sacz m ^ftkfi A'MT} ALBERT R. MANN LIBRARY AT CORNELL UNIVERSITY CORNELL UNIVERSITY LIBRARY 3 1924 054 697 002 The original of tliis book is in tlie Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http ://www. arch i ve . o rg/detai Is/cu31 924054697002 HEREDITY AND EVOLUTION IN PLANTS GAGER FRONTISPIECE Restoration of a scene along a sluggish creek in Texas and New Mexico during the late Carboniferous (Upper Pennsylvanian) and early- Permian times. The lowlands of this period doubtless swarmed with reptiles such as shown in the picture, and with other animals, now extinct. Some specimens of the giant "dragon-flies" had a spread of wings of two feet. The fern-like trees and the bushy plants in the fore- ground are Cycadofilicales. To the right of the water are wide stretches of the huge scouring rush {Calamites); on the left bank of the stream are the unbranched Sigillarias (still as prominent as earlier in the coal period), and on higher ground to the left the branched Lepidodendrons. One must view this scene as one of many such landscapes, with ever- varying detail, along streams and inlets. Cordaites, which in later Devonian time made the first great forests of which there is record, is still present, though not shown. So, too, there are hidden in the recesses of the forest the forerunners of the modern coniferous types, as well as other forms destined to give rise to the angiosperms. (Landscape from Williston, adapted from Neumayr.) HEREDITY AND EVOLUTION IN PLANTS BY C. STUART GAGER DIRECTOR OF THE BROOKLYN BOTANIC GARDEN WITH lis ILLUSTRATIONS PHILADELPHIA P. BLAKISTON'S SON & CO, 1012 WALNUT STREET 1920 Copyright, 1920, by P. Blakiston's Son & Co. T II K M A !■ I. K Tit K K K V O K IC V A To the Memory of BENJAMIN STUART GAGER " What a science Natural History will be when . . all the laws of change are thought one of the most important parts of Natural History." — Charles Darwin. (Letter to J. D. Hooker.) PREFACE The present little book was originally intended to be merely a reprint of Chapters XXXI to XXXVIII of the author's Fundamentals of Botany. The reprinting of those chapters was suggested by comments received from various correspondents, who pointed out that the subject matter which they cover had not been elsewhere presented in so concise a treatment in one volume, and in a manner suited, not only to beginning students, but also to more general readers. The chapter on Experimental Evolu- tion has received the approval of the author of the mu- tation theory, as being an accurate presentation of the essentials of that theory. " I have especially appreci- ated," writes Professor de Vries, " the statement of the difference between fluctuating and saltative variation, which is, to my mind, the real empirical basis for the theory, far more than the experiments on mutation with single plants. The relation of my view to Darwinism is misunderstood by many authors, and it is a great satis- faction to me that you have outlined it in such a plain way." In the preparation of the copy for reprinting, consider- able new matter has been added, certain sentences and paragraphs, pertinent only to an elementary text-book, have been omitted, and others recast, and several fresh illustrations have been introduced, either as new or as substitutes. XU PREFACE Chapters X, Geographical Distribution, and XIII, The Great Groups of Plants, and the Bibliography are new. No attempt has been made to cite the voluminous periodical literature in the Bibliography, but needless to state, this has been freely consulted and drawn upon. Numerous citations are given as foot-notes, especially in Chapter X. In going over the chapters it also became evident that since, in order to read them understandingly, one must have a clear conception of the facts of the lift history of a vascular plant, it would be best to introduce from the Fundamentals of Botany the three chapters (viz. XII- XIV) on the life history of the fern. As stated in the Preface to that book, while the ultimate problem of botany is the development of the kingdom of plants, the more immediate and fundamental problem is the development of the individual plant. "Ontogeny is fundamental because without a knowledge of its processes the processes of phylogeny cannot be comprehended. Phylogeny is the ultimate problem because its complete solution in- volves an orderly description of all the phenomena of plant life, and their relation to each other." The author is specially indebted to Dr. 0. E. White, curator of plant breeding in the Brooklyn Botanic Garden, for a careful reading of the entire manuscript and for many valuable suggestions; also to Mr. Norman Taylor, curator of plants, in connection with Chapter X, and to Dr. Alfred Gundersen, associate curator of plants in the same institution, for numerous constructive criticisms in connection with Chapter XIII. The diagram show- ing the apparent affinities and approximate geological distribution of the main groups of vascular plants (p. PREFACE Xin 248) originated with Dr. Gundersen, but has been modi- fied, as here printed, in certain details for which the author alone is responsible. Grateful acknowledgment is made to Dr. Ralph E. Cleland for photo prints of Figs. 74 and 75 from negatives made by him on the summit of Mt. Madison (Adirondacks) ; and to Prof. Harvey W. Shimer, author, and The Macmillan Co., publisher, for permission to reproduce Fig. 66. If the following pages shall prove to be a source of re- liable and readable elementary information to those in- terested in the subjects treated, the object of the book will be accomplished. Brooklyn Botanic Garden, March 25, 1920. C. Stuart Gager. CONTENTS Chapter I. LiEE History of a Fern I[. Life History of a Fern (Concluded) III. Fundamental Principles IV. Heredity v.. Experimental Study of Heredity VI. Evolution Vil. Darwinism VIII. Experimental Evolution IX. The Evolution of Plants X. Geographical Distribution XI. Paleobotany ... ... XII. The Evolution of Plants {Concluded) XIII. The Great Groups of Plants Bibliography. . . . .... Index . . Pace I 20 • 31 4S • • SS • 79 . . . . • . 90 . . . lOI . . 124 • 139 183 . . 201 ... 243 ... 252 .... 257 HEREDITY AND EVOLUTION IN PLANTS CHAPTER I LIFE fflSTORY OF A FERN 1. Life History. — Every plant, in the course of its ex- istence, passes through a series of changes in orderly Fig. I.- — A fern {Anisosorus hirsutus), showing portion of the stem above ground. sequence. Like an animal, every plant begins life as a single cell, the egg, or the equivalent of an egg. Except in some of the lower forms, the egg develops into an HEREDITY AND EVOLUTION IN PLANTS Fig. 2. — Portion of the rhizome of the common brake {Plcris aquilina) showing a cross-section view at the right. Fio. 3. — Cross-section of the rhizome of the bracken fern {Pleris aqui- lina), showing the tissue systems. Greatly magnified. LIFE HISTORY OF A FERN 3 embryo, and the embryo matures into an adult. By a series of more or less complicated processes the adult eventually gives rise to another egg, like the one from which it came, thus completing one life-cycle and initiat- ing another. These various changes constitute the life %j «* "■^^ ^^^- ■-^ Jj^ ^^gT->r,«v^^^^^^HH d|^H^T^gi3K^^ 1 r w €^ ^^ ^^\ / / ^\ 'L"Jt ^^ / ..,-ai i^^S^^ g -JlI [A^ 'T'^ M'~""w^M KMC ^^(m K'. ^■'^ t. 1 ^^v"? '■ Fig. 4. — Tree ferns on the military road between Cayey and Caguas, Porto Rico. (Photo by M. A. Howe.) history of the individual. The various stages of life history common to most plants are nowhere more clearly illustrated than in the ferns. 2. Description of a Fern Plant. — The more common ferns of temperate regions have underground stems or rhizomes (sometimes called root-stocks), so that only the 4 TIKREDITY AND EVOLUTION IN PLANTS leaves appear above ground.^ The stem may be branched or unbranched. When branched, the branches are pro- duced without reference to the insertion of the leaves, in contrast to the habit of higher plants of forming branches only in the upper angle (axil) between the stem and leaf-stalk. There is always a terminal btid at the ). — A, Upper epidermis; B, lower epidermis of the leaf of the fern, Drynaria meyeniana. (Camera lucida drawing.) tip of the fern-stem (and of the branches when any oc- cur) ; and the leaves are usually attached just back of this tip. The stems are commonly (though not always) covered by hairs or scales (Fig. i), and on their older portions, at some distance back from the tip, may be seen the scars, or the ends of leaf-stalks, left by old leaves that ' The leaves of ferns are often called fronds. LIFE HISTORY OF A FEEN have died and fallen away. The rhizome bears true roots (Fig. 2), and its tissues are differentiated into epider- mal, fundamental, mechanical, and conducting systems (Fig. 3). In tropical countries there are "tree ferns," FiQ. 6. — Osmimda Clayt-oniana. Young sporophylls, showing circinate vernation. Note the spore-bearing pinna;. with upright stems, and this type of fern is common among the fossil plants of earlier geological ages (Fig. 4). There are also climbing ferns. 3. Two Kinds of Fern-leaves. — Careful examination of the leaves of certain mature ferns will disclose the fact 6 HEREDITY AND EVOLUTION IN PLANTS that they are not all alike. Some of them are merely foliage-leaves, and do not differ in any essential point from the foliage-leaves of higher plants, such as the maple or lily; they possess stomata for the exchange of gases and Fig. 7. — Portions of the sporophylls of two ferns to show the sori. On the left Poly podium punclatum (L.) Sw. ; on the right a variety of Pleris longifoUa, with sporangia marginal on the pinnules. water-vapor with the outer air (Fig. 5), and they also resemble the leaves of higher plants in their internal struc- ture. All fern-leaves, however, have a very characteristic arrangement in the embryonic or bud condition, being LIFE HISTORY OP A FEEN coiled up from the tip. As the leaves grow they unroll, and in some ferns, at certain stages, they often closely re- semble the neck of a violin (Fig. 6). The leaf -blade Fig. 8. — Sporophylls of two ferns. At the left, a species of Polypodium {Phymatodes) , having no indusium; at the right, Diplazium zdamcum. possesses veins of iibro-vascular bundles that pass down the leaf -stalk and through the stem to the roots. Because of the possession of these vascular bundles, ferns (and all other plants of which this is true) are called vas- 8 HEREDITY AND EVOLUTION IN PLANTS cular plants. These leaves perform all the functions performed by the foliage-leaves of other plants, the most important of which are the manufacture of organic, car- bohydrate food from inorganic raw materials (photosyn- thesis), and the giving off of water vapor from within (transpiration). 4. Spore-bearing Leaves. — The second type of fern- leaf bears, on its underside, numerous "fruit-dots" or sort (singular sorus) (Figs. 7 and 8). These structures have to do with reproduction. A single sorus of such a fern Fig. 9. — Cross-section through the marginal sorus of a sporophyll of the bracken fern {Pteris aquilina). I, palisade layer; fb, vascular bundle; sp, sporangium; in, indusium. (Greatly magnified.) as, for example, Polypodium, is composed of a cluster of tiny stalked cases. The cases contain minute unicellular reproductive bodies called spores, and the entire structure is a sporangium. The place where the sporangia are attached to the leaf is the sporangiophore^ (Fig- 9), and over all is often found a thin membranous covering, the indusium (Figs. 9 and 10). In some ferns the indusium is lacking, and the sorus is naked. Spore-bearing leaves are called sporophylls, and plants that bear sporophylls are called sporophytes. ^AlsQ called receptacle, LIFE inSTOEY OF A FEJIN Fig. io. — Cyrtomium fakatum. Under (dorsal) surface of a portion of a sporophyll, showing the numerous sori on the pinnae. Fig. II.' — Fern leaves, showing various degrees of subdivision or brandl- ing of the blade. A, Phyililis; B, Polypodium; C, Pleris; D, Adianlum. HEREDITY AND EVOLUTION IN PLANTS 5. Types of Foliage-leaf.— In some ferns the foliage- leaf presents a simple, unbranched blade, and petiole; but in other species the blade is variously branched. In such cases the larger, primary divisions are called pinna, and the secondary subdivisions pinnules. Illustrations of these various types are shown in Fig. ii. 6. Sporangia. — As noted above, each sporangium con- sists of a spore-case borne on a stalk (Fig. 1 2) . The struc- ture of the case varies considerably in various groups of ferns, but it usually possesses walls only one cell thick, with a clearly differentiated region, the annulus, composed of cells whose radial and inner cell-walls are greatly thick- ened. Various types of spore-cases are illustrated in 12.- — Sporangia of an undetermined species of fern; li, lip-cells; an, annulus; st, stalk; sp, mature spores. Each of the four nuclei in the upper cells of the stalk is in the terminal cell of one of the tour vertical rows of cells that compose the stalk. Fig. 13. Among the group of ferns which are now most common, and to which the bracken fern (or "brake"), the maiden-hair fern, the common pol3q5ody, and others belong, the sporangium always originates from a single epidermal cell. Ferns whose sporangia thus originate are called leptosporangiate ferns (C/. p. 29). The walls of their spore-cases are always only one cell thick, and LIFE HISTORY OF A FERN II always possess some form of annulus. As the sporangia mature the spore-case itself becomes differentiated into two distinct kinds of tissue, namely, vegetative tissue on the outside, forming the wall and reproductive tissue within, from which the spores are developed. 7. Niunber of Spores.^The number of spores pro- duced by a vigorous fern is a great revelation to one who has never given such matters . careful thought. Pro- FiG. 13.— Types of fern sporangia. A, Loxsoma CunninghaiAl; E, Gleichenia circinata; C, Todea barbara; D, Thyrsopteris elegans; E, Matan& peclinala; F, Lygodmm japonicum. (Redrawn from various sourd^.) ' „■!'-:■*'■ ^ fessor Bower, of Glasgow, has called attention to thjs,f act in" the following words : , -' '"' "A rough. estimate may be made of the numerical output of spores ffom a large plant of the Shield fern, as follows: In each sporangium 48' spores may be formed; a sorus will consist of fully 100 sporangia, usually more; 20 is a moderate estimate of the sori on an average pinna; there may be fuUy 50 fertile pinnae on one well-developed leaf, and a strong plant would bear 10 fertile leives. 48 X ico X 20 X 3° X 10 = 48,000,000. The output of spores on a strong plant in the single season will thus, on a moderate estimate, approach the enormous number of fifty millions." 8. Types of Sporophylls — In many ferns the leaves serve both vegetative and reproductive functions in about ' Bower gives this number as the characteristic output for the species Aspidium Filix-mas. In other species the number may be 64. 12 HEREDITY AND EVOLUTION IN PLANTS equal degree, as in the case of Polypodium mentioned above. In some species, however, there are two kinds of leaves — one devoted entirely to vegetative functions, and another to the reproductive, or spore-producing function (Fig. 14); between these two extremes all grades of transi- tion are found (Fig. 15). But however widely the sporo- FiG. 14. — The cinnamon fern {Osmiinda cinnamomea), showing foliage' leaves and sporophylls. phyll departs from a foliage-leaf in appearance, it must, nevertheless, be regarded as morphologically a leaf. As partial evidence of the true foliar nature of sporophylls, there may be cited the interesting experiment of Atkinson, who, by removing the true foliage-leaves just beginning to unfold in the spring, was able to induce developing sporo- ' phylls to alter their character, and become transformed LIPE HISTORY OF A FERN 13 Fig. 15. — Clayton's fern {Osmunda Claytoniana) , showing sporophylls in the center, surrounded by foliage leaves. 14 IIl'REDITY AND EVOLUTION IN PLANTS into foliage-leaYes. Similar results were also obtained by Goebel. These experiments indicate that foliage-leaves and sporophylls are very closely related to each other, 1 - r f '^^Sm*-[ V J ; ►--. c*-' vSj Fig. i6. — -Portion of a leaf of a fern {Tectoria cicutariq) that bears bulbils on both the upper and lower surfaces of its leaves. Plantlets develop from the bulbils while they are still attached. and demonstrate clearly that foliage-leaves may be pro- duced by the sterilization of spore-bearing leaves. The interesting question here naturallyjarises as to whether, in the evolutionary development of the plant kingdom LIFE HISTORY OF A TERN IS Fig. 17. — Walking fern (Camptosonis rhizophyllus). The smaller,, lower plant originated at the tip of a leaf of the larger plant, and one of its leaves has, in turn, struck root. l6 HEREDITY AND EVOLUTION IN PLANTS through long geological ages, foliage-leaves have in gen- eral originated by the sterilization of spore-bearing organs . 9. Vegetative Multiplication. — In addition to repro- duction by spores, ferns may also be propagated vege- tatively in at least four ways. By one of these methods, the rhizome is cut into several pieces, and from every piece that contains a bud a new plant will develop. By Fig. i8. — ^A Boston fern (Nephrolepis), reproducing vegetatively by- means of runners or stolons. The parent plant is in the round pot. (After R. C. Benedict.) the second method, the plant is progagated by means of bulbils, which occur on the foliage-leaves of several species. In the fern Tectoria cicutaria, bulbils occur on both the upper and under sides of the leaf (Fig. i6). These bulbils fall to the ground, and under suitable conditions of light, moisture, and temperature give rise to new fern-plants. One of the ferns native to the eastern United States {Cystopteris bulbifera) received its specific name from the LIFE HISTORY OF A FEEN 1 7 fact that it bears bulbils. A third method is illustrated in the very interesting "walking fern" (Camptosorus rhizophyllus) , where the tips of the long acuminate leaves rest upon the moist ground, take root, and develop an entire new plant at the distance of the leaf's length from the parent fern (Fig. 17). The result of several repeti- tions of this suggested the common name "walking fern." A fourth inethod is by means of stolons or "runners" (Fig. 18). 10. Dispersal of Spores. — After the spores are mature the essential need is that they become dispersed, so that they may find favorable conditions of moisture, tem- perature, Ught, and soil for development; for, with rare exceptions, such conditions do not obtain within the spore-case. Moreover, if the spores remained within the sporangia they would be so greatly crowded that only a very small percentage of them would be able to develop into new plants. When the spores are ripe the spore-case opens, and by various movements the spores are expelled, often to a considerable distance; by wind and other agencies they may be carried still further from the parent plant. 11. Germination of Spores. — After dispersal, and under favoring conditions of temperature, moisture and light the spore begins to absorb water, and soon commences to grow. As the internal pressure increases, the walls of the spore are burst apart, and a tiny tube, the germ-lube or protonema (first thread), begins to develop. This process is germination. Shortly, near the wall of the spore, a smaller, slender tube develops as a branch of the germ- tube (Fig. 19). This is the first of innumerable root-like bodies, or rhizoids, which will help to hold the new plant i8 HEREDITY AND EVOLUTION IN PLANTS Fig. ig. — Germination of the spores of a fern, a, Before germination; b, early stage, showing protonema (pr.), and first rhizoid (rh); c, d, e, f, successive stages in the development of the prothallus. Fig. 20. — Prothallus of a fern. Archegonia on the (central) cushion near the notch; antheridia among the rhizoids, below. (After Margaret C. Ferguson.) LIFE mSTOEY OF A FERN 1 9 firmly to the soil, and also serve to take in water and dis- solved mineral nutrients. 12. The Prothallus. — Before the germ- tube has greatly enlarged, it becomes divided into two cells, and then, by successive cell-divisions, into an increasing number. Meanwhile chlorophyll bodies begin to appear, but nextr in the rhizoids. The final product of these cell-divisions and growth is a tiny, flat, green body, often (but not always) heart-shaped, with a central portion, the cushion, several cells thick, and a marginal part, the wings, of only one cell in thickness. Because of its flatness this little plant (for such it is) is called a thallus; and because it precedes, in the order of reproduction, the new sporophyte, it is called the prothallus (Fig. 20). It is usually possible to divide the prothallus into right and left halves, similar in shape and in other characters, and hence it is said to possess bilateral symmetry. CHAPTER II LIFE HISTORY OF A FERN (Concluded) The prothallus, as just described, bears little resem- blance, indeed, to the fern plant with which we are com- monly familiar. In fact , the relation between the two was not understood, nor even suspected, until about 1848, when Count Lesczyc-Suminski, a Polish botanist, first gave a connected description of the life history of the fern. We shall now proceed to follow the steps which lead from the prothallus to the new sporophyte. 13. Dorso-ventral Differentiation. — ^The appearance of the first root-like body, or rhizoid, was noted above. As the prothallus develops the rhizoids become more and more numerous, forming a mass of fine thread-like bodies on the under side, opposite the notch, of the heart-shaped prothallus. The presence of rhizoids, and of other struc- tures soon to be described, makes it easy to distinguish at once the surface that bears them from the opposite surface. Since the surface bearing the rhizoids lies nor- mally next to the substratum it was called the ventral surface, while the opposite surface was called dorsal. As now used, the terms dorsal and ventral are morphological terms, and have no reference to the manner in which the prothallus lies. Normally the ventral surface is the under one and the dorsal surface the upper, but the application of the terms would not be changed if the differentiated prothallus should happen, by any chance, to lie upside LIFE HISTOKY OF A FERN down. The dorsal surface would then be the under surface, and the ventral surface the upper one. Organisms or organs having two such surfaces clearly distinguishable are said to have dorso-ventral diferentiation. Among many- other structures thus differentiated are foliage-leaves, sporophylls, man, fishes, and other animals. In buds the dorsal surface of leaves is the upper or outer surface; when foliage leaves are fully expanded the dorsal surface is commonly underneath, and the ventral surface above. Fig. 21.- — Archegonia of a fern (Adiantum). A, young archegonium; B, mature; C, top view, showing terminal cells of the four rows of wall cells; V, wall of venter; e, egg; v.c.c, ventral canal-cell; n.c, neck-canal; sp, sperms entering the neck-canal. A and B in longitudinal section. 14. Reproductive Organs: Archegonia. — Examination of the ventral surface of a mature prothaUus with a lens will reveal near the notch and on the cushion, several tiny flask-shaped bodies, the archegonia. Each arche- gonium consists of a wall, one cell thick, and contents (Fig. 2i). The neck projects away from the surface 2 2 HEREDITY AND EVOLUTION IN PLANTS arid is usually slightly curved, while the remainder, the venter, is imbedded in the tissue of the cushion. As the archegonium approaches maturity it is seen to contain three cells; a long neck-canal cell, nearly fiUing the neck, an egg-cell or ovum, filling the venter, and between these two a ventral-canal cell. The egg is the female reproduc- tive cell. As it matures, the other two cells become disin- tegrated into a mucilaginous mass that fills the neck-canal. Since the archegonia contain the eggs they are the female reproductive organs. Fig. 22. — Portion of a clross-section of a prothallus of a fern [Adian- tum), showing an antfieridium (an), and sporogenous cells within. (Drawn from preparation of E. W. Olive.) 15. Reproductive Organs : Antheridia.^Search among the rhizoids will reveal another class of organs, the an- theridia, globular and also having walls only one cell thick. These are the male reproductive organs. At maturity they contain a large number of tiny motile cells, composed chiefly of a coiled nucleus, and able to swim about in water by the vigorous lashing of numerous little thread-Hke cilia attached to one end. These are the sperms, or male reproductive cells (Figs. 22 and 23.) LIFE HISTORY OF A FEEN 23 16. Fertilization. — Neither the eggs nor the sperms are able, independently, to reproduce their kind. In order to accomplish this they must unite, and the fusion of the sperm and egg is fertiHzation. One of the most significant facts about fertilization in ferns is that free water is re- quired, in order that the sperms may reach the egg by their own locomotion. When the antheridia and archegonia Fig. 23. — Fern prothallus; cross-sections showing antheridia {an), sperms {sp), and rhizoids {rh). Below at the right is a sperm {sp) greatly enlarged. are mature, a suitable amount of water (such as would result from a rain or a copious dew), soaking through the archegonial walls, will cause the mucilaginous matter in the neck-canal to swell. This in turn will rupture the archegonia at their distal ends, and a portion of the con- tents of the neck-canal will become extruded, while the egg will remain in the venter. The same conditions of 24 HEREDITY AND EVOLUTION IN PLANTS moisture will cause the rupture of the antheridia, and the sperms will be set free (Fig. 23) . The mucilaginous matter extruded from the archegonia contains a substance (malic acid, in some ferns) which stimulates the sperms to swim toward it. This they are enabled to do by the free external water. On reaching the archegonia, they enter it, and swim down the neck-canal to the egg. The sperm that first reaches the egg penetrates it, and passes through Fig. 24.' — ^Fertilization in the fein, Onoclea. A, longitudinal section of archegonium, showing the egg in the venter, and numerous sperms passing down the neck-canal. B, an egg-cell in the venter. One sperm has entered the nucleus, three sperms have failed to enter the egg. (After W. R. Shaw.) its cytoplasm until it reaches the egg-nucleus, with which it fuses, thus completing the act oi fertilization (Fig. 24). As soon as one sperm enters the egg-cell, the latter at once forms a fertilization-membrane about itself, through which the remaining sperms cannot enter. LIFE mSTdEY OF A FERN 25 17. Nature of the Fertilized Egg.— It will at once be recognized that the fertilized egg, resulting from a union with the sperm, possesses a double or diploid nature.^ In recognition of its dual nature it is called the oosperm (egg and sperm).'' The oosperm, however, like the un- FiG. 25. — Young embryo of a maidenhair fern (Adiantum conoinnum) , still surrounded by the archegonium, which has grown in size. L, leaf; S, stem; R, root; F, foot. (After Atkinson.) fertilized egg, is still only one cell, though its nucleus com- prises substances contributed by both egg and sperm. In some cases the egg and sperm that unite in fertilization may come from different parents; their fusion is then called cross-fertilization. ' As distinguished from the unfertilized egg, which is of a single, or haploid nature. ^ The term Sospore is often used here, but this term lacks the advan- tage of indicating the real nature of the fertilized egg. 26 HEREDITY AND EVOLUTION IN PLANTS 18. Development of the Fertilized Egg.^ — After fertili- zation the egg begins to develop, undergoing a series of nuclear and cell-divisions, accompanied by increase in size. The cell-wall of the first division (in all of the family Pol)^odiaceae) is parallel to the axis of the archegonial neck. The second wall, at right angles to the first, di- vides the oosperm into four cells. The beginning of these divisions marks the beginning of the embryo. By further cell-divisions each of the first four cells develops a mass of embryonic tissue. The two cells on one side of the first wall formed represent, the one the embryonic stem, and the other the embryonic leaf, or cotyledon. One of the two cells on the opposite side of the first wall, develops into the embryonic root, while the other develops into an organ peculiar to the embryonic stage, and known as the foot (Fig. 25). The function of the foot is to absorb nourishment for the young embryo from the prothallus. The need of such an organ becomes apparent when it is recalled that the oosperm, and consequently the embryo, lie free in the venter of the archegonium, without any organic or structural connection with the prothallus. This necessary connection is early established by the foot. 19. Growth of the Embryo. — ^As the embryo continues to grow, the root develops first. The advantage of this wiU become evident when we remember that the primary and most fundamental need of the young plant is water, which is taken in by the roots; The next most funda- mental need is nourishment, and as plant food is manufac- tured in chlorophyll-bearing organs, and usually in leaves, we would expect the early development of leaves. Such is the case, the growth of the first leaf being second- ary only to that of the root, and in advance of the stem. LIFE fflSTORY OF A FERN The development of the stem follows, and finally spore- bearing leaves appear (Fig. 26). We then have, an organism similar to that with which we started — a full- grown fern-plant, capable of producing spores, which can develop into prothallia again, with antheridia and arche- gonia, producing sperms and eggs, and so on. Thus we see that the steps in the life history of a fern constitute a life-cycle. At whatever point or with whatever struc- FiG. 26. — Prothallia of a fern, i, Before the sporophyte had appeared; 2-s, with sporophytes attached; I, cotyledon or first leaf of the sporophyte; V, circinate vernation of a leaf; s, mass of soil. ture we start, if we follow the course of development we are brought back again to the same point, or the same kind of structure with which we began. 20. Simpler Ferns. — In addition to the leptosporan- giate ferns, which have served as a basis for the general- ized description given above, there is another group, having a more primitive , type of organization. Repre- sentatives of this group include the "moonworts" (species ,of Botrychium, Fig. 27), and the "adder's tongue" {Oph- o HEREDITY AND EVOLUTION IN PLANTS ioglossum vulgatum, Fig. 28). The species of Botry- chium usually (though not invariably) possess but one Fic. 27. — Kattlesnake fern {Bolrychium virginianum (L.) Sw.). foliage-leaf, and a fertile spike, both of which are more or less branched. Abnormal forms are not uncommon in which the fertUe spike is more or less sterilized, sometimes LIFE HISTORY OF A FERN 29 being entirely so; while in other cases sporangia occur on the foliage-leaf. As in the re- placement of sporophylls by sterile leaves in the ostrich fern, Onoclea struthiopteris (para- graph 8), these abnormalities indicate the close relationship between leaves and spore-bear- ing organs, and clearly show that the latter may be com- pletely transformed, by sterili- zation, into foliage-leaves. In Ophioglossum the foliage- leaf and spore-bearing spike are both unbranched, the latter suggesting an adder's tongue, whence the name, Ophioglossum. In both Ophioglossum and Botry- chium the sporangia originate from a group of epidermal and sub-epidermal cells, and are consequently imbedded in the surrounding tissue . Their walls are more than one cell in thick- ness, the annulus is lacking, and they open by a slit. Ferns of this type a.Te culled eusporangiate {Cf. p . I o) . Their pro thallia are usually fleshy and subterranean, bear the antheridia and arche- gonia on the dorsal instead of on the ventral surface, and are per- FiG. 28. — Adder's tongue fern {Ophioglossum vvlgatum L.). B, runner or stolon. 30 IIKREDITY AND EVOLUTION IN PLANTS ennial, often living on after the sporophyte has died. In general the sporophyte possesses less sterile tissue in pro- portion to fertile tissue than is the case with the lepto- sporangiate forms. These characters mark the group as more primitive than the leptosporangiate ferns, and they are much less numerous, only about loo species being known from the entire world, while of the leptosporangiate ferns between 3,000 and 4,000 species have been described. Recent studies of the vascular anatomy of the Ophio- glossaceas have disclosed features in common with the Osmundaceae and Polypodiacese. The fact that the vas- cular bundles of the fertile spike originate in the same manner as those extending into the pairs of pinnae of the sterile segment points to the conclusion that the fertile spike represents, or is homologous with, two fused pinnae at the base of a fern leaf. From this and other evidence the Ophioglossaceae, while "simpler" in structural fea- tures, have been regarded as not having had a strobilar origin (by progressive sterilization^) from the liverworts, and as not standing in the ancestral line of the modern lep- tosporangiate ferns, but as having themselves been derived at a very early period from a primitive fern stock closely related to the Osmundaceae. On the other hand, Camp- belP has adduced evidence for the derivation of the fertile spike, of Ophioglossum from a sporogonium like that of the liverwort, Anthoceros. This and other evidence indicates that the Ophioglossaceae, and the eusporangiate ferns as a group, are the oldest fern stock, and this conclusion is supported by the geological record, for the oldest known fossil ferns are eusporangiate. Further investigation is necessary before the question can be definitely settled. 'Cf. pp. 379, 432, and 574 infra. 'Campbell, D. H., Amer. Nat. 41: 139-159. 1907. CHAPTER III FUNDAMENTAL PRINCIPLES 21. Two Kinds of Reproduction. — In the two preced- ing chapters attention has been called to three ways of ob- taining new fern-plants, namely, by spores, by vegetative multiplication, and by fertilized eggs. The first two methods may be grouped together as asexual, while the second is sexual, as shown in the following table. Reproduction Asexual, in- volving cell- divisions only. Sexual, in- volving cell- fusions. By the giving off of multi-cellular por- tions or outgrowths of vegetative tissue. By the giving off of special reproductive bodies of one or few cells, called spores. Artificial (slips, cuttings, etc.). Natural (tubers, bulbs, gemmas). 22. Vegetative Multiplication. — Vegetative multipli- cation may be accomplished either without or with the intervention of man. In the first case the plant produces special reproductive bodies such as tubers, bulbs, offsets and stolons, which become separated from the plant with- out assistance, and develop into new individuals. In the second case a similar result is accomplished through the removal by the gardener of portions of the parent plant, such as slips, cuttings, leaves {e.g., in the begonia), or by bending branches over until they touch the ground, and there take root, after which the newly rooted portion 31 32 HEREDITY AND EVOLUTION IN PLANTS may be severed from the parent plant. This is called layering. The production of new individuals by the arti- ficial methods of the gardener is called propagation; but between these methods and multiplication by special bodies, given off spontaneously by the plant, no hard and fast line can be drawn. Some plants, for example, be- come layered without the gardener's assistance; other plants (as the willow), by self-pruning, spontaneously give off branches from which new plants may develop; while, on the other hand, the gardener may cut a tuber, such as the "potato " into a number of pieces, from each of which a new plant will develop. In this practice artificial propagation and vegetative multiplication are combined. 23. Reproduction by Spores. — The essential fact about a spore is that it is an individual cell or small group of cells, produced primarily for reproductive purposes, given off by the plant, and capable by itself of producing a new individual. The essence of all reproduction is the separation of the reproducing cell or body from the parent plant. If a bud or a bulb remains attached to the plant that formed it, it produces only a branch or other organ, but not a new individual. So, also, a spore must be sepa- rated from the parent plant in order to reproduce the latter. In many cases spores may germinate before they are set free, but the separation must come sooner or later. 