'nVtrfTfttr^ frtWWf^ftiWtp**, .\ j^eto fiorfe g>tate CoIIese of Agriculture m Cornell Untbetiecitp 3tt)aca, iBt. S. ICifirarp Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924052310335 'Copyright, 1905 By FREDERIC E. CIvEMENTS and IRVING S. CUTTER All rights reserved ptcee of Jacob 'nortb & Companj I,iNCOi.N, Nebraska PREFACE The present volume is intended as a handbook for investigators and for advanced students of ecology, and not as a text-book of the subject. An elementary text-book covering the same field, but adapted to the needs of undergraduate students, is in preparation. The handbook is essentially an account of the methods used by the author in his studies of the last eight years, during which a serious attempt has been made to discover and to correlate the fundamental points of view in the vast field of vegetation. No endeavor is made to treat any portion of the subject exhaustively, since a discussion of general methods and general principles is of much greater value in the present condition of ecology. The somewhat unequal treat- ment given the different subjects is. due to the fact that it has been found possible to develop some of these more rapidly than others. Finally, it must be constantly kept in mind that ecology is still in a very plastic condition, and in consequence, methods, fiuidamental principles, and matters of nomen- clature and terminology must be approached without prejudice in order that the best possible development of this field may be attained. Grateful acknowledgment for criticisms and suggestions is made to Pro- fessor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound, who have read the text. The author is under especial obligations to Doctor Edith S. Clements for the drawings of leaf types, as well as for reading and crit- icising the manuscript. Professor Goodwin D. Swezey, Professor of As- tronomy in the University of Nebraska, has kindly furnished much material for the determination of the sun's altitude, and consequent light intensities, and has read the section devoted. to light. Mr. George A. Loveland, Di- rector of the Nebraska Section of the U. S. Weather Bureau, has contributed !Tiany helpful suggestions to the discussion of rheteorological instruments. To Nella Schlesinger, Alice Venters, and George L. Fawcett, advanced stu- dents in experimental ecology, the author is indebted for many experiments which have been used in the discussion of adjustment and adaptation. Acknowledgment is also made to the following for various cuts : Henry J. Green, Brooklyn, New York ; Julien P. Friez, Baltimore, Maryland ; C. H. Stocking Co., Chicago, Illinois ; Draper Manufacturing Co., New York city ; Gundlach-Manhattan Optical Co., Rochester, New York; Rochester Optical Co., Rochester, New York; Bausch and Lomb Optical Co., Rochester, New York. FREDERIC EDWARD CLEMENTS. The University of Nebraska, May, 1905. CONTENTS Chapter I. The Foundation of Ecology THE NEED OF A SYSTEM 1. The scope of ecology 2. Ecology and physiology Historical Development 3. Geographical distribution 4. The plant formation 5. Plant succession 6. Ecological phytogeography 7. Experimental ecology 8. Ecology of the habitat 9. The evidence from historical development Present Status of Ecology 10. The lack of special training 11. Descriptive ecology 12. The value of floristic 13. Reconnaissance and investigation 14. Resident investigation 15. The dangers of a restricted field Applications of Ecology 16. The subjects touched by ecology 17. Physiology and pathology 18. Experimental evolution 19. Taxonomy .... 20. Forestry 21. Physiography 22. Soil physics 23. Zoogeography 24. Sociology ■ . THE ESSENTIALS OF A SYSTEM 25. Cause and effect: habitat and plant . 26. The place of function PAGE I I 2 2 3 4 4 5 6 6 7 8 8 9 9 10 II II 12 14 15 15 15 16 16 17 VI CONTENTS Chapter II. The Habitat CONCEPT AND ANALYSIS 27. Definition of the habitat 28. Factors ..... Classification of Factors 29. The nature of factors 30. The influence of factors Determination of Factors 31. The need of exact measurement 32. The value of meteorological methods 33. Habitat determination 34. Determinable and efficient differences Instrumentation 35. Methods 36. Method of simple instruments 37. Method of automatic instruments 38. Combined methods . . . . CONSTRUCTION AND USE OF INSTRUMENTS 39. The selection of instruments Water-content 40. Value of different instruments Geotome methods 41. The geotome Soil borers Taking samples of soil Weighing Computation • . - Time and location of readings 42. 43- 44. 45- 46. 47- 48. 49. Location of readings Depth of samples Check and control instruments Physical and Physiological , Water 50. The availability of soil water 51. Terms 52. Chresard determination ynder control 53. Chresard readings in the field ■. 54. Chresard values of different soils CONTENTS Vil Records and Results page 55. The field record 35 56. The pernianent record 36 57. Sums and means 36 58. Curves 37 Humidity 59. Instruments 37 Psychfometers 60. Kinds 37 61. The sling psychrometer 38 62. Readings 39 63. Cog psychrometer 39 64. Construction and use 40 65. Hygrometers 40 Psychrographs 66. The Draper psychrograph 41 67. Placing the instrument 42 68. Regulating and operating the instrument ... 43 69. The weekly visit 44 Humidity Readings and Records 70. The time of readings 44 71. Place and height 45 72. Check instruments 45 y2)- Humidity tables 46 74. Sums, means, and curves 47 Conversion scale for temperatures 75. Records 48 Light 76. Methods • 48. The Photometer yy. Construction 49 78. Filling the photometer 50 79. Making readings 50 80. The Dawson-Lander sun recorder 51 81. The sciagraph 52 Standards 82. Use 53 83. Making a standard 53 84. Kinds of standards 54 Readings 85. Time / . . . . 55 Chart for determining sun's altitude .... 57 Vlll CONTENTS 86. Table for determining apparent noon 87. Place Table of intensity at various angles Reflected and Absorbed Light 88. The fate of incident light 89. Methods of determination 90. Leaf and epidermis prints Expression of Results 91. Light records .... 92. Light sums, means and curves Temperature 93 Thermometers 94. Air thermometers 95. Soil thermometers 96. Maximum-minimum thermometers 97. Radiation thermometers 98. Thermographs . Readings 99. Time . , . . 100. Place and height Expression of Results loi. Temperature records 102. Temperature sums and means 103. Temperature curves 104. Plant temperatures Precipitation 105. General relations 106. The rain gauge 107. Precipitation records Wind 108. Value of readings 109. The anemometer no. Records Soil III. 112. 113- 114. "5- Soil as a factor The value of soil surveys The origin of soils The structure of soils Mechanical analysis CONTENTS ii6. Kinds of soils 117. The chemical nature of soils Physiography 118. Factors .... Altitude 119. Analysis into factors 120. The barometer Slope 121. Concept 122. The clinometer 123. The trechometer Exposure 124. Exposure 125. Surface 126. Record of physiographic factors 127. Topography Biotic Factors 128. Influence and importance 129. Animals 130. Plants PAGE 79 8q 80 81 82 83 83 84 85 85 86 86 86 87 87 METHODS OF I-IABITAT INVESTIGATION 131 88 Method of Simple Instruments 132. Choice of stations . 88 133. Time of readings 89 134.. Details of the method 89 135. Records 91 Method of Ecograph Batteries 136 92 Expression of Physical Factor Results 137. The form of results 94 Factor Records 138. 94 Factor Curves 139. Plotting 95 140. Kinds of curves 96 141. Combinations of curves ....... 96 142. The amplitude of curves 98 Factor Means and Sums 143 98 CONTENTS Chapter III. The Plant STIMULUS AND RESPONSE General Relations 144. The nature of stimuli 145. The kinds of stimuli . 146. The nature of response 147. Adjustment and adaptation 148. The measurement of response 149. Plasticity and fixity . 150. The law of extremes . 151. The method of working hypotheses Hydroharmose Adjustment 152. Water as a stimulus ..... 153. The influence of other factors upon water 154. Response ...... 155. The measurement of absorption . 156. The quantitative relation of absorption and transpiration 157. Measurement of transpiration 158. Field methods 159. Expression of results .... 160. Coefficient of transpiration Adaptation 161. Modifications due to water stimuli 162. Modifications due to a small water supply 163. The decrease of water loss . "164. The increase of water supply • 165. Modifications due to an excessive water supply 166. Plant types . . . > 167. Xerophytic types 168. Types of leaf xerophytes . 169. Types of stem xerophytes . 170. Bog plants .... 171. Hydrophytic types Photoharmose Adjustment 172. Light as a stimulus 173. The reception of light stimuli 174. The response of the chloroplast 175. Aeration and translocation . PAGE 100 100 loi 102 103 104 105 106 107 107 108 109 III 113 114 116 117 118 118 118 121 121 122 122 123 125 126 127 129 131 132 134 CONTENTS 3?1 176. The measurement of responses to light Adaptation 177. Influence of chloroplasts upon form and structure 178. Form of leaves and stems 179. Modification of the epidermis .... 180. The differentiation of the chlorenchym 181. Types of leaves ....... 182. Heliophytes and sciophytes .... EXPERIMENTAL EVOLUTION 183. Scope 184. Fundamental lines of inquiry 185. Ancestral form and structure 186. Variation and mutation 187. Methods Method of Natural Experiment 188. Selection of species . 189. Determination of factors 190. Method of record Metiiod of Habitat Cultures 191. Scope and advantages 192. Methods 193. Transfer 194. Modification of the habitat Method of Control Cultures 195. Scope and procedure . 196. Water-content series . 197. Light series PAGE 135 138 139 140 142 144 144 145 146 146 147 149 149 151 152 153 153 154 156 157 158 160 Chapter IV. The Formation METHODS OF INVESTIGATION AND RECOKDi 198. The need of exact methods 161 Quadrats 199. Uses 161 200. Possible objections 163 Kinds of Quadrats and Their Use 201. Size and kinds 164 202. Tapes and stakes ........ 164 203. Locating quadrats 165 Xll CONTENTS The List Quadrat page 204. Description 165 205. Manner of use 166 206. Table of abundance 166 The Chart Quadrat 207. Description and use . . ..... 167 208. The chart 168 209. Mapping . . . 168 210. Factors and photographs 170 The Permanent Quadrat 211. Description and uses 170 212. Manner of use 172 The Denuded Quadrat 213. Description 173 214. Methods of denuding and recording .... 174 215. Physical factors 175 Aquatic Quadrats 216. Scope 175 Transects 217. The transect 176 The Line Transect 218. Description and method 176 219. The location and size I77 The Belt Transect 220. Details 178 The Permanent Transect 221. Advantages i79 222. Details I79 The Denuded Transect 223. . . ......... 180 The Layer Transect 224 180 Ecotone Charts 225 181 The Migration Circle 226. Purpose 182 227. Location and method 182 228. The denuded circle 183 229. Photographs 183 Cartography 230. Value of cartographic methods 183 CONTENTS XIU 231. Standard scale 232. Color scheme 233. Formation and vegetation maps 234. Continental maps Photography 235- 236. 237- 238. 239- 240. 241. 242. Formation 243- 244. 245- 246. The camera and its accessories The choice of a camera The use of the camera The sequence of details The time of exposure . Developing .... Finishing and Succession Herbaria Concept and purpose . Details of collecting Arrangement Succession herbaria . PAGE 184 184 185 187 188 188 190 191 192 193 194 19s 196 197 197 198 DEVELOPIiIENT AND STRUCTURE 247. Vegetation an organism 199 24S. Vegetation essentially dynamic 199 249. Functions and structures 199 Association 250. Concept 200 251. Causes 201 252. Aggregation 203 Kinds of Association 253. Categories 204 254. Stratum association 204 255. Ground association 205 256. Species guild association 206 257. Light association 206 258. Water-content association 208 THE DEVELOPMENT OF THE FORMATION 259 210 Invasion 260 • 210 Migration 261 210 XIV CONTENTS 262. Mobility .... 263. Organs for dissemination Contrivances for dissemination Position of disseminules Seed production . Agents of migration 264 265, 266. 267, 268. The direction of migration Ecesis 269. Concept 270. Germination of the seed 271. Adjustment to the habitat Barriers 272. Concept 273. Physical barriers 274. Biological barriers 275. Influence of barriers Endemism 276. Concept 277. Causes 278. Significance Polyphylesis and Polygenesis 279. Concept 280. Proofs of polygenesis 281. Origin by polyphylesis Kinds of Invasion 282." Continuous and intermittent invasion 283. Complete and partial invasion 284. Permanent and temporary invasion Manner of Invasion 285. Entrance into the habitat 286. Influence of levels Investigation of Invasion 287. Succession 288. Concept .... 289. Kinds of succession . Primary Successions 290 291. Succession through, elevation 292. Succession through volcanic action 293. Weathering .... CONTENTS XV 294. Succession in residuary soils 295. Succession in colluvial soils 296. Succession in alluvial soils 297. Succession in aeolian soils 298. Succession in glacial soils Secondary Successions 299 300. Succession in eroded soils 301. Succession in flooded soils 302. Succession by subsidence 303. Succession in land slips 304. Succession in drained or dried soils 305. Succession by animal agency 306. Succession by human agency 307. Succession in burned areas . 308. Succession in lumbered areas 309. Succession by cultivation . 310. Succession by drainage 311. Succession by irrigation 312. Anomalous successions 313. Perfect and imperfect successions 314. Stabilization .... Causes and Reactions 315 316. Succession by preventing v^^eathering 317. Succession by binding aeolian soils 318. Succession by reducing run-off and erosion 319. Succession by filling with silt and plant remains 320. Succession by enriching the soil . 321. Succession by exhausting the .soil- 322. Succession by the accumulation of humus . 323. Succession by modifying atmospheric factors Laws of Succession 324 Classification and Nomenclature 325. Basis 326., Nomenclature 327. Illustrations Investigation of Succession 328. General rules 329. Method of alternating stages PAGE 243 244 245 246 247 247 247 248 249 249 249 250 250 251 252 253 253 253 254 254 25s 256 257 258 259 260 261 262 263 264 264 267 267 270 270 271 XVI CONTENTS PAGE 330. The relict method 272 THE STRUCTURE OF THE FORMATION 331 274 Zonation 332. Concept 274 Causes of Zonation 333. Growth 275 334. Reactions 276 335. Physical factors . : 276 336. Physiographic symmetry 278 Kinds of Zonation 337 279 338. Radial zonation 280 339. Bilateral zonation 280 340. Vertical zonation 280 341. Vegetation zones 281 Alternation 342. Concept 283 343. Causes 284 344. Competition ........ 285 345. Kinds of alternation 289 The Formation in Detail 346. The rank of the formation ....... 292 347. The parts of a formation 295 34S. Nomenclature of the divisions ...... 299 349. The investigation of a particular formation . . . 299 Classification and Relationship 350. Basis 300 351. Habitat classification 301 352. Nomenclature 302 353. Developmental classification 304 354. Regional classification 304 355. Mixed formations 304 EXPERIMENTAL VEGETATION 356. Scope and methods 306 Method of Natural Habitats 357. Natural experiments , 307 Method of Artificial Habitats 358. Modification of habitat . 