which penetrate the thickening layers. These pores or PITS may be either circular (Fig. 84), elliptical, or elongated. The pits in adjoining cells converge, and would form one continuous canal, were it not that the unthickened primary cell wall persists as a CLOSING MEMBRANE between two converging pits. As a result of the continued thickening of the cell wall, the canals of several pits often unite, and so BRANCHED PiTs are formed. Such branched pits have usually very narrow canals, and occur for the most part only in extremely thick and hard cell walls, as, for instance, in those of SECT. I MORPHOLOGY 77 the so-called sclerotic cells or sclereides. Simple pits may, on the other hand, expand on approaching the primary cell wall. The structures known as BORDERED PITS (Fig. 83) are but a special form of such expanded simple pits. In bordered pits the closing membrane is thickened at the centre to form a ToRUS (Fig. 83, C). I HI | { { Fic. 85.--Part of two Fic. 86.— A, Part of an annular tra- sieve- tubes: of the cheid ; B, part of a spiral tracheid ; Pine, Pinus sylvestris, C, longitudinal section through part showing _ sieve - pits. of a reticulate vessel, showing perfor- (x 540.) ated partition wall, s. (x 240.) By the curving to one side or the other of the closing membrane, the torus may so act as to close the pit canal (Fig. 83, B). Bordered pits ; are only formed in cells which are soon to lose their living contents and thus serve merely as channels for conducting water. The bordered pits apparently act as valves. Seen from the surface a bordered pit appears as two concentric rings (Fig. 83, A). The smaller, inner ring represents the narrow opening of the Fic. $87.—Part of transverse section of a stem of Impatiens pit into the cell cavity ; parviflora. e, Epidermis ; c, collenchyma ; p, thin-walled the larger, outer ring parenchymatous cells ; i, intercellular space. (x 300.) indicates the junction of the wall of the PIT CHAMBER with the primary cell wall. Very large pits between adjoining living cells have often thin places in their closing membrane, and are then spoken of as compound pits. A special example of such pits is afforded by the SIEVE-PITS, in 78 BOTANY PART I which the closing membrane, in that case called the SIEVE-PLATE, is perforated by fine openings or pores (Fig. 85). : In cases where the greater part of the cell wall remains unthick- ened, it is characterised rather by a description of its thickened than unthickened portions ; it is in this sense that the terms annular, spiral, and reticulate are used (Fig. 86). Just as in the case of cells with bordered pits, annular, spiral, and reticulate cell walls are only acquired by cells that soon lose their contents, and act in the capacity of water-carriers. Such wall thickenings serve as mechani- cal supports, to give rigidity to the cells, and to enable the cell walls to withstand the pressure of the surrounding cells. CoLLEN- CHYMATOUS cells are living cells, the walls of which are thickened principally at the corners (Fig. 87). Cells on the surface of plants Fic. 88.—Part of transverse section of aleaf of have usually only their outer walls Ficus elastica. ¢, Cystolith ; e, e, *, triple: thickened (Fig. 100). By the layered epidermis ; p, palisade parenchyma ; . . é % spongy parenchyma. (x 240.) thickening of cell walls at special points, protuberances projecting into the cell cavity are formed; in this way the formations known as CYSTOLITHS arise. Certain large cells in the leaves of the Indiarubber plant (Ficus elastica) con- tain peculiar clustered bodies, formed by the thickening of the cell wall at a single point (Fig. 88). In their formation a stem-like body or stalk first protrudes from the cell wall; by the addition of freshly-deposited layers this becomes club-shaped, and, by continued irregular deposits, it finally attains its clustered form. So far only centripetal wall thickenings have been described. Cells, the walls of which are centrifugally thickened, can naturally only occur where the cell walls have free surfaces. The outer walls of hairs generally show small inequalities and projections. The surface walls of spores and pollen grains (Fig. 89) show a great variety of such centrifugally developed protuberances, in the form of points, ridges, reticulations, and bands of an often complicated internal structure. The Origin and Growth of the Cell Wall.—The cell wall is a product of the protoplasm. When a previously naked protoplast, as a swarm-spore of an Alga, envelops itself with a cell wall, this is effected, as is now generally believed, by the transformation of its SECT, I MORPHOLOGY 79 protoplasmic membrane into a cell wall. The newly-formed partition wall, resulting from cell division, is developed from the cell plate, which is also of cytoplasmic origin. ‘The new lamelle of a cell wall in process of thicken- ing are also derived from the protoplasmic membrane of the enclosed cytoplasm. The growth in thickness of a cell wall by the deposition of successive lamelle is termed GROWTH BY APPOSITION. The growth in surface of cell walls may, in many cases, be attributed to the deposition ‘of new lamelle Fic. 89.4, Pollen-grain of Cucurbita Pepo in 3 uw surface view, and partly in optical section, simultaneously accompanying the rendered transparent by treating with oil of distension of the old. The sub- lemons (x 240); B, part of transverse section sequent growth in thickness of of pollen grain of Cucurbita verrucosa. (x 540.) the single lamelle of the cell walls, by the interpolation of new particles of cell-wall substance between the old, is designated GROWTH BY INTUSSUSCEPTION. Cell Wall Substanee.—The transformation of the cell plate, or of the protoplasmic membrane of the cytoplasm, into lamelle of the cell wall, is accompanied by a change in their substance. The granules of the cell plate disappear and apparently dissolve, while the lamellae of a cell wall are eventually formed from the solution. Possibly the lamelle of cell walls possess a crystalline structure similar to that of starch grains, with which they seem to correspond in many structural peculiarities and in the double refraction of their layers. The most important constituent of cell walls is CELLULOSE. With the exception of the Fungi it is present in the cell walls of all plants. GILson succeeded in obtaining cellulose in a state of crystallisation. He treated a plant section for a time with cuprammonia, then washed the section carefully with ammonia of a suitable concentration, and afterwards with distilled water. In the cells of sections treated in this manner he found cellulose crystals in the form of spherites or dendrites. Cellulose is a carbohydrate of which the chemical composition is expressed by the general formula (CgH,,0;)n. It is insoluble in either dilute acids or alkalies. By the action of concentrated sulphuric acid it is converted into dextrose. After treatment with sulphuric or phosphoric acid, iodine will colour it blue; it shows a similar reaction when exposed to the simultaneous action of a concentrated solution of certain salts, such as zinc chloride or aluminium chloride, and of iodine. Accordingly, chloroiodide of zinc, on account of the blue colour imparted by it, is one of the most convenient tests for cellulose. The cell walls never consist entirely of pure cellulose, but contain a considerable amount of other substances, which are not stained blue 80 BOTANY PART [ by chloroiodide of zinc. In unlignified cell walls PECTOSE is parti- cularly prominent. It is easily distinguished by the readiness with which it dissolves in alkalies, after being previously acted upon by a dilute acid. Susceptibility to certain stains, for example congo red, is a characteristic of cellulose ; while other stains, such as safranin and methylene blue, colour pectose more deeply. According to MANcIN, the partition wall formed in the higher plants during cell division consists almost wholly of pectose ; the next developed lamine, the secondary cell-wall layer, of a mixture of cellulose and pectose ; the last formed, or tertiary layer, chiefly of cellulose. If the secondary layer of the cell wall remain unlignified, the amount of pectose contained in it increases with age and helps to strengthen the MIDDLE’ LAMELLA, or primary cell-wall layer. Among the substances entering into the composition of cell walls, in addition to cellulose and pectose, mention must be made of CALLOSE. It is characterised by its insolubility in cuprammonia and solubility in soda solution, and in a cold 1 per cent solution of caustic potash. It is coloured a red brown by chloroiodide of zinc, with aniline blue it takes an intense blue, and with corallin (rosolic acid) a brilliant red. Its presence in the higher plants is limited to a few special cases ; it envelops the sieve-pits and is always present in éalcified cell-wall layers, as, for example, in cystoliths (Fig. 88). According to Manan, callose exists in the cell walls of the Fungi and Lichens, generally in combination with cellulose, or more rarely with pectinaceous substances. GILSON asserts, on the other hand, that the cell walls of all the Fungi that he has thoroughly investigated, consist of a special nitrogenous substance, which he has called mycosrn, and considers that it corresponds to animal chitin. This chitin, according to Ginson, takes the same place in the cell walls of the Fungi as cellulose in the cell walls of the higher plants. In addition to chitin, the cell walls of Fungi always contain carbohydrates. Where cell walls become LIGNIFIED or SUBERISED, it is particularly the secondary layer that receives the wood or cork substance, while the tertiary or internal layer retains its cellulose character. The lignification is occasioned by the deposition in the cell wall of certain substances, among which are always coniferin and vanillin. It is these two substances which give the so-called wood reactions,—a violet colour with phloro- glucin and hydrochloric acid, a yellow colour with anilin sulphate. With chloroiodide of zine a lignified cell wall becomes yellow, not blue. Suberised cell walls take a yellowish brown colour with chloroiodide of zinc ; with caustic potash, a yellow. Van WIssELINGH has lately disputed the presence of cellulose in suberised cell walls, and regards the cork substance or sUBERIN as a fatty body, which is composed of glycerine esters and other compound esters, as well as of one or more other substances which are infusible, insoluble in chloroform, and decomposed by a solution of caustic potash. CUTINISATION, which is similar to but not identical with suberisa- tion, is usually due to the subsequent deposition of cutin in cellulose cell walls. VAN WissELINGH has shown that phellonic acid, which is always present in suberin, is constantly absent in cutin. Cutin withstands better the action of SECT. I MORPHOLOGY 81 caustic potash. In other respects, the reactions given by cutinised cell walls with chloroiodide of zinc or solutions of caustic potash are almost identical with those of suberised cell walls. While after lignification cell walls are still permeable to both water and gases, suberisation or cutinisation renders them impervious. Accordingly, suberised and cutinised cell walls are found especially in the surface of plants, as a means of protection and preservation. The layers of the cell walls of some cells, particularly the super- ficial cells of certain fruits, as of Sage, and of numerous seeds, such as Flax and Quince seeds, become mucilaginous, and swell in water to a slime or vegetable mucus, which, according to G. KLEBS, serves the purpose of attaching the seeds to the soil. The internal cells of some leguminous seeds with a mucilaginous endosperm, such as the seeds of the Carob tree (Ceratonia Siliqua), have similar mucilaginous layers, which serve as reserve substance. Firm cell walls can also be trans- formed into GUM, as is so often apparent in Cherry and Acacia trees, portions of whose woody cells often succumb to GUMMOSIS. The several varieties of gums and vegetable mucus react differently, accord- ing as they are derived from cellulose, callose, pectose, or from allied substances. According to Mancin they may be microchemically distinguished by their reaction with ruthenium red, which stains only such as are derived from pectose or related substances, such as the mucilage of the seeds of the Cruciferae and Quince (Cydonia), the mucus cells of the Malveae, the gums of the Cherry and Acacia, the gum tragacanth from Astragalus gummifer. The mucus of Orchid tubers, on the other hand, is related to cellulose, and remains uncoloured with the same reagent. The cell walls of the seeds of many Palms, as also those of Ornithogaluim (Fig. 84), have strongly developed thickening layers, which are full of pits. These thickening layers are lustrous white, and, as in the case of the seeds of the Palm, Phytelephas macrocarpa, may attain such a degree of hardness as to be technically valuable as vegetable ivory. Such thickening layers may contain other carbohyd- rates in addition to cellulose; thus the cell walls of the seeds of Tropaeolwm and Paeonia contain an AMYLOID, which turns blue even with iodine alone. These thickening layers are dissolved during germination, and are accordingly to be considered as a reserve substance of the seeds. Cell walls often become coloured by tannin or derivative sub- stances ; in this way, for instance, the dark colour is produced which is often seen in the shells of seeds and in old wood. The colours of the woods of economic value are due to such discoloured cell walls. Inorganic substances are often deposited in large quantities in old cell walls. Among such substances calcium oxalate is often met with, commonly in crystal form; also calcium carbonate, although perhaps not so frequently. In the eystoliths of Ficus elastica (Fig. 88) so much calcium carbonate is deposited that it effervesces with hydrochloric acid. In many plants, as, for instance, most of the Characeae, the quantity of calcium carbonate in their cell walls is so great as to G 82 BOTANY PART I render them stiff and brittle. Silica is also present in the superficial cell walls of the Gramineae, Equisetaceae, and many other plants. Cell Forms.—As cytoplasm is a viscous fluid, and would tend, if unimpeded, to take a spherical shape, it may be assumed that the natural and primary form for cells is spherical. Such a shape, how- ever, could only be realised by cells which, in their living condition, were completely free and unconfined, or in such as were able to ex- pand freely in all directions. Newly-developed cells, which are in intimate union, are, at first, always polygonal. Through subsequent growth their shape may change. The cubical cells of the growing point either elongate to a prism or remain short and tabular. If the growth is limited to certain definite points, and is regular, they become stellate ; if irre- gular, their outline is correspondingly un- symmetrical. In consequence of energetic growth in length, fibre-like, pointed cells are developed. If the walls of such cells become much thickened, they are called SCLERENCHYMA fibres (Fig. 90, 4). These show diagonal markings, due to their elon- gated pits, which are generally but few in number. When fully developed, the living contents of such cells are small in amount and frequently they contain only air. In the last case, they merely act.as mechanical supports for the other parts of the plant. Cells some- what similar, but shorter and considerably wider, not sharpened at the ends, and pro- vided with bordered pits, are called TRa- CHEIDS (Fig. 90,8). The tracheids, in their fully developed condition, never have any living contents, but serve as water- carriers for the plant. So long as they remain active, they contain only water and isolated air-bubbles ; their active functions afterwards cease, and they become filled Serer ee ee cee with air. Tracheids, which are specially fibre; B, ‘a tracheid ; C, part of elongated, and at the same time have only a spiral tracheid; D, part of a & narrow lumen, and, like the sclerenchy- latex tube. (4, B,C, x 100; 2, matous fibres, serve merely mechanical Cae Hee) purposes, are known as FIBRE TRACHEIDS. Very long tracheids with a wide lumen and thin walls, functioning, like typical tracheids, as water-carriers, are distinguished as vasiform or VASCULAR TRACHEIDS. They are characterised by the annular, spiral, or reticulate markings of their thickening layers, and may also be provided with bordered pits. A B I@® QO©_g _O.9:06 2 _ © © O@© 2 etore, be the arrow, ouly the thickness d is utilised in those parts of plants which aaa ts Oued me are in_an actively growing state, In organ the thickness D”. such cases where greater rigidity is re- quired than can be maintained by cell turgidity and tissue tension, it is secured by the development of COLLENCHYMA (p. 78). This tissue, according to AMBRONN’S re- searches, in addition to its extreme resistance to tearing, possesses the power of elongating under the influence of the force of growth. The SECT. II PHYSIOLOGY 171 more capable it is of growth the more it responds to the growth in its neighbourhood. It forms, so to speak, the CARTILAGINOUS TISSUE of plants. In many organs, as for example in leaf-stalks, collenchyma is the permanent strengthening tissue. Since, as has already been pointed out, the resistance of the mechanical elements to flexure is greater the farther they are removed from the centre of an organ, it will be readily seen that, while a flattened, outspread organ can be easily bent, if it were folded or rolled together, its power of resisting a deflecting force would be increased. In accordance with this principle many leaves become plaited or rolled (Fig. 170), and so acquire a sufficient rigidity without the assistance of any specially developed mechanical tissues. II. Nutrition By nutrition are understood all the processes of METABOLISM, or the chemical transformation and conversion of matter carried on by plants in the production and appropriation of their food-supply. Without nourishment and without new building material no growth or development is possible. As the processes of elaboration and secretion are continuous, if the food-supply is not kept equal to the demands made upon it, the death of the organism from starva- tion must ensue, while a continuance of its growth and further development is only possible when there is a surplus of the elaborated food material. The Constituents of the Plant Body.—By means of chemical analysis the constituent substances of plants have been accurately ascertained. It requires, however, no analysis to realise that a part, often indeed the greater part, of the weight of a plant is derived from the water with which the whole plant is permeated. Water not only fills the cavities of living, fully-developed cells, but it is also present in the protoplasm, cell walls, and starch grains. By drying at a tempera- ture of 110°-120° C. all water may be expelled from vegetable tissues, and the solid matter of the plant will alone remain. The amount of dried substance in plants varies according to the nature and variety of the plant and of the particular organ. In woody parts it constitutes up to 50 per cent of their weight, but in herbaceous plants amounts to only 20 or 30 per cent. In more succulent plants and fruits it makes up only 5-15 per cent of their total weight; in water-plants and Algae, 2-5 per cent, while everything else is water. The dried substance of plants is combustible, and consists of organic compounds, which contain but little oxygen, and are converted by combustion into simple inorganic compounds, for the most part into carbonic acid and water. The elements CARBON, HYDROGEN, and OxyGEN form the chief constituents of the combustible dried substance. 172 BOTANY PART I Next to them in quantity is NITROGEN, which is derived principally from the protoplasm. After combustion of the dried substance of plants there always remains an incombustible residue, the asH, consisting of the mineral substances contained in the plant. As these mineral substances undergo transformation during the process of combustion, they are found in the ash in different chemical combinations than in living plants. From numerous analyses made of the ash of a great variety of plants, it has been determined that nearly all the elements, even the less frequent, are present in plants. In addition to the four already named, the elements found in the ash of plants are sulphur, phosphorus, chlorine, iodine, bromine, fluorine, selenium, tellurium, arsenic, antimony, silicon, tin, titanium, boron, potassium, sodium, lithium, rubidium, calcium, strontium, barium, magnesium, zinc, copper, silver, mercury, lead, aluminium, thallium, chromium, manganese, iron, cobalt, and nickel. Many of these elements, indeed, occur only occasionally and acci- dentally, while others—sulphur, phosphorus, chlorine, silicon, potassium, sodium, calcium, magnesium, and iron—are met with in almost every ash. As might be inferred from the irregular occurrence of many of the elements, they are not all necessary for nutrition, and although their occasional presence in a plant may sometimes change certain of its special characteristics (thus the presence of zinc produces the so- called calamine varieties, such as, for example, Thlaspi alpestre var. calaminare, Viola lutea var. calaminaria, etc.), they do not exercise a decisive influence upon its existence. The Essential Constituents of Plant Food.—Chemical analysis, while enabling us to determine the substances present in plants, does not show how far they are essential for nutrition. From culture experiments, in which the plants are grown in a medium of which the constituents are known, and kept under chemical control, it has been ascertained that, in addition to earbon, hydrogen, oxygen, and nitrogen, which form the principal part of the combustible elements of the dry substance of plants, sulphur, phosphorus, potassium, ealeium, magnesium, and iron are absolutely indispensable to the growth of all green plants. In the absence of even a single one of these elements no normal development is possible. According to Moxiscu, only nine of these elements are required by the Fungi. It is not, however, iron, as might be supposed, but calcium, that is unessential. On the other hand, the ten substances named suflice for the nutrition of most green plants ; but it is not to be denied that certain other substances are of use in the plant economy and of advantage to growth, although not indispensable. Thus, for example, Buckwheat flourishes better when supplied with a chloride, and the jwesence of silica is advantageous as contributing to the rigidity of the tissues. It has also been discovered that by the presence of certain substances, in them- selves of no nutritive value, the absorption of actual nutritive matter is increased (cf. p. 175). In the case even of the very poisonous copper salts, experience SECT. IL PHYSIOLOGY 173 has taught that when they are brought into contact with the leaves (by sprink- ling the plants with solutions to prevent the inroads of insects), they exercise a beneticial influence on the formation of chlorophyll, and increase assimilation, transpiration, and the length of life. The nutritive substances are, naturally, not taken up by plants as elements, but in the form of chemical compounds. CARBON, the essential component of all organic substances, is obtained by all green plants solely from the carbonic acid of the atmosphere, and is taken up by the green leaves. All the other constituents of the food of plants are drawn from the soil by the roots. HyproGEn, together with OXYGEN, is obtained from water, although the oxygen is derived also from the atmosphere and from many salts and oxides. Nu?TROGEN is taken up by the higher plants only in the form of nitrates or ammonium salts. As the ammonia of the soil formed by the soil bacteria from organic decaying matter is transformed by the help of other so- called nitrifying bacteria into nitrites, and eventually into nitrates, only the nitrogen combined in the nitrates need be taken into consideration. Bacteria, as contrasted with the higher plants, are particularly characterised by their attitude towards nitrogen. In addition to the bacteria, which, by their nitrifying capability, are of service to green plants, there are other soil bacteria which set free the nitrogen of nitrogenous compounds and thus render it unservice- able for the nutrition of green plants, On the other hand, other forms of bacteria convert the free nitrogen of the air into compounds (amides?) which serve not only for themselves, but also for the higher plants as convenient nitrogenous food material. This remarkable nitrifying power of bacteria ,.has led to a lite partnership (SyMBrosrs) between them and some of the higher plants (Leguininosac). In such symbiotic relations the bacteria provide the higher plants with nitrogen in a form in which it may be assimilated, while, in turn, they are supplied with the carbon compounds essential for their nutrition (¢/. p. 211 and Fig. 186). SuLPHURand PHOSPHORUS form, like nitrogen, important constituents of protoplasm. All proteid substances contain sulphur. The sulphur is taken into plants in the form of sulphates ; phosphorus in the form of phosphates. Porasstum, unlike sodium, is essential to plant life, and is presumably active in the processes of assimilation and in the forma- tion of protoplasm ; it is introduced into plants in the form of salts, and constitutes 3-5 per cent of the weight of their dried substance. Macnesium, like potassium, participating in the most important synthetic processes of plants, is found in combination with various acids, particularly in reservoirs of reserve material (in seeds to the extent of 2 per cent) and in growing points (in leaves only 4 per cent). Cacrum also is taken up in the form of one of its abundant salts, and in considerable quantities (2-8 per cent). Calcium plays an im- portant part in the metabolic processes of plants, not indeed as an actual constituent of protoplasm, but as a vehicle for certain other 174 BOTANY PART I essential substances, and, through its capacity to form compounds, as a means of fixing and rendering harmless hurtful by-products. IRON, Sy Fi sD b ey a RE —— pap © Say ee SS Se eo; So SX "ae L> ony = = ‘IS; B KS LS) gy I Fic. 171.—Water-cultures of Fagopyrum esculentum. I. In nutrient solution containing potassium ; IJ., in nutrient solution without potassium. (After Nope, reduced. ) come abnormally developed. although of the greatest importance in the formation of chlorophyll, is present in plants only in small quantities. In order to determine the nutritive value of different substances the method of WATER-CULTURE has proved pafticu- larly useful (Fig. 171). In these culture experiments the plants, grown either directly from the seed or from cuttings, are cultivated in distilled water to which have been added certain nutritive salts. If all the essential nutritive salts are present in the culture solution, even larger plants, such as Indian Corn, Beans, etc., will grow to full strength and mature seeds as well as if grown in earth. It is not necessary in these ex- periments to provide carbon compounds in the nutrient solution, as plants do not derive their carbon supply through their roots, but, with the help of their leaves, from the carbonic acid of the atmosphere. The young plants would grow for a time just as well in pure distilled water as in the nutrient solution; but as the supply of nourishment stored in the seeds became exhausted, they would gradually cease to grow, and die. If one of the essential constituents of plant food be omitted from the nutrient solution, although the young plants would grow better than in the dis- tilled water, they would in time be- When, for example, a plant is grown in a nutrient solution containing all the essential food elements except iron, the new leaves developed are no longer green, but are of a pale yellow colour ; they are “CHLOROTIC,” and not in a condition to decompose the carbonic acid of the atmosphere and nourish the plant. Upon the addition, however, of a mere trace of iron to the solution the chlorotic leaves in a very short time acquire their normal green colour. So long as the necessary nutritive substances are provided, the form in which they are offered to the plants, as well as the proportionate strength of the nutrient SECT. IL PHYSIOLOGY 175 solution (if not too concentrated), may vary. Plants have the power to take up these substances in very dilferent combinations, and are able to absorb them in other proportions than those in which they occur in the soil. In concentrated nutrient solutions the absorption of water is increased ; conversely, in very dilute solutions it is the salts that are chiefly taken up. The presence also of certain substances often exerts an active and generally beneficial influence upon the capacity for absorbing other substances: thus, calcium salts increase the absorp- tion of potassium and ammonium salts. The following are the proportions of one among the many nutrient solutions recommended : Distilled water . : 1000 to 1500 grams. Potassium nitrate Le oy Magnesium sulphate . 5 On 5a Calcium sulphate : ‘ WS be Calcium or potassium phosphate ‘ Ord sy To this solution a trace of some iron salts should be added. The solution should be kept in the dark to prevent the development of algoid growths, and occasionally aerated during the culture experiment. As a most important result of such culture experiments, it has been demonstrated that only the ten elements already named are necessary for the growth of plants; all other elements, although present in plants in large quantities, are of subordinate value to plant life. This is true, for instance, of SopruM, which in combination with CHLORINE actually predominates in some plants, and occasions the characteristic development of many of the succulent salt-plants ; and also of SILICON, which, as silica, is so abundantly deposited in the cell walls of many plants—Lquisetuceae, Grasses, Sedges, Diatoms (in the ash of Wheat-straw 70 per cent, and of Kquisetaceae 70-97 per cent)— that, after combustion of their organic substances, it remains as a firm siliceous skeleton, preserving the structure of the cell walls. The hardness and firmness of the cell walls are so greatly increased by these siliceous deposits that some of the Hquisetuceae are even used for polishing and scouring ; while the margins of grass blades, from a similar deposition of silica in their cell walls, are often rendered sharp and cutting. The silicified cell walls of Diatoms occur as fossils, and form deposits of SILICKOUS EARTH (Kieselguhr) in some geological for- mations. The value of the siliceous concretions, termed “ Tabasheer,” that are found within the joints of Bamboo has not, as yet, been satisfactorily explained. ALUMINIUM, although like silica everywhere present in the soil, is only in exceptional instances taken up by plants. ‘Aluminium has been detected in the ash of Lycopodiaceous plants ; ‘Lycopodium complanatum contains a sufficient quantity of acetate of ‘aluminium to render the sap useful as a mordant. The same salt is found also in Grapes. On the other hand, although scarcely a trace of iodine can be detected by an analysis of sea-water, it is found, neverthe- less, in large quantities in sea-weeds, so much so that at one time they formed the principal source of this substance. 176 BOTANY PARTY I It was first asserted by C. SPRENGEL, and afterwards emphasised by Lrepic, that the mineral salts contained in plants, and once sup- posed to be products of the vital processes of the plants themselves, were essential constituents of plant food. Conclusive proof of this important fact was, however, first obtained by the investigations of WIEGMANN and POLSTORFF. The actual proportions of the more important ash constituents of some well- known plants can be seen from the following table of ash analysis by Wotrr. The table also shows exactly what demands those plants make upon the soil, that is, what substance they take away from it, in addition to the nitrates which do not appear in the ash. The great difference brought out by the table in the proportions of the more important phosphoric acid and of the less essential silica and lime contained in Rye and Pea seeds, as compared with the amounts of the same substances in the straw, is worthy of especial notice. Ashin | 100 Parts of ash contain i 100 parts of | Plants: dry solid 7 matter. | KO |NagO| CaO | MgO |Fes03|Mn30q] P2053] SO3 | SiOg | Cl Rye (grain) 2-09 | 82510} 1-47] 2°94] 11-22] 1-24 47-74| 1:28) 1:37| 0-48 Rye (straw) 4-46 22-56] 174] 8-20) 310) 1-91 6-53] 4:25 | 49-27] 2-18 Pea (seeds) 2°73 /43°10] 0-98} 481] 7°99] 0°83 35°90] 3-42} 0-91) 1°59 Pea (straw) $ 513 22°90 | 4:07 | 36°82, 804) 1°72 8:05] 6:26] 6S3] 5°64 Potato (tubers) 3-79 60°06 | 2°96] 2°64} 4°93] 1°10 16°86] 6°52] 2°04] 3:46 Grape (fruit) 519 56°20 | 142) 10°77 | 4:21] 0°37 15°58 | 5°62] 2°75) 1°52 ‘Tea (leaves) 5:20 34°30 | 10°21] 14°82] 5°01] 5:48 14°97 | 7°05} 5:04] 1:84 Coffee (beans) . 3°19 62°47 | 1°64] 6°29} 9°69] 0°65 13:29 | 3°80) 0°54] 0°91 Tobacco (leaves) 17°16 29°09 | 3°21 | 36°02; 7°36) 1:95 466 | 607) 577] 671 Cotton (fibres) . 114 39°96 | 13°16 | 17752) 5°36) 0°60 a 10°68 | 5:94] 2-40] 7°60 Spruce (wood) Q-21 19°66 | 1°37 | 83°97 | 11-27 | 1:42 | 23°96) 2°42] 2°64) 2°73) 0°07 Plants which require a large amount of potassium, such as the Potato, Grape- vine, and Coffee-tree, are termed potash plants. In the preceding table the figuresido not express absolutely constant propor- tions, as the percentage of the constituents of the ash of plants varies according to the character of the soil; thus, the proportion of potassium in Clover varies from 9 to 50 per cent ; the proportion of calcium in Oats from 4 to 38 per cent. The Proeess of Absorption.—As all matter taken up by plants must, as a rule, pass through continuous cell walls, it must be absorbed in a liquid or gaseous state. The only exception to this rule occurs in the amceboid forms of the lower plants (dAmoebae and Plasmodia), which, as they have no cell walls, are in a condition to take up and again extrude solid matter (small animals, living or dead, also plants and particles of inorganic substances). The fact that plant cells are completely enclosed by continuous walls renders it necessary that food, to pass into the cell, must be either liquid or gaseous. In this condition the constituents of plant food are, however, imperceptible, and thus the manner of plant nutrition remained for a long time a mystery, and it was only during the SECT. IL PHYSIOLOGY 177 last century that the nature of the nourishment and nutritive processes of plants was recognised. Plant nourishment is dependent upon the permeability of the cell walls to liquids and gases. Although impervious to solids, the cell walls of living cells are permeated with “imbibed” water; and to this “IMBIBITION WATER” in the cell walls, together with the physical char- acter of the cell walls themselves, are due their flexibility, elasticity, and ductility. The permeability of cell walls for imbibition water is only possible within certain limits, so that they thus retain the character of solid bodies. Treated with certain chemical reagents (potassium hydrate, sulphuric acid, etc.) cell walls become swollen and gelatinous, or even dissolve into a thin mucilaginous slime. This change in their character is due to an increase in the amount of their imbibition water, induced by the action of the chemicals ; other- wise, the water imbibed by ordinary cell walls is limited in amount. The walls of woody cells take up by imbibition about one-third of their weight ; the cell walls of some seeds and fruits and of many Algae absorb many times their own volume. THE CELL WALLS ARE NOT ONLY PERMEABLE TO PURE WATER, BUT ALSO TO SUBSTANCES IN SOLUTION. This fact, that the cell wall offers no resistance to the diffusion of crystalloid bodies when in solution, is of the utmost importance to plant nutrition; cell walls, on the other hand, which are scarcely or not at all permeable to liquids (cuticularised walls), take no part in the absorption of plant nourishment, except in so far as they may still be permeable to gases. In order that liquids may enter by osmosis into the living cell, they must first pass through the protoplasm, 7.e. the lining of the cell wall. LIVING PROTOPLASM is not, however, like the cell walls, equally per- meable to all substances in solution, but, on the contrary, COMPLETELY EXCLUDES CERTAIN SUBSTANCES, WHILE ALLOWING OTHERS TO PASS THROUGH MORE OR LESS READILY. Moreover, it is able to change its permeability according to circumstances, and thus THE OUTER PROTOPLASMIC MEMBRANE HAS THE POWER OF DECISION, whether a substance may or may not effect an entrance into the cell. Similarly the INNER PROTOPLASMIC MEMBRANE exercises a similar but often quite distinct power over the passage of substances from the proto- plasm into the cell sap. The same determinating power is exercised by these membranes in the transfer of substances in a reverse direction. On account of the selection thus exercised by the protoplasm, it is possible that, in spite of continued osmotic pressure, the contents of a cell are often of quite a different chemical nature from the immediately surrounding medium. To this same peculiar quality of the proto- plasmic membranes is also due the SELECTIVE POWER of cells, manifested by the fact that different cells, or the roots of different plants, appropriate from the same soil entirely different compounds ; so that, for instance, one plant will take up chiefly silica, another N 178 BOTANY PART I lime, a third common salt, while the aluminium, on the other hand, is rejected alike by all three. ‘The action of sea-weeds in this respect is even more remarkable; living in a medium containing 3 per cent of common salt, and but little potassium salts, they nevertheless accumulate much larger quantities of potassium than sodium. In addition they store up phosphates, nitrates, and iodine,—substances which are all present in sea-water in such small quantities as scarcely to be detected by chemical analysis. That osmosis may continue from cell to cell, it is essential that the absorbed material must become transformed into something else, either by the activity of the protoplasm or by some other means. Local accumulations of sugar or other soluble reserve material in fruits, seeds, bulbs, and tubers would otherwise not be possible; for osmotic action, if undisturbed, must in the end lead to the uniform distribution of the diffusible substances equally throughout all the cells. But if equilibrium is prevented by the transformation of the diffusible substances into others that are indiffusible, the osmotic currents towards the transforming cells will continue, and the altered and no longer diffusible substances will be accumulated in them. In this manner glucose passing into the cells of tubers or seeds becomes converted into starch. As a result of this a constant movement of new glucose is maintained towards these cells, which thus become reservoirs of accumulated reserve material. From the power of protoplasm to regulate osmotic currents, in that by reason of its permeability it allows the osmotic forces to operate, or, on the other hand, may modify and altogether prevent them, it is apparent that here also, just as in the case of the rigidity of plants, osmosis, although a purely physical phenomenon, is controlled by the protoplasm and rendered serviceable to plant life. Water and Mineral Substances The fact that water is essential to the life of all living organisms is so obvious that, in the infancy of natural history and philosophy, from THALES to EMPEDOCLES, water was regarded as the original principle of all existence, at least of the organic world. Even so late as the six- teenth century it was held by VAN HxELMoNT, the first to investigate experimentally the question of the nutrition of plants, that the whole substance of plants was formed of water. If the importance of water in this respect was greatly overrated, the universal necessity of water for all vital processes is still recognised in the present more advanced stage of scientific knowledge. Without water there can be no life. THE LIVING PORTIONS OF ALL ORGANISMS ARE PERMEATED WITH WATER ; it is only when in this condition that their vital processes can be carried on. Protoplasm, the real vehicle of life, is, when living, SECT, II PHYSIOLOGY 179 of a viscous, thinly fluid consistency, and when freed from its water either dies or becomes perfectly inactive. The circumstance that protoplasm, when in a state of inactivity, as in spores and seeds, can often endure a certain degree of desiccation for a limited time, forms no exception to this rule. During such periods its actual vital functions entirely cease, and only renew their activity when water is again supplied. In most plants desiccation occasions death, and it is always to be regarded as due to some special provision or exceptional quality when entire plants or their reproductive bodies can be again brought to life by a subsequent supply of water. Thus, for example, some Algerian species of Jsoetes, and the Central American Selaginella lepidophylla, can withstand droughts of many months’ duration, and on the first rain again burst into life and renew their growth. In like manner many Mosses, Liverworts, Lichens, and Algae growing on bare rocks, tree-trunks, etc., seem able to sustain long seasons of drought without injury. Seeds and spores, after separation from their parent plants, remain productive for a long time ; seeds of Mimosa, which had been kept dry for over sixty years, proved as capable of germination as those of recent growth. A similar vitality was shown by moss spores which had lain ina herbarium fifty years. The often-repeated assertion concerning the germination of wheat found with Egyptian mummies (‘‘mummy- wheat’) has, however, been shown to be erroneous. Many seeds lose their power of germination after having been kept dry for only a year ; others, even after a few days ; and others again, as the seeds of the willow, cannot endure drying at all. It must not be forgotten that in all these instances a certain amount of hygroscopic water is retained by plants even when the air is quite dry. Over the sulphuric acid of the desiccator, seeds retain for weeks 6 per cent or more of their weight of water. The withdrawal of this hygroscopically absorbed water kills all vegetable tissues without exception. Apart from permeating and energising the cells, water has other and more varied uses in plant life. It is not only directly indis- pensable for the solution and transportation of the products of metabolism, but also indirectly, in that its elements, hydrogen and oxygen, are made use of in organic compounds in plant nutrition. Water thus used (cf. p. 200) may be designated CONSTITUTION WATER. It is also necessary for the turgidity and consequent rigidity of paren- chymatous cells (p. 165); it is of use in the process of the growth of plant cells, which take it up in large quantities, and, through their consequent expansion, enlarge their volume with but little expenditure of organic substance. A further and still more important service which water performs for plants consists in THE CONVEYANCE AND INTRODUCTION INTO THE PLANT BODY OF THE NUTRIENT SUBSTANCES OF THE SOIL. Although a large amount of water is retained in the plant body (up to 96 per cent in succulent tissues) for the maintenance of rigidity and enlarge- ment of the organs, a still larger quantity of the water taken up by the roots passes through the plant merely as a medium for the trans- port of nourishment, and is again discharged through the leaves by 180 , BOTANY PART I evaporation. By this TRANSPIRATION from the aerial part of plants, the water passing into them from the roots escapes, and at the same time, by preventing saturation, which would otherwise be produced, tends to maintain a continuous upward movement of the water. The current of water thus produced is accordingly termed the TRANSPIRA- TION CURRENT. As the result of evaporation only water, in the form of vapour, and gases can escape from the plant. AS THE WATERY FLUID ABSORBED BY THE ROOTS CONTAINS SALTS, OXIDES, AND OTHER NON- VOLATILE SUBSTANCES IN SOLUTION, THESE ON EVAPORATION ARE LEFT IN THE PLANT AND GRADUALLY INCREASE IN QUANTITY. This accumulation of mineral salts is absolutely necessary for the plant, for the nutrient water taken up by the roots is so weak in mineral substances (it contains but little more solid matter than good drinking- water), that the plant would otherwise obtain too little food if it were only able to take up as much water as it could retain and make use of. ALL THOSE CONTRIVANCES IN PLANTS, THEREFORE, WHICH RENDER POSSIBLE OR PROMOTE EVAPORATION, OPERATE CHIEFLY IN THE SERVICE OF NUTRITION. Were transpiration not in the highest degree useful and even necessary for the acquisition of mineral substances, provision would certainly have been made by plants to restrict it within the smallest possible limits. For transpiration increases the amount of water required by plants disproportionally to their powers of absorption, and exposes them, moreover, to the danger of perishing through the insufficiency of their water-supply. Herbaceous land plants evaporate, in a few days, according to the calculations of Sacus, more than their own weight of water. A Tobacco or Sunflower plant will lose by evaporation in one day as much as a litre of water; and it has been estimated that trees lose in the same way 50-100 litres daily. In spite of the increased danger of drying up, as the result of evaporation, special provision is made by plants for facilitating trans- piration (p. 188). To supply the increased demands for water thus produced there is set up a strong current of water containing nutritive salts in solution, which passes through the plants, and after yielding up its solid constituents, escapes in the form of invisible aqueous vapour. Thus plants, in order to obtain their nutrient substances, proceed in the same manner as the smaller animals (Sponges, Ascidians), which draw in and maintain a continual flow of water through their bodies, in order to retain as food the nourishing particles suspended in it. The Absorption of Water.—“ Water,” as here used, it must always be remembered, does not mean chemically pure water, but rather a DILUTE WATERY SOLUTION OF VARIOUS SUBSTANCES IN THE ATMO- SPHERE, FROM THE MINERAL SALTS OF THE EARTH, AND FROM ORGANIC HuMus. In this connection it is also necessary to emphasise the fact that LIVING PLANTS DO NOT ABSORB THIS NUTRIENT WATER IN- SECT. II PHYSIOLOGY 181 ACTIVELY AND INVOLUNTARILY, as a sponge, but through the peculiar selective power of their cells (p. 177) they exercise a choice from among the substances available. The simpler and less highly developed plants, which are but slightly differentiated, are able to absorb water through the surface of their whole body. This is also generally true of all submerged aquatic plants, even of the Phanerogams. Water-plants which obtain their nourish- ment in this way often either possess no roots (Utricularia, Salvinia), or their roots serve merely as mechanical hold-fasts. With plants living on dry land the conditions are quite different; their stems and leaves develop in the air, and they are restricted to the water held by capillarity in the soil. In order to obtain this water in sufficient quantities, special organs are necessary, which may spread themselves out in the soil in their search for water. These organs must absorb the water from the soil, and then force it to the aerial portions of the plant. This office is performed for a land plant by its root system, which, in addition to providing the supply of water, has also the task of mechanically sustaining the plant, and withstanding all influences which could lead to a disturbance of equilibrium by loosening the hold of the plant on the earth. Conversely, loose soil is naturally bound together by the branching roots ; and on this account plants have an economic value in holding together loose earth, particularly on dykes and land subject to inundation. If the development of the root system of a germinating Bean or Oak be observed, it will be found that the growing root of the embryo at once penetrates the soil and pushes straight downwards. Lateral roots are then given off from the main axis, and, growing either horizontally or diagonally downwards, penetrate the earth in the neighbourhood of the primary root. These lateral secondary roots in turn develop other roots, which radiate in all directions from them, and so occupy and utilise the entire soil at their disposal. The branching of the root system can proceed in this manner until, within the whole region occupied by the roots of a large plant, there is not a single cubic centimetre of earth which is not penetrated and exhausted by them. All plants do not form a deep-growing tap-root like that of the Oak, Silver Fir, Beet, Lucerne, etc. ; some confine themselves to utilising the superficial layers of the soil by means of a thickly-branched lateral root system (Pine, Cereals). The agriculturist and forester must, accordingly, take into consideration the mode of branching and growth of the roots of a plant just as much as the habit of growth of its aerial portions. Plants which make use of different layers of soil may be safely cultivated together in the same soil, and succeed one another in the same ground. For similar reasons, in setting out trees along the borders of fields, the deep-rooted Elm should be preferred to the Poplar, whose roots spread out near the surface. Gardeners are in the habit of cutting off the tap-roots for the sake of conveni- 182 BOTANY PART I ence in transplanting or for pot culture, and also to force a more vigorous develop- ment of the lateral roots. Desert or xerophilous plants, according to the observations of VoLKENs, send out deeply penetrating roots, which only branch profusely on reaching depths where they find water. In order to secure a still more intimate contact with the particles of the soil, there are produced from the surface of roots small, exceed- ingly numerous and fine, cylindri- cal bodies, which penetrate the smallest interstices of the soil, and fasten themselves so closely to its Fic. 172.—Tip ofa root-hair with adhering smallest particles as to seem actu- particles of soil. (xcirca 240.) . ally grown to them (Fig. 172). These ultimate branches of the root system, which discover the very smallest quantity of moisture, and seek out the most con- cealed crevices in their search for nourish- ment, are the ROOT-HAIRS (p. 95),—delicate tubular outgrowths of the epidermal cells. Although they have the diameter of only a medium-sized cell, and appear to the naked eye as fine, scarcely visible, glistening lines, they often attain a length of several millimetres and enormously enlarge the ab- sorbing surface of their parent root. Accord- ing to F. Scowarz the epidermal surface of the piliferous zone of the roots of Piswm, which has 230 root-hairs to the square milli- metre, is thus increased twelvefold. The root-hairs do not cover the whole surface of roots, not even in the youngest roots, but only a comparatively small zone, a short distance above the growing root-tip. Soon after they have attained their greatest length, and have come into the closest contact with the earth particles, they die off. New root-hairs are developed to supply their place, so that a zone of root-hairs is thus constantly maintained just above the root-tip; while beyond this advancing zone of hairs the root epidermis becomes again Fis. 173.—Seedling of Carpinus completely divested of root-hairs (Fig. 173). fawus m Zone of root- : : a te , ot-tip ; h, hypo- To be convinced of this fact, it is only necessary —cotyl; iur, main root; sw, to carefully pull up a young plantlet growing in _ lateral roots; 2, 1, leat; ¢, a loose and not too dry soil, as such a condition —“Pi°#15 % cotyledons. is especially favourable for the development of root-hairs. Each root, AA Ah ye SECY. II PHYSIOLOGY 183 just above the tip, will be found clothed for a short distance with earth particles held fast by root-hairs, which thus mark the zone occupied by them. The older parts of roots, even in plants which persist for many years, take no part in the process of absorption. They envelop them- selves with cork, increase their conducting elements by growth in thickness, and function exclusively in the transfer of the water absorbed by the younger portion of the roots. Even in the young roots the absorption seems principally confined to the regions covered with root-hairs, or, in case no root-hairs are developed, to a correspond- ing zone of the root epidermis. Through the intimate union of the youngest roots with the soil, they are able to withdraw the minute quantity of water still adhering to the particles of earth, even after it appears perfectly dry to the sight and touch. There still remains, however, a certain percentage of water, held fast in the soil, which the roots are not able to absorb. Thus, Sacus found that the water left by a Tobacco plant, and which it could not absorb, amounted in cultivated soil to 12 per cent, in loam to 8 per cent, and in sand to 14 per cent. The root-hairs seem to take up chiefly the substances held by the soil by means of its ABSORPTIVE POWER. The absorptive power of soil depends, partly, upon chemical changes taking place within it, but partly also on physical processes (the superficial adhesive force of its particles). The chemical changes are especially concerned with the retention of ammonium and potassium salts, as wellas phosphates ; the former as difficultly soluble silicates or double silicates, while phosphoric acid is held in combination with calcium or iron. Magnesium and calcium salts are, on the contrary, but slightly absorbed. They are, like the chlorides, the nitrates, and, in part, also the sulphates, easily displaced ; in soil treated with a solution of saltpetre, for example, the potassium will remain in combination in the soil, while calcium nitrate passes off in solution. Humus acids contribute, to a certain extent, to the chemical changes occurring in soil, as do also soil bacteria, which possess strongly oxidising and reducing powers. The absorptivity of the soil, which, moreover, is not absolute, and varies with different soils (sandy soil absorbs poorly), operates advantageously for plants by the consequent rapid accumulation of large supplies of food-material for their gradual absorption. The absorptive power of soil for water is due to its capacity to retain water by capillarity, so that it does not run off. Of the soils investigated by Sacus, cultivated soil retained in this way 46 per cent, loam 52 per cent, and sand only 21 per cent of water. The activity of the roots in providing nourishment is not only manifested in overcoming the adhesive and absorptive power of the soil. The young roots, and especially the root-hairs, in addition to the carbonic acid exhaled by them, and which, no doubt, also aids in loosening the soil, excrete a stronger acid, by means of which they dissolve otherwise insoluble substances. Roots growing upon a polished plate of marble will so corrode it that an etched pattern of 184 BOTANY PART I their course and direction is thus obtained. By placing the roots upon litmus paper, it may be demonstrated that the corrosion is due to the action of an acid. The nutrient water with which the cell walls of the epidermal cells and root-hairs first become permeated is taken up by the epidermal cells, and thence passes through the cortical cells and the endodermis (p. 113) to the central cylinder of the root. The Distribution of the Nutrient Water—1. Root-PRESSURE.— The causes which determine the direction and strength of the movement of the water through the living cells of the root-cortex into the vascular bundles are not yet fully understood. The fact that the water does actually pass into them, but at times indeed is forced into them with a considerable pressure, may be easily demonstrated. If the stem of a strongly-growing plant, such as the Sun- flower, Dahlia, or Indian Corn, be cut off close above the ground, and the cut surface dried and then examined with a magnifying-glass, water will, in a short time, be seen to exude from the severed ends of the bundles. By close inspection, it is also possible to determine that the water escapes solely through the vascular or woody portion of the bundles. When the soil is kept warm and moist the out- flow will be greater, and will often con- tinue for several days. During this time, a halflitre or more of water will be dis- charged. This water, as analysis shows, is not pure, but leaves on evaporation a residue of inorganic and organic sub- stances. Again, if a hollow glass tube be placed on the root-stump and tightly fastened by rubber tubing, the exuded fiuid will be forced up the glass tube to a considerable Fic, 174.