24. Sexual Reproduction. — In marked contrast to reproduction by spores, is the reproduction by means of sperms and eggs, involving cell- and nuclear-fusions, known as fertilization. Eggs and sperms are called gametes,^ the egg being the female gamete, the sperm the male gamete. The diploid cell, resulting from the union of two gametes, is called a zygote, and this term is often extended ' From the Greek word, yijuos {gamos), meaning marriage. FUNDAMENTAL PRINCIPLES 33 to apply to tHe resulting diploid organism through all stages of its development to maturity. 25. Two Kinds of Generations. — A study of the life history of the fern disclosed two distinct phases or genera- tions, one bearing spores, and therefore called the sporo- phyte (spore-bearing plant), the other bearing gametes and for that reason called the gametophyte (gamete-bearing plant). The gametophyte of the fern was seen to be entirely independent of the sporophyte, capable of manu- facturing its own food by means of its own chlorophyll, not dependent upon any other plant, and in some groups being perennial, living on from year to year, and giving rise to sporophytes that live for only one season. The sporophyte, on the other hand, is at first, entirely de- pendent upon the gametophyte for its nutrition, living as a parasite upon the prothaUus, from which it absorbs its nourishment by means of the special organ, the foot. Gradually, however, the sporophyte puts forth roots, capable of taking in water and dissolved mineral sub- stances from the soil, and chlorophyll-bearing organs the fronds or leaves), capable of manufacturing organic food. As the sporophyte becomes independent, the gameto- phyte (with few exceptions, as noted above), perishes. A comparison of the two generations shows that the sporophyte is the much more complex of the two, being clearly differentiated into roots, and leafy shoot. The difference in the origin of these two generations results in a very fundamental difference in the nature of all the cells in each. Since the sporophyte is derived from an oosperm (zygote), formed by the fusion of the two, gametes, all of its cells are diploid, containing material derived from both its male and female parentage. The 34 HEREDITY AND EVOLUTION IN PLANTS gametophyte, on the other hand, being derived from a sin- gle reproductive cell (the spore), without nuclear or cell-fu- sions, is composed of cells of a single or haploid nature. 26. Alternation of Generations. — Our study of the fern also brought out another fact of very fundamental importance. Sporophytes do not produce sporophytes, nor gametophytes, gametophytes ; but there is always an alternation of generations, sporophytes producing gametophytes, and gametophytes, sporophytes. The order of sequence in the life-cycle is as follows: sporophyte — >spore — >gametophyte— >8ametes — ^oosperm — >sporophyte. The order of structures and processes involved in the life-cycle is as follows : OUTLINE OF LIFE HISTORY OF A FERN Gametophyte (prothallus) J T Antheridium Archegonium i i Sperm (male gamete) Egg (female gamete) Ferlilizalion Oosperm (zygote) li Embryo II Mature sporophyte (mature zygote) Sporophyll Sporangium Spore-mother-cell TTTT Spore Spore Spore Spore I Gametophyte Reduction FUNDAMENTAL PRINCIPLES 35 The fact of a cycle in the hfe history is brought out clearly in the following diagram : Fig. 29. — Diagram of life-cycle of a fern. 27. Reduction. — Since the sporophyte (descended from the diploid oosperm) has cells of a double nature, resulting from fertilization, and since the spores which give rise to the gametophyte are of a single (or haploid) nature, there must occur, at some stage in the life of the sporo- phyte, a process of reduction, restoring the cells, made diploid by fertilization, to the haploid condition. Pains- taking studies of cellular structure and processes has disclosed the fact that this reduction takes place during the two successive divisions (tetrad-divisions) of the spore- mother-cell, resulting in the formation of four spores. The diploid condition persists in all the cells of the sporophyte, and through every cell-division, up to the two divisions preceding spore-formation, just as the single or haploid condition persists in all the cells of the gameto- phyte, up to the very act of fertilization. 36 HEREDITY AND EVOLUTION IN PLANTS 28. Nature and Method of Reduction.— In order thoroughly to understand fertilization and reduction one must have a knowledge of the structure and behavior of the nucleus in cell-division and cell-fusion. This subject is too difl&cult and too extended to be thoroughly treated Fig. 30. — Diagram illustrating various stages of indirect nuclear division (mitosis). A, resting nucleus of the mother-cell; B, formation of nuclear skein or spirem; C, longitudinal splitting of the spirem; D, the chromosomes (four in number) have been formed by the transverse seg- mentation of the spirem; E, chromosomes arranged on the equator of the nuclear spindle; F and G, early and late anaphase, the chromosomes moving to the pales of the spindle; H, formation of daughter spirems; I, resting stage of the two daughter-cells. in an introductory study, but the salient facts are as follows. . The nucleus of all cells comprises at least four substances: nuclear sap, a threadwork of linin, and a substance called chromatin;'^ all these are enclosed by a nuclear membrane. In the non-dividing nucleus the ' Because it stains readily when treated with certain aniline dyes. rUNDAMENTAL PRINCIPLES 37 chromatin is distributed on the hnin threads in the form of minute granules (Fig. 30.) At one of the stages preliminary to nuclear division the linin network, with the chromatin, becomes transformed into a thickened skein, Fig. 31. — Diagram illustrating various stages in tlie reduction division (maiosis) of a spore-mother-cell of a plant; A , resting stage of the mother- cell-nucleus; B, the nuclear skein or spirem, in synizesis (during synapsis); C, the spirem after synapsis, showing its double (diploid) nature; the dot- ted line indicates the segmentation of the spirem into two diploid chromo- somes, each of which has split longitudinally in D; E, the diploid chromo- somes on the equator of the spindle of the first (heterotypic) division; F, late anaphase; G, metaphase of the second or homotypic division; H, late anaphase of same, two haploid chromosomes approaching the poles of each spindle; I, the four daughter-cells (spores) of the tetrad. which shortly becomes split into two, throughout its entire length. The skein finally becomes divided transversely into a number of double chromatin bodies or chromosomes. The number of these chromosomes is characteristic, and always the same for each species of plant. The nuclear 38 HEREDITY AND EVOLUTION IN PLANTS membrane then disappears, and, by a complicated mechan- ism, not entirely understood, the two halves of the chro- mosomes are separated and carried apart to opposite sides of the cell. After this division of the nucleus, a new cell- wall forms, dividing the entire cellinto halves; new nu- clear membranes develop, and the chromosomes in each FUNDAMENTAL PRINCIPLES 39 daughter-nucleus becomes gradually retransformed into a resting nucleus, like the one with which we started. In reduction (Fig. 31) a new resting nucleus is not organized after the first nuclear division by which the number of chromosomes in each nucleus is reduced by one-half, but this division is followed at once by a second. This is the process of tetrad-division, by which four spores are formed from each spore-mother-cell. The reduced number of chromosomes persists throughout the gametophyte-phase, including the formation of both egg and sperm. When the latter unite, the nucleus of the zygote will, of course, possess the doubled number of chromosomes, which then persists throughout the body of the sporophyte (mature zygote),' until the stage of spore- formation is again reached. These facts are shown dia- grammatically in Fig. 32. 29. Inheritance. — It is, of course, common knowledge that men do not gather grapes of thorns, nor figs of thistles. A given species of fern always reproduces the same species, and this is true of all plants. It requires only a brief reflection to realize that this must be so, for the beginning of every living thing is always merely a piece of an antecedent organism, the parent. The off- spring would, therefore, naturally partake of the nature of its parent — it is a piece of it — was originally a part of it. Resemblance between ancestor and descendant is commonly regarded as inheritance, but only a little careful thinking will lead us to see that resemblance and inheritance are by no means synonymous. The real nature of inheritance is well illustrated by the inheritance of property by a son from his father. The thing inherited is not an external appearance, but a material substance 40 HEREDITY AND EVOLUTION IN PLANTS (land, buildings, a business), which is handed from one to another. So it is in reproduction. That which one generation of plants inherits from another is the substance of the reproductive cells — the protoplasm of the spore, oosperm, tuber, or bulb — plus a certain characteristic organization of this protoplasm, and the effects of its past history. 30. Inheritance Versus Expression. — That inherit- ance and expression are not the same thing is plainly shown in the life history of the fern, for the gametophyte clearly derives its living matter by inheritance from the sporophyte, and the sporophyte, in turn, its living matter from the gametophyte, and yet the two generations look so little alike that men for centuries knew them both with- out recognizing the fact that they were merely two dif- ferent phases in the life history of the same species of plant. So, often, among human beings, children may bear very little if any resemblance to their parents, but may closely resemble their grandparents. Clearly we do not inherit the color of our eyes or hair, the shapes of our noses, the peculiarities of our voices, or our mental traits from our parents, nor even from our more remote ancestors. What we do inherit is a tiny particle of proto- plasm having a certain characteristic composition, struc- ture, and past history. This protoplasm is capable, under certain combinations of circumstances, of developing into a mature organism, resembling the one from which it came, but under other combinations of circumstances the external appearance — the expression — -may resemble that of the parent only a very little, or not at all. In- heritance may therefore he defined as the recurrence in successive generations, of a similar cellular constitution.^ 'Following Johannsen, Cf. p. 67. FUNDAMENTAL PRINCIPLES 41 Expression of this cellular condition is greatly modified by circumstances, which are never precisely the same for any two individuals (C/. p. 48) . 31. Variation. — The preceding sentence explains, in part, why it is that no two individuals are ever precisely alike — ^precisely similar circumstances surrounding de- veloping organisms never occur twice; that is, the environ- ment varies. Besides this, internal changes may take place in the reproductive cells. For either one or both of these reasons, constant variation is the rule for living things. This subject will be considered more at length in Chapters V and VI. 32. Adjustment to Envirotunent. — By the term environment is meant all the circumstances that surround a cell, tissue, or organism at any given time, or throughout its existence. The environment of tissues and organs includes surrounding tissues and organs, and the environ- ment of cells includes the neighboring tissues and cells. The most essential thing in the life of every plant or animal is to keep in harmony with its environment. Every change of environment necessitates an adjustment on the part of the plant in order to maintain this harmony. Adjust- ments are most easily made when the plant is young and plastic, and especially while it is developing to maturity. If the amount of water in the soil is diminished the young plant will send its roots deeper, if hght is entirely cut off no chlorophyll will form. A leaf, or the prothallus of ferns, is bilaterally symmetrical partly because the environment is uniform on all sides; the same organs have dorso-ventral differentiation largely because the environment is unlike above and below. The motility of sperms is an adjustment to water in the environment. Thus, variations in the 42 HEREDITY AND EVOLUTION IN PLANTS environment may result in different expressions of in- heritance, just as variations in inheritance would be followed by differences in expression, even in an unchang- ing environment. In order correctly to understand a plant nothing is more necessary than to remember that its characteristics are the result, not of its inheritance alone, nor of its environment only, but of the interaction between the two. 33. Struggle for Existence. — In paragraph 7 atten- tion was called to the fact that a moderate-sized fern pro- duces each year about 50,000,000 spores. If each one of these spores ultimately produced a mature fern-plant, and if we allowed only i square foot of "elbow-room" for each plant, the progeny of one parent only, in one season would require at least 50,000,000 square feet, or nearly i^^ square miles. If each of these plants in turn, produced 50,000,000 offspring the next season, the descendants of only one fern plant would, in only two years, cover the stupendous area of over 83,000,000 square miles, or an area equal to that of the North American Continent. It has been calculated that a single plant of hedge mustard may produce as many as 730,000 seeds. If each seed developed another full-grown plant, and if the plants were distributed 73 to each square meter, there would be enough mustard plants to cover an area equal to 2,000 times the dry surface of the earth. One may easily imagine the result if all the seeds produced by one of our large forest trees were able to mature. And yet the total number of any given kind of fern, of hedge mustard, or of forest tree does not appreciably change from year to year. The reason, of course, is that not all of the spores and seeds produced are allowed to come to maturity. PUNBAMENTAL PRINCIPLE'S 43 The direct result of the enormous number of spores and seeds produced is a struggle for existence — for sufl&cient soil, water, light, and food to insure a healthy, mature plant. 34. Elimination of the Unfit. — ^As a result of variation certain individuals will succeed better than others in the struggle for existence. Those most poorly adapted to their surroundings will perish, and only the more vigorous ones — those best adjusted to their surroundings — will persist. The result of this struggle for existence was called by Herbert Spencer the "survival of the fittest." What really takes place in nature is the elimination, by death, of the unfit. Darwin called this natural selection, implying that the result is similar to that when plant breeders select out of a progeny the best individual for further breeding. What really takes place in nature, however, is not so much the selection of the fittest, but a rejection of the unfit. Thus, among the 50,000,000 progeny of a single fern-plant, some are sure to have a weaker constitution than others; to develop a weaker root- system, less chlorophyll in their leaves, a less number of sporophylls or spores, or to be inferior in other ways. The result will be that, in the course of only a few years, the descendants of the most vigorous or otherwise superior plants will alone be left to perpetuate the race. 35. Problems to Solve. — In the preceding paragraphs we have called attention to a number of the problems which arise from the study of a fern. Some of these have been partially solved — ^probably none of them has been completely solved. In fact, we may say that our igno- rance of life-processes greatly exceeds our knowledge. Very much more remains to be ascertained than has al- ready been found out; for example, what is protoplasm? 44 HEREDITY AND EVOLUTION IN PLANTS Nobody really knows. We have analyzed the substance chemically, we have carefully examined and tried (but without complete success) to describe its structure. We know it is more than merely a chemical compound. It is a historical substance. A watch, as such, is not. The metal and parts of which a watch is made, have, it is true, a past history; but the watch comes from the hands of its maker de novo, without any past history as a watch. But not so the plant cell. It has an ancestry as a cell; its protoplasm has what we may call a physiological mem- ory of the past. It is what it is, not merely because of its present condition, but because its ancestral cells have had certain experiences. We can never understand a plant protoplast by studying merely it ; we must know something of its genealogy and its past history. What is the origin of the sporophyte, and how did there come to be two alternating generations? What is the meaning of fertilization; what the mechanism and laws of inheritance? How did there come to be on the earth such plants as ferns? What was the origin of life? What is life? No one can give complete answers to these ques- tions; but the purpose of the study of botany is to help fit us to seek the answers intelUgently. To those who are interested in problems of this sort, nothing can be more fascinating, nor more profitable. It is the aim of the fol- lowing chapters to give a brief, elementary resume of the method employed and the results obtained during the past fifty years by investigators in their attempts to solve two of the more important of these problems, namely, the nature and mechanism of inheritance and the causes and course of plant evolution. CHAPTER IV HEREDITY 36. Importance of the Study. — i. To Pure Science.— No knowledge is more fundamental than a correct under- standing of the aws of heredity. Its fundamental im- portance to pure science becomes evident at once when we consider that, since evolution has been accomplished by the descent of one organism from another, there have been one or more unbroken lines of inheritance from the dawn of plant life to the present. Hence, until we know the laws of heredity, we cannot fully understand expression, reproduction, development, variation, sex, or evolution. 2. To Applied Science: — Correct ideas concerning he- redity are absolutely essential to such phases of applied science as animal and plant breeding. In the light of such knowledge the breeder can avoid making useless experi- ments, and can accomplish desired results more quickly, more cheaply, and with greater certainty of success. 3. To Man. — ^A correct knowledge of the principles of heredity is vital to mankind; no knowledge is more so. To realize this, we have only to reflect that our own characters are very largely the result of inheritance from our ances- tors; and not only our characters, but bur physical char- acteristics, our vigor of m'nd and body, our capacity for education, our susceptibility to disease, and often the actual existence of some disease within our bodies or minds. 45 46 HEEEDITY AND EVOLUTION IN PLANTS 37. Heredity Reduced to Its Lowest Terms.— We may study heredity under the very simplest conditions in the descent of one-celled organisms, such as Pleurococcus. Thi^ plant, a unicellular green alga, is a globule of proto- plasm, containing chlorophyll, and surrounded by a cellulose cell-wall (Fig. 33). But why is it globular, why does it contain chlorophyll, why has it a cell- wall of cellulose? Why is it not eUiptical, why is it not red in- stead of green, why does it have a cell-wall, instead of existing naked like the plasmodium of a slime- mold, why is its cell-wall of cellulose, rather than of Hgnin or chitin? The short answer is, because its ancestors, for ages and ages, have Fig. 33. -Individual gggd tj^e characteristics which plants of green slime '^ [Pleurococcus vulgaris) now characterize Pleurococcus showing the tendency of plants. But that Only puts the the cells to remain ^iQj^|3g^j,]^g^j^jj^^lg£^jjgj^yjj^ber attached after cell-divi- . . _, , sion, thus causing transi- of generations. The real reason is, tions from a one-celled to because the P/eMrocc»ccM5 protoplasm a multi-cellular plant, possesses a physical and chemical constitution — or in other words a under normal external (C/. Fig. 34.) mechanism — that, under normal external conditions, manufactures green pigment instead of red, cellulose in- stead of lignin, or any other substance, at the surface, and makes the cell-wall of even resistance to the osmotic pressure within, thus producing a sphere and not an ellip- soid, or filament, or any other shape. 38. What is Inheritance. — When the Pleurococcus cell divides, this wonderful, invisible mechanism — the certain HEREDITY 47 definite physical and chemical constitution — is transmitted to each of the daughter-cells; each, in other words, re- ceives Pleurococcus protoplasm. This protoplasm, with its definite organization, constitutes the inheritance. The daughter-cells do not inherit a spherical shape (as is evident from Fig. 2,z), but a definite kind of protoplasm, cell-sap Fig. 34. — Fkurococciis vulgaris. Sections of one-, two-, and four-celled plants, showing the nuclei and the large chlorophyll bodies (chb) to which the green color of the plants is due. In D, the larger chloroplast is shown in perspective. (Camera lucida drawings from a microscopic preparation by E. W. Olive.). (Cf. Fig. 33.) of certain osmotic properties, and surface cellulose of even elasticity, so that, in surroundings uniform on all sides, a spherical shape must finally result. The shape is an expression of the inheritance for the given environment. Under different external conditions the expression might he different; but the inheritance would be the same. The chlorophyll in the daughter-cells^ immediately after cell- 48 HEREDITY AND EVOLUTION IN PLANTS division, is a direct inheritance, but the chlorophyll subse- quently manufactured, and the green color which it gives to the plant, are not inherited; they are expressions of the inheritance-T^which in this instance is a chloroplastid (Fig. 34) that reproduces itself by division, and manufac- tures chlorophyll in the presence of sunhght. Under abnor- mal external conditions the mechanism may not act, or may act abnormally, so that yellow pigment appears instead of green — or in darkness no pigment at all. In either case the inheritance is the same, but the expression varies. A modern writer (J. Arthur Thomson) has defined inheritance as all that an organism has to start with. It is the protoplasmic substance, with all its potentialities, passed on from parent to offspring. 39. Inheritance Versus Expression. — In the light of this information, obtained by a study of the simple Pleuro coccus, we are able to understand that what we inherit from our parents or grandparents, is not a certain shape of nose, a certain characteristic gait, a musical or mathe- matical bent of mind, a quick temper, but a substance (protoplasm) possessing a very delicate, intricate, and characteristic constitution or mechanism. Under certain conditions this inheritance may so express itself as to cause resemblance in some physical or mental trait; or it may find a quite different expression, as when parents of medium height have tall children, or parents musically inclined have children that do not care for music; or sweet- peas, having white flowers only, produce, when crossed, peas having colored flowers. Or again, not all that is in- herited may be expressed; this is illustrated when children resemble, not their parents, but their grandparents. Here the parents transmitted an inheritance which, in them, found no expression. HEREDITY 49 A remarkable illustration of inheritance without expres- sion is seen in the case of the alternation of generations (pages 33-35). The inital protoplasm of the sporophyte is all inherited through the fertilized egg from the game- FiG. 35. — Vegetative propagation of Haworthia sp. Tlie new plantlet forms on the flower stalk, below the flower-cluster. Ultimately it falls to the ground and takes root, becoming established as an independent plant. tophytes, but most of the gametophytic characters do not appear in the sporophyte, nor do the typically sporophytic characters find expression in the gametophy te. ^ (C/. p . 40 .) ' The chlorophyll, of course, is an exception. But the osmotic strength of the cell-sap is a different expression in gametophyte and sporophyte, otherwise the young sporophyte could not live parasitically upon the gametophyte. so HEREDITY AND EVOLUTION IN PLANTS 40. Inheritance Versus Heredity. — As stated above, the inheritance is that which is actually transmitted from parent to offspring. The fern-spore, for example, is the inheri- tance of the fern gametophyte from the sporophyte. Heredity is the genetic relationship that exists between suc- cessive generations of organisms. The relation between two brothers and their parents is similar — it is one of heredity; their inheritance may be quite different. 41. Inheritance and Reproduction. — Inheritance is, of course, inseparably hnked with reproduction and may be studied in connection with the three following types : 1. In vegetative propagation, e.g., by means of tubers, cuttings and "slips," bulbs and bulbils, gemmae, "run- ners," scions, vegetative rejuvenation or "budding" (Fig. 35), etc., the new plant is obviously only a portion of the vegetative tissue of the parent plant, iso lated and growing independently by itseK. The separationof the propagating piece from the parent is often (though not always) mechan- ical and artificial. The protoplasm remains unaltered by the act of separation, reduction divisions of cell-nuclei are not involved, and the inheritance, except in bud-varia- tions, is unaffected by the change. This is evident in those cases where the isolated piece is grafted upon another plant; the specific or varietal characteristics of the scion are seldom, if ever, affected by the stock. Thus, in the experiment illustrated in Fig. 36, a tomato stem was grafted upon a tobacco plant, and upon the tomato were grafted three other species — Solanum nigrum, Solanum integrifolium, and Physalis Alkekengi. Each species was apparently not in the least altered by this drastic change in the conditions of its life. 2. In asexual reproduction by spores the situation is quite similar to that in vegetative propagation, but in HEREDITY SI certain cases there is abundant opportunity lor the proto- plasm to become more or less altered during the compli- cated changes that accompany the division of the cell- nucleus. This is notably the case in the chromosome re- V f%.:. W^^^Cii^ \ wt X^ I 1:: .J«: 1 ^ ^*^^, Fig. 36. — Graft of tomato {Lycopersicum escuUntum) on tobacco (Nicoliana tabacum). On the tomato are grafted Solanum nigrum, S integrifolium, and Physalis Alkekengi. (Graft made by Mr. M. Free.) duction divisions preceding spore-formation in the sporo- phy tes of higher plants (p . 3 7) , especially when the plant is a hybrid; and in spore-formation in the sporangia produced from the zygospore of some of the filamentous fungi, like Rhizopus or Mucor, the common black mold of bread. In 52 HEREDITY AND EVOLUTION IN PLANTS the latter case the nuclear divisions, some time preceding spore-production, result in separating out the female (+) and male (— ) strains, so that the spores in a given sporan- gium are unlike as to sex — some being female (+), some male (— ), (Fig. 37). This will be discussed more fully in the next chapter. Such changes result merely in dis- tributing the heritable units (genes) of the mother-cell Fig. 37. — Sexual reaction between a hermaphroditic Mucor and (-|-) and (— ) races of a dioecious species. Diagrammatic representation of a Petri dish culture showing a heterogamic hermaphroditic Mucor (J) in the center separated by channels on either side from the {-{■') and (— ) races, respectively, of a dioecious species. Sp., sporangia containing spores by means of which the plant may be reproduced nonsexually. 1-6, stages in development of a hermaphroditic zygospore from unequal male and female gametes. A, sexual reaction between a ( — ) filament and a female gamete. B, sexual reaction between a (-|-) filament and a male gamete. C, a male zygospore formed by stimulus of contact with a (-|-) filament. (After Blakeslee.) unequally to the daughter-cells, but introducing nothing new; they may, however, result in the complete loss of one or more heritable units, or in the formation of a new one, not existent in the parent. In the latter two cases we recognize a mutation. No hard and fast line can be drawn between the various kinds of asexual reproduction; there are various degrees of transition between reproduction HEREDITY 53 by spores, gemmae, bulbs and tubers, and the artifically severed buds and scions used in grafting and "slipping." 3. In sexual reproduction there intervene between par- ents and offspring, not only the complicated reduction divisions involved in the formation of the gametes, but also the nuclear and cell-fusions accomplished by the union of the egg and sperm in fertilization (Fig. 38). Both proc- esses — the formation of the gametes, and their fusion — • Fic. 38. — Fertilization in the white pine {Finns Slrobus). The smaller sperm-nucleus (above) is imbedded in the (larger) egg-nucleus. The fu- sion of the nucleoplasms will finally become more intimate. (After Professor Margaret C. Ferguson.) offer almost unlimited opportunities for alterations of the protoplasm — especially that of the nucleus. This method of reproduction, therefore, has the very greatest interest and importance for the study of heredity. In the fertilized egg^ are united inheritanc-es from two parents — from two distinct lines of ancestry — -protoplasms (germ-plasms) with two entirely different histories extending back into the ' The fertilized egg (as Thomson has pointed out) is the inheritance, and becomes, in the mature individual, the inheritor. 54 HEREDITY AND EVOLUTION IN PLANTS past, no one knows how far. How will these two inheri- tances affect each other when they intermingle in the fertilized egg? Will one tend to inhibit or check certain characteristics or functions of the other ; will they evenly blend, so as to produce an expression intermediate between that of the parents; or may entirely new substances be formed or new combinations take place, resulting in an en- tirely new expression in the offspring? 42. Methods of Study. — To endeavor to answer the questions just asked is as fascinating an occupation as it is important, and the answers are significant for man, as well as for plants. It is indeed, a fortunate thing that prin- ciples ascertained by studjdng plants apply equally to man and other animals, since plants are so much easier to handle in experimental investigations. We may go about the answering of these questions in either of two ways. We may gather large numbers of statistics to measure and analyze (statistical or biometrical method), or we may employ the experimental method. The method of biometry enables us to deal with a larger number of individuals, but the material studied is usually a mixed population, whose history is only imperfectly known, the conditions are more complex, and little if at all under control. By the experimental method it is not necessary to deal with such large numbers; we may choose carefully pedigreed material, about the history of which we have more or less accurate knowledge, and we may greatly simpHfy and control the conditions under which we make our observations. The ..largest advance toward the solu- tion of the problems of inheritance has been made by the experimental method, in the form first employed success- fully by Gregor Mendel. This method will be briefly explained in the next chapter. CHAPTER V EXPERIMENTAL STUDY OF HEREDITY 43. Gregor Mendel. — Two of the most important contributions ever made to biological science, namely, Fig. 39. — Gregor Mendel, at the age of 40. His theory of alternate inheritance (Mendelism) , based largely on experiments with the garden pea, is the most important and most fruitful contribution ever made to the study of inheritance. Mendel's laws of heredity, and his method of investigating them, were made by a teacher who studied plants as a pas- time because he loved to do it. This man was Gregor 55 S6 HEREDITY AND EVOLUTION IN PLANTS Mendel, a monk in the monastery at Briinn, Austria, where he finally became abbott. In order to understand his work clearly the student should familiarize himself the various characters of the edible or garden pea, the chief plant with which Mendel worked. 44. Mendel's Problem. — Mendel was much interested in problems concerning the origin and evolution of species. It was largely this interest that led him to hybridize {i.e., cross-pollinate) plants of different species and varieties, and observe the behavior of the resulting hybrids in succes- sive generations. The problem which he endeavored to solve was the law or laws "governing the formation and development of hybrids,"' with special reference to the laws according to which various characters of parents appear in their offspring. 45. Mendel's Method. — He recognized that, in order to solve the problem, attention must be given to at least four points, as follows: 1. To start with pure-breeding strains. 2. To consider each character separately. 3. To keep the different generations distinct. 4. To record, for the progeny of each generation sepa- rately, the proportions in which the various characters appear. No previous student had recognized the fundamental importance of these requirements. 46. Choice of Material.— Mendel realized that the success of any experiment depends upon choosing the most suitable material with which to experiment. He laid down the requirements as follows : ' All the quotations in this chapter are from an English translation of Mendel's original paper. His form of expression has been preserved as far as possible, even when the "quotes" are omitted. EXPERIMENTAL STUDY OE HEREDITY 57 1. "The experimental plants must necessarily possess constant differentiating characters."^ 2. "The hybrids of such plants must, during the flower- ing period, be protected from the influence of all foreign pollen, or be easily capable of such protection. 3. "The hybrids and their offspring should suffer no marked disturbance in their fertility in the successive generations." Mendel also called attention to the advantage of choos- ing plants which, like the peas, are easy to cultivate in the open ground and in pots, and which have a relatively short period of growth. 47. Characters Chosen for Observation. — "Each pair of differentiating characters [have been thought to] unite in the hybrid to form a new character, which in the pro- geny of the hybrid is usually variable. The object of the experiment was to observe these variations in the case of each pair of differentiating characters, and to deduce the law ac- cording to which they appear in successive generations. The experiment resolves itself therefore into just as many separate experiments as there are constantly differentia- ting characters presented in the experimental plants." The following were the characters chosen for observation : 1. The difference in the shape of the ripe seeds (round and smooth vs. angular and wrinkled). 2. The difference in the color of the cotyledons (pale or bright yellow, or orange vs. light or dark green). 1 Differentiating characters are those in respect to which the two species or varieties to be crossed differ. The possession of chlorophyll by the leaves of peas, for example, is a common character. " Common characters are transmitted unchanged to the hybrids and their progeny." The color of the corolla (for example, white in one species and purple in the other) is a differentiating charctcter, serving to differentiate or distinguish one species from another, S8 HEREDITY AND EVOLUTION IN PLANTS 3. The difference in the color of the seed-coat (white vs. gray, gray-brown, leather-brown, with or without violet spotting, etc.)- 4. The difference in the form of the ripe pods (deeply constricted between the seeds and more or less wrinkled, or the opposite). 5. The difference in the color of the unripe pods (hght or dark green vs. vivid yellow). 6. The difference in the posi- tion of the flowers {i.e., axial vs. terminal, on normal vs. fasciated stems). 