307 CONTENTS XVU PAGE 359. Denuding 308 360. Modification of the formation by transfer .... 309 Method of Control Habitats 361. Competition cultures 310 362. Details of culture methods 311 Glossary . 314 Bibliography ' 324 RESEARCH METHODS IN ECOLOGY CHAPTER I. THE FOUIs[DATION OF ECOLOGY The Need of a System 1. The scope of ecology. The clue to the field of ecology is found in the Greek word, oTkos, home. The point of view in the following treatise is constantly that which is inherent in the term itself. Ecology is therefore considered the dominant theme in the study of plants, indeed, as the central aJid vital part of botany. This statement may at first appear startling, if not unfounded, but mature reflection will show that all the questions of botanical science lead sooner or later to the two ultimate facts: plant and habitat. The essential truth of this has been much obscured by detached methods of study in physiology, morphology, and histology, which are too often treated as independent fields. These have suffered incomplete and unsymmetric development in consequence of extreme specialistic tendencies. Analytic methods have dominated research to the exclusion of synthetic ones, which, in a greatly diversified field, must be final, if botanical knowl- edge is something to be systematized and not merely catalogued. Physiol- ogy in particular has suffered at the hands of detached specialists. Orig- inally conceived as an inquiry into the origin and nature of plants, it has been developed strictly as a study of plant activities. It all but ignores the physical factors that control function, and the organs and tissues that reflect it. 2. Ecology and physiology. There can be little question in regard to the essential identity of physiology and ecology. This is evident when it is clearly seen that the present difference between the two fields is superficial. Ecology has been largely the descriptive study of vegetation ; physiology has concerned itself with function; but, when carefully analyzed, both are seen to rest upon the same foundation. In each, the development is incom- plete : ecology has so far been merely superficial, physiology too highly spe- cialized. The one is chaotic and unsystematized, the other too often a minute study of function under abnormal circumstances. The greatest need of the former is the introduction of method and system, of the latter, a broadening of scope and new objectives. The growing recognition of the identity of the two makes it desirable to anticipate their final merging, and 2 THE FOUNDATION OF ECOLOGY to formulate a system that will combine the good in each, and at the same time eliminate superficial and extreme tendencies. In this connection, it becomes necessary to point out to ecologist and physiologist alike that, while they have been working on the confines of the same great field, each must familiarize himself with the work and methods of the other, before his preparation is complete. Both must broaden their horizons, and rearrange their views. The ecologist is sadly in need of the more intirnate and exact methods of the physiologist: the latter must take his experiments into the field, and must recognize more fully that function is but the middleman be- tween habitat and plant. It seems probable that the final name for the whole field will be physiology, although the term ecology has distinct advantages of brevity and of meaning. In this event, however, it should be clearly recognized that, although the name remains the same, the field has become greatly broadened by new viewpoints and new methods. HISTORICAL DEVELOPMENT 3. Geographical distribution. The systematic analysis of the great field of 'ecology is essential to its proper development in the future. A glance at its history shows that, while a number of essential points of attack have been discovered, only one or two of these have been organized, and that there is still an almost entire lack of correlation and coordination between these. The earliest and simplest development of the subject was concerned with the distribution of plants. This was at first merely an off-shoot of taxonomy, and, in spite of the work of Humboldt and Schouw, has per- sisted in much of its primitive form to the present time, where it is repre- sented by innumerable lists and catalogues. Geographical distribution was grounded upon the species, a fact which early caused it to become stereo- typed as a statistical study of little value. This tendency was emphasized by the general practice of determining distribution for more or less arti- ficial areas, and of instituting comparisons between -regions or continents too little known or too widely remote. The fixed character of the subject is conclusively shown by the fact that it still persists in almost the original form more than a half century after Grisebach pointed out that the forma- tion was the real unit of vegetation, and hence of distribution. 4. Tlie plant formation. The corner-stone of ecology was laid by Grise- bach in 1838 by his recognition of the plant formation as the fundamental feature of vegetation. Earlier writers, notably Linne (1737, 1751), Biberg (1749), and Hedenberg (1754), had perceived this relation more or less clearly, but failed to reduce it to a definite guiding principle. This was a natural result of the dominance of descriptive botany in the i8th century, HISTORICAL DEVELOPMENT 3 by virtue of which all other lines of botanical inquiry languished. This tendency had spent itself to a certain degree by the opening of the 19th century, and both plant distribution and plant physiology began to take form. The stimulus given the former by Humboldt (1807) turned the attention of botanists more critically to the study of vegetation as a field in itself, and the growing feeling for structure in the latter led to Grise- bach's concept of the formation, which he defined as follows : "I would term a group of plants which bears a definite physiognomic character, such as a meadow, a forest, etc., a phytogeographic formation. The latter may be characterized by a single social species, by a complex of dominant species belonging to one family, or, finally, it may show an aggregate of species, which, though of various taxonomic character, have a common peculiarity; thus, the alpine meadows consist almost exclusively of perennial herbs." The acceptance of the formation as the unit of vegetation took place slowly, but as a result of the work 'of Kerner (1863), Grisebach (1872), Engler (1879), Hult (1881, 1885), Goeze (18S2), Beck (1884), Drude (1889), and Warming (1889), this point of view came to be more and more preva- lent. It was not, however, until the appearance of three works of great importance, Warming (1895), Drude (1896), and Schimper (1898), that the concept of the formation became generally predominant. With the growing recognition of the formation during the last decade has appeared the inevitable tendency to stereotype the subject of ecology in this stage. The present need, in consequence, is to show very clearly that the idea of the formation is a fundamental, and not an ultimate one, and that the proper superstructure of ecology is yet to be reared upon this as the foundation. 5. Plant succession. The fact that formations arise and disappear was perceived by Biberg as early as 1749, but it received slight attention until Steenstrup's study of the succession in the forests of Zealand (1844 prox.). In the development of formations, as well as in their recognition, nearly all workers have confined themselves to the investigation of particular changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming (1S90), and others have added much to our detailed knowledge of formational development. Notwithstanding the lapse of more than a half century, the study of plant successions is by no means a general practice among ecologists. This is a ready explanation of the fact that the vast field has so far yielded but few generalizations. Warming (1895) was the first to compile the few general principles of de- velopment clearly indicated up to this time. The first critical attempt to systematize the investigation of succession was made by Clements ( 1904) , . though this can be considered as little more than a beginning on account of 4 THE FOUNDATION OF FXOLOGY the small number of successions so far studied. Future progress in this field will be conditioned not only by the more frequent study of develop- ir.ental problems by working ecologists, but also, and most especially, by the application of known principles of succession, and by the working out of new ones. 6. Ecological phytogeography. Until recent years, the almost universal tendency was to give attention to formations from the standpoint of vegeta- tion alone. While the habitat was touched here and there by isolated work- ers, and plant functions were being studied intensiveily by physiologists, both were practically ignored by ecologists as a class. The appearance of Warming's Lehrbuch der oecologischen PHanzengeo graphic (1896) and of Schimper's Pflanzengeographie auf fhysiologischer Grundlage (1898) rem- edied this condition in a measure by a general discussion of the habitat, and by emphasizing the importance of the ecological or physiological point of view. Despite their frank recognition of the unique value of the habitat, the major part of both books was necessarily given to what may be termed the general description of formations. For this reason, and for others arising out of an almost complete dearth of methods of investigation, ecology is still al- most entirely a floristic study in practice, although there is a universal recog- nition of the much greater value of the viewpoint which rests upon the relation between the formation and its habitat. 7. Experimental ecology. Properly speaking, the experimental study of ecology dates from Bonnier^ (1890, 1895), though.it is well understood that experimental adjustment of plants to certain physical factors had been the subject of investigation before this time. The chief merit of Bonnier's work, however, lies in the fact that it was done out of doors, under natural conditions, and for these reasons it should be regarded as the real begin- ning of this subject. Bonnier's experiments were made for the purpose of determining the effect of altitude. Culture plots of certain species were located in the Alps and the Pyrenees, and the results were compared with control cultures made in the lowlands about Paris. In 1894 he also made ' Bonnier, G. Les Plantes Arctiques Compar^es aux MSmes Espfeces des Alps et des Pyrfodes. Rev. Gen. Boti 6:505. 1894. Cultures Experimentales dans les Alps et les Pyr€ndes. Rev. Gen. Bot. 2:514. 1890. Recherches Experimentales sur I'Adaptation des Plantes au Climat Alpin. Ann. Nat. Sci. 7:20:218. 1895. Bonnier, G., et Ch. Flahault Modifications des v^g^taux sur I'influence des conditions physiques du milieu. Ann. Nat. Sci. 6:7:93. 1878. HISTORICAL DEVELOPMENT 5 a comparative study of certain polydemic species common to the arctic islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these meth- ods are fundamental to field experiment, but the results are inconclusive, inasmuch as altitude is a complex of factors. As no careful study was made of the latter, it was manifestly impossible to refer changes and dif- ferences of structure to the definite cause. In a paper that has just ap- peared, E. S. Clements (1905) has applied the method of polydemic com- parison to nearly a hundred species of the Rocky mountains. In this work, the all-important advance has been made of determining accurately the de- cisive differences between the two or more habitats of the same species in terms of direct factors, water-content, humidity, and light. In his own investigations of Colorado mountain vegetation, the author has applied the method of field cultures by planting seeds of somewhat plastic species in habitats of measured value, and has thought to initiate a new line of re- search by applying experimental methods to the study of vegetation as an organism. In connection with this, there has also been developed a method of control experiment in the plant house under definitely measured differ- ences of water and light. 8. Ecology of the habitat. Since the -time of Humboldt, there have been desultory attempts to determine the physical factors of habitats with some degree of accuracy. The first real achievement in this line was in the measurement of light values by Wiesner in 1896. In 1898 the writer first began to study the structure of habitats by the determination of water- content, light, humidity, temperature, wind, etc., by means of instruments. These methods were used by one of his pupils, Thornber (1901), in the study of a particular formation, and by another, Hedgcock (1902), in a critical investigation of water-content. Two years later, similar methods of measuring physical factors were put into operation in connection with experimental evolution under control in the plant house. E. S. Clements (1905), as already indicated, has made the use of factor instruments the foundation of a detailed study of polydemic species, i. e., those which grow in two or more habitats, and which are, indeed, the most perfect of all ex- periments in the production of new forms. In a volume in preparation upon the mountain vegetation of Colorado, the writer has brought the use of physical factor instruments to a logical conclusion, and has made the study of the habitat the basis of the whole work. Out of this investigation has come a new concept of vegetation (Clements 1904), namely, that it is to be regarded as a complex organism with structures and with functions susceptible of exact methods of study. O THE FOUNDATION OF ECOLOGY 9. The evidence from historical development. This extremely brief resume of what has been accomplished in the several lines of ecological research makes evident the almost complete absence of correlation and of system. The whole field not merely lacks system, but it also demands a much keener perception of the relative value of the different tendencies already developed. It is inevitable from the great number of tyros, and of dilettante students of ecology in comparison with the few specialists, that the surface of the field should have received all of the attention. It is, however, both unfortunate and unscientific that great lines of development should be entirely unknown to all but a few. There is no other department of botany in which the superficial study of more than half a century ago still prevails to the exclusion of better methods, many of which have been known for a decade or more. It is clear, then, that the imperative need of ecology is the proper coordination of its various points of view, and the working out of a definite system which will make possible a ready recogni- tion of that which is fundamental and of that which is merely