—Vigorous exudation of water height. How great the force of this as the result of root-pressure from Pressure 18 may be shown by attaching ti Ripe eat ae an § tube to the stump and closing it to the lass tube 9 by means of MER mercury (Fig. 174). The column the rubber tubing c. The water Of mercury will in some cases be forced WV, absorbed by the roots from the to a height of 50 or 60, and under favour- soil, is pumped out of the vessels One 2 3 of the stem with a force suficient @ble conditions to 100 or more centi- to overcome the resistance of the metres, thus indicating a root-pressure column Ob MercULy ws which may sometimes considerably exceed one atmosphere, and is of sufficient power to raise a column of water 6, 8, and 13 metres high. SECT. IL PHYSIOLOGY 185 If, instead of the effects of the pressure, the volume of water exuded each hour be observed, the remarkable fact will be demonstrated that the roots regularly discharge more water at certain hours than at others (PEniopiciry oF Roor- PRESSURE. When it was shown that the roots were capable of exercising so great a pressure, it was at first believed that the ascent of the sap to the tops of the highest trees was due to root-pressure. This, however, would be impossible in view of the following considerations. The volume of water supplied by root- pressure is not sufficient to satisfy the quantity given off by evaporation. On the contrary, by moderately vigorous transpiration, such as takes place on a summer day, the root-pressure is of a negative character. Thus, if an actively evaporating plant be cut off near the root, no outflow of water will take place. On the other hand, the stump will energetically draw in water supplied to it; and not until it has become saturated does the force of the root-pressure make itself apparent. In plants growing under natural conditions, the root-pressure is only effective on damp, cool days, or at nights, when the transpiration is greatly diminished. In spring, when the roots are beginning their activity, the conditions are most favour- able, the wood is full of water, and the transpiring leaves are not yet unfolded. When the wood is injured, ‘‘sap” is exuded in drops from the vessels and tracheids. The so-called BLEEDING from wounds or cut stems is chiefly due to root-pressure, but it is also augmented by the pressure exerted by the living cells of the wood (wood parenchyma, medullary rays). THE OUT-FLOWING SAP OFTEN CONTAINS, IN ADDITION TO NUMEROUS SALTS, CONSIDERABLE QUANTITIES OF ORGANIC SUBSTANCES (dissolved albuminous matter, asparagin, acids, and especially carbohydrates). The amount of saccharine matter in the sap of some plants is so great that sugar may be profitably derived from it. The sap of the North American sugar maple, for example, contains from 2 to 3 per cent of sugar, and a single tree will yield 2-3 kilos. The sap of certain plants is also fermented and used as an intoxicating drink (palm wine, pulque, a Mexican beverage made from the sap of the Agave, etc.) The bleeding which takes place on warm, sunny winter days from wounds or borings in trees is not due to root-pressure, but to- purely physical causes. It is brought about by the expansion of the air-bubbles in the tracheal elements of the wood, and may be artificially produced at any time in winter by warming a freshly- cut piece of wood; when the wood is allowed to cool, the air contracts and the water in contact with the cut surface will be again absorbed. IL Tue Warer-CONDUCTION IN PLants.—In living plant-tissues the cells of which require more or less water for their growth, for the maintenance or augmentation of their turgidity, and to supply the water lost by transpiration, there is a constant transfer of water from one cell to another. This transfer between the adjacent cells takes place much too slowly to equalise the great amount of water lost by evaporation from the foliage of a tall tree. IN ORDER TO TRANSFER THE WATER, QUICKLY AND IN LARGER QUANTITIES, FROM THE ROOTS TO THE LEAVES, PLANTS MAKE USE, NOT OF THE LIVING PARENCHYMA, BUT OF THE WOODY PORTION OF THE VASCULAR 186 BOTANY PART I BUNDLES. The woody elements which thus conduct the water have no protoplasm ; they are to be regarded as dead cells, in which the last office of the protoplasm was to give the walls their peculiar structure. II]. Tue TRANSPIRATION CuR- RENT.—It has long been known that the ascending transpiration current in woody plants, which is directed to the points of greatest consumption, flows solely through the wood. It had been observed that plants from which portions of the cortex had been removed, either purposely or accidentally, remained nevertheless perfectly fresh. The adjoining figure, taken from one of the first books in which the vital processes in plants were accurately described (ESSAYS ON VEGETABLE STATICS, by STEPHEN HALgs, 1727), shows the method employed in proving this fact experimentally (Fig. 175). At Z in the branch 0 all the Fic, 175.—HA es’ experiment to show the ascent tissues external to the slender of the sap in the wood. Although the cortex wood have been removed. Since has been entirely removed at Z,and the wood the leaves of this branch remain alone left, the leaves of the branch b remain as fresh as those on the uninjured branche; 8 fresh as those of the branch ¢, x, vessel containing water. Facsimile of the it is evident that the transpira- illustration in Hawes’ Vegetable Statics, tion current must pass through a the wood and not through the cortical tissues. On the other hand, when a short length of the wood is removed from a stem, without at the same time unduly destroying the continuity of the bark, the leaves above the point of removal will droop as quickly as on a twig cut off from the stem. It has also been shown by experiment that in herbaceous plants the vascular portions of the bundles provide for the conduction of the ascending currents. As Sacus demonstrated by spectroscopical analysis, a dilute solution of lithium nitrate taken up by an uninjured plant first ascends in the wood before it passes laterally into the other tissues. By means of the same solution, Prirzer and Sacus determined the velocity of the movement of the transpiration current, which naturally varies according to the plant and the effect of external conditions upon transpiration ; under favourable circumstances it attains a rate of 1-2 metres an hour. This method of showing the exclusive share of the wood in the con- SECT. II PHYSIOLOGY 187 duction of the water, and, also, of determining the maximum velocity of the transpiration current, from observations based on the path and rate of movement of-a coloured solution taken up by a plant, is not free from objection; for the colouring matter would not pass through the stem at the same rate as the water in which it is dissolved, but would be drawn out and held back by the cells. The employment of coloured solutions will, however, be found instructive for merely showing the course of the transpiration current. The transparent stems of the Balsam, Impatiens parviflora, and the white floral leaves of Lilies, Camellias, Mock Orange, etc., in which the coloured vascular system will stand out as a fine network, are especially adapted for such an experiment. In water-plants and succulents, in which little or no transpiration takes place, the xylem is correspondingly feebly developed. In land plants, on the other hand, and especially in trees with abundant foliage, the wood attains a much greater development. All the wood, however, of a larger stem does not take part in the task of water- conduction, but only the younger, outer rings. Where there is a distinction between heart- and sap-wood, under no conditions does the heart- wood take part in the conduction of the water, which is transferred exclusively by the younger rings of the sap-wood. The character of the forces which cause the ascent of the trans- piration currents is still unexplained. Transpiration itself only makes a place for the inflowing water ; it does not furnish the force which is necessary to rapidly convey a large volume of water for a considerable distance through the wood. Every operation by which work is accomplished implies a corresponding expenditure of force; and the force which is capable of raising great masses of water to the tops of a tall Poplar or of a Eucalyptus 150 m. high, must be consider- able. But, as yet, all efforts to determine the nature of this force have been fruitless, and all previous suppositions have been shown to be untenable.- ; It has been already explained that the RooT-PprEssune cannot exert such a force during transpiration (p. 184). Osmoric FoRcES act too slowly to be of any value, and, moreover, there is no fixed distribution of osmotic substances that would account for such a current. The transpiration current cannot be due to cAPILLARITY. In the first place, con- tinuous capillaries are entirely wanting in some plants (the Conifers, for example), and in the stems of others they are only present for comparatively short distances. Secondly, the concave menisci in the elements of the wood are not in relation with any level or convex surface of water, in which case alone they could have effect. Thirdly, the height to which liquids can rise by capillary attraction, and it would be less in the vessels and tracheids than in a glass tube, does not approach the height of an ordinary tree; and, finally, the rate of ascent induced by capillarity decreases so greatly with the increasing height of the fluid, that so copious a flow of water as occurs in plants would be impossible. ATMOSPHERIC PRESSURE has, also, been shown not to be the cause of the trans- piration current. It is true that the vessels and tracheids of vigorously transpiring plants contain rarefied air between the short columns of water. This is evident from the way in which stems cut under mercury become penetrated by it. But 188 BOTANY PART I as the water-courses in plants are all completely shut off from the outer atmo- sphere, the external atmospheric pressure could have no effect. The rarefied air within the plants, moreover, shows no such regularity in its distribution that.it could possibly give rise to so continuous a flow of water. Further, as the atmospheric pressure can only sustain the weight of a column of water 10 m. high, while the sap of a Begonia ascends 60-100 m., the inadequacy of the atmospheric pressure to give rise to such a movement must be admitted. The supposition that the water ascends in the form of vapour through the cavities of the wood, and is afterwards condensed in the léaves, is untenable, as is at once obvious from a consideration of the anatomical structure of the wood, the interruption of its cavities by short columns of water, and the temperature of the plants themselves. And, moreover, the special task of the transpiration current, to transfer the nutrient salts, could not be accomplished if such a supposi- tion were true. It has also been suggested that all of these processes might be aided by THE CO-OPERATION OF THE LIVING CELLS which are so abundant throughout the wood, and which have command of active osmotic forces, to the service of which they could unite a regulative irritability. Later investigations, however, have shown that poisonous solutions, which would at once kill all living protoplasm, are regularly transported, in great quantities, to the summits of the loftiest Oaks and Firs. Thus the supposition that the living elements in any way co-operate in the ascent of the transpiration current is absolutely precluded. The view most generally accepted at the present time, that THE TRANSPIRATION CURRENT ASCENDS IN THE CAVITIES OF THE WOOD THROUGH THE VESSELS AND TRACHEIDS, seems to be supported by observation as well as by the structural features of the wood, but leaves the question as to the cause of the movement still unanswered. Sacus, in his THEORY OF IMBIBITION, sought to solve the problem by supposing that THE WATER ASCENDED IN THE SUBSTANCE OF THE LIGNIFTED ’ WALLS, and that the upward movement was due to the force of molecular attrac- tion, and to the disturbance of the equilibrium existing between the water and the substance of the cell walls. In more recent attempts to account for the ascent of the sap, the direct transfer to the root cells of the force of suction arising from the transpiring green leaves, has been regarded as resulting from the internal cohesion of the water itself. On such a supposition, however, no evidence is furnished that the suction would, in itself, be sufficient to induce a movement like that of the transpiration current. ° The Giving-off of Water.—The requisite amount and essential concentration of the nutrient water supplied by the transpiration current are maintained only by the constant discharge of the accumu- lating water. This may occur in two ways, either more profusely by the evaporation of the water through the cell walls in the form of vapour—that is, by transpiration—or less copiously and also less fre- quently by the actual exudation of drops of water. I. Transprration.—In their outer covering of cork, cuticle, and wax, plants possess a protection from a too rapid loss of water. A Pumpkin, with its thick cuticle and outer coating of wax, even after it SECT. 11 PHYSIOLOGY 189 has been separated from its parent plant for months, suffers no great loss of water. A potato is similarly protected by a thin layer of cork from loss of water through evaporation. ‘The green organs of plants, on the other hand, as they are active in the processes of nutri- tion, and must be able to get rid of their surplus water in order to secure the proper concentration of their nutrient salts, make little use of such protective coverings. On the contrary, they are provided with special contrivances for promoting evaporation. The cell walls of all living organs are saturated with water, and, when the cuticle of the epidermis is not too strongly developed, water is constantly evapor- ated, even from uninjured cells, in amounts varying with the area of the exposed surfaces. From this point of view, it will be seen that THE FLAT EXPANSION OF FOLIAGE LEAVES RENDERS THEM ADMIRABLY ADAPTED FOR THE WORK OF TRANSPIRATION. Evaporation is also promoted by the numerous STOMATA (AIR-PORES) which penetrate the epidermis, and which give the air, saturated with watery vapour, an opportunity to escape from the intercellular spaces. Although the stomata are so small that neither dust nor water can pass through them into the plant, they are usually present in such enormous numbers (p. 94) that their united action compensates for their minute- ness. When it is taken into consideration that a medium-sized cabbage leaf (Brassica oleracea) is provided with about eleven million, and a Sunflower leaf with about thirteen million air-pores, it is possible to estimate how greatly evaporation must be promoted by these fine sieve-like perforations of the epidermis. The stomata also afford plants a means of REGULATING EVAPORA- TION. The pores, which are the mouths of intercellular spaces, are surrounded by GUARD-CELLS. As the term guard-cell suggests, these cells have the power of closing the pore. THE CLOSING AND OPENING OF THE STOMATA ARE ACCOMPLISHED THROUGH A CHANGE IN THE TURGIDITY IN THE GUARD-CELLS. In consequence of their peculiar wall thicken- ings, elasticity, and lateral attachment, a change of turgidity affects the size and shape of the guard-cells in such a way that, by diminished turgidity, they become flatter and close the air-passage, while an increase of turgidity has the contrary effect and opens them (Figs. 176, 177). In many plants the so-called accessory cells (p. 94) participate in various ways and degrees in these processes, depending upon the special structure of the whole apparatus. The opening and closing of the stomata may be effected by either external or internal stimuli; but such stimuli affect different plants in a different manner. Generally speaking, the stomata begin to close on the diminution of. the water-supply ; they open, on the other hand, when active transpiration is advan- tageous (in light, in moist air, etc.). The quantity and quality of the sub- stances held in solution in the nutrient water react in a remarkable manner upon the stomata. The size of their opening is decreased, and the quantity of water evaporated is therefore lessened when more than the usual amount of 190 BOTANY PART 1 nutrient salts is present in the transpiration current; as in that case if, through continued evaporation, the nutrient water should become too concentrated, it might act disastrously upon the plant. Alkalies usually tend to increase turgidity, while acids diminish it. It has already been pointed out, in describing the morphology of Ht ‘ Fic. 176.—Stoma of Helleborus sp. in transverse section. The darker lines show the shape assumed by the guard-cells when the stoma is open, the lighter lines when the stoma is closed. (After ScHWENDENER.) The cavities of the guard-cells with the stoma closed are shaded, and are distinctly smaller than when the stoma is open. the stomata, that they are chiefly to be found on the surfaces of the leaves. THE LEAVES ARE ACCORDINGLY TO BE CONSIDERED AS SPECIAL ORGANS OF TRANSPIRATION (and assimila- tion, p. 196). This is also evident from the manner in which the vascular bundles branch after entering the leaves. | As a large water-main divides into a network of smaller pipes where the consumption of the water takes place, so a leaf-trace bundle, after its long and uninterrupted course through the stem, suddenly branches as soon as it enters the leaf-blade. The adjoining illustration (Fig. 178), showing the nervature or dis- : tribution of the vascular bundles in a Fic. 177.—Stoma of a perianth- 3 . leaf of Galtonia candicans. s, Crataegus leaf, will convey some idea of the Guard-cell with diminished extensive branching which the bundles of a ote = ae leaf undergo, especially when it is taken straight ; S’, turgescent guard- Into consideration that only the macro- cell with curved lateral wall, scopic and none of the finer microscopic Meee te DEMS © onichinos are represented in the figur (After Lerroes.) g Pp e figure, By means of this conducting system, a copious supply of nutrient water can be delivered directly from the roots to every square millimetre of the leaf. There is, how- ever, a special reason why the leaves are so abundantly supplied. They are the actual laboratories of plants, in which, out of the carbonic acid of the atmosphere and the water, and nutrient salts of SECT. I PHYSIOLOGY 191 the soil, the organic building material of the plant-body is produced. For similar reasons, it is in the leaves that the broad expansions of tissue for the special promotion of transpiration are found. The amount of water actu- ally evaporated from the leaf surfaces in the per- formance of their vital functions is almost in- credible. For instance, a strong Sunflower plant, of about the height of a man, evaporates in a warm day over a litre of water. It has been estimated that an acre of cabbage plants will give off two million litres of water in four months, and an acre of hops three to four millions. The quantity of water daily required to maintain the water-supply of a single large tree, amounts to many litres. The water evaporated in the five months from June to November from an Oak standing perfectly free and \ apart, and having about : 700,000 leaves, has been Fic. 178.—Course of the vascular bundles (venation) in a estimated at 111,22 5 kilo- leaf of Crataegus. (From a photograph ; natural size.) grams. According to Direrricu, for every gramme of dry, solid matter produced, there is, on the average, 250-400 grams of water evaporated. EXPERIMENTAL DEMONSTRATION OF TRANSPIRATION.—The evaporation from plants, although imperceptible to direct observation, may be easily demonstrated, and its amount determined by the help of a few simple appliances. One method of doing this is to weigh a plant before and after a period of vigorous evaporation, and thus determine the amount of water actually lost. Or, if the water evaporated by a plant placed under an air-tight bell-jar be absorbed by calcium chloride or concen- trated sulphuric acid, it will only be necessary to determine the increase in weight of the absorbing substance to estimate the amount of water given off by evapora- tion. The amount of water taken up by a plant may also be shown by so arranging the experiment that the water passes in through a narrow tube, as then even a small consumption of water will be quickly indicated by the rapid lowering of the water-level, which will be the more rapid the smaller the bore of the tube. 192 BOTANY PART I The important part taken by the stomata in the process of transpiration may be easily shown, according to SraHL, by means of the cobalt reaction, or the change in colour of dark-blue dry cobalt chloride to light rose upon absorption of water. In making this experiment a leaf placed between strips of paper which have been previously saturated with this cobalt salt and then thoroughly dried, is laid between glass plates. The paper on the side of the leaf most abundantly supplied with Fic. 179.—Suction of a transpiring shoot. The leafy shoot is fitted so that it is air- tight in a glass tube filled with water and with the lower end immersed in a vessel of mercury. The mercury is drawn up the tube by the suction exerted by the transpiring shoot. (From DrtTMEr’s Physiol. Prakt.) stomata will then first change its colour, and that too the more rapidly the more widely open are the stomata. The cobalt reaction can thus also be utilised to deter- mine any variation in the size of the open- ings of the stomata. It is evident from these and similar ex- periments that more water is evaporated in a given time from some plants than from others. These variations are due to differ- ences in the area of the evaporating surfaces and to structural peculiarities (the number and size of the stomata, presence of a cuticle, cork, or hairy covering, etc.). But even in the same shoot transpiration is not always uniform. This is attributable to the fact that, both from internal and external causes, not only the size of the openings of the stomata varies, but also that transpiration, just as evaporation from a surface of water, is dependent upon external conditions. Heat, as well as the dryness and motion of the air, increases transpiration for purely physical reasons ; while light, for physio- logical reasons, also promotes it. From both physical and physiological causes, transpiration is much more vigorous during the day than night. Plants like Impatiens parviflora, which droop on warm days, become fresh again at the first approach of night. Suction In TRANSPIRING SHoots.—A shoot, the cut end of which is placed in water, shows by remaining fresh that it must be able to draw up water to its ex- treme tips. The force of suction exerted by such a transpiring foliage shoot may be demonstrated, by fitting the cut end in a long glass tube filled with water in such a manner that it shall be air-tight. Thus arranged, the shoot will be able to sustain and raise a column of water 2 metres high. If the lower end of the tube be inserted in mercury, as shown in the adjoining figure (Fig. 179), it will be found that even the heavy mercury will be lifted by the transpiring shoot to « consider- SECT. II PHYSIOLOGY 193 able height. Vigorous coniferous shoots absorb water through the cut end with a force of suction equal to one atmosphere, and are thus able to raise the mercury to a height equal to the barometric pressure (760 mm.). The complete exclusion of the external atmosphere is absolutely requisite for the existence of such « suction-force, a condition actually fulfilled in the water-courses of plants. I. Exupation or Warer.—The discharge of water in a liquid state by direct exudation is not of so frequent occurrence as its loss by evaporation in the form of vapour. Early in the morning, after a damp night, drops of water may often be found on the young leaves of Indian Corn, and also on the leaves of Alchemilla and the Garden Nasturtium. These drops gradu- ally increase in size until they finally fall off and are again replaced by smaller drops. These are not dew-drops, although they are often mistaken for them; on the contrary, these drops of water exude from the leaves themselves. They are discharged near the apex of the leaves of the Indian Corn, but in the case of Alchemilla from every leaf-tooth, and of the Nasturtium from the ends of the seven main nerves (Fig. 180). The Fic. 180.—Exudation of drops of water drops disappear ase thee Sin becomes from a leaf of Tropacolum majus. higher and the air warmer and relatively drier, but can be produced artificially if a glass bell-jar be placed over the plant, or the evaporation in any way diminished. Whenever plants become overcharged with water through the activity of the roots, it is discharged in drops. These are pressed out of special water-pores (p. 95), and sometimes even from the stomata and clefts in the epidermis ; while in Datura they have even been observed to exude directly through the walls of the epidermis. It is possible to cause similar exudations of water by forcibly injecting water into a cut shoot. Such exudations of water are particularly apparent on many Aroids, and drops of water may often be seen to fall within short intervals, sometimes every second, from the tips of the large leaves. From the leaves of a species of Colocasia the exuded drops of water are even discharged a short distance. In Spathodea, a tropical member of the Bignoniaccae, the space enclosed by the calyx, in which the young floral organs are developed, is filled with water. Again, in unicellular plants, especially some of the Fungi (Mucor, Pilobolus, Phycomyces), the copious exudation of water is very evident. The water in this case is pressed directly through the cell walls. The organs for the discharge of water, which HABERLANDT has collec- tively termed hydathodes (pp. 91, 99, 114), in some instances, like Oo 194 BOTANY PART I animal sweat-glands, actively press out the water; or, on the other hand, they may simply allow it to filter through them when the internal pressure has attained a certain strength. It would almost seem that, in case of inactive transpiration, such exudations of liquid water supplied the place of evaporation, were it not that the out-pressed liquid is not pure water, as in transpiration, but always contains salts and, sometimes, also organic substances in solution. In fact, the quantity of salts in water thus exuded is often so abundant that after evaporation a slight incrustation is formed on the leaves (the LIME-SCALES on the leaves of the Saxifrages). In some instances, also, the substances in solution in the water seem to be exuded with a purpose, as in the case of the SECRETIONS OF THE NECTARIES and of the DIGESTIVE GLANDS OF INSECTIVOROUS PLANTS (p. 215), and of the discharges of the viscid STIGMATIC FLUID. The superfluous water is discharged by a few plants, the Pumpkin, for example, into the cavities of their stems and leaf-stalks, and is again absorbed from these reservoirs when needed. Special Contrivances for regulating the Water-supply.— Almost all the higher plants possess in the power to close their stomata a special means of checking transpiration during a temporary insufficiency of the water-supply. In districts subject to droughts of weeks or months’ duration, only such plants can flourish as are able either to withstand a complete drying up without injury (p. 179), or to exist for a long time on a scanty supply of water. This last case is only rendered possible by the extreme reduction of transpiration, or by the formation of organs in which, in times of a superfluity of water, it may be retained for later use. Such protection against excessive transpiration is afforded by the formation of cork or cuticular coverings, by the reduction in the number and size of the stomata, and also by their occurrence in cavities or depressions. The rolling up of the leaves, as well as the development of thick growths of hair and the assumption of a vertical position to avoid the full rays of the sun, are also measures frequently adopted to lessen transpiration. The most efficient protection, however, from too great a loss of water by transpiration is undoubtedly obtained by the reduction of the transpiring surfaces, either through a diminution in the size of the leaves or through their complete disappearance. The upright position of the leaves, or the substitution of expanded, perpendicularly directed leaf-stalks for the leaves (PHYLLODIA), particularly characterises the flora of Australia. A clothing of hair, on the other hand, protects the leaves of many South African Proteaceae (e.g. Leucadendron argentewm). Some of the Gramincae (Stipa capillata, Festuca, alpestris, Sesleria tenuifolia, S. punctoria, etc.) roll or fold their leaf-blades, in times of drought, by means of special hinge-like devices, into narrow tubes, and so maintain a sufficient supply of water by diminishing the transpiration from their stomata. Reduction of the leaves is illustrated by the desert forms of Genista and Sarothamnus and by the Cypress-like Conifers. A complete disappear- ance of the whole leaf surface takes place in most Cact?, in which also the stems become swollen and converted into water-reservoirs (Fig. 25). A similar develop- ment of succulent swollen stems frequently occurs in the Huphorbiaceae (Fig. 181), in the Compositac (Kleinia articulata), Stapeleae, and many other plant families found in arid regions. It has been estimated that the amount of water evaporated by a Melon-Cactus is reduced by its succulent development to 755 of that given SECT, II PHYSIOLOGY 195 off by an equally heavy climbing plant (Aristolochia). Instead of the stem the leaves themselves may become succulent, as in the House-leek and other species of Sempervirum, also in many species of Sedum, Aloe, and Agave. Both stem and leaves are equally succulent in many species of Mesem- bryanthemum. In other plants, the paren- chyma of their stem tubers (epiphytic Orchids) or of their thickened roots (Owalideae) serve as water-reservoirs. Epiphytic Bromeliaceac catch the rain-water in reservoirs formed by their closely-joined leaves, and then eagerly take it up through the scaly hairs which cover the leaf surfaces, as in species of Tillandsia. Again, many epiphytic Orchids and Aroids collect the rain-water in a swollen sheath de- veloped from the epidermis of the aerial root (velainen radicwm, p. 100). In the case of other epiphytic Orchids, Aroids, and Ferns (Aspleniwm Nidus, for instance), the humus ——— : and other material caught in receptacles formed Fs. 181.—Zuphorbia globosa. The re- by the leaves or aerial roots act like a sponge duced eaves May Delseewom shy Up ier i : eo globose shoots. in taking up and retaining water, while the absorptive roots penetrate into these moist, compost-like masses and absorb both water and nutrient substances. Many species of Frudlania (a Liverwort common on Beech trees) possess, on the other hand, special water-sacs on the under side of their thallus (Fig. 319). A particularly remarkable contrivance for maintaining a constant supply of water is exhibited by the epiphytic Dischidia Rafflesiana, a number of whose leaves form a deep but small-mouthed urn, into which the roots grow. It would seem at first sight unnecessary that plants like the Mangrove tree, which stand with their roots entirely in water, should require protection against too rapid transpiration; but, as this tree grows in salt or brackish water, it is necessary to reduce the amount of water absorbed, in order to prevent a too great accumulation of salt in the tissues. The Absorption of Carbon (Assimilation) In any attempt to distinguish the relative importance of substances utilised in plant nutrition, carbon undoubtedly ranks first. Every organic substance contains carbon, and there is no other element which could supply or take part in the formation of so many or such a variety of substances, both in living organisms and in the chemical laboratory. Organic chemistry, in short, is merely the chemistry of carbon compounds. It requires no chemical analysis to realise that plants actually contain carbon, although in an imperceptible form. Every burning splinter of a match shows, by its carbonisation, the presence of this element. An examination of a piece of charcoal in which the finest structure of the wood is still distinguishable, shows how abundant is the carbon and how uniformly distributed. Estimated by weight, the 196 BOTANY PART I carbon will be found to make up about half the dry weight (when freed from water) of the plant. Whence do plants derive this carbon? The “humus” theory, accepted for a long time, assumed that the humus of the soil was the source of all the supply ; and that carbon, like all the other nutrient substances, was taken up by the roots. That plants grown in pure sand free from humus, or in a water-culture, increase in dry substance, and consequently in carbon, clearly demonstrates the falsity of this theory. The carbon of plants must therefore be derived from other sources ; and, in fact, the carbon in humus is, on the contrary, due to previous vegetable decomposition. The discovery made at the end of last and the beginning of the present century, that THE CARBON OF PLANTS IS DERIVED FROM THE CARBONIC ACID OF THE ATMOSPHERE, and is taken up by the action of the green leaves, is associated with the names of the Dutchman INGENHOUSS, and the Geneva Professors SENEBIER and THEO. DE SAUSSURE. This discovery is one of the most important in the progress of the natural sciences. It was by no means easy to prove that the invisible gaseous exchange between a plant and the atmosphere constitutes the chief source of nourish- ment; and it required the courage of a firm conviction to derive the thousands of pounds of carbon accumulated in the trees of a forest, from the small proportion contained in the atmosphere. 10,000 litres of air contain only 4-5 1. of carbonic acid, which weigh 8-10 grams ; #r of this weight is oxygen, however, and only #, carbon. Accordingly, 10,000 litres of air contain only 2 grams of carbon. In order, therefore, for a single tree, having a dry weight of 5000 kilos, to acquire its 2,500,000 grams of carbon, it must deprive 12 million cubic metres of air of their carbonic acid. From the consideration of these figures, it is not strange that the discovery of IncENHOUSS was unwillingly accepted, and afterwards rejected and forgotten. Lrzpic was the first in Germany to again call attention to this discovery, which to-day is accepted without question. The immensity of the numbers just cited are not so appalling when one considers that, in spite of the small percentage of carbonic acid in the atmosphere, the actual supply of this gas is estimated at about 3000 billion kilos, in which are held 800 billion kilos of carbon. This amount would be sufficient for the vegetation of the entire earth for a long time, even if the air were not continually receiving new supplies of carbonic acid through the respiration and decomposition of organisms, through the combustion of wood and coal, and through volcanic activity. An adult will exhale daily about 900 grams CO, (245 grams C). The 1400 million human beings in the world would thus give back to the air 1200 million kilos of CO, (340 million kilos C). The CO, discharged into the air from all the chimneys on the earth is an enormous amount. The Krupp works at Essen, according to Hanssn, send out daily into the atmosphere about 2,400,000 kilos of carbon. The whole carbon supply of the atmosphere is at the disposal of plants, as the CO, becomes uniformly distributed by constant diffusion. Not all plants, nor indeed all parts of a plant, are thus able to. abstract the carbon from the carbonic acid of the air. Only such organs as are coloured green by chlorophyll are capable of exercising SECT. IL PHYSIOLOGY 197 this function, for the chlorophyll bodies themselves are the laboratories in which this chemical process, so important for the whole living world, is carried on. From these laboratories is derived the whole of the carbon which composes the organic substance of all living things, plants as well as animals. Animals are unable to derive this most essential element of their bodies from inorganic sources, They can only take it up in organic substances, which have been previously formed in plants. Such plants, also, as are without chlorophyll, as, for example, the Fungi and some of the higher parasitic plants, are dependent for their nutrition upon organic substances previously formed by the chlorophyll bodies of other plants. Within the past ten years it has, indeed, been repeatedly determined that certain nitrifying bacteria have the power of forming a small amount of organic substances from carbonates, carbonic acid, and ammonia. The process by which the organic carbon compound is derived must, however, be altogether different from that of green plants, as the bacteria contain no chlorophyll, and their nutritive activity is in no way dependent upon the light. Roots and other organs unprovided with chlorophyll, and also the colourless protoplasm in the green cells themselves, are similarly dependent upon the activity of the chloroplasts. In the red-leaved varieties of green plants, such as the Purple Beech and Red Cabbage, the chlorophyll is developed in the same manner as in the green parent species, but it is hidden from view by a red colouring matter in the epidermis: in the case of the brown and red Algae, on the other hand, the chlorophyll pigment is concealed by a colouring matter, which is contained in the chromatophores along with the chlorophyll. The derivation of carbon from carbonic acid and its conversion into organic substances is termed ASSIMILATION. In its broadest sense, and especially in the animal kingdom, the word assimilation is used for all nutritive processes by which the nourishment is built up into the substance of an organism. But in Botany the meaning of the term has gradually been restricted, and now by assimilation the carbon assimilation of the chlorophyll granules alone is understood. Moreover, all the other so-called processes of assimilation are dependent upon carbon assimilation. The chlorophyll bodies, however, cannot independently produce organic substances from carbonic acid and water, but require the co-operation of light. The chlorophyll apparatus is unable to assimilate without light, although all the other requirements are present for active assimilation. A definite amount of heat is also naturally necessary for chlorophyll activity, just as for any other vital process. The vibrations of the ether perceptible as light, supply the energy for the decomposition of carbonic acid and the production of carbon, just as other vibrations of ether, in the form of heat, supply the energy requisite for the working of a steam-engine. Not all light vibrations 198 BOTANY PART I are equally capable of arousing the assimilatory activity. Just as the rays of different refrangibility differ in their action, both upon the eye and the photographic plate, so they have a different effect upon assimilation. It would be natural to suppose that the chemically active rays, the blue and violet, which decompose silver salts and other chemical compounds, would also be the most effective in promoting the assimilatory activity of the chlorophyll bodies. Exactly the contrary, however, has been shown to be the case. The highly refractive chemical rays have little or no effect on assimilation ; the red, orange, and yellow rays, that is, the so-called illuminating rays of the spectrum, are on the contrary the most active. In the blue-green fresh-water Algae, and also in the brown and red Seaweeds, in which the chromatophores contain true chlorophyll in addition to their peculiar special colouring matter, the maximum assimilation takes place, according to ENGEL- MANN, in another part of the spectrum than it does in the case of green plants. The assimilation in these Algae seems indeed to be carried on in the part of the spectrum, the colour of which is complementary to their own. All the rays of the mixed white light are usually at the disposal of plants growing freely in the open air ; only the Seaweeds found in deep water (at the most but 200 m. deep) grow in a prevailing blue light, while the deeper-lying tissues of land plants live in red light, as this penetrates further into the parenchymatous tissues. In studying the effect of different kinds of light upon assimilation, it is customary either to use the separate colours of the solar spectrum, or to imitate them by means of coloured glass or coloured solutions. For such experiments it will be found convenient to make use of double-walled bell-jars filled with a solution of bichromate of potassium or of ammoniacal copper oxides. Plants grown under jars filled with the first solution, which allows only the red, orange, and yellow rays to pass through, assimilate almost as actively (90 per cent) as in white light (100 per cent). Under the jars containing the second solution, which readily permits the passage of the photo-chemical rays, assimilation is extremely low (5-7 per cent). But little is known with regard to the processes carried on in green cells during assimilation ; and although it is evident that only the green chlorophyll bodies are capable of assimilating, it is still by no means clear what part the green chloro- phyll pigment performs. The pigment which may be extracted from the protoplasm of the chlorophyll bodies makes up only a small part of their substance, and gives no reaction from which its operations may be inferred. The light absorbed by the chlorophyll pigment also stands in no recognisable relation to the requirements of assimilation ; for the blue and violet rays, which are inoperative in assimilation, are most strongly absorbed (see p. 59). It has not as yet been determined what part the mineral constituents of the transpiration current take in the process. On the other hand, the protoplasmic body of the chloroplasts cannot assimilate when the green pigment is not present; that is, when from any cause the corpuscles are prevented from turning green. For, as the existence of the green pigment is dependent upon the presence of iron, upon a proper temperature, and, with few exceptions (Ferns, Conifers), upon the action of light, its formation in the chlorophyll bodies may be prevented by depriving them of the requisites for its development. The chromatophores will then remain yellow (in leaves) or white (in stems), and no longer assimilate. SECT, IL PHYSIOLOGY 199 As a result of the chemical processes involved in the decomposing activity of assimilation, only the special end-product and one by- product are at present known. Sacus discovered that the organic compound, first to be detected as the special ultimate products of assimilation in the higher plants, is a CARBOHYDRATE, which may either remain in solution, or in the form of STARCH GRAINS may become microscopically visible at the points of its formation. In the case of the lower plants, in the Algae, for example, the first visible product is often not starch but a fatty oil. A short time after assimilation begins, in sunshine, sometimes within five minutes, minute starch grains appear either in the centre or on the margins of the chloroplasts. These grains gradually enlarge until, finally, they may greatly exceed the original size of the chloroplasts. Should, however, the assimilation cease, which it regularly does at night, then the starch grains are dissolved and as soluble carbo- hydrates (glucose, etc.) pass out of the cell. In some plants (many Monocotyledons) there is no starch formed in the chloroplasts, but the products of assimilation pass in a dissolved state directly into the cell sap. In exceptional cases, however, starch is also formed where there is a surplus of glucose, sugar, and other substances, as, for example, in the guard-cells of Monocotyledons. This seems then to be a reserve substance rather than a special product of assimilation. In Tropacolwm, for instance, the formation of cane- sugar precedes the production of starch in the chloroplasts. The formation of starch may be shown to be a direct result of assimilation by means of the “jodine reaction,” and without the aid of a microscope. If a leaf cut from a plant pre- viously kept in the dark until the starch already formed in the leaves has become ex- hausted, be treated with a solution of iodine after being first discoloured in hot alcohol, it will in a short time assume a yellowish brown colour, while a leaf vigorously assimilating in the light will, with the same treatment, take Fic. 12—a leaf showing the a blue-black colour. In Fig. 182 the result of the iodine, reaction is shown on a leaf, part of which had been covered with a strip of dark paper or tinfoil. The cells darkened by the overlying paper or foil formed no starch, while those exposed to the light are shown by the iodine reaction to be full of it. A green leaf kept in air devoid of car- iodine reaction. Part of an assimilating leaf was covered witha strip of tinfoil. After- wards, when treated with a solution of iodine, the part of the leaf darkened by the overlying tinfoil, having formed no starch, gave no colour reaction. (3 nat. size.) bonic acid, although fully exposed to the light, will similarly form no starch. 200 BOTANY PART 1 Sensitive leaves, like those of many Leguminosae, often suffer more under such conditions than when the possibility of assimilation is pre- cluded by their being deprived of light. The by-product arising from the assimilatory process is PURE OXYGEN. The volume of oxygen thus set free is equal to the volume of carbonic acid taken in. If plants assimilate in a known quantity of air containing carbonic acid gas, its volume will therefore remain the same. The chemical process of assimilation resulting in the de- composition of the carbonic acid may be thus expressed : 6CO,-+5H,0 =C,H,,0; +60, (Starch) From this chemical equation it is evident that WATER IS REQUISITE 7 YOR THE PROCESS OF ASSIMILA- TION. The actual composition of starch corresponds rather to a multiple of the above symbol, or n(C,H,,0,), so that the whole equation should be multiplied by n. The oxygen given off by green plants, although not perceptible when they are growing in the open air, becomes apparent in the case of water-plants. It was, in- deed, through the evolution of bubbles of oxygen from water- plants that IncENHOUSs first had his attention called to the assimi- latory activity of leaves. To see this process, it is only necessary to place a cut stem of a water- plant in a vessel of water exposed to the sunshine, when a continuous series of small bubbles of gas will at once be seen to escape from the 5 : intercellular passages intersected Fans a. iq) DY the cut. The gas thus evolved OC ap Leveuliaetad ae eile thowils Fic, 183.—Evolution of oxygen froin assimilating g : plants. In the glass cylinder C, filled with (Fig. 183), and will be found to water, are placed stems of Elodea canadensis ; be chiefly oxygen, but containing the freshly-cut ends of the stems are intro- also traces of nitrogen and car- duced into the test-tube R, which is also full . . of water. ‘The gas-bubbles B, rising from the bonic acid taken up from the cut surfaces, collect at S. H, stand to sup- water. As water absorbs much port the test-tube. less oxygen than carbonic acid (at a temperature of 14° C. 100 vols. of water will dissolve only 3 SECT, II PHYSIOLOGY 201 vols. of oxygen, but 100 vols. of carbonic acid), the escaping bubbles of oxygen become visible; whereas the flow of the carbonic acid dissolved in the water to the assimilating plant is imperceptible. Artificially conducting carbonic acid through the water increases, to a certain degree, the evolution of oxygen, and thus the assimilatory activity. Similarly an artificial increase of carbonic acid in the air is followed by increased assimilation. In sunshine assimilation attains its maximum in air containing about 8 per cent of carbonic acid ; with a higher percentage it begins to decrease. If the amount of carbonic acid gas be increased two hundred times (from 0°04 per cent to 8 per cent in the atmosphere), the formation of starch is only increased 4-5 times. Carbon monoxide (CO) cannot be utilised by green plants ; it cannot take the place of the carbon dioxide, and is poisonous to plants. Under the same external conditions, the assimilatory activity of different plants may vary from internal causes. In the same time and with an equal leat’ surface, one plant will form more, and another less carbohydrates. In this sense, it is customary to speak of a ‘‘ specific energy of assimilation,’ which is partly due to the different number and size of the chloroplasts, as well as to a difference in the air-spaces and consequent aeration of the leaves, but, without doubt, has also its cause in their greater or less energy. As examples of medium assimilatory activity, the leaves of the Sunflower and Pumpkin may be cited. | Under conditions favourable for assimilation, the leaves of these plants form in a summer day of fifteen hours about 25 grams starch per square metre. The carbon for the formation of the starch was supplied in this case from 50 cubic metres of air. A room of 120 cubic metres would accordingly contain enough carbonic acid for 60 grams of starch. From these figures a faint conception may be gained of the enormous activity of the assimilatory processes, which are necessary to furnish the yearly grain supply of a large country. The Utilisation of the Products of Assimilation The Formation of Albuminous Substances. —The chlorophyll bodies supply plants with organic nourishment in the form of a carbohydrate. Although the greater part of the organic plant sub- stance consists only of carbohydrates, as, for example, the whole framework of cell walls, yet the living, and consequently the most important component of the plant-body, the protoplasm, is composed of albuminous substances. These albuminous substances have a com- position altogether different from that of the carbohydrates. In addition to carbon, oxygen, and hydrogen, they also contain nitrogen, sulphur, and frequently phosphorus, the nitrogen indeed in consider- able proportion (about 15 per cent). THERE TAKES PLACE AGCORD- INGLY WITHIN PLANTS A NEW FORMATION OF ALBUMINOUS SUBSTANCES FROM THE CARBOHYDRATES. ‘There are certain indications that this formation is, in part, accomplished within the green cells of the leaves, 202 BOTANY PART I but it must also be carried on in cells devoid of chlorophyll, as, for instance, in those of the Fungi. As little is known concerning the process of the synthesis of the albuminous substances of plants as concerning the formation of the carbohydrates from the carbonic acid and water. It has generally been supposed that they are formed from the carbohydrates and mineral substances already mentioned, as these are known to be transported to the region where the formation of protoplasm occurs, and are there consumed. The carbohydrates utilised in this process seem to be principally GLucosr (both grape-sugar, dextrose, CgsH,,0,+H.0, and fruit-sugar, levulose, CsHy.0g) and MALTOSE: (CgH220,,+ HO) ; for, whatever may be the form of the original carbohydrate, whether starch, inulin, cane-sugar, reserve-cellulose, or glycogen, glucose or maltose is always the first product formed from it. The mineral nitrates, sulphates, and phosphates take part in the process, chiefly in the form of potassium and magnesium salts. Nitrogen and sulphur are liberated from the nitrates and sulphates, with decomposition of the acid radicals ; while of the phosphates, the acid group is utilised in the formation of nuclein in the cell nucleus. Calcium salts, although they take no direct part in these processes, seem, nevertheless, to be indispensable. Their importance, indeed absolute necessity, for most plants, is due to their functioning as a medium for conveying the mineral acids, and for neutralising, or precipitating, injurious by-products which are produced in the formation of albumen. The.most frequent of these by-products is oxalic acid (CyH.0,), which, either as a free acid or as a soluble potassium salt, acts as a poison upon most plants. The oxalate of potassium, which is first formed from the potassium nitrate, reacts with the calcium salts present, with the formation of calcium oxalate, which is only slightly soluble and, as it accumulates, crystallises out and thus becomes harmless. Wherever the formation of albumen or nuclein takes place, oxalic agid is formed, the calcium salts of which may usually be found in adjacent cells often in enormous quantities, in the form of aggregates of crystals, raphides, or crystal sand. The process of the formation of oxalic acid, or its potassium salt, might be con- ceived of as taking place according to the following theoretical equation : CsHy»O, +; 2NO,K = C,HsN.O; + K,C,0, + H,O + 30 Glucose Potassium nitrate Asparagin Potassium oxalate Water Oxygen Starting with glucose and potassium nitrate, there would be formed in addition to potassium oxalate, water, and oxygen (which for the most part is consumed in the respiration, but also in many cases, as free oxygen, may be detected or estimated), an amido compound, ASPARAGIN, C2H3(NH2) (CONH2) (COOH). Asparagin is a body which, like oxalic acid, is widely distributed throughout the vegetable kingdom. Particularly large accumulations of this substance (first discovered in Asparagus) are found in etiolated seedlings of many Papilionaceae (1 litre of sap from Bean seedlings contains about 12-15 grams), always, however, under circum- stances which suggest the possibility that Asparagin participates in the synthesis of the albuminous substances. In all probability its formation precedes that of the ultimate proteid substances, Asparagin is soluble in water and watery sap, aud so is in a position to permeate the cell wall, which the colloid albuminous substances are not able to do in the same degree. Transfer of the Produets of Assimilation When proteid substances are to be conveyed through the tissues, as, for example, from seeds rich in proteids into the seedlings, they SECT. II PHYSIOLOGY 203 first become dissolved and form soluble amides. They are in this form transferred to places where in combination with carbohydrates and mineral acids they are used anew in the formation of albumen. Besides asparagin, there are still other but less widely distributed amides found in plants, as LEUCIN, TYROSIN (which, like asparagin, will crystallise out on treat- ment with alcohol as glistening spherites), GLUTAMIN (in the Pumpkin), BETAIN (in the Turnip), also ALLANTOIN, ete. In addition to the transfer of nitrogenous constructive material through the parenchymatous tissues, the LONG-DISTANCE TRANSPORT OF THE READY-FORMED ALBUMINOUS SUBSTANCES seems rather to take place through the open sieve-tubes of the bast. It appears to be in the sieve-tubes, which contain, during life, albuminous substances, starch grains, and drops of oil, that the conduction of organic substances is effected from the leaves to the roots. In fact, it was long ago con- cluded that the increased thickening of the cortical layers observed just above wounds made by ringing trees, was due to the interruption and detention of a flow of nourishing sap through the bast towards the roots. The transfer of the carbohydrates through unbroken cell walls to the various points of consumption can only be accomplished when they are in solution. In case they are not already dissolved in the cell sap, in the form of glucose, maltose, sugar, or inulin, they must first be converted into soluble substances. This is of the highest importance for ‘the transfer and utilisation of starch and reserve cellulose. They are converted by the influence of DIASTASE into glucose or maltose. Diastase belongs to those peculiarly acting substances termed UN- ORGANISED FERMENTS OR ENZYMES, which possess the remarkable power of decomposing or transforming certain organic compounds without themselves becoming changed or consumed in the process. By virtue of this property they are enabled to transform unlimited quantities of certain substances. The best known of the unorganised ferments are DIASTASE, which converts starch into MALTOSE, INVERTIN, EMULSIN, MYROSIN, as well as the PEPTONISING FERMENTS in insecti- vorous plants and in the latex of various plants. These ferments are proteinaceous substances, which in many of their chemical reactions resemble living protoplasm, but with which they must not be con- fused. Their power of exciting fermentation is not due to any vital property ; they are simply chemical substances, and like them, when in solution, may be precipitated, etc., without losing any of their active principles. Diastase, for example, may be extracted from germinating barley seeds by water or glycerine. After it has been precipitated by means of alcohol and dried to a powder, it may again be dissolved in water, and will still be in a condition to transform enormous quantities of starch, especially if in the form of paste, into sugar. Other substances similar to diastase, and also capable of dissolving starch, are widely distributed throughout the vegetable kingdom, and are classed together as diastatic ferments. They are especially abundant in starchy germinating seeds, 204 BOTANY PART I as well as in tubers and bulbs, in leaves and young shoots. They have also been found, strange to say, in organs where there was no starch for them to act upon. The diastatic transformation and dissolution of the starch is accomplished in a peculiar manner. ‘The starch grain is not dissolved as a homogeneous crystal, unifornly from the surface inwards, but becomes corroded by narrow canals, until it is finally completely disorganised and falls into small pieces (Fig. 184). The transformation of the starch formed in the chlorophyll corpuscles during the day, takes place, as a rule, at night; for in the daytime the action of the diastatic ferment is weaker, and is also counter- balanced by the formation of new starch. The glucose which is thus produced in the leaves passes out of the mesophyll cells into the elon- gated cells of the vascular bundle-sheaths. The glucose and maltose are transferred in these CONDUCTING SHEATHS through the leaf-stalks into the stem. Thence they are Fic. 1S4.