7. The difference in the length of the stem (the extremes chosen were "tails" 6 to 7 feet, and "dwarfs" }i feet to i}2 feet in height). 48. Artificial Hybridizing. — The edible pea is commonly self- fertilized; therefore, to make crosses it is necessary carefully to TT TVT .1. J r remove the stamens of one flower Fig. 40. — Method of pro- tecting flowers from foreign before the anthers have begun to pollen by paper bags, in shed their pollen, and then place plant-breeding experiments. jj f another flower On the (After O. E. White.) ^ . rr.-, n stigma. The flowers must then be carefully guarded, e.g., by tying paper bags over them (Fig. 40), to prevent other pollen being deposited by insects or otherwise. In this way the experimenter knows just what characteristics enter into the hybrid. ^r i - M 1 ^ ^ ^iH^^^ i 1 m j^B II M H !■ IR EXPERIMENTAL STUDY OF HEREDITY 59 Careful record is kept of all data, and plants produced in this way, with ancestral characters noted and recorded, are called pedigreed. Plantings of such plants are called pedigreed cultures. In many species, in "making the cross" {i.e., doing the cross-pollinating) great care must be taken to avoid con- tamination from foreign pollen, of which the air may be full. The fingers and all instruments are usually rinsed in alcohol before each operation, to insure killing any foreign pollen that might be present. Numerous other precautions are also taken. When the hybrid plants are mature, careful observations of whatever character is under observation are made and recorded. Whenever possible the observation should be quantitative. 49. Mendel's Discoveries. — We may illustrate Men- del's results in a simple manner by choosing, as the pair of contrasted characters, smooth and wrinkled seeds of the pea. Removing all the stamens from flowers of a variety having smooth seeds, he polHnated those flowers with pollen from a plant bearing wrinkled seeds. It should now be kept clearly in mind Just what the inheritance of the fertilized egg is in such a case. From the pistillate plant the inheritance, contributed by the egg-cell, included the protoplasmic properties (whatever they may be) which, when free to produce their effect, cause smooth seeds; from the staminate parent the in- heritance, contributed by the sperm-cell, included the protoplasmic properties, which, when free to act, cause wrinkled seeds. I. Law of Dominance. — What Mendel actually found by his experiments was that, in such a cross, all the seeds 6o HEREDITY AND EVOLUTION IN PLANTS of the hybrid plants are smooth. The inheritance was "smooth" and "wrinkled," but the expression was of only one type — smooth. A character thus expressed, to the exclusion of another, in the first filial (Fi) genera- tion Mendel called dominant, and the phenomenon he called dominance; the other character is recessive. From such observations Mendel formulated the law of domi- nance, as follows: When pairs oj contrasting characters are combined in a cross, one character behaves as a dominant over the other, which is recessive. By similar experiments Mendel found that, in the coty- ledons, yellow is dominant over green, tallness over dwarf - ness, axial flowers over terminal, and so on. Such pairs of contrasting characters are called allelomorphs. 2. Law of Segregation. — But what will happen if the first filial (Fi) generation is inbred or self-pollinated. Its inheritance included factors that make for both "smooth" and "wrinkled," but the expression was of one kind only. The experiment was made, and Mendel found that the second filial (F2) generation included plants, part of which possessed only smooth seeds, while the others had only wrinkled seeds (Fig. 41). "Transitional forms were not observedin any experiment." This illustratesin a striking way the difference between inheritance and expression, for a character cannot appear in a plant (or animal) unless the plant possesses the factor or factors for that character. Now, except for the comparatively rare cases where mutation occurs, the factors in the F2 generation must have been derived by inheritance from the germ-cells of the Fi generation; but the experiment shows that they did not come to expression there. The same law is illustrated in the crossing of a sweet variety of maize (having wrinkled EXPERIMENTAl. STUDY OF HEREDITY 6l grains) with a starchy variety (having smooth grains) . In this cross starchiness is dominant over sweetness (Fig. 42). Fig. 41. — Mendelian segregation in the edible pea [Pisum salmini) Full explanation in the text. (C/., Fig. 42.) 50. Ratio of Segregation. — But now we come to that feature of Mendel's experiments which, perhaps more than 62 HEREDITY AND EVOLUTION IN PLANTS Fig. 42. — Mendelian segregation in maize, a, the starchy parent; 6, the sweet parent; C, the first hybrid (Fi) generation, produced by crossing 0, and b, showing the dominance of starchiness; d, the second hybrid (F2) generation, showing the segregation of starchiness and sweetness with the ratioof three starchy to one sweet (wrinkled) grain. Lower row, daughters of d; e, /, and g resulted from planting starchy grains. One ear in three is pure starchy, the other two mixed; h, result of planting sweet (wrinkled) seed. They are pure recessives, and the ear is pure sweet. (After East.) (C/. Fig. 41.) EXPERIMENTAL STUDY OF HEREDITY 63 anything else, made them superior to all others that had preceded. He carefully counted the number of plants bearing each kind of seed, and found that the number of smooth-seeded plants was to those with wrinkled seeds as 3 : i. 51. Theory of Pxirity of Gametes. — When the wrinkled seeds (one-fourth of the total crop) were sown they all bred true to wrinkledness — their descendants of the F3 generation bearing only wrinkled seeds. The expression was aUke in every case. The gametes that united to produce these plants were therefore considered pure for "wrinkledness;" that is, it was inferred that they did not carry any inheritance tending to produce smoothness of seed. 52. Not All Dominants Alike. — But when the seeds of the F2 plants, having only smooth seeds, were sown it was found that the dominants were not alike, except in external appearance. The seeds, though all appeared smooth, carried different inheritances. One-third of them {i.e., one-fourth of all the seed produced by the F2 generation) bred true to smoothness, being therefore pure, or homozygous for smoothness; the other two-thirds of the dominants {i.e., one-half of all the seed produced) again segregated in the ratio of 3:1 — one-fourth wrinkled and three-fourths smooth, showing that they were hetero- zygous; that is, that they still carried inheritance from both the wrinkled and smooth-seeded grandparents. If we designate the first parental generation by P, the dominant character (whatever it may be) by D, and the recessive character by R, then. the facts above described may be diagrammed as follows : 64 HEREDITY AND EVOLUTION IN PLANTS DsXRcf P (ist Parental generation) D (R) Fi (ist Hybrid generation) I ■J T 3D iR F2 (2d Hybrid generation) iD + 2D(R) i I D 3D iR R F. (3d Hybrid generation) 53. Significance of the MendeKan Ratio. — The ratio 3 : I or, as it appears on analysis, i : 2 : x , is the ratio that one might expect, or that might be predicted, on the basis of chance. Students of algebra will recognize in it the essence of the familiar square of a -\- b, namely, a^ + 2ab + b'^, where a and b each equal i. In the plants the multiplication of inheritances (produced in fertilization) was as follows: eggs (s + w) X sperms (5 + w) = 55 + 2sw + ww where w = wrinkling and s — absence of wrinkHng, i.e., smoothness. 54. Theory of Purity of Gametes. — The above ratio is what we would expect if half of the egg-cells and half of the sperm-cells in a heterozygous plant (one of the Fi generation), carried only character-units or determiners^ that make for smoothness; the other half only those factors that make for wrinkling, giving 5 and w egg-cells, and 5 and w sperm-cells in equal numbers. Therefore, in pollination the chances would be equal that an s-egg would 1 The substance or condition (protoplasmic constitution), whatever it is, in the germ-cells that corresponds to any given character of the plant is variously referred to by the terms /actor, determiner, gene {— producer), character-unit, and others, Th?se terms are essentially synonyms. EXPERIMENTAL STUDY OF HEREDITY 65 be fertilized with either an 5-sperm or a w-sperm, giving {s -\-w) X (s + w) = 55 + 2SW + WW. Since 5 is dominant over w the product should be written : 55 + 5(w) + s{w) + WW giving in external appearances 35 + iw. Since the re- sult actually observed is what it would be if the gametes were thus "pure" /or smoothness and wrinkling, Mendel concluded that they really are, and moreover that each character behaves as a unit, appearing and disappearing in its entirety. 55. Character-tmits versus Unit-characters. — ^As just stated, Mendel held that the various visible characters of his plants (dwatfness, for example) behaved as units, either appearing in their fullness, or not appearing at all. From more careful observations we know that such is not the case. A blossom may, for example, be more or less pink, an odor more or less strong, dwarfs are not all the same height, but fluctuate around a mean. We conclude therefore that characters do not behave as units, and that the conception of "unit-characters" is erroneous. The evidence does, however, seem to justify the conclusion that the factor or factors, whatever they may be,^ that are causally related to the given character do behave as units. We may therefore designate them as character-units. Since they are causally or genetically related to the character they have been called genes (from the root of the Greek word, genesis). They are more commonly known as factors. Quite probably, in many if not all cases, more than one factor is involved in the production of any given character. ' Substance or condition, we know not what, within the germ-ceils. 66 HEREDITY AND EVOLUTION IN PLANTS 56. Applications of Mendel's Law. — Over loo pairs of structural and color characters have been found, in plant breeding, to behave more or less closely in accord- ance with the MendeUan conception. In peas alone over 20 pairs of characters are expressed in successive genera- tions, in accordance with this law. Among the more striking results which are explainable upon Mendelian theory are the following: 1. Mottled beans have been produced in the Fi genera- tion by crossing two varieties, neither of which had mottled seeds. Various types appeared in the F2 generation. 2. Jet black beans have appeared in the Fi generation from a cross between two varieties, one of which had pure white seeds, the other light yellow. Vanous shades and colors appeared in the F2 generation. 3. In one case three distinct varieties of beans, breed- ing true to white seeds (when self ed^), were crossed with the same variety of red bean. In the Fi generation each cross gave a different color — one blue, another black, and the third brown. A varied assortment of colors appeared in each case in the F2 generations. 4. Two varieties of sweet peas, each breeding true to white flowers, when crossed gave, in the Fi generation, nothing but purple-flowered offspring, resembling the wild sweet pea. A medley of white, pink, purple, and red-flowered plants appeared in the F2 generation. Num- erous other cases might be cited, all of which would have been unsolvable riddles except in the light of Mendelism. 57. Inheritance and Environment. — Emphasis should be laid on the fact that the behavior of any plant, and the ' The pollination of a flower with its own pollen, or with pollen from an- other flower of the same plant, is called selfing. EXPERIMENTAL STUDY OF HEREDITY 67 characters it manifests, are the result of the combined influence of inheritance and environment. A bean seed- hngj[is green, not merely because it has inherited chloro- plastids, but because it develops in sunKght; without sunlight the green color could not come into expression. If we vary any factor of environment (temperature, mois- ture, illumination, food) the expression of the inheritance may be altered, just as truly as though the inheritance were changed. The characteristics expressed by any plant {or animal) are the result of the combined action of inheri- tance and environment. It is of fundamental concern to a man, not only to be "well-born" {eugenics), but also to be "well-placed" {euthenics), although the former, according to present day conceptions appears to be more important. 58. Johannsen's Conception of Heredity. — The con- ception that inheritance, as previously noted, is not the transmission of external characters from parent to off- spring, but the reappearance, in successive generations, of the same organization of the protoplasm with reference to its character-units, was first developed by Johannsen, of Copenhagen, Denmark, who proposed the term "genes." "The sum total of all the 'genes' in a gamete or zygote," is a genotype. Inheritance is the recurrence, in succissive generations, of the same genotypical constitution of the pro- toplasm. Johannsen does not attempt to explain the nature of the genes, "but that the notion 'gene' covers a reaHty is evident from Mendelism." This conception of heredity is diametrically opposed to the older and popular conception, but is much more closely in accord with]^the facts revealed by recent studies of plant and animal breeding (C/. p. 40). 68 HEREDITY AND EVOLUTION IN PLANTS 59. Pure Line Breeding. — Johannsen also originated the "pure line" theory — a theory which has done much toward elucidating the problems of selection. He and his followers regard genetic factors as fixed and un- varying. Hence the results obtained in selective breeding of a given variety of maize for high or low oil content, or of a given variety of beans for larger or smaller size of seed, would be interpreted on this theory, as the isolation or separation of pure strains from a "mixed population" or "impure" variety. In practical language, several true breeding varieties of beans, differing in seed size, might be obtained by selection from what appeared to be a "pure" variety with considerable variation in size of seeds.' 60. Value of Mendel's Discoveries. — The discoveries that, in inheritance, certain characters are dominant over certain others; that a given inheritance (e.g., conditions associated with seed-color, odor, eye-color, stature, musi- cal abihty, insanity, tendency to some disease) may be carried and transmitted to offspring by an adult who gives no outward signs of carrying the inheritance; that, under certain conditions of breeding, some characters (the re- cessive ones), whether good or bad, may become perma- nently lost; that dominant characteristics are certain to reappear in some of the offspring — all of these truths, learned by the study of a common garden vegetable, will be recognized at once as of enormous importance to the breeders of plants and animals, and above all to man- kind, in connection with our own heredity. They point the way to the explanation of such enigmas as the pro- verbial bad sons of pious preachers, spendthrift children ' A detailed discussion of Johannsen's method of "pure line" breeding belongs to more advanced studies. EXPERIMENTAL STUDY OF HEREDITY 69 of thrifty parents, talented offspring of mediocre parents, blue-eyed children of brown-eyed parents,^ and so on. 61. Increased Vigor from Crossing. — Experiments with pedigreed cultures have disclosed a principle of the utmost practical importance for the plant breeder. A careful analysis of a field of Indian corn {Zea Mays) has disclosed the fact that any given variety is very complex, being heterozygous for many characters; in other words any horticultural variety is a composite of numerous elemen- tary species, and is therefore heterozygous for most of its characters. When pollination is allowed to take place in the corn field without interference by man, both crossing and selling occur. As a result the yield, in bushels per acre, remains about stationary, or gradually becomes less and the variety changes and deteriorates by the segregation and recombination of the numerous elementary species that compose it. By artificial self-pollination for several generations (e.g., (five or more) less complex strains result, which are homo- zygous for one or more characters, and the yield per acre may thus become greatly reduced.^ If now, two of these simplified strains, homozygous for many characters, and 1 If both parents have blue eyes the children with rare exceptions have blue eyes; if one parent has brown eyes and one blue, the children may be both blue- and brown-eyed, or all brown-eyed, for brown eye-color tends to be dominant over blue color. When both parents have brown eyes, part of the children may have blue eyes and part of them brown, or they may all be brown-eyed. As used here, the term "brown-eyes" means all eyes having brown pigment, whether in small spots (gray eyes), or traces (hazel eyes), or generally distributed (brown, or sometimes black, eyes). The term "blue eyes" designates only those cases in which brown pig- ment is entirely lacking. ^If a high-yielding strain was separated out by selection, the yield, would of course be increased above the average of the mixed field. 70 HEREDITY AND EVOLUTION IN PLANTS EXPERIMENTAL STUDY OF HEREDITY 71 having a low yield per acre, are crossed, there results an Fi hybrid progeny that is heterozygous for all of these characters. This heterozygosity is correlated with a greatly increased vigor; the plants are much larger, and the yield per acre is enormously increased (Fig. 43). Thus in one experiment of this kind the average yield of the heterozygous horticultural variety was 61.25 bushels per acre. After self-fertilization for several generations the yield became reduced to 29.04 bushels per acre; but in the Fi generation of a cross between two of these self- fertilized strains the yield per acre rose at once to 68.07 bushels. In the F2 generation the yield again fell to 44.62 bushels. From this, and numerous other experiments, it is found that the biggest corn crop is to be obtained by finding the strains that will produce the largest yield when crossed, and then using for seed the grains of the first-generation hybrids each year. 62. Breeding for Disease-resistance.— Biffen, in England, crossed a wheat of poor quahty, but resistant to rust disease {Puccinia glumariim), with a superior variety but very susceptible to the disease. Suscepti- bility proved dominant in the Fi generation, but in the F2 generation disease-resistant forms appeared, of superior Fig. 43. — Zea Mays. In tie experiment, the results of which are here illustrated, nine strains of Indian corn were selected according to the number of rows of kernels on the cob, varying from 8 to 24 rows. These were pollinated by hand each year, with mixed pollen, in such manner that self-pollination was entirely prevented. An average ear of each strain is shown in the first row above. In the second row is shown an average ear of each strain after self-fertilization for five generations. Note the resulting decrease in the number of rows, lack of filling out of the ears, and other marks of inferiority. The last row shows the remarkable and immediate increase of vigor resulting in the Fi generation of hybrids be- tween various pairsof the selfed strains. (Photo supplied by G. H. ShuU.) 72 HEREDITY AND EVOLUTION IN PLANTS quality, which bred true for resistance. The water- melon, in the southern states, is subject to a very de- structive disease which causes a wilting of the vines and consequent loss of fruit. By crossing the ordinary non- resistant watermelon with the closely related common citron, which is wilt resistant, W, A. Orton, of the United States Department of Agriculture, produced a water- melon resistant to this disease. Numerous other illus- trations might be given. This is becoming one of the common and successful methods of combating plant disease. 63. Unsolved Problems. — ^Like all truly great con^ tributions to science, Mendel's discoveries have raised more questions than they have answered. Therein hes, in part, their great value. So, also, the most important effect of Darwin's work was that it set men to asking questions. The history of botany, as of all natural science since 1859, is chiefly the attempts of men to answer the questions raised by Darwin, or stimulated in their own minds by his books. So with Mendel and de Vries; biological science, since igoo, has been largely occupied in trying to answer the questions raised by these men. What are these questions? There is not space here even to ask them all, much less to endeavor to answer them even briefly; but they include the following large problems : I. Are acquired characters inherited? In other words, do characteristics acquired after birth by the body or mind of the parent, either by its own activity or as a re- sult of the immediate effects of environment, influence the germ-cells so as to alter the inheritance which they transmit? Some say yes, others say no; others say, only EXPERIMENTAL STUDY OF HEREDITY 73 in part. There seems to be evidence both ways, but the bulk of the evidence and the weight of scientific opinion is against the inheritance of acquired characters as here defined. We can arrive at the correct answer only by care- ful experimentation, that is, by asking questions of nature.^ 2. Can the inheritance of a strain be artificially altered? This is a question of the very first importance. If the inheritance could be so altered the marvels that breeders might perform would be greatly increased. A blue rose (the despair of all plant breeders) might possibly be pro- duced by sufficiently careful and extended experiment- ing; disease and undesirable traits of character might be eUminated from certain tainted family strains by artificial treatment. On the other hand, by an unfortunate com- bination of circumstances, most undesirable and re- grettable results (e.g., a weed poisonous to cattle, or a new and virulent disease-causing bacterium) might be experi- mentally produced. The experiment has been made of exposing the ovaries of flowers to the rays of radium, and of injecting them with various chemical substances, with an idea of altering the physical or chemical nature of the egg-cells, and thus altering the inheritance. The results of such experiments, so far tried, need to be further con- firmed before we can say with certainty that the result sought has been accomplished. 3. How may dominance he explained? Why is tallness dominant over dwarfness, brown eye-color over blue, any one character over any other? At present we can only speculate on these questions. 4. What is the mechanism of inheritance? In other words, by what arrangement and interaction of atoms 'On the inheritance of acquired characters, see Thomson, J, A., Heredity. London, 1908. Chapter VII. 74 HEREDITY AND EVOLUTION IN PLANTS and molecules is it made possible that the peculiar tone of one's voice, the color of a rose, the odor of a carnation, the evenness (or otherwise) of one's disposition, may be transmitted from one generation to another? How may it be transmitted through one generation, without causing any external expression, and reappear in the second gen- eration removed? Is the cytoplasm the carrier, or the chromatin, or both combined, or neither? Is the transfer accomplished by Uttle particles {pangens), as de Vries contends, or by chondriosomes, or otherwise? We do not definitely know, but many careful investigations point to the chromatin as the bearer of the hereditary factors. 64. Weismaimism. — It was a botanist, Nageh, who first recognized and clearly stated that inheritance must depend upon a least quantity of matter, and numerous experi- ments by both botanists and zoologists soon made it evident that the hereditary substance is in the cell- nucleus, rather than in the cytoplasm surrounding the nucleus. Nageli called the hereditary substance idio- plasm. Observations of the germ-cells of plants by Stras- burger, and of the germ-cells of animals by O. Hertwig, led them to conclude that the chromosomes of the dividing nucleus (Fig. 30 )are the locus of the hereditary substance. The subsequent evidence, upon which this conclusion rests is too voluminous, and some of it too technical, to be pre- sented here in any detail. ^ As an illustration there may be cited the experiment of Boveri who removed the nucleus from the egg-cell of one species of sea-urchin, and then caused the remaining cytoplasm to be fertilized with a sperm-cell of another species of sea-urchin; the result- ing larva possessed only paternal characters. ' See Morgan, T. H. The physical basis of heredity. Philadelphia, 1919. EXPERIMENTAL STUDY OF HEREDITY 75 Weismann expanded the above conception of hereditary substance by calling attention to the fact that it must contain elements, not only from one individual or pair, but from a long Hne of ancestors. He called the idioplasm (of Nageh) in the germ-cells germ-plasm, and the heredi- tary units, "necessary to the production of a complete individual," he called ids. Each id contains a full com- plement of hereditary factors necessary to produce a perfect plant or animal. The germ-plasm corresponds to the deeply staining chromatin of the cell-nucleus, and the ids are grouped together in idants, which correspond, in general, to the chromosomes. Weismann further postulated that the ids were composed of "primary con- stitutents," which he called determinants, and that every character independently inherited has its own determinant in the germ-plasm. Finally Weismann postulated that each determinant is a complex of hiophors (the ultimate units of matter in the Kving state), each biophore being composed of (non-living) chemical molecules. Thus we rise through his categories as follows, from atom to mole- cule, from molecule to biophore, from biophore to deter- minant, from determinant to ids, from ids to idants (chromosomes), which are composed of the hereditary substance or germ-plasm; schematically as follows: germ-plasm (chromatin) idant (chromosome) id. determinant {factor, of Mendel) biophore {biogen, of Verworn) molecule atom The germ-plasm is continuous from generation to genera- tion, and therefore possesses a kind of physical immortahty. 76 HEREDITY AND EVOLUTION IN PLANTS 65. Relation of ■Weismannism to Mendelism. — It will readily be recognized that the "determinants" of Weis- mann are the "factors" of Mendelian nomenclature. Morever, it follows logically from Weismann's theory that acquired characters are not inherited, an inference that agrees with observation and experiment. NageU, director of the botanic garden in Munich, transplanted specimens of Hawkweed (Hieraceum) from the high Alps to the lower altitude and changed climate of his garden, and these plants began to manifest new characters which reappeared in successive generations for more than a decade. This looked like the inheritance of acquired characters, but when the plants were subsequently taken back to the high Alps, they failed to manifest the charac- ters expressed in the botanic garden, reverting to their former alpine characteristics. Thus it is seen that the reappearance of the new characters in successive genera- tions in the botanic garden was not due to inheritance of these acquired characters, but to the continuity of the new environment. The inheritance had not been altered though the expression of it had. This is in agreement with what we should expect from the definition of inheritance given on page 50.^ 66. Eugenics. — Students of biology have been quick to recognize the fact that, if we correctly understand the laws of heredity, we are in a position to apply them, not only to plants and the lower animals, but to mankind. The application of the laws of heredity in a way to produce a healthier and more efficient race of men constitutes the practice of eugenics. ^ The underlying principles of eugenics ' See also pages 48 and 66-67. ' The word eugenics is from two Greek words meaning well born. EXPERIMENTAL STUDY 01" HEREDITY 77 are of course, very largely those of heredity. Eugenics is the applied science based upon the pure science of heredity. The main problem of eugenics is how to eliminate human beings with a tendency to any physical or mental weakness making for poverty, misery, ignorance, and crime; and how to increase the number of individuals physically, mentally, and morally more robust and sound; and withal how, if possible, to raise the standard of all desirable human traits. A careful study of heredity and eugenics will make possible a much more intelligent and efl&cient program for charity work and social betterment. 67. Investigations Since Mendel. — It must be re- membered that Mendel's most valued contribution was not the observations which he made and recorded con- cerning the garden pea, nor the hypotheses which he ad- vanced on the basis of those observations, but this method of procedure, whereby he elevated the study of heredity to the rank of an exact science. As in the case of all hypotheses, the task for science is to subject them to the most searching tests, to see if they invariably agree with facts, and may be accepted as in all probability embody- ing the actual truth — may be elevated to the rank of theories. The testing of Mendelism has been occupying all the best talents of many investigators since the re- discovery of Mendel's publication, about 1900. Many biologists are still skeptical, a few reject the hypotheses, and still others believe they contain the germ of truth, but must be more or less modified. Whether they prove to he erroneous or true is not so important, hut it is impor- tant for us to know which is the case. True or not, they, like nearly all working h3T)otheses (natural selec- tion, mutation, nebular hypothesis, atomic hypothesis 78 HEREDITY AND EVOLUTION IN PLANTS in chemistry, etc.) are rendering, or have rendered, a priceless service to science by pointing the way to further study, which enriches our knowledge of the living world, including ourselves, and therefore increases the intelli- gence with which we may order our own conduct and lives. If the study of plants had rendered no other service to mankind than this contribution of an effective method of ascertaining the laws of heredity, it would have amply justified all the arduous labor that men have devoted to it for 2,000 years. ^ ' Only one of the simplest cases worked out by Mendel is summarized in this chapter. A more thorough study of his experimental results and theories must be reserved for more advanced study. CHAPTER VI EVOLUTION 68. Doctrine of Special Creation. — In the time of Linnaeus, the "father of botany," men believed that the seven "days" of creation left the world substantially as we now find it. The stars and planets, mountains and oceans, plants and animals were created once and for all, and continued without important change until the present. In the beginning, as now, there were the same oceans and hiUs, the same kinds of plants, and the same kinds of animals. Nor, it was believed, are any fundamental changes now in progress. Creation was not continuous; it took place within a brief period (seven "days"), and then ceased; after that the Creator merely watched over the objects of his handiwork. Opposed to this doctrine is the theory of evolution. 69. Meaning of Evolution. — Evolution means gradual change. Applied to the natural world the theory of evolution is the direct opposite of the doctrine of special creation. It teaches that things were not in the beginning as we now find them, but that there has been constant though gradual change. Creation is regarded, not as having taken place once and for all, but as being a con- tinuous process, operating from the beginning without ceasing — and stUl in progress. 70. The Course of Evolution. — The theory teaches that the gradual changes have been from relatively simple conditions to those more complex. The compli- 79 8o HEREDITY AND EVOLUTION IN PLANTS cation has been two-fold: (i) simple individuals, whether mountains, rivers, planets, animals, or plants, have become more complex (e.g., compare the structure of the plant, Pleurococcus, a simple spherical cell, with that of the fern) ; (2) the relation between living things, and between them and their surroundings has become more complex {e.g., compare a unicellular bacterium, with its relatively simple life relations, with the clover plant, highly organized, and related to water, air, soil, light, temperature, gravity, bacteria (in its roots), and insects (for cross-pollination)). Most of the steps of evolution have been progressive, toward higher organization, greater perfection of parts, increased efficiency of function, as, for example, from algae having one or a few cells only, to flowering plants, like roses and orchids; but not all the steps have been in this direction. Some of the steps have been regressive, toward simpler organization, less perfection of parts, decreased efficiency of function, as, for example, from green algae to the non-green, alga-like fungi (Phycomy- cetes, such as bread mold) , from independence to parasitism (mistletoe and dodder), or to saprophytism (Indian pipe and toad-stools). The thirty odd species of the Duckweed family, related to the Arum family (Jack-in-the-pulpit, calla, skunk cab- bage, sweet flag, etc.), illustrate regression; they comprise the simplest, and some of them the smallest of all flowering plants. The plant body of Lemna is a tiny disc-shaped, thallus, having a central vein (vascular strand) with or without branches. Each plant has one root with no vascular tissue. The flowers, borne on the margin or upper surface of the thallus, have one simple pistil and only one stamen (Fig. 44). The dozen or more species EVOLUTION of Woljfia possess still simpler bodies, somewhat globose, with neither roots, veins, nor other organs, except flowers; even flowers are unknown in some species {e.g., Wolffia populifera, Fig. 44). Wolffia punctata measures only 0.5-0.8 mm. long. The plants are fittingly described in the manuals as "minute, alga-like grains," floating on or ecr^ Fig. 44. — Lemnaceae. e, /, Wolffia papuUfera. modified.) a, b, c, Lemna Irisiilca; d, Wolffia punctata; (Redrawn from Britton and Brown, slightly just beneath the surface of still water. Some botanists consider the plant body as morphologically a frond, others as a leafless stem. Since the first plant-body from the seed is only a matured cotyledon, or seed-leaf, Goebel considers that it cannot be interpreted as other than a free-living leaf. These tiny, simple plants are considered to have 82 HEREDITY AND EVOLUTION IN PLANTS originated by regressive evolution, their simplification being closely correlated with a reversion from dry land to an aquatic habit of life. A similar reduction of structure is found in the tiny floating ferns, Sahinia and Azolla. 71. Inorganic Evolution. — The process of evolution is not confined to living things, but, as indicated above, applies to all nature. Even the chemical elements are believed to have been produced by evolutionary changes, and to be even now in process of evolution. This is one of the results of the recently discovered phenomenon of radioactivity, which is essentially the transformation of the atoms of one chemical element into those of another. Fossil remains of marine animals and plants, found im- bedded in the rocks on mountain summits, indicate, with- out possibility of reasonable doubt, that what is now mountain top was formerly ocean bottom. The mountain has come to be, by a series of gradual changes. Rivers and valleys are constantly changing so that the present landscape is the result of evolutionary processes ; climates have changed, as we know from the fact that fossil re- mains of tropical plants are now found in the rocks in arctic regions; the atmosphere and the water of the ocean have reached their present condition as the result of gradual transformations extending over aeons of time; even the stars and planets, like our own earth, are coming gradually into being, undergoing changes of surface and interior condition, and ceasing to exist. Nothing is constant except constant change. The main problem of astronomy is to ascertain and record, in order, the evolutionary changes that have resulted in the present system of suns and planets. The main problem of geology is to ascertain and record, in order, the evolutionary steps that have resulted in the present condition of the earth. KVOLUTION 83 72. Fitness of the Environment. — Biological literature has always taken account of what has been called "adapta- tion," or the fitness of living things for life in the surround- ings or environment where they are placed. But a recent writer,^ has elaborated the complimentary notion of the fitness of the environment. Recognizing living things as "mechanisms which must be complex, highly regulated, and provided with suitable matter and energy as food," he shows that the present inorganic environment is the best conceivable. Inorganic evolution has resulted, among other things, in the occurrence of large quantities of water and carbon dioxide; their physical and chemical properties, and those of the ocean, together with the chemical properties of the elements, carbon, oxygen, and hydrogen, and their numerous compounds, "are in character or in magnitude either unique or nearly so, and are in their effect favorable ' ' to the organisms with which we are familiar, and which possess the three fundamental characteristics of complexity, regulation, and metabolism. The elements carbon, hydro- gen, and oxygen, says Henderson, are uniquely and most highly fitted to be the stuff of which life is formed, and of the environment in which it exists. 