collateral. The historical development, as is well understood, can furnish but a slight clue to this. It is a fact of common knowledge that the first development of any subject is general,, and usually superficial also. True values come out clearly only after the whole field has been surveyed. For these reasons, as will be pointed out in detail later, the newer viewpoints are regarded as either the most important or the most fundamental. Experimental ecology will throw a flood of light upon plant structure and function, while exact methods of studying the habitat are practically certain of universal appli- cation in the future. PRESENT STATUS OF ECOLOGY 10. The lack of special training. The bane of the recent development popularly known as ecology has been a widespread feeling that anyone can do ecological work, regardless of preparation. There is nothing in modem botany' more erroneous than this feeling. The whole task of ecology is to find out what the living plant and the living formation are doing and have done in response to definite complexes of 'factors, i. e., habitats. In this sense, ecology is practically coextensive with botany, and the student of a local flora who knows a few hundred species is no more competent to do ecological work than he is to reconstruct the phylogeny of the vegetable kingdom, or to explain the transmission of ancestral qualities. The com- prehensive and fundamental character of the subject makes a broad special training even more requisite than in more restricted lines of botanical in- quiry. The ecologist must first of all be a botanist, not a mere cataloguer of plants, and he must also possess a particular training in the special meth- PRESENT STATUS 7 ods of ecological research. He must be familiar with the various points of attack in this field, and he must know the history of his subject thoroughly. Ecology affords the most striking example of the prevalent evil of Ameri- can botanical study, i. e., an indifference to, or an ignorance of the literature of the subject. The trouble is much aggravated here, however, by the breadth of the field, and the common assumption that a special training is unnecessary, if not, indeed, superfluous. Ignorance of the important eco- logical literature has been a most fertile source of crude and superficial studies, a condition that will become more apparent as these fields are worked again by carefully trained investigators. 11. Descriptive ecology. The stage of development of the subject at the present time may be designated as descriptive ecology, for purposes of dis- cussion merely. This is concerned with the superficial description of vege- tation in general terms, and results from the fact that the development has begun on the surface, and has scarcely penetrated beneath it. The organic connection between ecology and floristic has produced an erroneous impres- sion as to the relative value of the two. Floristic has required little knowl- edge, and less preparation : it lends itself with insidious ease to chance jour- neys or to vacation trips, the fruits of which are found in vague descriptive articles, and in the multiplication of fictitious formations. While there is good reason that a record should be left of any serious reconnaissance, even though it be of a few weeks' duration, the resulting lists and descriptive articles can have only the most rudimentary value, and it is absurd to regard them as ecological contributions at all. No statement admits of stronger emphasis, and there is none that should be taken more closely to heart by botanists who have supposed that they were doing ecological work. An almost equally fertile source of valueless work is the independent treatment of a restricted local area. The great readiness with which floristic lists and descriptions can be made has led many a botanist, working in a small area, or passing hurriedly through an extended region, to try his hand at formation making. From this practice have resulted scores of so-called formations, which are mere patches, consocies, or stages in development, or which have, indeed, no existence other than in the minds of their dis- coverers. The misleading definiteness which a photograph seems to give a bit of vegetation has been responsible for a surplus of photographic for- mations, which have no counterparts in nature. Indispensable as the photo- graph is to any systematic record of vegetation, its use up to the present time has but too often served to bring it into disrepute. There has been a marked tendency to apply the current methods of descriptive botany to vegetation, and to jegard every slightly different piece of the floral covering o THE FOUNDATION OF ECOLOGY as a formation. No method can yield results -further from the truth. It is evident that the recognition and limitation of formations should be left abso- lutely to the broadly trained specialist, who has a thorough preparation by virtue of having acquainted himself carefully vi^ith the development and structure of typical formations over large areas. 12. The value of floristic. In what has been said above, there is no in- tent to decry the value of floristic. The skilled workman can spare the material. which he is fashioning as readily as the ecologist can work without an accurate knowledge of the genera and species which make up a particular vegetation. Some botanists whose knowledge of ecology is that of the study or the laboratory have maintained that it is possible to investigate vege- tation without knowing the plants which compose it. Ecology is to be wrought out in the field, however, and the field ecologist — none other, in- deed, should bear the name — understands that floristic alone can furnish the crude material which takes form under his hands. It is the absolute need of a thorough acquaintance with the flora of a region which makes it impossible for a traveler to obtain anything of real ecological value in his first journey through a country. As the very first step, he must gain at least a fair knowledge of the floristic, which will alone take the major part of one or more growing seasons. This information the student of a local flora already has at the tip of his tongue; in itself it can not constitute a contribution to ecology, but merely the basis for one. In this connection, moreover, it can not be used independently, but becomes of value only after an acquaintance with a wide field. Floristic study and floristic lists, then, are indispensable, but to be of real value their proper function must be clearly recognized. They do not constitute ecology. 13. Reconnaissance and investigation. In striving to indicate the true value and worth of ecological study, it becomes necessary to draw a definite line between what we may term reconnaissance and investigation. By the former is understood the preliminary survey of a region, extending over one or two years. The objects of such a survey are to obtain a compre- hensive general knowledge of the topography and vegetation of 'the region, and of its relation to the other regions about it. The chief purpose, how- ever, is to gain a good working acquaintance with the flora: a reconnais- sance to be of value must do this at all events. Certain general facts will inevitably appear during this process, but they will invariably need the con- firmation of future study. It would be an advantage to real ecology if reconnaissance were to confine itself entirely to the matter of making a careful floristic survey. Investigation begins when the inquiry is directed PRESENT STATUS 9 to the habitat, or to the development and structure of the formation which it bears, i. e., when it takes up the manifold problems of the oocos. Such a study must be based upon floristic, but the latter becomes a part of in- vestigation only in so far as it leads to it. Standing by itself, it is not ecological research : it is the preparation for it. This distinction deserves careful thought. The numerous recruits to ecology have turned their at- tention to what lay nearest to hand, with little question as to its value, or to where it might lead. The result has been to make reconnaissance far out- weigh investigation in amount, and to give it a value which properly be- longs to the latter. Furthermore, this mistaken conception has in many cases, without doubt, prevented its leading to valuable research work. 14. Resident investigation. Obviously, if reconnaissance is a superficial survey, and investigation thorough extensive study, an important distinction between them is in the time required. While one may well be the result of a journey of some duration, the other is essentially dependent upon resi- dence. In the past the great disparity between the size of the field and the number of workers has made resident study too often an ideal, but in the future it will be increasingly the case that a particular region will be worked by a trained ecologist resident in it. This may never be altogether true of inaccessible and' sterile portions of the globe. It -may be pointed out, how- ever, that, between the tropics and the poles, residence during the summer or growing period is in essence continuous residence. In the ultimate an- alysis, winter conditions have of course some influence upon the develop- ment of vegetation during the summer, but the important problems which a vegetation presents must be worked out during the period of development. For temperate, arctic, and alpine regions, then, repeated study during the growing period for a term of years has practically all the advantages of continuous residence. For all practical purposes, it is resident study. 15. Tlie dangers of a restricted field.. In the resident study of a par- ticular region, the temptation to make an intensive investigation of a cir- cumscribed area is very strong. The limits imposed by distance are alone a sufficient explanation of this, but it is greatly increased by the inclination toward detailed study for which a small field offers opportunity. This .temptation can be overcome only by a general preliminary study of the larger region in which the particular field is located. The broader outlook gained in this way will throw needed light upon many obscure facts of the latter, and at the same time it will act as a necessary corrective of the ten- dency to consider the problems of the local field in a detached manner, and to magnify the value of the distinctions made and the results obtained. ^O THE FOUNDATION OF ECOLOGY Such a general survey has the purpose and value of a reconnaissance, and is always the first step in the accurate and detailed investigation of the local area or formation. Each corrects the extreme tendency of the other, and thorough comprehensive work can be done only by combining the two methods. When the field of inquiry is a large area or covers a whole re- gion, the procedure should be essentially the same. A third stage must be added, however, in which a more careful survey is made of the entire field in the light of the thorough study of the local area. The writer's methods in the investigation of the Colorado vegetation illustrate this procedure. The summers of 1896, 1897, 1898 were devoted to reconnaissance; those of 1 899-1904 were given to detailed and comprehensive study by instrument and quadrat of a highly diversified, representative area less than 20 miles square, while the work of the final summer will be the application of the results obtained in this localized area to the region traversed from 1896-98. This is practically the application of methods of precision to an area of more than 100,000 square miles. It also serves to call attention to another point not properly appreciated as yet by those who would do ecological work. This is the need of taking up field problems as a result of serious fore- thought,, and not as a matter of accident or mere propinquity. A carefully matured plan of attack which contemplates an expenditure of time and energy for a number of years will yield results of value, no matter how much attention an area may have received. On the other hand, an aimless or hurried excursion into the least known or richest of regions will lead to nothing but a waste of time, especially upon the part of the ecologist, who must read the articles which result, if only for the purpose of making sure that there is nothing in them. APPLICATIONS OF ECOLOGY 16. The subjects touched by ecology. The applications of .ecological methods and results to other departments of botany, and to other fields of research are numerous. Many of these are both intimate and fundamental, and give promise of affording new and extremely fruitful points of view. It has already been indicated that ecology bears the closest of relations to morphology and histology on the one side, and to physiology on the other — that it is, indeed, nothing but a rational field physiology, which regards form and function as inseparable phenomena. The closeness with which it touches plant pathology follows directly from this, as pathology is nothing more than abnormal form and functioning. Experimental work in ecology is purely a study of evolution, and the facts of the latter are the materials with which taxonomy deals. Forestry has already been termed "applied APPLICATIONS 1 1 ecology" and in its scientific aspects, which are its foundation, it is precisely the ecology of woody plants, and of the vegetation which they constitute. Apart from botany, the physical side of ecology is largely a question of soil physics, and of physiography. On the other hand, vegetation is coming more and more to be regarded as a fundamental factor in zoogeography and in sociology. Furthermore, with respect to the latter, it will be pointed out below that the principles of asso'ciation which have been determined for plants, viz., invasion, succession, zonation, and alternation, apply with almost equal force to man. 17. Physiology and pathology. The effect of ecology in emphasizing the intrinsically close connection between physiology and morphology has al- ready been mentioned. Its influence in normalizing the former by forcing it into the field as the place for experiment, and by directing the chief at- tention to the plant as an organism rather than a complex of organs, is also rapidly coming to be felt. Ecology will doubtless exert a corrective in- fluence upon pathology in the near future. This is inevitable as the latter ceases to be the merely formal study of specific pathogenic organisms, and turns its attention to the cause of all abnormality, which is to be found in the habitat, whether this be physical, as when the water-content is low, or biotic, when a parasitic fungus is present. The relative ease with which specific diseases can be studied has helped to obscure the essential fact that the approach to pathology must be through physiology. Much indeed of the observational physiology of the laboratories has been pathology, and it will be impossible to draw a clear line between them until precise experi- ment in the habitat has come into vogue. 