—Different, stages of corrosion shown by the conveyed to the young shoots starch grains of germinating Barley. and buds or carried down to the roots; in short, they are finally transported to places where they are required for the nutrition of the plant. The glucose and maltose often become converted into other carbohydrates during their passage from one organ to another, particularly into starch. Starch thus formed from other carbohydrates, and not directly by assimilation, is often referred to as TRANSITORY STARCH, and is usually distinguishable by the smaller size of the grains. At the points of consumption these carbohydrates are again converted into glucose, in which condition alone they seem adapted for direct nutrition. The Storage of Reserve Material All the products of assimilation are not at once consumed. In spite of this, however, assimilation is continued, and the surplus products beyond the requirements of immediate consumption are accu- mulated as RESERVE MATERIAL for future use. Large amounts of such reserve material are accumulated by the American Agave during many periods of vegetation, to be finally expended in nourishing the immense inflorescence with its hundreds of flowers and fruits. In our herbs, bushes, and trees, as the yearly growth and consequent consumption cease at the end of each vegetative period, and as the assimilating organs have by that time attained their greatest expansion and efficiency, the surplus of reserve material is the greatest at the close of SECT. II PHYSIOLOGY 205 the season, and is stored in special RESERVOIRS OF RESERVE MATERIAL. All growth of the succeeding year, either of the plants themselves or of their embryonic offspring, is dependent upon the existence somewhere of a supply of reserve material, which may be utilised by the plant until the organs of assimilation are developed. Reserve materials will accord- ingly be found stored in different forms in the cells of the embryo, or in the surrounding tissues of the seed, in underground rhizomes, tubers, bulbs, and roots, or in the cortical layers, the medullary rays, the wood parenchyma (especially the fibres), and the medulla of persistent stems. Conveyed to these depositories of reserve material, the glucose and maltose are again converted into other carbohydrates, usually starch, which is formed from them by the activity of the starch-producing leucoplasts. In other cases the reserve carbohydrates take the form of cane-sugar, inulin, or reserve cellulose (eg. vegetable ivory in the fruit of Phytelephas). Still more remarkable is the transformation of carbohydrates into fats and oils, occurring in the ripe and ripening seeds of many plants, in fruits (Olive), and also in strictly vegetative tissues. In winter the starch in the wood of many trees also becomes converted into oil, but in the succeeding spring it is again changed to starch. It is finally, at the opening of the buds, converted into glucose or maltose, and conveyed by the transpiration current to the young shoots. Other receptacles of reserve material contain scarcely any carbo- hydrate, but on the other hand there is much more albuminous matter in the form of thick protoplasm, aleurone grains, protein crystals, and fats (seeds of Ricinus). That in the germination of young plants similar tissues with protoplasm, nucleus, cell wall, etc., are formed from these different materials, seems to indicate that all these constructive materials are of almost equal value to the plants. This is due to the fact that plants can, apparently without difficulty, transform the carbo- hydrates, fats, or albuminous substances one into the other, a result not yet accomplished by chemical processes. Other Products of Metabolism The chemical activity of the vegetable cell is by no means exhausted in the production of the substances mentioned : the increasing number of chemical com- pounds found to be derived from the first product of assimilation is a matter of con- tinual sur[itise. Of most of them neither the manner of their formation nor their full importance in metabolism is understood. The conditions are not even fully known which are necessary for the formation and /unctional activity of the orncanic Actps (malic, tartaric, citric, etc., which may in part be considered as products of imperfect respiration) and tannins, although both are so frequent in plants. The function of the GLUCOSIDEs is also imperfectly understood. These are nitrogenous and non-nitrogenous compounds, and are not widely distributed. They are soluble in water, and by the action of ferments are broken up into glucose and other deri- vative products. In the Amygdalaceac they appear as AMYGDALIN, in the Solan- aeceae as the poisonous sOLANIN, in the Cruciferae (mustard seeds) as MYRONIC ACID, in the bark of the Horse-chestnut as the extremely fluorescent mSCULIN, in species of Digitalis as the poisonous DIGITALIN. CoNIFERIN, which is contained 206 BOTANY PART I in lignified cell walls, and especially in the cambial sap of the Conifers, is also in- cluded in the glucosides. Coniferin has recently acquired an economic value, as from it VANILLIN, the aromatic principle of vanilla, may be artificially produced. In this process the coniferin is decomposed, through the action of a ferment or acid, into glucose and coniferylalcohol, through the oxidation of which its aldehyde, vanillin, is formed. It is as yet unknown what part in the metabolic processes of plants is per- formed by the BITTER PRINCIPLES, such as the LUPULIN of Hops, ALoIN of Aloes, ABSYNTHIN of Wormwood. There is the same uncertainty with regard to the functions of the ALKALOIDS. Since most alkaloids, sTRYCHNIN, BRUCIN, VERATRIN, CONIIN, MUSCARIN, ATROPIN, QUININ, MORPHIN, CODEIN, COFFEIN (thein), THEO- BROMIN, ACONITIN, NICOTIN, PILOCARPIN, COCAINE, etc., are violent poisons, their vegetable bases and repugnant bitter principles furnish a certain protection to plants against destructive animals. This, however, does not preclude the possibility that they, like the poisonous oxalic acids, may at the same time have an important physiological significance. The colouring matters and ethereal oils, although in actual weight present only in small quantities, make themselves particularly noticeable to the senses of sight and smell. They probably represent only by- and end-products of meta- bolism ; and, with the exception of chlorophyll, take no further part in the vital processes of plants, except in so far as they are beneficial to the general well- being by enticing (e.g. flowers, fruits) or repelling animals. Their biological signifi- cance is accordingly much better known than their physiological function. Just as the ethereal oils are frequently found in special excretory receptacles, the resins, gum-resins, and gum-mucilages, which are also excretion products, are usually deposited in canals or glandular cavities (p. 88), and are often mixed with ethereal oils. Whether their formation in the particular instances is necessary for the carrying out of the normal processes of metabolism is altogether uncertain. They are, at any rate, useful to plants when wounded, serving as a protection against evaporation and the attacks of parasites. ’ The significance of the so-called india-rubber (cAourcHoUC) and GUTTA-PERCHA (in the latex, p. 73) in the economy of the plant is still less known. In addition to these substances, there also occur in latex, resins, ethereal oils, alkaloids (in opium), starch grains and other carbohydrates, oil-drops and albuminous substances. The presence of these substances in the latex, valuable as constructive material, and occasionally also of active enzymes (peptonising ferments are found in the milky juice of Ficus Carica and Carica Papaya), gave rise to the suggestion that the latex cells and tubes function in the transport of the nutrient matter. It has, however, been found that, even in starved plants, the latex remains unconsumed ; and the present knowledge of these often caustic and poisonous saps is limited to their external utility in the economy of plant life. By their obnoxious properties they defend plants from the attacks of enemies. Also, in the event of plants being wounded, the latex is pressed out either by the surrounding turgescent tissue or by the tension of the elastic walls of its own cells, and forms, as it quickly coagulates in the air, an efficient covering for the wound. In other plants, especi- ally in trees, wound-gum serves the same purpose (p. 81). Special Processes of Nutrition Parasites, Saprophytes, Symbionts, and Inseetivorous Plants.— The acquisition of organic nutritive substances through the activity of SECT. II PHYSIOLOGY 207 assimilating green cells is the most frequent, and is consequently con- sidered the normal method of plant nutrition. Other modes of nutrition are only possible at the cost of organic substances already produced by the assimilating activity of green cells. The de- pendent relations existing between the colourless and green cells, and between the leaves and roots of all green plants, have already been pointed out. Just as in the case of the cells devoid of chlorophyll, some plants also forgo all attempts to develop an adequate chlorophyll apparatus, and by so doing lose all ability to provide themselves with nourishment from the inorganic matter about them. Great numbers of such colourless plants derive their nourishment from the bodies of dead animals and plants. All organic matter at one time or another falls into the power of such plants as are devoid of chlorophyll; it is chiefly due to their decomposing activity in the performance of the nutritive processes that the whole surface of the earth is not covered with a thick deposit of the animal and plant remains of the past thousands of years. These peculiar plants are not satisfied with the possession of the lifeless matter alone ; they even seize upon living organisms, both animal and vegetable, in their search for food. It is chiefly the vast number of Fission-Fungi (Bacteria) and true Fungi which nourish themselves in this way as PARASITES (upon living organisms) or as SAPROPHYTES (upon decaying remains of animals and plants). But even some species of the most widely separated families of the higher phanerogamic plants have also adopted this method of obtaining food. As a result of this modification of their manner of life, the organi- sation and functions of these higher plants have undergone the most remarkable transformation. From the corresponding changes in their external appearance, it is evident how far-reaching is the influence exercised by the chlorophyll. With the diminution or com- plete disappearance of the chlorophyll, and consequent adoption of a dependent mode of life, the development of large leaf surfaces, so especially fitted for the work of assimilation, is discontinued. The leaves shrink to insignificant scales, for with the loss of their assimi- latory activity the exposure of larger surfaces to the light is no longer essential for nutrition. For the same reason active transpiration becomes unnecessary ; the xylem portion of the vascular bundle re- mains weak, and secondary wood is feebly developed. In contrast to these processes of reduction resulting from a cessation of assimilation, there is the newly-developed power in the case of parasites to penetrate other living organisms and to deprive them of their assimilated products. In saprophytic plants, however, where the question is merely one of absorbing nourishment from organic remains, the external adaptations for taking up nourishment continue more like those for absorbing 208 BOTANY PART 1 the mineral salts from the soil, for it then depends only upon an intimate union with the decaying substances. Cuseuta europaca (Fig. 185) may be cited as an example of a parasitic Phanero- gam, a plant belonging to the family of the Convolvulaceae. Although, through the possession of chlorophyll, it seems to some extent to resemble normally assimi- Fic. 185.—Cuscuta ewropaea. On the right, germinating seedlings. In the middle, a plant of Cuscvte parasitic on a Willow twig ; b, reduced leaves ; Bl, flower-clusters. On the left, cross-section of the host-plant W, showing haustoria H of the parasite Cus, penetrating the cortical parenchyma and in intimate contact with the xylem v and the phloem ¢ of the vascular bundles ; s, ruptured cap of sheathing sclerenchyma. lating plants, in reality the amount of chlorophyll present is small, while the leaves are reduced to mere scales. And as the devices for a parasitic acquisition of nourish- ment are so easily seen, much more so, for instance, than in parasites which attack their host-plant underground, it will be at once evident that Cuscuta (Dodder) affords an example of a wonderfully well-equipped parasite. The embryonic Cuscwta plantlet, coiled up in the seeds, pushes up from the ground in the Spring, but even then it makes no use of its cotyledons as a source SECT. II PHYSIOLOGY 209 of nourishment ; they always remain in an undeveloped condition (Fig. 185, at the right). Nor does any underground root system develop from the young rootlet, which however soon dies off. The seedling becomes at once drawn out into a long thin filament, the free end of which moves in broad circles, and so inevitably discovers any plant, available as a host, that may be growing within its reach. In case its search for a host-plant is unsuccessful, the seedling is still able to creep a short distance further at the expense of the nourishing matter drawn from the other extremity of the filament, which then dies off (¢) as the growing ex- tremity lengthens. If the free end, in the course of its circular movements, comes ultimately into contact with a proper nourishing plant, such as, for example, the stem of a Nettle or a young Willow shoot (Fig. 185, in the centre), it twines closely about it like a climbing plant. Papillose protuberances of the epidermis are developed on that side of the parasitic stem in contact with the host-plant, and pierce the tissue of the host. If the conditions are favourable, these PRE- HAUSTORIA are soon followed by special organs of absorption, the HAusTORIA (#). These are peculiarly developed adventitious roots which arise from the internal tissues of the parasite, and possess, in a marked degree, the capability of penetrat- ing to a considerable depth into the body of the host-plant by means of solvent ferments and the pressure resulting from their own growth. They invade the tissues of the host, apparently without difficulty, and fasten themselves closely upon its vascular bundles, while single hyphal-like filaments produced from the main part of the haustoria penetrate the soft parenchymatous cells and absorb nourishment from them. . A direct connection is formed between the xylem and phloem portions of the bundles of the host-plant and the conducting system of the parasite, for in the thin-walled tissue of the haustoria there now develop both wood and sieve-tube elements, which connect the corresponding elements of the host with those of the parasitic stem (Fig. 185, at the left). Like an actual lateral organ of the host- plant, the parasite draws its transpiration water from the xylem, and its plastic nutrient matter from the phloem of its host. The haustoria of Orobanche (Broom rape), another parasite, penetrate only the roots of the host-plant, and only its light yellow or reddish-brown or amethyst-coloured flower-shoot appears above the surface of the ground. Orobanche, like Cuscuta, also contains a small amount of chlorophyll. Both are dreaded pests ; they inflict serious damage upon cultivated plants, and are difficult to exterminate. Many parasitic plants, especially the Rafflesiaceac, have become so completely transformed by their parasitic mode of life that they develop no apparent vegetative body at all; but grow altogether within their host-plant, whence they send out at intervals their extraordinary flowers. In the case of Pilostyles, a parasite which lives on some Asiatic species of Astragalus, the whole vegetative body is broken up into single cell filaments, which penetrate the host-plant like the mycelium of a fungus. The flowers alone become visible and protrude from the leaf-axils of the host-plant. In addition to these parasites, which have come to be absolutely dependent upon other plants for their nourishment, there are certain parasites which, to judge by external appearances, seem to be quite independent, for they possess large green leaves with which they are able to assimilate vigorously. In spite of this, however, these plants only develop normally, when their root system is in connection with the roots of other plants by means of disc-shaped haustoria. Thestwm, belonging to the Santalaceae, and the following genera of the Lhynan- P 210 BOTANY PART I thaceae, Rhinanthus, Euphrasia, and Pedicularis, may he mentioned as examples of plants showing these peculiar conditions. Another member of the same natural order, Melanypyrwm, has, on the other hand, developed a saprophytic mode of life. The Mistletoe (Viscwm album), although strictly parasitic, possesses, nevertheless, like many of the allied foreign genera of the Loranthaceae, fairly large leaves well supplied with chlorophyll, and fully able to provide all the carbohydrates required. Humus plants, like some of the Orchidaceae (Neottia, Coralliorrhiza, etc.), and the Monotropeac, are restricted to a purely saprophytic mode of nutrition, and to that end utilise the leafmould accumulated under trees. The thick roots or rhizomes of these plants offer so little surface for the absorption of nourishment, that it appears as if the threads of the Fungi, which are always found knotted and coiled together in their outer cells, and the free ends of which spread out in surrounding humus, must in some way co-operate in their nutritive processes. The roots of green plants which live in a soil rich in decaying vegetable matter possess similar fungoid growths which, as in the above-mentioned Orchids, lie partly rolled up in the root-cells, and in part spread out in the humus. Interwoven masses of hyphe sometimes so thickly surround and encircle the young root-tips that a direct absorption by the roots from the soil is rendered impossible. These give rise to a formation known as Mycorruiza. In this manner, according to Frank, the root-tips of the forest-forming Cupuliferae and Contferae, as also of many Hricaceae, are always covered by a fungus sheath. This fungus vegetation appears to be in no way injurious, but, on the contrary, probably of benefit, at least, judging from the results of culture experiments made with these plants without mycorrhiza. As yet, the mutual relations existing between the Fungi and the flowering plants is not fully understood ; possibly their connection may be a sym- biotic one, in which the fungus hyphe perform for the trees the functions of the root-hairs, and, in turn, receive from the tree a part of their nourishment. A marvellous relation between roots and Bacteria exists in the case of the Leguminosae. It has long been known that peculiar outgrowths, the so-called ROOT-TUBERCLES, are found on the roots of many Legu- minosae (Bean, Pea, Lupine, Clover, etc.) (Fig. 186). Within the last few years, the astonishing discovery. has been made that these tubercles are caused by certain Bacteria, chiefly by Rhizobiwm legumi- nosarum (Bacillus radicicola). These Bacteria penetrate through the root-hairs into the cortex of the roots, and there give rise to the tuber- cular growths. These tubercles become filled with a bacterial mass, consisting principally of swollen and abnormally developed (hypertro- phied) BacTERIOIDS, but in part also of Bacteria which have remained in their normal condition. The former seem to be eventually consumed by the host-plant, while the latter remain with the dead roots in the soil, to provide for future reproduction. As the experiments of HELLRIEGEL and the investigations of NoBBE, BEYERINCK, and FRANK prove, SECT, IL PHYSIOLOGY 211 we have here another example of symbiosis, in which the Leguminosae furnish carbohydrates to the Bacteria, which, in turn, possess the power of taking up free nitrogen, and passing it on to the host in an available form (p. 173). This is at least certain; the Leguminosae with such tubercles contain at maturity more nitrogen than could have been pro- cured from the nitrates and other substances in the soil in which they grow. In addition to increasing the supply of nitrogen, the presence of Lhizobium seems to exert a favourable influence on the growth of its host-plant. Peas and Lupines do not thrive well in even the richest soil, if it has been sterilised, and the formation of the tubercles prevented. On the addition of other unsterilised soil in which the Bac- teria are known to exist, the tubercles will then appear on the roots, the plants become at once stronger, and show by their in- creased growth a greater activity of their metabolic processes. While parasitism or saprophytism is of rare occurrence among the higher plants, and confined to single species, in which it often occurs only irregularly and is depen- dent upon the environment, among the lower plants it is more general; large families with innumerable genera and species are found completely devoid of chlorophyll 16. 18—A root of Vicia Haba, fe . with numerous root-tubercles. (Fungi and Bacteria), and altogether para- —(Reancea.) sitic or saprophytic in their mode of life. Of the Fungi and Bacteria some are true parasites, and are often restricted to certain special plants or animals, or even to distinct organs; others, again, are strictly saprophytic in their habit, while others may be either parasitic or saprophytic, according to circum- stances. What renders the conduct of these lower organisms parti- cularly striking, is the peculiarity possessed by many of them of not fully utilising all of the organic matter at their disposal; but, on the contrary, so decomposing and disorganising the greater part of it by their fermentative activity that their own development soon becomes restricted. When Yeast-fungi develop in a litre of grape-juice they use very little of it for their own nourishment, but by far the greater part of it becomes decomposed by the fermentation they induce. As a result of this fermentation, together with the pro- duction of carbonic acid, the grape-sugar solution becomes converted 212 BOTANY PART I into an alcoholic liquid, containing small amounts of glycerine, succinic acid, and ester-like compounds in which the yeast itself can no longer thrive. The nourishing material of the litre of grape-juice could have supported a vastly larger quantity of yeast had the fermentation not set in. In the same manner, when Mucor-fungi attack an apple, they not only take the small amount of organic matter necessary for their sustenance, but at the same time convert the whole apple into a soft decaying mass. In addition to this peculiar nutritive activity, intramolecular respiration (p. 219) is also active in the promotion of fermentation and putrefaction. A considerable degree of heat is also evolved in the course of these processes. The utilisation of this heat in making hot-beds is a familiar practice. The heat produced by damp fermenting hay or raw cotton may often become so great that spontaneous combustion ensues. In germinating Barley an increase in temperature of from 40 to 70 or more degrees has been observed. The development of so much heat in this case is not due solely to the respiration of the barley seeds, but, according to CoHN, to the decom- posing activity of a fungus (Aspergillus fumigatus). The spontaneous combustion of raw cotton is, on the other hand, caused by a Micro- coccus, Coagulated albumen and thick gelatine are rendered fluid by many Fungi and Bacteria, while the escaping gases (carbonic acid, sulphuretted hydrogen, ammonium sulphide, ammonia, etc.) show how deep-seated is the decomposition. It is by similar processes of decomposition that dead organic matter becomes thoroughly dis- organised and rendered harmless. To the decomposing action of Fungi and Bacteria is due the severity of many diseases which they produce in living organisms (potato disease, wheat smut, cholera, typhus, diphtheria, anthrax, etc.). By the possession or formation of substances (alexine, antitoxine), which react as specific poisons upon the infecting Bacteria, plants, and particularly animals, in turn protect themselves against the attacks of such micro-organisms. It is due to a knowledge of this fact that the science of Therapeutics has been enabled to cope more and more successfully with infectious diseases. Fungi and Bacteria, in addition to the power, dangerous to them- selves, of disorganising their own nutrient substratum by fermentation and putrefaction, also possess the capability of making an unsuitable substratum suitable for their sustenance. By means of inverting ferments they can convert an unsuitable cane-sugar into an available grape-sugar, and by their diastatic ferments they are able to form starch from glucose and maltose. As is evident from their thriving upon such various substrata, Fungi have the power of producing from the most different carbon compounds (and also from nitrogenous mineral compounds such as ammonium tartrate, or even ammonium carbonate) protoplasm, cell wall, nuclein, fat, glycogen, etc. It is also an astonishing fact that, SECT. IT PHYSIOLOGY 213 while certain Fungi and Bacteria do not require free oxygen for their development (Anaerobionts), others (the so-called aerobiotic forms) are unable to develop or indeed to exist without oxygen. While many Fungi inflict far greater injury upon their host-plants by the decomposition they induce, than by the withdrawal of the nutritive substances, others produce a different effect. The Rust-fungi, for instance, do comparatively little injury to their host; while the relation between host and Fungus in the case of the Lichens has been shown to be absolutely beneficial. The Lichens were formerly considered to be a third group of the lower Cryptogams and of equal value with the Algae and Fungi. It is only in recent years that the astounding discovery was made by DE Bary and verified by the investigations particularly of SCHWENDENER, ReEss, and STautL, that the body of the Lichens is not a single organism, but in reality consists of Algae (e.g. fission- Algae), which also exist in a free state, and of Fungi, which for the most part belong to the Ascomycetes. The Fungus hyphae within the Lichen weave themselves around the Algae; and while the latter occupy the upper or outer side of the leaf-like or cylindrical thallus as the more favourable position for assimilation, the hyphae come into the closest contact with them and absorb from them part of their assimilated products. The Fungi in return provide the Algae with nutrient water, and enable them to live in situations in which they could not otherwise exist. As a result of this close union with the Fungi, the Algae are in no way exhausted, but become more vigorous than in their free condition, and reproduce themselves by cell division. As both symbionts, the Algae as well as the Fungi, thus derive mutual advantage from their consortism, Lichens form one of the most typical examples of vegetable symbiosis. The cause of the regular appearance of the fission-Algae Nostoc and Anabaena in the roots of the Cycudeae and in the leaves of Azolla and other water-plants is much less easy to explain. In connection with these cases of symbiosis between plants, mention may here be made of the similar symbiotic relation existing between animals and plants. Like the Lichen-fungi, the lower animals, according to Branpr, profit by an association with unicellular Alyce by appropriating their assimilated products with- out at the same time disturbing the performance of their functions. Fresh-water Polyps (Hydra), Sponges (Spongitla), Ciliata (Stentor, Parameciwm), also Helio- zoas, Vermes (Planaria), and Amoebae (A. proteus) are often characterised by a deep green colour, due to numerous Algae which they harbour within their bodies, and from the products of whose assimilation they also derive nourishment. In the case of the Radiolarias, the so-called ‘‘ yellow cells,” which have been distinguished as yellow unicellular Seaweeds, function in the same way as the green Algae in the other instances. Another remarkable example of symbiosis in which the relation- ship is not one merely of simple nutrition, has been developed between certain plants and ants. The so-called ANT-PLANTS (Afyrmecophytae) offer to certain small extremely warlike ants a dwelling in convenient cavities of the stems (Cecropia), in hollow thorns (Acacia spadicigera and sphaerocephala, Fig. 187, WV), in swollen 214 BOTANY PART I and inflated internodes (Cordia nodosa), or in the labyrinthine passages of their large stem-tubers (Myrmecodia). At the same time the ants are provided with tood in the case of the Cecropias and Acacias in the form of albuminous fatty bodies (‘‘ food bodies,” Fig. 187, #'), and by the Acacias also with nectar. The ants in exchange guard the plants most effectively against the inroads of animal foes as well as against other leaf-cutting species of ants, which, in the American tropics, kill trees by completely and rapidly divesting them of their entire foliage. These same leaf-cutting ants live in symbiosis with a Fungus (Rozites gongylophora). Upon the accumulated leaves (‘‘ Fungus-gardens’’), according to MOLLER, the ants make pure cultures of the fungus mycelium, whose peculiar nutritive outgrowths serve them exclusively for nourishment. Other familiar examples of symbiosis are those existing between flowers and birds or insects. The flowers in these instances provide the nourishment, usually nectar or pollen, but sometimes also the ovules (Yucca-moth and the gall-wasp of the figure), while the animals are ywic. 187.—Acacia sphaerocephala, I, Leaf and part of stem; S, hollow thorns in which the ant live ; F, food-bodies at the apices of the lower pinnules; N, nectary on the petiole. (Reduced.) IT, Single pinnule with food-body, F. (Somewhat enlarged.) instrumental in the pollination. Here also each symbiont is dependent upon the other. In the case of the unintentional dissemination of fruits and seeds by the agency of animals, the symbiotic relations are less close. Of all the different processes of supplementary nutrition employed by plants, those exhibited by Insectivorous Plants in the capture and digestion of animals is unquestionably the most curious. Although they are green plants and in positions to provide their own organic nourishment, they have, in addition, secured for themselves, by peculiar contrivances, an extraordinary source of nitrogenous organic matter, by means of which they are enabled to sustain a more vigorous growth, and especially to support a greater reproductive activity, than would otherwise be possible without animal nourishment. It is not accidental that the plants which have become carnivorous are, for the most part, either inhabitants of damp places, of water swamps, and moist tropical woods, or that they are epiphytes. The nitrogenous SECT. II PHYSIOLOGY 215 and phosphoric salts of the soil are not obtained by them in the same quantities as in the case of the more vigorously transpiring land-plants. This is very evidently the case in the Sundew (Drosera), which is loosely attached by a few roots upon a thick spongy carpet of Bog-moss, and must find in the animal food a valuable addition to its nitrogenous nourishment. A great variety of contrivances for the capture of insects are made use of by carnivorous plants. The leaves of Drosera are covered with stalk-like out- growths (‘‘tentacles”’), the glandular extremities of which discharge a viscid acid secretion (Figs. 188 and 115). Any small insect, or even larger fly or moth, which comes in contact with any of the tentacles is caught in the sticky secre- tion, and in its ineffectual struggle to free itself it only comes in contact with other glands and is even more securely held. Excited bythe contact stimulus, all the other tentacles curve over and close upon the captured insect, while the leaf-lamina itself becomes concave Fic. 188.—A leaf of Drosera rotundifolia, The stalked glands and their secretions serve for the capture and digestion of insects. (After Darwin, enlarged.) and surrounds the small prisoner more closely. The secretion is then discharged more abundantly, and contains, in addition to an increased quantity of formic acid, a peptonising ferment. The im- prisoned insect, becoming thus completely covered with the secre- tion, perishes. It is then slowly digested, and, together with the secretion itself, is absorbed by the cells of the leaf. . In Pinguicula it is the leaf- margins which fold over any small insects that may be held by the minute epidermal glands. In species of Utricularia (Fig. 34), growing frequently in stagnant Fic. 189.—A leaf of Dionaea muscipula, showing the water, small green bladders (meta- sensitive bristles on its upper surface, which, in the morphosed leaf-tips) are found on parts shaded, is also thickly beset with digestive the tips of the dissected leaves. glands. (After Darwin, enlarged.) In each bladder there is a small opening closed by an elastic valve which only opens inwards. Small snails and crustaceans can readily pass through this opening, guided to it by special out- growths ; but their egress is prevented by the trap-like action of the valve, so that in one bladder as many as ten or twelve crustaceans will often be found imprisoned 216 BOTANY PART I at the same time. The absorption of the disorganised animal remains seems to be performed by forked hairs which spring from the walls of the bladder. More remarkable still, and even better adapted for its purpose, is the mechanism exhibited by other and now well-known insectivorous plants. In the case of Venus Fly-trap (Dionaea), growing in the peat bogs of North Carolina, the capture of insects is effected by the sudden closing together of the two halves of the leaves (Fig. 189). This action is especially due to the irritability of three bristles on the upper side of each half-leaf (the leaf surfaces themselves are much less sensitive). Upon the death of the insect caught by the leaf, a copious excretion of digestive sap takes place from glandular hairs on the leaf surlace, followed by the absorption of the products of the digestive solution. In the case of other well-known insectivorous plants (Wepenthes, Cephalotus, Sarra- cenia, Darlingtonia), the traps for the capture of animal food are formed by the leaves which grow in the shape of pitchers (Figs. 38, 190). These trap- like receptacles are partially filled with a watery fluid excreted from glands on their inner surfaces. En- ticed by secretions of honey to the rim of the pitcher (in the case of Nepenthes), and then slipping on the extraordinarily smooth surface below the margin, or guided by the downward-directed hairs, insects and other small animals finally fall into the fluid and are there digested by the action of ferments and acids. In Sarracenia and Cephalotus, GOEBEL was unable to discover any digestive ferments; but in Cephalotus, however, it was possible to determine Fic. 190.—Pitchered leaf of a that the secretions have antiseptic properties. The Nepenthes. A portion of the jiq-like appendage at the opening of the pitcher of lateral wall of the pitcher has e been removed in order to ~epenthes, Sarracenia, and Cephalotus does not shut ; show the fluid (F), excreted by its function seems to be merely to prevent foreign the leaf-glands. (Reduced.) substances from falling into the pitcher, and _parti- cularly to keep out the rain. The entrance to the tubular leaves of Darlingtonia is under the helmet-like extremity, and therefore a lid is unnecessary. III. Respiration It is a matter of common knowledge that animals are unable to exist without breathing. In the higher animals the process of respira- tion is so evident as not easily to escape notice, but the fact that plants breathe is not at once so apparent. Just as the method of the nutrition of green plants was only discovered by experiment, so it also required carefully conducted experimental investigation to demon- strate that PLANTS ALSO MUST BREATHE IN ORDER TO LIVE; that, like animals, they take up oxygen and give off carbonic acid. Even Liepia in 1840, in his epoch-making work (Die organische Chemie in SECT. 11 PHYSIOLOGY 217 threr Anwendung auf Agricultur und Physiologie), showing the applica- tion of organic chemistry to agriculture and physiology, refused to believe in the respiration of plants. Although the question had already been thoroughly investigated by SAUSSURE in 1822, and by DutTRocuet in 1837, and its essential features correctly interpreted, LIEBIG pronounced the belief in the respiration of plants to be opposed to all facts, on the ground that it was positively proved that plants on the contrary decomposed carbonic acid and gave off the oxygen. He asserted that it was an absurdity to suppose that both processes were carried on at the same time; and yet that is what occurs. ASSIMILATION AND RESPIRATION ARE TWO DISTINCT VITAL PRO- CESSES CARRIED ON INDEPENDENTLY BY PLANTS. WHILE IN THE PROCESS OF ASSIMILATION green PLANTS ALONE, AND ONLY IN THE LIGHT, DECOMPOSE CARBONIC ACID AND GIVE OFF OXYGEN, all PLANT ORGANS WITHOUT EXCEPTION BOTH BY DAY AND BY NIGHT TAKE UP OXYGEN AND GIVE OFF CARBONIC ACID. Organic substance, obtained by assimilation, is in turn lost by respiration. A seedling grown in the dark so that assimilation is impossible, loses by respiration a considerable part of its organic substance, and its dry weight is considerably diminished. It has been found that during the germination of a grain of Indian Corn, a full half of the organic reserve material is consumed in three weeks. That green plants growing in the light accumulate a considerable surplus of organic substance, is due to the fact that the daily production of material by the assimilatory activity of the green portions is greater than the constant loss which is caused by the respiration of all the organs. Thus, according to BOUSSINGAULT’S estimates, in the course of one hour’s assimilation a plant of Sweet Bay will produce material sufficient to cover thirty hours’ respiration. The question may be asked, why then is respiration essential to life? It cannot be that its importance for plants arises from the loss of substance; that would be but a mere waste of material which had been previously elaborated by the plant. A means of judging of the importance of respiration is afforded by the behaviour of the plants themselves when deprived of oxygen. By placing them, for example, under a jar containing either pure nitrogen or hydrogen, or in one from which the air has been exhausted, it will then be found that all vital activity soon comes to a stand-still ; plants, previously growing vigorously, cease their growth ; the streaming motion of the protoplasm in the cells is suspended, as well as all external movement of the organs. Motile organs of plants become stiff and rigid and sink into a death-like condition. If oxygen be admitted, after not too long an interval, the interrupted performance of the vital function is again renewed. A longer detention in an atmosphere devoid of oxygen will, however, irrevocably destroy all traces of vitality; as in every condi- tion of rigor internal chemical changes take place, which, by a pro- longed exclusion of oxygen, lead to the destruction and disorganisation 218 BOTANY PART I of the living substance. THE PRESENCE OF OXYGEN IS NECESSARY TO THE CHEMICAL PROCESSES TAKING PLACE WITHIN THE CELL, IN ORDER TO MAINTAIN THE LIVING SUBSTANCE IN A CONDITION OF NORMAL ACTIVITY. The absorption of oxygen and the evolution of carbonic acid by living plants can be demonstrated both qualitatively and quantitatively by simple experiments. From what has already been said of the contradictory nature of assimilation and respiration, it will be at once apparent that these experiments must be conducted either in the dark or on portions of plants devoid of chlorophyll. Coloured or white flowers, roots, germinating seeds and Fungi furnish suitable objects on which, at any time, the gaseous interchange occurring during respiration may be observed. The more abundant the protoplasm and the more energetic its vital activity, so much the more vigorous is the respiration. The best results are obtained, therefore, from young portions of plants in an active state of growth. It should also be mentioned that from the following experiments only the carbonic acid and not the whole of the products of the respiratory activity are determined. From theoretical considerations, and also from exact chemical analysis, it has been definitely established that, IN ADDITION TO CARBONIC ACID, WATER IS FORMED FROM THE ORGANIC MATTER BY RESPIRATION. The absorption of oxygen and the formation of carbonic acid may be clearly shown by the following experiments (Fig. 191). A flask (B) filled with young mush- rooms or Composite flowers is inverted with its mouth in an open vessel of mercury (Q), and a few centimetres of caustic potash solution introduced within its neck. In the same degree as the carbonic acid produced by respiration is absorbed by the caustic potash, the volume of air in the flask will be reduced and the mercury will rise in the neck. After a time, the ascent of the mercury ceases and it remains stationary. Ifthe quantity of air remaining in the flask be estimated, it will be found that it has lost a fifth of its original volume ; this means, however, that the whole of the oxygen (which makes up one-fifth of the atmospheric air) has been absorbed. If caustic potash is not used in this experiment to absorb the ex- haled carbonic acid, the mercury remains at its natural level, or, in other words, the volume of air in the flask remains unchanged. From this experiment it is apparent that the volume of oxygen absorbed is equal to the volume of carbonic acid evolved, as expressed by the formula aaa, This equivalence of volume 2 between the oxygen absorbed and the carbonic acid exhaled exists only in cases where the oxygen is used exclusively for respiration, and not where it is con- sumed in transforming the contents of the cells, as is observed in the germination of seeds rich in fat, and in the interchange of gases in the case of the succulents. In the germination of seeds rich in fat, the fat is converted into carbohydrates richer in oxygen. The oxygen consumed remains combined in the plant. On the other hand, in the case of the succulents, their peculiar power of effecting oxidation during the night and subsequent deoxidation in the light, modifies the gaseous interchange of respiration. The absorption of oxygen in the respiration of plants can also be shown by the fact that a flame, held in a receptacle in which plants have been kept for a time, is extinguished. If a lighted taper be thrust into a glass cylinder which has been partially filled with flowers or mushrooms, and then tightly covered and allowed to remain for a day, it will be extinguished, as the oxygen of the air in the cylinder will all have been absorbed. The carbonic acid exhaled in respiration can SECT, IT PHYSIOLOGY 219 be quantitatively determined from the increase in the weight of the caustic potash by which it has been absorbed, or from the amount of barium carbonate precipi- tated by conducting the respired carbonic acid through baryta water (to which some BaCl, has been added). In this last experiment it will of course be necessary to free the air from all traces of carbonic acid before it is admitted to the respiring plants. Intramolecular Respiration.—In the middle of the seventies PrLUGER made the surprising discovery that, frogs are not only able to live for some time in an atmosphere devoid of oxygen, but even continue to exhale carbonic acid. From similar investigations it was found that plants also, when deprived of oxygen, do not die at once, but can prolong their life for a time and evolve car- bonic acid. Under these cir- cumstances it is apparent that both elements, the carbon as well as the oxygen, must be derived from the organic sub- stance of the plants themselves: the oxygen can only be ob- tained through some unusual process of decomposition carried on within the plant. This form of respiration has consequently been described as intramole- cular. The amount of carbonic acid produced in a given time by intramolecular respiration is usually less than that given off Fic. 191. 7 Weperiment in respiration. The i santo im the same time during normal MX (2) tly ed with fever which respiration. There are plants, Through the absorption of the carbonic acid ex- however (for instance, Vicia ee peetion by Lun of tortie Faba), whose seedlings, fhe ani Pa? he mercury ises in the neck o: atmosphere of pure hydrogen, will exhale for hours as much carbonic acid as in the ordinary air. During intramolecular respiration all growth ceases and abnormal processes of decomposition take place, whereby alcohol and other products are formed. It had formerly been believed that the inciting cause of respiration was the oxidising activity of the oxygen, which was thought to act upon the living 220 BOTANY PART I substance in the same way as upon an easily oxidised body. But the discovery of intramolecular respiration led to a new conception of the processes of normal respiration. According to it, the protoplasm seems by its vital activity constantly to produce one or more substances which greedily seize upon oxygen. The aflinity of these substances for oxygen is so great that, in case no free oxygen is at their disposal, they decompose and take it from the protoplasmic substance itself (just as chlorine has the power of decomposing other compounds to combine with hydrogen). Plants breathe, accordingly, not as a result of the decomposing oxidation of the oxygen in the air, but they absorb oxygen because respiration is essential to the performance of those metabolic processes on the continuance of which their own vitality depends. RESPIRATION, LIKE NUTRITION AND GROWTH, Is AN EXPRESSION OF A PARTICULAR VITAL ACTIVITY OF THE PROTOPLASM. From this standpoint, it is at once evident that respiration becomes intensified with every increase in the vital activity, and on the other hand, decreases with every diminu- tion of the vital functions. To understand the physiological reason or the existence of such a vital pro- cess as respiration is more difficult. The behaviour of plants in an atmosphere tree from oxygen demonstrates, at all events, that normal respiration is requisite for the vital activity of the protoplasm ; that, through it, in a word, the equilibrium of the living substance is disturbed, and so the stimulus given to further mole- cular movements and renewed vital activity. Through the disturbing activity of respiration, the energy of the protoplasm is continually aroused, and the latent forces, accumulated through the operation of the vital processes, are again set free: it is, in other words, the specific source of all vital energy. In intramolecular respiration, the necessity for oxygen disturbs the equilibrium in an unnatural way, and sets free forces, which lead, not to the continuancé of the vital activity, but to the destruction of the living substance. That specific vital energy can be otherwise derived than through the utilisation of free oxygen is shown in the case of the Anaerobionts (p. 213), which live and multiply without the presence of free oxygen. The formation of ferric hydroxide by the so-called IRON BACTERIA, as well as the production of sulphuric acid by the SULPHUR BACTERIA, is probably the result of an attempt on the part of those micro- organisms to substitute other sources of energy for normal respiration. The energy gained by the absorption of oxygen is accompanied by a loss of combustible organic substances. This loss is first felt by the protoplasmic body itself, but is soon made good again at the expense of the carbohydrates and fats; so that no permanent loss of protoplasmic substance from respiration is perceptible, but A VISIBLE DIMINUTION OF THE CARBOHYDRATES AND FATS CAN BE DETECTED. Heat produced by Respiration.—Respiration is, chemically and physically considered, a process of oxidation or combustion, and, like them, is accompanied by an evolution of heat. That this evolution of heat by plants is not perceptible is due to the fact that considerable quantities of heat are rendered latent by transpiration, so that transpiring plants are usually cooler than their environment ; and also to the fact that plants possess very large radiating surfaces in propor- tion to their mass. The spontaneous evolution of heat is easily shown experimentally, if transpiration and the loss of heat by radiation are prevented and vigorously respiring plants are selected. Germinating SECT. IT PHYSIOLOGY 221 seeds (Peas), if examined in large quantities, show under proper conditions a rise in temperature of 2° C. The greatest spontaneous evolution of heat manifested by plants has been observed in the inflorescence of the raceae, in which the temperature was increased by energetic respiration 10°, 15°, and even 20°C. Also in the large flower of the Victoria regia temperature variations of 15° C. have been shown to be due to respiration. One gramme of the spadix substance of an Araceae exhales, in one hour, up to 30 cubic centimetres CO, ; Fria. 192.—Experiment to show the direct communication of the external atmosphere with the internal tissues of plants. The glass tube R, and the leaf P, are fitted air-tight in the bottle G; upon withdrawal of the air in the bottle by suction on the tube R, the external air penetrates the intercellular spaces of the leaf, through the stomata, and escapes in the form of small air-bubbles from the cut surface of the leaf-petiole. (From Dermer’s Physiol. Pract.) and half of the dry substance (the reserved sugar and starch) may be consumed in a few hours as the result of such vigorous respirations. That other processes, in addition to respiration, co-operate in the production of heat is apparent from the fact that the amount of heat evolved does not vary proportionally to the carbonic acid exhaled. The high temperature (up to 70° C.) observable in germinating Barley does not result from respiration alone, but is due to the decomposing activity of a Fungus. The Movement of Gases within the Plant.—The entrance of 222 : BOTANY PART I oxygen into the plant body is not accompanied by any respiratory movements, as in the case of animals; but takes place solely through diffusion. Those cells which are in direct contact with the air or water can absorb their requisite oxygen directly ; while cells in the midst of tissues are dependent upon; the oxygen which can diffuse through the surrounding cells. Such a diffusion from cell to cell would not, however, be adequate, in the case of the vast cellular bodies of the higher plants, to provide the living cells of the interior with a sufficient supply of oxygen. This is accomplished by means of the air-spaces, which, as INTERCELLULAR PASSAGES, penetrate the tissues in all directions and so bring to the protoplasm of the inner cells the air entering through the STOMATA and LENTICELS (p. 143). That the intercellular spaces were in direct communication with each other and also with the outer atmosphere, was rendered highly probable from anatomical investigation, and has been positively demonstrated by physiological experiment. It is, in fact, possible to show that air forced by moderate pressure into the intercellular passages makes its escape through the stomata and lenticels; and conversely, air which could enter only through the stomata and lenticels can be drawn out of the intercellular passages. The method of conducting this experiment can ‘be seen from the adjoining figure (Fig. 192). Through the cork of the bottle (G), which is partially filled with water, a glass tube (£) and a leaf (P) are inserted ; when the air in the bottle (Q) is drawn out by suction through the glass tube (£), a stream of air- bubbles passes out through the intercellular spaces of the severed leaf- stalk, and is maintained by the air entering through the stomata of the leaf-lamina. By a similar experiment it can also be shown that in corky stems the communication between the intercellular spaces in the medullary rays, cortex, and wood and the external atmosphere is maintained through the lenticels. The movement of the gases within the intercellular spaces is due partly to the diffusion, induced by the constant interchange of gases caused by respiration, assimila- tion, and transpiration, and partly to their own instability, arising chiefly from modifications of the temperature, pressure and moisture of the surrounding atmosphere, but which is also increased by the move- ment of the plants themselves, through the action of the wind. Intercellular air-spaces are extensively developed in water and marsh plants, and occupy the greater part of the plant body. The submerged portions of water- plants unprovided with stomata secure a special INTERNAL ATMOSPHERE of their own, with which their cells maintain an active exchange of gases. This internal atmosphere is in turn replenished by the diffusion taking place with the surround- ing atmosphere. In marsh-plants, which stand partly in the air, the large inter- cellular spaces form connecting canals through which the atmospheric oxygen, without being completely exhausted, can reach the organs growing deep in the swampy soil, surrounded by marsh-gas and otherwise cut off from any communica- tion with the atmosphere. SECT. II PHYSIOLOGY 223 Phosphorescence.