73. Organic Evolution. — Developmental changes in living things constitute organic evolution. Such changes are manifested in the development of an individual from a spore or an egg. The development of a mature in- dividual is ontogeny. The development of a group of related forms (genera, famihes, orders, etc.) is phytogeny. The chief problem of biology is to ascertain and record, in order, the evolutionary changes that have resulted in ' Henderson, Lawrence J. The fitness of the environment. New York, 1913. 84 HEREDITY AND EVOLUTION IN PLANTS the appearance of life and the present condition of living things. The major problem of botany is to record, in order, the evolutionary steps that have culminated in the present con- dition of the plant world. Organic evolution means that, after the first appearance of life, all living things, plant or animal, have been derived from preexisting living things, in other words, that the present method of formation of living things, by the reproduction of organisms already existing, has always been the method — "Omne vivum ex ovo" (all life from an egg), "omne vivum e vivo" (all life from preexisting life). 74. Method of Evolution. — To recognize that evolution is the method of creation still leaves unanswered the im- portant question as to the method of evolution. By what process was the gradual development of the living world accomplished ? Various hypotheses have been elaborated in answer to this question. We can here only briefly outline three of the most important ones. I. Agassiz's Hypothesis. — The great teacher and student of nature, Louis Agassiz, believed that the vast array of plant and animal species, past and present, had no material connection, but only a mental one; that is, they merely re- flected the succession of ideas as they developed in the mind of the Creator, but were not genetically related to each other. "We must . look to some cause outside of Nature, corresponding in kind to the intelligence of man, though so different in degree, for all the phenomena connected with the existence of animals in their wild state. . . Breeds among animals are the work of man: Species were created by God."^ • Agassiz, L. "Methods of Study in Natural History," Boston, 1893, pp. 146, 147. EVOLUTION 8S But to state that species were created by God does not satisfy the legitimate curiosity of the scientific man. What he wishes to know is : By what method was creation accomphshed? God might have worked in various ways. Now, the study of Nature has never revealed to us but one method by which living things originate, and that is by descent from preexisting parents. Agassiz's hypothesis Fig. 45. — Louis Agassiz. (From Ballard's "Three Kingdom.") contradicts this. All oaks now-a-days are derived by descent from preexisting oaks, but the first oak, accord- ing to the doctrine of special creation, was created by supernatural means ; it had no ancestors. The chief objec- tion to the acceptance of this hypothesis is that the more profoundly and accurately we study living things, the more obvious it becomes that truth Hes in another direction. 86 HEREDITY AND EVOLUTION IN PLANTS 2. Lamarck's Hypothesis. — The noted French naturahst, Lamarck, taught that all living things have been derived from preexisting forms; that the effects of use and disuse caused changes in bodily structure; that these changes were inherited and accentuated from generation to genera- tion; that, being of use, those individuals possessing the changes in greatest perfection survived, while others per- FlG. 46. — Water buttercup {Ranuncahis aquatilis), showing aerial leaves [a), and aquatic leaves {w). f, fruit. Drawn from herbarium specimen. ished; and that the derivation of new species is thus ac- counted for in a simple and logical manner. By continual reaching for tender leaves on high branches, the long neck of the giraffe was gradually produced, the slight gain in length in one generation being transmitted by inheritance to the next, and so on. The main thesis of Lamarck, as stated by himself, is as follows : EVOLUTION 87 "In animals and plants, whenever the conditions of habitat, exposure, climate, nutrition, mode of life, et cetera, are modified, the characters of size, shape, relations be- tween parts, coloration, consistency, and, in animals, agility and industry, are modified proportionately." As illustrating the direct effect of environment on organ- isms, Lamarck chose a plant, the water-buttercup {Ran- unculus aquatilis), which may grow in marshy places, or im- mersed in water (Fig. 46). When immersed, the leaves are all finely divided, but when not immersed, they are merely lobed. While plants are more passive, and are affected by their surroundings directly, through changes in nutrition, light, gravity, and so on, animals react to environmental changes in a more positive and less passive manner. Thus, in the words of Lamarck:^ Important changes in conditions bring about impor- tant changes in the animals' needs, and changes in their needs bring about changes in their actions. If the new needs become constant or durable, the animals acquire new habits. . . . Whenever new conditions, becoming constant, impart new habits to a race of animals these habitual actions lead to the use of a certain part in preference to another, or to the total disuse of a part which is now useless . The lack of use of an organ, made constant by acquired habits, weakens it gradually until it degenerates or even disappears entirely." Thus, "it is part of the plan of organization of reptiles, as well as of other vertebrates, that they have four legs attached to their skeleton . . . but snakes acquired the habit of glid- ' Translated from his Philosophie Zoologique, vol. I, pp. 227, 223, 224, 248. 88 HEREDITY AND EVOLUTION IN PLANTS ing over the ground and concealing themselves in the grass ; owing to their repeated efforts to elongate themselves, in order to pass through narrow spaces, their bodies have acquired a considerable length, not commensurate with their width. Under the circumstances, legs would serve no purpose and, consequently, would not be used, long legs would interfere with the snakes' desire for gliding, and short ones could not move their bodies, for they can only have four of them. Continued lack of use of the legs in snakes caused them to disappear, although they were really included in the plan of organization of those animals." On the other hand, "the frequent use of an organ, made constant by habit, increases the faculties of that organ, develops it and causes it to acquire a size and strength it does not possess in animals which exercise less. A bird, driven through want to water, to find the prey on which it feeds, will separate its toes whenever it strikes the water or wishes to displace itself on its surface. The skin uniting the bases of the toes acquires, through the repeated sepa- rating of the toes, the habit of stretching; and in this way the broad membrane between the toes of ducks and geese has acquired the appearance we observe to-day." If such modifications are acquired by both sexes they are transmitted by heredity from generation to generation. This hypothesis is known as "the inheritance of acquired characters." One of the weaknesses in Lamarck's hypothesis appears in his illustration of the snake. If we should grant that inheritance of the effects of disuse of the legs might possi- bly explain their absence in snakes, still it would not ex- plain the origin of the snake's desire to glide. That is, of EVOLUTION 89 course, as much a characteristic of the snake as the absence of legs. Other arguments against the vahdity of Lamarckism are: first, that no one has ever been able to prove, by ex- periment or otherwise, that the effects of use (the so-called F:g. 47. — Jean Baptiste Lamarck (1744^1829). He elaborated the hypothesis of organic evolution by inheritance of the effects of use and disuse. "acquired characters") are inheritable, while innumerable facts indicate that they are not; second, the hypothesis could apply only to the animal kingdom, since plants in general have no nervous and muscular activities like those of animals. A hypothesis of organic evolution, to be valid, must apply equally to both plants and animals. 3. Darwin's Hypothesis. — This will be outlined in the next chapter. CHAPTER VII DARWINISM 75. Darwin and Wallace. — The question of the method of evolution continued to be debated, with no satisfactory solution in sight, until 1859,^ when Charles Darwin published the greatest book of the nineteenth century, and one of the greatest in the world's history, the Origin of Species. ^ This book was the result of over 20 years of careful observation and thought. It consisted of the elaboration of two principal theories: (i) that evolution is the method of creation; (2) that natural selection is the method of evolution. By a strange coincidence Alfred Russell Wallace, also by many years of thoughtful observation, reading, and reflection, had independently formulated the conception of natural selection in far-off Ternate, and embodied his ideas in a paper which he sent to Darwin for the purpose of having it read before the Royal Society. The paper, with its accompanying letter, reached Darwin on June 18, 1858, while the latter was engaged in writing out his own views on a scale three or four times as extensive as that afterward followed in the Origin of Species. As a result of the unsurpassed magnanimity of the two men, and their generous attitude toward each other, it was 1 This date should be memorized. It is one of the most important in the whole history of human thought. = The full title of the book was "The Origin of Species by Natural Selection, or the Preservation of Favored Races in the Struggle for Life." 90 DARWINISM 91 arranged to have a joint paper by Darwin and Wallace presented to the Society. This paper, entitled "On the tendency of species to form varieties; and on the per- petuation of varieties and species by natural means of selection," was presented at a special meeting of the Society Fig. 48.— Charles Darwin. The publication of his "Origin of Species, ' in 1859, revolutionized human thought, and gave direction to all scientific and philosophic thinking from that time to the present. on July I, 1858, being read by the secretary in the absence of both Darwin and (of course) Wallace. 76. Early Antagonism to Evolution.— The concep- tion that evolution (as distinguished from periodic, super- natural interventions of the Deity) is the method of 92 HEREDITY AND EVOLUTION IN PLANTS creation was arrived at independently by Darwin, but was not ilew with him. As we have just seen, it was proposed by Lamarck. Greek philosophers 2,000 years previously had suggested the idea; but it had never won the general acceptance of the educated world, partly because it was feared to be anti-religious, partly because it was never substantiated by sufi&ciently convincing evidence, and partly because of the antagonism of a few men of great influence in the world of intellect. Men preferred to fol- low a leader, more or less blindly, rather than take the pains to examine the voluminous evidence for themselves, and accept the logical conclusion without prejudice or fear, wherever it might lead them, or however much it might contradict all their prejudice and preconceived notions. But truth will always, in the end, command recognition and acceptance, and there is now almost no scientific man who does not regard evolution as axiomatic. It is one of the most basic of all conceptions, not only in the natural and the physical sciences, but also in history, sociology, philosophy, and religion; it has, indeed com- pletely revolutionized every department of human thought-! 77. Darwinism. — It is the second of the above men- tioned theories, i.e., natural selection,, that constitutes the essence of pa,rwinism. The theory is based upon five fundamental facts, which are matters of observation, and may be verified by anyone, as follows: I. InheHiance.^-Cha,ractens.tics possessed by parents tend to reappear in- the next or in succeeding generations. We are all- familiar with the fact that children commonly resemble one or both parents, or a grandparent or great grandparent, in -some characteristic. From this we infer DARWINISM 93 that something hag been inherited from the ancestor which causes resemblance in one or more characters — physical or mental. 2. Variation. — But the expression of the inheritance is seldom, if ever, perfect. Eyes are a little less or a little more brown; stature is never just the same; one-half the face may resemble a given ancestor more than another; petals may be more or less red or blue; no two oranges taste exactly alike; no two maple leaves are of precisely the same shape. There is inheritance, but inheritance is usually expressed with modifications or variations of the ancestral type. 3. Fitness for Environment. — It is common knowledge that living things must be adjusted to their environment. Poor adjustment means sickness or weakness; complete or nearly complete lack of adjustment means death. Water-lilies, for example, cannot live in the desert, cacti cannot live in salt marshes; cocoanuts cannot be grown except in subtropical or tropical climates, edelweiss will not grow in the tropics. This is because these various kinds of plants are so organized that they cannot adjust themselves to external conditions, beyond certain more or less definite limits or extremes. A cactus is fit to live in the desert because it is protected by its structure against excessive loss of water, and has special provision for storing up water that may be used in time of drought. Deciduous tress are fitted to live in temperate regions, partly because their deciduous habit and their formation of scaly buds enables them to withstand the drought of winter. Negroes live without discomfort under the trop- ical sun because they are protected by the black pigment in their skin. And so, in countless ways, we might illus- 94 IIF.-REDITY AND EVOLUTION IN PLANTS trate the fact that all living things, in order to flourish, must be adjusted to their surroundings. 4. Struggle for Existence. — The clue to the method of evolution first dawned upon Darwin in 1838, while reading ]\Ialthus on "Population." Malthus emphasized the fact that the number of human beings in the world increased in geometrical ratio (by multiplication), while the food sup- ply increased much less rapidly by arithmetical ratio (by addition). Therefore, argued Malthus, the time will soon be reached when there will not be food enough for all; men will then struggle for actual existence, and only the fittest (i.e., the strongest, the fleetest, the most clever or cunning) will survive. In pondering this h)^o thesis Darwin at once saw its larger application.^ There are always more progeny produced by a plant or an animal than there is room and food for, should they all survive. Darwin showed that the descendants of a single pair of elephants (one of the slowest breeders of all animals) would, if all that were born survived, reach the enormous number of 19,000,000 in from 740 to 750 years. ^ But the total number of elephants in the world does not appre- ciably increase: evidently many must perish for every one that lives. '"In October 1838," says Darwin, "that is, 15 months after I had begun my systematic inquiry, I happened to read for amusement 'Malthus on Population,' and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result of this would be the forma- tion of new species. Here then I had at last got a theory by which to work." '' One pair of elephants produces an average of only one baby elephant in 10 years, and the breeding period is confined to from about the 30th to the 90th year. DARWINISM 95 Linnaeus, a century before Darwin, had called attention to the fact that if an annual plant produced only two seeds a year, and each of the plants from these seeds, in turn, produced two seeds the second year, and so on, there would, if all the individuals lived, be a million plants at the end of twenty years. But, few species breed as slowly as that. According to Kerner, the common broad-leaved plantain (Plantago major) produces 14,000 seeds annually; shep- herd's purse (Capsella Bursa-pastoris), 64, 000; and tobacco, 360,000. The number of seeds produced each year by the orchid, Acropera, was carefully estimated by Darwin at 74,000,000. But these figures are wholly surpassed by the ferns. Professor Bower estimates the number of spores produced each year by a well grown specimen of the shield fern (Nephrodium filix-mas) at from 50,000,000 to 100,000,000, while the estimate for the fern Angiopteris has been placed at 4,000,000,000 spores for a single leaf. One plant may have as many as 50 or more spore-bearing leaves. It has been pointed out that, at these rates of increase, unrestricted, a given species of plant would, in two or three years, cover an area several thousand times that of the dry land. But nothing of the sort occurs. There must, therefore, be an intense struggle for existence, in which the vast majority of individuals perish. Darwin^ gives the following illustration: "Seedlings, also, are destroyed in vast numbers by various enemies; for instance, on a piece of ground 3 feet long and 2 wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357 no less than 295 were destroyed, chiefly by slugs and 1 "Origin of Species" (New York, 1902 edition), pp. 83, 84. g6 HEREDITY AND EVOLUTION IN PLANTS insects. If turf which has long been mown, and the case would be the same with turf closely browsed by quadru- peds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants; thus out of 20 species growing on a little plot of mown turf (3 feet by 4) nine species perished, from the other species being allowed to grow up freely." "Struggle for Existence" Used in a Large Sense. — "I should premise," said Darwin, "that I use this term in a large and metaphorical sense including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which only one on an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistle- toe is dependent on the apple and a few other trees, ^ but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, may jTiore truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on them ; and it may metamorphically be said 'to struggle 1 In the above quotation, Darwin is undoubtedly referring to the European mistletoe {Viscum album). The American mistletoe (Phora- dendron flavescens) is found in the eastern and central United States on various deriduous-leaved trees, including the sour gum and red maple. DARWINISM 97 with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience sake the general term of Struggle for Existence." S- Survival of the Fittest. — In this struggle for existence only those best suited to their environment will survive. The dandelion from the seed that germinates first secures the best light; the one that sends down the longest and most vigorous root-system, that produces the largest, most rapidly growing leaves will survive, and will tend to trans- mit its vigorous qualities to its progeny. Less vigorous or less "fit" individuals perish. To this phenomenon Herbert Spencer applied the phrase, "survival of the fit- test." Darwin called it "natural selection," because it was analogous to the artificial selection of favored types by breeders of plants and animals. It will be readily seen, however, that the process in nature is not so much a selec- tion of the fittest, as a rejection of the unfit; the unfit are eliminated, while the fit survive. It has been suggested that "natural rejection" would be a better name than "natural selection." "Variations neither useful nor in- jurious," said Darwin, "would not be affected by natural selection." 78. Difficulties and Objections. — The publication of Darwin's "Origin of Species" aroused at once a storm of opposition. Theologians opposed the theory because they thought it eliminated God. Especially bitter antagonism was aroused by Darwin's suggestion that, by means of his theory "much light will be thrown on the origin of man and his history." The unthinking and the careless thinkers accused Darwin of teaching that man is descended from monkeys. Neither of these accusations, however, 98 HEREDITY AND EVOLUTION IN PLANTS is true. Darwinism neither eliminates God, nor does it teach that monkeys were the ancestors of men. By slow degrees, however, men began to give more care- ful and unprejudiced attention to the new theory, and not to pass adverse judgment upon it until they were sure they understood it. "A celebrated author and divine has written to me," says Darwin, "that he has gradually learnt to see that it is just as noble a conception of the Deity to believe that He created a few original forms capa- ble of self-development into other and needful forms, as to believe that He required a fresh act of creation to supply the voids caused by the action of His laws." And in closing his epoch-making book, Darwin called attention to the fact that, in the light of evolution, all phases of natural science possess more interest and more grandeur. "When we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his compre- hension; when we regard every production of nature as one which has had a long history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, in the same way as any great mechanical invention is the sum- ming up of the labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting — I speak from experience — does the study of natural history become!" "It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each DARWINISM 99 Other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Diver- gence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers having been orginally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved." 79. Objections from Scientists. — Objections to Dar- win's theory were also brought forward by scientific men — partly from prejudice, but chiefly because they demanded (and rightly) more evidence, especially on certain points which seemed at variance with the theory. For example, they said, no one has ever observed a new species develop from another; this ought to be possible if evolution by natural selection is now in progress. The absence of "connecting links," or transitional forms between two related species was noted; the presence of apparently useless characters (of which there are plenty in both animals and plants) was not accounted for; and the geologists and astronomers claimed that the time required 100 IIKREDITY AND EVOLUTION IN PLANTS for evolution to produce the organic world as we now behold it is longer than the age of the earth as understood from geological and astronomical evidence. There is not space here to summarize the answers to all these objections. Sufi&ce it to say that scientific investi- gation since Darwin's time has given us reasonably satis- factory answers to most of them, so that now practically no scientific man doubts the essential truth of evolution; it is the corner stone of all recent science, the foundation of all modern thought. 80. The Modem Problem. — But Darwinism left us with a very large and very fundamental problem unsolved. Upon what materials does natural selection act in the formation of species? Obviously the "fittest" survives, but what is the origin of the fittest! This problem Darwin- ism did not solve. The solution of it is one of the most fundamental and "important tasks now being undertaken by biologists. The most effective attack is by the method of experimental evolution, which forms the subject of the next chapter. CHAPTER VIII EXPERIMENTAL EVOLUTION 81. A New Method of Study. — Previous to Darwin's time the study of plants and animals, was carried on chiefly by observations in the field. The science was largely descriptive — a record of what men had observed under conditions over which they did not endeavor to exercise any control; it was accurately named "Natural History" — a description of Nature. But Darwin and a few of his contemporaries, especially among botanists, began to make observations under conditions which they determined and largely regulated. In this way the problems were simplified, observation became more accurate, and the endeavor was made to assign the probable causes of the observed phenomena. With the introduction of this experimental method, science began to make rapid strides, and, more than ever before, facts began to be, not only recorded, but interpreted and explained. 82. Hugo de Vries.^The director of the Botanic Gar- den in Amsterdam, Holland, Hugo de Vries, was among the first to demonstrate that the method of experiment may be applied to the study of the origin of species. His plan was to secure seed of a given species from a plant which he believed to be pure with reference to a given character, that is, not contaminated or mixed by being cross-pollinated with another variety or species. The ro2 HEREDITY AND E\'OLUTION IN TLANTS characters of the parent plant were carefully noted and recorded by photographs and written descriptions, and by preserving dried and pressed herbarium specimens. The plants of the second generation were carefully guarded from being cross-pollinated, and thus "pure" seed were secured for a third generation. This was continued often for 25 or 30 generations of the plant, requiring as many Fig. 49. — Hugo de Vries. His pioneer studies of osmosis resulted in fundamental contributions to our Icnowledge of that subject; his mutation- theory is one of the most important contributions to the study of evolu- tion since Darwin. years when a species produced only one crop of seed a year. Very careful records and preserved specimens were kept of the plants of each generation, and accurate com- parisons were made to see if any individuals showed a tendency to vary widely from their parents in any sig- nificant way, such as showing entirely new characters, not expressed in the parents, or failing to manifest one or more of the characters of the parent. EXPERIMENTAL EVOLUTION 103 83. Two Kinds of Variation. — One of the first results of de Vries's painstaking work was the demonstration of what he believed to be a fundamental difference between two distinct kinds of variation — continuous (or fluctuating) and discontinuous (or saltative, i.e., leaping). 84. Continuous Variation. — Continuous variation is quantitative— a. case merely of more or less. It deals with averages. Some flowers on a red-flowered plant may be lighter or darker red, but, in a series of generations, the Fig. 50. — Fluctuating variation in the leaves of an oak {Quercus chry- solepis), a, all the leaves of a twig; h, younger leaves of a, twig; c, con- secutive leaves; d, some leaves on one season's growth of a twig. (After Copeland.) average of a large number in each generation does not vary, and the departure from the average never exceeds certain limits. The flowers of a given species may have a certain characteristic odor, but the odor may be stronger in some flowers than in others, or in some individual plants than in others , The plants grown from a handful of beans of the same variety may vary in height within limits, but the average^ height of a large number will not vary in successive generations, and will be characteristic 104 HEREDITY AND EVOLUTION IN PLANTS of the species or variety. In other words, continuous or fluctuating variation is variation about a mean. It may be illustrated by the bob of a swinging pendulum, which continually fluctuates within definite limits about the mean position assumed when the pendulum is at rest (Fig. 56). All plants and animals manifest fluctuating variation in all their characters (Fig. 50), and such variations are largely, if not entirely, dependent upon the environment. A slight change in the kind of food elements supplied, or in the amount of water or sunlight available will make the leaves or petals a deeper or a paler color. Rich soil, fa- voring a more abundant food supply, will cause a greater average growth than poor soil, but unless the seed for future generations is selected from the tallest plants, and the richness of the soil is maintained, the plants will revert to their normal, lower average of height. In other words, the average height of the plants of any given variety is a constant (unvarying) character, except that it may be temporarily altered by careful selection of seeds from the tallest or shortest individuals, or by choosing the largest or the smallest seeds from any given plant, or by making the soil richer or poorer, or otherwise. When the selection ceases, and the soil is maintained at average fertility, the characteristic average height of the plants is restored. 85. Illustrations of Continuous Variation. — In a quart of beans, for example, there are no two seeds of precisely the same proportion or size; some are longer, some shorter. De Vries describes^ an experiment in which about 450 beans were chosen from a quantity purchased in the market, and the lengths of the indi- 'De Vries. "The Mutation Theory," vol. 2, p.- 47, Chicago, 1909 EXPERIMENTAL EVOLUTION lOS viduals measured. The length varied from 8 to i6 millimeters, and in the following proportions : Millimeters Number of beans. 10 The beans were then placed in a glass jar divided into nine compartments, all the beans of the same length in the same compartments. When this was done it was found that the beans were so grouped that the tops of the columns in the various compartments followed a curve, known as Quetelet's' curve (Fig. 51). This curve may be plotted by erecting vertical lines (ordinates) at intervals of one millimeter on a horizontal line or base, the height of each vertical line being pro- portionate to the number of beans having the length in- dicated in figures at its base. This curve shows the freq- 108 12 167 13 106 14 33 15 7 16 I Fig. 51. — Demonstration of Quetelet's law of fluctuating varia- bility in the length of seeds of the . . , common bean (Phaseolus milgaris). uency of occurrence of seeds description in the text. (Redrawn dimension from de Vries.) of any given ■ So named from its discoverer, Qu6telet (Ket-lay). As de Vries states: "For a more exact demonstration a correction would be necessary, since obviously the larger beans fill up their compartment more than a similar number of small ones." Io6 HEREDITY AND EVOLUTION IN PLANTS between the two limits or extremes, and is therefore often referred to as a curve of frequency. It should be noted that, in the case illustrated, the greatest frequency (in- dicated by the highest point of the curve) very nearly coincides with the average dimension ; in other words, the more any given character departs from the average for that character, the less frequent is its occurrence. In another experiment, ears of corn, harvested from the same crop, were measured and found to vary in length Fig. 52.— Curve of fluctuating variation (Qufitelet's curve), formed by arranging 82 ears of corn in ten piles, according to the length of the ears. The extremes were 4.5 and g inches. The ears were taken from unselected material from a field of corn. (After Blakeslee.) from 4H inches to 9 inches; the largest number of ears (20) were 7 inches long. The greater the departure from this length, in either direction, the fewer the individuals; for the lengths 4 inches and 9 inches the frequency was zero. When the ears were arranged in piles according to their length, the tops of the piles indicated the curve of frequency (Fig. 52). The curve of frequency indicates the quantitative dis- tribution of any character or quality when its occurrence is dependent largely upon chance. This is strikingly EXPERIMENTAL EVOLUTION 107 Fig. S3- — Photograph of beans rolling down an inclined plane and accumulating at the base in compartments, which are closed in front by glass. The exposure was long enough to cause the, moving beans to appear as caterpillar-like objects hopping along the board. If we assume that the irregularity of shape of the beans is such that each may make jumps either toward the right or toward the left in rolling down the board, the laws of chance lead us to expect that in very few cases will these jumps be all in the same direction, as indicated by the few beans collected in the compartments at the extreme right and left. Rather the beans will tend to jump in both right and left directions, the most probable condition being that in which the beans make an equal number of jumps to the right and to the left, as shown by the large number accumulated in the central compartment. If the board be tilted to one side, the curve of beans would be altered by this one-sided influence. In like fashion, a series of factors — either of environment or of heredity — if acting equally in both favorable and unfavorable directions, will cause a collection of ears of corn to assume a similar variability curve, when classified according to their relative size. Such curves, called Qu6telet's curves, are used by biometricians in classi- fjang and studying variations in plants and animals. (Photo by A. F. Blakeslee. Legend slightly modified from Journal of Heredity, June, 1916.) I08 HERIf.DITY AND EVOLUTION TN PLANTS illustrated by the grouping of bean seeds rolled down a smooth inclined plane, and collected in receptacles at the bottom (Fig. 53). The seeds are started rolling midway between the edges of the plane; the chances are about equal for some of the seeds to fall into the outside compart- ments, but the odds are vastly in favor of their landing at or near the center. Thus they group themselves so that the tops of the piles form a curve of chance variation. When the result is influenced in one direction more than in another the crest of the curve will be nearer one extreme than the other, and the curve is to that extent skew. The curve of bean seeds in Fig. 53 is slightly skew toward the right-hand extreme. Suggest one or more reasons why. 86. Fluctuating Variation and Inheritance. — When the ancestry is not mixed or hybrid the curve of frequency of any character in one generation ordinarily tends to recur in successive generations of descendants, providing the environment remains essentially the same. ' 87. Discontinuous Variation. — ^Long before Darwin, students of plants and animals had observed a different kind of variation than continuous — one which was not quantitative but qualitative, resulting in the expression of new characters, or of a new curve of frequency; that is, in fluctuation about a new mean. Plants from some of the seeds of a red-flowered specimen bear flowers, not that vary from deeper to paler red, but that suddenly, at one step, have become pure white; one or more seeds from an odorless plant may give rise to individuals whose flowers are sweet-scented; or vice versa, odorless specimens ' The behavior of hybrid descendants is a special case described in Chapter XXXVII. teXPERIMENTAL EVOLUTION 109 45 40 35 30 25 20 15 10 5 Fig. S4- — Curves of variation, illustrating the diflference between fluctuation about a given mean, and the appearance of a new mean, i.e., mutation. At the right, variations in the stature of Oenothera Lamarchiana; at the left, variation in the stature of Oenothera nanella, a form derived from 0. Lamarchiana by mutation. (After de Vries.) / / / / \ / \ s / 1 / \ \ ■ / / } \ / / \ / \^ / \ / \ \ 10 10 15 15 20 ■20 ■25 25 30 r30 ^35 35 40 i40 -45 70-75 1 8065 1 9095 1 100 7*80 85-90 95-100 ^^^^^^Hs^ ^^^^^H^.