18. Experimental evolution. As a result of the extremely fragmentary character of the geological record, nothing is more absolute, than that there can be no positive knowledge of the exact origin of a form or species, ex- cept in those rare cases of the present day, where the whole process has taken place under the eye of a trained observer. The origin of the plant forms known at present must forever lie without the domain of direct knowl- edge. If it were possible by a marvel of ingenuity and patience to develop experimentally Myosurus from Selaginella, this would not be absolutely conclusive proof that Myosurus was first derived in this way. When all is said, however, this would be the very best of presumptive evidence. It must also be recognized that this is the nearest to complete proof that we shall ever attain, and with this in mind it becomes apparent at once that evidence from experiment is of paramount importance in the study of evo- lution ('the origin of species). 12 THE FOUNDATION OF ECOLOGY The phase of experimental ecology which has to do with the plant has well been called experimental evolution. While this is a field almost wholly without development at present, there can be little question that it is to be one of the most fertile and important in the future. Attention will be di- rected first to those forms which are undergoing modification at the present time. The cause and direction of change will be ascertained, and its amount and rapidity measured by biometrical methods. The next step will be to actually change the habitat of representative types, and to determine for each the general trend of adaptation, as well as the exact details. By means of the methods used and the results obtained in these investigations, it will be possible to attack the much more difficult problem of retracing the devel- opment of species already definitely constituted. This will be accomplished by the study of the derived and the supposed ancestral form, but owing to the great preponderance of evolution over reversion, the study of the an- cestral form will yield much more valuable results. The general application of the methods of experimental ecology will mark a new era in the study of evolution. There has been a surplus of literary investigation, but altogether too little actual experiment. The great value of De Vries' work lies not in the importance of the results obtained, but in calling attention to the unique importance of experimental methods in con- tributing to a knowledge of evolution. The development of the latter has been greatly hindered by the dearth of actual facts, and by a marked ten- dency to compensate for this by verbiage and dogmatism. This is well illus- trated by the present position of the "mutation theory," which, so far as the evidence available is concerned, is merely a working hypothesis. An in- credible amount of bias and looseness of thought have characterized the discussion of evolution. It is earnestly to be hoped that the future will bring more work and less argument, and that the literary evolutionists will become less and less reluctant to leave the relative merits of variation and mutation to experiment. 19. Taxonomy. Taxonomy is classified evolution. It is distinct from descriptive botany, which is merely a cataloguing of all Icnown forms, with little regard to development and relationship. The consideration of the lat- ter is peculiarly the problem of taxonomy, but the solution must be sought through experimental evolution. The first task of the latter is to determine the course of modification in related forms, and the relationships existing between them. With this information, taxonomy can group forms accord- ing to their rank, i. e., their descent. The same method is applicable to the species of a genus, and, in a less degree, perhaps, to the genera which con- stitute a family. The use to which it may be put in indicating farnily re- APPLICATIONS 15 lationships will depend largely upon the gap existing between the families concerned. While interpretation will always play a part in taxonomy, the general use of experiment will leave much less opportunity for the personal equation than is at present the case. Taxonomy, like descriptive botany, is based upon the species, but, while there may exist a passable kind of de- scriptive botany, there can be no real taxonomy as long as the sole criterion of a species is the difference which any observer thinks he sees between one plant and another. The so-called- species of to-day range in value from mere variations to true species which are groups of great constancy and definiteness. The reasons for this are obvious when one recalls that "spe- cies" are still the product of the herbarium, not of the field, and that the more intensive the study, the greater the output in "species." It would seem that careful field study of a form for several seasons would be the first requisite for the making of a species, but it is a precaution which is entirely ignored in the vast majority of cases. The thought of subjecting forms presumed to be species to conclusive test by experiment has appar- ently not even occurred to descriptive botanists as yet. Notwithstanding, there can be no serious doubt that the existing practice of re-splitting hairs must come to an end sooner or later. The remedy will come from without through the application of experimental methods in the hands of the ecol- ogist, and the cataloguing of slight and unrelated differences will yield to an ordered taxonomy. Experimental evolution will solve a taxonomic problem as yet untouched, namely, the effect of recent environment upon the production of species. It is well understood that some species grow in nature in various habitats without suffering material change, while others are modified to constitute a new form in each habitat. It is at once clear that these forms (or ecads) are of more recent descent than the species, i. e., of lower rank. It must also be recog-nized that a constant group and a highly plastic one are essen- tially different. If constancy is made a necessary quality of a species, one is a species, the other is not. If both are species, then two different kinds must be distinguished. Among the species of our manuals are found many ecads, alongside of constant and inconstant species. These can be distin- guished only by field experiment, and their proper coordination is possible only after this has been done. Indeed, the whole question of the ability or the inability of environmental variation to produce constant species is one that must be referred to repeated and long-continued experiment in the field. A minor service of considerable value can be rendered taxonomy by working over the diagnosis from the ecological standpoint. Many ecological facts are of real diagnostic value, while others are at least of much interest, and serve to direct attention to the plant as a living thing. The loose use of 14 THE FOUNDATION OF ECOLOGY terms denoting abundance, which prevails in lists and manuals, should be replaced by the exact usage which the quadrat method has made possible for vegetation. The designation of habitats could be made much more exact, and the formation, as well as the habitat-form or ecad, and the vege- tation-form or phyad, should be indicated in addition. The general terms drawn from pollination, seed-production, and dissemination might also be included to advantage. 20. Forestry, if the purely commercial aspects be disregarded, is the ecology of a particular kind of vegetation, the forest. Therefore, in point- ing out the connection between them, it is only necessary to say that what- ever contributes to the ecology of the forest is a contribution to forestry. There are, however, certain lines of inquiry which are of fundamental im- portance. First among these, and of primary interest from the practical point of view, are the questions pertaining to the distribution of forests and their structure. Of even greater significance are the problems of forest development, movement, and of reforestation, which are comprised in succession. The gradual invasion of the plains and prairies by the forest belt of the east and north is in full conformity with the laws of invasion, and the ecological methods to be employed here serve not merely to de- termine the actual conditions at present, but also to forecast them with a great deal of accuracy. The slow but certain development of forests on new soils, and their more rapid re-establishment where the woody vegetation has been destroyed by burning or lumbering, are ordinary phenomena of suc- cession, for which the ecologist has already worked out the laws, and de- termined the methods of investigation. Having once ascertained the original and adjacent vegetation and the character of the habitat, the ecologist can indicate with accuracy not only the character of the new forest that will appear, but also the nature of the antecedent formations. A full knowledge of the character and laws of succession will prove of the greatest value to the forester in all studies of forestation and reforestation. Forests which now seem entirely unrelated will be seen to possess the most intimate de- velopmental connection, and the fuller insight into the life-history gained in this way will have a direct bearing upon methods of . conservation, etc. It will further show that the forester must know other vegetations as well, since grassland and thicket formations have an intimate influence upon the course of the succession, as well as upon the advance of a forest frontier. One of the greatest aids which modern ecology can furnish forestry, however, is the method of determining the physical nature of the habitat. So far, foresters have been obliged to content themselves with a more oi" less superficial study of the structure of forest formations, without being APPLICATIONS 15 able to do more than guess at the physical causes which control both struc- ture and development. This handicap is especially noticeable in the case of forest plantings in non-forested regions, where it has been impossible to estimate the chances of success, or to determine the most favorable areas except by actual plantations. Equipped with the proper instruments for measuring water-content, humidity, light and temperature, the ecologist is able to determine the precise conditions under which reproduction is occur- ring, and to ascertain what non-forested areas offer the most nearly similar conditions. A knowledge of habitats and the means of measuring them enables the forester to discover the causes which control the vegetation with which he is already familiar, and to forecast results otherwise hidden. Fur- thermore, it makes it possible for him to enter a new region and to deter- mine its nature and capabilities at a minimum of time and energy. 21. Physiography. Physiographic features play an important part in de- termining the quantity of certain direct factors of the habitat. Perhaps a more important connection between physiography and ecology is to be found in succession. The beginning of all primary, and of many secondary suc- cessions is to be sought in the physiographic processes which pr-vduce new habitats, 'Or modify old ones. On the other hand, most of the reactions which continue successions exert a direct influence upon the form of the land. The most pronounced influence of terrestrial successions is found in the stabilization which their ultimate stages exert upon land forms, even where these are highly immature. The chief effect of aquatic successions is to be found in the "silting up" and the formation of new land, which result from the action of vegetation upon silt-bearing waters. The closeness of the re- lation between succession and the forms of the land has led to the application of the term "physiographic ecology" to that part of the subject which deals with the development of vegetation, i. e., succession. 22. Soil physics. This subject is as mucii a part of ecology as is forestry. It is intrinsically that subdivision of ecology which deals with the edaphic factors of the habitat, and their relation to the plant. Since the basis is physics, there has been a general tendency to overvalue the determination of soil properties, and to ignore the fact that these are decisive only when considered with reference to the living plant. As the soil contains the water which is the factor of greatest importance to plants, soil physics is an especially important part of ecology. Its methods are discussed under the habitat. 23. Zoogeography. Since animals are free for the most part, and hence not confined so strictly to one spot as plants, their dependence upon the l6 THE FOUNDATION OF ECOLOGY habitat is not so evident. The relation is further obscured by the fact that no physical factor has the direct effect upon them which water or light exerts upon the plant. Vegetation, indeed, as the source of food and pro- tection, plays a more obvious, if not a more important part. This is especially true of anthophilous insects, but it also holds for all herbivorous animals, and, through them, for carnivorous ones. The animal ecology of a particular region can only be properly investigated after the habitats and plant formations have been carefully studied. Here, as in floristics, a great deal can be done in the way of listing the fauna, or studying the life habits of its species, without any knowledge of plant ecology; but an adequate study must be based upon a knowledge of the vegetation. Although animal formations are often poorly defined, there can be no doubt of their exist- ence. Frequently they coincide with plant formations, and then have very definite limits. They exhibit both development and structure, and are sub- ject to the laws of invasion, succession, zonation, and alternation, though these are not altogether similar to those known for plants, a fact readily explained by the motility of animals. Considered from the above point of view, zoogeography is a virgin field, and it promises great things to the student who approaches it with the proper training. 