—The same conditions which accompany respira- tion also give rise to the production of light or phosphorescence in a limited number of plants, particularly in Fungi and Bacteria. This phosphorescence at once disappears in an atmosphere devoid of oxygen, only to reappear on the admission of free oxygen. All the circum- stances which facilitate respiration intensify phosphorescence; the converse of this is also true. According to the results of investigations concerning the phosphorescence of animals, from which that of plants does not probably differ in principle, the phosphorescence is not directly dependent upon the respiratory processes, but is due to the production by the protoplasm of a special colloid substance in the form of globules or granules, which give out light when undergoing crystallisation. On free exposure to the atmosphere, and under proper conditions of moisture and temperature, this phosphorescent substance, even after its removal from the living organism, is still capable of giving out light for a long time. The best-known phosphorescent plants are certain forms of Bacteria which develop on the surface of fish and meat, and the mycelium, formerly described as “ Rhizomorpha,” of the fungus Agaricus melleus. As further examples of spontaneously luminous Fungi may be cited Agaricus olearius, found growing at the foot of olive trees in South Europe, and other less familiar Agarics (4g. igneus, noctilucens, Gardneri, ete.). The phosphorescence of decaying wood is also, without doubt, due to the growth of Fungi or Bacteria. Of plants taking part in the phosphorescence seen in water, the most important are Pyrocystis noctiluca, an Alga, and the spontaneously luminous Bacteria. The so-called phosphorescence of the Moss, Schistostega, and of some Selaginellas and Ferns, has nothing in common with actual phosphorescence, but is produced solely by the reflection of the daylight from peculiarly formed cells (Fig. 325). The phosphorescence observed in some sea-weeds results, on the other hand, from the fluorescence and opalescence of certain of their albuminous substances, or from the iridescence of their cuticular layers. IV. Growth The size which plants may attain varies enormously. A Mush- room seems immeasurably large in contrast with a Micrococeus, but inexpressibly small if compared with a lofty Californian Sequow. A Bacillus of the size of a Mushroom, or a Mould-Fungus of the height of a Sequoia, are, with their given organisation, physiologically as incon- ceivable as a Mushroom with the minuteness of a Micrococcus. The size of an organism accordingly is an expression of its distinct individu- ality, and stands in the closest relation to structure and conditions of life, and in each individual varies within certain narrow limits. However large a plant may be, and however innumerable the number of its cells, it nevertheless began its existence as a single cell, 224 BOTANY PART I microscopically small and of the simplest structure. To attain its final size and perfect development it must grow, that is, it must enlarge its body and undergo differentiation. Even for the minute unicellular bacteria growth is essential, as they multiply chiefly by cell division. Each daughter cell must grow and attain the dimensions of the parent cell, or in a few years the capacity for existence itself will be lost through their continually decreasing size. It is in fact impossible to conceive of a plant where perfect de- velopment is not the result of growth. If a growing Oak or Cedar be compared with the single spherical egg-cell from which it has arisen, it is at once clear that by the term growth we mean not only an increase in volume, but include also a long series of various developmental stages, and external and internal modi- fications. A mere increase in volume does not necessarily imply growth, for no one would say that a dried and shrivelled turnip grows when it swells in water. On the contrary, active growth may be accompanied by a considerable loss of substance, as is shown by the sprouting of potatoes kept in a dark cellar. Water is lost through transpiration as well as organic substance through respiration, and yet the new shoots show true growth. In the lower organisms growth is exhibited in its most simple form. In an Ameba or a Plasmodium growth is simply an increase in their substance ; in a unicellular Alga or in a Fungus it means, in addition to this, an enlargement of their cell walls. In the higher plants the processes of growth are far more complicated and various, so that, according to Sacus, four chief phases of growth can be distin- guished, which, however, are not sharply separated, but merge imper- ceptibly one into the other. 1. The embryonic phase, or the first origination of new cells or organs, according to their proper position and number. 2. The formative phase, or the continuance of the embryonal development, and the assumption of a definite form. 3. The phase of elongation of the already formed embryonal organs. 4. The phase of internal development and completion of the tissues. The Embryonal Development of the Organs Plants, in contrast to the higher animals, continually develop new organs. These arise either from tissues retained in their embryonic condition, as at the growing point, or they have their origin in regions which have already more or less completely attained their definite form. The leaves and shoots spring directly from the tissues of the growing point; the first lateral roots, however, make their appearance at some distance from the growing point, where a perceptible differen- tiation of the tissues has already taken place. SECT. 11 PHYSIOLOGY 225 Leafy shoots may also take their origin from old and fully- developed tissues, which again assume an embryonic character, accom- panied by an accumulation of protoplasm and renewed activity in cell division. But as this only occurs in exceptional cases, shoots which thus arise out of their regular order are termed ADVENTITIOUS. The manner of the Formation of New Organs at the Growing Point has already been described (p. 149). It is only necessary here to again call attention to the fact that the young organs develop in acropetal succession, so that the youngest is always nearest the apex. This is, in fact, the most natural method in consideration of the apical growth of the axes. In spite of that, however, special cases are known in which the young organs arise at some distance from the growing point, and between older organs (in the inflorescence of Typha). The point from which new organs arise, and the number which develop, are chiefly dependent upon inherited internal disposi- tion. Although external conditions exert in this respect but small influence, it has been recognised that the available space, and the subsequent pressure of the older organs of the vegetative cone, as well as the torsion of the axis, operate in determining the ultimate position of new organs on the parent axis. The influence of other factors, light, gravity, chemical and mechanical stimuli, which at certain times in the later life of the tissues are of extreme importance, have usually but little effect on the embryonal development. Yet, on the other hand, the position of the first division wall of the ger- minating spore of Marsilia is determined by the action of gravity, and the direction of the first wall (as well as of the preceding nuclear division) in the spore of Lquisetum is determined by its relative position to the light. In Adventitious Formations, on the contrary, the influence of external forces is often very evident, as, for example, in the formation of climbing-roots, which in the Ivy and other root-climbers are de- veloped only on the shaded side of the stem. In the Alga Cuwlerpa the new leaf-like organs arise only on the illuminated side of the parent organ. It is, on the other hand, the force of gravity which excites the formation of roots on the under side of underground rhizomes. It is also due to gravity that the growing points of shoots are formed only from the upper side of the tubers of Thiadiantha dubia, or that new twigs develop, for the most part, from the upper side of the obliquely growing branches of trees. Contact stimuli, on the other hand, determine the primary inception and point of development of the haustoria of Cuscuta (p. 208). The sexual organs of Fern prothallia are always developed on the side away from the light; that is, in normal conditions on the under side, but in case of artificial illumina- tion on the upper side. As a result of one-sided illumination and the stimulus of gravity, together with the favouring influence of moisture, the rhizoids spring Q 226 BOTANY PART I only from the under side of the gemme of Marchantia, so that eventually the two originally similar sides assume an altogether different anatomical structure. Many adventitious formations are the result of definite external causes ; as, for example, the galls induced by the stings of insects and the deposits of animal eggs and larve (cf. p. 155). The development of adventitious formations is especially induced by MUTILATION of plants. NEW FORMATIONS are in this manner produced at points from which they would never have arisen on the uninjured plants. In the case of Pelargoniums, Oleanders, Willows, and many other plants, it is possible to induce the formation of roots wherever the shoots are cut. In other plants, however, there seem to be certain preferred places, such as the older nodes, from which, under the same circumstances, roots develop. In like manner new shoots will appear in the place of others that have been removed. IN THE DEVELOPMENT OF NEW FORMATIONS ON A MUTILATED PLANT THOSE VERY ORGANS ARISE, OR PREFERABLY ARISE, OF WHICH THE PLANT HAS BEEN DEPRIVED. Rootless shoots develop first of all new roots. Roots and root-stocks deprived of their shoots form first new shoots. In these processes there is manifested an internal reciprocity in the formative growth of organs, which has been termed the CORRELATION OF GROWTH. Correlation of growth is often, also, very apparent in the normal development of the organs of uninjured plants. It is due to this that scales of buds are developed in their special form rather than as foliage leaves. For, as GoEBEL showed, it is possible by artificial means, as, for example, by the timely removal of the leaves of the parent shoot of Aeszulus, Acer, Syringa, Quercus, or in the case of Prunus Padus, by cutting off the upper extremity of the shoots, to induce the formation of normal foliage leaves in the place of the scales. The vigorous growth which ensues in the fruit and in the fruit-coverings after fertilisation and development of the embryo in the ovule, affords another example of correlation; for, in case no fertilisation of the egg-cell occurs, all those changes which produce a ripe fruit from the flower do not take place ; and, instead, another correlative process is manifested by which the now useless organs are discarded. Certain plants, especially those modified by cultivation, form an exception to this: in many varieties of banana, in the seedless mandarin, and in the variety of raisins known as sultana, etc., although no seeds capable of germination are produced, the formation of a so-called fruit is nevertheless continued. Even in these instances it is essential for the formation of fruit that there shall have occurred a previous pollination of the stigma, or the fertilisation of the ovules, which, howeyer, do not mature. In some few exceptional cases, however, as in the Fig, even this impetus to fruit formation is not necessary. The manner of the formation of conducting tissues in plants, and also their anatomical development, are regulated by correlation. From these few instances it may be seen how the principle of correlation affects the most various of the vital processes, even under normal conditions, and how the har- monious development and function of the single members of the plant body are controlled by it. The polarity manifested by plants should also be considered as a special example SECT. 11 PHYSIOLOGY 227 of the correlation existing between the different parts of the plant body. This polarity is particularly apparent in stems and roots, and finds its expression in the tendency of every small piece of a stem to develop new shoots from that end which was nearer the stem apex, while the roots take their rise from the other end. Pieces of roots in like manner send out roots from the end originally nearer their apex, and shoots from the end towards the stem. In accordance with this principle, detached pieces of stems produce new shoots from their ‘‘shoot-pole,” and injured roots new roots from their ‘‘root-pole.” This polarity, particularly investigated by Vécurine and Sacus, makes itself apparent in even the smallest pieces of stems or roots, and may, in this respect, be compared to the magnetic polarity exhibited by every small piece of a magnet. Unlike poles of a plant may readily be induced to grow together, while like poles may only be brought to do so with difficulty, and then do not develop vigorously. As a result of such experiments, a radial polarity has also been recognised by VécuTinG in stem and root tissue: thus, for instance, pieces of a stem or root, inserted in a lateral incision of a similar organ, become united with it, if they are so placed that the side originally outermost occupies the same relative position in the new organ, but if this position is altered no such union takes place. Leaves take, in respect to polarity, a special position, for they are not organically included within new formations derived from them. Thus, from the basal end of a leaf, an entire plant, with roots, stem, and leaves, may arise, while the regenerative leaf itself gradually dies. It is of especial interest to observe the effect of external influences upon the position of new formations, when they come into opposition to the internal disposition of the plants themselves. In this respect, the behaviour of different species. varies greatly. In one, the internal factors predominate, that is, the new formations appear quite independently of external conditions ; in another, the external influences of the moment prevail ; but the internal disposition of the plant, when thus constrained for the time being, ultimately makes itself apparent and the new formations never develop vigorously. A willow twig, planted in « reversed position, with the shoot-pole in the ground, will produce roots, and from the root-pole may even produce shoots. These, how- ever, usually soon die and their place is supplied by other stronger shoots arising from the shoot axis just above the roots. It is only by the most careful suppression of any such developments that the shoots from the root-poles may be kept alive. In so-called ‘‘creeping” trees, the formation of side branches from the upper side of the hanging branches is favoured by external conditions, but the internal polarity prevents their vigorous development, and those formed soon die. In the cultivation of grapes and fruit-trees this peculiarity is utilised to produce short- lived, fruit-producing shoots by bending over the vines or training the branches of the trees in the cultivation of wall fruits. On the other hand, in some cases the internal polarity is easily overcome by external influences. It is sometimes suffi- cient merely to reverse the erect thallus of Bryopsis, one of the Siphoneae, to con- vert the former apical portion into a root-like tube which penetrates the substratum and fastens itself to the grains of sand. It has also been positively determined, although otherwise such cases are unknown among the higher plants, that the growing points of the roots of Neottia and of certain Ferns (Platyceriwm, Aspleniwm esculentum) may be converted through some inherent tendency into the vegetative cone of a stem. The correlation phenomena manifested in the formation of new organs have the greatest practical importance, for the propagation of plants by cuttings or grafting is based upon them. 228 BOTANY PART I In artificial reproduction detached pieces of plants are made use of for the purpose of producing a fresh complete plant. In many cases this is easily done, but in others it is more difficult, or even impossible. The favourite and easiest method is by means of cuttings, that is, the planting of cut branches in water, sand, or earth, in which they take root (Pelargonia, Tradescantias, Fuchsias, Willows, etc. ). Many plants may be propagated from even a single leaf or portion of a leaf, as, for instance, is usually the case with Begonias. The young plants spring from the end of the leaf-stalk, or from its point of union with the leaf-blade, or from the ribs, particularly when they are artificially broken or incised. In other cases the leaves, while still on the parent plant, have the power to produce adventitious buds, and, Fic. 193.—Different modes of grafting J, Crown grafting ; IJ, splice grafting ; IJ, bud grafting ; TI, stock ; FZ, scion, in this way, give rise to new plants (see Vegetative Reproduction, p. 279). Even from roots or pieces of roots it is also possible to propagate some few plants. An example of this is afforded by Zpecacwanha, whose roots are cut in pieces and then sown like seeds. The Dandelion possesses the same capability of developing from small portions of the root, and to this peculiarity is due the difficulty with which it is destroyed. In grafting or budding, cuttings from one plant are inserted in another, so that they grow together to form physiologically one plant. The union is accomplished by means of a callus (p. 144), formed by both the scion and the adopted stock. Vessels and sieve-tubes afterwards develop in the callus, and so join together the similarly functioning elements of both parts. Such an organic union is only possible between very nearly related plants, thus, for example, of the Amygdalaccac, the Plum, Peach, Almond, Apricot, may readily be grafted one upon the other, or of the Pomaceae, the Apple with the Quince ; but not the Apple with the Plum. SECT. 1 PHYSIOLOGY 229 In spite of the apparent physiological union between the old stock and the newly-formed growth, from a morphological standpoint they lead an altogether separate and distinct existence. In its structural character, forms of tissues, mode of secondary growth, formation of bark, etc., each maintains its own individu- ality. In special cases it has been affirmed that they do mutually exert, morpho- logically, a modifying effect upon each other (Graft-hybrids). In practice several different methods of inserting cuttings are in use, but only the more important can be mentioned here. GRAFTING is the union of a shoot with a young and approximately equally- developed wild stock. Both are cut obliquely with a clean surface, placed together, and the junction protected from the entrance of water and fungi by means of grafting wax. CLEFT OR TONGUE GRAFTING is the insertion of weaker shoots in a stronger stock. Several shoots are usually placed in the cut stem of the stock, care being taken that the cambial region of the different portions are in contact, that the cortex of the shoots is in contact with that of the stock. In other methods of grafting the cut end of the shoot is split longitudinally and the cut shoot inserted in the periphery, or a graft may be inserted in the cortex or in the side of the stock. In grafting in the cortex the flatly-cut shoot is inserted in the space cut between the bark and the splint wood (Fig. 193, J). In lateral grafting, the shoot, after being cut down, is wedged into a lateral incision in the stock. A special kind of grafting is known as BuppING (Fig. 193, ZZZ). In this process a bud (‘‘eye”’) and not a twig is inserted under the bark of the stock. The ‘‘eye” is left attached to a shield-shaped piece of bark, which is easily separated from the wood when the plants contain sap. The bark of the stock is opened by a T-shaped cut, the ‘‘eye” inserted, and the whole tightly covered. Occasionally some of the wood may be detached with the shield-shaped piece of bark (budding with a woody shield). In the case of sprouting buds, the budding is made in spring; in dormant buds, which will sprout next year, in summer. Budding is especially used for roses and fruit-trees. The Phase of Elongation For the performance of their proper functions, the embryonic rudiments of the organs must complete their external development. They must unfold and enlarge. This subsequent enlargement of the embryonic organs of plants is accomplished in a peculiar and economical manner. While the organs of animals increase in size only by a corresponding increase of organic constructive material and by the formation of new cells rich in protoplasm, and thus require for their growth large supplies of food substance, plants attain the chief part of their enlargement by the absorption of water—that is, by the incorporation of an inorganic substance which is most abundantly supplied to them from without, and to obtain which no internal nutritive processes are first necessary. The elongation of a plant organ to its definite extension, whereby it is often enlarged a hundred or thousand fold, may be compared with the extension of certain animal organs by means of an influx of water, as occurs in the case of the 230 BOTANY PART I Coelenterata or Echinodermata. When an ambulacral foot of a star- fish or a sea-urchin is lengthened from a millimetre to several centi- metres by filling with water from the water-vascular system, the water has in this instance the same biological significance as in the elongation of the plant organs, except that in the latter case the process is not of repeated occurrence. The great advantage resulting from this easy method of enlarge- ment is apparent from a consideration of the importance of a large external surface for the nutrition of a plant. Assimilation is just so much the more productive, the larger the exposure of green surface, and the more accessible it is to the surrounding carbonic acid. In like manner, the superficial enlargement is exceedingly advantageous as regards the absorption of nourishment from the soil. It is accord- ingly of great economic value biologically that the growth through elongation is accomplished chiefly by the absorption of water. The absorption of water by living cells does not take place with the same rapidity and without interruption as in the case of porous bodies. Before the cells can take up additional water they must enlarge by actual processes of growth. The water, penetrating the young cells by imbibition or by the force of osmotic pressure, is uniformly distributed through the protoplasm, which fills the cell ; in case the protoplasm is already abundantly supplied with water, it is instead accumulated in vacuoles (Fig. 50). As the vacuoles contain also organic and inorganic matter in solution, they exert an attractive force and give rise to further absorption of water. The sap of the vacuoles would, in turn, soon be diluted and its attractive force diminished, were it not that the regulative activity of the proto- plasm soon provides for a corresponding increase of the dissolved salts, so that the concentration and attractive force of the sap are continually being restored or even increased. The separate vacuoles thus enlarged ultimately flow together into one large sap-cavity in the middle of the cell, while the protoplasm forms only a com- paratively thin layer on the cell walls, which now exhibit considerable surface- growth. During this increase in the volume of the cell, the protoplasm has experienced but little augmentation of its substance, or other modifica- tion. The enlargement of the cell has been almost wholly produced by the increased volume of water in the sap cavity, which, to dis- tinguish it from the “nutrient water,” “imbibition water,” and “constitution water” of the plant, may be designated “ inflation water.” As is often observed with the occurrence of many vital phenomena, the rate of distension of the walls with the inflation water is not uniform, but BEGINS SLOWLY, INCREASES TO A MAXIMUM RAPIDITY, AND THEN GRADUALLY DIMINISHING ALTOGETHER CEASES. As ail the cells of equal age in an organ go through this process of inflation at the same time, the phenomena of increase and decrease in the rate of growth are apparent in the growth of the organ, and give rise to GRAND PERIODS OF GROWTH. Minor periods, or fluctuations in the rate of growth, occurring within the grand periods, are due to irregu- SECT. II PHYSIOLOGY 231 larities in the swelling of the cells, occasioned by change of temperature, light and other influences operative on growth. The large amount of water absorbed by the growing organ in the process of elongation does not lessen its rigidity, but, on the contrary, it is to the turgor thus maintained that the rigidity is due (ef. p. 165). Osmotic pressure seems also to take an important part in the growth of the cell wall itself. Cells in which the turgor is destroyed by a decrease in the water-supply exhibit no growth of their cell walls ; it is thus evident that the distension of the cell walls is physically essential for their surface-growth. This distension is in itself, however, by no means the cause of their growth; the internal physiological conditions of the growth of the cell walls are dependent upon the vital activity of the living proto- plasm. Without the concurrent action of the protoplasm, there is no growth in even the most distended cell wall; on the contrary, active growth of the cell wall may take place with the existence of only a small degree of turgor tension, A CORRESPONDENCE BETWEEN THE TURGOR TENSION OF THE CELL WALLS AND THE AMOUNT OF GROWTH CANNOT, UNDER THESE CONDITIONS, BE EXPECTED, nor can, on the other hand, the conclusion be drawn that turgor tension is inoperative in the processes of growth. The importance of the turgor tension is variously estimated, according to the conception of the manner in which the growth in substance of the cell walls takes place. There have been for some time two conflicting theories in regard to this. According to one, the growth of the cell wall is due to the inter- polation of new particles of constructive material between the already existing particles of the cell-wall substance (INTUSSUSCEPTION) ; in the other theory, the assumption of the interpolation of new particles is disputed, and growth in surface is attributed to the plastic (inelastic, not resuming its original position) expansion of the distended cell wall. As in this case the growing membrane would continu- ally become thinner, its growth in thickness results from the repeated deposition of new layers (APPOSITION) of substance on the internal surface of the original wall. It is, however, a question of purely theoretical interest, by which of the methods the growth of the cell membrane is effected in particular cases. While, in general, neither of these views is inconsistent with the external phenomena of growth, in some special cases intussusception, and in others apposition, seems to offer the more satisfactory explanation. It is, in fact, not improbable that the crowTH or THE CELL WALLS IS DUE TO BOTH PROCESSES. It is evident that at least some degree of turgor tension is necessary for the existence of this form of expansion. To support the theory of intussusception it has also been found necessary to suppose that the new particles are not interpolated until the spaces between the particles of the cell-wall substance has been enlarged by the distension of the wall itself. The process of elongation has so far been considered only in relation to the single cell, preparatory to the consideration of the phenomena presented by the growth of multicellular organs. The operations of growth in plant organs proceed very slowly ; so slowly as to be, in general, imperceptible. The stamens, however, of many Gramineae grow so rapidly that their elongation is evident, even to the naked eye. An increase in length of 1°8 mm. a minute has been observed in the stamens of Triticum (Wheat). This approximately corresponds to the rate of movement of the minute-hand of a watch. In comparison with it, the next known most rapidly growing organ 232 BOTANY PART I is the leaf-sheath of the Banana, which shows an elongation of 1-1 mm., and a Bamboo shoot, an increase in length of 0:6 mm. per minute ; while most other plants, even under favourable circumstances, attain but a small rate of elongation (0'005 mm. and less per minute). In order to measure the growth in length of a plant, it is customary to magnify in some way the actual elongation for more convenient observation. This may be effected by means of a microscope, which magnifies the rate of growth correspondingly with the distance grown. With a high magnifying power the growing apex of a Fungus hypha seems to advance across the field of vision as if impelled by an invisible power. For large objects, the most convenient and usual method of determining the rate of growth is by means of an AUXANOMETER. The principle of all auxanometers, however they may differ in construction, is the same, ryt en Fic. 194.—Simple and self-registering auxanometer. For description see text. and is based-upon the magnification of the rate of growth by means of a lever with along and short arm. In Fig. 194, at the left, a simple form of auxanometer is shown. The thread fastened to the top of the plant to be observed is passed over the movable pulley (7), and held taut by the weight (g), which should not be so heavy as to exert any strain on the plant. To the pulley there is attached a slender pointer (2), which is twenty times as long as the radius of the pulley, and this indicates on the scale (§) the rapidity of the growth, magnified twenty-fold. By a growth in the length of the plant-stem of {mm., the pointer would accord- ingly register 4mm. ~ Self-registering auxanometers are also used, especially in making extended observations. In Fig. 194, at theright, is shown one of simple construction. The radius of the wheel (2) corresponds to the long arm, and the radius of the small wheel (r) to the short arm of the lever, in the preceding apparatus. Any movement of the wheel, induced by the elongation of the shoot, and the consequent descent of the weight (@), is recorded on the revolving drum (C) by the pointer attached to the weight (Z), which is, in turn, balanced by the counterweight (JV). The drum is SECT. IL PHYSIOLOGY 233 covered with smoked paper, and kept in rotation by the clock-work (UY). If the drum is set so that it rotates on its axis once every hour, the perpendicular distances between the tracings on the drum will indicate the proportional hourly growth. The grand periods in the growth of an organ, due to the internal causes, are clearly shown by such self-registering auxanometers by the gradual increase and final de- crease in the perpendicular distances, represent- ing the increment of growth. Srrruu found the daily growth in length of a root of Lupine, ex- pressed in tenths of millimetres, to be: 58, 70, 92, 97, 165, 192, 158, 137, 122, 83, 91, 59, 25, 25, 8, 2, o. For the first internode of the stem, growing in the dark, the daily growth observed was: 8, 9, 11, 12, 35, 43, 41, 50, 51, 52, 65, 54, 43, 37, 28, 18, 6, 2, 0. The grand periods of growth, that is, the gradual increase from nil to a maximum, and the succeeding decrease to nil again, are, how- ever, not evident throughout the whole of a root ; during the growth in length only a small portion of a root is actually, at one time, in process of elongation. In roots of land-plants the growing region extends over only about one centimetre of the extreme tip, often indeed over only $ centimetre ; while all the rest of the root has already completed its growth in length. This may be made clear by marking off with india-ink, near the tip of a root, narrow zones of equal width, which would thus also be made up of cells of nearly equal size. In Fig. 195, J, is shown a germinating Bean, Vicia Faba, whose root-tip has been marked in this way; Fig. 195, IZ, represents the same root after 22 hours of growth. The marks have become separated demiona ofthe roots tipot Picle eb, by the elongation of the different zones, but in 7 "The root-tip divided by marking different degrees, according to their position. with india-ink into 10 zones, each The greatest elongation is shown by the trans- — 1 mm. long. IJ, The same root after verse zone 3; from there the growth in length twenty-two hours ; by the unequal growth of the different zones the decreases towards the younger zones (2 and 1), lines have beeome separated by as well as towards the older (4 to 10). This unequal distances. (After SAcHs.) peculiar distribution of growth is but the result of the grand periods of growth of the cells in zones of different ages. In the millimetre-broad zones of a root of Vicia Faba Sacus found, after twenty-four hours, that the increase in growth, expressed in tenth-millimetres, was as follows :— Zones: J., II., II., IV., V., VI., VIJ., VIII, IX., X., XT. Increase: 15, 58, 82, 85, 16, 13, 5, By. 2h ay. Oh The elongating region in shoot axes is generally nuch longer than in roots, and usually extends over several centimetres, in special cases even over 50 or more centimetres. The distribution of the increase corresponds in stems, as in roots, with the grand periods of growth of the cells. Even by INTERCALARY GROWTH, where the region of elongation is not confined to the apex but occurs in any part of 0 Fia. 195.—Unequal growth of different 2384 BOTANY PART I the organ, generally at its base (leaves and flower-stalks of many Monocotyledons), grand periods of growth are also apparent. A shoot of Phaseolus multiflorus which was divided, from the tip downwards, into transverse zones 3°5 mm. broad, showed in forty hours, according to SAcHs, in zones: I., IL., III., IV., V., VI, VII., VIII., [X., X., XI., XII. an increase of 20, 25, 45, 65, 55, 30, 18, 10, 10, 5, 5, 5 tenth-millimetres. ’ This periodicity in the growth in length occurs even when the external influences affecting growth remain unchanged, and is determined by internal causes alone. External Influences upon Growth.—External factors often take an active part in the process of elongation, either as retarding or accelerating influences. As growth is itself a vital action, it is affected by any stimulus acting upon the protoplasm ; on the other hand, as it is also a physical function, it is modified by purely physical influences. Growth is particularly dependent upon temperature, light, moisture, the supply of oxygen, and the existence of internal pressure and tension. The INFLUENCE OF TEMPERATURE is manifested by the complete cessation of growth at a temperature less than 0° or higher than 40°-50°. Between the MINIMUM and MAXIMUM temperatures, at which growth ceases, there lies an OPTIMUM temperature (p. 163), at which the rate of growth is greatest. This optimum temperature usually lies between 22° and 37°C. The three CARDINAL POINTS OF TEMPERA- TURE here given include a wide range, as they vary for different species and even for individual plants of the same species. In tropical plants the minimum temperature may be as high as+10°C., while those of higher latitudes, where the first plants of spring often grow .through a covering of snow, as well as those of the higher Alps and polar regions, grow vigorously at a temperature but little above zero. In like manner, the optimum and maximum temperatures show great variation in different species of plants. The optimum does not usually lie in the middle between the minimum and maximum, but is nearer the maximum. The INFLUENCE OF LIGHT makes itself felt in a different manner from changes of temperature. Light as a general rule retards growth. This is apparent from observations on stems and roots grown in the dark, and is also true in regard to the growth of leaves, if the disturbing effects resulting from long-continued darkness be disregarded. Too great an intensity of light causes a cessation of the growth of an organ, while feeble illumination or darkness increases it. The effect of darkness upon the growth of plants is, however, differently manifested according to its duration, whether it be continuous, or interrupted, as in the changes of night and day. Long-continued darkness produces an abnormal growth, in that the growth of certain organs is unduly favoured, and of others greatly retarded, so that a plant grown altogether in the dark presents an abnormal appearance. The stems of Dicotyledons, in such case, become unusually elongated, also soft and SECT, II PHYSIOLOGY 235 white in colour. The leaf-blades are small and of a yellow colour, and remain for a long time folded in the bud (Fig. 196, ZL). A plant grown under such conditions is spoken of as “ etiolated.” This diminution in the size of the leaf-blades and the elongation of the stem (and leaf-stalks) are not manifested by all plants, nor under all circumstances. The stems, for instance, of certain Cacti are much shorter when grown in the dark than in the light. Similarly, the leaves of varieties of the Beet (Beta) grow as large, or even larger, in the dark than in the light; this is also true, under conditions favourable to nutrition, of the leaves of other plants (Cucurbita). In the shade of a forest leaves often become larger than in full daylight. They are then proportionally thinner, and the palisade cells which, in leaves fully exposed to the light, are in close contact, become pointed below, and thus produce intercellular spaces between them. In this way the modifying influence of light of diminished intensity is apparent in the internal structure of such scotophilous leaves. Flowers, however, if sufficient constructive material be provided by the assimilating leaves, develop, according to SAcus’ observations; as well in the dark as in the sunlight, except that they are sometimes paler in colour. If, however, the assimilatory activity of the green leaves be reduced or destroyed by depriving them of light, many plants, as VécHTING found, form only inconspicuous or cleistogamous flowers. The tissues of etiolated stems and leaf-stalks are fuller of water and thinner-walled than in normally growing plants. Even the roots of such plants are often found to be less strongly developed. The supply of reserve material at the disposal of plants growing in the dark is utilised, together with the help of an unusual amount of inflation water, in the elongation of the axis. This elongation of the shoot axis, resulting from growth in darkness, is of especial value in the development of young plants from underground tubers, rhizomes, and seeds; for p.. 196. two gcitealae in this way the light is quickly reached, and _sinapis alba, of equal age : they are the sooner capable of independent a nee dark: nutrition. The advantage derived from a rapid inary dagient momma. elongation is especially apparent when the leaves must themselves reach the light by their own elongation. This is often necessary, particularly for the leaves of Monocotyledons, which spring from bulbs and rhizomes. They act just as the stems of Dicotyledons, and attain an abnormal length in the dark. From what has already been said it would seem that plants must grow more rapidly during the night than day, and this is actually the 236 BOTANY PART I case where other conditions affecting growth remain the same by night as by day. A too low temperature during the night may, how- ever, completely counteract the accelerating influence of darkness upon the growth. Just as the rays of light of different wave-length and refrangi- bility were found to be of different value in the process of assimilation, so growth is by no means equal in differently-coloured light. Iv 1s To - THE STRONGLY REFRACTIVE, SO-CALLED CHEMICAL, RAYS THAT THE Fic, 197.—Two leaves of Ranunculus Purshii. L, An aerial leaf; 1’, a submerged water-leaf. (After GOEBEL.) INFLUENCE OF LIGHT ON GROWTH IS DUE: the red-yellow end of the spectrum acts upon many plants in the same manner as darkness. Moisture exerts a twofold influence upon growth. It acts as a stimulus, and also, by diminishing transpiration, increases turgidity. Plants in damp situations are usually larger than those grown in dry places, and in fact may differ from them in their whole habit and mode of growth. Direct contact with water seems frequently to exert a special influence upon the external form of plants. Amphibious plants, that is, such as are capable of living both upon land and in water, often assume in water an entirely different form from that which they possess in air. This variation of form is particularly manifested in the leaves, which, so long as they grow in water, are finely dissected, while in the air their leaf-blades are much broader (Fig. 197). The leaf-stalks and internodes also often exhibit a very different form in air and water, and undergo the same abnormal elongation as in darkness. This is especially noticeable in submerged water-plants, whose organs must be brought to the surface of the water (young stems and leaf- stalk of Trapa natans, stem of Hippuris, leaf-stalk of Nymphaca, Nuphar, Hydro- charis). Such plants are enabled by this power of elongating their stems or leaf- stalks to adapt themselves to the depth of the water, remaining short in shallow SECT, IL PHYSIOLOGY 237 water and becoming very long in deep water. The pressure of the water upon the tip of the growing organ, as well as the insufficient supply of oxygen, seem to act upon the growth, in this instance, as regulating stimuli. The great importance of free oxycEn has already been alluded to in connection with respiration (p. 219). Without gaseous or dissolved oxygen in its immediate environment the growth of a plant entirely ceases. Mechanical Influence.—Pressure and traction exert a purely mechanical influ- ence upon growth, and also act at the same time as stimuli upon it. External pressure at first retards growth; it then, however, according to Prrrrer, stimu- lates the protoplasm and occasions the distension of the elastic cell walls, and frequently also an increase of turgor. As a consequence of this increased turgor the counter-resistance to the external pressure is intensified. If the resistance of the body exerting the pressure cannot be overcome, the plasticity of the cell walls renders possible a most intimate contact with it; thus, for instance, roots and root-hairs which penetrate a narrow cavity so completely fill it that they seem to have been poured into it in a fluid state. It would be natural to suppose that the effect of such a tractive force as a pull would accelerate growth in length, by aiding and sustaining turgor expansion. But the regulative control exercised by the protoplasm over the processes of growth is such that mechanical strain, as HrcLEr has shown, acts upon growth to retard it (except in the maximum of the grand periods). The elastic resistance and rigidity of cell walls are in- creased by the action of a strain; such a strain may also induce the formation of collenchyma and sclerenchyma which would not otherwise have been developed. The Internal Development of the Organs The internal development of the organs is only completed after they have finished their elongation and attained their ultimate size. They are then first enabled to fully exercise their special function. To this end cell cavities usually become more or less fused, and the cell walls thickened, often in a peculiar and characteristic manner (p. 75). In the case of plants equipped for a longer duration of life, a growth in thickness follows the growth in length (p. 119). Periodicity in Development, Duration of Life, and Continuity of the Embryonic Substance The periodically recurring changes in the determinative external influence, especially in light and temperature, occasioned by the alter- nations of day and night and of the seasons, cause corresponding periodical variations in the growth of plants. These variations do not follow passively every change in the condition of the external influences. On the contrary, the internal vital processes of plants so accommodate themselves to a regular periodicity that they continue for a time their customary mode of growth, independently of any external change. The nightly increase of growth, which is especially noticeable after midnight in the curve of growth, and the retardation of growth, specially marked after mid-day, will continue to be exhibited 238 BOTANY PART I for some time in prolonged darkness when the temperature remains constant, thus under these conditions Helianthus tuberosus has been observed to continue its regular DAILY PERIODS for two weeks, affording an example of the inexplicable occurrence of so-called AFTER-EFFECTS, which are frequently mentioned in a later chapter. Still greater is the influence exerted on the life of plants by the alternation of winter and summer, which in the plants of the colder zones has rendered necessary a well-marked winter rest. This is not in reality an absolute rest ; for although the outwardly visible pro- cesses of development and growth stand still, the internal vital pro- cesses, although retarded, never altogether cease. The ANNUAL PERIODS of growth occasioned by climatic changes, which are ren- dered so noticeable by the falling of the leaves in the autumn, and the develop- ment of new shoots and leaves in the spring, have stamped themselves so indelibly upon the life of the trees and shrubs of the temperate zones, that, when culti- vated in tropical lands where other plants are green throughout the year and blossom and bear fruit, they continue to lose their leaves and pass for a short time at least into a stage of rest. The Oak and Beech have become so habituated to this annual periodicity that they never depart from it ; other trees again gradually accustom themselves to the new conditions, as the Cherry and Peach, for instance, which in Ceylon have become evergreen trees. The Peach is reported to produce flowers and fruit throughout the entire year ; while the Cherry, like many other trees of the temperate zone, ceases altogether to bear flowers in tropical climates. It is due to a similar habituation to an annual periodicity that in some cases it is so difficult, or even altogether impossible, to induce plants by artificial culture to flower out of season. The behaviour of different species also varies in this respect ; in general, those flowers accommodate themselves best to forcing which, like the Hyacinth, Crocus, Tulip, Syringa, and Cornus mas, naturally flower early. That the internal vital processes are not promoted by artificial heat to the same extent as growth in length, is at once perceptible from the abnormal appearance of many forced plants whose leaves and flowers do not attain their full development (the flowers of the Lily of the Valley, when forced artificially, develop even before the leaves). Low temperature, especially frost, is often of advantage in the preparatory vital processes during the period of rest; this is made evident by the accelerated transformation of the reserve material, and by the active growth in spring. Although to so many plants winter is the season of rest and cessation from growth, other plants, ¢.g. certain Lichens and Mosses, seem to find in the warmer days of winter the most favourable conditions of vegetation ; and in summer, on the contrary, either do not grow at all or only very little. Similarly, many spring plants attain their highest development, not in summer, but during the variable weather of March and April, and, for the most part, they have entered upon their rest period when the summer vegetation is just awakening. In countries where there are alternate rainy and dry periods, the latter generally correspond to the winter period of vegetative rest. Duration of Plant Life-—The life of a plant, during the whole of its development, from its germination to its death, is dependent upon external and internal conditions. In the case of ‘the lower vegetable SECT. II PHYSIOLOGY 239 organisms, such as Algae and Fungi, their whole existence may be completed within a few days or even hours, and indeed some of the higher herbaceous plants last only for a few weeks, while the persistent shrubs and trees, on the other hand, may live for thousands of years. After the formation of the seeds, there occurs in many plants a cessation of their developmental processes, and such a complete exhaustion of vitality that death ensues. Such an organic termination of the period of life occurs in our annual summer plants, but also takes place with plants in which the preparatory processes for the formation of fruit have extended over two or more years, as in the case of the 10 to 40-year-old Agave, which, after the formation of its stately inflorescence, dies of exhaustion. In plants, on the other hand, which in addition to the production of flowers and fruit accu- mulate also a reserve of organic substance, and, with their reproductive organs, form also new growing points, life does not cease with the production of the seeds. Such plants possess within themselves the power of unlimited life, the duration of which may only be terminated by unfavourable external conditions, the ravages of parasites, injuries from wind, and other causes. The longevity of trees having an historical interest is naturally best known and most celebrated, although, no doubt, the age of many other trees, still living, dates back far beyond historical times. The celebrated Lime of Neustadt in Wiirtemberg is between 800 and 1000 years old ; the age of the Fir of Béqué is estimated at 1200 years, and a Yew in Braburn (Kent) is at least as old. A stem of a Sequoia in the British Museum has, with 1330 annual rings, a diameter of 4°5 m., and, according to CARRUTHERS, must have originally been 28°5 m. in circumference. An Adansonia at Cape Verde, whose stem is 8-9 m. in diameter, and a Water Cypress, near Oaxaca, Mexico, are also well-known examples of old trees. Of an equally astonishing age must have been the celebrated Dragon-tree of Orotava, which was overturned in a storm in 1868, and afterwards destroyed by fire. The lower plants also may attain a great age; the apically growing mosses of the calcified Gymnostomum clumps, and the stems of the Sphagnaceae, metre-deep in a peat-bog, must certainly continue to live for many hundred years. In thus referring to the ages of these giant plants, it must not be understood that all the cells remain living for so long a time, but rather that new organs and tissues are developed, which continue the life of the whole organism. All that is actually visible of a thousand-year-old Oak is at most but a few years old. The older parts are dead, and are either concealed within the tree, as the pith and wood, or have been discarded like the primary cortex. The cells of the original growing point have alone remained the whole time alive. They continue their growth and cell division so long as the tree exists; while the cells of the fundamental tissue arising from them, 240 BOTANY PART I and destined for particular purposes, all lose their vitality after a longer or shorter performance of their functions. The cells of the root-hairs often live for only a few days; the same is also true of the glandular cells and trichomes of stems and leaves. The wood and bast fibres, as also the sclerenchymatous cells, lose their living protoplasm after a short time. Entire organs of long-lived plants have frequently but a short existence; the sepals, petals, and stamens, for example. The foliage leaves, also, of deciduous trees live only a few summer months and then, after being partly emptied of their contents, are discarded. Before the falling of leaves a separative layer is first formed in the elongating leaf-base (p. 148); while a layer of cork, formed either before or after the leaf- fall, closes the leaf-scar. The formation of ice in the absciss layer, as may easily be observed after the first frost, facilitates the separation of the leaf from the stem. The leaves even of evergreen plants continue living but a few years, before they too fall off. Small twigs, especially of Conifers, are also subject to the same fate. The cells of the medullary rays afford the best examples of long-lived cells con- stituting permanent tissues. In many trees, as in the Beech, living medullary ray cells over a hundred years old have been found, although, for the most part, they live only about fifty years. Continuity of the Embryonic Substance. —While the cells of the permanent tissue have thus but a limited activity, the vitality of the embryonal tissues is unlimited, and never terminates from natural causes. From such embryonal tissue the growing points of perennial plants are formed, and the growing points of their descendants, as Sacus has pointed out, are also derived from it, through the substance of the sexual cells. The embryonic substance does not change; it produces new individuals, which live but a short time, but is itself perpetuated in their offspring ; it continues always productive, always rejuvenescent and regenerative. The thousands and thousands of generations which have arisen during the past ages were its products ; it continues living in the youngest generations with a capacity for production still unabated and undiminished. The single organism is perishable ; its embryonic substance, however, is imperishable and unchangeable, and continually gives rise to new tissues. Considered from this standpoint, the difference between short- and long-lived plants, between annual herbs and thousand-year-old trees, appears in quite another aspect. From the embryonic substance of the oldest trees there are produced, each year, new leaves and shoots, which, however, remain united with the dead remains of former years. In annual herbs, on the other hand, the embryonic substance in the embryo becomes separated each year from the dead plant, and develop- ing new leaves, stems, and roots, forms a completely new individual. The old and well-known maxim of Harvry’s, “Omne vivum ex ovo,” is, in other words, only the expression of the principle of the con- tinuity of the embryonic substances. And similar to it, in its continual SECT. 11 PHYSIOLOGY 241 vitality and organic imperishability, is the substance of the lowest unicellular organisms, continually reproducing themselves by division and ever changing into new individuals. V. The Phenomena of Movement In every living organism there is constantly occurring in the course of the metabolic processes an active movement and _ transposi- tion of substance. As these movements are for the most part molecular they are generally imperceptible; but that they actually take place is demonstrated with absolute certainty by the local accumulation and diminution of substance, shown both by weighing and by the results of chemical analysis. There are also other forms of movement which play an important part in the physiology of every organism, and on which its vital processes are to a large extent dependent: these are the movements due to heat and the related conditions of vibration resulting from light, electricity, ete. Apart from the movements of this class, which may take place within organisms which, externally, are apparently at rest, there occur also in plants actual CHANGES IN POSITION, externally noticeable but usually of gradual operation ; yet in special cases they may involve rapid motion. These movements may be carried on either by the whole plant or by single organs. Reference is here made only to the SPONTANEOUS MOVEMENT resulting from the activity of a plant organism itself, and this should not be confused with the PASSIVE movements due to externally operating mechanical agencies, such as water and wind, which, although they have a certain importance for plant life, will not be here considered. ProropLasM itself is capable of different movements. Naked ‘ protoplasmic bodies almost always show slow movements resulting in a gradual change of position ; but cells enclosed by cell walls possess also the power of INDEPENDENT LOCOMOTION, often indeed to a considerable extent. Multicellular plants, however, as a rule ultimately attach themselves, by means of roots or other organs, to the place of germina- tion, and so lose for ever their power of locomotion, except in so far as it results from growth. A gradual change in position due to growth is apparent in plants provided with rhizomes, the apical ex- tremities of which are continually growing forward, while the older portions gradually die off. A yearly elongation of 5 cm. in the apical growth of the rhizomes would result, in twenty years, in moving the plant a distance of one metre from its original position. A seedling of Cuscuta in its search for a host plant illustrates the power of maintaining, for a time, a creeping movement over the surface of the soil; a growing Caulerpa (Fig. 250) likewise exhibits in the course of years a similar advancing movement. In addition to these move- R 242 BOTANY PART I ments, occasioned by a growth in length, plants firmly established in the soil possess also the power of changing the position and direction of their organs by means of CURVATURE and RoTATION. In this way the organs are brought into positions necessary or advantageous for the performance of their function. By this means, for example, the stems are directed upwards, the roots downwards; the upper sides of the leaves turned towards the light; climbing plants twined about a support, and the stems of seedlings so bent that they break through the soil without injury to the young leaves. Movements of Naked Protoplasts The creeping movements of naked protoplasts, such as are shown by an amceba or plasmodium, in the protrusion, from one or more sides, of protuberances which ultimately draw after them the whole protoplasmic body, or are themselves again drawn in (Fig. 198), are distin- guished as AM@EBOID MOVEMENTS. These movements resemble, externally, the motion of a drop of some viscid fluid on a surface to which it does not adhere, and are chiefly due, according to BERTHOLD, to superficial tension, which the protoplasm can at different points increase or diminish, by means of its quality of irritability. (By means of irregular changes of surface- tension similar amceboid movements are . also exhibited by drops of lifeless fluids.) Fic. 198.