* j^^^^^H ^M ■ ^^K^^^^ii^^l ^^^^^k ^^1 f^^^^^^^l ^^^^H v^^j^^^^^^^^^H ^^H 'if^^^^^^^^^K^^ ^^H '^^^K^^^^^^^k PI m ^B^ '. ^^^Bji^^ 0k'''^M^' ^MBrffn^?^''fcw(B ' ~^S^ ^H M ^8 ^^^hhS Bp^ J^^^JWBKB^ '^^^^^^mS^i^^^S^^Si ^^^CT nf -^SKS^J^^vm^^m, ^i^'^^SSeSBI^^^^wh HnHMl Fig. 74. — ^Lapland rhododendron (Rhododendron lapponicum) . Photo- graphed on the summit of Mt. Madison, New Hampshire, June 25, 1917, by Ralph E. Cleland. surface, which is often firm enough to walk upon. At length these disappear wholly, and give place to the Lap- land rhododendron (Fig. 74), Labrador tea, dwarf birch, and alpine willows, all of which, after rising a few inches above the ground, spread out over the surface of the l60 HEREDITY AND EVOLUTION IN PLANTS nearest rock thereby gaining warmth, which enables them to exist in spite of tempest and cold. These in their turn give place to the Greenland sandwort, the diapensia (Fig. 75), the cassiope, and others, with arctic rushes, sedges, and Uchens, which flourish on the very summit."^ According to Flint, there are about fifty strictly alpine species on these summits, found nowhere else in New England and New York, except on similar summits, such as Mt. Katahdin in Maine, and Mt. Marcy and Mt. Mclntyre in New York State. Incidentally, it may be remarked that a similar state- ment may be made for the animal life. Writing of the insects, Scudder says^ that, "in ascending Mt. Washing- ton, we pass, as it were, from New Hampshire to northern Labrador; on leaving the forests we first come upon animals recalling those of the northern shores of the Gulf of St. Lawrence and the coast of Labrador opposite Newfound- land; and when we have attained the summit, we find insects which represent the fauna of Atlantic Labrador and the southern extremity of Greenland." 118. Effects of Continental Glaciation. — The above mentioned and other similar cases of discontinuity are satisfactorily explained by the advance and retreat of the 1 Among numerous species that have been recorded from both Labra- dor and the peaks of the White Mountains, there may be mentioned the following: Salix argyrocarpa, S. phylicifolia, S. herbacea, S. uva-ursi, Comandra limda, Arenaria groenlaniUca, Silene acaulis, Oxyria digyna, Cardamine bellidifoUa laxa, Saxifraga rivularis, Sibhaldia procunibens, Empetrum nigrum, Epilobium Hornemannii, Loiseleuria procumbens, Rhododendron Lapponicum, Phyllodoce coerulea, Cassiope hypnoides, Arctostapphylos alpina, Vaccinium coespitosum, Diapensia Lapponica, Veronica alpina var. unalaschensis, Gnaphalium supinum. ' Scudder, Samuel H. Distribution of insects in New Hampshire. In Hitchcock, C. H. The geology of New Hampshire, i :34i. 1874. GEOGRAPHICAL DISTRIBUTION l6l continental glacier during the Ice Age. With the advance of the ice all vegetation was either exterminated or com- pelled to migrate southward. With the subsequent retreat of the ice northward the glaciated region was gradually re-occupied by the encroachment of vegetation from the south, and of this flora the arctic species could become permanently re-established only in what are now the arctic regions, and in the arctic or sub-arctic climate of the higher mountain tops, forming there what is known as a relict flora. ^ It has been suggested that, in theory, alpine plants on high mountain peaks south of the region covered by the continental ice sheet, should not be related to arctic and sub-arctic forms. In harmony with this idea Wallace has cited the volcanic Peak of Teneriffe (Pico de Teyde), in the Canary Islands, 12,000 feet high, where, above the timber line, von Buch found only eleven species of plants, eight of which appeared to be endemics ; but all of them were related to the plants of the same general region, growing at lower levels. However, seed-distribution by birds and winds and other agencies has been going on continually since the continental ice sheet began to recede, with the result that arctic-alpine and subarctic-alpine plants are numer- ous in the alpine zone of higher peaks below the southern limits of continental glaciation. Thus the snowy cinque- foil {Potentilla nivea) is found, not only throughout the arctic regions, but also in the Alps, in alpine Asia, and in the Rocky Mountains as far south as Utah and Colo- 1 The eflfect of continental glaciation on the distribution of plants was first noted by Edward Forbes, but was also worked out independently by Darwin several years previous to the publication of Forbes's paper. (Darwin, C. Life and Letters, I : 71-72, 372. New York, igor. See also The Origin of Species, 2: 152. New York, 1902.) l62 HEREDITY AND EVOLUTION IN PLANTS rado. According to Rydberg'- there is evidence that it has spread not only in the earUer postglacial period, but also in recent years. The common arctic and sub-arctic grass, Phleum alpinum, occurs as far south as Arizona and S"^TO 4i* ' •' V- ,'''.^'-> i^lfi^ Fig. 75. — Diapensia lapponica. Photographed on the summit of Mt. Madison, New Hampshire, June 25, 1917, by Ralph E. Cleland. California and the Sierra Madre of Mexico, and also in Patagonia. It is also found throughout the Montane zone, from which it might have spread to the subalpine, following the woods throughout the whole mountain system. Other similar cases might be cited. ^ Letter from Dr. P. A. Rydberg to the author. GEOGRAPHICAL DISTRIBUTION 163 119. Escapes from Ctiltivation. — Every case of discon- tinuous distribution must be carefully analyzed by itself, and care must be taken not to adopt unwarranted con- clusions. Thus, certain cases of discontinuity are ex- plained by the escape from cultivation of forms introduced by human agency for economic uses, and thus have no scientific significance. The presence in the Hawaiian Islands of such economic plants as sugar-cane, cocoanut, and others is an apparent case of discontinuity, but these plants are known to have been introduced there by man, and to have escaped from cultivation. Campbell thinks that the candle-nut tree (Aleurites moluccana, the source of a commercial oil) and the mountain apple {Eugenia malaccensis) , which now constitute the chief elements in the lowland forests of Hawaii, were also introduced by man, and are therefore only apparent cases of disconti- nuity. Among numerous illustrations of this in North America may be mentioned the paper mulberry {Brousone- tia papyrifera), white mulberry {Morus alba), hemp, {Cannabis saliva), stinging nettle {Urtica dioica), the day lily {Hemerocallis fulva), all natives of Europe and Asia, and the tree, Paulownia, a Japanese species now becoming established as an escape from cultivation in New York, New Jersey, the District of Columbia, and Georgia. The last two species were introduced into North America as ornamental plants, the hemp and white mulberry, of course, as economic plants, the latter in connection with the raising of silk worms. Attention has recently been called to the wide and rapid spread of the Japanese honeysuckle {Lonicera japonica) introduced in the Eastern United States from Asia. Twenty-five or thirty years ago this was a comparatively 164 HEREDITY AND EVOLUTION IN PLANTS rare cultivated vine, but since that time, according to Miss Andrews,^ 'it has spread over practically the whole of the Eastern States, from the Gulf of Mexico to the estuary of the Hudson, making itself equally at home in the low hammocks of the Coastal Plain, on the old red hills of the Piedmont region, on the stony ramparts of the Lookout Plateau, and onward for a thousand miles up the great Appalachian Valley." The adaptabihty of the plant, as indicated by this description of its habitats, in no doubt a large factor in its rapid spread, for while it is a profuse bloomer under cultivation, it tends to become weedy, as it grows wild, blossoming rarely and therefore setting few seeds. But its wide distribution must have been accom- plished by the dissemination of its seeds, and in this Miss Andrews believes that the most probable agents are birds, to whose feet the small, inconspicuous nutlets, "embedded in a mucilaginous pulp," readily adhere. Several species (e.g., the fleabane, Pluchea fcetida) are found in shallow fresh water or fresh or salt water marshes from southern New Jersey to Florida, and then across 120 miles of salt water in Cuba. In this case it seems clearly evident that the seeds have been able to undergo transportation across the Florida strait within comparatively recent times. Examples might be multi- pKed, and in such cases discontinuous distribution has httle evolutionary significance for the particular species concerned, though the facts may serve to throw hght upon other cases that are significant. 120. Endemism. — On the basis of the evolution theory every species originated in some one area (its center of dis- ' Andrews, E. F. The Japanese honeysuckle in the Eastern United States. Torreyo, 19 : 37-43. Mch. 1919. GEOGRAPHICAL DISTRIBUTION 165 tribution), where it was at first endemic,^ and whence it gradually spread as far as it could. This is well illustrated in the distribution of the Verbenaceae, one of the higher and therefore more recent families of flowering plants, com- prising about 75 genera and 1300 species, occurring widely throughout tropical and temperate regions. Of 104 species belonging to various genera in the Philippines, 60 per cent., according to Lam,^ are apparently endemic. These endemic forms have undoubtedly been derived from the 39 non-endemic species, and will, in the course of time, spread from the Philippines to neighboring islands and thence to the mainland. About 85 per cent, of the flora of Hawaii is endemic,^ and even the strand flora, while cosmopolitan on the whole (the general rule for coastal vegetation), is nearly 40 per cent, endemic, a surprisingly high percentage. From the facts of geographical and geological distri- bution, Wallace deduced the following law:* Every species has come into existence coincident both in time and space with a pre-existing closely allied species." "The law here enunciated," said Wallace, "not merely explains but necessitates the facts we see to exist, while the vast and long-continued geological changes of the earth readily account for the exceptions and apparent discrepancies that here and there occur." And again, "this law agrees 1 Endemic: found in a given region, but not elsewhere. 2 Lam, H. J. The verbenaceae of the Malayan Archipelago. Gronin- gen, 1919. ' Including, for example, all the native Hawaiian palms, belonging to the genus Priichardia. See MacCaughey, Vaughan. Bull. Torrey Bot. Club, 45: 259-277. July, igi8, and Plant World 21 -.317-328. Dec.,1918. * Wallace, Alfred Russel. On the law which has regulated the introduc- tion of new species. Annals and Mag. of Nat. Hist. 16, Ser.2 : 184-196. Sept. 1855. i66 HEREDITY AND EVOLrTION IN PLANTS with, explains and illustrates all the facts connected with the following branches of the subject: ist, the system of natural affinities; 2d, the distribution of animals and plants in space; 3d, the same in time . . . 4th, the phenomena of rudimentary organs." And Wallace goes on to show, in detail the bearing of the law upon each of the four points enumerated. ^^P!!'^^H ^^H^f^ ..'i^^H HpHHH^^-^, M ji^^^^^l^^^l ^^wSi^JH^H^^^^^^^^^H Fig. 76. — Alfred Russel Wallace C1823-1913). Co-discoverer, with Darwin, of the principle of natural selection. Noted student of geo- phical distribution. A quotation from Darwin is also pertinent here: "It is . . . obvious," said Darwin, "that the individuals of the same species, though now inhabiting distant and isolated regions, must have proceeded from one spot, where their parents were first produced for, as has been explained, it is incredible that individuals identically the same should have been produced from parents specifically distinct." 121. Mutation and Discontinuous Distribution. — Read- ing Darwin's statement in the light of the mutation theory GEOGRAPHICAI, DISTRIBUTION 1 67 of de Vries, we must of course recognize that, if a mutating species were widely distributed, different individuals of the species in widely separated locahties and even with a dis- continuous distribution, might throw the same mutants. (Enothera Lamarckiana, for example, threw the same ele- mentary species (mutants) in experimental pedigree cultures in Holland and in various localities in the United States.^ Had 0. Lamarckiana (contrary to fact) been widely distributed in nature, such mutants as 0. gigas, 0. scintillans, 0. laevifolia, and others would possibly (or even probably) have appeared in different and widely sepa- rated stations, and these elementary species might con- ceivably (and not improbably) have become established as true species of the systematist. When, therefore, we find a given species (or a larger group) in widely separated localities, but not in the intervening regions, we must (barring the phenomenon of mutation referred to above) conclude, either that it has been able to migrate across barriers where it could not become established (as when seeds of land plants are carried by ocean currents across barriers of salt water), or else it has formerly had a con- tinuous distribution, but has subsequently died out in regions between its present localities ; in the latter case it is referred to as a relict endemic. When these localities are distant hundreds or, as is often the case, thousands of miles from each other, one can readily understand that species having such discontinuity of distribution must, other things being equal, be older than species having continuity of distribution; they must have existed long enough for the changes above mentioned to have taken place. This principle is confirmed by the evidence of fossils. A striking case is that (cited by Chodat) oi Zelkowa, 'See pp. 114-117. 1 68 HEREDITY AND EVOLUTION IN PLANTS related to our modern elms. This genus comprises only four living species, which occur in only three widely separated areas, namely, the far East (Eastern China and Japan), the area between the Black and the Caspian Seas (Caucasia), and islands in the eastern Mediterranean Sea. But a study of the fossil evidence shows that during a preceding geological age this genus had a very extended distribution, including central Europe, the Iberian penin- sula, Iceland, southeast Greenland, Labrador, western North America, and Alaska. Owing to profound changes of climate, in the transition from one geological age to another, Zelkowa was apparently unable to survive, except in the two restricted and widely separated areas where it is now found. 122. Continuous Distribution. — Continuous distribution is of two types: ubiquitous, like the bracken fern, and isolated, like the redwoods. Sequoia. In the latter case two suppositions are possible: either the species or genus is very new and has not had time to spread {indigenous endemic) ; or it is very old and a relict endemic, as defined above. Which of these two alternatives is correct for any given case may be ascertained only on the basis of comparative anatomic evidence, or on fossil evidence, or on both. The motile sperms and the structure of the wood of the maiden-hair tree {Ginkgo biloba), for example, point without question to affinities with an older type of seed- bearing plants, the Cycads. In the case of the genus Sequoia, with only two living species, the coast redwood {S. sempervirens) and the giant redwood {S. gigantea), restricted in range to one state, California, the fossil evidence shows that these two species are the meager GEOGRAPHICAL DISTRIBUTION 169 remains (relict endemics) of a genus of several species, which, in Tertiary times, was widespread over most of the northern hemisphere (Fig. 77). By a Uke balancing of evidence we are able to ascertain that the ubiquitous fern family, Polypodiacece, with some 200 genera and about 3,000 species, is a com- paratively modern group, while the Osmunda family. Fig. 77. — Map showing the known geographical distribution of Se- quoia during the Cenozoic era. The cross indicates the only known loca- tion of living specimens. (After E. W. Berry.) with only two (or possibly three) living genera and some ten species, and with wide but discontinuous distribution, is much older. The greater antiquity indicated for the Osmundaceas by the facts of their geographical distribu- tion is also attested by fossil evidence, and further by the nature of their spores. The spores when mature contain chlorophyll, and this fact, of itself, indicates antiquity; for this and other structural and physiological reasons. 170 HEREDITY AND EVOLUTION IN PLANTS they quickly perish unless they find at once suitable conditions for germination and development. Thus they could not spread rapidly over large areas. In the light of these facts the only logical inference is that their wide and discontinuous distribution must have required a vast period of time. The tulip tree, represented now by only one genus (Liriodendron) and one or possibly Fig. 78. — Map showing the known geographical distribution of the bald cypress {Taxodium) in the Tertiary and Pleistocene. Tertiary dis- tribution, shaded; Pleistocene occurrences north of its present limits, in dots; present distribution, black. (From Shimer, after E. W. Berry.) two species, and with discontinuous distribution (Eastern North America and China), represents an old type now, perhaps, on the way to extinction. A similar statement may be made for Sassafras, for the bald cypress {Taxodium, Fig. 78), and numerous other groups. In general it may be said that groups considered relatively more primitive or ancient on morphological or paleonto- GEOGRAPHICAL DISTRIBUTION 171 logical grounds, are characterized by few genera and a restricted or (if wide) discontinuous distribution. Thus the Barberry family, one of the relatively primitive groups of dicotyledons, contains only about lo genera and over 130 species, found in temperate North America and Asia, temperate South America, and sparingly in the tropics; the Nymphaeaceae (Water lily family), more primitive than the Berberidaceae, contains only eight genera and about 50 species, of wide but discontinuous distribution. In contrast there may be mentioned the gamopetalous Potato family (Solanacese), with about 70 genera and 1,600 species, found generally on every continent, and in New Zealand, Hawaii, Australasia, and other oceanic and continental islands, and specially abundant in the tropics; and also the still more highly developed Madder family (Rubiacece), with as many as 355 genera and 5,500 species, also of almost cosmopolitan distribution. As a final example among families of flowering plants, there may be mentioned the Orchidaceae, the most highly developed of the Monocotyledons, and, on morphological grounds, possibly the most recent family of seed-bearing plants. This family contains about 430 genera and over 5,000 species, of almost cosmopolitan distribution, most abundant in the tropics, and gradually diminishing toward the poles. The seeds of orchids are very tiny, and the embryo consists of a few undifferenti- ated cells. They are capable of rapid and wide distribution (Fig. 78a). In the Nympheaceae is the relatively primitive genus, Nelumbo, containing only two species, one the lotus {N. luiea), in North America, the other the Oriental lotus (N. nucifera), in Asia and Australasia. In the 172 HEREDITY AND EVOLUTION IN PLANTS Rubiaceae is the genus Mitchella, also relatively primitive, and containing only two species, one in Japan, the other the Partridge berry (M. repens) in North America. 123. Evidence from the Distribution of Liverworts. — The geographical distribution of the lower cryptogams (below the ferns and their allies) has not been the subject of as extensive study as that of the ferns and flowering plants, but the evidence marshalled by Campbell^ in 1907, concerning the distribution of the liverworts (Hepaticai), illustrates in a striking manner the importance of the Fig. 78a. — Seed capsule and seeds of an orchid. facts of geographical distribution in endeavoring to determine the question of the relative age of a group of plants. It had been argued by Scott, in 1906, that the liverworts were probably of comparatively recent origin because of the almost entire absence of fossil remains in the Paleozoic rocks. But, as Scott himself records, impres- sions have been described from Paleozoic strata of plant forms that can be assigned only to the Hepaticae, and indeed to one of the most highly organized groups — the ' Campbell, Douglas Houghton. On the distribution of the Heptaticae, and its significance. New Phytologist 6 : 203-2 12. Oct. 1907. GEOGRAPHICAL DISTRIBUTION 1 73 Marchantiacese. This, of course, means a long period of evolutionary development from similar forms to the more complex, preceding the geological age of the rocks containing the fossil record, and one such bit of positive evidence fully substantiated, is of itself sufficient to establish the antiquity of the liverworts. Moreover, when such testimony is in agreement with the evidence derived from other sources, such as comparative mor- phology and geographical distribution, the fact of anti- quity would seem to be reasonably well established. Now, in addition to the evidence of comparative morphology, there are, as Campbell points out, certain facts of dis- tribution that can only be satisfactorily interpreted on the basis of the comparative antiquity of the liverworts.^ The liverworts are a widely distributed group ; some of the genera are cosmopolitan, i.e., they are found practically everyTvhere, in all continents, climates, and habitats, and widely on oceanic islands. Riccia and Marchantia are cosmopolitan genera of continuous distribution. Other genera are of wide, but discontinuous distribution, such, for example, as Targionia, a genus containing only two species, which are found in Southern and Western Europe, Africa, Java, Australia, and Western America, , but are absent from Eastern America and from most of Asia. The famUiar Lunularia cruciata of our greenhouses has a distribution similar to Targionia in the eastern hemisphere, but is unknown in the western hemisphere except where introduced. 1 Throughout the discussion of liverworts I have drawn freely on Camp- bell's article, cited above, and have, to a certain extent, adopted his wording, asking the reader and the author quoted to accept this statement in lieu of frequent quotes. 174 HEREDITY AND EVOLUTION IN PLANTS A third type of distribution is that of limited range, such as has been mentioned above for the venus's fly- trap and the giant redwood trees. Among genera thus distributed are Wiesnerella Javanica Schiff., known at present only from Mt. Gedeh, in Java, and Geothallus tuberosus Campbell, known only from near San Diego, California. These ranges may ultimately be extended, as was that of Treubia insignis, known for a time only from Mt. Gedeh, but later found by its original discoverer in New Zealand. As already noted, in order to become widely distributed, either continuously or discontinuously, a plant must either 1. Have reproductive bodies capable of rapid distribu- tion over wide areas, or 2. Possess sufficient antiquity to have been in process of dissemination for a comparatively long period of time. In the former case, its reproductive bodies must be of such nature as to resist unfavorable environment and vicissi- tudes, during transit over long distances, and be able to establish themselves readily in the new habitat, especially in competition with the plants already established, and pos- sibly also in an unfavorable environment. Now the spores , of many of the most widely distributed Hepaticcs are not of this nature. We can hardly explain the present distribu- tion of such widespread tropical genera as Dendroceros, Monoclea, and Dumortiera, says Campbell, by the theory that their spores could be carried across the wide ocean barriers that separate the regions where they now occur, as the spores are not of the type that could be carried long distances wi thout perishing. Since there are no connecting forms in the higher latitudes that could explain the passage of these forms from one tropical zone to the other, we can GEOGR.VPmCAL DISTRIBUTION 17S only assume that these genera are the little changed descendants of ancient, widely distributed types. Although making a special search for Liverworts on Krakatoa in 1906, Campbell found no specimens, nor up to that time had any other collector. Professor Treub, of the Botanic Garden at Buitenzorg, Java, had reported two species of mosses. "Inasmuch as Krakatoa is within sight of Java and Sumatra, both of which have an extremely rich hepatic flora, the absence of these plants from the new flora of Krakatoa is, to say the least, worthy of note." In a similar way Campbell argues that the wide distribution of mosses (cosmopoUtan in the case of the genus Sphagnum), combined with the inabiUty of their reproductive bodies to withstand trans- portation over great distances, indicates a great antiquity for the group; and this inference is substantiated by the meager but positive evidence of fossil remains. In a later discussion of the origin of the Hawaiian flora, Campbell^ notes that the filmy ferns, since they are hydrophytic with a rain-forest habit, and are, therefore, not suited to transportation over wide stretches of ocean, must have existed in Hawaii since those islands were connected with some mainland, now submerged. The relatively shallow water between Hawaii and the Australasian-Malaysian regions, as compared to the great depths between Hawaii and North America, in- dicate a former mainland connection to the west, and this inference is further substantiated by the great pre- ponderance of Australasian-Malaysian plants in Hawaii over those represented in America. In this connection 1 Campbell, D. H. The origin of the Hawaiian flora. Mem. Torrey Bot. Club, 17: 90-96. June, 1918. 176 HEREmXY AND EVOLUTION IN PLANTS is should be noted that a considerable proportion of the species of the strand vegetation of Hawaii are endemic, but many of the introduced littorals are known to be transported by ocean currents from the north Pacific.^ 124. Distribution of Algae.^ — And finally, to bring all the great phyla under brief review, it may be mentioned that facts of distribution of the Algae point to a great antiquity for the group. This is not only in harmony with the generally accepted evidence from comparative morphology, but is substantiated by fossil remains, in early Paleozoic rocks, of calcareous Siphonogamous forms related to liv- ing calcareous forms. The absence of fossil remains of non-calcareous green forms is readily explained by the delicate nature of their tissues. 125. Hypothesis of "Age and Area." — As noted above (p. 165), an endemic species is one found in a given local- ity but not elsewhere. According to some botanists^ endemism is a criterion of youth. The area occupied by a species within a given country, argues Willis, varies directly with its age within that country; that is, the longer it has been a part of the flora, the wider the area it occupies, so long as conditions remain constant. But Wilhs enumer- ates various conditions that would interfere with the operation of this law, including "chance" {i.e., causes not understood) , the action of man (clearing of forests,' etc.) , ' Twenty-one littorals and eleven pseudo-littorals, out of a total of over 75, are listed as endemic by Vaughan MacCaughey. Bull. Torrey Bot. Cte6, 45: 259-277. July, 1918. ' Willis, J. C. The relative age of endemic species and other controver- sial points. Ann. Bot. 31:189-208. April, 1917. James Small (see p. 148) has characterized Willis's Age and Area hypothesis, as the most im- portant contribution to geographical botany since the Origin of Species. ' Macrozamia Moorei is being systematically exterminated in Australia because it is poisonous to cattle. GEOGRAPHICAI DISTRIBUTION 177 interposition of barriers (mountains, broad deserts, salt water areas, sudden changes of climate from one district to the next, geological changes, natural selection, local adapt- ation (the possession of a character useful in one country but not in another), the dying out of occasional old species, the arrival of a migrating species at its cHmate limit, et cetera. But on the whole the endemic species, says Willis, are the youngest. As an illustration of the operation of the hy- pothesis of age and area, Small {I.e., p. 25-30) mentions numerous Compositae which have limited distribution, although there would seem to be practically no limit to the distance their pappose seeds can be transported by wind. They are Umited (endemic) because they are young. According to another view,^ endemic species are the oldest species of a region; they are either relicts, and thus very ancient, or they represent types which have been in the region so long that their original characters have been lost. The latter are indigenes, and are spoken of as indigenous to the country. Endemics, according to Sinnott, contain a greater percentage of trees than do wides (or polydemics)- but, according to the same author, trees and shrubs are older than herbs, and therefore the endemic woody species must be older than the herbaceous element of a given flora. The hypothesis of WiUis demands that herbs be considered as an older form of vegetation than trees and shrubs, which, others argue, is contrary to a mass of evidence. Trees are more common as endemics (in Ceylon, e.g., twice as common), notwithstanding the fact that they spread less rapidly than herbs. After its 'Sinnott, Edmund W. The "age and area" hypothesis and the problem of endemism. Ann. Bot. 31:209-216. April, igi?. ' "Wides" and "polydemics" are used as antonyms of endemics. 178 HERICDITY AND JA'^LUl'ION IN PLANTS first rapid spread, says Sinnott, a species becomes less common the older its age of occupation. Reviewing these two theories, Taylor^ holds that, in the flora of the vicinity of New York at least, endemism is not a criterion of antiquity nor of youth, for while many endemics of the flora of New York and vicinity are very recent (as the hypothesis of Willis would require), and while some of them are even found in the geologically recent portion of the area (one. Hibiscus occuliroseus, being a salt marsh plant and therefore very 'new'), other forms are relict endemics (p. 167), and could not, therefore, be of very recent origin. ^ As an example of relict (and therefore old) endemics (outside the local flora region of New York, there may be cited the well known case of the giant and coast redwoods {Sequoia gigantea and S. sempervirens), and the begonia, Hilldebrandia sandwicensis , endemic in Hawaii.' An example of an indigenous (and therefore relatively recent) endemic, is the well-known insectivorous plant, Venus fly-trap {Dioncea muscipula) , a genus having only one species, i.e., monotypic (Fig. 79) . This unique plant is found in sandy swamps, only in a narrow strip of country ' Taylor, N. Endemism in the flora of the A-icinity of New York. Torreya 16: 18-2^. Jan. 1916. ' Five cases of apparently relict endemism are cited by Taylor from the vicinity of New York. Torreya 16 : 18-27. Ja-"- 1916. ' The Begoniaceae have scarcely any representatives in the islands of the southern, equatorial, and Northern Pacific, but are abundant in the Andes region of South America and Mexico. The endemic begonia of Hawaii is regarded by MacCaughey (Bot.Gaz. 66:273-275. Sept. 1918) as one of several bits of evidence that "at one time in the history of the Pacific basin the Hawaiian islands were much more closely associated with the Andean and South Pacific regions than they are at present. See also P- 175. GEOGRAPHICAL DISTRIBUTION 1 79 about ten miles wide and extending about 40 miles south of Wilmington, North Carolina. The yellow waterlily (Nym- phcBa mexicana Zuccarini) ^ may also be cited as an aquatic Fig. 79. — Venus fly trap {Dionaa muscipula) . example of an indigenous endemic, being known only from Florida, Texas, and Mexico. = ' Castalia flava Greene (1888). 2 Conard, Henry S. The waterlilies, p. 167 and 213. Carnegie Insti- tution of Wasliington, Publication No. 4. 1905. l8o HEREDITY AND EVOLUTION IN PLANTS Again, as Taylor points out, most of the recent endem- ics in the New York flora are not woody, the proportion of woody species among the endemics (17 per cent.) being essentially the same as for the entire flora (18.2 per cent.) Most of the endemics are probably accounted for by generic and specific instability, that is, by the ten- dency of existing forms to vary, at or near the edge of their range, and for the variations to become estabUshed. At least one is a case of "habitat" endemism; that is, the endemic species is confined to a given locality because suited to the environment afforded by that locaUty. This is illustrated by Prunus Gravesii, a saxitile form of the beach-plum (P- mariiima). Many factors are involved in the phenomena of en- demism, and here, as in the case of discontinuous geo- graphical distribution, each case must be carefully analyzed by itself. In view of our present restricted knowl- edge, we can generalize only with extreme caution. 126. An Illustrative Study.' — As an illustration of the apphcation of evidence from various sources in an en- deavor to decide the relative age of two large groups of plants, herbs and woody plants ■ (trees and shrubs), there may be mentioned the recent work of Sinnott and Bailey,' who marshalled evidence from paleobotany, anatomy, phylogeny, and phytogeography, as bearing on the rela- tive antiquity of herbaceous and woody plants. Very briefly summarized , their argument runs as follows : I. A study of fossil plants shows that the remains of 1 Sinnott, Edmund W. and Irving W. Bailey. Investigations on the phylogeny of the Angiosperms : No. 4. The origin and dispersal of herbaceous Angiosperms. Ann. Bot. 112: 547-600. Oct. 1914. The phraseology of the authors is freely incorporated in the above very brief summary. GEOGRAPHICAL DISTRIBUTION l8l Angiosperms in earlier geological periods were almost all woody. The number of herbaceous forms increases as we pass from older to more recent strata. Fossils of herbaceous plants are rarely found in Cretaceous rocks but become increasingly abundant throughout the Ter- tiary. Caution is necessary here, however, for the foUage and other parts of herbs are more tender and deUcate than those of woody plants, and therefore less hable to be preserved as fossils. This evidence is significant only in connection with evidence derived from other sources. 2. A study of the comparative anatomy of stems indi- cates that the continuous ring of wood, which character- izes the stems of all trees and shrubs, is a more primitive character than the separate fibro-vascular bundles of herbaceous stems. It is suggested that a change from a woody to an herbaceous type may have resulted from regional decrease in the activity of the cambium layer, from which the wood is formed by cell-division followed by hgnification. 3. Evidence from phylogeny shows that the more primitive groups of Angiosperms and their probable an- cestors are composed overwhelmingly of woody plants. In more than half of the famiUes of Dicotyledons there are no herbaceous species, and the few families which are entirely herbaceous are almost all insectivorous plants, water plants, parasites, or monotypic famihes,^ and hence can lay no claim to great antiquity. Also, there is a much larger proportion of woody plants in the lower groups of Angiosperms (Apetalae and Polypetalae) than in the higher groups (Sympetalse.) 4. From a study of plant geography we learn that 1 /I monotypic family is a famUy having only one genus. l82 HEREDITY AND EVOLUTION IN PLANTS dicotyledonous herbs preponderate in north temperate regions, and woody plants in the tropics. The latter climate probably approaches more nearly to that under which Angiosperms first appeared. Herbs, having a short life cycle (one to two or three seasons) are able to survive periods of intense cold in the form of seeds, and would, therefore, survive in larger numbers than woody plants on the advance and retreat of the continental ice sheet of the Glacial period. This would account for the fact of a much smaller proportion of woody plants in the flora of Europe, for these could not migrate southward, as the ice encroached, since the mountain ranges there have a general east-west trend (in contrast to the general north- south trend of American ranges), and southern migration would necessitate an ascent to high altitudes that would be fatal to temperate or subtropical species. The above facts are not cited as established, but only to illustrate a method. There is also evidence and argu- ment suggesting the opposite conclusion, namely, that herbaceous plants are older than woody. CHAPTER XI PALEOBOTANY 127. The Scope of Paleobotany.