24. Sociology. In its fundamental aspects, sociology is the ecology of a particular species of animal, and has in consequence, a similar close con- nection with plant ecology. The widespread migration of man and his social nature have resulted in the production of groups or communities which have much more in common with plant formations than do formations of other animals. The laws of association apply with especial force to the family, tribe, community, etc., while the laws of succession are essen- tially the same for both plants and man. At first thought it might seem that man's ability to change his dwelling-place and to modify his environ- ment exempts him in large measure from the influence of the habitat. The exemption, however, is only apparent, as the control exerted by climate, soil, and physiography is all but absolute, particularly when man's depend- ence upon vegetation, both natural and cultural, is called to mind. The Es.sentials of a System 25. Cause and effect: habitat and plant. In seeking to lay the foundation for a broad and thorough system of 'ecological research, it is necessary to scan the whole field, and to discriminate carefully between what is funda- mental and what is merely collateral. The chief task is to discover, if possible, such a guiding principle as will furnish a basis for a permanent and logical superstructure. In ecology, the one relation which is precedent ESSENTIALS OF A SYSTEM l^ to all Others is the one that exists between the habitat and the plant. This relation has long been known, but its full value has yet to be appreciated. It is precisely the relation that exists between cause and effect, and its fun- damental importance lies in the fact that all questions concerning the plant lead back to it ultimately. Other relations are important, but no other is paramount, or able to serve as the basis of ecology. Ecology simis up this relation of cause and effect in a single word, and it may be- tftat this ad- vantage will finally cause its general acceptance as the proper name for this great field. In the further analysis of the connection between the habitat and the plant, it is evident that the causes or factors of the habitat act directly upon the plant as an individual, and at the same time upon plants as groups of in- dividuals. The latter in no wise decreases the importance of the plant as the primary effect of the habitat, but it gives form to research by makiug it possible to consider two great natural groups of phenomena, each character- ized by very different categories of effects. Ecology thus falls naturally into three great fundamental fields of inquiry : habitat, plant, and formation (or vegetation). To be sure, the last can be approached only through the plant, but as the latter is not an individual, but the unit of a complex from the formational standpoint, the formation itself may be regarded as a sort of multiple organism, which is in many ways at least a direct effect of the habitat. In emphasizing this fundamental relation of habitat and vegeta- tion, it is imperative not to ignore the fact that lieither plant nor formation is altogether the effect of its present habitat. A third element must always be considered, namely, the historical fact, by which is meant the ancestral structure. Upon analysis, however, this is in its turn found to be the product of antecedent habitats, and in consequence the essential connection between the habitat and the plant is seen to .be absolute. 26. The place of function. In the foregoing it is understood th^t the im.mediate effect of the^'physical factors of the habitat is to be found in the functions of the plant, and that these determine the plant structure. Func- tion has so long been the especial theme of plant physiology that methods of investigation are numerous and well known, and it is unnecessary here to consider it further than to indicate its general bearing. The essential sequence in .ecological, research, then, is the one already indicated, viz., habitat, plant, and formation, and this will constitute the order of treatment in the following pages. That portion of fioristic which is not mere de- scriptive botany belongs to the consideration of the formation, and in con- sequence there will be no special treatment of floristic as a subdivision of ecology. CHAPTER II. THE HABITAT Concept and Analysis 27. Definition of tlie liabitat. The habitat is the sum of all the forces or factors present in a given area. It is the exact equivalent of the term en- vironment, though the latter is commonly used in a more general sense. As an ecological concept, the habitat refers to an area much more definite in character, and more sharply limited in extent than the habitat of species as indicated in the manuals. Since the careful study of habitats has scarcely begun, it is impossible to recognize and delimit them in an absolute sense. Visible topographic boundaries often exist, but in many cases, the limit, though actual, is not readily perceived. Contiguous habitats may be sharply limited, or they may pass into each other so gradually that no real line of demarcation can be drawn. Whatever variations they may show, however, all habitats agree in the possession of certain essential factors, which are universally present. On the other hand, a few factors are merely incidental and may be present or absent. The relative value and amount of these is probably similar for no two habitats, though the latter readily fall into groups with reference to the amount of some particular factor. 28. Factors. The factors of a habitat are water-content, humidity, light, temperature, soil, wind, precipitation, pressure, altitude, exposure, slope, surface (cover), and animals. To these should he added gravity and polarity, which are practically uniform for all habitats, and may, in consequence, be ignored in this treatise. Length of season, while it plays an important part in vegetation, is clearly a complex and is to be treated under its constituents. Of the factors given, all are regularlv found in each habitat, though some are not constantly present. The first live, water-content, humidity, light, temperature, and soil are the most important, and any one may well serve as a basis for grouping habitats into particular classes with reference to quantity. As will be pointed out later, however, water-content and light furnish the most striking differences between habitats, and offer the best means of classification. As habitats are inseparable from the formations which they bear, the discussion of the kinds of habitats is reserved for chapter IV. FACTORS 19 Classification of Factors 29. The nature of factors. The factors of a habitat are arranged in two groups according to their nature: (i) physical, (2) biotic. In the strict sense, the physical factors constitute the habitat proper, and are the real causative forces. No habitat escapes the influence of biotic factors, howr ever, as the formation always reacts upon it, and the influence of animals is usually felt in some measure. Physical factors are further grouped into (i) climatic and (2) edaphic, with respect to source, or, better, the medium in which they are found. Climatic, or atmospheric factqrs are humidity, Hght, temperature, wind, pressure, and precipitation. Axiomatically, the stimuli which they produce are especially related to the leaf. Edaphic or soil factors are confined to the 'Soil, as the term denotes, and are im- mediately concerned with the functions of the root. Water-content is by far the most important of these; the others are soil composition (nutrient- content), soil temperature, altitude, slope, exposure, and surface. The last four are of a more general character than the others, and are usually re- ferred to as physiographic factors. Cover, when dead, might well be placed among these also, but as it is little different from the living cover in effect, it seems most logical to refer it to biotic factors. 30. The influence of factors. While the above classification is both ob- vious and convenient, a more logical and . intimate grouping may be made upon the influence which the factor exerts. On this basis, factors are divided into (i) direct, (2) indirect, and (3) remote. Direct factors are those which act directly upon an important function of the plant and produce a formative effect: for example, an increase in humidity produces an im- mediate decrease in transpiration. They are water-content, humidity, and light. Other factors have a direct action: thus temperature has an im- mediate influence upon respiration and probably assimilation also, but it is not structurally formative. Wind has a direct mechanical effect upon woody plants, but it does not fall within our definition. Indirect factors are those that affect a formative function of the plant through another factor; thus a change in temperature causes a change in humidity and this in turn calls forth a change in transpiration; or, a change in soil texture increases the water-content, and this affects the imbibition of the root-hairs. Indirect factors, then, are temperature, wind, pressure, precipitation, arid soil compo- sition. Remote factors are, for the most part, physiographic and biotic: they require at least two other factors to act as middlemen. Altitude affects plants through pressure, which modifies humidity, and hence transpiration. Slope determines in large degree the run-off during a rain-storm, thus 20 THE HABITAT affecting water-content and the amount of water absorbed. Earthworms and plant parts change the texture of the soil, and thereby the water-content. Indirect factors often exert a remote influence -also, as may be seen in the effect which temperature and wind have in increasing evaporation from the soil, and thus reducing the water-content. This distinction between . factors may . seem insufficiently grounded. In this event, it should be noted that it centers the effects of all factors upon the three direct ones, water-content, humidity, and light. If it further be recalled that these are the only factors which produce qualitative structural changes, and that the classification of ecads and formations is based upon them, the validity of the distinction is clear. The Determination of Factors 31, The need of exact measurement. Any serious endeavor to find in the habitat those causes which are producing modification in the plant and in vegetation can not stop with the factors merely. The next step is to de- termine the quantity of each. It is not sufficient to hazard a guess at this, or to make a rough estimate of it. Habitats differ in all degrees, and it is impossible to institute comparisons between them without an exact measure of each factor. Similarly, one can not trace the adaptations of species to their proper causes unless the quantity of each factor is known. It is of little value to know the general effect of a factor, unless it is known to what degree this effect is exerted. For this purpose it becomes necessary to appeal to instruments, in order to determine the exact amount of each factor that is present in a particular habitat, and hence to determine the ratio between the stimulus and the amount of structural adjustment which results. The employment of instruments of precision is clearly indispen- sable for the task which we have set for ecology, and every student that intends to strike at the root of the subject, and to make lasting contributions to it, must familiarize himself with instrumental methods. One great benefit will accrue to ecology as soon as this fact is generally recognized. The use of instruments and the application of results obtained from them demand much patience and seriousness of purpose upon the part of the student. As a consequence, there will be a general exodus from ecology of those that have been attracted to it as the latest botanical fad, and have done so much to bring it into disrepute. 32. The value of meteorological methods. At the outset there must be a very clear understanding that weather records and readings have only a very general value. This is in spite of the fact that the instruments em- ployed are of . standard precision. An important reason for this lack of FACTORS 21 value is that readings are not made in a particular habitat ; as a rule, indeed, they are made in towns and cities, and hence are far removed from masses of vegetation. They are usually taken at considerable heights, and give but a general indication of the conditions at the level of vegetation. The chief difficulty, however, is that the factors observed at weather stations — tem- perature, pressure, wind, and precipitation — are those which have the least value for the ecologist. It is true that a knowledge of the temperature and rainfall of a great region will afford some idea of the general character of its vegetation. A proper understanding of such a vegetation is, however, to be gained only through the exact study of its component formations. Ecology has already incurred sufficient censure as a subject composed of very general ideas, and the use of meteorological data, which can never be connected definitely with anything in the plant or the formation, should be discontinued. This must not be understood to mean that meteorological in- struments can not be used in the proper place and manner, i. e., in the habitat. 33. Habitat determination. It is self-evident that determinations of factors by instruments can only be of value in the habitat where they are made. In other words, a habitat is a unit for purposes of measuring its factors, and measures of one habitat have no exact value in another. This fact can not be overstated. Thus,'while it is perfectly legitimate, and indeed highly desirable, to locate thermographs in different mountain zones for ascertaining the rate at which temperature decreases with altitude, the data obtained in this way are not directly applicable in explanation of plant or formation changes, except when the same species occurs at different al- titudes. Special methods are valuable and often absolutely necessary, but in view of the fact that the plant as well as the formation is the definite product of a definite habitat, the fundamental rule in instrumentation is that complete readings must be made within a habitat for that habitat alone. This necessarily presupposes a certain preliminary acquaintance with the habitat to be investigated, as it is imperative that the station for making readings be located well within the formation, in order to avoid transition conditions. In vegetation, there are as many habitats as formations, and in addition a large number of new and denuded habitats upon which suc- cessions have not yet started; a knowledge of each formation or succession must rest ultimately upon the factors of its particular habitat. 34. Determinable and efficient differences. The instruments employed in studying habitats can not be too exact, as there is no adequate knowledge as yet concerning the real differences which exist between related or con- tiguous formations. This is particularly true of differences which are 22 THE HABITAT efficient in producing a recognizable structural change in plant or formation. Investigations made by the writer have shown that standard instruments will measure differences of quantity quite too small to produce a visible re- action. Efficient differences are not the same for different factors, and perhaps also for the same factor when found in various combinations. They vary widely for different species, being in direct relation to the plasticity of the latter. The point necessary to bear in mind in formulating methods for habitat investigation and in making use of instruments is that standard in- struments should be used for the very reason that we do not yet know the relation between determinable and efficient differences. On the other hand, it is unnecessary to insist upon. absolute exactness as soon as it is found that the determinable difference lies well within the efficient one. This by no means indicates that instruments are not to be carefully standardized and frequently checked, or that accurate readings should not be made. It means that a slight margin of error may be permitted in a machine which registers well within the efficient difference for that factor, and that instru- ments that read to the last degree of nicety are not absolutely necessary. In the fundamental work of determining efficient differences, however, instru- ments can not have too great precision. Moreover, these differences must be based upon the most plastic species of a formation, and the readings must be made under normal conditions. Instrumentation 35. Methods. In the field use of instruments two methods have been de- veloped. The first in point of time was the method of simple instruments, devised especially for class work, and capable of being used only where a number of trained students are available. The method of automatic instru- ments was an immediate outgrowth of this, due to the necessity which con- fronts the solitary investigator of being in different habitats at the same time. In the gradual evolution of this subject, it has become possible to combine the two methods in such a way as to retain all the advantages of the automatic method, and most of those of the method of simple instruments. 