—Amceboid movement, The arrows indicate the direction and In the SWIMMING MOVEMENTS BY energy of the movement; the crosses, MEANS OF CILIA, on the contrary, the een nee MT rent Whole protoplasmic body is not involved eing the principal movement is P 2 =e from H te V, but at any moment it but it possesses special organs of motion ee ee a ae in the form of whip-like FLAGELLA or caren one Brahe: cia. These may be one, two, four or more in number, and arranged in various ways (Figs. 69, 70). They move very rapidly in the water and impart considerable velocity to the protoplast, often giving it at the same time a rotatory movement. While the swiftest ship requires 10-15 seconds to travel a distance equal to its own length, the velocity with which these protoplasmic bodies are impelled by their cilia, in a second, is two or three times their length, although, owing to their diminutive size, the distance travelled by them in an hour would amount to only about a metre. The protoplasmic body is conveyed by the motion of the cilia in a definite direction, which is so regulated by the action of stimuli that it may be instantly changed. In this way the direction and velocity of the ciliary movements are made SECT. II PHYSIOLOGY 243 serviceable to the protoplasmic organisms through the irritability of the protoplasm. Gravity and light, certain substances in solution, and mechanical hindrances are the principal influences which regulate the movements of free-swimming protoplasmic. bodies and cells. The direction of the movements of the swarm-spores of Algae are chiefly determined by the light. So long as they remain in darkness they move through the water in all directions ; but as soon as they are illuminated from one side only, a definite direction in their movements is perceptible. They move either straight towards the light or turn directly away from its source. Their retrogressive movements from the light occur either in case of too intense illumination, or at a certain age, or through some unknown disturbing irritation. The advantage of such HELIOTACTIC MOVEMENTS (phototactic) is at once apparent when the part taken by the swarm-spore in the life of an Alga is considered. In order to provide for the future nutrition of the stationary Alga into which it afterwards develops, it must seek the light. If a point with suitable (that is, not too intense and not too weak) illumination be attained, then the swarm-spore must attach itself by the end which carries the cilia: to do this it must turn itself from the light towards a dark object. On the other hand, as the swarm-spores do not come to rest at all in absolute darkness, but swim continuously until thoroughly exhausted, the possibility of their attaching themselves in a spot devoid of light is excluded, and where the new plant could not assimilate. The SWARM-SPORES of water Fungi and motile Bacteria, according to PFEFFER’S investigations, are chiefly influenced in their movements by the unequal distribution of dissolved, solid, or gaseous matter (oxygen) in their environment. According to their momentary requirements and their sensitiveness to stimuli, they move either towards or away from the points of highest concentration. As the result of similar CHEMOTACTIC MOVEMENTS spermatozoids approach the female sexual organs. PFEFFER has demonstrated that the spermatozoids of Ferns are enticed into the long necks of the archegonia by means of malic acid: while the archegonia of the Mosses attract the spermatozoids by a solution of cane-sugar. In such cases an extremely small quantity of dissolved substance is often a sufficient stimulus to call forth active chemotactic movements ; a 0°001 per cent solution of malic acid suffices for the attraction of Fern spermatozoids. The movements of amcebe and plasmodia are similarly induced by external influences. These naked protoplasts live not only in water (amcebe), but also in moist substrata (plasmodia, amczbee), and seem to possess the power of seeking out situations with more moisture, or of avoiding them (before the formation of spores): their movements are also influenced by the direction of currents in the water (rheotaxis). In eases where cells enclosed by cell walls (Sphaerella pluvialis) swim freely about by means of cilia, the cilia spring from the protoplasm and pierce the cell walls. 244 BOTANY | PART I Diatoms and Desmids exhibit quite a different class of movements. The Diatoms glide along, usually in a line with their longitudinal axes, and change the direction of their movements by oscillatory motions. From the manner in which small particles in their neighbourhood are set in motion, it was concluded that special organs of motion probably protrude, like pseudopodia, through openings in their hard silicified shell ; while more recently, in a few instances, HAUPTFLEISCH has been able to render visible the protoplasmic motile organs. The protrusion of a transparent thread of mucilaginous matter is claimed to have been seen by BUTscHLT and LauTERBORN in the case of a large Diatom which propelled itself by this means. This means of locomotion resembles that of the nearly related Desmids, which, it has been shown, maintain their peculiar movements with the help of a similar mucilaginous protrusion. The pendulous advancing movements of the filamentous Oscillariae and Spirulinae are also said to be dependent upon similar mucilaginous exudations. The mechanism of the movements of Spirogyra is still unexplained. The Movements of Protoplasm within Walled Cells Although plants which are firmly attached and stationary exhibit no such locomotory movements, the protoplasm within their cells does possess a power of movement. Such internal protoplasmic movements are especially active in the non-cellular Siphoneae, in the elongated internodal cells of the Characeae (Fig. 167), and often in the hairs of many plants, as well as in the leaf-cells of some aquatic plants. The active protoplasmic currents in Caulerpa move along its outer walls and around the internal cellulose bands, stretching from wall to wall in the manner of an immense imprisoned plasmodium. The three following different forms of protoplasmic movement within cell cavities may be distinguished: ‘CIRCULATION, ROTATION, and ORIENTATION. In the case of CIRCULATORY MOVEMENT the different currents of protoplasm, although often quite close together, flow in different directions. This motion is seen most frequently in cells of which the nucleus is suspended in the centre of the cell cavity by means of protoplasmic threads. In these threads continuous protoplasmic currents flowing towards and away from the nucleus connect the protoplasm enveloping it with the protoplasm clothing the cell wall (Fig. 53). Sometimes, even in extremely fine threads of protoplasm, two currents may be seen to pass each other (¢g. in the stamens of Tradescantia, the stinging hairs of Urtica, and the bristles of Cucurbita). In the ROTATORY MOVEMENT the protoplasm moves along the cell wall in one direction only, dragging with it the nucleus and often also the chlorophyll grains. In an elongated cell, in which the rotation usually takes place in the direction of the longitudinal axis, as the protoplasm forms one united body, there must be a strip of immovable protoplasm which separates the rotating masses. This stationary part is termed the NEUTRAL or INTERFERENCE ZONE. The rapidity of the movement diminishes towards the cell wall, and the layer of SECT. II PHYSIOLOGY 245 protoplasm directly contiguous to the cell wall is not in motion. The rotatory movements are easily seen in Chava and Nitella, where they take a spiral course, and they are also very energetic in the cells of the leaves of Elodea canadensis and of Vullisneria spiralis, and also in the root-hairs of Hydrochuris morsus ranae and Trianea bogotensis. The cause of these movements, which may take different directions in adjoining cells, and may also continue after the protoplasm has been drawn away from the cell walls by plasmolysis (p. 166), is not yet understood. It is, however, known that the continuance and activity of such protoplasmic movements, the existence of which was first observed by Corti in 1772, and later rediscovered by TREVIRANUS in 1807, are dependent on factors which, in general, support and promote the vital activity ; while the presence of free oxygen and proper conditions of temperature seem to be particularly favourable to them. Through the study of sections in the cells of which currents had been induced in the protoplasm, by the injuries sustained in their preparation and by other abnormal conditions, grave errors have arisen concerning the existence of such protoplasmic movements in cells, in which under normal conditions they cannot be observed. The presence of protoplasmic currents in a cell may, in fact, indicate either an energetic vital activity, or, on the other hand, be merely a symptom of a pathological or, at least, of an excited condition of the protoplasm. The movements of orientation of the protoplasmic body do not proceed in the same uninterrupted manner as the circulatory and rotatory movements. They are also usually so gradual as to be only recognisable through their operations. They are induced by changes in the external influences, especially as regards the intensity of the light, and result in producing a definite position of the protoplasmic bodies, as, for example, the orientation of the chlorophyll grains with regard to the light. Movements of this kind have been most frequently observed in Algae, in sub- merged Duckweed (Lemna trisulea), in the prothallia of Ferns and Mosses ; but similar movements can also be observed in the higher plants. In the cells of the filamentous Alga Mesocarpus, the chloroplasts, in the form of a single plate suspended length-wise in each cell, turn upon their longitudinal axes according to the direction and intensity of the light. In light of moderate intensity, according to STAHL’s observations, they place themselves transversely to the source of light, so that they are fully illuminated (transverse position) ; when, on the other hand, they are exposed to direct sunlight, the chlorophyll plates are so turned that their edges are directed towards the source of light (profile position). A similar protection of the chloroplasts against too intense light, and their direct exposure, on the other hand, to more moderate illumination, is accomplished, where they are of a different form and more numerous, by their different disposition relative to the cell walls. In moderate light the chlorophyll bodies are crowded along the walls, which are transverse to the direction of the rays of 246 BOTANY PART I light (Fig. 199, 7). They quickly pass over to the walls parallel to the rays of light, however, as soon as the light becomes too intense, and so retreat as far as possible from its action (Fig. 199, 8). In darkness or in weak light the chloroplasts group themselves in still a third way (Fig. 199, WV), the advantage of which is not altogether clear. The form of the chlorophyll bodies themselves undergoes modification dur- ing changes in their illumination ; in moderate light they become flattened, while in light of greater intensity they are rounded and thicker. As a special mode of protection against too intense light, the chloroplasts of the Siphoneae (and the same thing is observed in many plants) become balled together in separate clumps. In correspondence with the changes in the position of the chloro- plasts, the colouring of green organs natur- ally becomes modified. In direct sunshine they appear lighter, in diffused light a darker green. The attention of Sacus was first called to the phenomena of the move- ments of the chloroplasts, by the accidental observation that the shadow of a thermo- meter was represented in dark green on a Fic. 199.—-Varying positions taken by Jeaf otherwise directly illuminated by the the chlorophyll grains in the cells of Siti Lemna trisulea in illumination of dif- : ‘ ferent intensity. 7, in diffuse day- Wounds and one-sided cell-wall thicken- light; S, in direct sunlight; N, at ings likewise give rise to orientation move- night. The arrows indicate the direc- ments, as they occasion a crowding together HR OE Ene Het Ro eE STRELA) on one side of the nucleus and protoplasm. Movements producing Curvature The movements of the organs of stationary plants, unicellular as well as multicellular, are accomplished by means of curvatures. In an organ that has grown in a straight line the longitudinal sides are all of equal length ; in an organ that is curved, however, the concave side is necessarily shorter than the convex side. When, accordingly, the opposite sides of a pliable organ become of unequal length, the organ must curve toward the shorter side (Fig. 168). Inequality in the length of the opposite sides may result from various causes. A curvature occurs if the length of one side remains constant, while the SECT. II PHYSIOLOGY 247 opposite side becomes shorter or longer, and also from the unequal elongation or contraction of both sides, and similarly from the elonga- tion of one side and the contraction of the other. Such curvatures most frequently occur in plants as a consequence of UNEQUAL GROWTH. More rarely they are due to the different length of the opposite sides, resulting from unequal TURGOR TENSION. This is principally the case in fully-grown organs, as in leaf-cushions (p. 268) and stamens. A third source of curvature is found in the unequal amount of water taken up by IMBIBITION, and the consequent unequal distension of the cell walls on the opposite sides of an organ. 1. Hygroscopic Curvatures (Imbibition Movements) As the cell walls of actively living cells are always completely saturated with imbibition water, hygroscopic curvatures are exhibited only by dry and, for the most part, dead tissues ; although occasionally they also take place in living tissues which can endure desiccation without injury, as in the cases of Mosses, Lichens, and Selaginella lepidophylla (p. 179). The hygroscopic move- ments in any case, however, are due to the physical properties of the cell walls, and have no direct connection with the vital processes, except in so far as the capacity of cell walls to swell and take up large quanti- ties of imbibition water is due to the protoplasm by which they were formed. The activity of the proto- plasm in the formation of the cell walls is likewise manifested in their anatomical structure, in their strati- fication and striation, and in the position of the pits, as well as in the arrangement and disposition of the cells themselves. The absorption of imbibition water by cell walls is accompanied A 2B by an increase in their volume, and Fic. 200.—Fruit of Erodium grwinwm. A, in conversely the volume of the cell {)* 0 condition, sailed; 2, moist aad walls is diminished by the evaporation . of the imbibition water. Accordingly, whenever unequal amounts of water are held by the cell walls on the different sides of an organ, either through unequal absorption or evaporation, hygroscopic movements 248 BOTANY PART I are produced, which result in the curvature of the organs. In many cases the organs of plants are especially adapted to such movements, by means of which, in fact, important operations are often accom- plished, as, for example, the dehiscence of seed-vessels and the dis- semination and burial of seeds. The rupture of ripe seed-vessels, as well as their dehiscence by the opening of special apertures (Papaver, Lychnis, Antirrhinum, etc.), is a consequence of the unequal contraction of the cell walls due to desiccation. At the same time, through the sudden relaxation of the tension, the seeds are often shot out to a great distance (Tricoceae, etc.). In certain fruits not only curvatures but torsions are produced as the result of changes in the amount of water they contain, e.g. Erodiwm gruinwm (Fig. 200), Stipa pennata, Avena sterilis, by means of which, in conjunction with their stiff barb-like hairs, the seeds bury themselves in the earth. The opening and closing of the involucre of many Compositae (Erigeron, Carlina, etc.) at the time of the ripening of the seeds, and the changes in the position of the pappus-hairs (Taraxacum, Tragopogon, etc.), are also due to hygroscopic movements resulting from variations in the amount of moisture in the atmosphere. In dry weather the pappus is spread out in the form of an umbrella, but in wet weather it closes up. The opening of anthers and sporangia, the rupture of moss-capsules, and the dissemination of the spores of the Equisctaceac, Hepaticae, and Myxonvycetes are also effected by similar movements. Anthers and sporangia possess peculiarly thickened cells (fibrous cells, annulus), by the contraction of which their dehiscence is produced. The opening of the moss-sporangium is, in like manner, due to the hygroscopic movements of the teeth of the peristome, while the sporangia of the Liverworts are provided with specially thickened spiral bands (elaters), which, like the capillitia of the Myxomycetes, effect the discharge of the spores. In the case of the Eguwisetaceae the outer walls of the spores themselves (the perinium) take the form of four arms, which, like elaters, are capable of active movements, by means of which numbers of spores become massed together before germinating, and the isolation of the dicecious prothallia prevented. In order to call forth imbibition movements the actual presence of liquid water is not necessary ; for, through their hygroscopicity, cell walls have the power of absorbing moisture from the air. They are hygroscopic, and for this reason the ensuing movements are also often termed hygroscopic movements. 2. Growth Curvatures Movements from which curvatures result are, for the most part, produced by the unequal growth of living organs. The unequal growth is due, partly to internal causes which are still undetermined, and partly to the operation of external influences which can be posi- tively demonstrated and defined. The movements resulting in the first case are spontaneous, and are called AUTONOMIC MOVEMENTS or NUTATIONS; in the second case the movements are the result of external stimuli, and are distinguished as irritable or PARATONIC MOVEMENTS. Autonomie Movements are most plainly apparent in young actively-growing organs, although nutations have been shown to be SECT. 11 PHYSIOLOGY 249 exhibited by all growing plants, as their tips do not grow forward in a straight line, but, instead, describe irregular elliptical curves. These movements, which Darwin termed CIRCUMNUTATIONS, while often not perceptible to the eye, are very noticeable in some special organs. The unfolding of most leaf and flower buds, for example, is a nutation movement which, in this instance, is induced by the more vigorous growth of the inner side of the young leaves. The same unequal growth manifests itself most noticeably in the leaves of Ferns and many Cycudece. In the same manner, movements of nutation are caused in other lateral organs when growth is more energetic on either the upper side (EPINASTY) or on the lower side (HYPONASTY). From the nutation of the shoots of Ampelopsis quinquefolia a curvature is produced which continuously advances with the increased growth ; so that, by means of its hooked extremity, a shoot is better enabled to seek out and cling to a support. When the unequal growth is not confined to one side, but occurs alternately on different sides of an organ, the nutations which result seem even more remarkable. Such movements are particularly apparent in the flower-stalk of an Onion or of Yucca jfilamentosa, which, although finally erect, in a half-grown state often curves over so that its tip touches the ground. This extreme curvature is not, however, of long duration, and the flower-stalk soon becomes erect again and bends in another direction. Thin and greatly elon- gated organs must, from purely physical reasons, quickly respond to the effects of unequal growth. The thread-like tendrils of many climbing plants, so long as they are in a state of active growth, afford excellent objects for the observation of nutation movements. If the line of greatest growth advances in a definite direction around the stem, its apex will exhibit similar rotatory movements (REVOLVING NUTATION). This form of nutation is characteristic of the tendrils and shoots of climbing plants, and renders possible their peculiar mode of growth. The SO-CALLED REVOLVING NUTATION OF TWINING PLANTS is not an AUTONOMIC MOVEMENT, and will be considered later with the paratonic movements. Paratonie Movements.—The phenomena of paratonic movements are of the very greatest importance to plant life, for through their operations the organs of plants first assume such positions in the air, or water, or in the earth as are necessary for the performance of their vital functions. A green plant which spread its roots over the surface and unfolded its leaves below ground could not exist, even though all its members possessed the best anatomical structure. The strongest roots would become dried up without the necessary absorption of water, and the leaves could not assimilate in the dark. The organisa- , tion and specific functions can have effect only when the root penetrates the soil. Similarly, the leaves are efficient only when exposed to air and light. Seeds are not always so deposited in the soil, with the embryonal stem directed upwards and the radicle down- 250 BOTANY PART I wards, that their different organs can, merely by direct growth, attain at once their proper position. A gardener does not take the trouble to ascertain, in sowing seed, if the end which produces the root is directed downwards or the stem end upwards, he knows that in any position the roots grow into the ground and the stems push themselves above the surface. Plants must accordingly have in themselves the power of placing their organs in positions best adapted to the condi- tions of their environment. That is only possible, however, when the externally operative forces and substances can so influence the growth of a plant that it is constrained to take certain definite directions. The same external influences excite different organs to assume quite different positions. Through the influence of gravity, the tap- root grows directly downwards in the soil, while the lateral roots take a more or less diagonal direction. The main stem grows perpendi- cularly upwards ; it, like the primary root, is oRTHOTROPIC. The lateral branches, on the other hand, just as the secondary roots, assume an inclined position and are PLAGIOTROPIC. The apical extremities of shoots are constrained to seek the source of light; the leaves, on the contrary, under the same influence place their surfaces transversely to the direction of the rays of light. The different positions assumed by an organ when acted upon by external influences has been termed by SacHS ANISOTROPY. In addition to the purely morphological structure of the plant body, anisotropy also determines essentially its external form and appearance. That all these paratonic movements cannot result merely from the action of external forces alone will be at once recognised if it be taken into consideration that anisotropic but in other respects similar organs are affected differently by the same influences, and that even the same organs react differently at different ages ; and that, moreover, the external forces produce effects which bear no relation to their usual physical and chemical operations. It will, on the contrary, be at once apparent that they are rather the result of definite processes of growth, arising from an irritability to stimuli induced by external influences (cf. p. 161). In order that external influences may produce such stimuli, plants must be sensitive to stimuli, that is, the stimuli must produce in them certain modifications with which, in turn, certain definite vital actions are connected. The precise manner in which an external influ- ence produces an internal stimulation within an organism is not at present known. In order that an external physical force can operate as a stimulus, there must exist within the living substance definite struc- tures or organs which are influenced by it. When the position of an organ is dependent upon the direction of an external influence, its sensitive structure must possess polarity. But for such a polar struc- ture to be of any effect, it must have a definite orientation ; so it is necessary to assume that the DIRECTIVE STIMULI ARE RECEIVED BY SECT, 11 PHYSIOLOGY 251 THE RESTING PORTIONS OF THE PROTOPLASM, THAT IS, BY THE SURFACE LAYER. The movements of growth occasioned by external stimuli are, for the most part, movements in response to directive stimuli which lead to a definite position of the organ, relatively to the direction of the operative influence. The principal external stimuli that come into consideration are light (and electricity), heat, gravity, chemical influ- ences (oxygen, nutritive substances, water, etc.), impact and friction. As the points of greatest nritability in plants or their organs are often more or less removed from the points where the effect of the stimulation is manifested, a propagation of the stimulation must take place. Thus, a stimulus received by a non-motile organ may be conveyed to an organ capable of motion, and there produce movement. In the case of roots, for example, the geotropic stimulus is received by the non-motile root-tip, while the movement is induced in the part of the root in process of elongation. The capacity of organs to assume a definite direction by means of curvatures of growth is distinguished, according to the nature of the particular inciting stimulus, as heliotropism, geotropism, hydrotropism, etc. ; and these again are either POSITIVE or NEGATIVE, according as the direction taken by the curvature is towards or away from the irritating stimulus ; while plant organs which place themselves more or less transversely to the line of action of the operative forces are termed DIATROPIC. As a special result of diatropism, a transverse position is assumed which is exactly at right angles to the direction in which the influence which acts as the stimulus is exerted. Dorsiventral organs, in particular, exhibit a tendency to assume diatropic and even transverse positions. A. Heltotropism The importance of light to plant life is almost incalculable. It is not only absolutely essential for the nutrition of green plants, but it has also a powerful effect upon the growth and general health of the plant organs. Deprived of light for any length of time, leaves and flowers usually fall off; fully developed, vigorous organs of green plants soon become yellow in the dark, and droop and die. Prolonged darkness acts like a poison upon those portions of plants accustomed to the light. On the other hand, exactly the reverse is true of plants or organs whose normal development is accomplished in darkness. Upon them the light has a most injurious, even fatal, effect, as may be easily observed in the case of Fungi and Bacteria. The hygienic importance of daylight in dwelling-places is due to the destructive action of light upon such forms of plant life. That some plants seek the light, while others avoid it, is not surprising in view of the adaptability which organisms usually exhibit in respect to the in- fluences with which they come in contact in the natural course of their development. A good opportunity for the observation of heliotropic phenomena 252 BOTANY PART I is afforded by ordinary window-plants. The stems of such plants do not grow erect as in the open air, but are inclined towards the window, and the leaves are all turned towards the light as if seeking help. The leaf-stalks and stems are accordingly POSITIVELY HELIOTROPIC. In contrast with these organs the leaf-blades take up a position at right angles to the rays of light in order to receive as much light as possible. They are diaheliotropic, or TRANSVERSELY HELIOTROPIC, in the strictest sense (Fig. 201). If among the plants there should be one with aerial roots, (‘Zlorophytum for instance, an example of NEGATIVE HELIOTROPISM will be afforded, as the aerial roots will be found to grow away from the window and turn towards the room. For more exact investigation of heliotropic movements it is necessary to be able to control more accurately the source and direction Fic. 201.—Heliotropic curvature of a seedling of Galium Aparine, resulting from one-sided illumina- tion; in 1 the apex is ina line with the direction of the light, the leaves at right angles to it; in 2, with the illumination from the opposite direction, the same plant has quickly changed the position of its apex, while the cotyledons are only beginning to assume their heliotropic position. (Somewhat enlarged.) of the light. This can be best accomplished by placing the plants in a room or box, lighted from only one side by means of a narrow opening or by an artificial light. It then becomes apparent that the: direction of the incident rays of light determines the heliotropic position ; every alteration in the direction of the rays produces a change in the position of the heliotropic organs. The apical ends of positively heliotropic organs will be found to take up the same direction as that of the rays of light. The exactness with which this is done is illustrated by an experiment made with Pilobolus crystallinus. The sporangiophores of this Fungus are quickly produced on moist horse or cow dung. They are positively heliotropic, and turn their dark sporangia towards the source of light. When ripe these sporangia are shot away from the plant, and will be found thickly clustered about the centre of the glass covering a small aperture through which the light has been admitted ; a proof that the sporangiophores were all previously pointed exactly in that direction. Upon closer investigation of the manner in which the POSITIVE SECT. IT PHYSIOLOGY 253 HELIOTROPIC CURVATURE of an organ is accomplished, it is found THAT THE SIDE TURNED TOWARDS THE LIGHT GROWS MORE SLOWLY, THE SIDE AWAY FROM THE LIGHT MORE RAPIDLY THAN WHEN ILLUMINATED FROM ALL srpEs. This may be readily shown by previously marking with Indian ink regular intervals from one to two millimetres apart on the opposite sides of the organ. After the curvature has taken place the intervals between the marks will be found to be much farther apart on the shaded side than on the side turned to the light. As compared with the elongation under normal conditions of growth, the marks on the illuminated side have remained nearer together, while those on the shaded side have drawn farther apart ; that is, the growth in the case of a positive heliotropic curvature has been retarded on the illuminated side and promoted on the shaded side. It also becomes evident, from observation of the ink-marks, THAT CURVATURE TAKES PLACE ONLY IN THE PORTIONS OF STEMS— ‘ STILL IN PROCESS OF GROWTH, AND THAT THE CURVATURE IS GREATEST WHERE THE GROWTH IS MOST VIGOROUS (Fig. 201). The curvature is then only a result of unequal growth induced by one-sided illumination. It was formerly believed that the increased growth of the shaded side was pro- duced by the beginning of etiolation, and that the diminished growth on the illumin- ated side was due to the retarding effect which light exerts upon growth in length (p. 234). Other heliotropic phenomena were found to be at variance with this explanation of heliotropism. Unicellular perfectly transparent Fungus hyphe are also subject to positive heliotropic curvature, although in this instance there can be no shaded side; on the contrary, the side of a hypha turned away from the light is especially illuminated on account of the refraction of the light rays. The fact, too, that negative heliotropic curvatures also take place renders it evident that heliotropism cannot be due to one-sided etiolation ; for in negative heliotropism the side most directly illuminated is the one that grows more rapidly, although the retarding effect of light on the normal growth in length of negatively heliotropic organs is equally operative (roots, rhizomorpha). It is evident from these considerations that it is not the difference in the intensity of the light which causes the heliotropic curvatures, but the direction in which the most intense light rays enter the organs. LIGHT ACTS AS A MOTORY STIMULUS WHEN IT PENETRATES AN ORGAN IN ANY OTHER DIRECTION THAN THAT WHICH CORRESPONDS WITH THE POSITION OF HELIOTROPIC EQUILIBRIUM. The heliotropic curvatures are most strongly produced, just, as in the case of the heliotactic movements of freely moving swarm- spores, by the blue and violet rays, while red and yellow light exerts only an extremely slight influence, or none at all. It is due to the fact that the red-yellow and blue-violet rays are always present together in daylight, that the heliotropism of the leaves is of advantage to their assimilatory activity. Sensibility to heliotropic influences is prevalent throughout the vegetable kingdom. Even organs like the roots of trees, which are 254 BOTANY - PART I 2 never under ordinary circumstances exposed to the light, often exhibit heliotropic irritability. Positive heliotropism is the rule with aerial vegetative axes. Negative heliotropism is much less frequent; it is observed in aerial roots, and sometimes also in climbing roots (Ivy, Ficus stipulata, Begonia scandens), in the hypocotyl of germinating Mistletoe, in many, but not all, earth roots (Sinapis, Helianthus), in tendrils (chiefly in those with haptera or holdfasts), and in the stems of some tendril-climbers. By means of their negative heliotropic character, the organs for climbing and attachment turn from the light towards their support, and are pressed firmly against it.. Negative heliotropic curvatures are occasionally produced, not in the region of most rapid growth, but in the older and more slowly growing portions of the stem. The stems of Tropacolum majus, for example, exhibit positive heliotropic curvatures in the region of their greatest elongation ; but lower down the stems, with the retardation of their growth, they become negatively heliotropic. TRANSVERSE HELIOTROPISM is confined almost solely to leaves and leaf-like assimilatory organs, such as Fern prothallia and the thalli of Liverworts and Algae. In these organs transverse heliotropism, in conformity with its great utility for assimilating, predominates over all other motory stimuli. Thus it is possible to cause the leaf-blades of a Maiva or a Tropaeolum to turn completely over by illuminating their under surfaces by means of a mirror. The leaf-blades themselves, and also the thalli of the Cryptogams, are capable of carrying on transversely heliotropic movements, while the movements of the growing portions of leaf-stalks seem to be influenced by their leaf- blades. In too bright light the transverse position of the leaves becomes changed to a position more or less in a line with the direction of the more intense light rays. In assuming a more perpendicular position to avoid the direct rays of the mid-day * sun, the leaf-blades of Lactuca Scariola and the North American Silphiwm lacinia- twm necessarily take the direction of north and south, and so are often referred to as COMPASS PLANTs. (As regards the vertical position of phyllodes, in connection with which may be mentioned the vertically-placed leaves of many Myrtaceae and Proteaceae, see p. 195.) The heliotropic character of organs may change through the activity of external influences, and also at different stages of their development and growth ; just as in the case of the heliotactic swarm-spores, the higher plants in ordinary light may be positively, and in very intense illumination negatively heliotropic. The youngest portion of the shoots of Ivy and T'ropacolum are positively heliotropic, while the lower and older portions turn away from the light. The flower-stalks of Linaria cymbalaria are at first positively heliotropic. After pollination, how- ever, they become negatively heliotropic, and as they elongate they push their fruits into the crevices of the walls and rocks on which the plant grows, and thus assure the lodgment of the seeds and the possibility of their future germination. SECT. IE PHYSIOLOGY 255 B. Geotropism That the stems of trees and other plants should grow upwards and their roots downwards, is such a familiar occurrence and so necessary for the performance of their respective functions as to seem almost a matter of course. Just as in the discovery of gravitation, it required an especially keen spirit of inquiry to lead to the investigation of this everyday phenomenon. ‘The fact that everywhere on the earth, even on the sides of the steepest mountains, stems take a perpendicular direction ; and that, while buried in the earth, this same direction is assumed with certainty by germinating seeds and growing shoots ; and chiefly the fact also that a shoot, when forced out of its upright position, curves energetically until it is again perpendicular, led to the supposition that the cause of these phenomena must be in a directive force proceeding from the earth itself. The correspondence in the behaviour of a stem in always assuming a perpendicular position, with the continued maintenance of the same direction by a plumb-line, suggested at once the force of gravitation, and the English investigator Knicut, in 1809, demonstrated that the attraction of gravitation, in fact, exerted an influence upon the direction of growth. As KNIGHT was not able to nullify the constantly operative influence of gravity upon plants aud so directly prove its influence, he submitted them to the action of centrifugal force—an accelerative force operating like gravity upon the masses of bodies, and which had, in addition, the advantage that it could be increased or diminished at will. KNicHT made use of rapidly rotating, vertical wheels, upon which he fastened plants and germinating seeds in various positions. The result of his experiments was that the stems all turned towards the centre of the wheel and the roots directly away from it. On wheels rotating in a horizontal plane, where, in addition to the centrifugal force, the one- sided action of gravitation was also still operative, the shoots and roots took a definite middle position ; the shoots and roots still grew in opposite directions, but their line of growth was inclined to the plane of rotation, at an angle dependent upon the rotating velocity. The position thus assumed was evidently the result of the combined action of the centrifugal force and gravity, which was manifested in the directions taken by the plants according to their comparative strength and respective influence on growth. In this way it was posi- tively ascertained that terrestrial gravitation determines the positions of plant organs in respect to the earth. Later, it was also shown that not only the perpendicular direction of stems and primary roots, but also the oblique or horizontal direction taken by lateral branches, roots, and rhizomes, is due to a. peculiar reaction towards the force of gravitation. The property of plants to assume a definite position with respect to the direction of gravitation is termed GEOTROPISM. It is customary 256 BOTANY PART I also, as in the case of heliotropism, to speak of positive and negative geotropism, diageotropism, and transverse geotropism, according to the position assumed by the plant or organ with respect to the centre of the earth. Still another form of geotropic irritability, lateral geotro- pism, renders possible the winding of stem-climbers. Negative Geotropism. — All vertically upward growing organs, whether stems, leaves (Liliiflorae), flower-stalks, parts of flowers, or roots (such as the respiratory roots of Avicennas, Palms, etc.), are negatively geotropic. In case such negatively geotropic organs are forced out of their upright position, they assume it again if still capable of growth. As in heliotropism, GEOTROPIC CURVATURE RESULTS FROM THE INCREASED GROWTH OF ONE SIDE AND THE RETARDED GROWTH OF THE OPPOSITE SIDE; and the region of greatest growth is, in general, also that of the greatest curvature. In negatively geotropic organs, growth is accelerated on the side towards the earth ; on the upper side it is retarded. In consequence of the unequal growth thus induced, the erection of the free-growing extremity is effected. After the upright position is again attained, the one-sided growth ceases and the organ continues to grow in an upward direction. The process of negative geotropic movement is dependent: (1) upon the vigour of the existing growth ; (2) upon the sensibility of the organ ; (3) upon the fact that the stimulus of gravity works most energetically when the apex of the ortho- tropic organ is removed about 135° from its position of geotropic equilibrium ; the more nearly the zone capable of curvature approaches this position, the stronger is the motory stimulus ; (4) and, also, upon the fact that after a stimulus has ceased to act upon » plant, the induced stimulation continues to produce so- called AFTER-EFFECTS, just as by a momentary stimulus of light an after-perception persists in the eye. These considerations determine the actual course of the directive movement of geotropism, which, as will be seen from the adjoining figure (Fig. 202), does not consist merely of a simple, continuous curvature. The numbers 1-16 show, dia- grammatically, different stages in the geotropic erection of a seedling grown in semi- darkness and placed in a horizontal position (No. 1). The growth in the stem of the seedling is strongest just below the cotyledons, and gradually decreases towards the base. The curvature begins accordingly close to the cotyledons, and proceeds gradually down the stem until it reaches the lower, no longer elongating, portions. Through the downward movement of the curvature, and partly also through the after-effect of the original stimulus, the apical extremity becomes bent out of the perpendicular (No. 7), and in this way a curvature in the opposite direction takes place. Thus, under the influence of the stimulus, the stem bends backwards and forwards, until, finally, the whole growing portion becomes erect and no longer subject to the one-sided action of the geotropic stimulus. (A good example of ex- cessive curvature beyond the vertical is afforded by vigorously growing aerial shoots of Hippuris vulgaris.) Analogous phenomena to those here described are exhibited in the case of all paratonic curvatures of growth. In case of a different distribution and rapidity of growth, or of the unequal sensitiveness, rigidity, or thickness of the organ, as well as in the case of a difference in its position at the commencement of the curvature, the process, as indicated in the figure, is corre- spondingly modified. SECT. IL PHYSIOLOGY Positive Geotropism, on the other hand, is observable in tap-roots, in many aerial roots, and in the leaf-sheaths of the cotyledons of many monocotyledons which penetrate the earth during germination. All these organs, when placed in any other position, assume a straight downward direction and afterwards maintain it. Formerly, it was believed that this re- sulted solely from their weight and the pliancy of their tissues. It is now known that this is not the case, and that posi- tive geotropic, like negatively geotropic movements, are possible only through growth. The power of a downward curving root-tip to penetrate mercury (specifically much the heavier), and to overcome the resistant pressure, much greater than its own weight, proves con- clusively that positive heliotropism is a manifestation of a vital process. Posi- tive geotropic curvature is due to the fact that THE GROWTH OF AN ORGAN IN LENGTH IS PROMOTED ON THE UPPER SIDE, AND RETARDED, EVEN MORE STRONGLY, ON THE SIDE TURNED TOWARDS THE EARTH. A young germinal root of Vicia Fuba, growing vertically, elongated equally on all sides 24 mm.; when placed _hori- zontally, it exhibited a growth of 28 mm. on the upper and of only 15 mm. on the lower side. A root of Castanea vescu, with a growth in a vertical direction of 20 mm., showed, in a horizontal position, a growth of the upper side of 28 mm. and of the under of only 9 mm In these experiments, by marking with Indian ink, the unequal elongation in the downward curvature may be demon- strated by the greater divergence of the marks on the upper than the lower side; it is also evident that, as in negative heliotropism, the curvature takes place in the region of greatest elongation (Fig. 203). As the portion 14 13 Fic. 202.—Different stages in the pro- cess of geotropic movement. The figures, 1-16, indicate successive stages in the geotropie curvature of a seedling grown in seimi-darkness ; at 1, placed horizontally ; at 16, ver- tical. For description of intermedi- ate stages, see text. (Diagrammatic. ) of a root capable of elongation is very short, no excessive over- s 258 BOTANY PART I curvature, as in the case of negatively geotropic stems, takes place. Diageotropism.—Most lateral branches and roots of the first order are diageotropic, while branches and roots of a eS ei I higher order stand out from their parent organ in all directions. DIAGEOTROPIC ORGANS ARE ONLY IN A POSITION OF EQUILIBRIUM WHEN THEIR LONGITUDINAL AXES FORM A DEFINITE ANGLE WITH THE LINE OF THE ACTION OF GRAvITY. If forced from their normal inclina- tion they return to it by curving. A special instance of diageotropism is exhibited by strictly horizontal organs, such as rhizomes and stolons, which show a strictly TRANSVERSE GEO- TROPISM, and, if removed from their normal position, their growing tips always return to the horizontal. A more complex form of geotropic orientation is manifested by dorsiventral organs. These, in contrast to radial organs, such as most Fic, 203.—Geotropic cuva- roots and stems, are not developed on a uni- ait P elaedl form plan on all sides, but show two usually horizontally; I, after externally perceptible different sides—a dorsal seven hours; II, after and a ventral side. The foliage leaves of twenty-three hours; Z, a fe a fixed index, (After Sacus.) Most dicotyledons and zygomorphic flowers (Antirrhinum, Aconitum, ete.) furnish pronounced examples of dorsiventral structure. All such dorsiventral organs, just as radial organs that are diageotropic, form a definite angle with the direction of gravity, but are only in equilibrium when the dorsal side is uppermost. If, in spite of the proper inclination of the longitudinal axis, the dorsal side should lie underneath, it elongates until it comes back again into a dorsal position. A state of torsion often results from the orientation movements of dorsiventral organs to recover from abnormal positions. Similarly, a torsion must also, of neces- sity, occur when a geotropic organ, which has become curved over toward its parent axis, turns itself about so as to face outwards (ExoTROPISM). The rotation of the ovaries of many Orchidaceae, of the flowers of the Lobeliaceac, of the leaf-stalks on all hanging or oblique branches, of the originally reversed leaves (with the palisade parenchyma on the under side) of the Alstroemeriac, and of Allium ursinwm, all afford familiar examples of torsion regularly occurring in the process of orientation. Stem - Climbers. —In addition to the better-known forms of geotropism already mentioned, stem-climbers exhibit a peculiar and only recently recognised geotropic movement, by means of which they are enabled to twine about upright supports. This movement depends upon the geotropic promotion of the growth of one side (not, as in negative or positive geotropism, of the upper or lower portions). Thus a geotropic curvature in a horizontal plane is produced (LATERAL GEO- SECT. IL PHYSIOLOGY 259 TROPISM), resulting in a revolving motion of the shoot apex. Stem- climbers occur in very different plant families ; and although an upward growth is essential to their full development, which they do not attain if left on the ground, their stems are not able of themselves to main- tain an erect position. The erect stems of other plants, which often secure their own rigidity only through great expenditure of assimilated material, are made use of by stem-climbers as supports on which to spread out their assimilatory organs in the free air and light. The utili- sation of a support produced by the assimilatory activity of other plants is a peculiarity they possess in common with other climbers, such as tendril- and root-climbers. Unlike them, however, the stem-climbers ac- complish their purpose, not through the use of lateral clinging organs, but by the capacity of their main stems to twine about a support. The first internodes of young stem-climbers, as a rule, stand erect. By further growth the free end curves energetically to one side, and assumes a diageotropic, more or less oblique or horizontal position. At the same time the inclined apex begins to revolve in a circle either to the right or to the left. This is the movement which it has been customary to speak of as “revolving nutation,’ but which it is better to term REVOLVING MOVEMENT. The expression “ nutation” is not in this case correct, as by it are understood autonomic movements; while THE REVOLVING MOVEMENTS OF STEM-CLIMBERS RESULT FROM THE EX- TERNAL SIMULUS OF GEOTROPISM, which causes a promotion of growth in either the right or left side of the young internodes of the inclined shoot apex. As a result of this, a movement towards the other side is induced. On account of the direct connection of the apex of the shoot with the lower erect internodes, this revolving movement necessarily gives rise to a similar rotation of the revolving apex on its longitudinal axis. Through this rotation the torsion, which would otherwise be produced by the revolving movement of the inclined tip of the shoot, is released. (This process will at once become apparent by imitating the movement with a rubber tube.) Thus the apex of a stem-climber sweeps round in a circle like the hand of a watch, and rotates at the same time like the axle to which the hand is attached. Through this rotation of the shoot apex, the part of the stem subjected to the action of the lateral geotropism is constantly changing ; and the revolv- ing movement once begun, must continue, as no position of equilibrium can be attained. Without the constant and unchanging action of gravitation in determining the direction of the revolving movement, the twining of a shoot continuously about a support is hardly conceivable. It is accordingly not without reason that the re- volving movement is a continuous, fixed, geotropic movement, and not an autonomic nutation without definite directive force. Lateral geotropism is a physiological requisite for the climbing, and the existence of stem-climbers as such_is dependent upon this peculiar form of geotropism. To this dependence, however, is also due the fact that stem-climbers can only twine about upright or slightly inclined sup- 260 BOTANY PART I ports. This is, it is true, a limitation to their power of climbing, but one which is not without advantage, for the plauts are thus constrained to ascend to freer light and air. When an upright support occurs anywhere in the immediate neigh- bourhood of the apex of a climbing shoot it is sure to be discovered. The apical extremity, of which the movement is but little disturbed by the leaves, which remain for a long time undeveloped, is forced by its lateral geotropism against the support, and byits next revolutions twines around it. If the support be thin, the coils, at first almost horizontal, are only loosely wound about it. Later they become more spiral, and so wind more tightly. This is accomplished by the ulti- mate predominance of negative geotropism in the coiled portions of the stem, which tends con- tinually to draw out the coils and make the stems upright. This action of negative geotro- pism is well shown in the case of shoots which have formed free coils without a support (Fig. 204). By the resistance offered by the supports to the complete elongation of the spiral stems, the shoots are held firmly in position. In many twining plants the roughness of their surfaces (due to hairs, bristles, hooks, furrows) also Fic. 204.—Free coils formed ‘ : i mar by a shoot of Ipomoca pur- ASSists in preventing the shoots sliding down purec. (From Detver’s their supports. The autonomic torsion arising hysiol, Pract. : : : Ee ae in the older portions of the stems is also of assistance in holding climbing plants, especially those with furrowed stems, tightly wound about their supports. The twining of stem- climbers, as well as the attachment to their supports, is due to geo- tropic processes of growth, and not, as in tendril-climbers, to contact stimuli. In addition to the autonomic torsions, a torsion from purely mechanical causes is necessarily manifested in the elongation of the coils of a twining stem which are at first nearly horizontal, so far at least as it is not equalised by the free movement of the apex. (To make this form of torsion apparent, it is only necessary to hold firmly the inner end of a horizontal coil of rubber tubing, and draw out the other end until the tube is straight. Ifa mark has previously been drawn along one side, say the convex side of the tube, its position, after the tube has been extended, will show clearly the actual torsion that has taken place.) From their manner of winding, stem-climbers can twine only around slender or, at the most, moderately thick supports. Here again is a limitation to their powers of climbing; but in this instance also the limitation has its advantages, for by climbing the trunks of large shade trees, these plants, which require the unobstructed light, would be placed in an unfavourable position. SECT, IL PHYSIOLOGY 261 The direction of the revolving movements, and accordingly also of the windings, of most stem-climbers is constant. The twining stems are for the most part SINISTRORSE (Convolwulus, Phaseolus, Pharbitis, etc.). Seen from above, the windings run from the north towards the west ; that is just the reverse of the movement of the hands of a watch. Viewed from the side, the windings ascend the support from the left below to the right above (Fig. 205). Drxrrorsr stem-climbers with Fic, 205.—A sinistrorse stem-climber, Fic. 206.—A dextrorse stem-climber, Pharbitis hispida. The upper Myrsiphyllum asparagoides. The leaves remain small for a long short lateral shoots have de- time. veloped phyllocladia. windings from east to west occur less frequently (Hop, Honeysuckle, Polygonum Convolvulus, etc.). In the example chosen for illustration (Myrsiphyllum asparagoides, Fig. 206) the undeveloped condition of the lateral members in comparison with the elongated internodes of the stem is very apparent. A very few plants, such as Blumenbachia lateritia, Hibbertia dentata, and Scyphantus, seem able to climb equally well either to the right or to the left. A. similar irregularity is shown in Solanum Dulcamara, which, however, rarely winds, and then only under special circumstances. 262 BOTANY 4 PART I When the apex of a sinistrorse shoot points towards the north, it is the east side of which the growth is promoted by geotropism ; in dextrorse climbers, on the contrary, the growth of the west side is more rapid. That the same stimulus affects in different plants the growth of opposite sides, may be explained by the difference in the arrangement of their irritable structures (through their reversed position) within the organ. From the fact that the promotion of growth occurs always on the same side, it will be apparent that the apex of an inverted twining stem must unwind from its support. (Concerning the behaviour of stem-climbers on the Klinostat, compare p. 264.) Curvature of Grass-Haulms.—All the examples of geotropic movements, so far observed, took place in the growing portions of plants, and whether occurring in unicellular or multicellular organs, were due to a disturbance of the course of growth. A curvature even of lignified twigs can also be produced by the one-sided stronger growth of the cambium and of the young secondary tissues. Even many-year-old branches of Conifers are all able, although slowly, to exhibit geotropic curvatures. THE NODES OF GRASSES SHOW THAT RESTING "TISSUES ALSO CAN BE EXCITED TO NEW GROWTH BY THE STIMULUS OF GRAVITATION. The knot-like swellings on the haulms of the Grasses are not nodes in a morphological sense, but are cushion-like thickenings of the leaf-sheaths above their actual in- sertion on the shoot axis. The part of the stem thus enveloped is very tender and flexible. When a grass-haulm is laid horizontally, which not unfrequently occurs through the action of the wind or rain, the nodes will begin to exhibit an energetic growth on their lower sides. As the upper sides of the nodes take no part in the growth, but are instead frequently shortened through pressure and loss of water, knee-like curvatures are formed at the nodes, by means of which the haulm is again quickly brought to an erect position (Fig. 207). In this way laid corn is able to right itself. Similar curva- tures to those of the grass nodes may be produced in the true nodes of the grass- like Dianthi, and of the Polygonaceae (Poly- Fic. 207.—Geotropic erection of a gonum, Rumen) and Commelinaceae (Trades- grass-haulm by the curvature of cantia) anode. 