— The study of fossil plants, though of course a phase of botany, constitutes a science by itself, not only covering a special subject matter, but having its own methods (technique), and pos- sessing a large literature. It is called paleobotany. One cannot pursue this study without a knowledge of the anatomy and morphology of living forms. This is neces- sary in order to interpret the meaning of plant fossils, which often occur only in small fragments of the entire plant. Moreover, one must have a good knowledge of at least the elements of geology, since fossils are found in rocks. One must not only know the geological age to which the fossil-bearing rock he studies belongs, but also something of the geological processes by which fossils, and even the rocks themselves, are formed. 128. What is a Fossil? — A fossil is any remains of a plant or animal that lived in a geological age preceding the present; these remains are preserved in rocks.' There are two methods of preservation, namely, incrustation and petrifaction. Incrustations are merely impressions or ' By an extension of the term we also speak of fossil footprints of ani- mals, fossil ripple marks, et celera. The word fossil is derived from the Latin }odere (to dig), and originally signified anything dug up. 183 1 84 HEREDITY AND EVOLUTION IN PLANTS casts resulting from the encasement of the organ or organism in the rock-forming material. The tissue itself either decayed or became carbonized, leaving only the ■■■>•#• Fig. 8o. — Fossil incrustations of the foliage of two species of Spheno- phyllum from the coal measures of Missouri. (From U. S. Geological Survey.) impression of its surface features. The well-known "fossil fern-leaves," found in coal mines, are of this nature. The tissues of the plant were transformed into coal, PALEOBOTANY 185 leaving the impression or cast on the adjacent shale. The first stage in this process may often be observed in the autumn, when impressions of recently fallen leaves are made on the surface of wet mud. Obviously from such fossils we can learn nothing of internal structure (Fig. 80). Petrifactions are formed by the gradual replacement of the organic tissue by mineral matter, usually carbonate of lime (CaCOs) or silicic acid (H4Si04). In this process the tissues become soaked with a saturated solution of the given mineral, which is gradually deposited from solu- tion, and takes the place of the original organic matter. By this means the most minute details of microscopic structure are preserved, even in some cases the nuclei and other cell-contents (Figs. 97 and 100). 129. Conditions of Fossil-formation. — In order to understand how fossils come to be formed, we must pic- ture to ourselves certain geological processes now in operation— the initial stages of rock-formation. Rocks are of two kinds, igneous and sedimentary. Igneous rocks result from the cooling of molten lava poured out on the surface or injected into crevices by volcanic action. Such rocks never contain fossils, as the intense heat necessary to melt the rock destroys all trace of organic matter. Sedimentary rocks are formed by the deposit under water of the sediment formed by weathering and erosion and transported by streams. This deposit may occur along the flood-plains or at the mouths of streams empty- ing into inland lakes or into the ocean. In addition to rock-sediment eroded from the surface of the land, streams also transport quantities of plant (and animal) frag- i86 HEREDITY AND EVOLUTION IN PLANTS M p -a ■- PALEOEOTANY 187 ments, leaves, stems, pieces of bark, fruit, flowers, pollen and spores, roots, and even entire plants. These natur- FiG. 82. — Diagram illustrating the gradual filling up of lakes by the encroachment of vegetation, and also the stages in the origin of peat and marl deposits in lakes. The several plant associations of the Bog series, displacing one another, belong to the following major groups: (i) O. W., open water succession; (2) M., marginal succession; (3) S., shore succes- sion; (4) 5., bog succession, comprising the bog-meadow (Bm), bog-shrub (Bs) and bog-forest (Bf); and (s) M. P., mesophytic forest succession (C/. Fig. 81.) (After Bray.) aIly~become buried in the mud and sediment wherever deposition takes place, and when the deposit becomes 18» HEREDITY AND EVOLUTION IN PLANTS converted into rock the organic remains may become con- verted into fossils by either of the processes described above. Swampy regions are especially favorable to the preservation of plant and animal remains as fossils, as is illustrated in Figs. 8i and 82. 130. Metamorphism.- — After sedimentary rocks are once formed they are subject to various changes. The amorphous carbonate of lime, of limestone rocks, may be transformed into crystals of calcite until marble results; thin flakes of mica may form in clay rock in thin sheets, transforming the rock into slate; vegetable deposits in the form of peat may become transformed into anthracite coal and graphite; molten lava poured out on the surface or into crevices of sedimentary rocks may fuse the adja- cent material, causing contact metamorphism; while the heat engendered over larger areas by mountain folding, or by the weight of superincumbent strata^ may cause regional metamorphism. Obviously such changes, espe- cially those caused by heat, result in the complete de- struction of all plant or animal remains or impressions, and thus fossil records over large areas, and representing vast periods of geologic time, have been obliterated. 131. Stratification of Rocks.— Changes in the relative level of sea and land have occurred many times in the geological past, so that submerged areas of sedimentation in one period have become areas of dry land, undergoing erosion in another; and vice versa, areas of erosion have become areas of sedimentation. As a result of this, rocks occur in layers,^ the deeper lying layers (with ex- ^ Some rocks are buried under more than 40,000 feet of strata, and the temperature increases approximately i°F. for every 50 to 60 feet of depth. ^ Several layers form a stratum, or bed. PALEOBOTANY I 89 ceptions readily explained by geologists) being older than those above, or nearer the surface. Moreover, as a result of a second submersion following elevation and erosion, subsequent layers were often deposited with an uncon- ormity on the weathered and eroded surface under- neath. By the presence of fossil imprints of rain drops, foot- prints, ripple marks, and mud cracks, and by the character of the plant and animal fossils which they contain, we know that most sedimentary rocks were deposited in shallow water, not far from the shore line. But since these same rocks may have a thickness of thousands of feet we know the area of sedimentation must have been slowly sinking while the sediment was being deposited. As a result of the enormous pressure of the overl3dng material, of the deposit of cementing substances from solution, and of other causes, the sedimentary deposits became, in time, converted into solid rock. 132. Classification of Rock Strata. — By a study of the fossils which the rocks contain, geologists have been able to classify the various strata according to their age. As a result of the period of erosion, indicated by un- conformity, the transition from the stratum of one age to that of another is often abrupt, the fossils in successive periods being quite characteristic of the given stratum or period. In other cases, as for example between the Silurian and Devonian in New York State, there is no unconformity, and this renders it more difficult to decide just where the plane of division lies. The names and order of occurrence of the known rock strata are given in the following table, the older rocks being at the bottom, the most recently formed at the top. I go Era TIEREDITV AND E\(ILU'1I(IN IN PLANTS Table II. — Table of Geological Time Period Holocene Quaternary (recent, or the pres Pleistocene (ice age) Cenozoic Pliocene Tertiary r Miocene Oligocene Eocene Upper Cretaceous Lower Cretaceous Mesozoic Secondary r (Comancbean) Jurassic Triassic Permian Upper Carboniferous (Peniisylvanian) Lower Carboniferous Paleozoic Primary (Mississippian) Devonian Silurian Ordovician Cambrian Archean Huronian Laurentian 133. Paleogeography. — By changes in the relative level of the land and sea, above referred to, rocks contain- ing fossils may be elevated as dry land, and frequently as mountains, so that remains of marine organisms, as well as of others, are often found at high elevations. In some cases forests near the seashore have been submerged, and covered over with sediment, then elevated again as dry land, so that subsequent excavations have revealed the fossilized trunks and stumps (Figs. 83 and 84). Thus PALEUBOTANY 191 Fig. 83. — Fossil tree stumps in a carboniferous forest, Victoria Park, Glasgow. (Cf. Fig. 84.) (After Seward.) Fig. 84. — Part of a submerged forest as seen at low tide on the Cheshire coast of England. (C/. Fig. 83.) (After Seward.) 192 HEREDITY AND EVOLUTION IN PLANTS it is seen that, by a study of fossils, we may not only learn of their structure and thus fill in many of the gaps in the evolutionary sequence left by a study of forms now living, but we may also learn of the distribution of plants and animals in previous geological ages — in other words, we have the basis for a science of fossil geography or paleogeography. 134. Plant Migrations.— With the development of Paleogeography, a clearer conception of the location and changes of the continental areas of the past is gradually being gained. As a consequence, plant geography is a sub- ject of increasing interest to the paleobotanist. More- over, geology, the fossil record, and the present zonal grouping of plants indicate that, in the past, the polar areas, then much warmer than now, must have been fruit- ful in new species.^ High mountains or plateaus are also suggested as homes of plastic races. ^ In the tropics en- vironments are more nearly static, and, it is reasonable to suppose, less likely to favor variation. It is knownthat once established, many species move most readily along the geologic formation which supplies the exact soil con- stituents most favorable to their growth, the rate of movement often being rapid. Flotation of seeds is also a factor. The facts here briefly cited rest on the obser- vations of a large number of investigators, extending over more than a century. 135. Distribution of Plants in Time. — In addition to the distribution of plants in space (plant geography), the problem of their distribution in geologic time is one of ' Owing to the precession of the equinoxes these areas undergo an ex- treme variation in the length of winter and summer of 37 days every, 1 2,934 years. 2 CJ. pp. 148-149. PALEOBOTANY 193 absorbing interest and importance. The following table indicates the known distribution of the various plant groups from the earliest geologic time to the present. Table III. — Distribution op Plants in Geologic Time' IV. Spermato- phyta Cycadophyta 1 to a < Monocotyled iiies Dicotyledoiieo Comanchean to present Comanchean to present (Fossil record scant) Permian to present Permian to present Devonian to Jurassic Permian to present Devonian toComanchian Oaks Grasses a § a 5 Gnetales Coniferales Ginkgoales Cordaitales Cycadales Cycadofilicales Ephedra Pines Ginkgo Cordaites Cycads Neuropteria Ill Lepidophyta Calamophyta Pteridophyta Lycopodiales Equisi tales Sphenophyllales Filicales Devonian to present Devonian to present Devonian to Permian Devonian to present Club mosses Horsetails S p h e n - phyllum Ferns II. Bryophyta Musci Hepaticae Tertiary to present , Tertiary^ to present Mosses Liverworts I. Thallophyta Fungi Algae Diatomeaa Schizophyta Myxomycetffi Silurian to present Pre-Cambrian to present Jurassic to present Pennsylvanian to present (Fossil record lacking) Fungi Seaweeds Diatonas Bacteria Slime-molds 'Modified from Shinier. ^See, however, p. 172. 136. Gaps in the Fossil Record. — In the Origin of Species Darwin called attention to the paltry display of fossils in our museums, as evidence of how little we really know of the plant and animal Hfe of past ages. "The number, both of specimens and of species, preserved in 194 HEREDITY AND EVOLUTION IN PLANTS our museums," says Darwin, "is absolutely as nothing compared with the number of generations which must have passed away during a single formation." The meagerness of the record is, of course, due in part to the relatively small area explored in proportion to the whole ; but there are other reasons much more serious, because they represent opportunities lost forever. Among them are metamorphosis, explained above, and the fact that many of the organisms of the past were composed wholly or largely of soft tissues, which were entirely destroyed, by decay or otherwise, in the process of rock-formation. Such plants, for example, as Spirogyra and many other algae, the fleshy fungi, and, among animals, jelly-fish, earthworms, and others, would form fossils only under exceptionally favorable circumstances, if at all. But there is an even more effective cause of oblitera- tion of the fossil record in the long-continued erosion and denudation represented by unconformity in the rock strata. In many cases only a small proportion now re- mains of the thickness of a rock stratum originally de- posited, and all traces of the plant and animal life that may have existed on the denuded area have thus been ob- literated forever. These blank intervals between suc- cessive periods were of vast duration. "I look at the geological record," said Darwin, "as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may PALEOBOTANY I95 represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced.'" These views have received added emphasis from the recent development of Paleo- geography. 137. Factors of Extinction. — The question may natu- rally arise, "Why did the species common in previous geo- logical ages die out, giving place to newer forms?" The answer is found in the facts of struggle for existence and survival of the fittest. In the words of the great American botanist, Asa Gray, species may continue only "while the external conditions of their being or well-being con- tinue." The struggle may be with other organisms or with the physical conditions of the environment. Among the more important factors of extinction, may be men- tioned the following: 1. Struggle ■with Other Plants for Adequate Space. — This is illustrated in a simple way by the crowding out of culti- vated plants by weeds in a neglected garden, or of grass by dandehons or chickweed in a lawn. By more rapid ger- mination and growth, and by other "weedy" character- istics, the weeds get the start of the cultivated plants, occupying all available space, and choking them out. 2. Attacks of disease-causing parasites, e.g.. chestnut trees by a parasitic fungus, elm tress by the elm tree beetle. 3. Changes of Environment too Great or too Rapid to Per- mit of Readjustment. — Plants are plastic organisms, and can adapt or readjust themselves to considerable environ- mental change, but there are limits of speed and amount of change beyond which readjustment is not possible, and the plant must consequently perish. If such changes 1 Darwin, C. "Origin of Species," vol. 2, p. 88. New York, 1902. 196 HEREDITY AND EVOLUTION IN PLANTS involve the entire area of distribution of the species con- cerned, the species will, obviously, become extinct. The following nine factors (paragraphs 4-12) are specific instances of this. 4. Diminished Water Supply. — ^Aquatic plants may be destroyed by the draining of a pond or lake; hydrophytic forms by the drying up of a swamp. Sometimes forms suited to conditions of moderate water supply (hydro- phytes) are destroyd by the conversion of wide areas into desert regions, as has doubtless occurred. If such changes are gradual, resting spores (e.g., Spirogyra), winter buds (e.g., Utricularia, Elodea, Vallisneria), and seeds readily transported by wind (e.g., cat- tail) enable the species to become reestablished in a new location, but not so when the changes are too abrupt, or cover too wide an area. 5. Temperature changes, when too abrupt, too extreme, or too long continued. When the continental ice-sheet advanced southward during the glacial period, many forms, adapted only to temperate conditions, became ex- tinct. Fossils of extinct tropical plants are found in Greenland, which is now undergoing a glacial period. 6. Volcanic eruptions, such, for example, as those of Mount Pelee, which occurred in 1902, on the island of Martinique, W. I., often destroy all signs of hfe over a radius of many miles. In the states of Washington, Oregon, and Idaho floods of molten lava, covering thou- sands of square miles, have, during a previous geological age, been poured out over the surface, forming a wide plateau. A great volcanic eruption in Alaska, in prehistoric times, covered an area of over 140 square miles with a deposit of ash and pumice varying in thickness from a PALEOBOTANY I 97 few inches near the margin to some 300 feet near the crater. In 1883 the eruption of Krakatoa, in the Straits of Sunda, killed practically all the plants and animals on an island of five square miles in area, and on neighboring islands ; a part of the island was completely blown away, leaving only deep water. So recently as 191 2 the eruption of Katmai, in Alaska, spread a layer of ash nearly a foot deep over the entire surface of Kodiak Island, one hundred miles from the volcano, and killed all the herbaceous vege- tation, leaving only trees and bushes. It is almost certain that many species of plants and animals have become ex- tinct by such agencies. Not only the lava, but poisonous gases that fiiU the air during volcanic eruptions, may prove fatal to plant and animal Ufe. 7. Encroachment of salt water in coastal regions, caused by changes in the level of the land, resulting in the killing of fresh-water vegetation. According to Fernald, one of the sundews, Drosera filiformis, is known to occur in only two regions, namely along the Gulf coast from Flor- ida to Mississippi, and along the Atlantic coast from Mary- land to Massachusetts (Fig. 85). Its extinction in the intervening region is explained by the subsidence and drowning of a former high continental shelf, along which this and other species migrated northward during the late Tertiary. If a similar subsidence should occur in the two Umited regions where the species is now found it would become extinct unless, by some combination of circumstances, it could migrate and become established in new localities. It is not unlikely that species have often been exterminated in this way. 8. Encroachment of Fresh Water over Land Areas. — Previous to about the year 1900, the Salton basin, in 1 98 HEREDITY AND EVOLUTION IN PLANTS lower California, was a saline area of a so pronounced desert type that its flora contained less than 140 species of ferns and flowerng plants, five of which were endemic. During the winter of 1904-1905 the fresh waters of the Colorado River began to debouche into this basin, and by early 1907 had formed a brackish lake, over 80 feet deep and of about 450 square miles in area, known as the Salton Sea. At the end of ten years it still had an area of some- what less than 300 square miles. Some three or four hundred years previously the entire Salton Basin was Fig. 85. — Sketch map showing the geographical distribution of the sun- dew, Drosera filiformis. (After M. L. Fernald.) occupied with a lake of over 2,000 square miles in area, which, in turn, had dried up and given place to the desert conditions above mentioned. It is not improbable that such drastic changes as this may have resulted in the obliteration of one or more species, though the flora was not well enough known previous to the last inundation to make a definite statement on this point possible. For example, the presence there of endemic species was not known until the recent botanical survey of the region lying between the late water level and that of the ancient PALEOBOTANY I 99 sea. According to MacDougal,^ if the water had risen in 1907 to its ancient level of three or four hundred years ago, it would have destroyed all these endemic species. 7 . Transformation of fresh water lakes into salt lakes, as in the case of the Caspian Sea, and the Great Salt Lake of Utah (i 8 per cent, salt^ . This change gradually extermi- nates plant and animal life until the given body of water becomes a true "dead" sea, where practically nothing remains alive, as in the Dead Sea (24 per cent. salt). A more extreme case yet is Lake Van, in Turkey, where saHne matter constitutes over one-third of the contents. In the last stages of such transformations the lake may give place to a salt marsh or plain (salina). South of Lake Titicaca, in the Andes Mts. of BoUvia, ar several sahnas, one of some 4000 square miles in area, with a layer of salt three or four feet thick. 10. Disturbance of Symbiotic Relationships. — The inter- relationships of organisms are very complex, affording innumerable opportunities for extinction by a disturbance of adjustments. Shade-loving forms in a forest may perish by the destruction of those affording the shade; obligate parasites may perish from the destruction of the necessary host; plants dependent upon certain insects for cross-pollination may perish on account of the extinction of the necessary insects. 11. Diminution of Carbon Dioxide in the Atmosphere. — There are reasons for thinking that in certain past ages the atmosphere was richer than now in carbon dioxide, and that that condition was favorable to the development of certain vegetatively vigorous species which cannot live in an atmosphere like the present, having a smaller per- centage of carbon. • In a letter to the author. 2CO HEREDITY AND EVOLUTION IN PLANTS 12. Denudation of the Land Surface. — In the course of ages even lofty mountains are planed down by erosion, and the arctic and sub-arctic species of the high altitudes thus undergo extinction. Furthermore, erosion may be coupled with general subsidence. In fact, not only do geologists now recognize numerous old mountain "roots," such for example as the Adirondack region of New York State, but there are also abundant evidences of periodic emergencies and subsidence of areas of continental extent, quite throughout geologic time. The climatic and other environmental disturbances accompanying such changes would inevitably result in the extinction of certain species. (See also If 129.) CHAPTER XII THE EVOLUTION OF PLANTS (Concluded) 138. Evidences from Fossil Plants. — The study of fossil plant remains has greatly enlarged our knowledge of the course of plant evolution, filling in gaps derived from the study of living forms, and affording new facts, not disclosed by the study of plants now living. Like the study of comparative anatomy and life histories, paleo- botany teaches us that there has been a gradual evolu- tionary progress from the simple to the more complex, but it has also disclosed the fact that some of the complex forms are much more ancient than had been inferred from the study of living plants only. 139. Discovery of Seed-bearing Ferns.— For example, remains of seed-bearing plants, quite as highly organized as those of to-day, are found far back in the earliest fossil- bearing strata of the Paleozoic. Great forest types ex- isted as early as the Devonian. Later in the Carboniferous occur many seed-bearing ferns. These have been called Cycadofilicales (cycadaceous ferns), or, by some, Pterido- sperms. Recent studies have disclosed the fact that most of the fossil plants from the Carboniferous coal- bearing strata, formerly thought to be ferns, are not even cryptogams, but are these fern-Hke seed-bearing plants. The best known pteridosperm is Lyginodendron oldhamium (Fig. 86), first described from fossil leaves, in 1829, as , a tree-fern, under the name Sphenopteris Hoeninghausi. After investigations extending over nearly 90 years, "we are 202 HEREDITY AND EVOLUTION IN PLANTS now in position to draw a fairly complete picture of the plant as it must have appeared when living. "It was in effect a little tree-fern, with long, slender, sometimes branched, stem, 4 centimeters or less in diame- FiG. 86. — Lyginodendron oldhamium. Pinna of a microsporophyll, found in an ironstone nodule. Before its identity was established this specimen was named Cr ossolheca Ilocninghausi. The somewhat peltate fertile pinules on the ultimate branches, bear each a fringe of micro- sporangia about 3 mm. long. The appearance has been likened to that of a fringed epaulet. (After Scott, from a photo by Kidston.) ter, and provided with spines by means of which it prob- ably climbed on its neighbors. The foliage was disposed spirally and consisted of relatively very large, finely divided fronds with small, thick pinnules with revolute THE E\-OLUTION OF PLANTS 203 margins, suggesting a xerophytic or halophytic habitat. The stem in the lower portion gave rise to numbers of slender roots, some of which appear to have been aerial in their origin. These gi;ew downward and often branched where they entered the soil. ti tt Fig. 87. — Young leaf of the Cycad, Bowenia serrulata. Comparison of this with a leaf of the fern Angiopteris (Fig. 88) shows how difficult it might be to decide from a fossil leaf whether the plant was a cycad or a fern. [Cf., also, Fig. 91.) (Photo from specimen in Brooklyn Botanic Garden.) "The stems, roots, and petioles, and even the pinnules, have been found calcified and so beautifully preserved that their entire structure can be made out with certainty. Without going into a technical description of these organs, it may be said that the stem when young, and before secondary growth has begun, has a very strong resemblance 204 HEREDITY AND EVOLUTION IN PLANTS to the stem of [the fern] Osmunda, but when more mature certain cycadean characters appear to predominate."^ Its foliage and other characters closely resemble some of our modern tree-ferns (C/. Figs. 87 and 88), but more Fig. 88. — ^Leaf of a fern {Angiopteris evecia). {Cf. Fig. 87.) careful study of the calcified specimens of much beauty, found in calcareous nodules (the so-called English "coal balls"''), has disclosed both the microsporophylls, ' Knowlton, F. H. American Fern Journal, 5: 85. iQiS- * Coal balls are "concretions of the carbonates of lime and magnesia which formed around certain masses of the peaty vegetation as centers and, through inclosing and interpenetrating them, preserved them from the peculiar processes of decay which converted the rest of the vegetation into coal. In them the mineral matter slowly replaced the vegetable matter, molecule by molecule, thus preserving the cellular structure to a remarkable degree. Such balls are especially frequent in the coal of certain parts of England (Lancashire and Yorkshire)." Shimer, H. W. An introduction to the study of fossils," p. 53. London, 1914. THE EVOLUTION OF PLANTS 205 bearing pollen-sacs, and the megasporophylls, bearing, not merely megasporangia, but true seeds. The ovule has a pollen-chamber, like the cycads, except that it projects a bit through the micropyle, and, strange as it may seem, fossil pollen-grains have been discovered, well preserved within this chamber. The seeds, about 3-^ inch long, have been described as resembling little acorns, enclosed like hazelnuts in smaller glandular cupules (Fig. 89). They are similar to those of the cycads, except that they are not known to have organized an embryo with cotyle- FiG. 8g. — Restoration of a seed of Lyginodendron oldhamium (Lagenos- tema Lomaxi), from a model by H. E. Smedley. (After Scott.) dons and caulicle. Instead, the tissues of the female gametophyte only are so far found, retained within the megasporangium, which is enclosed in the integument. In this connection it is of interest to note that the seeds of some modern plants (e.g., orchids) do not possess differ- entiated embryos, but whether this is a primitive or a reduced character is not certain. The pollen was formed in spindle-shaped pollen-sacs, having two chambers, and borne in clusters of four to six on the under side of little oval discs, from 2 to 3 millimeters long. These structures 206 HEREDITY AND EVOLUTION IN PLANTS are found on pinhules of ordinary foliage leaves, resem- bling the sporophylls of certain ferns (Fig. 90) rather than the stamens of modern flowers. The discovery of the seed-bearing character of the fern- like plants of the Paleozoic has been called the most im- portant contribution of paleobotany to botany ever made. It was predicted by Wieland, of Yale University, nearly two years before it was announced by Oliver and Scott. It is now believed that seed-bearing plants of the pterido- vSrvw^ Fig. 90. — Top, lateral pinna from a leaf of Marailia fraxinea. (After Bitter.) Below at left, synangium of same. (After Bitter.) At right, cross-section of the synangium. (After Hooker-Baker.) sperm type were nearly as numerous in the Paleozoic as were the cryptogams. 140. Significance of the "Pteridospenns." — The close resemblanc^ of the pteridosperms to ferns, on the one hand, and to modern cycads on the other, justifies the conclu- sion that they represent a "connecting Unk" between the true ferns and the cycads, and that the modern cycads have descended from the same ancestry as the modern ferns, each developing along somewhat different lines. THE EVOLUTION OF PLANTS 207 It was in recognition of their vegetative resemblances that the Pteridosperms were first called (by Potonie) Cycado- filices, now Cycadofilicales. Van Tieghem tersely de- scribed them as "phanerogams without flowers." 141. A Modem Fern-like Cycad.— One of the modern cycads {Stangeria paradoxaY is of much interest in this Fig. gi.-^Slangp.ria paradoxa Moore. Specimen from the cycad house at the New York Botanical Garden, bearing, at the apex of the stem a carpellate cone. (Photo from New York Botanical Garden.) connection. So closely does it resemble a certain fern {Lomaria) that the botanist Kunze, who first described it when it was brought from Natal to the botanic garden at Chelsea, England, supposed it was a fern, and named it Lomaria eriopus. The specimen possessed no fruit, which would have helped to identify it. Its leaves, with circinate 1 Stangeria paradoxa Moore = Stangeria eriopus (Kunze) Nash. 208 HEREDITY AND EVOLUTION IN PLANTS vernation, have a pinnately compound blade, and leaflets with pinnate dichotomous yenation. Two or three years later another botanist, examining it more closely, pro- nounced it a "fern-like Zamia or a Zamia-like fern." These facts show how puzzling the specimen was, and how Fig. 92. — To the le£t, Ca.ca4eoidea dacotensis Macbride. Longitudinal section of a silicified specimen of a bisporangiate cone (unexpanded flower), so taken that the pinnules of the microsporophylls on both sides of the central axis, or receptacle, are successively cut throughout their entire length. The lines indicate the planes of various sections through the cone, published in Wieland's" American Fossil Cycads." To the right Cycado- cephalus Sewardi Nathorst. Microsporangiate cone, natural size, preserved as an impression on a flat slab. From a fossil-bearing bed of the Trias, at Bjuf, Southern Sweden. (Left figure from Wieland, right figure from Nathorst.) closely a plant may resemble both a cycadophy te and a fern. In a sense this plant may be called a living fossil. Speci- mens have since come into flower in botanic gardens, and the typical cycadaceous cones (Fig. 91) leave no doubt that the plant is a true cycadophyte. THE EVOLUTION OF PLANTS 209 142. Derivation of New Types. — Attention should here again be called to the fact that the theory of evolution does not teach that one given species becomes transformed into another, but simply that new species are descended from older forms which may or may not continue to exist. It is not supposed, for example, that ferns developed into Fig. 93. — Cycadeoidea dacoknsis. Semi-diagrammatic sketch of a flower (bisporangiate cone), cut longitudinally; one sporophyll folded, and one (at the right) arbitrarily expanded. At the center is the apical, cone- shaped receptacle, invested by a zone of short-stalked ovules and inter- seminal scales. The pinnules of the sporophylls bear the compound sporangia (Synangia). Exterior to the flower are several hairy bracts. About three-fourths natural size. (After Wieland.) cycads, and cycads into higher gymnosperms, but that there has been an unbroken line of descent (possibly more than one) in the plant kingdom, that closely related forms (like ferns and cycads) have descended from a common ancestral type which may or may not now be found. We must not, in other words, expect necessarily to find in 14 210 HEKliUlTY AND INVOLUTION IN PLANTS fossil forms the direct ancestors of those now living, although a study of their structure is of the greatest value in ena- bling us to understand the genetic relationships of the great groups of plants. 143. Ancestors of the Angiosperms.^ — Just as the Cyca- dofilicales indicate the ancestry of the cycads, so fossil types of Cycadophyta have been discovered which are Fig. 94. — Cycadcoidea dacotensis (?). Photomicrograph of a young seed (X 15), showing a sterile scale on either side. Between them pro- jects the entire length of the tube through which the micropyle extends. The partially collapsed nucellus is distinctly shown in the center. (After Wieland.) interpreted by some paleobotanists as ancestors of the modern angiosperms. Other investigators, however, dissent from this view and consider that we have not yet sufi&cient knowledge of fossil forms to be justified in desig- nating the ancestors of the Angiosperms. This differ- ence of opinion is largely due to the meagerness of the available evidence. As one writer has stated it, "A THE EVOLUTION OF PL'ANTS 211 trayful of flowers may be all the record of the Pterido- sperms from the Devonian on. The gaps in the evidence are always enormous." Although the Cycadophyta are now a very insignifi- cant element in the earth's flora, in the Mesozoic period ^H'^^^M Ma il ^^^^1 w M '^^^H ^^■^^^ i wt^"^' im^m^!^^ ^ ^ ^ ^fllE^fiW^ t^eSSS^M' m ' ■'■' T"^ H' Fig. 95. — Macrozamia spiralis. Tip of the trunk, showing three lateral cones, inserted in the axils of leaves. Photo from specimen in Brooklyn Botanic Garden. (C/. Fig. 96.) they form about one-third of the recovered vegetation of the land. One order, the Hemicycadales (Bennettitales^), then had a cosmopolitan distribution and seemingly was as important as the Dicotyledons are now. Over 30 species of the petrified stems have been found in the Mesozoic ' In his paper on the Classification of the Cycadophyta (Am. Jour, Sci. 47 '• 391-406. June, 1919), Wieland states simple and good reasons" for letting the name Bennettitales fall into disuse, and substituting there- fore the term Hemicycadales (half-cycads). 212 HEREDITY AND EVOLUTION IN PLANTS terrains of the United States, the Black Hills of South Dakota alone yielding a score. The Isle of Portland forms Fig. 96. — Cycas circinalis. Tip of trunk, showing numerous leaf- stalks, and the large terminal cone. Photo from specimen in Brooklyn Botanic Garden. (C/. Fig. 95.) were called Cycadeoidea by the celebrated geologist Buck- land. The original name of the order was derived from THE EVOLUTION OF PLANTS 213 Fig. 97. — Cycadeoidea Widandi. At left, a finely preserved trunk bearing many ovulate cones with seeds approaching maturity, and a lesser number of either young or abortive cones. ]', Receptacle of a shed or non-preserved cone with surrounding bracts yet present; f", two cones broken away during erosion, with a portion of the basal infertile pedicel yet remaiiiing; m, four cones eroded down to the surface of the armor, in this instance about or a little beneath the level of the lowermost seeds; y, three of the dozen or more very young cones, in some cases known to be simply ovulate and to be regarded as having aborted or else as be- longing to a later and sparser series of fructifications than the seed-bearing cones present, the latter unquestionably representing the culminant fruit- producing period in the life of this cycad; s (over lower arrow), the ovulate strobilus, shown at the right, in its natural position, this photograph having been made before the cone was cut out by a cylindrical drill. X 0.5. At right, longitudinal section of the small ovulate strobilus cut from its natural position on the trunk as denoted by the arrow s, in photograph I. c (upper arrow), seed with dicotyledonous embryo preserved, cotyle- dons being similarly present in the lowermost seed on the left-hand side of the strobilus; s, traces of hypogynous staminate disk; 6, bracts; I, leaf bases. X S- (After Wieland.) 