36. Method of simple instruments. By simple instruments are denoted those that do not record, but must be read by the observer at the time. They are standard instruments of precision, but possess the disadvantage of requiring an observer for each one. They are well illustrated by the thermometers and psychrometers used by the Weather Bureau. In the hands of trained 'observers the results obtained are unimpeachable ; in fact, standard simple instruments must be constantly employed to check automatic INSTRUMENTS 23 ones. As physical factors vary greatly through the day and through the year, it is all-important that the readings in habitats which are being com- pared should be made at the same instant. This requires a number of ob- servers ; as many as twelve stations have been read at one time, and there is of course no limit to the number. It is very important, also, that observers be carefully trained in the handling of instruments, and in reading them ac- curately and intelligently at the proper moment. In practice it has been found impossible to do such work in elementary classes, and, even in using small advanced classes, prolonged drill has been necessary before trust- worthy results could be obtained. When a class has once been thoroughly tiained in making accurate simultaneous readings, there is practically no limit, other than that set by time, to the valuable work that can be done, both in instruction and investigation. 37. Method of automatic instruments. The solitary investigator must replace trained helpers by automatic instruments or ecographs. These have the very great advantages of giving continuous simultaneous records for long periods, and of having no personal equation. They must be regulated and checked, to be sure, but as this is all done by the same person, the error is negligible. There is nothing more satisfactory in resident investigation than a series of accurate recording instruments in various habitats. Eco- graphs have two disadvantages. The chief perhaps is cost. The expense of a single "battery" which will record light, water-content, humidity, and temperature is about $250. Another difficulty is that they can be used only within a few miles of the base, since they require attention every week for regulation, change of record, etc. While this means that ecographs in their present form are not adapted to reconnaissance, this is not a real dis- advantage, as the scattered observations possible on such a journey can best be made by simple instruments. 38. Combined methods. The best results by far are to be obtained by the combined use of simple and aiitomatic instruments. This is particularly true in research, but it applies also to class instruction. The ecographs afford a continuous, accurate basal record, to which a single reading made at any time or place can be readily referred for comparison. On the other hand, it is an easy matter to carry a full complement of simple instruments on the daily field trips, and to make accurate readings in a score or more of forma- tions in a single day. An isolated reading, especially of a climatic factor, has little or no value in itself, but when it can be compared with a reading made at the same time in the base station by an ecograph, it is the equivalent of an automatic reading. This method renders a set of simple instruments 24 THE HABITAT more desirable for a long trip or reconnaissance than a battery of automatic ones. It is practically impossible to carry the latter into the field, and in any event a continuous record is out of the question. As there are other tasks at such times also, it becomes evident that the taking of single readings which can be compared with a continuous record ofifers the most satisfactory solution. Construction and Use of Instruments 39. The selection of instruments. In selecting and devising instruments for the investigation of physical factors, emphasis has first been laid iipon accuracy. This is the result of a feeling that it is better to have instruments that read too minutely than those which do not make distinctions that are sufficiently close, particularly until more has been learned about efficient differences. On the other hand, no hesitation has been felt in employing in- struments which are not absolutely accurate, when it was clear that the error was less than the efficient difference. Similarly, the margin of error practically eliminates itself in the case of simultaneous comparative readings, when the instruments have been checked to the same standard. Simplicity of construction and operation are of great importance, especially in saving time where a large number of instruments are in operation. Expense is likewise to be carefully considered. It is impossible to have too many in- struments, bvit cost practically determines the number that can be obtained. It is further necessary to secure or invent both simple and automatic instru- ments for all factors, except such invariable ones as altitude,- slope, etc. Simple instruments must be of a kind that can be easily carried, and so con- structed that they can be used at a minimum of risk. The sling psychro- meter, for example, is very readily broken in field use, and it has been replaced by a protected modification, the rotating form. In describing the construction and operation of the many factor instru- ments, there has been no attempt to make the treatment exhaustive. Those instruments which the author has found of greatest value in his own work are given precedence, and the manner of using them is described in detail. Other instruments of value are also considered, though with greater brevity. Some of the most complex and expensive ones have been ignored, as it is altogether improbable that they can come into general use in their present form. While the conviction is felt that the methods described below will enable the most advanced investigators to carry on thorough work, it is hoped that they will be seen to be so fundamental, and so attractive, that they will appeal to all who are planning serious ecological study. INSTRUMENTS 25 WATER-CONTENT 40. Value of different instruments. The paramount importance of water- content as a direct factor in the modification of plant form and distribution gives a fundamental value to the methods used for its determination. Au- tomatic instruments for ascertaining the water in the soil are costly, in ad- dition to being complicated, and often inaccurate. For these reasons, much attention has been given to developing the simpler but more reliable methods in which a soil-borer or geotome is used. The latter is simple, inexpensive, and accurate. It can be carried easily upon daily trips or upon longer re- connaissances, and is always ready for instant use. In the determination of physiological water-con- tent, it is -practically indis- pensable. . Indeed, the readiness with which geo- tome determinations of water-content can be made should hasten the universal recognition of the fact that it is the available, and not the total amount of water in the soil, which deter- mines the effect upon the plant. Geotome Methods 41. The geotome. In its simplest form, the geotome is merely a stout iron tube with a sharp cutting edge at one end and a firmly attached handle at the Fig. 1. Geotomes and soil can. other. The length is variable and is primarily determined by the location of the active root surface of the plant. In xerophytic habitats, generally a longer tube is necessary than in mesophytic ones. The bore is largely determined by the character of the soil ; for example, a larger one is neces- sary for gravel than for loam. Tubes of small bore also tend to pack the soil below them, and to give a correspondingly incomplete core. The best results have been obtained with geotomes of J^-i inch tube. Each geotome 26 THE HABITAT has a removable rod, flattened into a disk at one end, and bent at the 'other, for forcing'out the core after it has been cut from the soil. Sets of geotomes have been made in lengths of 5, 10, 12, 15, 20, and 25 Inches. The 12- and iS-inch forms have been commonly used for herbaceous formations and layers. They are marked in inches so that a sample of any lesser depth may be readily taken. Such a device is very necessary for gravel soils and in mountain regions, where the subsoil of rock lies close to the surface. 42. Soil borers. There is a large variety of soil borers to choose from, but none have been found as simple and satisfactory for relatively shallow readings as the geotome just described. For deep-rooted plants, many xerophytes, shrubs, and trees, borers of the ^auger type are necessary. These are large and heavy, and of necessity slow in operation. They can not well be carried in an ordinary outfit of instruments, and the size of the soil sample itself precludes the use of such instruments far from the base station, except on trips made expressly for obtaining samples from deep-seated layers. For, depths from two to eight feet, the Fraenkel borer is per- haps the most satisfactory, except for the coarser gravels ; it costs $14 'or $20 according to the length. For greater depths, or when a larger core is desirable, the Bausch & Lomb borer, number 16536, which costs $5.25, should be made use of. This is a ponderous affair and can be employed only on special occasions. On account of the size of samples obtained by these borers, it is usually most satisfactory to take a small sample from the core at different depths. Frequently, indeed, a hand trowel may be readily used to obtain a good sample at a particular depth. X Fig. 2. Fraenkel soil borer. Fig. 3. Ameri- can soil borer. 43. Taking samples of soil. In obtaining soil samples, the usual practice is to remove the air-dried surface, noting its depth, and to sink the geotome with a slow, gentle, boring movement, in order to avoid packing the soil. This difficulty is further obviated by deep notches with sharp, beveled edges which are cut at the lower end. In obtaining a fifteen-inch core, there is also less compression if it be cut five inches at a time. Repeated tests have shown, however, that the single compressed sample is practically as trust- worthy as the one made in sections. The water-content of the former constantly fell within .5 per cent of that of the latter, and both varied less WATER-CONTENT 27 than I per cent from the dug sample used as a check. As soon as dug, the core is pressed out of the geotome by the plunger directly into an air-tight soil can, Bottles may be used as containers, but tin cans are lighter and more durable. Aluminum cans have been devised for this purpose, but on account of the expense, "Antikamnia" cans have been used instead. These are tested, and those that are not water-tight are rejected, although it has been found that, even in these, ordinary soils do not lose an appreciable amount of water in twenty-four hours. The lid should be screwed on as quickly as possible, and, as an added precaution, the cans are kept in a close case until they have been weighed. The cans are numbered consecutively on both lid and side in such a way that the number may be read at a glance. The numbers are painted, as a label wears off too rapidly, and scratched numbers are not quickly discerned. 44. Weighing, Al- though soil samples have -been kept in tight cans outside of cases for several days without losing a milligram of moisture, the safest plan is to make it a rule to weigh cans as quickly as possible after bring- ing them in from the field. Moreover, when delicate balances are available, it is a good practice to weigh to the milligram. At remote bases, however, and particularly in the field, and on reconnaissance, where delicate, expensive instruments are out of place, coarser balances, which weigh accurately to one centigram, give satisfactory results. The study of efficient water-content values has already gone far enough to indicate that differences less than i per cent are neg- ligible. Indeed, the soil variation in a single square meter is often as great as this. The greatest difference possible in the third place, i. e., that of 9 milligrams, does not produce a difference of .1 of i per cent in the water-content value. 'In consequence, such strong portable balances as Bausch & Lomb 1.2308 ($2), which can be carried anywhere, give entirely Fig. 4. Field balance. 