1, Placed horizontally, s s 4 both sides Gu, 0). of -the wiode Modifications in the character of the geo- being of equal length; 2, the tropism, as of the heliotropism, of an organ ue ~ nee may be occasioned by the operation of in- ened; asaresult ofthecurvature ternal as well as external influences. Such the grass-haulm has been raised changes in their geotropic position frequently a a macs occur, as VOCHTING has demonstrated, dur- ing the development of flower-buds, flowers, and fruits (buds and flowers of Papaver, flowers and fruits of Aguilegia, Delphinium, Aconitum, and in the burial of the fruit of 7rifolium subterraneum, Arachis hypogaea, etc.). SECT. II PHYSIOLOGY 263 Of the changes in the geotropic conditions of plant organs due to external causes, those are particularly noticeable which result from a failure of a sufficient supply of oxygen, by which, for example, roots and rhizomes are made negatively geotropic. And even more im- portant are the modifications arising from the action of light, by which the geotropic irritability of rhizomes and foliage leaves may be so modified or weakened as to permit of more advantageous heliotropic positions. C. Hydrotropism, Culoritropism, Thermotropism, ete. Whenever any external force or substance is important to the vital activity of a plant or any of its organs, there will also be found to be developed a corresponding irritability to their influences. Roots in dry soil are diverted to more favourable positions by the presence of greater quantities of moisture. The force of this POSITIVE HYDRO- TROPISM may be so great as to overcome the geotropic equilibrium of the roots, and thus give rise to hydrotropic curvatures. Conversely, the sporophores of many mould Fungi avoid moisture. To this property is due the fact, so advantageous for the distribution of the spores, that their sporangiophores grow directly away from a moist substratum. Corresponding to the chemotactic irritability of Bacteria and spermato- zoids, roots, fungus hyphe, and pollen tubes exhibit positive and negative CHEMOTROPIC CURVATURES. These vary according to the concentration of the solution, so that an attractive substance, at a higher concentration, may act repulsively. THERMOTROPISM (due to the stimulus of heat), RHEOTROPISM (occasioned by the direction of water currents), and AEROTROPISM, a form of chemotropism, are additional phenomena, which have been distinguished as arising from the special action of external stimuli, and which stand in direct rela- tions to certain vital requirements of plants. In the case of ELECTROPISM, which has also been demonstrated in plants, no such essential relations have been discovered ; the disposition of plant organs in a direction contrary to that of an electric current, seems in no way to affect their growth. The fact of the existence of electropism in plants shows clearly that an irritability may be present, from which no direct benefit is ordinarily derived, and which accordingly could not have been attained by natural selection. D. The Method of Slow Rotation—The Klinostat All the curvatures of growth previously discussed have been in- duced by the one-sided action of stimuli, the source of which determined the direction of the movements as well as the position of equilibrium. An influence operating equally on all sides is unable to produce a curvature in an organ of which the irritability is equally developed on all sides. In like manner no curvatures can take place when the plant is 264 BOTANY PART I uniformly rotated, with a velocity sufficient to preclude the continuous operation of a stimulus on any one point long enough to occasion a one-sided growth. As in that case, no one side will be exclusively acted upon, but the growth of all will be equally promoted or retarded ; the action of external influences, although exerted in only one direction, will be equalised. On this account the “method of slow rotation,” originally instituted by SACHS, is of great assistance in the observation and investigation of the phenomena of movements. By means of it, heliotropic movements due to one-sided illumination may be prevented without the necessity for either exposing the plants to the injurious effects of continued darkness, or providing for an equal illumination on all sides. This method is, moreover, of especial value in investigating the movements due to the action of gravitation, for it is not possible to exclude its influence, as it is those arising from light, definite temperature, oxygen, etc. WHEN PLANTS ARE SLOWLY ROTATED ON A HORIZONTAL AXIS, THE ONE-SIDED ACTION OF GRAVITATION IS ELIMINATED AND GEOTROPIC CURVATURE IS THUS PREVENTED in organs which react equally on all sides. The rotations are best produced by the KLINOSTAT, an instrument by means of which an exactly horizontal axis is rotated by clock-work. That geotropic curvatures of radial organs are, in fact, precluded by means of the klinostat, furnishes a re- markable corroboration -of the result of KNIGHT’s experiments, and may also be regarded as a further proof that such curvatures are due to terrestrial gravitation. Through the equalisation of the action of external directive influences, radial portions of plants exhibit on the klinostat only such movements as arise from internal causes. The most important of these autonomic movements are those resulting in epinastic and hyponastic curvatures (p. 249), and the retrogression of recently formed paratonic curvatures through longitudinal extension (autotropism). Such autonomic movements should not be confused with those exhibited by dorsiventral organs on the klinostat, in consequence of the unequal irritability of their different sides. Through the special irritability of the dorsal side (p. 258) of foliage leaves and zygomorphic flowers, it is during their rotation more strongly acted upon by geotropic influence than the ventral side; as a result of4his curva- tures are produced which so closely resemble those resulting from epinasty that they were for a long time actually considered as such. When stem-climbers are rotated on the klinostat, their revolving movement ceases, the part of the stem capable of growth unwinds and straightens, and afterward exhibits only irregular nutations. It is thus evident from their behaviour that their winding and par- ticularly their revolving movements are dependent upon geotropism. E. Curvatures induced by Contact Stimuli The protoplasm of plants, like that of animals, exhibits an irritability to contact, whether momentary or continuous. This is apparent, SECT. IT PHYSIOLOGY 265 not only from the behaviour of the naked protoplasmic bodies of spermatozoids, swarm-spores, plasmodia, and amcebe, but also from the reactions manifested by walled cells and by whole organs, the functions of which may be so disturbed by the action of mechanical stimuli that death ensues. The almost universal irritability of vegetable protoplasm to mechanical stimulation is utilised by a number of plants for the production of movements which lead to their ultimate attachment to the irritating body. Tendril-climbers, in particular, have developed this irritability to contact stimuli as a means of attaching themselves to supports ; and in that way are enabled to elevate their assimilating and also their reproductive organs into more favourable situations. In the case of twining plants which possess similar powers of climbing, the process of elevation, as has already been shown, is accomplished by means of the geotropic irritability of the stems themselves. In the case of tendril-climbers, on the contrary, the attachment to the support is effected, not by the main axis of the plant, but by lateral organs of different morphological character. These may either main- tain, at the same time, their normal character and functions, or, as is usually the case, become modified and as typical tendrils serve solely as climbing organs. The support operates, moreover, not as a hindrance to a movement previously induced, as in the case of stem- climbers, but itself produces curvature in the tendrils in consequence of contact or friction. THE CONTACT OF A TENDRIL WITH A SOLID BODY ACTS UPON ITS GROWTH IN SUCH A WAY THAT THE ELONGATION OF THE CONTACT SIDE IS ARRESTED, WHILE THAT OF THE OPPOSITE SIDE IS PROMOTED. As a result of this, a sharp curvature of the tendril ensues, which coils it about the support. The more slender the tendrils and the stronger their growth, the more easily and quickly this process occurs. Through the tendency of the curvature to press the tendrils more and more firmly against the support, deep impressions are often made by them upon yielding bodies, soft stems, rubber tubing, etc. In the more typically developed tendrils the curvature does not remain restricted to the portions directly subjected to the action of the contact stimulus. Apart from the fact that, in the act of coiling, new portions of the tendrils are being continually brought into contact with the support and so acted upon by the stimulus, the stimulation to curvature is also transferred to the portions of the tendril not in contact with the support. Through the action of the propagated stimulus, not only is the free apex of the tendril turned more quickly around the support, but a tendency to curvature is imparted to the portion of the tendril between the support and the parent shoot. As it extends between two fixed points, this tendency causes it to coil spirally, like a corkscrew. With the spiral coiling, a torsion is produced, and, on account of the fixed position of the two end 266 BOTANY PART I points, it cannot be exerted in one direction only, the spiral, for purely mechanical reasons, coils partly to the left and partly to the right. PoInTs OF REVERSAL (a) a thus oceur in the windings which, in equal numbers to the right and to the left, equalise the torsion (Fig. 208). Through the spiral coiling of the tendrils the parent-stem is not only drawn closer to the support, but the tendrils themselves, through their consequent elasticity, are enabled to withstand the injurious effects of a sudden shock. Advantageous changes also take place in the ana- tomical structure of the ten- Fic. 208.-—Portion of a stem of Sicyos angulatus with drils after they are fastened tendril ; x, point of reversal. to the supports. The young tendrils, after their elongation, exhibit active nutations, and thus the probability of their finding a support is enhanced. During this time they remain soft and flexible, while the turgor rigidity of their apices is maintained only by collenchyma. In this condition they are easily ruptured, and have but little sustaining capacity. As soon, however, as a support is grasped, the coiled-up portion of the tendrils thickens and hardens, while the other part lignifies, and becomes so strengthened by sclerenchymatous formations that the tendrils can, finally sustain a strain of many pounds. When the tendrils do not find a support. they usually dry up and fall off, but in some cases they first coil them- selves into a spiral. The tendrils of many plants (Colaca, Cissus) are irritable on all sides ; others, on the contrary, on only the lower side (tendrils of Cucwrbitaceae and others with hooked tips) ; while others possess extremely sensitive shoots (J/utisia). In some cases the tendrils quickly coil themselves to the support, but others coil more slowly (Passiflora, Sicyos, Bryonia); while in other tendrils the supports are very slowly grasped (Sinrilax, Vitis). According to PrEFFER’S investigations, it is of great importance to the tendrils in the performance of their functions that they are not induced to coil by every touch, but only through CONTACT WITH THE UNEVEN SURFACE OF SOLID BODIES (as thus adjacent cells become unequally affected). Rain-drops consequently never act as a contact stimulus ; and even the shock of a continued fall of mercury produces no stimulation. Tendril-climbers are not, like twining plants, restricted SECT. IL PHYSIOLOGY 267 to nearly vertical supports, although, on account of the manner in which the tendrils coil, they can grasp only slender supports. A few tendril- climbers are even able to attach themselves to smooth walls. Their tendrils are then negatively heliotropic, and provided at their apices with small cushion-like outgrowths, which may either develop independently on the young tendrils (1mpelopsis Ieitehii), or are first called forth by contact irritation (Ampelopsis hederacea). Through their sticky ex- cretions these cushions become fastened to the wall and then grow into disc-like suckers, the cells of which come into such close contact with the sup- porting wall that it is easier to break the lignified tendrils than to separate the hold- fasts from the wall. Fig. 209 represents the tendrils of Ampelopsis Veitchit. (Vitis in- constans). The suckers occur on its young tendrils in the form of knots. In Ampe- lopsis hederacea the suckers are only pro- duced as the result of contact, and the tendrils of this plant require thin supports. Sometimes, as in the case of Lopho- spermum scandens (Fig. 210), the leaf-stalks, although bearing normal leaf-blades, be- come irritable to contact stimuli and function as tendrils. Of leaf-stalks which thus act as tendrils, good examples are afforded by Tropaeolum, Mauwrandia, Solanum jasminoides, Nepenthes, etc. The subsequent modifications occurring in more perfectly developed tendrils are not noticeable in the case of petiolar tendrils, although the ; coiling portion of the leaf-stalks of Solanwm Fre. 209.—Portion of a climbing 5 Secs dah + shoot of Ampelopsis Veitchii (Vitis jasminoides do become strongly thickened jreonstuns). The tendrils (R) and lignified; while the leaf-blades of have fastened themselves to a Clematis, by remaining small for a time, i wall by means of hold- enhance the tendril-like character of their a, leaf-stalks, and by bending backwards also assist in maintaining the initial contact with a support. At other times the midribs of the leaf- blades themselves become prolonged, and assume the function of tendrils (Gloriosa, Littonia, Flagellaria). In many species of F'umaria and Corydalis, in addition to the leaf-stalks, even the leaf-blades of the leaflets twine around slender supports, while the parasitic shoots of Cuscuta (Fig. 185) are adapted for both twining and climbing. F. Curvatures of growth due to Variations in Light and Temperature The flowers and foliage leaves of many plants exhibit the peculiarity that their different sides (the upper and under sides of foliage-leaves and 268 BOTANY PART I leaf-stalks, the inner and outer sides of floral leaves) show an unequal growth in response to even transitory and slight variations in tempera- ture and in the intensity of light. Whenever, on account of such variations, the growth of the under side of a leaf exceeds that of the upper side, the whole leaf moves upwards and towards the parent axis ; while if the growth of the upper side is the stronger, the leaf is depressed. Movements of this nature are especially noticeable in flower-leaves, Fic. 210.—Lophospermum scandens climbing by means of its tendril-like petioles. and bring about the opening and closing of the flower. A rise of temperature causes the flowers of the Tulip and Crocus, and also those of Adonis, Ornithogalum, and Colchicum, to open, while sudden cooling causes them to close. Tulips and Crocuses, if brought, while still closed, into a warm room, open in a very short time; with a dif- ference of temperature of 15°- 20° C., in from two to four minutes. Crocuses respond to an alteration in temperature of 4° C.; Tulips to a variation of 2°-3° C. In warm sunshine the spring or summer flowers are open for the visits of Fic. 211.—Composite flower of Leontodon hastilis, insects, but on a lowering of closed when kept in darkness, open when illu- temperature the sexual organs inated. (From DETMER’ iol. Pract, ininated. (From DETmMER’s Physiol. Pract.) are covered up and protected. The stronger growth of one side occurs in this case either at the base or upper part (Colchicum) of the perianth leaves. SECT, II PHYSIOLOGY 269 The composite flowers of Zaraxacum, Leontodon, and other Com- posites, also the flowers of Nymphaca, Cacti, etc., open when illuminated, and close when kept in darkness (Fig. 211). Variation of light produces also unequal growth in foliage-leaves, particularly in those of the Chenopodiaceae, Curyophyllaceae, and Balsaminaceae, and cause them to assume so-called SLEEP POSITIONS. In many instances the movements of the floral leaves are produced by varia- tions of light as well as of temperature ; for example, the flowers of the Tulip and Crocus open in the light and close in the dark, although the temperature remains constant. In the case of opposing external influences, the resulting direction of the movement of the flower-leaves is determined by the one which is predominant. The dependence of these movements upon different, and often opposing, influences, together with the continuance of movements induced by previously operative influences (after-effects, pp. 256, 272), was for a long time a difficult problem, and obscured the discovery of their true cause. These movements, occasioned by variations in the illumination and temperature, must not be confused with those of heliotropism and thermotropism ; in both of which the movement induced in an organ is dependent upon its relative position with respect to the source of the light or heat, and not upon the varying intensity of the stimulus. 3. Movements due to Changes of Turgor (Movements of Irritability) The various movements hitherto considered are, to a large extent, the result of the action of forces acting on growth. These move- ments were therefore confined to organs, or parts of organs, still in a state of growth. In contrast to the almost universal immobility of all fully-grown organs, it is particularly interesting to find that some plants have found a means of carrying on vigorous movements without the assistance of growth. It has already been shown (p. 166) that through the pressure of increasing turgidity the elastic cell walls become greatly distended and the cell cavity largely expanded, while, on the other hand, the cell walls shrink and the cell becomes smaller when the turgor is diminished (Fig. 167). It is to these changes in volume, which thus result from alterations in turgor, that the varying movements of fully-developed living organs are due. Such variation movements occur only in leaves (foliage and flower leaves). These movements are especially noticeable in the compound leaves of the Leguminosae and Ovalideae, and also in the leaflets of Aursilia (a water-fern). In the motile regions of these leaves special masses of tissue are, both physiologically and anatomically, adapted for the promotion of this form of movement. This tissue appears externally as a firm cushion or PULVINUS, sharply distin- guished from the rest of the leaf-stalk, and is the direct cause of the leaf move- 270 BOTANY PART I ments. Anatomically considered, the pulvinus consists, for the most part, of strongly turgescent parenchyma with very elastic cell walls. The vascular bundles and mechanical elements, which, in other portions of the leaf-stalk, have an approximately circular arrangement, unite in the pulvinus in the form of a single flexible strand, and so offer little opposition to the movement of the leaf resulting from the curvature of the motile region (cf. Fig. 165, 4). The parenchyma of the pulvinus forms a thick enveloping layer about this axial strand, by means of which, through the pressure arising from a difference in the turgescence of its opposite sides, a movement of the whole leaf-blade is brought about, similar to that of the outspread hand by the motion of the wrist. These variation movements are either autonomic, when the variations of turgor are due to no recognisable external influence, or paratonic, when the turgor is regulated in a definite way by the action of external stimuli. Autonomie Variation Movements.—A remarkable example of this form of movement is furnished by the small lateral leaflets of Desmodium (Hedysarwm) gyrans, a papilionaceous plant growing in the damp Ganges plains. In a moist, warm atmosphere (22°-25°) these leaflets make circling movements which are in no way disturbed by variations in the intensity of the light, and which are of such rapidity that their tips describe a complete circle in 1-3 minutes. The auto- nomic variation movements of Trifolium and Oxalis take place, on the contrary, only in darkness. Thus the terminal leaflets of 7rifoliwm pratense exhibit oscillatory movements in the dark with an amplitude which may exceed 120°, and are regularly repeated in periods of 2-4 hours ; but on exposure to light the leaflets cease their oscillations and assume a fixed light position. Paratonie Variation Movements are chiefly induced by variation in the intensity of the light, by the stimulus of gravitation, and by mechanical irritation (shock, friction), and also, but more rarely, by variations of temperature. The pulvini of leaves may be affected by several different stimuli; the leaves of Mimosa pudica, for example, are set in motion by the action of light, and also by the stimulus of a shock, and in addition, exhibit autonomic movements. A change from light to darkness, as from day to night, occasions NYCTITROPIC MOVEMENTS, or the so-called SLEEP MOVEMENTS. In the day or light position, which is the same as that of diaheliotropic foliage- leaves, the leaf-blades are perpendicular to the incident rays of light. With the commencement of darkness the leaves or the single leaflets fold either upwards with their upper surfaces inward, or downwards with their lower surfaces together, and so remain until the diurnal position is again assumed on recurring illumination. The turgor of the whole motile organ of the Bean, for instance, increases with darkness, but in the upper half more (4-5 atmospheres) than in the lower; while the turgor of the motile organ is decreased by illumination, the upper half in particular loses the rigidity acquired by the tissue- SECT, II PHYSIOLOGY 271 tension, and, in consequence of the resulting superior pressure of the lower half, the leaf is raised again to its diurnal position. According to Darwin, the leaves are protected from too great a loss of heat by radiation by the assumption of the nocturnal position. This loss of heat.may sometimes be very considerable, so much indeed, that nyctitropic leaves, forcibly retained in their diurnal position, were frozen, while adjacent ‘‘sleeping” leaves sustained the night temperature without injury. As sleep movements are also manifested by plants growing in tropical climates, where no injurious nocturnal diminution of temperature occurs, the advantage accruing from the sleep position in the previous instance is not explanatory of the nyctitropic behaviour of leaves in all cases. Sleep movements are particularly noticeable in Phaseolus, Trifolium, Robinia, Acacia lophantha, -lmicia zygomeris (Fig. 212), Mimosa pudica, ete. Too intense light frequently causes the change from the diurnal Fic. 212.—Amicia zygomeris, showing diurnal and nocturnal position of leaves. position, and a movement either towards or away from the nocturnal position. The leaflets of the common Locust (Robinia pseudacacia) are folded downwards at night. In ordinary diffuse daylight they assume their diurnal, outspread position ; but, if exposed to the direct rays of the mid-day sun, they turn obliquely upwards. In many plants ALTERATIONS IN THE INTENSITY OF THE LIGHT CHANGE THE GEOTROPISM OF THE MOTILE ORGANS; the sleep movements are then accomplished by the help of geotropic variation movements (Phaseolus, Lupinus). The change from the diurnal to the nocturnal position continues for a time to take place, even in constant darkness or prolonged illumination. The leaves themselves seem to have a tendency to pass at regular intervals from one condition to the other. The daily periods are the result of the stimulus imparted by the light, the periodic action of which induces the regular changes of position. If, however, the external stimulus ceases to operate, the internal disposi- 272 BOTANY PARI I tion still continues for some time to give rise to visible after-effects (pp. 256, 272), until finally, from the abnormal conditions, an abnormal state of rigour (light rigour, dark rigour) and symptoms of disease are manifested. Only a few plants respond with pronounced variation movements to mechanical irritation (shock, friction, injury). Formerly, these alone were considered irritable plants, as in the vegetable kingdom only the apparent mechanical irritations, from which visible movements resulted, were then regarded as stimuli. Of irritable plants in this sense, mention has already been made of Dionaea muscypula (p. 215), whose leaves when touched on the upper side, especially if the bristles are disturbed, fold together. ‘lhe most Fic. 213.—AMimosa pudica, with leaves in normal, diurnal position; to the right, in the position assumed on stimulation : B, flowers. familiar example of this irritability to mechanical stimuli is furnished by Mimosa pudicu, a tropical leguminous shrubby plant, which owes its name of sensitive plant to its extreme sensitiveness to contact. The leaves of this plant are doubly compound (Fig. 213). The four secondary leaf-stalks, to which thickly-crowded leaflets are attached left and right, are articulated by well-developed pulvini with the primary leaf-stalks; while they, in turn, as well as the leaflets, are similarly provided with motile organs. ‘Thus all these different parts are capable of independent movement, and the appearance of the entire leaf becomes, in consequence, greatly modified. In their unirritated, light position (Fig. 213, on the left) the leaf-stalk is directed obliquely upwards, while the secondary petioles with their leaflets are extended almost in one plane. Upon any vibration of the SECT, II PHYSIOLOGY 273 leaf, in favourable conditions of temperature (25°-30° C.) and moisture, all its parts perform rapid movements. The leaflets fold together, and, at the same time, move forward, the secondary petioles lay themselves laterally together, while the primary leaf-stalk sinks downwards (Fig. 213, on the right). Leaves thus affected, if left undisturbed, soon resume their former position. The behaviour of the leaves is still more marvellous when only a few of the leaflets are acted upon by the stimulus. This is easily demonstrated by holding a burning match near the leaflets of one of the pinne. The leaflets directly affected by the Hame fold quickly upwards, and this movement is performed successively by each pair of leaflets of the pinna until the articulation with the primary leaf-stalk isreached. The stimulation is then conveyed to the other pinne, the leaflets of which go through the same movement in a reverse order ; finally, the secondary petioles themselves draw together. Suddenly, when the whole process seems apparently finished, the main leaf-stalk in turn makes a downward movement. From this leaf the stimulus is able to travel still further through the stem, and it may thus induce movement in leaves 50 cm. distant. The movements of the pulvini are due solely to differences in turgidity. It has been observed that a sudden escape of water into the intercellular spaces takes place out of the cells of the lower or irritable side of the pulvinus of the primary leaf-stalk. According to the recent investigations of HABERLANDT, the conduction of the stimulus does not appear to be accomplished by the movement of the water thus discharged, but by the mucilaginous contents of sacs which are situated in the phloem portion of the vascular bundles, and which are easily affected by variations in the hydrostatic pressure. The position of an irritated leaf resembles externally its sleep or nocturnal position, but in reality the turgor tension of the pulvinate motory organ is different. IN THE NOCTURNAL POSITION THE TURGOR IN THE DIFFERENT SIDES OF THE PULVINUS IS UNEQUALLY INCREASED, and its rigidity, as a whole, is therefore increased ; in the position assumed after a shock the turgidity of the upper and lower sides is UNEQUALLY DIMINISHED, and as a result of this process the pulvinus loses its rigidity. Robinia, Oxalis acetosella and Biophytwm (Oxalideae) exhibit similar, although less active, movements, under the influence of mechanical stimuli. The state of rigour sometimes occurring in motile organs may also be best observed in Mimosa, for, although so sensitive to the action of external influences, it does not exhibit its irritable movements at all times. Whenever the temperature of the surrounding air falls below a certain degree, no movements take place and the whole plant passes into a condition known as COLD RIGOUR, while, on the other hand, at a temperature of about 40°, HEAT RIGOUR occurs. DROUGHT RIGOUR is induced, just before wilting, by an insufficient supply of water, and a DARK RIGOUR by a prolonged retention in darkness. T 274 BOTANY PART I In a vacuum, or on exposure to hydrogen and other gases—chloroform vapour, etc.—-movement also ceases, partly on account of insufficient oxygen, and partly from the actual poisonous action of the gases them- selves. If the state of rigour is not continued too long, the original irrita- bility will again return on the restora- tion of normal conditions. The movements of irritability exhibited by the staminal leaves of some Berberidaceae (Berberis, Mahonia) and Compositae (Cynareae and Liguliflorac) bear a certain relation to those of foliage leaves. The bow- shaped filaments of the stamens of the Compositae straighten upon mechanical irritation. As they frequently contract 10-20 per cent of their length, the style becomes extended beyond the anther-tube (Fig. 214). The reduction in the length of the filaments is accompanied by a moderate increase in their thickness, due to the elastic contraction of the cell walls, and the consequent expulsion : : § of water into the intercellular spaces. The jacea with perianth removed : 4, sta- : ° mens in normal position; B, stamens stamens of Berberis and Mahonia are only contracted ; c, lower part of tubular Sensitive to contact on the inner side near perianth; s, stamens; «, anther-tube; their base, and as their contraction occurs g, style; P, pollen. (After PrerFeR, only on the inner side, the anthers are thus culareed:) brought into contact with the stigma. Ex- amples of variation movements of carpellary leaves may be seen in the flowers of Mimulus, Strobilanthes (Goldfussia), Martynia, Torenta, and other plants. The two lobes of the styles of these flowers fold together when irritated. Similarly, in the flowers of Stylidiwm, a sudden upward movement of the bent style occurs when it is irritated by a touch ; but the style then loses its sensitiveness. Fic. 214.—A single flower of Centaurea VI. Reproduction The life of every plant is of limited duration. Death ensues, sooner or later, and the decayed remains form a part of the surface soil. All existing plants are descended from ancestral forms. A spontaneous generation of new organisms from lifeless matter does not, as far as experience teaches, take place, and all existing vegetable life owes its existence to the capacity inherent in all organisms of reproducing their kind. Reproduction is accordingly a vital power which must be exercised by every existing plant species. In special cases, it is true, abnormal forms, sports or monstrosities, are produced unlike their parent plants; but although they grow vigorously and develop a strong vitality, they have lost the capability of giving rise to equally strong descendants, or are unable to compete successfully SECT. II PHYSIOLOGY 275 with wild plants in the struggle for existence, and consequently would soon die out were they not protected and multiplied by artificial means. A great number of our cultivated plants belong to this class of artifici- ally maintained plant forms. It is also evident from the very nature of reproduction that in the production of new organisms a process of rejuvenation is continually being carried on. The formation of independently existing offspring necessitates also their separation from the parent plant. The formation of a new bud by a tree would never be distinguished as reproduction so long as the bud remained in connection with the tree as a part of its life. But if the bud became separated from the tree and continued its existence as an independent plant, that would constitute a form of reproduction, and, in fact, this actually takes place in many plants. The conditions of the outer world make the still further demand upon reproduction, that from it a multiplication of the species should result. As the germs after separation from the mother plant do not always find the conditions necessary for their development and so, for the most part, perish, the extinction of the whole species would soon result if a plant produced but a single germ. That in reproduction care is taken for the multiplication of the individual in an almost spendthrift manner, is shown by a consideration of the innumerable spores produced by a single mushroom, or by the thousands of seeds contained in the fruit capsule of an orchid. SEPARATION, REJUVENATION, and MULTIPLICATION of the individual are accordingly the essential requisites of reproduction. These requirements are fulfilled by plants in the most varied manner. ach great division of the vegetable kingdom has adopted its own special method; and each family and genus, or even the different species, are characterised by some peculiar feature of their manner of reproduction. Systematic botany is so essentially based upon the different development of the reproductive organs and their functions, that it consists for the greater part of special descriptions of the processes of reproduction in the vegetable kingdom. Numerous and varied as these processes are, they are in reality but modifications of two different and distinct modes of reproduction. The simpler of these, or VEGETATIVE REPRODUCTION, consists in the formation of cells or cell bodies which, after their separation from the parent plant without undergoing any further change, either germinate at once, or develop into new organisms after a period of rest. This mode of reproduction, in which the growth and development of the parent plant are directly continued, is also distinguished as MONO- GENETIC, VEGETATIVE, or ASEXUAL reproduction. In SExuAL REPRODUCTION, the second of the two modes of propa- gating vegetable life, two kinds of reproductive cells are first formed, but neither is directly capable of further development, and both perish in 276 BOTANY PART I a very short time, unless opportunity is given for their fusion with each other. Not until one cell (the female) has fully taken up and become inseparably united with the other cell (the male), does it acquire the capacity of development and growth. This mode of reproduction is designated SEXUAL or DIGENETIC reproduction. The physiological significance of sexual reproduction is not at once apparent. In many plants the vegetative mode of reproduction is sufficient to secure the necessary multiplication of the species, so that plants are able to continue without sexual reproduction. Many Fungi, for instance, are reproduced only vegetatively ; the cultivated Banana, many Dioscoreaceae, and varieties of the Grape, Orange, Straw- berry, no longer reproduce themselves sexually, but are propagated solely in a vegetative manner. The Garlic, which forms small bulbs in place of flowers, the White Lily, and Ranunculus Ficaria, which reproduces itself by root tubers, are hardly able to produce good seeds. They multiply exclusively by asexual methods without suffering any degeneration. Continued reproduction by vegetative means used to be regarded as necessarily injurious. Since monogenetic reproduction is sufficient for the preservation of the species, sexual reproduction must answer some purpose not attained by the vegetative mode of multiplication, for otherwise it would be altogether superfluous that the same plant, in addition to the vegetative, should also possess the sexual form of reproduction, which is so much more- complicated and less certain. Even the Mould Fungus (Mucor Mucedo), whose vegetative spores (conidia) are very widely distributed, occasionally develops sexual reproductive cells in specially formed sexual organs. In many of the lower plants (Algae and Fungi) it has been shown that the development of sexual cells is dependent upon definite external influences. KLEBS has demonstrated, in fact, that it is possible by regulation of the external conditions to induce the non-cellular Alga Vaucheria. to produce at will either non-sexual swarm-spores or sexual cells. In many plants unfavourable external conditions apparently give the impetus to a sexual mode of reproduction. The sexual product (zygospores of Algae) seems better able than the vegetative germs (swarm-spores of Algae) to remain a long time at rest, and so withstand the disastrous effects of an unfavourable environment. No inference can be drawn, however, from the function of the sexual germs in this instance concerning the necessity for the existence of a sexual, in addition to a vegetative, mode of reproduction; for in other cases it is the vegetative re- productive bodies, as, for example, the spores of Ferns and Horsetails, which are especially equipped for a period of enforced rest. What makes digenetic reproduction essentially different from monogenetic is the UNION OF THE SUBSTANCES OF THE PARENTS AND THE CONSEQUENT TRANSMISSION AND BLENDING OF THE PATERNAL AND MATERNAL PROPERTIES. As special care is almost always taken SECT. II PHYSIOLOGY 277 in sexual reproduction to ensure that the uniting cells have been developed from different individuals of the same species, an equal- ising influence is exerted which tends to maintain the permanence of the species as a whole. Any accidental variations in the form or properties of one individual of a species would, through crossing with others normally developed, disappear in the descendants, while the descendants by vegetative reproduction would retain them. A phenomenon of not infrequent occurrence, and one which shows, on the other hand, the persistency with which inherited attributes are retained in sexual reproduction, is the unexpected reappearance in the descendants of the attributes of former generations (ATAVISM). While, on the one hand, sexual reproduction tends to maintain the unchangeability of the form by abolishing isolated variations, on the other hand, variations may be confirmed in the descendants when they were similarly manifested by both parents. As a result of the union between individuals of different varieties, or species, or even of differ- ent genera (cf. Hybridisation, p. 289), offspring may be produced which, if not sterile, have a remarkable tendency to variation and so to the formation of new forms. It is in this influence exerted upon the quality that the chief difference between sexual and vegetative reproduction is shown. By VEGETATIVE REPRODUCTION THE QUANTITATIVE MULTIPLICATION OF THE INDIVIDUAL IS SECURED, WHILE BY SEXUAL REPRODUCTION A QUALITATIVE INFLUENCE IS EXERTED, which is of the greatest import- ance for the continued existence of the species. Sexual reproduction might therefore be spoken of as the QUALITATIVE reproduction of the species, and vegetative reproduction as the QUANTITATIVE repro- duction of the individual. The vegetatively produced progeny consist of unmixed descendants ; the sexually produced offspring, on the, other hand, are the result of a blending of the parents. Vegetative Reproduction Vegetative reproduction, the purely quantitative character of which as a mere process of multiplication has been emphasised, exists generally throughout the vegetable kingdom, and but few plants, some of the Conifers and Palms, are altogether devoid of it. Mention has already been made in considering artificial propagation (p. 228) that, from the separate parts or single cells, or even from the naked energides (Siphoneac) of many plants, the regeneration of a new and perfect individual may ensue. In vegetative reproduction the process is similar except that the separation of the part from the parent plant is an organic one, occurring in the natural course of development. The vegetative form of reproduction is manifested in various aspects, and may be distinguished as a multiplication by means of multicellular vegetative bodies (budding), or by single cells (spore-formation). 278 BOTANY PART I Multiplication by Multicellular Vegetative Bodies (Budding) often consists merely in the separation of lateral shoots, or in a division of a single plant into several. In this way the lateral shoots of the Water Fern, dzolla, through the death and disruption of the older parts of the parent axis, become separated from one another and con- tinue their growth as independent plants; similarly, separate plants originate from the vegetative body of the Duckweed (Lemna). Multiplication by stolons, rhizomes, and tubers results in a similar formation of independently existing plants. As may be seen in the Strawberry, Potato, Ranunculus repens, etc., the shoots produced from many of the axillary buds of the widely outstretched stolons take root and form new plants. In cases where the runners themselves eventually die, the parent plant becomes finally surrounded by a colony of entirely independent plants. Instead of forming runners, the single tuber may divide (Corydalis solida), and in this way give rise to two, four, or more new tubers. New bulbs are produced in the leaf-axils of the bud-scales of bulbs, while brood buds (bulbils, gemmz) are frequently developed on aerial vegetative organs. Bulbils are found on the inflorescence in the place of the flowers in many species of Allium, in the grass Pow bulbifera, and also in Polygonum viviparum. In Lilium bulbiferum; Dentaria bulbifera, etc., the bulbs in the axils of the leaves are specially constructed with a view to detachment from the parent plant. The swollen leaves contain reserve food material, and frequently develop roots before falling from the plant. In Ranunculus Ficaria the roots of the axillary buds are full of reserve food material, and resemble grains of corn. When the plant dies the bulbils remain on the ground, and have given rise to the fable of showers of grain. Bulbils or gemme are met with also among the Mosses, Liverworts, and Charas. The winter buds of many water-plants (Hydrocharis, Utricularia, Potamogeton crispus, Lemna, etc.) have a peculiar biological significance. They are formed in the autumn, and sink to the bottom of the water; in the succeeding spring they rise to the surface and form new plants. In addition to the instances just cited, in which the vegetative reproductive bodies take their origin from points where lateral shoots are normally formed, they may also appear in places where no shoots are normally developed. Thus the adventitious formations often found on leaves, particularly on the leaf-blades, serve the purpose of reproduction. Just as the leaves of the Begonia, after they have been cut off, are able to give rise to new plants, in other cases the leaves possess this power while still growing on the parent plant. Some Ferns afford specially characteristic examples of this (Aspleniwm decussatum, A. Fabianum, A, bulbiferum, A. viviparum) ; adventitious buds are produced on their lamine, which develop into small rooted plants, which then fall off and complete their development (Fig? 215). The adventitious buds of Cystopteris bulbifera take the form of bulbils with small] swollen leaves. Adventitious plantlets are frequently formed also on the leaves of Cardamine pratensis, and Cardamine amara manifests a similar tendency. One of the best known examples of such adventi- tious formations is afforded by the leaves of the tropical Bryophyllwm, in whose marginal indentations the brood plantlets develop in great numbers. Gemme are abundantly produced on the thallus of many Hepaticae (Marchantia, Lunularia), SECT. II PHYSIOLOGY 279 and by their continuous growth the gemme capsules (Fig. 316, 0) are always kept well filled. A most remarkable instance of adventitious budding sometimes occurs, in which adventitious buds, which have arisen in the nucellus of the ovule, grow into the Fic, 215.—Asplenium Fabianum.'~ A young plant (7), with leaves and roots (W), has sprung from the leaf (JZ) of the older plant. embryo-sac, and there develop just as if they were embryos; examples of this phenomenon may be found in Evonymus, Citrus, Punkia (Fig. 216), Coclebogyne. Formerly it was thought that such a POLYEMBRYONY was due to the existence of numerous egg-cells in one embryo- sac; but more thorough investigation has shown, however, that it arises from the vegetative formation of ADVENTI- TIousGERMS, At the same time the egg- cell previously existing in the embryo- sac is able to continue its development after fertilisation, but is usually pre- vented from so doing by the adventi- tious or nucellar embryos. The seeds in such cases would no longer contain the pro- ducts of sexual reproduction, but would be degraded to organs of vegetative multi- plication. The adventitious germs in the polyembryonic seed are, however, so far dependent upon sexual reproduction, that for the most part they only attain Fic. 216.--Vegetative formation of embryos in their development in case fertilisation has Funkia ovata (Hosta coerulea) by the budding previously taken place ; but in Coedebo- of the nucellus ; , nucellus with cells in gyne, one of the Australian Buphorbiaccac, process of forming is rudiments (ae) of the of which usually only female specimens used peweuies soe eee : ens , 8 g are found in cultivation, the adventitious ally-produced embryo. (After STRASBURGER.) germs develop without the stimulus of fertilisation. This plant, accordingly, affords another example of ApocAMy, or of the substitution of a vegetative for a sexual mode of reproduction, such as occurs in different degrees in certain Ferns, Athyriwm filix femina var. cristatum, Aspidium Saleatum, Todea africana, and Pteris erctica. In the last-named example the sexual 280 BOTANY PART I organs are no longer formed, although the young plants arise, by a vegetative process of budding, from exactly the same part of the prothallium where the archegonia would have been developed. In the case of Aspidiwm filix mas var. cristatwm the apogamy seems to have resulted from cultivation. In a broad sense the develop- ment of bulbils in the place of flowers, in the species of Adliwm, might be con- sidered as an example of apogamy. PARTHENOGENESIS, or the development of an egg-cell without previous fertilisa- tion, might also be viewed as an instance of the same phenomenon in plants with more advanced sexual differentiation. In only one case, Chara crinita, has parthenogenesis been positively proven. The female plants of this species of Chara are widely distributed throughout Northern Europe, and develop normal plants from their egg-cells, although the male plants are found only in Asia and in South Europe, so that fertilisation could not have taken place. The egg-cell of Chara crinita has thus lost its special sexual character without altering its external appear- ance. The essential sexual attribute of being incapable of further development, without fusion with a male cell, has disappeared ; it has become a vegetative cell. Vegetative Multiplication by Single Celis (Spore - Formation). ——As in the case of multicellular vegetative bodies, multiplication can be effected also through the separation of single cells. Strictly speak- ing, this manner of multiplication actually takes place whenever a division of the vegetative body occurs in unicellular Bacteria, Fungi, and Algae. Cells which serve the purpose of vegetative reproduc- tion, and have a special form and method of development, are first met with in the higher Cryptogams. They are frequently formed in special organs or receptacles. Such organs, in the case of the Fungi, are the sporangia or conidiophores, and the more complicated fructifications in or on which the spores are formed. Instead of spores with cell walls many Algae develop swarm-spores, which propel themselves in the water by means of cilia, and are thus enabled to seek out positions favourable for germination (cf. p. 243). In all higher Cryptogams (Mosses, Ferns, Egwisetaceac, etc.) the vegetative reproductive cells are produced in peculiar multicellular sporangia, which open spontane- ously by hygroscopic movements when the spores have reached maturity. Among the higher Cryptogams there is not developed from the spore a daughter plant similar to the parent, but there results an entirely differently organised structure, which, by sexual reproduction, produces a plant bearing spores, and similar to the original form. Sexual Reproduction For the purpose of sexual reproduction two kinds of cells, male and female, are produced. Although neither alone is capable of develop- ment, the actual reproductive body is formed by the fusion into one cell of two such sexually differentiated cells. It has already been pointed out that through such a union of two distinct cells, qualitative changes may arise in the resulting organism, which would not have been possible had it been produced by merely vegetative processes. SECT, II PHYSIOLOGY 281 As it is thus necessary in sexual reproduction not only to provide for the production of male and female cells, but also to ensure their union, it becomes at once evident that, for sexual reproduction, the organs must have a different morphological and anatomical structure than if they were designed solely for vegetative activity. The sexual organs accordingly often exhibit a special and peculiar form, which differs materially in appearance from the vegetative parts of a plant. The Union of Sexual Cells (Fertilisation).—Leaving out of con- sideration the necessary external contrivances to that end, fertilisation is accomplished by means of a chemotactic or chemotropic stimulus (pp. 243, 268). It is generally the non-motile egg-cells or female sexual organ which exert an attractive influence upon the motile male cells; as, for instance, in the case of the Mosses, where the sper- matozoids are enticed within the archegonia by a solution of cane- sugar, or, as in Ferns, where they are similarly stimulated by malic acid. When, however, there is no difference in the external form of the male and female cells, then both are usually motile, and the attrac- tion seems to be exerted mutually. This is probably the case with the motile and externally similar sexual cells (GAMETES) of the lower Cryptogams, particularly of the Algae (Fig. 69). In the conjugation of the Conjugatae, however, although both sexual cells are externally alike, one cell alone is usually motile, and passes through the connecting canal to the other; and in the Mucaceae, though the egg-cells are ejected from the mother plant, they have not themselves any power of movement, while the male cells or spermatozoids, by means of their cilia, are capable of independent motion. This capacity of the male cells for independent movement is common to most Algae, with the exception of the Florideae, by which the walled male cells are passively conveyed to the female organ by the water. Throughout the whole group of the higher Cryptogams, the male cells are motile spermato- zoids, capable of seeking out the non-motile egg-cells concealed within the archegonia. But in the sexually differentiated Fungi the male substance usually remains enclosed in special hyphe which press them- selves close against the female organs, and, by the perforation of the intervening cell wall, the fusion of their contents is rendered possible. The fertilisation of the Phanerogams is accompanied by a perforation of the intervening cell walls similar to that which occurs in the Fungi. In this case the male cell is enclosed within the pollen grain; the female, as a naked egg-cell, is included in the embryo-sac, which in turn lies in the ovule, and in the Angiosperms the ovule is again enclosed within the ovary. The double-walled pollen grains possess no independent power of movement, but are conveyed to the female sexual organs by the assistance of external agencies (animals, currents of air or water). The pollen grain then grows out into a tube which is acted upon by chemotropic (including hydrotropic and aerotropic) influences, and grows like a fungus-filament through the tissues of the ovary and 282 BOTANY ‘PART I ovule until it penetrates to the ege-cell in the embryo-sac; whereupon the union of the sexual cells is easily effected (Fig. 71). — To render certain the accomplishment of this POLLINATION, or con- veyance of the pollen to the female sexual organs, special and often complicated contrivances are made use of by the different Phanero- gams, according to the means of conveyance upon which they are dependent. Plants of which the pollen is carried by wind are designated AnEmopnitous. As this method of conveyance depends upon the chance of wind direction, an enormous amount of pollen characterises wind-fertilised plants. Such enormous quantities of pollen are often taken up from pine forests by the wind that clouds of pollen fill the air. The surface of Lake Constance in spring is so thickly covered with pollen that it is coloured yellow (‘‘ the lake blooms,” it is then said), and in the Norwegian fiords, at a depth of 200 fathoms, the pollen of Conifers, according to F. C. No.1, forms for a time the principal nourishment of the Rhizopod Saccamina, The male flowers of such anemophilous plants are accordingly either freely exposed to the wind in Catkins (Coniferac, Amentaccae), or the versatile anthers, as in the Grasses, depend from long, lightly- swaying filaments. The pollen grains themselves do not stick together but escape from the opened anthers in the form of fine powder. The pollen grains of many Conifers are rendered extremely buoyant and easy of conveyance by the wind by two sac-like protrusions of the exine. In some anemophilous plants the pollen is discharged by the sudden extension of the filaments, previously rolled up in the bud (Urticaceae, e.g. Pilea), or by the hygroscopic tension of the anthers. The female organs are also often especially adapted for the attachment of the pollen thus floating in the air. The stigmas either spread out like a brush (Corylus), or are finely feathered or provided with hairs (Grasses, Walnut), or drawn out into long threads (Indian Corn). In the Conifers, with freely exposed ovules, the grains of pollen are caught and retained in a drop of fluid exuded from the micropyle, into which they are gradually drawn as the fluid dries up. In other Conifers whose ovules are concealed in the cone of the female inflorescence, scale-like formations catch the pollen and conduct it to the sticky opening of the young ovules. For the fertilisation of the higher plants, the presence of water is not so essential as it is for most Cryptogams. Only a few sub- merged Phanerogams make use of the agency of water for effecting their pollination, and are, on that account, termed HYDROPHILOUS PLANTS. The pollen of the submerged Zostera exhibits certain peculiarities, distinctly referable to the necessity of effecting fertilisation under water. It does not form round grains, but in their place elongated thread-like filaments devoid of an exine, which, as they have the same SECT. 11 PHYSIOLOGY 283 specific weight as the surrounding water, are easily set in motion by the slightest currents, and are thus brought into contact with the stigmas. In the case of the submerged water-plants, Vullisneria, Elodea, and species of Hnhalus, found in the Indian Ocean, the pollination is accomplished on the surface of the water. Thus, for example, the male flowers of /allisneria, after separating from the parent plant, rise to the surface of the water, where they open and float like little boats to the female flowers, which, by the elongation of their spirally coiled flower-stalks, ascend, at the same time, to the surface of the water, only to become again submerged after fertilisation. In the great majority of Phanerogams pollination is effected by means of animals. By enticing in various ways insects, birds, or snails, plants are enabled not only to utilise the transporting power but also the intelligence of animals in the service of pollen-con- veyance. The pollination is then no longer left to chance; and as the transport of pollen to the sexual organs becomes more assured, the necessity for its formation in such enormous quantities as in ane- mophilous plants is obviated. For the most part, such plants (Fig. 219) are adapted to POLLINATION BY INSECTS (ENTOMOPHILY). For their nourishment, plants offer not only the sugary sap, which, as nectar, is excreted from different parts of the flowers, but also the pollen itself, which furnishes a nitrogenous food material and which, together with the honey, is kneaded by bees into bee-bread. As additional means of enticement, and to attract animals from a distance to the nectar offered by the sexual organs, special perfumes and couspicuous colours have also been developed. The ATTRACTIVE-APPA- RATUS of plants is generally formed by the coloured floral leaves; by the outer floral leaves or calyx (Nigella, Aconitum), or by the perianth (Lily, Tulip), or as an extra-floral show apparatus, by the hypsophyllary leaves and parts of the shoot, which do not belong strictly to the flower (Astrantia major, Richardia aethiopica, Melampyrum, Dalechampia, Bougainvillea spectabilis). The pollen of the entomophilous, in contrast to that of the anemophilous plants, is not a dry powder, but its grains are stuck together with an oily mucilaginous fluid; in other cases, they are held together by their rough outer surfaces and can only be removed from the anthers by animals. The structure of the flower is so contrived, as CHRISTIAN CONRAD SPRENGEL first pointed out in 1793 in his famous work on the structure and fertilisation of flowers (“ Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen”), that the pollen grains must necessarily become attached to certain parts of the body of the animal visiting it in search of food, and so be conveyed to the sticky or hairy stigma of other flowers. The remarkable variety of means employed to secure pollination, and the wonderful adaptation shown by the flowers to the form and habits of different insects, border on the marvellous. In addition to the 284 BOTANY PART I stimulus of hunger, plants utilise the reproductive instinct of animals for securing their pollination. Not a few plants (Stapelia, Aristolochia, and members of the Araceae), by the unnatural colour of their flowers, combined with a strong carrion-like stench, induce carrion-flies to visit them and deposit their eggs; in so doing they effect, at the same time, the pollination of the flowers. In South America, instead of insects, it is the humming-birds which are especially active in the conveyance of pollen. In addition to such ORNITHOPHILOUS PLANTS whose pollination is accomplished through the agency of birds (Maregruvia nepenthoides, and different species of Feijoa and Abutilon), pollination in some cases is effected by means of snails (MALAcoPHILOUS PLANTS). To their instrumentality the flowers of Calla palustris, Chrysosplenium, and also the half-buried flowers of the well-known Aspidistra owe their pollination. Self and Cross Fertilisation.