214 HEREDITY AND EVOLUTION IN PLANTS the genus-name, Bennettites} Other fbrms, usually found as casts, are called Williamsonia, still others are known mainly as genera founded on leaf imprints. 144. Cycadeoidea." — In most of its purely vegetative characters, such as the anatomy of the stem and the Fig. 98. — Cycadeoidea Wielandi. Longitudinal section through the axis of a female inflorescence, or cone. /, old leaf-base; d, insertion of disc; J, erect seed, borne at summit of seed-pedicle inserted on convex receptacle; J, hair-covered bract. (After Wieland.) structure of the leaves, Cycadeoidea resembled modern cycads, but its reproductive branches were character- istically lateral, which is one of the most fundamental characteristics of the higher seed-bearing plants of to- day. Only two modern cycads {Macrozamia and Bow- '■ Cycadeoidea Bucklan'd = Bennetiites Carruthers. THE EVOLUTION OF PLANTS ' 2IS enia) have lateral seed-bearing cones (Fig. 95) ;^ in the other genera the carpellate cones are terminal (Fig. 96) . Various structural characters of Cycadeoidea are shown in Figs. 92-100. In Cycadeoidea dacotensis the "flower," which in some specimens was 5 inches long, was a strobilus, consisting of a thick axis on the lower part of which were numerous Fig. 99. — Cycadeoidea ingens. Restoration of an expanded bispor- angiate cone, or flower, in nearly longitudinal section. Restored from a silicified fossil. (After Wieland.) bracts arranged in spirals. The bracts surrounded a campanula of about 20 stamens. Each stamen was, in reality, a pinnately compound sporophyll, about 4 inches long, rolled in toward the center of the flower, and bear- ing two rows of compound microsporangia (pollen-sacs) on each leaflet. They thus closely resembled the sporo- phyll of a fern. ' The staminate cones of Zamia are lateral. 2l6 HEREDITY AND EVOLUTION IN PLANTS The axis of the flower terminated in a cone-shaped receptacle, bearing the stalked ovules, and numerous sterile scales (Figs. 97 and 98). The mature seeds often contain the well-preserved fossil embryos, with two cotyledons which quite fill out the nucellus, and show that there was little or no endosperm. These are char- acters never found in the lowest group of modern seed- FiG. 100. — Cycadeoidea Dartoni. Tangential section through outer tissues of the (fossilized) trunk, showing the very numerous seed-cones. The seeds are very small (the illustration being natural size), and nearly every one has a dicotyledonous embryo. There were over 500 such cones on the original stem. (After a photograph loaned by Prof. Wieland.) bearing plants (the Gymnosperms), but only in the highest group of Angiosperm^s, the Dicotyledons. In fact, the French paleobotanist, Saporta, called some of the Cycadeoids, Proangiosperms. 145. Relation of Cycadeoidea to Modem Angiosperms. — The question of the ancestry of the Angiosperms is the most important problem of paleobotany. Although the THE EVOLUTION Or PLANTS 21 7 Hemicycadales possess many of the primitive anatomical features that characterize the CycadofiHcales, their development of a bisporangiate strobilus with two sets of sporophylls, related to one another as they are in the flower of the Angiosperms, indicates a genetic relationship to that group, as does also the fact that the seeds, enclosed in a fruit, possess a dicotyledonous embryo, without endo- FiG. loi. — Flower of magnolia. {Cf. Fig. 102.) sperm. In other features the Hemicycadales are unlike the Angiosperms; the ovules, for example, are enclosed by sterile scales, instead of by the carpels on which they are borne, and the protrusion of the pollen-chamber through the micropyle signifies the gymnospermous type of fertilization. These and other comparisons indicate that the Hemi- cycadales were essentially Gymnosperms having certain 2l8 HEREDITY AND EVOLUTION IN PLANTS Angiospermous characters, and therefore, while they are not to be considered as the ancestors of the Angiosperms, it is probable that they and the modern dicotyledons are both descended from a common branch of the ancestral tree. Among modern plants, the flower of the magnolias most closely resembles that of Cycadeoidea in the spiral arrangement of its stamens and pistils (Figs. loi and 102). Just what significance should be attached to that fact has been disputed by students of morphology. The older view of the systematists rega,rded the primitive flower as more complex in structure, with pistils, stamens, and floral envelopes arranged spirally in centripetal or acropetal succession on a fleshy axis, as in Magnolia and other flowers of the order Ranales; other types of floral structure were con- FiG. io2.--Magnolia ^y^j.^^ ^^ derived from this one by Flower with perianth . , . . ^ removed, showing the^ reduction. This IS often referred to compound pistil, and four as the "Strobiloid theory of the of the stamens. Most of flower" (C/. pp. 132 and 134). the stamens have been , , . removed so as to bring ^ more recent view recognizes out their spiral arrange- that simple staminate or pistillate ment as shown by the flowers may, in some cases, be in- !tt»rT,mpnt ^ ^f^ FiV terpreted as derived by reduction from more complex forms, but re- gards the primitive flower as uni- attachment, lOI.) (C/. Fig. THE EVOLUTION OF PLANTS 219 sexual — in effect a microsporophyll or a megasporophyll, from which complex forms were derived by elaboration. This latter view, however, is not in harmony with avail- able evidence from fossil plants, such as that afforded by Cycadeoidea. "The strobiloid theory of the flower seems in the present state of our knowledge to stand alone as a working hy- FiG. 103. — Theoretical stages in the reduction (from Cycadeoidea to modern Angiosperms) of staminate discs represented as segments. A, any] common campanulate form with simple stamens (e.g., morning glory) ; B, hypothetical Cycadeoid reduced to a single synangium to each frond component; C, inner view of a sector of a WilUamsonia meocicana disc; D, sector of a Cycadeoidea dacotensis disc with the pair of shoulder spurs borne by each frond. (After Wieland.) pothesis. If we reject it, we are left without any historical clue to the origin of the floral structure of Angiosperms. If we accept it, the Primitive Angiosperm must be cred- ited with a flower resembling that of Magnolia or Lirio- dendron in general plan."^ From this it follows that the Magnoliacea must be among the most primitive, if not the most primitive, of all Angiosperms, as Wieland first and Hallier later and independently pointed out. ^ Sargant, Ethel. Thereconstructionofaraceof primitive Angiosperms. Ann. Bot. 22:121-186. April, 1908. 2 20 HEREDITY AND EVOLUTION IN PLANTS The gap between the stamen of Cycadeoidea and the type characteristic of modern Angiosperms is partially bridged by the genus Williamsonia (which has simple vs. pinnately compound stamens), and by another genus, Wielandiella, both older genera than Cycadeoidea (Fig. 103). From this it has been inferred that the Hemicycad- ales are a lateral branch, further removed than their ancestors from the direct evolutionary stock of the Angiosperms. 146. Origin of Dicotyledony. — Two problems of major importance are involved in the question of the evolution of Angiosperms, namely, the origin of dicotyledony and the origin of monocotyledony. Are dicotyledons more ancient than monocotyledons, or vice versa? Again, in the evolution of seed-bearing plants was the condition of polycotyledony antecedent to that of dicotyledony, or the reverse? This would be a comparatively easy question to answer if we had an unbroken series of fossil remains of the primitive and intermediate spermatophytes; but unfortunately such evidence has not yet been discovered. We know nothing of the embryos of the geological ances- tors of modern conifers. The Mesozoic gymnosperms {Cycadeoidea and other related genera) are known to have had dicotyledonous embryos, but these forms do not stand in the ancestral hne of the (polycotyledonous) conifers of to-day. To answer our question, therefore, we must, for the present, depend largely on the study of living forms. The evidence has seemed conflicting, and for nearly three- quarters of a century opinion has varied. Adanson and Jussieu, in the early nineteenth century, contended that polycotyledony was derived from dicotyledony by a spUt- ting of the primordia of two original cotyledons; Sachs THE EVOLUTION OF PLANTS (187s) lield the opposite opinion. Hill and de Fraine (1908-1910) are among the recent protagonists for the hypothesis that dicotyledons are the more primitive. One of the most recent studies is that by Bucholz^ who ex- J G Fig. 104. — Development of stem tip and cotyledons in Pinus Bank- siana. Dotted line represents plerome of root-tip; shaded area, meristem of stem tip; E, I, J, K, fusing cotyledons. (After Bucholz.) amined the embryos of pine, spruce, larch, juniper, balsam fir, cedar of Lebanon, and others. Many instances of the fusion of the primordia of cotyledons were found, but no evidence of cotyledonary splitting. This fusion has resulted in reducing the number of cotyledons, and, in ' Bucholz, John T. Studies concerning the evolutionary status of poly- cotyledony. Am. Journ. Bot. 6 : 106-119. March, 1919. 222 HEKEMTY AND EVOLUTION IN PLANTS certain species, in the formation of a coiyledonary ring, or tube. Bucholz interprets the facts set forth by him- self and other investigators as leading to the conclusion that the more primitive gymnosperms had numerous coty- ledons, that their number was reduced by the fusions of their primordia and, in some species, a cotyledonary tube or ring was formed. " Dicotyledony was attained either by a general fusion of many cotyledons in two groups, or Fig. 105. — Poly cotyledonous seedlings of dicotyledonous species. A-C, Silene odonlipetala, with hemi-tricotylous, tricotylous, and tetracotylous seedlings; D-H, Papamr Rhoeas (semi-double cultivated form), dicoty- lous, hemi-tricotylous, tricotylous, tetracotylous, and pentacotylous seedlings; /, Acer Pseudo-Plalantis, tetracotylous seedling. (All figures re-drawn from de Vries.) by an extremely bilabiate development of a cotyledonary tube" (Fig. 104). Tlie final conclusion of Bucholz, based on the evidence of comparative anatomy, supplemented by studies of development, is that the polycotyledonous condition is the more primitive, and the dicotyledonous one derived. On the basis of this theory, the rather common abnormal a ppearance of supernumerary cotyledons in dicotyledonous THK EVOLUTION OF PLANTS 223 seeds is to be interpreted as a reversion to a more primitive condition (Fig. 105).^ 147. Origin of Monocotyledony. — If the earliest Angio- sperms were dicotyledons, as now seems probable, the monocotyledons were probably derived from them by a process of simplification. Several hypotheses have been framed as to how the final result was accomplished, but the voluminous evidence and the conclusions can only be briefly summarized here. For nearly a century it has been generally accepted by botanists that the two seed-leaves or cotyledons of dicotyledonous plants were lateral organs, originating below the tip of the embryonic stem or hypocotyl, while the single cotyledon of monocotyledonous plants was considered as a terminal organ. The grass family offers a case in point. The embryo of Indian corn {Zea Mays), for example possesses a well developed cotyledon, called the scutellum; there is little or no trace of a second cotyledon. The embryos of many other grasses, however, possess an organ, the epiblast, homologous in position with the scutellum, and regarded by earlier botanists as a rudi- mentary cotyledon (Fig. 106). Recent studies of Coulter and Land leave little doubt of this as the correct interpre- tation of that organ. A study by Bruns (1882) of 82 genera of grasses, repre- • According to de Vries (The Mutation theory. 2: 393-456. Chicago, igio) tricotylous intermediate races do not arise by selection but by mutation, tricotyly being the expression of an ancestral character which is latent in the normal species. If the normal character is active and the anomaly semi-latent we have what de Vries calls a ' half-race;" if the normal character becomes latent and the anomaly active, we have a "constant variety." Sometimes an equilibrium is maintained in the ex- pression of the normal character and the anomaly, giving rise to a "middle race," or "eversporting variety." 224 HEREDITY AND EVOLUTION IN PLANTS senting 12 tribes, demonstrated the presence of the rudi- mentary cotyledon (epiblast) in 29 of the genera, repre- FiG. 106. — Diagram of longitudinal sections of grass-embryos (Gram- ineas) to illustrate the rudimentary cotyledon {epiblast). A-C, E-G, redrawn from J. M. Coulter, after Bruns; D, from nature. A, Zizania acflatica; B, Leersia clandestina; C, Leptochloa arabica; D, Triticum vtd- gare; E, Spartina cynosuroides; F, Trilicum vulgare; G, Zea Mays; i, scutellum; c, coleoptile; ii, epiblast. senting nine tribes; later studies by Van Tieghem (1897) disclosed the presence of an epiblast in 61 out of 91 genera examined. From these figures we may reasonably infer THE EVOLUTION OF PLANTS 225 that the majority of the so-called " monocotyledonous " grasses possess two cotyledons, one of which is more or less rudimentary, and that the grasses are primitive monocoty- ledons, representing a transitional stage from dicotyledons to the higher monocotyledons. Monocotyledony, then, as stated by Coulter, is simply one expression of a process common to all cotyledony, gradually derived from dicotyle- dony by reduction, and involving no abrupt transfer of a lateral organ to a terminal origin. Variations in the rela- tive size of the second cotyledon in grass embyros are illustrated in Fig io6. Henslow^ was among the first to suggest the origin of monocotyledons from dicotyledons. ^ Previous to the pub- Hcation of his paper, it was generally assumed that mono- cotyledons were the older group, and Henslow stated that no systematist of his day recognized any real points of con- nection between the two groups. He proposed the hy- pothesis that the monocotyledons were derived by the arrest of the development of one seed-leaf in a primitive dicotyledonous Angiosperm;' hence said Henslow, "only one elongates, its superior vigour carrying it on in a straight 1 Henslow, Rev. George. A theoretical origin of endogens from e.xogens, through self-adaptation to an aquatic habit. Journ. Linnean Soc. Boi. 19 : 485-528. May 15, 1893. ' The first to make the suggestion appears to have been Agardh, in his Larobok i Botanik, Part I. Malino, 1829-32. 'In discussing the origin of Angiosperms, Arber {Journ. Linnean Soc. Bot. 38 : 29-80. July, 1907) calls attention to the "Law of corresponding stages in evolution," namely, that in the evolution of seed-plants, the stages reached by different organs at any one period are dissimilar. From this law it follows that such a plant as a "primitive Angiosperm," in the strict sense of the term, that is, with all its organs primitive, never existed in reality. We must picture the ancestors of modern Angiosperms as having certain organs in a primitive stage of evolutionary development, others as more advanced toward the stage in which they are now found. IS. 2 26 HEREDITY AND EVOLUTION IN PLANTS line with the suspensor, finally making the cotyledon ter- minal." This he calls "the real interpretation of a mono- cotyledonous embryo." Henslow further inferred a very early origin of monocotyledons from dicotyledons, from the fact that so many of their orders contain very few gen- nera and monotypic groups, for groups of plants or animals with few members, are regarded, in general, as survivals, representing a lost ancestry. He recorded voluminous observations in support of his theory, and, among other evidence, called attention to " Dicotyledonous monocoty- ledons "such as Tamus communis (black bryony) , a tuberous rhizomed species of the Yam family, where the first foliage leaf, situated exactly opposite the cotyledon, is interpreted (with Dutrochet) as a second cotyledon; and to "Mono- cotyledonous dicotyledons," especially among aquatic species such as the water-chestnut {Trapa natans), where one cotyledon is arrested in its development. Other illus- trations, not mentioned by Henslow, include such forms as Dioscorea bonariensis and Pinguicula vulgaris (Fig. 107). Ranunculus Ficaria is not an aquatic, but it flourishes by the waterside, and is regarded by Henslow as descended from an aquatic form. About one-third of the orders of of monocotyledons are aquatic, as compared to only 4 per cent, in dicotyledons, and the monocotyledonous dicoty- ledons are all aquatic. The final conclusion of Henslow is, "that endogens [monocotyledons] have in the first place descended from very early types of exogens [dicotyledons] . . . ; and that, secondly, the more immediate cause of their origin was an aquatic habit of life assumed by certain primitive exogenous plants." Miss Ethel Sargant has more recently elaborated the hypothesis of the derivation of monocotyledons from THE EVOLUTION OF PLANTS 227 dicotyledons, by a fusion of the two cotyledons into one. ^ On this basis the single seed-leaf of monocotyledons is interpreted as homologous to the two seed-leaves of di- cotyledons. The evidence supporting this suggestion is derived largely from a study of the anatomy of monocoty- ledonous seedlings. "The young epicotyl of monocotyle- donous seedlings contains a single ring of collateral bundles which may even show traces of cambium, much resembling Fig. 107. — A-B, embryos of a "dicotyledonous monocotyledon," A, longitudinal section through an embryo of Tamus communis; B, Tamus communis, entire (.A and B enlarged after Solms-Laubach.). C-G, embryos of "monocotyledonus dicotyledons;" C, D, Dioscorea bonariensis, enlarged (after Beccari); E, Trapa natans, the water chestnut X H (after Barnfioud); F, Pingiiicula vulgaris; G, Pinguicula caudata. {P and G after Dickson, both greatly enlarged.) dicotyledons." Professor Jeffrey has also called attention to evidence that the anatomy of the stem of the hypotheti- cal ancestor of the Angiosperms was exogenous (dicotyle- donous). Miss Sargant has further pointed out that the few dicotyledons which possess but one seed-leaf (pseudo- monocots) are widely distributed through the dicotyle- donous famiUes, from Ranunculaceae to Umbilliferae, ^Annals of Botany 17: 1-88. Jan., 1903; Botanical Gazette 37: 325- 345. May, 1904, and other papers. 2 28 HEREDITY AND EVOLUTION IN PLANTS Primulaceae, and Nyctaginaceas, which indicates that the abnormality was not derived by inheritance from a com- mon ancestor; its explanation, therefore, must be sought in the influence of environment. Professor Henslow, as noted above, associated the monocotyledonous tendency with an aquatic habit of life, but Miss Sargant points out that all the pseudo-monocots possess some underground organ which is thickened as a tuber, suggesting that the signi- ficant ecological factor is a geophilous, rather than aquatic, habit. In further confirmation Miss Sargant notes that of twenty genera having their seed-leaves fused for some distance upward from the base, the majority have a tuberous hyocotyl. The dicotyledonous may-apple (Po- dophyllum), for example, with a geophilous habit has partially united cotyledons and a stem anatomy resembHng that of the monocots. The only exception to correlation of this nature is the mangrove (Rhizophora Mangle), a tropical tree whose seeds germinate in the air while still in the fruit. The monocotyledons are separated from the dicotyle- dons by seven characters as follows:^ 1. A single cotyledon. 2. Stem-anatomy. 3. Development of the embryo. 4. Parallel venation of leaves. 5. Short duration of primary root. 6. Seeds with endosperm. 7. Parts of the flower in threes. Of these characters, "four have been shown to appear frequently among geophytes, and to be useful to the plant growing under conditions which determine the geophilous ' As enumerated by Miss Sargant. THE EVOLUTION OP PLANTS 229 habit. They are therefore in all probability adaptations to that habit. Two more — stem anatomy and the ap- parently terminal cotyledon in the embryo — may be considered as direct consequences of such adaptations; the stem anatomy acquiring its peculiar features from the insertions of numerous broad-based leaves on a squat subterranean axis, and the embryonic cotyledonary number arising from the congenital fusion of two ancestral cotyle- dons. The seventh character — trimerous floral symmetry — bears no obvious re- lation to the geophilous habit, but is not inconsistent with it." Recent evidence as to how monocoty- ledony may have been derived from dicotyledony has been furnished by a study of the embryogeny of Agapanthus umbellatus L'Her (Fig. 108), a South African plant of the Lily family. The sequence of events is as follows.^ -"oj^ocotyledonous ^ embryo. B,aicoty- As the massive pro-embryo enlarges the ledonous embryo, r o t - e n d • elongates, thus remaining (Redrawn from narrow and pointed; while the shoot-end P^° J J^^ ^- J" ^• widens, becoming relatively broad and flattish. At this broad and fiat end the peripheral cells remain in a state of more active division than do the central cells, and form what is known as the cotyledonary zone. In this zone two more active points (frimordia) appear and begin to develop. Soon the whole zone is involved in more rapid growth, resulting in a ring or ' The above description closely follows Coulter and Land. The origin of monocotyledony. Bot. Gaz. 57: s°9-5i8, June, 1914- Fig. 108.— Agapanthus umbellatus . A, 230 HEREDITY AND EVOLUTION IN PLANTS tube, but with the primordia still evident. The cotyle- donary zone continues its growth until a tube of con- siderable length is developed, leaving the apex of the pro- embryo depressed. At this stage either one of two things may occur. As the cotyledonary zone continues to grow, the two primordia on the rim of the tube may continue to develop equally, forming two cotyledons; or one of the primordia may cease to grow, resulting in an embryo of only one cotyledon; in other words, the entire cotyledo- nary zone may develop under the guidance of only one growing point. It is not that one cotyledon is eliminated, but the whole growth is diverted into one. There thus develops what appears to be an "open sheath" and a ' 'terminal " cotyledon. In other words, according to Coulter and Land, mono- cotyledony is not the result of the fusion of two cotyledons, nor of the suppression of one; but is simply the con- tinuation of one growing point on the cotyledonary ring, rather than a division of the growth between two growing points. In a similar way, polycotyledony is the appear- ance and continued development of more than two growing points on the cotyledonous ring (C/. p. 222, and Fig. 104). We are not in possession of enough facts to construct a genealogical tree showing the derivation of Mono- cotyledons from Dicotyledons, nor the derivation of the original Angiosperm stock, but the table of Arber and Parkin (Table IV, p. 231) shows in a very general pro- visional way a possible course of events, and the ap- proximate geological period when the various advances were made, beginning with the Paleozoic Cycadofilices (Pteridosperms). The first step in the immediate evolution of the Angio- THE EVOLUTION OF PLANTS 231 Table iv (After Arber and Parkin) Recent Tertiary Mesozolc Palaeozoic Ranalian plex us Eu-anthoatrobilateae Pro-anthostrobilateae M onosporangiateae S Ampbiaporangiateae sperms, according to Arber/ was the transfer of the pollen- collecting surface from the ovule to the carpel or carpels, resulting in the stigma as now known. "It was this act which called the Angiosperms into being." Arber does not regard the Apetalous orders (Piperales, Amentiferous families, and Pandanales) as primitive Angiosperms, for that theory necessitates the view that the perianth arose de novo, by enation.^ He considers ' Arber E. A. Newell. On the origin of Angiosperms. Jour. Linnean. Soc. Bot. 38: 2g-8o. July, 1907. 2C/. p. 132. 232 HEREDITY AND EVOLUTION IN PLANTS the perianth an ancient structure, present in the ancestors of the Angiosperms, and inclosing an axis (" amphispor- angiate cone") bearing both megasporophylls and micro- sporophylls. Such a structure is called by Arber an "anthostroMlus." The term "flower," should be re- stricted to Angiosperms, and may be termed an "eu- anthostrobilus." The earlier form of anthostrobilus (such as occurs in modern Gymnosperms, and in the Mesozoic Benettitea£) is called a pro-anthostrohilus. The h5^o- thetical, direct ancestors of the Angiosperms are called " Hemiangiospermce," and the possible order of evolu- tionary development is conceived by Arber as follows: 5. AngiospermEe Mesozoic and Tertiary (Recent) Eu-anthostrobilata3. Mesozoic — ^Pro-anthostrobilatae. 4. Hemiangiospermse (Fossils unknown) 3. Cycadofilices 2. Heterosporous fern-lilie ancestor I Paleozoic — Non-strobilate I. Homosporous fern-like I ancestors. ancestor J 148. Ancestors of the Gymnosperms. — ^As far back as Devonian time, preceding the great coal period (Carbon- iferous), fossils have been found of a plant, Cordaites (of the order Cordaitales), common in that period, and having characters which indicate that it stands in the ancestral line of our modern conifers — that it and the conifers had a common ancestry. The leaves of Cordaites resembled those of the Kauri pines {Agathis) of the southern hemisphere (Fig. 109), or the leaflets of Zamia. They varied from a decimeter to over a meter in length. The male cones resembled those of the still living Ginkgo, each stamen having from four THE EVOLUTION OF PLANTS 233 to six microsporangia (pollen-sacs) on a stalk. The female cones resembled the male in general appearance, and the seeds resembled those of the Cycadofilicales (Fig. 94). The plant itself was a slender tree, some forms of which attained a height of over 100 feet. The Cordaitales formed the world's first great forests. They represent a Fig. 109. — Branch, with cones, of the Kauri pine (Agathis auslralis). (From the Gardener's Chronicle.) wide departure from the Cryptogams, and must be con- sidered as true seed-bearing plants. They were closely related to the Ginkgo — another living fossil, ranking next below the modern cone-bearing trees. We thus ascend from the ferns to the conifers by a series of transitional forms as follows (reading from the bottom, up) : 234 HEREDITY AND EVOLUTION IN PLANTS 6. Coniferales (modern cone-bearing trees). 5. Ginkgoales (primitive gymnosperms). 4. Cordai tales (transitional conifers). 3. Cycadales (true cycads). 2. Cycadofilicales (cycad-like ferns). I. Filicales (true ferns). 149. Relation of the Above Groups. — It must not be inferred that the above groups were derived one from the other by descent from lower to higher. They should be interpreted rather as samples remaining to show us, not the steps, but the kinds of steps through which the plant kingdom has passed in developing the more highly organ- ized, modern cone-bearing trees from more primitive forms hke the ferns. As stated above, it is doubtful if the actual transitional forms have been preserved, so that the entire history of development can probably never be written. 150. A Late Paleozoic Landscape. — The frontispiece illustrates the kind of landscape that must have been common in the latter part of the Paleozoic era along sluggish streams in certain regions such as Texas and New Mexico. Of the primitive vertebrates then abounding, only a few larger types are shown. The dragon-flies of that time are known to have had a spread of wing amounting, in some cases, to as much as two feet. In the foreground, at the left, are representatives of the Cycadofilicales, some of them bushy, and others resembUng our modern tree ferns. At the right are dense thickets of Calamites, the ancient representatives of our modern scouring rushes {Equisetum). In the background, at the left, are the unbranched, Sigillarias, and the branched Lepidodendrons. The Cor- daitales, which formed the Devonian forests, were not yet extinct, but none is shown in the figure. Other forms, THE EVOLUTION OF PLANTS 235 ancestors of our modern conifers and angiosperms, must be imagined as hidden in the recesses of the forest. 151. Significance of the Fossil Record. — Before the brilliant discoveries in fossil botany, just outlined, were made, there had been (as stated in Chapter VI) a general tendency among botanists to consider the comparatively simple inoss-plants as an older type than the fern, and that either they or their close relatives were the ances- tors of Pteridophytes. As outhned in the same chapter, the sporogonium of the moss was regarded as representing the form from which, by elaboration of vegetative tissues and organs, the sporophyte of the fern was derived. This view was clearly expressed in 1884 by the noted botanist Nageli, who considered that the sporophyte of Pterido- phytes was derived from a moss-like sporogonium by the development of leafy branches. A consideration of the fossil record, however, makes it difficult to accept this hypothesis. Not only do we find, in the fossil forms described above, sporophytes that do not bear the remotest resemblance to the moss-sporo- gonium, but fossil mosses and liverworts have never been positively identified in either the Palaeozoic or the Meso- zoic rocks, ^ while the same rocks are rich in fossils of such advanced forms as the broad-leaved sporophytes of the Cycadofilicales and Cycadophytes. We must not, how- ever, hastily conclude, from this lack of evidence, that mosses and liverworts did not exist in those early ages. Quite possibly they were present when the Paleozoic rocks were being deposited, though doubtless not represented by the same genera, or at least not by the same species, as are now Hving. 'C/., however, p. 172. 236 HEREDITY AND EVOLUTION IN PLANTS 162. Smmnary of Results. — From what has been said, in this and in Chapter VI, we recognize that the method of evolution is to be ascertained chiefly by experiment — by studying living plants in action; but the course of evolution chiefly by the study of comparative morphology, with special attention to fossil forms, and supplemented by the facts of geographical distribution. Other points are necessary to complete the history of the evolution of plants ; the above paragraphs give only the barest outline of the problem, for the entire history is much too long and much too difficult to be treated here. To summarize; the facts now known have led some investigators to infer : 1. The origin of Angiosperms from Cycadophyta (pro- angiosperms) . 2. The origin of Cycadophyta from CycadofiUcales. 3. The origin of Cycadofilicales from Primofilices. ^ 4. The origin of Filicales from Primofilices. 5. The origin of Cordaitales from Primofilices. 6. The origin of Coniferales from Cordaitales. An ancestral tree embodying these views is shown in Fig. no. What was the origin of the Primofilices? Here, as often in every science, we have to acknowledge that we do not know; the group is a hypothetical one, and some investigators doubt its actual existence altogether. 153. Other Views. — (a) Other and equally competent students of the problem take exception to one or more of the six points tabulated above. Not all of their views can here be discussed, but mention may be made of that first elaborated by Jeffrey, of Harvard University. ' The term Primofilices, not hitherto used in this text, refers to a hypo- thetical, primitive fern stock. THE EVOLUTION OF. PLANTS 237 Ancestors of Pnmo-fil/ces Fig. 1 10. — Genealogical tree, showing the ancestral lines of the mod- ern plant orders, according to a monophyletic hypothesis. Full ex- planation in the text. (C/. Fig. in.) 238 HEREDITY AND EVOLUTION IN PLANTS According to this view, vascular plants appear at the beginning of the fossil record as two distinct series, the Lycopsida and Pteropsida. The Lycopsida, like the modern Lycopodiales, are characterized by the possession of small leaves (a primitive character), and by few spor- angia on the upper surface of the leaves. The Pteropsida, by contrast, like the modern Filicales, are in general, dis- tinguished by large leaves, having the numerous sporangia on the lower surface. The two groups also have well- marked anatomical differences. The Lycopsida reached their greatest development in the Paleozoic period, and now appear to be on their way to extinction. The Pterop- sida, on the other hand, although possessing many repre- sentatives in former geological ages, still maintain their full vigor, and are considered by this school of paleo- botanists to be in the direct ancestral line of our modern vascular plants, substantially as indicated in Fig. 11 1 . ^ {b) Greater precaution in drawing conclusions from the few known facts has led still other students of fossil plants to refrain from endeavoring to connect up the ancestral lines, claiming that while they may converge, indicating a common ancestry of the known forms in the geologic past, on the other hand they may not unite, or at least may not all converge toward the same ancestral type. In other words, it is suggested that fossil and modern plants had a polygenetic origin from the stage of .primitive protoplasm. Such views are illustrated in Table V (p. 240). It is seen from this diagram that our modern ferns have a long ancestral history, extending from the present back ' Scott restricts the name Lycopsida to the Lycopodiales, and proposes a third group, Sphenopsida, incuding the Equisetales, Pseudoborniales, Sphenophyllales, and Psilotales. Wieland has recently adduced reasons for using the term Hemicycadales. vs. Bennettitales. (C/. foot-note, p. 211.) THE EVOLUTION OF PLANTS 239 Anc&stors of- Lycoj^sidaand Pterops/da, Fig. III.— Genealogical tree, showing the ancestral line of the modem plant orders according to a polyphyletic hypothesis. Full explanation in the text. (C/. Fig. no.) 240 IIKREDITV AND EVOLUTION IN PLANTS to early Palaeozoic times; the same is true of our modern cycads, maidenhair tree (Ginkgo), club-mosses (Lyco- podiales), and horse-tails (Equisetales). The Coniferales may be traced back into the upper Carboniferous period, while the most highly developed of modern plants, the Angiosperms, appear to have come into existence as late as about the middle of the Mesozoic era, perchance as Table V Ascendancy Periods Persistence and relationship of great groups VII. Reign of Angiosperms Tertiary Cretaceous Comanchian V 1 1 1 d Us is rs u u Q (A .■2- 1 I u •i U Q. 0) 0) « < VI. Reign of Pro-angiosperms Jurassic Triassic Permian e a 3 s a 3 W a u >> 1-1 V. Reign of Acrogens (High- er Equisetes. Lycopods, etc.) Pennsylvanian Mississippian 2 ■3 L_ i •0 ! «8 : u ; >> :0 : a IV. Reign of Gymnospertns Devonian III. Reign of Early Land Plants Silurian Ordovician S. Actual Fossil Land Plant rec- ord begins 4. Primoiilices — Early Equisetes 3. Basal Plant Complex with va- riety of species II. Reign of Algse Cambrian Precambrian (Proterozoic) 2. Differentiation of Dry Land and Aquatic Plants (Fossil Algae abundant) I. Reign -of Primitive Life (Hypothetical) Old P r e c a m- brian (Archeozoic) (Fossil Algae begin) I. Primitive Protoplasm and Unicellular Life In the above table (after Wieland), the groups are to be considered as arranged on an unrolled cylinder, projected from a hemisphere; thus the phyletic lines are to be pictured as converging below toward. the pole, and the Cordaitales as coming between the Ginkgoales and Filicales, to both of which they are related. THE EVOLUTION OF PLANTS 241 I recently as 20 million years ago. The lateness of their appearance and the rapidity with which they have spread, until they are now the dominant element in the flora of the land, indicate how well they are adapted to their environment. "Nothing is more extraordinary in the history of the vegetable kingdom," wrote Darwin to Hooker, "than the apparently very sudden or abrupt development of the higher plants." "The construction of a pedigree of the Vegetable King- dom is a pious desire, which will certainly not be realized in our time; aU we can hope to do is to make some very small contributions to the work. Yet we may at least gather up some fragments from past chapters in the history of plants, and extend our view beyond the narrow limits of the present epoch, for the flora now living is after all nothing but one particular stage in the evolution of the Vegetable Kingdom."