28 THE HABITAT reliable results. The best procedure is to weigh the soil with the can. Turning the soil out upon the pan or upon paper obviates one weighing, but there is always some slight loss, and the chances of serious mishap are many. After weighing, the sample is dried as rapidly as possible in a water bath or oven. At a temperature ' of loo" C. this is accomplished ordinarily in twenty-four hours; the most tenacious clays require a longer time, or a higher temperature. High temperatures should be avoided, however, for soils that contain much leaf mould or other organic matter, in order that this may not be destroyed. When it is necessary on trips, soil samples can be dried in the sun or even in the air. This usually takes several days, however, 'and a test weighing is generally required before one can be certain that the moisture is entirely gone. The weighing of the (dried soil is made as before, and the can is carefully brushed out and weighed. The weight of aluminum cans may be determined once for all, but with painted cans it has been the practice to weigh them each time. 45. Computation. The most direct method of expressing the water- content is by per cents figured upon the moist soil as a basis. The ideal way would be to determine the actual amount of water per unit volume, but as this would necessitate weighing one unit volume at least in every habitat studied, as a preliminary step, it is not practicable. The actual process of computation is extremely simple. The weight of the dried sample, ziP-, is subtracted from the weight of the original sample, w, and the weight of the can, iv^, is likewise subtracted from w. The first result is then divided by the second, giving the per cent of water, or the physical water-content. The formula is: ;=fF. The result is expressed pref^ w — w' erably in grams per hundred grams of moist soil; thus 20/100, from which the per cent of water-content may readily be figured on the basis of dry or moist soil. 46. Time and location of readings. Owing to the daily change in the amount of soil water due to evaporation, gravity, and rainfall, an isolated determination of water-content has very little value. It is a primary re- quisite that a basis for comparison be established by making (i) a series of readings in the same place, (2) a series at practically the same time in a number of different places or habitats, or (3) by combining the two methods, and following the daily changes of a series of stations through- out an entire season, or at least for a period suiificient to determine the approximate maximum and minimum. The last procedure can hardly be carried out except at a base station, but here it is practically indispensable. It has been followed both at Lincoln and at Minnehaha, resulting in a basal series for each place that is of the greatest importance. When such a WATER-CONTENT 29 base already exists, or, better, while it is being established, scattered readings may be used somewhat profitably. As a practical working rule, however, it is most convenient and satisfactory to mal7 the mean, i8 per cent, while the latter gives little or no. clue to the extremes. It is hardly necessary to state that means and extremes should be deter- mined for a certain habitat, or particular area of it, and that the results may be expressed with reference to holard and chresard. ,. ; 58. Curves. The value of graphic methods and the details of plotting curves are reserved for a particular section. It will suffice in this place to indicate the water-content curves that are of especial value. Simple curves are made with regard to time, place, or depth. The day curve shows the fluctuations of the water-content of one station from day to day or from time to time. The station curve indicates the variation in water from sta- tion to station, while the depth curve represents the different values at var- ious depths in the same station. These may he combined on the same sheet in such a way that the station curves of each day may be compared directly. Similar combinations may be used for comparing the day curves, or the depth curves of different stations, but these are of less importance. A combination of curves which is of the greatest value is one which admits of direct comparison between the station curves of saturation, holard, chresard, and echard. HUMIDITY 59. Instruments. As a direct factor, humidity is intimately connected with water-content in determining the structure and distribution of plants. The one is in control of water loss ; the other regulates water supply. Humidity as a climatic factor undergoes greater fluctuation in the same habitat, and the efficient difference is correspondingly greater. Accordingly, simple instruments are less valuable than automatic ones, since a continuous record is essential to a proper understanding of the real influence of humidity. As is the rule, however, the use of simple instruments, when they can be referred to an ecographic basis, greatly extends the field which can be studied. In investigation, both psj'chrometer and psychrograph have their proper place. In the consideration of simple instruments for obtaining humidity values, an arbitrary distinction is made between psychrometers and hygrometers. The former consist of a wet and a dry bulb thermometer, while the latter make use of a hygroscopic awn, hair, or other object. Psychrometers 60. Kinds. There are three kinds of psychrometer, the sling, the cog, and the stationary. All consist of a wet bulb and a dry bulb thermometer set in a case; the first two are designed to be moved or whirled in the air. The same principle is applied in each, viz., that evaporation produces a THE HABITAT G ' r\ Fig. 5. Sling psy- chrometer. decrease, in temperature proportional to the amount of moisture in the air. The dry biilb thermometer is an ordinary thermometer, while the wet bulb is covered with a cloth that can be moistened. The former indicates the normal temperature of the air, the latter gives the re- duced temperature due to evaporation. The relative humidity of the air is ascertained by means of the proper tables, from two terms, i. e., the air temperature and the amount of reduction shown by the wet bulb. The sling and the cog psychrometers alone are in general use. The stationary form has been found to be unre- liable, because the moisture, as it evaporates from the wet bulb, is not removed, and, in consequence, hinders evaporation to the proper degree. 61. The sling psychrometer. The standard form of this is shown in the illustration, and is the one used by the Weather Bureau. This instrument can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of a metal frame to which are firmly ■ attached two accurately standardized thermometers, reading usually from -30° to 130°. The frame is attached at the uppermost end to a handle in such fashion that it swings freely. The wet bulb thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth for the wet bulb should be always of the same texture and quality ; the standard used by the Weather Bureau can be obtained from the instrument makers. A slight difference in texture mal-. •00 g a) Fig. 10. Conversion scale for temperatures. 48 THE HABITAT These njay be combined in a series for the comparison of readings made at various heights in the stations. The day or point curve shows the fluctuations during the day of one point, and the station curve the variation at different heights in the same station. The curves of successive days or of different stations may of course be combined on the same sheet for comparison. Level and station curves based upon mean relative humidities are especially valuable. 75. Records. A field form is obviously unnecessary for the psychrograph. The record sheets constitute both a field and permanent record. The alti- tude and other constant features of the station and the list of species, etc., are entered on the back of the first record sheet, or, better, they are noted in the permanent formation record. For psychrometer readings, whether •single or in series, the following record form is employed: u 01 II § S ■3 3 1^ la "5 5 i t a s i < NOTES ii cn a 'I •0 a 15/8/'04 6:30 a.m. Spruce .... Brook bank 2500 m Mertensiare 1ft. 51° 46° 11$ m 2.9 Clear " " Half gravel Hiawatha . . " Asterare . . . " 56° 49° 7 Ui. 63i« 3.0 •' 6:45 pm. Spruce .... Brook bank " Mertensiare •• 54° 52° 2 i% m 4.2 '• 2cc. " " Half gravel Hiawatha . , " Asterare -. . . " 56° 32° 4 1% m 4.0 " 2cc. On page 47 is given a table for the conversion of Centigrade into Fahren- heit temperatures. This may be done mentally by means of the formula LIGHT 76. Methods. All methods for measuring light intensity, which have been at all satisfactory, arc based upon the fact that si lver salts blacken in the light. The first photographic method was proposed by Bunsen and Roscoe in 1862 ; this has been taken up by Wiesner and variously modified. After consider- able experiment by the writer, however, it seemed desirable to abandon all methods which require the use of "normal paper" and "normal black" and to develop a simpler one. As space is lacking for a satisfactory discussion of the Bunsen-Roscoe-Wiesner methods, the reader is referred to the works cited below.^ Simple photometers for making light readings simultaneously 'BuNSEN, R., AND RoscoE, H. Photometrische Untersuchungen. Poggendor£E's Annalen., 117:529. J 862. | Wiesner, J. Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete. Sitzb. Akad. Wiss. Wien., I, 1893. II, 1S95. LIGHT 49 or in series were constructed in 1900, and have been in constant use since that time. An automatic instrument capable of making accurate continuous records proved to be a more difficult problem. A sunshine recorder was ulti- mately found which yields valuable results, and very recently a recording photometer which promises to be perfectly satisfactory has been devised. Since the hourly and daily variations of sunlight in the same habitat are relatively small, automatic photometers are perhaps a convenience rather than- a necessity. The Photometer 77. Construction. The simple form of photometer shown in the illustra- tion is a light-tight metal box with a central wheel upon which a strip of Fig. 11. Photometer, showing front and side view. photographic paper is fastened. This wheel is revolved by • the thumb- screw past an opening 6 mm. square which is closed by meaiis of a slide working closely between two flanges. At the edge of the 'opening, and beneath the slide is a hollow for the reception of a permanent -Tight stand- ard. The disk of the thumbscrew is graduated into twenty-five parts, and these are numbered. A line just beneath the opening coincides with Untersuchungen iiber das photochemische Kiima von Wien, Cairo, und Buitenzorg (Java) Denksch. Kais. Akad. Wien., 64. 1896. Untersuchungen iiber den Lichtgenuss der Pflanzen im arktischen Gebiete. Sitzb. Kais. Akad. Wien., 109. 1900. 5° THE HABITAT the successive lines on the disk, and indicates the number of the exposure. The wheel contains twenty-five hollows in which the click works, thus mov- ing each exposure just beyond the opening. The metal case is made in two parts, so that the bottom may be readily removed, and the photographic strip placed in position. The water-photometer is similar except that the opening is always covered with a transparent strip and the whole in- strument is water-tight. These instruments have been made especially for measuring light by the C. H. Stoelting Co., 31 W. Randolph street, Chicago, 111. The price is $5. 78. Filling the photometer. The photographic paper called "solio" which is made by the Eastman Kodak Company, Rochester, N. Y., has proved to be much the best for photometric readings. The most convenient size is that of the 8 x 10 inch sheet, which can be obtained at any supply house in packages of a dozen sheets for 60 cents. New "emulsions," i. e., new lots of paper, are received by the dealers every week, but each emulsion can be preserved for three to six months without harm if kept in a cool, light- tight place. Furthermore,, all emulsions are made in exactly the same way, and it has been impossible to detect any difference in them. To fill the photometer, a strip exactly 6 mm. wide is cut lengthwise from the 8 x 10 sheet. This must be done in the dark room, or at night in very weak light. The strip is placed on the wheel, extreme care being taken not to touch the coated surface, and fixed in position by forcing the free ends into the slit of the wheel by a piece of cork 8-9 mm. long. The wheel is replaced in the case, turned until the zero is opposite the index line, and the instrument is ready for use. >79. Making readings. An exposure is made by moving the slide quickly in such a way as to uncover the entire opening, and the standard if the exposure is to be very short. Care must be taken not to pull the slide en- tirely out of the groove, as it will be impossible to replace it with sufficient quickness. The time of exposure can be determined by any watch after a little practice. It is somewhat awkward for one person to manage the slide properly when his attention is fixed upon a second hand. This is obviated by having one observer handle the watch and another the photometer, but here the reaction time is a source of considerable error. The most satis- factory method is to use a stop-watch. This can be held in the left hand and started and stopped by the index finger. The photometer is held against it in the right hand in such a way that the two movements of stopping the watch and closing' the slide may be made at the same instant. The length of exposure is that necessary to bring the tint of the paper to that of the LIGHT 51 standard beside it. A second method which is equally advantageous and sometimes preferable does away with the permanent standard in the field and the need for a stop-watch. In this event, the strip is exposed until a medium color is obtained, since very light or very deep prints are harder to match. This is later compared with the multiple standard. In both cases, the date, time of day, station, number of instrument and of exposure, and the length of the latter in seconds are carefully noted. The instrument is held with the edge toward the south at the level to be read, and the open- ing uppermost in the usual position of the leaf. When special readings are desired, as for isophotic leaves, reflected light, etc., the position is naturally changed to correspond. In practice, it is made an invariable rule to move the strip for the next exposure as soon as the slide is closed. Otherwise Fig. 12. Dawson-Lander sun recorder. double exposures are liable to occur. When a strip is completely exposed it is removed in the dark, and a new one put in place. The former is care- fully labeled and dated on the back, and put away in a light-tight box in a cool place. 