—It has already been pointed out that it is by sexual reproduction, in contrast to the vegetative mode of multiplication, that qualitative modifications are effected. Such qualitative changes are best attained when the sexual cells are derived from different individuals; although, when they spring from the same individual, through the recurrence of ancestral characteristics (atavism, p. 277), there is always the possibility of the appearance of descendants which differ greatly from those produced vegetatively, by the same plant. By such close fertilisation, however, no opportunity is given for a new blending with others of the same species. It is an old maxim founded on experience, that prolonged close-breeding produces a deteriorating effect, as the slightly injurious variations, which other- wise would have been equalised by cross-breeding, become augmented. It is in accordance with this same principle that, in the sexual reproduction of plants, varied and often complicated contrivances are manifested, which conduce to CROSS-FERTILISATION (union between sexual cells of different individuals), even when the individuals them- selves are HERMAPHRODITE and possess two kinds of sexual organs, as in the case of the majority of Phanerogams. As, however, self-fertilisation takes place also in a small number of plants, either regularly or from necessity, it is evident that what- ever may be the advantage derived from a union of two distinct individuals, it is no more essential for sexual reproduction than for vegetation multiplication. Though in consideration of the otherwise predominant tendency to cross-fertilisation, self-fertilisation, just as apogamy, appears to be a retrogression. Self-pollination, although regularly occurring, frequently fails to occasion self-fertilisation, as often the pollen will not develop pollen-tubes on the stigmas of the flower (self-sterile) by which it was produced, but only on those of different flowers (Secale cereale, Corydalis cava, Lobelia fulgens, Verbascum nigrum, ete.). The antipathy between the sexual organs of the same flower, in certain plants, so greatly exceeds the bounds of indifference that they act upon each other as SECT. IT PHYSIOLOGY 285 poisons. Thus, for example, it is known of certain Orchids that pollination with their own pollen causes the death of the flower, while in other cases the pollen is killed in a short time by the stigmatic fluid. In other instances, self-fertilisation occurs where cross-pollination either is not effected, or else in conjunction with it (Wheat, Barley, Canna, Viola species, Linwm usitatissimum, ete.). By many plants, in addition to the large flowers adapted to insect pollination, small, inconspicuous flowers are produced which, usually concealed under- ground or by the lower leaves, never open, and only bear seeds which have been produced by self-fertilisation. In some plants the majority of the seeds are derived from such CLEISTOGAMOUS flowers (Viola), and sometimes their seeds alone are fruitful (Polycarpum tetraphyllum possesses only cleistogamous flowers). As the greater number of such plants, however, in addition to the seeds of the self-fertilised small cleistogamous flowers, produce others resulting from the cross-fertilisa- tion effected in the larger flowers (Impatiens noli-tangere, Lamium amplexicaule, Specularia perfoliata, etc.), the ancestral plants of the cleistogamous generations, as well as their descendants, have, at least, the opportunity for cross-fertilisation open to them. Special contrivances for assuring the crossing of the sexual cells, particularly by preventing self-pollination, are found to exist throughout the whole vegetable kingdom. Self-pollination is most effectually avoided when the plants are unisexual, that is when both male and female plants lead a separate existence. Such DICECIOUS plants exist in almost all classes of plants from the lower Cryptogams to the most highly developed Phanerogams (many of the lower Algae, species of Fucus, Marchuntia, Polytrichuwm, Equisetaceae, Taxus, Hemp, Hops, Date-Palm, etc.). In Monacrous plants the male and female organs occur on different flowers, but the flowers are borne on the same plants. The fertilisation between different flowers is thus secured ; but even here crossing with other individuals is, for the most part, assured by dichogamy. The term DICHOGAMY is used to denote the fact that the male and female sexual organs attain their maturity at different times. When either the male or female sexual organ matures before the other, the self-pollination of morpho- logically hermaphrodite flowers is avoided and crossing assured. Both herma- phrodism and moncecism are more advantageous than diccism, as all the plants in such cases are able to produce seeds ; while in diccious plants the male flowers cannot be utilised for the direct production of seeds. Dichogamy secures crossing in such a simple manner, and is so easily attained by hermaphrodite plants, that it is of very general occurrence in the vegetable kingdom. According to the priority of the maturity of their sexual organs, plants are designated PROTANDROUS Or PROTOGYMOUS, Proranpry, the earlier maturing of the male sexual organs, is the more frequent form of dichogamy. It occurs in the flowers of the Geraniaccae, Campanulaceae, Compositac, Lobeliaceae, Umbelliferae, and in Epilobium, Digitalis, etc. of the Mal- vacee. The anthers, in this case, open and discharge their pollen at a time when the 286 BOTANY PART I stigmas of the same flowers are still imperfectly developed and not ready for pollina- Fic, 217.—hiflorescence of Plan- tago media with protogynous tlowers. The upper, still closed flowers (9) have protruding styles; the lower (6) have lost their styles, and disclose their elongated stamens. tion. Accordingly, PROTANDROUS FLOWERS CAN ONLY BE FERTILISED BY THE POLLEN OF YOUNGER FLOWERS. In the less frequent Prorocyny the female sexual organs are susceptible to fertilisation before the pollen of the same flowers is ripe ; so that the PRoTOGYNOoUS FLOWERS MUST BE FERTILISED BY THE POLLEN OF OLDER FLOWERS (Anthowanthum odoratum, Luzula pilosa, Serophularia nodosa, Helleborus, Magnolia, Plantago media, Fig. 217). A still more complicated method of effecting cross- fertilisation, because involving also morpho- logical and anatomical differences of structure, results from HETEROSTYLY, or the peculiarity of some species of plants of producing stigmas and anthers which vary in height in different individuals of the same species. A good example of heterostyled flowers is afforded by the Chinese Primrose (Fig. 218). This plant has two forms of flowers, long-styled (Z) and short-styled (K), while the positions of the stigmas and anthers in the two kinds of flowers are exactly reversed. The pollen grains of the short-styled flowers, moreover, are larger, and the stigmatic papille shorter, than in those with the longer styles (p, P, and x, V). The purpose of such morphological and anatomical differences existing between flowers of the same species was first understood after they were discovered by DARWIN to be a contrivance for cross-pollination. Fertilisation is most successful in such cases when the pollination of the stigmas is effected by the Fig. 218.—Primula sinensis; two heterostyled flowers from different plants. ZL, Long-styled ; K, short-styled flowers; G, style; S, anthers; P, pollen-grains; and N, stiginatic papilla of the long-styled form; p and n, pollen-grains and stigmatic papille of the short-styled form. (P, N, pn, X110.) pollen of anthers correspondingly situated. By such a “‘legitimate”’ fertilisation more and better seeds are produced than by ‘“‘illegitimate” fertilisation, and in SECT. II PHYSIOLOGY 287 some cases (Linwm perenne) legitimate fertilisation alone is productive. Legitimate fertilisation is rendered more certain by the fact that insects in visiting the flowers touch correspondingly placed sexual organs with the same portions of their body. The flowers of Primroses have styles of two different lengths (DIMORPHIC HETERO- STYLY); the same peculiarity is exhibited by Pulmonaria, Hottonia, Fagopyrum, Linwm. There are also flowers with TRIMORPHIC HETEROSTYLY(Lythrum Salicaria, and some species of Oxalis), in which there are two circles of stamens and three variations in the height of the stigmas and anthers. In a great number of flowers self-pollination is made mechanically impossible, as their own pollen is prevented by the respective positions of the sexual organ from coming in contact with the stigma (Hercocamy). In the Iris, for example, the anthers are sheltered under the branched petaloid style, upon whose lip-like stigma no pollen can come, unless through the agency of insects. In the Orchid- aceae and Asclepiadaceac self-pollination is rendered impossible both by the nature of the pollen masses and by their position. A complicated form of structural con- Fic. 219.—Pollination of Salviu prutensis. 1, Flower visited by a bumble-bee, showing the projec- tion of the curved connective from the helmet-shaped upper lip, and the deposition of the pollen on the back of the bumble-bee ; 2, older flower, with connective drawn back, and elongated style ; 4, the staminal apparatus at rest, with connective enclosed within the upper lip ; 3, the same, when disturbed by the entrance of the proboscis of the bee in the direction of the arrow; f, filament; c, connective ; s, the obstructing half of the anther. trivance, by means of which cross-pollination is secured, may be seen in a flower of Salvia pratensis (Fig. 219). The anthers of this flower are concealed in the upper lip of the corolla, from which the style, with its bilobed stigma, projects. When a bumble-bee visits the flower in search of honey, it must first with its proboscis push out of the way the small plate (s), formed of two sterile anther halves grown together. These are situated at the ends of the short arms of the connectives (c), which are so elongated that they might easily be mistaken for the filaments (f) of the stamens. The fertile anther halves are situated at the other ends of the connectives, and so are brought in contact with the hairy back of the bumble-bee when it pushes against the plate at the short ends of the lever-like connectives. The pollen thus attached to the bee will be brushed off its back by the forked stigma of the next flower it enters. Good examples of hercogamous flowers are afforded by the Papilionaceae, by Kalimia, whose anthers are held in pockets of the corolla, by Vinea, Aristolochia, etc. Hybridisation.—The union of two sexual cells is, as a rule, only possible when they are derived from closely allied plants ; it is only then that they exercise an attractive influence upon each other and 288 BOTANY PART I fuse together in the act of sexual reproduction. The sexual cells of Mosses and Ferns, apart from all other considerations, would not unite because the spermatozoids of Mosses are attracted to the female organs by sugar, while those of the Ferns are only stimulated by malic acid. In the case of Phanerogams, a mixed union of sexual cells is likewise prevented by various obstacles to pollination and fertilisation. Occasionally, however, the sexual cells of different varieties, species, or even genera have shown themselves able to unite and produce descend- ants capable of development. Such a union is termed HYBRIDISATION, or bastard-formation, and its products HYBRIDS or BASTARDS. Through the demonstration of the possibility of hybridisation, the sexuality of plants, for a long time doubted, was indisputably proven. (With this object in view, hybrids were raised in great numbers by KOLREUTER as early as 1761.) It also demonstrated that the real purpose of sexual union was the combination of the properties of both parents, for transitional forms are found among hybrids which in many characteristics resemble the male and in others the female ancestor, or they may show an equal combination of the characters of both. Less frequently it happens that the hybrid resembles one ancestor almost exclusively. In such a case the attributes of the other ancestor remain latent, and may appear quite unexpectedly, through atavism, in later generations. Had one species simple leaves and the other compound, their hybrid would have leaves more or less cleft ; or were the flowers of one parent species red and those of the other yellow, the hybrid frequently bore flowers with red and yellow markings (mosaic hybrids), or which were orange-coloured. If an early blooming form were crossed with a late bloomer, the hybrid would flower at a time intermediate between the two. From these and similar differences shown by hybrids, it became clear that the inherited characteristics of both the male and female cells were transmitted by sexual reproduction, and that the only function of the male fertilising substance was not, as was at one time believed, merely to give an impetus to the development of the egg-cell. A large number of spontaneous hybrids have been found which have arisen naturally from plants with a special capacity for hybridisation. That such natural hybrids do not oftener occur is due to the lack of an opportune time or space for their development, and also to the fact that in the case of pollination of flowers with different kinds of pollen, that of their own species seems always more effectual in effecting fertilisation. The more closely allied the parent plants, the more readily, as a rule, may hybrids between them be produced. Many families seem to incline naturally to hybridisation (Solanaceae, Caryophyllaccac, Iridaceae, etc.) ; others again develop hybrids only occasionally or not at all (Cruciferae, Papilionaceac, Urticaceac, Convol- vulaceae, etc.). Even in the same family the related genera and species exhibit great differences in the readiness with which they may be crossed. The Grape- vine and also the Willow are easily crossed with other species of their own genus, SECT. II PHYSIOLOGY 289 and the same is also true of the different species of Dianthus, while the species of Silene cross with each other only with difficulty. Species hybrids are easily produced from species of Nicotiana, of Verbascum, and of Gewm; on the other hand, it is very difficult to cross different species of Solanum, Linaria, or Poten- tilla. The hybridisation, however, of nearly allied forms is often impossible—the Apple with the Pear, for instance, although the Peach and Almond may be crossed, and also the species of even the different genera Lychnis and Silene, Rhododendron and Azalea, Aegilops and Triticwim, each according to their ‘‘ sexual affinity.” DERIVATIVE HYBRIDS arise when hybrids are crossed with one another, or with one of the original parent forms. In this way it has been possible to unite six species of Willow in one hybrid, and in the case of the Grape-vine even more species have been combined. It is only in rare cases, however, that the form of the hybrid remains constant in the succeeding generations. These exhibit more frequently a tendency to revert to one of the original ancestral forms. In addition to their inherited qualities HYBRIDS EXHIBIT NEW PECULIARITIES not derived from their parent forms. These are a MODIFIED FERTILITY, GREAT TENDENCY TO VARIATION, and often a MORE LUXURIANT GROWTH. ‘The fertility is often so enfeebled that the hybrids are sterile and do not reproduce themselves sexually. ‘This enfeeblement of the sexuality increases the more remote is the relationship of the ancestral forms. The tendency to variability is often greatly enhanced in hybrids, especially in those arising from the hybridisation of different varieties of the same species. Hybrids, particularly those from nearly related parents, produce more vigorous vegetative organs, they bloom earlier, longer, and more profusely than the uncrossed plants, while at the same time the flowers are larger, more brilliant, and exhibit a tendency to become double. The luxu- riance of growth and the increased tendency to produce varieties displayed by the hybrids have made the whole subject of hybridisation one of great practical as well as theoretical importance. It is doubtful if hybrid forms can be produced (graft-hybrids) by a vegetative union of portions of two different plants (grafting, budding). It will seem very improbable, as in all properly regulated experiments the vegetatively united forms have preserved their independent in- dividuality (p. 227). Alternation of Generations In the lower Cryptogams, as well as in the Phanerogams, vegeta- tive and sexual reproduction may exist, either side by side or following one another often in apparently irregular succession. After many generations have been produced in a vegetative way, in the cage of the Algae or Fungi, sexual organs suddenly appear ; but by both modes of reproduction descendants of similar appearance are pro- duced. Although in this case sexually and vegetatively produced generations succeed each other, it would not, strictly speaking, be U 290 BOTANY PART I considered as an example of the alternation of generations. This expression has been restricted to cases WHERE THERE IS A REGULAR ALTERNATION BETWEEN A VEGETATIVE AND SEXUAL GENERATION, EACH OF WHICH HAS AN ENTIRELY DIFFERENT ORGANISATION. A Fern-plant produces only asexual spores. By their germination, however, a Fern-plant is not produced, but in its place a diminutive plantlet, which remains without stem and leaves, without vascular bundles, and without any internal differentiation. This is the PROTHALLIUM, which in turn produces sexual organs with spermatozoids and egg-cells, from which a large Fern-plant is developed after fertilisation. In a similar manner, sexual and asexual generations alternate in the Mosses and in the Hydropterideac, Equisetinae, Lycopodinae. In the three last-named, as in the case of the Ferns, the prothallia are developed vegetatively from the spores of the large plant, and these again give rise sexually to an Eqwise- tum, a Lycopodium, etc. In the Hgwisctinae the spores are externally exactly alike, but some give rise to male, others to female prothallia. In the case of the Hydrop- terideae and the heterosporous Lycopodinae (Selaginellae, Isoeteae) the spores from which the male prothallia are derived are smaller (microspores) but more numerous than those which give rise to the female prothallia (macrospores). At the same time, the prothallium does not in all cases grow out of the spores as an independent plantlet, but remains within it and only exposes the sexual cells for purposes of fertilisation ; so that the male sexual cells are produced within the micro- | spores and the egg-cells within the macrospores. Thus, in the higher Cryptogams the alternating sexual generation, or the one producing the sexual cells, remains concealed within the spores. In Phanerogams (Gymnosperms and Angiosperms) the sexual generation has undergone even greater reduction. It has nevertheless been determined that the pollen grains of the Phanerogams correspond to the vegetatively produced microspores of the Vascular Cryptogams, and that in them the male sexual cells also arise through a process of division. Similarly, the embryo-sac of the Phanerogams, in which, in addition to the more or less reduced prothallium (synergide, antipodal cells), the female sexual cell (the egg-cell) occurs, must be regarded as the equivalent of the asexually produced macrospores. The young plant (the embryo), just as in Selaginelia, is also formed in the macro- spores—that is, in the embryo-sac. Viewed in this way, it is evident that an ALTERNATION OF GENERATIONS TAKES PLACE ALSO IN PHANEROGAMS. Hor- MEISTER, the discoverer of this most important fact, drew most ingenious infer- ences from it concerning the genetic connection of the higher with the lower plants, of Phanerogams with the Vascular Cryptogams. In the alternating generations are clearly manifested the essential functions of both modes of propagation—the quantitative, in the extra- ordinary multiplication by asexual reproduction ; the qualitative, in the sexual fusion. For while thousands of asexual spores are produced from a single Fern-leaf, from the prothallium of the sexual generation seldom more than one new Fern-plant arises, but that one plant derives a qualitative value from the cross-fertilisation necessitated by the dicho- gamy of the prothallia. Just as the Fern-plant can occasionally arise by budding (p. 279) directly from the prothallium, without the intervention of a sexual act, the formation of spores is also sometimes omitted, and the prothallia SECT. IT PHYSIOLOGY 291 can then spring directly from the Fern-leaf (APOSPORY, in varieties of Athyrium and Aspidium). The Dissemination and Germination of Seeds If the seeds after their separation from the parent plant simply fell upon the earth, the young seedlings would be injuriously restricted to the place already occupied by the parent plant, and would also spring up in such large numbers that they would mutually exterminate each other. The dissemination of the seeds thus becomes a necessity, and although a larger or smaller proportion perish in the process, a small number eventually find themselves in a favourable environment. For their DISSEMINATION, seeds make use of the same agencies as are employed for the conveyance of pollen. Thus their dispersion is effected by means of currents of air and water; by their forcible dis- charge from their receptacles ; by animals; and also by their accidental transportation by railroads and ships. To ensure the dispersal of seeds by the wind, all those contriv- Fic. 220.—Winged seed of Bignonia mucronata. (Nat. size.) ances are of use which serve to increase their superficial area with but small augmentation of their weight. Of this nature are the hairy appendages of seeds and fruit-walls, as in Gossypium, Hpilobium, Populus, Saliz, Typha, Clematis, and the fruits of the Compositae with their pappus, of Valeriana, ete. Compared with the accelerated fall in a vacuum, the retardation exerted by the resistance of the air (by which the opportunity for dispersal through the agency of the wind is enhanced) in the case of Cynaria Scolymus is, in the first second, as six to one. Similar adaptations for utilising the agency of the wind as a means of dispersal are the wing-like appendages formed from the expansion of the sepals (Dipterocorpus) or of the ovary (Acer, Fraxinus, Uimus, Polygonum, Robinia, Gleditschia, and the fruits of many Umbelli- ferae), or of the seeds themselves, as in the winged seeds of the Bignoniaceae (and many Ternstroemiaceae). In a Bignonia seed (Fig. 220), with its widely outspread, glossy 292 BOTANY PART I wings, the centre of gravity is so disposed that the seed floats lightly along through the air in an almost horizontal course, and with a motion like that of a butterfly. The seeds of Zanonia, one of the Cucurbitaceae, are very similarly equipped. In the Lime the subtending leaf which is attached to the inflorescence is retained to facilitate the dispersal of the seeds by the wind; and in the seeds of the Fir the winged appendages are derived from the tissue of the placental scale. The aerial transportation of seeds and fruits, winged only on one side, is accompanied by a continuous spirally twisting movement which assists to retard their fall. The diminutive size of many reproductive bodies, and the propor- tionate enlargement of their surface in comparison with their volume, increase their buoyancy. Microscopically small Fungi, spores, and Bacteria are in consequence easily transported by the wind. In the spores of Lycoperdon caelatum DINGLER found the retardation to be as 1 to 1000, which, according to NAGELI, could only be theoretically explained by the supposition that the retardation was intensified by a thin layer of air permanently adhering to the surface of the spores. Seeds and fruit are also frequently transported great distances by the agency of WATER. In the case of maritime plants the seeds are often especially adapted (water-tight tissues; large air-spaces serving as swimming-bladders, etc.) for transport by ocean currents. Through the possession of such devices, the seeds of West Indian plants are carried to Norway by the Gulf Stream, and the appearance of Cocoa- nut palms as the first vegetation on isolated coral islands is in like manner due to the adaptation of their fruits to transport by water. ANIMALS participate largely in the dissemination of seeds; either by eating the agreeably tasting and often attractively coloured fruit, and excreting the undigested seeds, or by their involuntary transportation of seeds and fruits which have become in some way attached to them. This is effected in many cases by hooks and bristles (Lappa, Guliwm Aparine, Bidens, Echinospermum, Xanthium, and the fruits of Medicago minima, so common in sheep’s wool and erroneously termed wool- lice). Or the seeds become attached to animals by means of some sticky substance ; in this way the seeds of the Mistletoe, which stick to the beaks of birds eating the berries, finally adhere to the branches of trees upon which the birds wipe their bills. The widespread distribution of fresh-water plants can only be accounted for through the agency of aquatic birds. The natural distribution of plants has been greatly modified by the interference of man, especially in these days of universal commercial intercourse by rail and sea. By their instrumentality not only have the useful plants been widely distributed over the earth, but the weeds have followed in the same way; and many a seed thus accidentally carried to other lands has finally found there a new place of growth. The forcible discharge of spores and seeds is effected by the SECT. II PHYSIOLOGY 293 sudden liberation of hygroscopic or tissue tensions. It has already been mentioned that the capillitia of the Myxomycetes and the elaters of the Liverworts serve for the dispersal of the spores. In the case of the Box (Buaus), the smooth».seeds are forcibly discharged by the contraction of the pericarp, liké a bean pressed between the fingers. The dry fruit of Hura crepitans bursts apart with a report like that of a pistol, and is scattered in pieces far and wide. The turgescence and elasticity of the cell-walls give rise to the tension which results in the forcible discharge of the sporangia of Pilobolus, and in the ejection of the ascospores of many Ascomycetes. The bursting and rolling up of the segments of the seed-vessels of Jimpatiens, by means of which the dispersal of the seeds is effected, are due to the sudden release of tissue-tensions. Similarly, the fruits of Momordica elaterium and Ecballiwm dehisce suddenly and eject the seeds with considerable force. It is unnecessary to cite further examples ; those already given may be sufficient to call attention to a few of the different means made use of for the dispersal of the reproductive germs. Germination.—The dry condition of the seed and the cessation of all vital activity render the resting germ extremely resistant to the action of external influences, and capable of maintaining its vitality during the course of its dissemination, until it is ultimately fixed in the earth. In effecting their PERMANENT LODGMENT IN THE SOIL, seeds are aided by the various STRUCTURAL PECULIARITIES OF THEIR SURFACE (furrows, bristles, hairs, etc.). The fruits of the Geraniaceae (Erodium, Fig. 200) and Gramineae (Stipa, Avena sterilis, and species of Aristida) are enabled, by means of movements due to hygroscopic torsion, to bury themselves in the ground. In the case of Trifolium subterranewm and Arachis hypogaea the same result is accomplished by the geotropic growth of the fruit-stalks, while the seed-capsules of Linaria cymbalaria are deposited in the crevices of walls and cliffs by the negative heliotropic movements of the fruit- stalks. Nuts, acorns, and seeds buried by squirrels or other animals in the ground and forgotten, or for any reason not made use of, often germinate. The seedlings of Mangrove trees, Rhizophora and Bruguiera, exhibit a most peculiar manner of growth to ensure their lodgment in the ground. The seed germinates in the fruit before it is detached trom the tree. When the radicle has attained a considerable length, the young seedling, separating either from the cotyledons or from the fruit-stalk, falls to the earth; it then bores into the mud and is thus enabled to commence its growth without delay. Many seeds and fruits acquire a more or less voluminous MUCILAGINOUS SHEATH, which serves a double purpose. Quince seeds, Flax seeds, seeds of the Plantain, of Crucifers, the fruits of Salvia Horminum, seed of Cuphea and Cobaea (in the mucilage cells of which delicate thickening bands are rolled up), afford the best-known examples of such slimy envelopes, which, in addition to fixing the seed to the 294 BOTANY PART I ground, serve to absorb water by holding it in their substance or drawing it in hygroscopically (cf. Mistletoe berries). Fruit-walls, by their spongy nature, may also serve as water-carriers (ripe fruits of Tropaecolum, Poterium spinosum, Medicago terebellum). The germination of seeds, once securely lodged in the soil, may begin immediately or after a longer or shorter PERIOD OF REST. The seeds of many Conifers do not germinate for several years. Some plants again, in addition to seeds which germinate in the first year, produce others which require a longer rest (Trifolium pratense, Robinia Pseudacacia, Cytisus Laburnum, Reseda lutea, etc.). Even under favourable circumstances such seeds do not germinate until a definite length of time has elapsed. Germination may be de- layed also by external conditions, and the vitality of the seed may still be retained for years. Thus, for example, on the removal of a forest from land that had been under cultivation for forty-six years, Prrzr found that a great variety of field- plants at once sprang up as soon as the requirements for their germination were restored. Germination, according to the observations of KLEBS, is introduced by true processes of growth, which result in THE RUPTURE OF THE SEED-COVERINGS. This is effected either by the growing radicle, or, in many Monocotyledons, by the cotyledon. In other seeds enclosed within a shell, the bursting of the shell through the growth of the endosperm or cotyledons precedes germination. In cases where the shell is very hard and does not consist of two halves easily separable by internal pressure (as in Cherry-stones), special places are often provided for the egress of the young seedling. At the end of a cocoa-nut, for example, such points of egress, behind the thinnest of which the embryo will be found emerging from the endosperm, are very easily seen. Through the extremely hard, thick shell of another Cocoa-palm, Cocus lapidea, there are three long germinal pores, while the seedling of fcrocomia sclerocarpa has only to push a loosely fastened plug out of the thick shell of the seed (Fig. 221). Similar contrivances are found in Fic. 221.—Section through the upper part of the the case of Pandanus, Canna, Typha, Potamo- fruit of Acrocomia sclero- geton, and many Dicotyledons (Tetragonia expansa, carpa. 8, The hard ° : : ‘ Sai eooplicaiok Medicago, and some species of Onobrychis and is pushed out of the Portulaca). SEEDLINGS PENETRATE THE SOIL by shell by the germinat- means of the elongation of the primary root, or of ing embryo, K; Z, endo- the h tyl al is th itl sperm. (AfterPritzer,) the hypocotyl, or also, as is the case with many Monocotyledons, through the movements of the geotropic cotyledons. After the descending part is firmly attached to the soil, by either root-hairs or lateral roots, THE UPWARD GROWTH COM- MENCES. In this process the cotyledons may either remain within the seed or unfold above ground. The first is often the case where the cotyledons are full of reserve material (Phaseolus multiflorus, Aes- SECT, II PHYSIOLOGY 295 culus, Quercus), or where their function is to absorb nourishment from the endosperm (in Palms and the scutellum of Gramineae). More fre- quently the cotyledons are pushed above ground, and may then be thick and filled with reserve nourishment, or thin and turning green on exposure to the light. In many Monocotyledons, as also in Ricinus, etc., the cotyledons, even if they afterwards appear above ground, may first take up the nutritive substances of the endosperm; while in the Conifers the cotyledons perform the same office above ground. THE COTYLEDONS ARE DRAWN FROM THE SEED by the curvature of the hypocotyl or of the petioles of the cotyledons (Smyrnium, Delphinium). The seed-coverings also are often further ruptured by the swelling of the hypocotyl (Cucurbita, etc.). The unfolding of the first leaves above ground is frequently accompanied by a CONTRACTION OF THE ROOT, occasioned by its distension in a transverse direction ; the seed- ling is in consequence drawn deeper into the soil, and its position rendered more secure. Even older plants, particularly those whose leaves form a radical rosette, notwithstanding their upward growth, are held close to the ground through a similar contraction of their roots. When its attachment in the soil is properly provided for, and after the first germ-leaves are unfolded, the young plant has acquired the capacity for self-sustenance, its further growth and development being dependent upon its own activity. PART II SPECIAL BOTANY SECTION I CRYPTOGAMS SPECIAL BOTANY SPECIAL Botany is concerned with the special morphology and physiology of plants. While it is the province of General Botany to investigate the structure and vital processes of the whole vegetable kingdom, it is the task of Special Botany to interpret the structure and vital processes of its separate divisions. The aim of General Morphology is to determine the phylogenetic derivation of the external and internal segmentation of plants, and to refer theirnumerousstructural peculiarities to the primitive form from which they have arisen. The purpose of Special Morphology, on the other hand, is to trace the development which has been reached in the different divisions of the plant kingdom, to understand the form of individual plants, and to trace the connection between one form and another. Thus the methods of special morphology are also phylogenetic, and furnish the basis for a NATURAL SYSTEM of classification of the vegetable organisms based upon their actual relationships. Although such a system must necessarily be very imperfect, as it is not possible to determine, directly and indisputably, the phylogenetic connection of different plants, but only to derive indirectly their relationships from morphological com- parisons, the aim which we set before us is none the less both legitimate and essentially justifiable. Such a natural system, founded on the actual relationship existing between different plants, stands in direct opposition to the ARTIFICIAL SYSTEM, to which has never been attributed more than a practical value in grouping the plants in such a manner that they could easily be determined and classified. Of all the earlier artificial systems, the sexual system proposed by CarL LINN#US in the year 1735 is the only one which need be considered. LINNAUS, in establishing his classification, utilised characteristics which referred exclusively to the sexual organs, and on this basis distinguished twenty-four classes of plants. In the last or twenty- fourth class he included all such plants as were devoid of any visible sexual organs, and termed them collectively Cryprocams. Of the 300 BOTANY PART IL Cryptogams there were at that time but comparatively few forms known, and the complicated methods of reproduction of this now large class were absolutely unknown. In contrast to the Cryptogams, the other twenty-three classes were distinguished as PHANEROGAMS or plants whose flowers with their sexual organs could be easily seen. LINNZUS divided the Phanerogams, according to the sexual character of their flowers, into such as possessed hermaphrodite flowers (Classes I.-XX.), and those in which the flowers were unisexual (XXI.-XXIII.). Plants with hermaphrodite flowers he again divided into three groups: those with free stamens (I-XV.), which he further distinguished according to the number, mode of insertion, and relative length of the stamens ; those with stamens united with each other (XVI-XIX.) ; and those in which the stamens were united with the pistil (XX.). Each of the twenty-four classes were similarly subdivided into orders. While some of the classes and orders thus constituted represent naturally related groups, although by the method of their arrangement in the artificial system they are isolated and widely removed from their proper position, they include, for the most part, plants which phylo- genetically are very far apart. Linnavs himself (1738) felt the necessity of establishing natural families in which the plants should be arranged according to their “ re- lationships.” So long, however, as the belief in the immutability of species prevailed, the adoption of a system of classification expressive of relationship and family could have no more than a hypothetical meaning, and merely indicated a supposed agreement between plants having similar external forms. A true basis for a natural system of classification of organisms was first afforded by the theory of evolution. The system adopted as the basis of the following description and systematic arrangement of plants is the natural system of ALEX- ANDER Braun, as modified and further perfected by KicHLER and others. : According to this system we have to distinguish between CRYPTO- GAms as the lower division, and PHANEROGAMS as the higher division of the plant kingdom. SECTION I CRYPTOGAMS The Cryptogams include an extraordinary variety of the most different plant forms, extending from unicellular organisms to plants exhibiting segmentation into stem, leaf, and root. The Cryptogams, however, are collectively distinguished from Phanerogams by the moee of their dissemination by SPORES, in contrast to that of the Phanerogams, which SECT. I CRYPTOGAMS 301 is effected by SEEDS; spores are formed also by Phanerogams, but they are not the immediate cause of the origin and development of new individuals. Seeds are multicellular bodies, within which is included the multicellular rudiment or EMBRYO of a plant ; while spores which, in the case of the Cryptogams, become separated from the mother plant, and give rise to a new and independent organism, are unicellular struc- tures. Cryptogams may therefore be termed SPORE PLANTS or Sporo- phytes, and Phanerogams SEED PLANTS or Spermaphytes ; although uniformity to previous usage and custom would recommend adherence to the older terms. The Cryptogams are divided into the three following groups :— I. The THALLOPHYTA, embracing a great variety of plants whose vegetative portion may consist of one or many cells in the form of a more or less branched thallus, II. The Bryopuyta, which include forms with a leaf-like thallus, as well as cormophytic forms, with evident segmentation into stems and leaves. The Bryophytes possess no true roots, and their conduct- ing bundles are of the simplest structure. II. The Preriporuyta, or Fern-plants, exhibit a segmentation into stems, leaves, and roots, and also possess true vascular bundles. While thus resembling the Phanerogams in structure, they differ from them in their mode of reproduction, and in their dissemination by means of spores. The Thallophytes and Bryophytes are also characterised as cellular plants, in contrast to the Pteridophytes or Vascular Crypto- gams, which, together with the Phanerogams, are collectively desig- nated vascular plants. I. THALLOPHYTA The Thallophytes may be divided according to their natural relationships into the following classes :— 1. Myxomycetes, Slime-Fungi. 6. Chlorophyceae, Green Algae. 2. Schizophyta, Fission-Plants. 7. Phaeophyceae, Brown Algae. 3. Diatomeae, Diatoms. 8. Rhodophyceae, Red Algae. 4, Peridineae, Dinoflagellates. 9. Characeae, Stoneworts. 5. Conjugatae, Conjugates. 10. Hyphomycetes (Ewmycetes), Fungi. Formerly it was customary to divide the Thallophyta comprised in these ten classes into the two groups of Algae and Fungi. The Algae are Thallophytes which possess chromatophores with colouring pigments, particularly chlorophyll; they are, therefore, capable of assimi- lating and providing independently for their own nutrition. The Fungi, on the other hand, are colourless and have a saprophytic or parasitic 802 BOTANY PART II mode of life. Such a method of classification, however, although possessing a physiological value, has no phylogenetic significance, as it gives no expression to the natural relationship of the Fungi to the Algae, from which they have been derived. Of the ten classes previously enumerated, the Schizophyta, Peridineae, and Rhodophyceae include both assimilating and colourless non-assimilating forms: the Diatomeae, Conjugatae Chlorophyceae, Phaeophyceae, and Characeae contain exclusively assimilating forms; the Mycomycetes and Hyphomycetes, on the con- trary, include exclusively colourless and not independently assimilating forms. By the term Algae in its restricted sense are understood only the Thallophytes represented in the classes 3 to 8; by Fungi, only the Hyphomycetes. To the ten classes of the Thallophytes may be added, as Class 11, the Lichens (Lichenes), in which the thallus affords an in- stance of a symbiosis of Algae and Fungi (p. 213). From a strictly systematic standpoint, the Fungi and Algae composing the Lichens should be classified separately, each in their own class ; but the Lichens, among themselves, exhibit such a similarity in structure and mode of life, that a better conception of their characteristic peculiarities is obtained by their treatment as a distinct class. As a rule the Thallophytes are distributed and multiplied by means of asexually produced spores, but with a varying mode of de- velopment in the different groups ; and also, although not in all classes, they exhibit a sexual mode of reproduction. This reproduction con- sists, in the simplest cases, in the production of a single cell, the ZYGOSPORE or ZYGOTE, by the union or CONJUGATION OF TWO SIMI- LARLY FORMED SEXUAL CELLS OR GAMETES. In many of the more highly developed forms, however, the gametes are differentiated as small male cells or SPERMATOZOIDS, and as larger female cells, the egg- cells or OOSPHERES. As a result of the fusion of an egg-cell and a spermatozoid, an OOSPORE is produced. The first form of sexual re- production or fertilisation is termed ISOGAMOUS, the second O0GAMOUS ; but these are connected by intermediate forms. Crass I Myxomycetes (Slime-Fungi) The Myxomycetes form an independent group of lower Thallo- phytes; in certain respects they occupy an intermediate position between plants and animals, and have in consequence also been termed Mycetozoa or Fungus-animals. They are represented by numerous species (about 50 genera), and are widely distributed over the whole earth. In their vegetative condition the Slime-Fungi consist of naked SEO. 1 CRYPTOGAMS 303 masses of protoplasm, the PLASMODIA, containing numerous small nuclei but utterly devoid of chlorophyll. In consequence they are reduced to a saprophytic mode of life upon decaying vegetable remains, or as parasites they often obtain their nourishment from living plants. The plasmodia (p. 51) are found most frequently in forests, upon soil rich in humus, upon fallen leaves, and in decaying wood. They creep about on the substrata, changing their form at the same time, and thrust out processes or pseudopodia, which may in turn coalesce. Their movements are regulated by the intensity of the light and heat to which they are exposed, and by the amount of moisture and nourishment supplied by the substratum. Although in the vegetative condition the plasmodia are negatively heliotropic and positively hydrotropic, these characteristics become changed when the process of spore-formation begins. The plasmodium then creeps out from. the substratum towards the light and air, and, after coming to rest, is con- verted into single or numerous and closely contiguous fructifications, according to the genus. On the periphery of each fructification an outer envelope or PERIDIUM is formed ; while internally the contents of the fructification separate into spores, each of which is provided with a nucleus, and enclosed by an outer wall. The spores thus formed have accordingly an asexual origin. In many genera, part of the internal protoplasm within the SPORANGIUM or spore-receptacle is utilised in the formation of a CAPILLITIUM, consisting of isolated or reticulately united threads or tubes. Upon the maturity of the spores, the peri- dium of the sporangium becomes ruptured, and the spores are dis- persed by the wind. In the case of the genus Ceratiomyxa, the process is somewhat simplified, as the fructification is not enveloped by a peridium, and the spores are produced at the extremities of short stalks. SEXUAL REPRODUCTION is entirely absent in the Myxomy- cetes. A good exampie of the development of the plasmodia from the spores is afforded by Chondrioderma difforme, a Slime-Fungus common on decaying leaves, dung, etc., upon which it forms small, round, sessile sporangia. The germination of the spores (a, Fig. 52, p. 51) may be easily observed when cultivated in an infusion of Cabbage leaves or other vegetable matter. The spore-wall is ruptured and left empty by the escaping protoplast. After developing a flagellum or cILIUM as an organ of motion, the protoplast swims about in the water, being converted into a SWARM-SPORE (Fig. 52, e-g), with a cell nucleus in its anterior or ciliated end, and a contractile vacuole in the posterior end of its body. Eventually the cilium is drawn in, and the swarm-spore becomes transformed into « Myxama@Ba (Fungus amceba), which creeps about, and, while undergoing constant alteration in its shape, at the same time it takes up food material by enclosing within its proto- plasmic body small particles of foreign matter. The amcbe have also the capacity of multiplication by division. In conditions unfavourable for their develop- ment they surround themselves with a wall, and as MicRocysTs pass into a state of rest from which, under favourable conditions, they again emerge as swarm-spores. Ultimately a number of the Myxameebe approach close together 304 BOTANY PART II (Fig. 52, 2) and coalesce, forming small plasmodia (Fig. 52, m), which in turn fuse with others into larger plasmodia (Fig. 52, 2). Both the amcbe and plasmodia are nourished by the small food particles taken up by the protoplasm, which also exhibits active, internal, streaming movements. After an interval of a few days the plasmodium creeps to the surface of the substratum to the air and light, and passing into a resting stage becomes at length converted into a white sporangium with a double wall, consisting of an outer, calcareous, brittle peridium and an inner and thinner enveloping pellicle which, in addition to the numerous spores, encloses also a poorly developed capillitium. The development of the other Myxomycetes is accomplished in a similar manner. Very large plasmodia, often over a foot in breadth, of a bright yellow colour and creamy consistency, are formed by the tan-pit Fungus Fuligo varians (Acthaliwm septicwm), and as the ‘‘ flowers of tan”’ are often found in summer on moist tan bark. If exposed to desiccation, the plasmodia of this Myxomycete pass into a resting state, and become converted into spherical or strand-like scLEROTIA; from which a plasmodium is again produced on a-further supply of water. Finally, the whole plasmodium becomes transformed into a dry cushion or cake- shaped fructification of a white, yellowish, or brown colour. The fructification, in this instance, is enveloped by an outer calcareous crust or rind, and is subdivided by numerous internal septa. It encloses numerous dark violet-coloured spores, and is traversed by a filamentous capillitium, in which are dispersed irregularly- shaped vesicles containing granules of calcium carbonate. A fructification of this nature, or so-called ethalium, consists, therefore, of a number of sporangia combined together, while in most of the Myxomycetes the sporangia are simple and formed singly. The structure and nature of the sporangia afford the most convenient means of distinguishing the different genera. The following species may be mentioned as exhibiting characteristic differences in the form of their sporangia. Stemonitis fusca forms simple, stalked, cylindrical sporangia (Fig. 222, A), which are often found standing in clusters on dead leaves, bark, etc. The stalk is pro- longed as a columella through the sporangium, and gives rise to a deli- cate, reticulate capillitium, within the meshes of which lie the dark- violet spores. The peyidium is thin and non-persistent. Arcyria punicea produces its spherical sporangia on rotten wood. They are simple, stalked, of a reddish-brown colour, and without a columella. At ma- turity the peridium ruptures circu- larly and the upper part falls off, whereupon the capillitium attached to the basal walls of the sporangiun Fic. 222.—Ripe fructifications, after discharge of the SpEnge OW audenly, end ae mee spores. 4, Stemonitis fusca (x10); B, Arcyria the spores (Fig. 222, B). Cribraria punicea (x 12); C, Cribraria rufa (Xx 32). rufa also develops its reddish- brown sporangia on rotten stumps of trees. They are simple and stalked, without either columella or capillitium. The sporangia open at the top, but the thickened portions of the fragile peridium persist after its rupture in the form of a net-work (Fig. 222, C). Leocarpus fragilis, SECT. I CRYPTOGAMS 305 with its reddish-brown oval sporangia, may frequently be found on moss, grass- hauls, etc. The sporangia are simple, and have a double peridium and a reticulate filamentous capillitium, but no columella (fig. 224). Trichia varia, one of the commonest species on decaying wood, has Fic. 224. — Leocarpus Sragilis. a Groups of sporangia upon Moss. a sessile globose SPor- Nat. size.) angium with a yellowish peridium, which, after rupturing, forms a dish-shaped receptacle. The capillitium is made up of delicate tubes strengthened by spiral thickenings, and having free extremities (Fig. 223). A few Slime-Fungi, termed collectively Acrasieae, exhibit a more simple mode of spore-formation. The spores on germination give rise directly to amcebe without the previous development of swarm-spores. The amcebe multiply by division, and without previously undergoing fusion form so-called aggre- gate plasmodia. In the process of spore-formation each ameba of such aggregate plasmodia surrounds itself with a wall and assumes the nature of a spore. Plasmodiophora Brassicae, one of the few parasitic Myxomycetes, causes tuberous swellings on the lateral roots of various species of Brassica. Its plasmodia fill the cells of the hypertrophied parenchyma of these swellings, and these, eventually dividing into Fic. 293.—Trichiu varia. 4, MWmMerous spores, are set free by the disorganisation of Closed and open sporangia the plant. The spores germinate like those of Chon- (x6); B, a fibre of the capil- drioderma, and the Myxamebe penetrate the roots litium (x 240); C, spores (X of a young Cabbage-plant. The formation of true oan sporangia, however, does not take place, and this Slime-Fungus represents a more simply organised or, in consequence of its parasitic mode of life, a degenerate Myxomycete. Crass II Schizophyta (Fission-Plants) The Schizophyta comprise only Thallophytes, having very simple structure; they may be either unicellular or filamentous, consisting of a row of cells, or they may assume the form of cell colonies. They have no sexual mode of reproduction, and multiply only by cell division or by asexually-formed spores. They include two orders—the Fission-Algae or Schizophyceae, and the Schizomycetes (Fission-Fungi or Bacteria). The cells of the Schizophyceae contain an assimilating blue-green colour- ing matter. The Schizomycetes, on the other hand, which are only Xx 306 BOTANY PART IL exceptionally provided with such a pigment, live either parasitically or saprophytically, and may be regarded as a derived form of Schizophyceae. Order 1. Schizophyceae (Fission-Algae) The Fission-Algae were formerly thought to show a variation from other Algae in the differentiation of their cells. It was customary to distinguish within the protoplasts of their walled cells an apparently homogeneous colourless CENTRAL-BODY, separated from the other portion of the cell contents by a delicate membrane, and possessing a greater capacity for taking up stains. According to the recent investigations of HEGLER, this central body has, however, the structure of a true nucleus, and undergoes indirect karyokinetic division. In certain of the filamentous forms, special cells, no longer capable of division, may contain several nuclei, the number of which is in such cases the result of fragmentation. The cell nucleus is surrounded by a coloured peripheral layer. This layer may be considered as equivalent to a chromatophore ; it contains, in addition to chlorophyll, a blue-green or verdigris-coloured pigament, termed phycocyanin, to the presence of which this group of the SCHIZOPHYTA owes its name of CYANOPHYCEAE or Blue-green Algae. There are also found within the cells, usually lodged in the periphery of the chromatophores, small granular bodies of an unknown significance, the so-called cyanophycin grains; while mucous globules are also disposed in the vicinity of the nucleus. In addition to these, vacuoles occasionally occur in the cells. The cell walls consist of cellulose, and often exhibit distinct stratification, and in many species they undergo a mucilaginous modification of their outer layers. Multiplication is effected in a vegetative manner, simply by the division of the whole contents of the cells and by the formation of partition walls. In the case of the unicellular forms, included col- lectively in the family of the Chroococcaceae the daughter cells separate after the division, and become either entirely isolated or remain as cell colonies in proximity with one another. In the filamentous forms or Nostocaceae, the daughter cells continue in contact and form cell rows. These cell filaments eventually break up into shorter segments, which repeat the process of multiplication and segmentation. It is from this mode of reproduction by the division or fission of the cells that the name Fission-Algae has been derived. The Fission-Algae represented by numerous species are universally distributed. They occur as floating water forms, attached to stones and plants, or they form mucilaginous or pubescent coatings on damp soil, moist rocks, tree-trunks, moss, etc. 1. Chroococcaceae.