^ 154. The Element of Geological Time.— How many years has it taken for the evolution of the higher Angio- sperms — that is, from the dawn of the fossil record in the Silurian period to the present? No one knows. From a study of the thickness of rock strata, and a knowledge of the probable time required for the depositing of those strata as sediment on the floor of the ancient oceans, and their elevation and denudation to their present condition by weathering and erosion, geologists have been able to suggest relative measures of geologic time. Paleozoic time is long, twice as long as Mesozoic time, and Meso- zoic time must be at least twice as long as Cenozoic time. The actual age of the earth is, however, a problem which engages the attention of physicists as well as geologists. 'Scott, D. H. "Studies in Fossil Botany,'' p. 3. 16 242 HEREDITY AND EVOLUTION IN PLANTS Sixty years ago Lord Kelvin gave a mean estimate of 100,000,000 years. With this estimate two geologists, Walcott and Geikie, have nearly concurred; but since the discovery of radium it has been estimated that certain carboniferous iron ores have an age of 140,000,000 years. Figures of such magnitude convey but little meaning to our minds; they are too large for us to grasp their real value. "Therefore," as Darwin has said, "a man should examine for himself the great piles of superimposed strata, and watch the rivulets bringing down mud, and the waves wearing away the sea-cliffs, in order to comprehend some- thing about the duration of past time, the monuments of which we see all around us." CHAPTER XIII THE GREAT GROUPS OF PLANTS 165. The entire question of taxonomic groups is very difficult and intricate, and there is at present a consider- able difference in opinion and usage, even among those equally competent to judge. As set forth in Chapter IX, the segregation and sequence of larger groups may be based chiefly upon the morphology of living plants, or upon that basis supplemented by the findings of anatomy (including embryology and histology), comparative life histories, and paleobotany.^ Manuals and "Floras" of systematic botany are, for the most part, arranged upon the former basis, which operates at present in the direction of conservatism and few changes in connection with the largest groups, or phyla. Regard for the evidence from other sources is more apt to result in conflict of opinion and more frequent revisions in the light of new studies, but it is also more apt to result in a closer approximation to the truth. In the former case the sequence of groups is chiefly based upon complexity of organization, proceeding from the simpler to the more complex. On this basis the monocotyledons, for example, would precede the dicotyle- dons, the order observed in the Manuals. In the latter case the sequence of groups attempts to indicate or reflect their order of development in time, as indicated by the data of paleobotany, comparative life- histories, comparative anatomy, and plant geography. On this basis the monocotyledons would follow the 'See, also, p. 236. 243 244 HEREDITY AND EVOLUTION IN PLANTS dicotyledons, as being derived from the latter by a process of simplification (C/. p. 223). The structural and ana- tomical evidence that eusporangiate ferns are more ancient than leptosporangiate ferns is rendered more cer- tain by the fact that the earliest fern fossils (in Paleozoic rocks) are eusporangiate; the leptosporangiate forms do not appear until later, and the fossils belong to families closely related to the more ancient eusporangiate group, while the fossils of more recent rocks show closer affinities with the modern living forms. (See, however, p. 30.) In any tabular arrangement including all the great groups or phyla every group must, of course, come defi- nitely after some one group and precede another. Thus, mosses logically fall between the Thallophytes and the fern allies ; but there is scarcely any evidence that they are phylogenetically related to the groups that follow them in the table. Strangely enough, there are few well- authenticated fossil remains of mosses (and those not below the Mesozoic), and it has even been seriously suggested that they may have developed from more complex groups by processes of reduction and simplifica- tions; but there is little, if any, evidence to indicate from what higher group they might have been thus derived, and the positive, though meager, fossil evidence is suffi- cient to render highly improbable, if not to nullify, the suggestion of derivation by reduction. The old group "Pteridophytes," of the manuals, including the true ferns and their "allies" (horsetails, lycopods, and little club-mosses), served a useful purpose before the recent researches in fossil botany; but the results of those studies have made it impossible consist- ently to maintain the group longer in its former con- THE GREAT GROUPS Or PLANTS 245 notation. The term " Pteridophyta " may still be used to advantage in a more restricted sense, as applying to the "true ferns," while the "fern allies" naturally fall into two other Divisions or phyla, namely the Club- mosses (Lepidophyta) and the Horsetails {Calamophyta of Bessey, or Arthrophyta of Berry). The discovery of the fossil seed-bearing ferns (Cycado- filicales) and their fossil and living relatives (Hemicycad- ales, Cycadales, Cordaitales, and Ginkgoales), all having cryptogamic (i.e., centripetal) wood,^ and all the living forms distinguished from the other gymnosperns by the possession of ciliated motile sperms , suggested the group to which Jeffrey has given the convenient and descriptive term Archigymnosperma (Early Gymnosperms) :in contrast to the Yews (Taxales), Conifers (Pines, Spruces, Hem- locks, Firs, Cypress, etc.), and Gnetales, which lack both those characters. To this latter group Jeffrey has given the name, Metagymnospermce (Late Gymnosperms). Other authors have suggested grouping the woody- stemmed and comparatively small leaved Cordaitales, Ginkgoales, and Coniferales together, and apart from ' The first formed woody tissue is primary wood or proloxylem. It is present when the organ (stem, root, etc.) is young, and its cell walls are thickened in rings or spirals and thus it can readily stretch as the organ elongates in growth. After growth in length has ceased, or has been greatly retarded, secondary wood or metaxylem, forms. The cell walls of this tissue have scalariform, reticulate, or pitted thickening, and thus they cannot readily stretch. In the vascular cryptogams (e.g., Club-mosses and related forms) the secondary wood forms inside the zone of primary wood; in the later or "higher" gymnosperms (Metagym- nosperms) this order of development is reversed; while in the ferns and lower gymnosperms (Archigymncsperms) the earlier development is cen- tripetal and the later centrifugal. Thus the mode of development of the woody tissue is an index of the evolutionary position of a, given form. 246 HEREDITY AND EVOLUTION IN PLANTS the Cycadean series (Cycadales, Hemicycadales (Bennet- titales) , Cycadofilicales) which have pithy steins; but some of the Cordaitean forms also have pithy stems and com- paratively large leaves. Here again, as so often, an attempt at a formal classification necessitates drawing an apparently sharp line where in fact one does not exist. As Professor Jeffrey' has said, the term Archigymno- spermse is one of convenience, and like most scientific terms falls short of covering the situation. On the basis of certain criteria {e.g., the structure of the wood), the Ginkgoales appear to be intermediate between the Coni- f erales and the Cordaitales. In fact, as Jeffrey^ has pointed out, the "living fossil," Ginkgo, may be regarded as a connecting link or transitional form between the Archi- gymnospermae and the Metagymnospermae. The relationship of Isoetes is one of the most difi&cult to determine among all the vascular cryptogams. Argu- ments for and against interpreting it as derived by re- duction from the Lepidodendron group are given by Lady Isabel Browne.' The secondary growth in thickness of its stem (in such a dwarfed form) must be regarded as a character of long standing, not recently acquired; plants in both groups have mucilage cavities. Isoetes resembles some of the Lepidodendrales {e.g., the so-called Stigmaria*) in the dichotomous branching of its roots. Other facts of structure {e.g., the occurrence of the sporangia on the upper side of the leaves) have also been interpreted 'Jeffrey, E. C Science, N. S. 47: 316. 1918. 2 Jeffrey, E. C. The anatomy of woody ptanls, p. 315. Chicago, igiy. 'Browne, Lady Isabel. The phylogeny and inter-relationships of the Pteridophyta. New Phy tola gist 7: 93, 103, 150, i8r, 230. 1908. * The fossil remains to which the generic name Stigtnaria was assigned have long been known to be the root-system of Sigillaria. THE GREAT GROUPS OF PLANTS 247 as pointing to the origin of Isoetes (by reduction) from the Lepidodendrales. One of the most cogent objections to this theory is the great amount of reduction which must be postulated; moreover, Isoetes has no cone, while most of the Lepidodendrales have. ' The absence of secondary growth in thickness of the stem and of a ligule on the leaves, combined with the possession of a biciliate sperm, in Lycopodium, would tend to preclude its close affinity with Isoetes. While certain features of sporophyte anatomy {e.g., the possession of a ligule) suggest Selag- inella, it seems difficult to accept a close relationship between Isoetes and the Selaginellales, since the sperms of the latter like those of Lycopodium are biciliate, while those of Isoetes are multiciliate. The possession of multi- ciliate, sperms and the structure of the archegonia suggest affinity with the eusporangiate pteridophytes, and notably with the Marattiales. Without going further into details which belong to a more advanced and technical treatise than this, and disregarding certain mooted points, or almost equally balanced choices like the one just mentioned, it may be said that the following tabular statement (pp. 249-251) reflects the present state of our knowledge concerning the rela- tionship and developmental sequence (phylogeny) of the great Divisions and Orders^ of the Kingdom of Plants. The same thing is shown diagrammatically in Fig. 112 (p. 248). The tabular statement aims, not only to indi- cate the relationship and sequence of groups, but also to help the student define the terms commonly met with in the established literature of botany. 1 Attention is called, in passing, to the uniform termination (-ales) of the plant Orders. 248 HEREDITY AND EVOLUTION IN PLANTS oLUBMOssza Eoassiins (lapldo- phyta) 11 (Calaao- phyts ) (Ftarldophyta Id restrloted SUED iJdSfS (Sponoatopbyta) Lower Gynuo— Bparms 1 Higher Gymno- Bperms Angto- eperma 15 «' Seed nake eper oi 1 dl m |8 lated I Seeds e&aloBed apersiB not oillftted nalce'd. eperliDB hot- Stem with rldgee & JotntB, leaveB nhorled , Bporejigla aeveral LeaveB amall. sporangia on upper Bide, woody cylinder oontinaoua LTCOFSIDJ, Euapor- Lepto- angiate aporan- giata Wittaont aeeda LeaTea large, aporangia on tuder aide, woody oylinder with fo-llar gapa PTEHOPSIDA Buaporan' giate I living plantB I well known foeail planta I foasila of doubtful affinity •f- exoluBlTely foaall groupa UAIH QROUFd OF TASCUIAR FLAHT3 Their apparent affinltiea and approzlJiiate geologioal diatrihution Fig. 112. THE GREAT GROUPS Of PLANTS 249 Table VI THE GREAT GROUPS OF THE KINGDOM OF PLANTS Main Groxjps of Non-vascular Plants Plants without "flowers" — Crvptogams (Nos. 1-5) Plant body usually a Ihallus; sexual organs usually one celled Thallophytes i No archegonia Chlorophyll-bearing Algig 10 Non-chlorophyll bearing Fungi ib Plant body thalloid or leafy; sexual organs usually several celled Bryophytes 2 Archegonia Protonema rudimentary or wanting, sporophytes with elaters Liverworts 2a Protonema well defined, sporophytes without elaters Mosses 26 1. Thallopliytes 10. Alga Cyanophyceae (Blue-green) Phaeophyceas (Brown) Chlorophycese (Green) Rhodophyceae (Red) lb. Fungi Myxomycetes (Slime-molds) Basidiomycetes (Spores on stalks) Schizomycetes (Bacteria) Including the Basidiolichenes Phycomycetes (Molds) Fungi imperfecti (Life histories Ascomycetes (Spores in sacs) imperfectly known). Including most lichens (Ascolichenes) 2-66, Arckegoniales; 2-6c, Embryophyla 2. Bryophytes 20. Liverworts Ricciales Jungfermanniales Marchantiales Anthocerotales 26. Mosses Spagnales (Peat mosses) Bryales (True mosses) Andreaeales (Black mosses) Main Groups of Vascular Plants Woody cylinder continuous, {i.e., without foliar gaps), leaves small, sporangia above— Lycopsida 250 HEREDITY AND EVOLUTION IN PLANTS 3-5, Vascular Cryptogams Stem smooth, leaves spirally arranged, sporangia single. Clubmosses 3 Stem with ridges and joints, leaves whorled, sporangia several Horsetails 4 Woody cylinder discontinuous (i.e., with foliar gaps), leaves large, sporangia below — Pteeopsida Without seeds Ferns 5 With seeds — Spermatophyta (Phanerogams) Seed Plants 6 Ovules naked, endosperm formed before fertilization . Gymnosperms 6 Sperms ciliated Early Gymnosperms 6a Sperms not ciliated Lale Gymnosperms 6b Ovules enclosed, endosperm formed after fertilization . Angiosperms 6c 3. Clubmosses {Lepidophyta (Bessey)) Lycopodiales Selaginellales Isoetales (?) Lepidodendrales (FoisU) Psilotales 4. Horsetails (Calamophyta (Bessey), Sphenopsida (Scott), Arthrophyla (Berry)) SpenophyUales {Fossil) Calamariales (Fossil) Pseudoborniales (Fossil) Equisetales 5. Ferns (Pteridophyta, in restricted sense; Filicinea). EusporangiatcB Leptosporangiatce Primofilices (Coenopteridese) Osmundales Marattiales Polypodiales Ophioglossales Marsiliales (Isoetales?) 6. Seed-Plants (60 &• 6J Gymnospermce of Brongniart) 6a. Early Gymnosperms (Cycadophyta (Nathorst) except Ginkgoales; Archigymnosperma (Jeffrey)) Cycadofilicales (Fossil) Cordaitales (Fossil) Hemicycadales (Wieland) = Ginkgoales Bennettitales of Potonifi (Fossil) Cycadales 6b. Late Gymnosperms (Coniferse (Hallier); Metagymnospermae (Jef- frey); Strobilophyta (Bessey)) Taxales Finales . , Araucariales Gnetales THE GREAT GROUPS OE PLANTS 251 6c. Angiosperms (Angiospermse; Anthophyta (Braun)) Two cotyledons, leaves net-veined, parts of the flower in s's or 4's — DiCOIYlEDONS ApetalcB (petals wanting) 1. Casuarinales 2. Piperales 3. Salicales 4. Myricales 5. Leitneriales 6. Balanopsidales 7. Juglandales 8. Fagales 9. Urticales 10. Proteales 11. Santalales 12. Aristolochiales 13. Polygonales 14. Chenopodiales Polypelalce (petals distinct — wanting in a few exceptional cases) t> — 1„= 3 Malvales 9. Parietales 10. Opuntiales 11. Thymeliales 12. Myrtales 13. Umbellales 1. Ranales 2. Papaverales 3. Sarraceniales 4. Rosales 5. Geraniales 6. Sapindales 7. Rhamnales or 6 7. xuiamnaieb Sympetala; Gamopetalce (petals more or less united) 1. Ericales 6. Plantaginales 2. Primulales 7. Rubiales 3. Ebenales 8. Valerianales 4. Gentianales 9. Campanulales 5. Polemoniales One cotyledon, leaves usually parallel- veined, parts of the flower in 3's I ' s — Monocotyledons 1. Naiadales 6. Arales 2. Pandanales 7- Xyridales 3. Graminales 8. Liliales 4. Palmales (Principes) 9. Scitaminales S- Cyclanthales (Synanthse) 10. Orchidales BIBLIOGRAPHY! DARWINISM AND GENERAL EVOLUTION Bateson, W. Problems of Genetics. Yale Univ. Press. New Haven, 1913. Bergson, Henri. Creative Evolution. New York, 1911. Candolle, Alphonse de. Origin of cultivated plants. London, 1904. Chamberlin, T. C, and others. Fifty years of Darwinism, etc. New York, 1909. Conn, W. H. The Method of Evolution. New York. 1900. Coulter, John M. Evolution, heredity and eugenics. (School Science series.) Bloomington, HI., 1916. Darwin, Charles. The Origin of Species by Means of Natural Selection. New York, 1902. (ist Edition, London, 1859). Darwin, Francis. The life and letters of Charles Darwin. New York, 1901. More letters of Charles Darwin. New York, 1903. Delage and Goldsmith. The theories of evolution. New York, 1913. Geddes and Thomson. The Evolution of Sex. London and New York. Gray, Asa. Darwiniana. New York, 1876. Hertwig, O. The biological problem of to-day. (Preformation or Epi- genesis?) Eng. translation by Chalmers Mitchell. New York. No date. Huxley, Leonard. Life and letters of Thomas Henry Huxley. New York, 1901. Judd, John W. The coming of evolution. Cambridge (Eng.), 1912. Kellogg, Vernon L. Darwinism To-day. New York, 1907. Lamarck, J. B. Zoological Philosophy (Eng. translation by H. Elliot). London, 1914. Lock, R. H. Recent progress in the study of variation, heredity, and evolution. 3d. Ed. London, 1911. Morgan, T. H. A critique of the theory of evolution. Princeton Univ. Press, r9i6. Osbom, H. F. The Origin and Evolution of Life. New York, 191 7. Osbom, H. F. From the Greeks to Darwin. Pemberton, Henry. The path of evolution. Philadelphia, 1902. Seward, A. C. (Editor.) Darwin and Modern Science. Cambridge (Eng.), and New York, 1909. Thomson, J. Arthur. Darwinism and human life. New York. '.Omitting periodical literature 252 BIBLIOGRAPHY 253 Thomson and Geddes. Evolution. New York, 1911. Wallace, Alfred Russel. Darwinism. 3d. Ed., London, 1901. . Fifty years of Darwinism: Modern aspects of Evolution. New York, 1909. MUTATION AND EXPERTMENTAL EVOLUTION Doncaster, L. The Determination of Sex. Cambridge Univ. Press. 1914. Gates, R. R. The Mutation Factor in Evolution. London, 1915. Lotsy, J. P. Evolution by means of hybridization. The Hague, 1916. Vries, Hugo de. The mutation theory. Eng. trans, by Farmer and Darbishire. Chicago, 1909. Vries, Hugo de. Species and Varieties, their origin by Mutation. Chicago, 1905. Vries, Hugo de. Intracellular pangenesis. Eng. trans, by C. Stuart Gager, Chicago, igio. Vries, Hugo de. Gruppenweise Artbildung. Berlin, 1913. Weismann, A. The evolution theory. Trans, by J. Arthur Thomson and M. R. Thomson. 2 vols. London, 1904. HEREDITY Babcock and Clausen. Genetics in relation agriculture. New York, 1919. Babcock and Collins. Genetics laboratory manual. New York, 191 8. Bailey and Gilbert. Plant breeding. New York, 1915. Bateson, W. Materials for the study of variation. London, 1894. Bateson, W. The Methods and Scope of Genetics. Cambridge, 1898. Bateson, W. Problems of Genetics. Yale Univ. Press. 1913. Bateson, W. Mendel's Principles of Heredity. Third Impression. Cambridge and New York. 1913. Castle, W. E. Genetics and eugenics. Cambridge (Mass.), 1916. Castle, Coulter, et al. Heredity and Eugenics. Chicago, 1912. Conklin,E. G. Heredity and Environment. Princeton Univ. Press, 1915. Coulter, John M. Fundamentals of plant breeding. New York & Chicago, 1914. Coulter (John M.) and Coulter (Merle C). Plant genetics. Cljicago, igrS. Darbishire, A. D. Breeding and the Mendelian Discovery. London and New York, 1913. Darwin. C. The variation of animals and plants under domestication. 2d Ed. New York, 1900. 2 54 HEREDITY AND EVOLUTION IN PLANTS Davenport, C. B. Heredity in Relation to Eugenics. New York, 191 1. Davenport, E. Principles of breeding. Boston, 1907. Doncaster, L. Heredity in the Ijght of recent research. Cambridge (Eng.), 1912. Mendel, Gregor. Experiments in plant hybridization. Eng. trans, by Royal Horticultural Sec. (In Bateson, W., Mendel's principles of heredity. Cambridge, 1909.) The original paper, Versuche iiber Pflanzen-Hybriden, was published in Verhandlungen des naturforschen- den Vereines in Briinn. Abhandlungen IV, Band 1865. Briinn, 1866. Morgan, T. H. et al. Mechanism of Mendelian heredity. New York, 1915- Morgan, T. H. The physical basis of heredity. Philadelphia, igrg. Popenoe and Johnson. Applied eugenics. New York, 1919. Punnett, R. C. Mendelism. 3d Ed. New York, 1911. Thomson, J. A. Heredity. London, 1908. Vernon, H. M. Variation in Animals and Plants. New York, 1903. Viies, Hugo de. Plant breeding: Comments on the experiments of Nilsson and Burbank. Chicago, 1907. Walter, Herbert E. Genetics: An introduction to the study of heredity. New York, 1915- Weismann, A. Essays upon heredity and kindred subjects. 2 vols. Trans. Oxford, i8gi, 1892. Weismann, A. The germ-plasm: a theory of heredity. Trans, by W. N. Parker and H. Ronnfeldt. London, 1893. EVOLUTION OF PLANTS Bailey, L. H. The Survival of the Unlike. New York, 1897. Bower, F. O. Plant life on land. Cambridge (Eng.), igir. Bower, F. O. The origin of a land flora. Cambridge, 1908. Campbell, D. H. Plant life and evolution. New York. 1911. Campbell, D. H. Lectures on the evolution of plants. New York, 1899. Coulter, J. M. The evolution of sex in plants. Jeffrey, E. C. The anatomy of woody plants. Chicago, 1917. Scott, D. H. Studies in fossil botany. 2d Ed. London, 1909. Scott, D. H. The evolution of plants. New York & London, 191 1. Seward, A. C. Links with the past in the plant world. Cambridge, igir. Seward, A. C. Fossil plants. 3 vols. Cambridge (Eng.), 1898-1917. Shimer, H. W. An introduction to the study of fossils. New York, 1914. Stopes, Marie C. Ancient plants. 1910. Wieland, G. R. American fossil cycads. Carnegie Institution of Wash- ington, Publication No. 34, Vols. 1-2, 1906. BIBLIOGRAPHY 255 PLANT GEOGRAPHY De Candolle, Alphonse. G^ographie Botanique raisonnee. Geneva, i8ss. Humboldt and Bonpland. Essai sur la g^ographie des plantes. Paris, 1805. Huxley, Leonard. Life and letters of Sir Joseph Daltoa, Hooker. New York, 1918. H^chant, James. Alfred Russel Wallace: Letters and reminiscences. New York, 1916. ScharS, Robert Francis. Distribution and origin of life in America. New York. The Macinillan Co. 191 2. (Treats chiefly of Animals.) Schimper, A. F. W. Plant geography upon a physiological basis. Eng. translation by Fisher. Oxford, 1903. Thistleton-Dyer, Sir William. The distribution of plants. In Darwin and Modern Science, Chapter XVI. Ed. by A. C. Seward. Cam- bridge, 1909. Wallace, Alfred Russel. Geographical distribution of animals. New York, 1876. . Island Life, 3d Ed. Revised. London, 191 1. . Darwinism. Chapter XII. The geographical distribution of organisms. 3d Ed. London, 1901. My life. New York, 1906. Warming, Eugene. Botany of the Faeroes. London, 1901-1908. (Especially the chapter on "The history of the flora of the Faeroes," pp. 660-681). PERIODICALS American Journal of Botany, Brooklyn Botanic Garden, Brooklyn, N. Y. American Naturalisit. The Science Press, Garrison, N. Y. Annals of Botany. Cambridge University Press, Cambridge, England. Botanical Abstracts, Baltimore, Md. Botanical Gazette, The University of Chicago Press, Chicago, 111. Ecology, Brooklyn Botanic Garden, Brooklyn, N. Y. Genetics. Princeton Univ. Press, Princeton, N. J. Journal of Heredity. Amer. Genetic Association, Washington, D. C. New Phytologist, London, Eng. Revue gfe6ral de Botanique, Paris, France. INDEX Acer Pseudo-Platanus, 222 Acquired characters, 72 Acropera, 95 Adianlum, 9, 21, 22 concinnum, 25 emarginatum, 126 Africa, 145 Agapantkus umbellatus, 229 Agassiz, Louis, 85 Agassiz's Hypothesis, 84 Agalhis, 232 austraiis, 233 Age and Area, Hypothesis, of, 176 Aleurites molvccara, 163 Algae, distribution of, 1 76 Alternation, homologous, 135 of generations, 132 ontogenetic hypothesis of, 136 Ambrosia crithmifoUa, 149 Anaphase, 36 Anatomy, evidence from compara- tive, 127 results of the method of com- parative, 129 Andes, 149 Angiopteris, 95, 203 evecta, 204 Angiosperm, primitive, 225 Angiosperms, ancestors of the, 210 Anisosorus hirsulits, i Annulus, 10 Antheridia, 18, 22, 23 Anthostrobilus, 232 Apical cell, 126 Aralia, 156 Archegoniates, 130 Archegonium, 18, 21, 24 Archigymospserma, 24s Arthropkyta, 243 Ascaris, 38 Ash, 113 Aspidium Filix-mas, 11 Autochthonous hypothesis, 142 Azolla, 82 Azores, 149, 150 Bananas, 113 Barberry family, 171 Beagle, 145 Bean pond, 186 Beans, 66, 104, log, 107 Bennettitales, 211 Bennettites, 214 BerberidaceEE, 171 Bibliography, 252 Biometry, 54 Biophors, 75 Birds, dispersal by, 149 Blakeslee, 52 Blue eyes, 69 Blue rose, 73 Bolivia, 149 Boston fern, 16, 109, 113 Botany, the major problem of, 84 Botrychlum Lunaria, 128 Boveri, 74 Bowenia, 214 serrulata, 203 Bower, Professor, ii Bracken fern, 2 Brake, 2 Brassica oleracea, 113, 124 2S7 2S8 INDEX Bread mould, 80 Broccoli, 112 Brousoneiia papyrifera, 163 Brown eyes, 69 Brown, Lady Isabel, 135 Brussels sprouts, 112 Bryophytes, 127 Bucholz, 127 Bud-sport, 114 Bulbils, 16 Cabbage, wild, 112 Cacti, thornless, 113 Calanwphyta, 245 Calla, 80 Camptosorus rhizophyllus, 15, 17 Canary Islands, i6i Cannabis sativa, 163 Capsella Bursa-pastoris, 95 Castalia flava, 179 tetragona, 158 Cauliflower, 112 Ceylon, 14s, 150 Character-units, 64 versus unit-characters, 65 Chromatin, 36, 73 CJiromosomes, 37 Chrysanthemum leucanthemum, 154 Cinnamon fern, I2 Classification, 121 evolution and, 122 Clayton's fern, 13 Cliff-cabbage, 124 Clover, 113 Coal balls, 204 Cocos Island, 143 Coleoptile, 324 Cone-flower, 114 Cordaitales, 233 Cordaites, 232 Corsica, 144 Cotyledonary ring,' 2 2 2 Cross-fertilization, 25 Crossing, increased vigor from, 6g Crossotheca Eoeninghausi, 202 _ Cultivation, escapes from, 163 Cultures, pedigreed, 59 Curve of frequency, 106 Cycadeoidea, 212, 214, 220 dacotensis, 208, 209, 210, 213, 219 Dartoni, 216 ingens, 215 Wielandi, 213, 214 Cycadocephalus Sewardi, 208 Cycadofilicales, 201, 207 Cycas circinalis, 212 Cyrlomium falcatum, 9 Cystopteris htdlifera, 16 Darwin, Charles, 90, 91 Darwinism, 90, 92 mutation theory to, 118 De Candolle, 144 Dendrobium attenualum, 147 Dendroceros. 174 Determinants, 75 Determiner, 64 de Vries, Hugo, 74, loi, 102 Diapensia lapponlca, 162 Dicotyledons, monocotyledonous, 226 Dicotyledony, origin of, 220 Dictyola, 13s, 137 dichotoma, 136 Diervilla, 154 Lonicera, 155 rivularis, 155 sessilifolia, 155 Differentiation, Dorso-ventral, 20 Dionaea muscipula, 178, 179 Dioscorea bonariensis, 226, 227 Diplazivm zelanicunt, 7 Diploid, 2S, 35 INDEX 259 Disease-resistance, breeding for, 71 Dispersal, means of, 142 Distribution, continuous, 168 types of, iS4 significance of geographical, 139 Division, heterotypic, 37 homotypic, 37 Dodder, 80 Dominance, Ian of, 59 Dominants, 63 Drosera filiformis, 197, 198 Drynaria meyeniana, 4 Dumortiera, 174 Earthworms, 194 Egg, 21, 24 Elementary species, 1 1 r Elephants, 94 Elodea, 196 Enation, 132, 135, 231 Endemism, 164 Environment, adjustment to, 41 fitness for, 93 fitness of the, 83 inheritance and, 66 Epiblast, 223, 224 Epidermis, 4 Equinoxes, precession of the, 192 Equisetum, 234 lelamateia, 126 Eriocaulon septangular e, 157 Eugenia malaccensis, 1 63 Eugenics, 76 Eupatorium, 156 Evening-primrose, 114 Evolution and classification, 122 Evolution, early antagonism to, 91 experimental, 118 inorganic, 82 meaning of, 79 method of, 84 organic, 83 Existence, struggle for, 42 Experimental evolution, loi Expression, inheritance versus, 40, 48 Extinction, factors of, 19s Factor, 64 Factors, 65 Falkland islands, 148 Fern, life-cycle of a, 35 Fern, life history of a, 1, 20, 34 Fertilization, 23, 24, 53 Fertilization-membrane, 24 Fimbristylis spathacea Roth, 146 Fittest, survival of the, 43, 97 Fleabane, 164 Foliage-leaf, 10 Foot, 25 Forbes, Edward, 161 Fossil, what is a, 183 Fossil-formation, conditions of, 185 Fossil record, significance of the, 23s Fronds, 4 Gaertner, 120 Gametes, 32 Gametophyte, 33 Gaps in the fossil record, 193 Genealogical tree, 237, 239 Gene, 64, 65 Generations, alternation of, 34 two kinds of, 33 Genoa, 144 Geothallus tuberosus, 174 Germination, 17 Germ-plasm, 75 Ginkgo, 139, 232, 233, 246 biloba, 168 Ginkgoales, 246 Glaciation, effects of continental, 160 Gleickenia circinata, n 26o INDEX Gmelin, 141 Grass-embryos, 224 Gray, Asa, 157 Green dahlias, 113 Green roses, 113 Groups of plants, the great, 243 (jymnosperms, ancestors of the, 232 early, 245 late, 24s Habit of life, consequences of an amphibious, 130 Half -race, 223 Uamamelis, 156 Haploid, 25, 3S Hawaii, 145, 150 Hawaiian flora, origin of the, 175 Hawkweed, 76 Haworthia sp., 49 Hedge mustard, 42 Helix hortensis, 157 Hemerocallis Juha. 163 Hendangiosperma, 232 Hemicycadales, 211, 238 Hepatica, 128, 172, 174 Heredity, 45 experimental study of, 55 inheritance versus, 50 Johannsen's conception of, 67 Heterozygous, 63 Hibiscus occuUroseus, 178 Hieraceum, 76 auranUacum, 154 Hilldebrandia sandwicensis, 178 Honey-locusts, thornless, 113 Humbolt, 140 Hurricane grass, 146 Hybridizing, artificial, 58 Hydrangea, 156 Hydrophytes, ig6 Idants, 7S Idioplasm, 74 Ms, 75 Incrustation, 183 Indian corn, 69, 223 Indian pipe, 80 Indigenes, 177 Indusium, 8 Inheritance, 39, 92 and environment, 66 and reproduction, 50 mechanism of, 73 versus expression, 40 versus heredity, 50 what is, 46 Isoetes, 246, 247 Jack-in-the-pulpit, 80 Jelly-fish, 194 Johannsen, 40, 67 Jordan, 120 Kale, 112 Kauri pine, 233 Keeling Islands, 143 Kerner, 95 Kingdom of plants, the great groups of the, 249 Knight, 120 Kohlrabi, 112 Kolreuter, 120 Krakatoa, 146, 149, 17s Labrador, 160 Lagenostema Lomaxi, 205 Lakes, filling up of, 187 Lamarck, Jean Baptiste, 89 Lamarckism, arguments against, 89 Lamarck's hypothesis, 86 Landscape, a late paleozoic, 234 Lang, 136 Layering, 32 Leaf, free-living, 81 Leersia clandestina, 224 INDEX 261 Lemna, 80 Irisulca, 81 Lepidodendron, 134, 246 Lepidophyta, 245 LeptocMoa arabica, 224 Leptosporangiate ferns, 10 Lesczyc-Suminski, Count, 20 Life-cycle, cytological, 38 Life-histories, evidence from, 125 Life history, 1, 3 Linin, 36 Linkage, 119 Linnaeus, 121 Linnaea borealis, 121 Liguidambar , 156 Liriodendron, 170, 219 Lomaria eriopus, 207 Lonicera japonica, 163 Loxsoma Cunninghami, 11 Lunularia cruciata, 173 Lycopersicum esculenlum, 51 Lycopodium, 134, 247 complanaium, 128 lucidulum, 156 Selago, 128 Lycopods, 134 Lycopsida, 238 Lyginodendron oldhamium, 201, 202, 205 Lygodium japoricum, 1 1 Macrozamia, 214 Moorei, 176 spiralis, 211 Madder family, 171 Magnolia, 156, 218, 219 flower of, 217 Magnoliacea, 219 Maidenhair fern, 25 Maiosis, 37 Maize, 60, 62 Malay archipelago, 143, 147, iSi Malthus, 94 Maraltia Douglasii, 126 fraxinea, 206 Marattiales, 247 Marble, 188 Marchantia, 173 polymorpha, 128 Marchantiaceae, 173 Marsilia vestita, 126 Matonia pecUnata, 11 Megasporophytes, 129 Membrane, nuclear, 36 Mendel, Gregor, 55 Mendelian ratio, significance of the, 64 Mendel, investigations since, 77 Mendelism, relation of Weismann- ism to, 76 Mendel's discoveries, 59 discoveries, value of, 68 law, applications of, 66 method, 56 problem, 56 Metagymnospermae, 245 Metamorphism, 188 contact, 188 regional, 188 Metaxylem, 245 Microsporophytes, 129 Migrations, plant, 192 Mistletoe, 80 American, 96 European, 96 Mitchella, 172 repens, 172 Mitosis, 36 Moluccas, 150 Monoclea, 174 Monocotyledon, dicotyledonous, 226, 227 Monocotyledony, 225 origin of, 223 262 INDEX Monophyletic hypothesis, 237 Morus alba, 163 Moss-roses, 113 Mt. Gedeh, 174 Mt. Washington, 158 Mucor, SI hermaphroditic, 52 Multiplication, vegetative, 16 Mutant, III Mutation and discontinuous dis- tribution, 166 examples of, iii Mutation theory to Darwinism, Relation of, 118 value of the, 120 Mutations, no Nageli, 74, 76 Naudin, 120 Nelumho, 171 lutea, 171 nucifera, 171 Nepenthes ampuUaria, 14s phyllamorpha, 147 Nephrodium filix-mas, g$ Nephrolepis, 16, 109 Nice, 144 Nicotiana tabacum, 51 Notothylus orbicularis, 126 Nymphaeaceae, 171 Nymphaea mexicana, 179 Oak, 103 Objections, difficulties and, 97 Oenothera biennis, 115 breiiistylis, 115, 116 gigas, 117, 167 laevifolia, 116, 167 Lamarckiana, 109, 114 IIS, 116, 167 nanella, 109 scintillans, 167 Onoclea, 24, is6 Ontogeny, 83, 136 evidence from comparative, 126 Oosperm, 2s, 33 Oospore, 25 Ophioglossum pendulum, 126 Orchid, seed capsule and seeds of an, 172 Orchidaceae, 171 Organs, origin of vegetative, 131 Orton, W. A., 72 Osmunda, 169, 204 Claytoniana, 5, 13, 126 cinnamomea, 12, 154 Japonica, iS4 regoKs, iS4 Osmundacea, 139 Paleobotany, the scope of, 183 Paleogeography, 190 Palisade layer, 8 Pangens, 74 Papaver Rhoeas, 222 Paulownia, 163 Pea, edible, 61 Peak of Teneriffe, 161 Pendulun, illustration of the, no Petrifaction, 183 Petrifactions, i8s Phaseolus vtdgaris, 105 Phleum alpinum, 162 Phoradendron flavescens, 96 Photosynthesis, 8 PhysaUs Alkekengi, $0, $1 Phycomycetes, 80 Phyllitis, 9 Phylogeny, 83 Phymatodes, 7 Pinguictda caudata, 227 vulgaris, 226, 227 INDEX 263 Pinus Banksiana, 221 Laricio, 126 Strobus, S3 Pisum sativum, 61 Plantago major, 95 Plantain, 95 Plant groups, sequence of, 130 Plants, evolution of, 124, 201 Pleurococcus, 80, 127 vulgaris, 46, 47 Plover, American golden, 151 Pacific golden, iji Phichea faetida, 164 Polydemics, 177 Polyphyletic hypothesis, 239 Polypodiacem, 139, 169 Polypodium, 7, 8, 9, 12 punctatum, 6 venosum, 128 Polysiphonia, 135 Porrella, 128 Poientilla niua, 161 Primofilices, 236 Prince's Island, 148 Priichardia, 165 Proangiosperms, 216 Pro-anthostrobilus, 232 Problem, the modern, 100 Propagation, 32 vegetative, 49, 5° Prothallus, 18, 19 Protonema, 17, 18 Protoxylem, 245 Frunus Gravesii, 180 maritima, 180 Pteridophs^ta, 245 Pteridophytes, 244 Pteridosperms, 2or significance of the, 206 Pteris, 9 aquiUna, 2, 8, 154 longifolia, 6 Pteropsida, 238 Puccinia glumarum, 71 Pure line breeding, 68 Purity of gametes, theory of, 63, 64 Quercus chrysolepis, 103 Qu6telet, 105 Qu6telet's curve, 106, 107 Qu^telet's law, 105 Race, middle, 223 Radium, 73, 242 Randies, 218 Ranunculus aqualilis, 86, 87 Ficaria, 226 Receptacle, 8 Reduction, 35 nature and method of, 36 Relict endemic, 167 Relicts, 177 Reproduction, inheritance and, 50 sexual, 32, S3 Rhizoid, 18, 17, 23 Rhizomes, 3 Rhizophora Mangle, 228 Rhizopus, SI Rhododendron lapponicum, 159 zerlicillatum, 147 Riccia, 126, 129, 173 trichocarpa, 128 Rock strata, classification of, i8g Rocks, stratification of, 188 Root-stocks, 3 Rubiacea, 171, 172 Rubus chamaemorus, 155 Rudbeckia sp., 114 Rust disease, 71 Salina, 199 Saltation, orthogenetic, 109 Salvinia, 82 Sap, nuclear, 36 264 INDEX Saprophytism, 80 Sardinia, 144 Sassafras, 170 Scarlet tanager, 151 Schizaea pusilla, iss Schouw, 142 Schouw's hypothesis, 141 Scutellum, 224 Seed-bearing ferns, discovery of, 201 Segregation, law of, 60 Mendelian, 61, 62 ratio of, 61 Selfing, 66 Self-pruning, 32 Semon, 143 Senecio, 148 Sequoia, 168, 169 gigantea, 168, ryS sempervirens, 168, 178 Seychelles, 145 Shepherd's purse, 95 Shield fern, 11, 95 Shirley poppies, 113 SigUlaria, 246 Silent odontipetala, 222 Siphonogamy, 133 Skunk cabbage, 80 Slate, 188 Srnall, James, 147 Snakes, 87 Solanaceae, 171 Solanum integrifoUum, 50, 51 nigrum, 50, 51 Solomon archipelago, 151 Sorus, 8 Sparlina cynoswroides, 224 Special creation, doctrine of, 79 Spencer, Herbert, 43, 97 Spencerites, 134 Spermatophytes, 130 Sperms, 21, 22, 23 Sphenophyllum, 184 Sphenopsida, 238 Sphenopteris Hoeninghausi, 201 Spiranthes Romanzoffiana, 157 Spirem, 36 Spirogyra, 194, 196 Sporangia, 8, 10, 1 1 Sporangiophore, 8 Spore-production, consequence of enormous, 131 Spores, dispersal of, 17 germination of, 17, 18 reproduction by, 32, 50 SporophyU, 9 Sporophylls, 6, 7, 8, 11 Sporophyte, 33 steps in the evolution of the, 1 3 2 Sporophytes, 8 St. Helena, 148 Stangeria eriopus, 207 paradoxa, 207 Sterilization, 14 progressive, 126, 128 Stigmaria, 246 Stolons, 16, 17 Strawberry, 113 Strobiloid theory, 218, 219 Struggle for existence, 94, 96, 153 Stumps, Fossil tree, 191 Sunflowers, red, 113 Sweet flag, 80 Sweet peas, 66 Switzerland, 14s Sykes, Miss, 134 Symmetry, bilateral, 19 Symplocarpus fcetidus, 155 Synangia, 209 Synapsis, 37 Synizesis, 37 Taal volcano, 152 Tamus communis, 226, 227 INDEX 265 Targionia, 173 Taxodium, 170 Tectoria cicutaria, 14, i6 Teneriffe, Peak of, 161 Tern, arctic, 151 Thallophytes, 130 Thomson, J. Arthur, 48 Thyrsopleris elegans, 11 Tierra del Fuego, 149 Time, distribution of plants in, 192 distribution of plants in geo- logic, 193 table of geological, 190 the element of geological, 241 Toad-stools, 80 Todea barbara, 11 Trajispiration, 8 Trapa natans, 226 Tree ferns, 3 Tree, hypothetical ancestral, 137 Treubia insignis, 174 Triticum vulgare, 224 Twin-flower, 121 Unconformity, 189 Unit-characters, character-units versus, 65 Urtica dioica, 163 Utricularia, 196 Vallisneria, 196 Variation, 4r, 93 and inheritance, fluctuating, 108 continuous, 103 curves of, 109 discontinuous, 108 Variation, fluctuating, 103, 106 illustrations of continuous, 104 two kinds of, 103 Variety, constant, 223 eversporting, 223 Vascular plants, 8 Venus fly trap, 179 Visctim album, 96 Vries, Hugo de, 74, loi, 102 Walking fern, 15, 17 Wallace, Alfred Russell, go, 166 Wanakena, 186 Warming, 145 Water, dispersal by, 149 Water buttercup, 86, 87 Water lily family, 171 Weismannism, 74 to Mendelism, Relation of, 76 West Indies, 146 White Mountains, 160 White pine, 53 Wides, r77 Wielandiella, 220 WiesnereUa Javanica, 174 WUUamsonia, 214, 220 mexicana, 219 Willow, 32 Wind, dispersal by, 143 Wolffia papidifera, 8i punctata, 81 Zamia, 2x5, 232 Zea Mays, 69, 223, 224 Zelkowa, 167 Zygospore, 51 Zygote, 32, 33