—The simplest forms of the Schizophyceae are included in this family. The genus Chroococeus consists solely of isolated, rounded cells, which are enveloped by a thin wall and have a blue-green colour. In other genera cell SECT. I CRYPTOGAMS 307 colonies are formed by the daughter cells, which result from division, remaining enclosed ina common gelatinous envelope, formed by the mucilaginous degeneration of their cell walls. Thus, the four-cornered, tabular cell colonies of the genus Merismopedia, often found floating in the water, are formed by repeated cell division, which is always in one plane and in two directions only. The cell colonies of Glococapsa, whose different species form, for the most part, olive-green or blue- green patches on damp walls and rocks, present a peculiar appearance, as shown in Fig. 225. The walls of the cells are mucilaginous and swollen. When a cell divides, the walls of the daughter cells also become mucilaginous, while at the same time they remain enclosed within the walls of the mother cell. In this manner, through division in three dimensions of space, a cubical or rounded colony composed of 2, 4, 8 or more cells is produced which eventually breaks up into daughter colonies. 2. Nostocaceac.—The simplest forms of this family, in which are included the Fic. 225.—Glococapsa polydermatica. A, Fic. 226.—A, Oscillaria princeps ; a, terminal cell; 8, ce, In process of division ; B, to the left, portions from the middle of a filament. In c, a dead shortly after division; C, a later stage. cell is shown between the living cells. 3B, Oscillaria (x 540.) Froelichii; b, with granules along the partition walls. (x 540.) most highly developed of the Fissiou-Algae, are merely filamentous rows of cells, unbranched and without any distinction of base or apex. This is the case in the genus Oscillaria (Fig. 226), whose single filaments are motile and exhibit peculiar gliding movements. The filaments consist of disc-shaped, blue-green cells, with numerous small'granules disposed in their peripheral protoplasm, which, as a rule, appears to be especially accumulated along the transverse walls (Fig. 226, 8). The terminal cells of the filaments are usually rounded. By the rounding off and separation of any two adjoihing cells the whole filament may break up into short germinal segments, termed HORMOGONIA, Which then grow out again into long filaments. In species in which the filaments are invested with thick sheathing walls, the hormogonia creep out of the cell envelope, leaving only the empty sheath remaining. The species of Oscillaria are found in tufts, either freely float- ing or growing upon damp soil. While in the case of Oscidlaria and in several other genera the cells are all alike, many Wostocaceae not only develop special cells, termed HETEROCYSTS, which seem to be incapable of further development, but also thick-walled resting cells or sporgs, This is the habit of the genus Nostoc, which is found growing on damp 308 BOTANY PART II soil or floating in water in the form of gelatinous masses, in which are embedded the unbranched cell filaments like rows of beads. Heterocysts poorly supplied with cell contents occur at irregular intervals (Fig. 227, 2) in these chains of cells, while from the vegetative cells, richer in contents, spores (sp) are produced. On germination these spores give rise to a new filament composed of similarly united cells (Fig. 227, B, C). In certain Nostocaceae the cell filaments are characterised by false branching. This pseudo- branching occurs when a cell of a filament becomes bent outwards and is pushed upwards by the con- tinued division of the lower cells, so that the upper portion assumes the appearance of a lateral branch. Many Cyanophyceae take part with the Fungi in the formation of Lichens. Some species also are endophytic and inhabit cavities in other plants. Thus, species of Fic. 227.—Nostoe Linckii. A, Fila. IVostoc are constantly found in the tissues ment with two heterocysts (%), of certain Hepaticac, in Lemna, and in the anda large number of spores (sp) ; se B, isolated spore beginning to TOOts of Cycas and Gunnera ; and similarly germinate; €, young filament a species of .fnabaena occurs in Azolla. ae a Spore. “(After Especially interesting are the floating io : forms of the Cyanophyceae, which rise in quiet water to the surface, and collect there in large masses. In the protoplasm of the cells of these species (¢.g. Glocotricha echinulata, Anabaent flos aquae, of fresh-water lakes) are found numerous vacuoles, which are filled with gas and render it possible for the Algae to float on the surface of the water. Order 2. Schizomycetes (Fission-Fungi, Bacteria) The Fission-Fungi differ from the Fission-Algae principally through the absence of an assimilating green pigment in their cells. In them, too, no cell nucleus has as yet been found, although, according to HeGLer, a cell nucleus is present in certain species which he investi- gated. Their protoplasm is colourless and always enclosed by thin cell walls. In a condition of plasmolysis, induced by means of a salt solution, the protoplasm becomes contracted, and shrinks from the cell walls, from which it may be concluded that within the cells of Bacteria there is a sap cavity surrounded by a peripheral cytoplasmic layer. Like the Fission-Algae the Fission-Fungi occur under a great variety of forms. The latter, however, are of a much smaller size, includ- ing in fact the smallest of known living organisms. The spherical cells of Micrococcus prodigiosus, which develops on cooked potatoes, bread, milk and meat, and is distinguished by the formation of a blood-red SECT. I CRYPTOGAMS 309 pigment, measure only 0°0005 mm. in diameter, while the rod-shaped cells of the Tubercle Fungus, Bacillus Tuberculosis, are only from 0:0015 mm. to 0:005 mm. long. The simplest form of Fission-Fungi are represented by minute spherical cells, Cocct. Forms consisting of short, rod-shaped cells are designated BACTERIUM ; those of the same shape but longer are known as Baciutus. Simple cell filaments are termed LEPToTHRIX ; spiral, closely-wound filaments are classified as SPIRILLUM, when more loosely wound as VIBRIO, and longer spiral filaments as SPIROCHATE. In the highest stage of their development the Fission-Fungi consist of cell filaments exhibiting false branching, as in certain of the Nostocaceae. As in the Fission-Algae, but more frequently, the cell walls become swollen and mucilaginous. In this condition of their development, termed ZoocLi@a, the Cocci, Bacilli, etc., appear to be embedded in a gelatinous mass, as in the Alga Nostoc. While most Bacteria have only one form throughout the whole course of their growth, and are accordingly spoken of as species of the genera Ificrococcus, Bacterium, Bacillus, etc., there are, on the other hand, so-called pleomorphic species which exhibit differences of form cor- responding to different stages in their life-history. Multiplication of the individual is accomplished vegetatively by the active division or fission of the cells; the preservation and dis- tribution of the species by the asexual formation of RESTING-SPORES. Bacteria may be divided into the follow- ing two groups, according to their mode of spore-formation :— 1. ARTHROSPOROUS BACTERIA, in which vegetative cells, just as in the case of Nostoc (Fig. 227), simply become thick- walled and converted into spores (¢f. Leuconostoc, Fig. 231, £). 2. ENDOSPOoROUS BACTERIA, in which the spores are formed within the cells by the contraction of the protoplasm and its investment with a new cell wall (éf. Bacillus subtilis, Fig. 230, B). Many Bacteria are motile. Their in- dependent movements are due to the vibration and contraction of fine proto- Fre. 228.—Bacillus subtilis. Swarming plasmic cilia. These flagella, according to cisaine eal ca ye hee ie A. FISCHER, are distributed over the whole 5p heii. A, aiken 7 hows 42 aria surface of the cells (e.g. Bacillus subtilis, 8} hows, with fully-developed Fig. 228, and also the Typhus Bacillus), . (Alters Ae BISCHER: or they are polar, and spring from a single ‘ point. A single, polar flagellum occurs in Vibrio cholerae; a polar terminal tuft of flagella in Bacteriwm termo; a lateral polar tuft 310 BOTANY a PART II in the swarm-spores of Cladothriz. The ciliary tufts may become so closely intertwined as to present the appearance of a ‘single thick flagellum. The cilia, \ although arising from a pro- i trusion of the cell protoplasm, if are never drawn within the | body of the cell, but undergo fi dissolution before the forma- H tion of spores . takes place. The ‘existence of such special flagella has not as yet been demonstrated in the Fission- Algae, so that, in this respect, there is a characteristic differ- ence between them and the Fission-Fungi. SSeSeesss = SS Se oS SS Sess: S555 The Fission- Fungi are repre- sented by numerous species, and have a world-wide distribution. Although they present but little variety of external form, the sepa- rate and scarcely distinguishable species exhibit numerous variations in their metabolic and nutritive pro- cesses (cf. also pp. 212, 197). A dis- tinction is also made between sapro- phytic and parasitic forms. To the former belong the morphologically most highly-developed species, of which the highest is represented by Cladothria dichotoma. This Fission- Fungus is found in stagnant water, and consists of falsely branch- ing, delicate filaments (Fig. 229) attached to stones and Algae, and forming a slimy coating over them. The filaments are composed of rod-shaped cells enclosed within an outer filamen- tous sheath. Multiplication occurs through the separation from the parent filament of longer or shorter branches, which pass into 4 swarm stage and eventually fall into still smaller rod-like segments. These segments either escape from the enveloping sheath or are set free by its dissolution. Eight or ten flagella spring from a point on the side of the cylindrical swarm segments or, as they are termed, rod-gonidia. After swarming, the rod-gonidia settle down, and attaching themselves to a support grow out into new filaments. There are also always found associated with Cladothrix numerous other sapro- phytic Bacteria, Vibriones, Spirilla, Cocci, Zooglee. It is doubtful whether these are all merely different stages in the development of Cladothrix. This view has certainly not been positively demonstrated as yet by actual continuous observation. Among the most common filamentous Fission-Fungi occurring in water are the Sulphur Bacteria (e.g. Beggiatoa alba), which form small. granules of sulphur in Fia. 229.—Cladothrix dichotoma. Part of a branched filament with rod-shaped cells; treated with fuchsin. (After FiscHer, x 540.) SECT, 1 : CRY PTOGAMS 311 their cells if sulphuretted hydrogen be present in their environment. Another filamentous Fission- Fungus, Crenothrix Kiihniana, in the sheaths of whose filaments deposits of hydrated oxide of iron are found, is of frequent occurrence in springs and water-pipes, where it forms brown slimy masses and renders the water unfit for drinking. In both of these last-named Schizomycetes the filaments, unlike those of Cladothriz, are unbranched. The majority of Bacteria, like these important water-bacteria, maintain a saprophytic mode of life. Their metabolic processes vary in correspondence with their numerous decomposition products, and are usually adapted to definite conditions of nutrition. Thus the Hay bacillus, Bacillus subtilis, develops in an infusion of hay. The spores are able to withstand the heat employed in making the infusion, and produce in from 12 to 15 hours, on the surface of the liquid, a gelatinous pellicle consisting of closely compacted parallel filaments. Each filament is composed of long rod-shaped cells in active process of division (Fig. 230, 4). After exhaustion of the nutrient substance of the infusion, an endogenous formation of spores takes a iy i i A | i oo ae IN Nt iti it ie i on VV ASN i i a ii MW ANS ak ai i Fic. 230.—Buvillus subtilis. A, Pellicle of parallel filaments (x 500); B, formation of spores (x 800). place within the cells of the filaments (B). In germinating, the walls of the spores become ruptured on one side and their elongating protoplasmic contents emerge as rod-shaped swarm-spores provided with numerous flagella (Fig. 228), and multiply further by division. Many saprophytic Bacteria are characterised by their capacity to induce ferment- ation and putrefaction, and in the operation of their metabolic processes are able to decompose certain organic compounds. Thus Leuconostoc mesenterioides occasions the mucous fermentation of beet-sugar. It forms gelatinous masses resembling frog-spawn, consisting of a number of polygonal colonies enclosing rosary -like chains of cells within the mucilaginous sheaths (Fig. 231, D). In its mode of spore-formation this species of Fission-Fungus closely resembles the Fission-Algae Nostoc. Special cells of the chain become larger and transformed into arthrospores (£). In the process of germination these spores become invested with a gelatinous sheath (B), and develop into thick but short rows or chains of cells (C). These unite into colonies, and these again into groups of colonies, thus forming large gelatinous masses similar to the original. The Vinegar bacterium, Bacterium acett, oxidises alcohol into vinegar ; Bacillus amylobacter occasions the butyric fermenta- tion ; Bacteriwm termo the putrefaction of albumen, meat, etc. 812 BOTANY PART IT Among the parasitic Bacteria there are numerous forms which may be described as harmless, as for example Sorcina ventriculi (Fig. 232, 4), which forms cubical masses of cocci in the stomach and intestines of man ; also the various Bacteria, Micrococcus, Spirillum dentium, Leptothria buccalis, ete. (Fig. 4, p. 11), which occur in the cavity of the mouth. Of dangerous or pathogenic Bacteria which have been demonstrated to be the cause of infectious diseases, mention may here be made of the following: Bacillus Tuberculosis, the cause of tuberculosis (Fig. 232, C) ; -@2 2 fe 2 8 3 (3) F Sad A B soft @ o 4, $200? 0906.98 8% Fic. 231.—Leuconostoce mesenterioides. A, Iso- Fic. 232.—A, Sareina ventriculi (x700); B, lated spores; B, C, formation of chain of cells with gelatinous sheath ; D, portion of mature zooglea ; HZ, formation of spores in Spirochaecte Obermeieri (x 950); C, Bacillus Tuberculosis, plasmolysis of contents occa- sioned by mode of treatment (x 1500); D, the filaments of the zooglea. (After Van TIEGHEM, X 520.) Vibrio cholerae (x950); LE, Streptococcus pyogenes (X950). (After BAUMGARTEN.) Vibrio cholerae asiaticaec, the comma bacillus of Asiatic cholera (Fig. 232, D) ; Spirochaete Obermeicri (Fig. 232, B), found in the blood of patients suffering from intermittent fever ; Bucillus Typhi, the bacillus of typhoid fever; the pyogenic Bacteria, Streptococcus pyogenes (Fig. 232, £) and Staphylococcus aureus ; Strepto- coceus Erysipelatis, occurring in the lymphatic glands of persons affected with erysipelas ; Bacillus Anthracis, the anthrax bacillus, with a mode of spore-formation similar to that of the Hay bacillus. Rhizobium Leguminosarum: (Bacillus radicicola) lives in symbiosis with the Leguminosae, and causes the formation of their root-tubercles. After multiply- ing enormously in the cells of the root-tubercles, the Bacteria eventually undergo transformation into bacterioids (see p. 211). Crass III Diatomeae (Diatoms) The Diatomeae constitute a large class of unicellular Algae, including about 1500 species. They usually occur associated together in large numbers, in both fresh and salt water, and also on damp soil. The individual cells or FRUSTULES are either solitary and free- swimming, or they are attached by means of gelatinous stalks, excreted SECT. I CRYPTOGAMS 313 by the cells themselves. Sometimes these chains remain connected and form bands or zigzag chains, or, on the other hand, they are attached and enclosed in gelatinous tubes, while in the case of the marine genus Schizonema they lie embedded in large numbers in a gelatinous branching thallus, often over 1 dem. in breadth. The cells also display a great diversity of shape; while generally bilaterally symmetrical, they may be circular or elliptical, rod- or wedge-shaped, curved or straight. The structure of their cell walls is especially characteristic; it is composed of two halves or VALVES, one of which over- laps the other like the lid of a box (Fig. 3, B, p. 11). The cells thus present two altogether different views, according to the position in which they are observed, whether from the GIRDLE (Fig. 3, B) or VALVE-SIDE (Fig. 3, 4). Both valves are so strongly impregnated with silica, that, even when subjected to intense heat, they remain as a siliceous skeleton, retaining the original form and markings of the cell walls. The walls of the cells, particularly on the valve side, are often ornamented with numerous fine, transverse markings or ribs, and also with small protuberances and cavities. In many instances (Fig. 3) a longitudinal line corresponding to an opening in the cell walls, and exhibiting swollen nodules at both extremities and in the middle, is distinguish- able in the surface of the valves. Forms provided with such a median suture or RAPHE are characterised by peculiar backward-creeping movements, resulting from the extrusion of protoplasmic protrusions from their longitudinal edges. Each frustule has always a central nucleus and one (Fig. 3) or two large or numerous smaller (Fig. 233, D) chromatophores embedded in its parietal protoplasm. These chromatophores or ENDOCHROME PLATES, as they are often called, are flat, frequently lobed, and of a brownish-yellow colour. In addition to chlorophyll they contain a golden brown colouring matter, termed DIATOMIN. Globules of a fatty oil are also included in the cell contents, and take the place of starch as an assimilation product. The Diatomeae multiply vegetatively by bipartition, which always takes place in one direction. In this process the two valves are first pushed apart from one another by the increasing protoplasmic contents of the mother cell, which then divides longitudinally and always in such a direction that each of the two new cells retains one valve of the original frustule. After the division of the protoplasm of the mother cell is accomplished, each daughter cell forms, on its naked side, a new valve fitting into the old one. The two valves of a cell are therefore of different ages. In consequence of this peculiar manner of division, as the walls of the cells are silicified and incapable of dis- tension, the daughter cells become successively smaller and smaller, until finally, after becoming reduced to a definite minimum size, they undergo transformation into AUXOSPORES. The auxospores are usually two or three times larger than the frustules from which they arise, 3i4 BOTANY PART II and by their further development they re-establish the original size of the cells. The formation of.auxospores is accomplished in various ways. In the case of Melosira, a free-swimming genus whose cells are joined together in chains, the single cells simply swell greatly in size, and secrete two new valves (Fig. 233, D). An altogether different mode of spore- formation is exhibited by the isolated, unattached cells of Cocconema lanceo- latum (Fig. 233, B). In this instance, two cells place themselves together side by side, and throwing off their valves, surround themselves with an envelop- ing gelatinous mass. Each naked protoplast, without, however, under- going conjugation, is then transformed into a single large auxospore, which ultimately becomes invested with a new cell wall. In other genera true conjugation occurs ; thus, in the case of Himantidium pectinale (Fig. 233, A), each auxospore is the result of the conjugation of two individuals. On Fig. 233.—Formation of auxospores. A, Himanti- the other hand, in the formation of in ee . ee the auxospores of Epithemia turgida a ee er a eae D tier (Big. 235, o); cook bd une fonjugating Prrrzer.) frustules first divides into daughter cells, which then, fusing two and two with the corresponding daughter cells of the other frustule, give rise to two auxo- spores. The auxospores do not pass through a period of rest, but begin at once to multiply by division. Countless numbers of Diatoms live in the ocean, and they constitute also a pro- portionately large part of the PLANKTON, that is, the free-swimming organic world on the surface of the sea. The plankton Diatoms have no middle suture or raphe on the surface of their valves, and are especially adapted to swimming or floating. To this end they are often provided with horn-like protuberances or membranous wings, which, like the contrivances of seeds for a similar purpose, greatly enhance their buoyancy. Diatoms occur also as fossils. Their silicified valves form a large part of the deposits of Srticrovs EARTH, Kieselguhr, mountain meal, etc., and in this form they are utilised in the manufacture of dynamite. On account of the extreme fineness of the markings of their valves, it is cus- tomary to employ certain species of Diatoms as test objects for trying the lenses of microscopes. Plewrosigma angulatwm is commonly used for this purpose, and, with a sufficiently strong lens, it is possible to distinguish on the surface of the S-shaped valves a system of fine markings, forming a network of six-sided meshes to the right and left of the raphe. SECT. I CRY PTOGAMS 315 Ciass IV Peridineae The Peridineae or Dinoflageliata were formerly classed with the lowest animals, but are, in reality, unicellular Thallophytes. - They live for the most part in salt water, and form, together with the Diatomeae, an important part of the plankton floating on the surface of the ocean. Their cell plasma contains a nucleus, a com- plicated system of vacuoles, and light yellow, tabular chromatophores. The pres- ence of these chromatophores in the Peridineae has, in particular, been considered indicative of their vegetable nature. The Peridineae are further characterised by two long protoplasmic cilia or flagella, to the vibrations of which the move- ments of the cells are due. The flagella spring from the ventral side of the cells, and lie in two furrows, which cross each other at right angles, on their surface (Fig. 234). Only a few Peridineae are entirely naked ; most of them have peculiarly sculptured cell walls, consisting of intersecting cellulose plates or ribs. They multiply by division, and in the autumn form thick-walled cysts, in which condition they pass the winter. Conjugation has not been observed. In addition to the forms which, like Algae, sus- tain themselves by means of assimilating yellow Pinna chromatophores, there occur also colourless Peri- * ee 284 Remdiminine Dipes, ven: : ral view. (After ScHILLING, dineae, whose chromatophores are only represented x 750.) by colourless leucoplasts. Such species, although nearly related to the brown Peridineae, live either as saprophytes or in the same way as animals. Gymnodinium hyalinum, a colourless, naked, fresh-water form, exhibits a mode of life resembling that of a Myxomycete. For the purpose of absorbing nourishment it loses its cilia and assumes the form of an amceba; in this condition it encloses and digests small Algae. Crass V Conjugatae In the class of the Conjugatae is included a large independent group of green, fresh-water Algae, comprising over 1000 species, in the form either of solitary cells or filamentous rows of cells. They derive their name from their peculiar mode of sexual reproduction, which consists in the CONJUGATION of two apparently similar cells, resulting in the formation of a zyGosPpoRE. They are in this respect sharply distinguished from all the other green Algae, the Chiorophyceae, from which they may be distinguished also by the absence of any asexual mode of spore-formation, and by the complicated structure of their green chromatophores. 316 BOTANY PART ll 1. ZYGNEMACEAE.—In this family, all of which are filamentous in character, the genus Spirogyra, with its numerous species, is the best known. It is commonly found in standing water forming unattached masses of intertangled green filaments. The filaments exhibit no dis- tinction of base and apex, and are composed of simple rows of cells, which vary in length in different species. Growth results from the divi- sion and elongation of the cells in one direction only (cf. Fig. 65, p. 64). Each cell has a large nucleus situated either in the peripheral proto- plasm or suspended in the centre of the cell by protoplasmic threads extending from the parietal protoplasm. The name of the genus, Spirogyra, is due to the peculiar spiral form of its green band-like chromatophores. These spiral bands lie in the parietal protoplasm, and contain numerous pyrenoids (p. 71). In Fig. 235 is represented a species with three such spiral chromatophores ; in other species their number is some- times less, some- times more. The chromatophores in the other genera of the Zugnemaveae ex- hibit a variety of form; thus, in the filaments of Zyg- nema the chromato- phores are star- Fic. 235.—Cell from a eevee : filament of Spirogyra ; CoNnJUGATION, in k, nucleus; ch, chro- the case of Spirogyra, matophore; , (x 200.) pyre- noid. is preceded by the development of con- verging lateral processes from the cells of adjacent filaments. When two pro- cesses from opposite cells meet (Fig. 236, A), their walls become absorbed at the point of contact, and the whole protoplasmic contents of one cell, after contracting from the cell wall, passes through the canal which is thus formed into the opposite cell. Fra. 236.—A, Conjugation of Spirogyra quinina (x 240). B, Spirogyra longata (x 150); 2, zygospore. The protoplasm and nuclei of the conjugating protoplasts then fuse together and form a zygospore invested with a thick wall, and filled with fatty substances and reddish-brown mucous globules. It is the SECY. I CRY PTOGAMS 317 function of the zygospore to act as a resting-spore, to tide over the winter or a period of drought, and eventually, on germination, to give rise to a new filament of Spirogyra. This form of conjugation, which is the one peculiar to most species, is described as scalariform (Fig. 236, A), as distinct from the lateral conjugation of some species, in which two adjacent cells of the same filament conjugate by the development of coalescing processes, which are formed near their trans- verse wall (Fig. 236, B). 2. MrsocaRracEAE.—The representatives of this family are also composed of filamentous rows of cells, but exhibit a difference in their mode of conjugation. In this case, in the process of conjugation, which is either scalariform or lateral, only a portion of the protoplasm of both conjugating protoplasts, together with their nuclei and a greater part of their chromatophores, passes into the connecting canal, and there, fusing into a zygospore, becomes separated from the parent cells by transverse walls. 3. In the DesmipiAcrag, the third family of the Conjugatac, are comprised the Fic. 237.—A, Cosmarium coelatum in process of Fic. 238.—Closterium moniliferum ; p, division ; B, Cosmarium Botrytis; C, the same pyrenoid ; AK, vesicle with erystals. with fully-developed zygospore ; D, Micrasterias (x 240.) Crux melitensis. (After RALFs.) unicellular forms. They are ornamented with delicate markings, and, like the Diatoms, exhibit a great variety of form (Figs. 237, 238). Their cells are composed of two symmetrical halves, separated, as a rule, from each other by a deep constriction, the isthmus. Each half contains a large, radiate, irregularly defined chromatophore, or a number of plate-like chromatophores united into one. Within the chromatophores are disposed several pyrenoids, while the nucleus lies in the centre of the cell in the constriction. The cells themselves display a great diversity of form and external configuration (Figs. 237, 238). The cell walls are frequently beset with wart- or horn-like protuberances. In some genera there is no constriction between the two halves of the cell. This is the case, for instance, in the crescent-shaped Closterium moniliferum (Fig. 238), whose two chromatophores consist of six elongated plates, united in the long axis of the 318 BOTANY PART II plant, while in each end of the cell there is a small vacuole containing minute crystals of gypsum in constant motion. Many Desmids are characterised by heliotactic movements ; they protrude fine mucilaginous threads through the cell walls, by means of which they can push themselves along, and take up a position in a line with the direction of the incident rays of light. Multiplication is effected by cell division. This is accomplished by the forma- tion of a partition wall across the middle of the cell after the nuclear division is completed. Each daughter cell eventually attains the size and form of the mother cell, by the outgrowth of a new half on the side towards the new division wall (Fig. 237, 4). Atter the completion of their growth, the two cells separate from each other. The conjugation of the protoplasts takes place, in the case of the Desmidiaceae, outside their cell walls. Two cells approach each other, and surround them- selves with a mucilaginous envelope. Their cell walls rupture at the constriction, and parting in half allow the protoplasts to escape, which then unite to form a zygospore. The zygospores of the Desmidiaceae frequently present a very character- istic appearance, as their walls are often beset with spines (Fig. 237, C). The four empty cell halves may be seen close to the spore. Cuass VI Chlorophyceae (Green Algae) In the Chiorophyceae are included the majority of the Algae pro- vided with green chromatophores. They group themselves naturally into three orders, according to the structure of the thallus: the Proto- coccoideae, which include all the unicellular forms, whether living as isolated cells or as cell colonies ; the Confervoideae, comprising forms consisting of simple or branched cell filaments or cell surfaces; the Siphoneae, with a thallus variously developed, but usually consisting of a single, multinuclear, tubular cell. Sexual reproduction has not been demonstrated for all species of the Chiorophyceae. In the simplest cases it is effected by the conjuga- tion of naked gametes, of similar form and equal size. The gametes, as distinct from those of the Conjugatae, are motile ciliated protoplasts, and are known as PLANOGAMETES. In other genera there is a differ- entiation of the sexual cells into a female non-motile egg-cell or OOSPHERE and a motile ciliated male cell or SPERMATOZOID. Examples of this advance from ISOGAMY to OOGAMY are afforded by each of the above three orders. In addition to asexual reproduction, the Chlorophyceae almost always exhibit an asexual mode of reproduction by the formation of motile ciliated SWARM-SPORES (ZOOSPORES) which resemble the plano- gametes. The cells in which the swarm-spores are formed are termed SPORANGIA ; similarly those producing gametes are designated GAME- SECT. I CRYPTOGAMS 319 TANGIA. Cells in which spermatozoids take their origin are termed ANTHERIDIA ; those giving rise to egg-cells, ooGoNIA. If the sexual form be derived from an asexual form of reproduction, all these organs, as well as those similarly named in the other classes of the Thallophytes, must be regarded as homologous. The Conjugautae and Characeae, as well as the three orders of the Chlorophyceae, also possess green chromatophores, and hence the designation Green Algae, in its widest, unrestricted sense, is also applicable to them. The Conjugatae, however, are sharply characterised by their peculiar manner of sexual reproduction. The Characeae also form a distinct group, and are marked off from the Chlorophyceae by the more highly advanced segmentation of their thallus and the more compli- cated structure of the female sexual organs and of the antheridia, both of which are enclosed within special enveloping receptacles, while the antheridia and oogonia of the Chlorophyceac are always devoid of any external covering of sheathing sterile cells. Order 1. Protococcoideae The Protococcoideae include only unicellular Algae, whose cells lead a separate existence, or are united into cell families with a definite or in- definite order of arrangement. They occur, for the most part, as freely- swimming, fresh-water forms, but are also found in damp places. The cells are uninuclear, and contain one or more chromatophores. In the simpler forms multiplication takes place vegetatively by cell division ; but, in most cases, asexual swarm-spores, provided with two cilia, are produced. Sexual reproduction, which does not occur in all genera, is effected by the conjugation of two exactly similar planogametes which fuse into a zygospore or zygote. The fertilisation of an egg by a motile spermatozoid is only known to take place in the case of Eudorina and Volvox. Many of the Protococcoideae are polymorphous, and assume, according to the season of the year and the conditions of their environment, different external forms corresponding to different stages in their development. Scenedesmus acutus, a polymorphous free-swimming form, very common every- where in water, is gener- ally found in small cell- families, consisting of four spindle-shaped cells lying close together (Fig. 239, i,k). Under certain conditions, however, this Alga passes into the Pal- mella stage, and it then appears as spherical cells, multiplying by cell divi- sion (a, b). Each of these Fic. 239.—Scenedesmus acutus. Different stages of development. cells may again divide (After Cuopat.) into four spindle-shaped cells, which, after escaping from the mother cell, either remain isolated (c, d, e) 320 BOTANY PART II orconnected together by fine threads (Dactylococcus stage, Fig. 239, y). By the longi- tudinal division of the cells of these forms the four-celled Scenedesmus family may again be produced (f, /, 7, &). No formation of swarm-spores occurs in this Alga. One of the simplest forms of this order is represented by the genus Chlorella, which multiplies solely by cell division. This genus is particularly interesting also from a biological standpoint, as its small round cells live symbiotically in the plasma of Infu- soria, in the cells of Hydra viridis, Spongilla fluviatilis, and other lower animals. Pediastrum (Fig. 240) may be cited as an example of a genus which gives rise to cell-families. Each cell-family forms a free-swimming plate, composed internally of polygonal cells, and on the margin it consists of cells more or less acutely crenated. The formation of asexual swarm-spores is effected in Pediastrum by the division of the contents of a cell into a number (in the case of the species illustrated, P. granulatum, into 16) of naked swarm- spores, each with two cilia. The swarm- spores, on escaping through the ruptured cell wall (Fig. 240, 4, 0), are enclosed in a common envelope. After first moving vigorously about within this envelope, they eventually collect together and form a new cell-family. Pediastrum possesses also an asexual mode of reproduction. The gametes are all of equal size, and, except that they are smaller and are pro- ; duced in greater numbers, they are other- Fic. 240.—Pediastrum granulatum. A, An old é ao. a cell-family : u, cells containing spores; b, wise: similar tothe awamm-spores, ‘They spores in process of extrusion (the other Nove freely about in the water, and in cells have already discharged their spores); conjugating fuse in pairs to form zygotes. B, cell-family shortly after extrusion of The further development of the zygotes a 4h DOUrS later nto cell-families is not yet fully known. In the spring the cell-families develop from peculiar, thick-walled, spiniferous resting-cells or PoLYHEDRA, the contents of which separate into swarm-spores, which escape enclosed in a common envelope, and give rise toa new family. The polyhedra are probably formed from swarm- spores developed in the zygotes. The Volvocaceae include also forms whose cells live either isolated or united into of Fie. 241.—A, B, Sphaerella pluvialis (x360): A, swarming cell; B, formation of swarm-spores. C-(, Sphaerella Bitschlit: C, formation of gametes (x 400); D, gamete ; B, conjugation of two gametes ; I, G, zygotes (x 800). (C-G after BLocHMANN.) colonies, but which, unlike the types of the Protococcoideae heretofore considered 2 SECT. I CRYPTOGAMS 321 are also provided in their vegetative state with cilia and surrounded by a delicate envelope. The cilia, usually two in number, project through this external envelope, and by means of them the Algae of this family are enabled to swim freely about. In this respect they continue their vegetative existence in that condition which, in the case of the other Protococcoideae, is only assumed transitionally by the swarm- spores. The multiplication of the Volvocaceae is effected by simple division of the ciliated cells ; their sexual reproduction by conjugating gametes or by means of egg- Fic, 242.—Volvox aureus. A, Colony with three eggs, o, shortly after fertilisation; a, spermato- zoid-packets in process of development; ¢, vegetative daughter colonies (x180); B, sper- matozoid- packet of 32 cells, seen from above; C, the same seen from the side (x 687); D, spermatozoids (x $24). (After L. KLErn.) cells fertilised by spermatozoids. The genus Sphacrella (Haematococcus) belongs to the simplest solitary forms of this family, the presence of some forms of which (particularly 8. plwvialis), on account of the hematochrome contained in their protoplasm, often impart a bright red colour to small pools of water in which they are found. Sphaerella nivalis, another species of this same genus, is also the cause of the so-called ‘‘red-snow”’ of the snowfields in high northern latitudes and in the Alps. The swarm-cells have a widely-distended envelope and two cilia (Fig. 241, A). They can withdraw their cilia and become resting-cells, which eventually separate again into several swarm-cells by the division of their protoplasmic contents Y 822 BOTANY PART IT (B): The gametes, which may be produced in large numbers (32 or 64) in every cell (C), possess two delicate cilia, a red eye-spot, and a chromatophore. After swarming, the gametes conjugate in pairs (#) and give rise to zygotes (Ff). The zygotes become invested witha thick wall, and serve as resting-spores (@). While the gametes of Sphacredla and of most other Volvocaceae are similar and of equal size, in the case of Eudorina and Volwox, which may also be considered as the most highly-developed forms of the whole order, the sexual cells are more differ- entiated, and assume the form of large passive egg-cells and small biciliate sperma- tozoids. The genus Volvox, as represented by the species }. globator and V. aureus (V. minor), found in small pools and ditches, forms hollow, spherical colonies (ccenobia), which are often large enough to be visible to the naked eye. The colonies are composed of numerous cells (up to 22,000), regularly distributed in a peripheral layer. The cells are connected laterally with each other by proto- plasmic threads, usually six in number, which extend through their distended cell walls (Fig. 242, 4), and from each cell two delicate cilia are given off externally. The Volvox colonies multiply vegetatively by the formation and final escape of new daughter colonies, resulting from the division of a single cell(A,¢). Spermatozoids and egg-cells are produced either in the same or different colonies. The spermato- zoids arise through the division of special cells (so-called antheridia) into numerous daughter cells, which eventually form tabular packets of elongated spermatozoids (B, C). The anterior extremity of the spermatozoids of Volvox aureus is colourless, and terminates in two cilia; in their opposite, posterior end the spermatozoids contain a bright green chromatophore. In the anterior portion there are a lateral red eye-spot, two contractile vacuoles, and w cell-nucleus (D). The egg-cells are produced by the enlargement of individual cells of the colony. They are large and green, non-motile, and surrounded by a gelatinous envelope (4, 0). After fertilisation by the spermatozoids, which, in swarming, escape into the interior of the hollow spherical colony, they become transformed into firm-walled resting oospores, which on germination gives rise to a new colony. The mother colony dies after the egg-cells have reached maturity. Order 2. Confervoideae The Confervoideac exhibit, as compared with the unicellular Proto- coccoideae, an advance in the external segmentation of the thallus. It is always multicellular, and, in most of the genera, consists of simple or branched filaments. The thallus of the marine genus Ulva (Ulta lactucu, SEA LETTUCE) has, however, the form of a large, leaf-like cell surface (Fig. 5,p. 12). Although a greater part of the Confervoideae live in fresh or salt water, where they are found either free-swimming or attached to some substratum by a colourless basal root-cell, a few aerial forms (Chroolepideac) grow on stones, trunks of trees, and, in the tropics, on leaves. To this family belongs the aerial Alga Trentepohliu (or Chroolepus) Jolithus, often found growing on stones in mountainous regions. The cell filaments of this species appear red on account of the hematochrome they contain, and possess a violet-like odour. The asexual reproduction of the Confervoideae is accomplished by the formation of ciliated swarm-spores, although in many cases they may also develop resistant resting-spores. SECT. I CRY PTOGAMS 323 Sexual reproduction is effected either by the fusion of plano- gametes (p. 319), or the sexual cells are differentiated as non-motile ege-cells and motile spermatozoids. Ulothri« zonata, almost everywhere abundant in fresh water, may serve as a type of the isogamous Confervoideac. The filaments of Ulothria exhibit no pronounced apical growth ; they are unbranched, attached by a rhizoid cell, and consist of single rows of short cells (Fig. 243, 4). Each cell contains a nucleus and one band-shaped, green chromatophore in the form of an almost complete hollow cylinder. Asexual repro- duction is effected by means of swarm-spores (1-8), which have four cilia (C), and are formed by division in any cell of the filament. The swarm-spores escape through a lateral opening (2) formed by absorption of the cell wall, and, after swarming, give rise to new filaments. The sexual swarm-cells, or planogametes, are formed in a similar manner by the division of the cells, but in much greater numbers. They are also smaller, and possess only two cilia. In other respects they resemble the swarm- spores, and possess a red eye-spot and one chromatophore. By the conjugation of the planogametes in pairs, zygotes (/-H) are produced, which, after drawing in their cilia, round themselves off and become invested with a cell wall. After a shorter or longer period of rest the zygotes are converted into uni- cellular germ plants (J), and give nee hifi ae SWarMeSDOLES (X), Fic. 243.—Ulothrix zonata. A, Young filament with which in turn grow out into new rhizoid cell r (x300); B, portion of filament with filaments. Ulothrix, like many fila- escaping swarm-spores ; C, single swarm-spore ; D, mentous Algae, passes into a so- formation and escape of gametes ; Z, gametes; F, G, called Palmella stage, in which, conjugation of two gametes ; H, zygote ; J zygote t meas after period of rest; K, zygote after division into under certain conditions, the Sep a swarin-spores. (After DoDEL-Port, B-K x 482.) rate cells of the filaments give rise by division to colonies of cells. The individual rounded cells thus produced have often been mistaken for species of Protococcoideac. In this manner, according to CHopArt, is formed the common Plewrococcus vulgaris, which occurs as the green covering on the trunks of trees, and consists of round cells which multiply by division, in which, however, the formation of swarm-spores has been suppressed in the course of adaptation to an aerial mode of existence. In its unicellular condition, accord- ing to CHopAT, the cells are round, and multiply by division ; they either remain 324 BOTANY PART II isolated or they may be united in groups of two or more ; but under some circum- stances they produce short, branched cell filaments. Cladophora is a genus comprising numerous species, including Cladophora glom- erata, a form specially abundant in rivers. It consists of branched filaments of long cells, growing in tufts attached to a support, and exhibiting well-marked apical growth (Fig. 6, p. 12). The cells, unlike those of Ulothrix, are multinuclear, and contain also numerous polygonal, closely-crowded chromatophores (Fig. 60, p. 59). By the protrusion and elongation of lateral outgrowths from the cells just below their upper transverse walls, the filaments become extensively branched ; while, in addition to their apical growth, they increase in length also by the division of the cells and the formation of new transverse walls (Fig. 66, p. 64). The swarm-spores of this species are biciliate (Fig. 244), and are formed in large numbers in the cells at the tips of the branches, from which they escape through an opening in the upper end of the lateral wall. Having completed their swarming, they become invested Fic. 244.—Cladophora glome- Fic. 245.—A, B, Oedogoniwm.: A, escaping swarm-spores ; B, free ria. Swarm-spore. (x 540.) swarm-spore. C, D, Oedogoniwm ciliatum: C, before fertilisa- tion; D, in process of fertilisation ; 0, oogonia; a, dwarf- males ; S, spermatozoid. (After PRinesHErs, x 350.) with a cell wall, and, after a period of rest, they eventually grow out into a new cell filament. In other species of Cladophora, smaller, sexual swarm-spores have also been observed which, as in the case of Ulothria, fuse together in pairs in the pro- cess of conjugation. The genera Ocdogonium and Bulbochacte may be quoted as examples of oogamous Confervoideae. While the thallus of the latter is branched, the numerous species of Oedogoniwm consist of unbranched filaments, each cell of which possesses one nucleus and a single parietal chromatophore composed of numerous united bands. The asexual swarm-spores of Oedogoniwm are unusually large and have a circlet of cilia around their colourless anterior extremity (Fig. 245, B). In this case the swarm-spores are formed singly, from the whole contents of any single cell of the filament (4), and escape by the rupture of the cell wall. For the purpose of sexual reproduction, on the other hand, special cells become swollen and differentiated into barrel-shaped oogonia. A single large egg-cell with a colourless receptive spot is formed in each oogonium by the contraction of its protoplasm, while the wall of the oogonium becomes perforated by an opening at a point opposite the receptive spot of the egg. At the same time, other, gener- ally shorter, cells of the same or another filament become converted into antheridia. SECT. I CRYPTOGAMS 325 Each antheridium gives rise either to one or, as is more generally the case, to two spermatozoids. The spermatozoids are smaller than the asexual swarm -spores, but have a similar circlet of cilia. They penetrate the opening in the oogonium and fuse with the egg-cell, which then becomes transformed into a large, firm-walled oospore. On the germination of the oospore its contents become divided into four swarm-spores, each of which gives rise to a new cell filament. In the adjoining figure (Fig. 246) a germinating oospore of Bulbochaete with four swarm-spores is represented. In some species of Ocdogonium the process of sexual reproduction is more complicated, and the spermatozoids are produced in so-called DWARF MALES. These are short filaments (Fig. 245, C, «) consisting of but few cells, and are developed from Fic. 246.—Bulbochacte intermedia. asexual swarm-spores (ANDROSPORES) which, after A, Oospore ; B, formation of four swarming, attach themselves to the female fila- eee le SermnALne 3 oospore. (After PrRiInGSHEIM, x ments, or even to the oogonia. In the upper cells 959) of the dwarf-male filaments thus derived from the androspores, spermatozoids are produced which are set free by the opening of a cap-like lid (Fig. 245, D, a). In consequence of the greater complication in the process of their sexual reproduction, the oogamous Confervoideae are considered to represent a higher stage of development than the isogamous forms. Order 3. Siphoneae The Siphoneae are distinguished not only from the Chlorophyceae but from all other Algae by the structure of their thallus, which, although more or less profusely branched, is usually composed of but one cell, or if it is multicellular, each cell contains several nuclei. In the first case, the cell wall encloses a single protoplasmic mass, in the peripheral portions of which are embedded the many nuclei and numerous small green chromatophores. In the class of the Hyphomycetes, the Phycomycetes, or Algal Fungi, exhibit the same characteristic structure, and may be regarded as probably derived from the Siphoneae. The Siphoneae comprise about forty genera, which, however, do not include a great number of species. They live for the most part in salt-water, although the species of /’aucheria thrive in fresh-water or are found as terrestrial Algae, growing on damp soil. Botrydiwm is also terrestrial, while some forms of the Siphoneae are endophytic, and live in the leaves of the higher plants. Sexual reproduction has advanced to oogamy only in the genus Vaucheria ; in other instances it is isogamous and the conjugating gametes are alike in form and size. The simplest form of the Siphoneuc is represented by Botrydiwm, to which genus belongs the cosmopolitan species Botrydiwm granulatum. This Alga grows on damp clayey soil, where it forms groups of green, balloon-shaped vesicles about two millimetres in breadth. The vesicles are attached to the ground by prolongations from the base, in the form of a branching system of filamentous rhizoids devoid of ’ 326 BOTANY es PART II chromatophores (Fig. 247, 4). The cell walls of the vesicle and rhizoids of each individual enclose but one protoplast. Multiplication may take place vegetatively, by budding, resulting in the outgrowth of a new vesicle from the aerial portion of the thallus. After enlarging ¢onsiderably in size and sending down rhizoids into the substratum, the young plantlet isolates itself from the mother vesicle by a new cell wall. Asexual reproduction is provided for by the formation of swarm- spores. In this process the whole plant becomes converted into a single sporangium by the division of its protoplasmic contents into numerous swarm-spores, which make their escape through an opening at the apex. Each swarm-spore has two to four chromatophores, but only a single cilium, which is situated at its anterior, colourless Fia. 247.—Botrydiwm granulutum. A,The Fic. 248.—Vaucheria sessilis. A, B, A sporangium in whole plant ; B, swarm-spore ; C, plano- process of formation ; C, D, E, formation of a swarm- gametes; a, a single gamete; b-c, two spore (X 95); F, swarm-spore (xX 25); G, portion of gametes in process of fusion; f, zygote. the colourless peripheral protoplasm in the anterior (A X 28; B, C x 540.) end of the swarm-spore (x 950). end (Fig. 247, B). The formation of swarm-spores occurs only when the thallus is covered with water. After coming to rest the heliotactic swarmers (p. 243) invest themselves with a cell wall and give rise to new plantlets. Sexual reproduction may also oceur. For this purpose, in summer or in times of drought, the proto- plasm of the vesicles becomes broken up into a number of rounded or angular non- motile spores or APLANOGAMETES. These spores may remain at rest, perhaps for a period of a year or more, until supplied with water, when numerous small sexual planogametes (C, a) are formed from their contents. These planogametes are each provided with two cilia and a red eye-spot, and, by conjugating in pairs, give rise to zygotes (b-/). The zygotes round themselves off and germinate, either directly or after a period of rest. The planogametes are also heliotactic. Through the for- mation of the gametes within the resistant resting-spores the latter acquire the character of gametangia. SECT. I Pe CRYPTOGAMS 327 The thallus of Vaucheria, the only oogamous genus of the Stphoneae, also cou- sists of a single cell attached to the substratum by means of colourless rhizoids ; but its aerial portion, unlike that of Botrydiwm, is branched and filamentous. The swarm-spores of Jwucheria are developed in special sporangia, cut off from the swollen extremities of lateral branches by means of transverse walls (Fig. 248, A-E). The whole contents of such a sporangium become converted into a single green swarm-spore. The wall of the sporangium then ruptures at the apex, and the swarm-spore rotating on its longitudinal axis forces its way through the open- ing. The swarm-spore (/’) is so large as to be visible to the naked eye, and contains numerous nuclei embedded in an investing layer of colourless protoplasm. It is entirely surrounded with a fringe of cilia, which protrude in pairs, one pair opposite each nucleus (@). Morphologically the swarm-spores of Vaucheria correspond to the collective individual spores of Botrydiwm. The sexual reproduction of Vaucheria is not effected like that of the other Siphoncae, by the conjugation of motile gametes, from which, however, as the earlier form of reproduction, it may be considered to have been derived. The oogonia and antheridia first appear as small protu- berances, which grow out into short lateral branches and become separated by means of septa from the rest of the thallus (Fig. 249, v, a). At first, according to OtrmManns, the rudi- ments of an oogonium contain numer- ous nuclei, of which all but one, the nucleus of the future egg-cell, retreat again into the main filament before the formation of the separative septum. Fic. 249.—Vaucheria sessilis. Portion of a filainent In its mature condition the oogonium tan ovgoium, anther 4; oo has on one side a beak-like projection (3 240.) containing only colourless protoplasm, while the rest of the oogonium is filled with numerous chromatophores and oil globules. The apical portion of the projection becomes mucilaginous, and is finally ruptured by the extrusion of a colourless drop of protoplasm from the egg-cell which, in the meantime, has been formed by ,the contraction of the contents of the oogonium. The antheridia, which are also multinuclear, are more or less coiled (a), and open at the tip to set free their slimy contents, which breaks up into a number of swarming spermatozoids. The spermatozoids, which are very small and entirely devoid of chromatophores, consist chiefly of nuclear substance. They collect around the receptive-spot of the egg-cell, into which one spermatozoid finally penetrates. After the egg-cell has been fertilised by the fusion of its nucleus with that of the spermatozoid, it becomes invested with a wall and con- verted into a resting oospore. : The marine Siphoneae, on account of the more complicated segmentation of their thallus, afford one of the most interesting types of algal development. The genus Caulerpa, represented by many species inhabiting the warmer water of the ocean, has a thick, creeping main axis or stem. Increasing in length by apical growth, the stem-like portion of the thallus gives off from its under surface profusely branched colourless rhizoids, while, from its upper side, it produces green thalloid segments which vary in shape in the different species. In Caulerpa prolifera (Fig. 250) these outgrowths are leaf-like, are frequently proliferous, and have only a limited growth. In other species they are pinnately lobed or branched. The whole 328 BOTANY PART II thallus, however branched and segmented it may be, encloses but one cell-cavity, which is, however, often traversed by a network of cross-supports or trabecule. The thallus of Codiwm, also a marine form, consists at first of a single cell, but in time develops lateral outgrowths which become thickly intertwined and cut off by transverse walls. In the case of Codiwm Bursa, the vegetative body thus formed has the shape of a hollow sphere, while the thallus of Codiwm tomentosum is cylindrical and dichotomously branched. The genus Bryopsis, on the other hand, has w delicate, pinnately-branched thallus. Although originally unicellular, the Fic. 250.—Caulerpa prolifera. The shaded lines on the Fic. 251.—Acetabularia medi- thallus leaves indicate the currents of protoplasinic move- terranca. (Nat. size.) ment; @, growing apex of the thallus axis ; b, b, young thallus lobes ; 7, rhizoids. (4 nat. size.) thallus develops lateral tubular branches that eventually become septated from it by the formation of transverse ,walls. Other marine Siphoneae become encrusted with calcium oxalate and calcium carbonate, and bear a resemblance to coral, e.g. Halimeda Opuntia, which resembles Opuntia on a small scale.