80. The Dawson-Lander sun recorder. "The instrument consists of a small outer cylinder of copper which revolves with the sun, and through the side of which is cut a narrow slit to allow the sunshine to impinge on a strip of sensitive paper, wound round a drum which fits closely inside the outer cylinder, but is held by a pjn so that it can not rotate. By means of 52 THE HABITAT a screw fixed to the lid of the outer cylinder, the drum holding the sensi- tive paper is made to travel endwise down the outer tube, one-eighth of an inch daily, so that a fresh portion of the sensitive surface is brought into position to receive the record." The instrument is driven by an eight-day clock placed in the base below the drum. The slit is covered by means of a flattened funnel-shaped hood, and the photographic strip is protected from rain by a perfectly transparent sheet of celluloid. The detailed structure of the instrument is shown in figure 12. This instrument may be obtained from Lander and Smith, Canterbury, England, for $35. In setting up the sunshine recorder, the axis should be placed in such a position that the angle which it makes with the base is the same as the altitude of the place where the observations are made. This is readily done by loosening the bolts at either side. The drum is removed, the celluloid sheet unwound 'by means of the key which holds it in place, the sensitive strip put in position, and the sheet again wound up. Strips of a special sensi- tive paper tipon which tlie hours are indicated are furnished by the makers of the instrument, but it has been found preferable to use solio strips in order to- facilitate comparison with the standards. The drum is placed on the axis, and is screwed up until it just escapes the collar at the top of the spiral. The clock is wound and started, and the outer cylinder put on so that the proper hour mark coincides with the index on the front of the base. As a sunshine recorder, the instrument gives a perfect record, in which the varying intensities are readily recognizable. Since the cylinder moves one-half inch in an hour, and the slit is .01 of an inch, the time of each exposure is ^2 seconds. This gives a very deep color on the solio paper, which results in a serious error in making comparisons with the standard. On account of the hood, diffuse light is not recorded when it is too weak to cast a distinct shadow. It seems probable that this difficulty will be over- come by the use of a flat disk containing the proper slit, and in this event the instrument will become of especial value for measuring the diffuse light of layered formations. The celluloid sheet constitutes a source of error in sunlight on account of the reflection which it causes. This can be prevented by using the instrument only on sunny days, when the protection of the sheet can be dispensed with. 81. The sciagraph. This instrument is at present under construction, and can only be described in a general way. In principle it is a simple photometer operating automatically. It consists of a light-tight box pref- erably of metal, which contains an , eight-day lever clock. Attached to the arbor of the latter is a disk 7 inches in diameter bearing on its circum- ference a solio strip i cm. wide and 59 cm. long. The opening in the box' LIGHT 53 for exposure is 6 mm. square and is controlled by a photographic shutter. The latter is constructed so that it may be set for 5, 10, or 20 seconds, since a single period of exposure can not serve for both sun and shade. The shutter is tripped once every two hours, by means of a special wheel revolv- ing once a day. Each exposure is 6 mm. square, and is separated by a small space from the next one. Twelve exposures are made every 24 hours, and 84 during the week, though, naturally, the daytime exposures alone are recorded. Comparisons with the multiple standard are made exactly as in the case of the simple photometer. The sciagraph is made by the C. H. Stoelting Co., Chicago, Illinois. Standards 82. Use. The light value of each exposure is determined by reference to a standard. When the photometer carries a permanent standard, each ex- posure is brought to the tint of the latter, and its value is indicated by the time ratio between them. Thus, if the standard is the resuir of a s-second exposure to full sunlight at meridian, and a reading which corresponds in color requires 100 seconds in the habitat concerned, the light of the latter is twenty times weaker or more diffuse. Usually, the standard is regarded as unity, and light values figured with reference to it, as .05. With the sciagraph such a use of the standard is impossible, and often, also, with the photometer it is unnecessary or not desirable. The value of each exposure in such case is obtained by matching it with a multiple standard, after the entire strip has been exposed. The further steps arc those already indicated. After the exact tint in the standard has been found, the length of the reading in seconds is divided by the time of the proper standard, and the result expressed as above. 83. Making a standard. Standards are obtained by exposing the photo- meter at meridian on a typically clear day, and in the field where there is the least dust and smoke. Exception to the latter may be made, of course, in obtaining standards for plant houses located in cities, though it is far better to have the same one for both field and control experiment. Usable standards can be obtained on any bright day at the base station. Indeed, valu- able results are often secured by immediate successive sun and shade read- ings in adjacent habitats, where the sun reading series is the sole standard. Preferably, standards should be made ait the solstices or equinoxes, and at a representative station. The June solstice is much to be preferred, as it represents the maximum light values of the year. Lincoln has been taken as the base station for the plains and mountains. It is desirable, however. 54 THE HABITAT that a national or international station be ultimately selected for this pur- pose, in order that light values taken in different parts of the world may be readily compared. 84. Kinds of standards. The base standard is the one taken at , Lincoln (latitude 41° N.) at meridian June 20-22. This is properly the unit to which all exposures are referred, but it has been found convenient to employ the Minnehaha standard as the base for the Colorado mountains, in order to avoid reducing each time. Relath-e standards are frequently used for tem- porary purposes. Thus, in comparing the light intensities of a series of formations, one to five standards are exposed on the solio strip before be- ginning the series of readings. Proof standards are the exposed solio strips, which fade in the light, and can, in consequence, be kept only a few weeks without possibility of error. The fading can be prevented by "toning" the strip, but in this event the exposures must be fixed in like manner before they can be compared. This process is inconvenient and time-consuming. It is also open to considerable error, as the time of treatment, strength of solu- tion, etc., must be exactly equivalent in all instances. Permanent standards are accurate water-color copies of the originals obtained by the photometer. These have the apparent disadvantage of requiring a double comparison or matching, but after a little practice it is possible to reproduce the solio tints so that the copy is practically indistinguishable from the original. The most satisfactory method is to make a long stroke of color on a pure white paper, since a broad wash is not quite homogeneous, and then to reject such parts of the stroke as do not match exactly. Permanent standards fade after a few month's use, and must be replaced by parts of the original stroke. Single standards are made by one exposure, while miUtiple ones have a series of exposures filling a whole light strip. These are regularly obtained by making the exposures from i-io seconds respectively, and then increas- ing the length of each successive exposure by 2 seconds. Single exposures of 1-5 seconds as desired usually serve as the basis for permanent stand- ards, but a multiple standard may also be copied in permanent form. Ex- posures for securing standards must be made only under the most favorable conditions, and the length in seconds must be exact. The use of the stop- watch is imperative, except where access may be had to an astronomical clock with a , large second hand, which is even more satisfactory. The length of time necessary for the series desired is reckoned beforehand, and the exposures begun so that the meridian falls in the middle of the process. Single standards are exceedingly convenient in photometer readings, but they are open to one objection. In the sunshine it is necessary to make in- stant decision upon the accuracy of the match, or the exposure becomes too LIGHT 55 deep. In the shade where the action is slower, this difficulty is not felt. For this reason it is usually desirable to check the results by a multiple stand- ard, and in the case of sciagraph records, where the various exposures show a wide range of tint, light values are obtainable only by direct comparison with the multiple standard. The exact matching of exposure and standard requires great accuracy, but with a little practice this may be done with slight chance of error by merely moving the exposure along the various tints of the standard until the proper shade is found. The requisite skill is soon acquired by running over a strip of exposures several times until the comparisons always yield the same results for each. The margin of error is practically negligible when the same person makes all the comparisons, a,nd in the case of two or three working on the same reading the results diverge little or not at all. The efficient difference for light is much more of a variable than is the case with water-content. It has been determined so far only for a few species,, all of which seem to indicate that appreciable modification in the form or structure of a leaf does not occur until the reduction in intensity reaches .1 of the meridian sunlight at the June sol- stice. The error of comparison is far less than this, and consequently may be ignored, even in the most painstaking inquiry. Readings 85. Time. The intensity of the light incident upon a habitat varies peri- odically with the hour and the day, and changes in accord with the changing conditions of the sky. The light variations on cloudy days can only be de- termined by the photometer. While these can not be ignored, proper com- parisons can be instituted only between the readings taken on normal days of sunshine. The sunlight varies with the altitude of the sun, i. e., the angle which its rays make with the surface at a given latitude. This angle reaches a daily maximum at meridian. The yearly maximum falls on June 22, and the angle decreases in both directions through the year to a minimum on December 22. At equal distances from either solstice, the angle is the same, e. g., on March 21 and September 23. At Lincoln (41" N. latitude) the extremes at meridian are 73° and 26°; at Minnehaha (39°) they are 75" and 28°. The extremes for any latitude may be found by subtracting its distance in degrees north of the two tropics from 90. Thus, the soth parallel is 26.5" north of the tropic of Cancer, and the maximum altitude of the sun at a place upon it is 63.5°. It is 73.5° north of the tropic of Capri- corn, and the minimum meridional altitude is 16.5°. The changes in the amount of light due to the altitude of the sun are pro- duced by the earth's atmosphere. The absorption of light rays is greatest near the horizon, where their pathway through the atmosphere is longest, 5^ THE HABITAT and it is least at the zenith. The absorption, and, consequently, the relative intensity of sunlight, can be determined at a given place for each hour of any sunshiny day by the use of chart 13. This chart has been constructed for Lincoln, and will serve for all places within a few degrees of the 40th parallel. The curves which show the altitude of the sun at the various times of the day and the year have been constructed by measurements upon the celestial globe. Each interval between the horizontal lines represents 2 de- grees of the sun's altitude. The vertical lines indicate time before or after the apparent noon, the intervals corresponding to 10 minutes. If the rela- tive intensity at Lincoln on March 12 at 3 :oo p.m. is desired, the apparent noon for this day must first be determined. A glance at the table shows that the sun crosses the meridian on this day at 9 minutes 53 seconds past noon at the 90th meridian. The apparent noon at Lincoln is found by adding 26 minutes .\g seconds, the difference in time between Lincoln and a point on the 90th meridian. When the sun is fast, the proper number of minutes is taken from 26 minutes 49 seconds. The apparent noon on March 12 is thus found to fall at 12:37 ^■^■, and 3:00 p.m. is 2 hours and 23 minutes later. The sun's altitude is accordingly 36°. If the intensity of the light which reaches the earth's surface when the sun is at zenith is taken as i, the table of the sun's altitudes gives the intensity at 3:00 p.m. on March 12 as .85. For places with a latitude differing by several degrees from that of Lin- coln, it is necessary to construct a new table of altitude curves from the celestial globe. It is quite possible to make a close approximation of this from the table given, since the maximum and minimum meridional altitude, and hence the corresponding light intensity, can be obtained as indicated abo-\'e. For Minnehaha, which is on the losth meridian, and for other places on standard meridians, i. e., 60°, 75", 90°, and 120° W., the table of apparent noon indicates the number of minutes to be added to 12 noon, standard time, when the sun is slow, and to be subtracted when the sun is fast. The time at a place east or west of a standard meridian is respectively faster or slower than the latter. The exact difference in minutes is obtained from the dif- ference in longitude by the equation, 15"=! hour. Thus, Lincoln, 96° 42' W. is 6° 42' west of the standard meridian of 90" ; it is consequently 26 min- utes 49 seconds slower, and this time must always be added to the apparent noon as determined from the chart. At a place east of a standard meridian, the time difference is, of course, subtracted. The actual differences in the light intensity froin hour to hour and day to day, which are caused by variations in the sun's altitude, are not as great as might be expected. For example, the maximum intensity at Lincoln, June 22, is .98 ; the minimum meridional intensity Decerriber 22 is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00 a.m. and 6:00 p.m* LIGHT 57 approximately) ; between 8:00 a.m. and 4:00 p.m. the range in intensity is from .90 to .98 merely. On December 22, the greatest intensity is .52, the least .20 (the latter at 8 :oo a.m. and 4 :oo p.m. approximately). If the grow- ing season be taken as beginning with the 1st of March and closing the 1st of October, the greatest variation in light intensity at Lincoln within a period of lojiQljrs with the meridian at its center (cloudy days excepted) is from .33 to .98. In a period of .SJuiurs, the extremes are .65 to .98, i. e., the greatest variation, .3, is far within the efficient difference, which has been put at .9. a\h\u$t Inhnsdif. 90' ■1.000 so] ffb 7d .m i(f .f*r S(f 13:1 vo- .9 of 3(f 1%^ iCf .4VJ IS , ,T,?Y Fig. 13. Chart for the determination of the sun's altitude, and the corresponding light intensity. For the growing period, then, readings made between 8 :oo a.m. and 4 :oo p.m. on normal sunshiny days may be compared directly,, without taking into ac- count the compensation for the sun's altitude. Until the efficient difference has been determined for a large number of species, however, it seems wise to err on the safe side and to compensate for great differences in time of day or year. In all doubtful cases, the intensity obtained by the astronomical 58 THE HABITAT method should also be checked by photometric readings. A slight error probably enters in, due to reflection from the surface of the paper, and to temperature, but this is negligible. 86. Table for determining apparent noon DATE TIME LIN EQUATION N COLN OON DATE TIME LIN EQUATION N COLN OON Sun slow: -\- 26r n. 49s. Sun slow: + 26n 1. 49s. January 1.. 3m. 47s. 12 31 P.M. July 5.. 4m. 19s. 12 31 P.M. 6.. 6 7 :33 10.. 5 7 32 11.. 8 12 :35 20.. 6 6 33 16.. 10 3 37 August 4.. 5 53 33 21.. 11 35 .38 14. . 4 30 31 26.. 12 48 40 19.. 3 28 30 31.. 13 41 40 2i.. 2 13 29 February 10. . 14 27 41 29.. 48 28 20. . 13 56 41 Sun fast: — March 2.. 12 18 39 September3. . 45 26 7.. 11 10 38 8.. 2 25 24 12.. 9 53 37 13.. 4 9 23 17.. 8 29 36 18.. 5 55 21 22.. 6 59 34 . 23.. 7 41 19 27.. 5 27 32 28.. 9 23 17 April 1.. ■3 55 31 October 3.. 10 59 16 6.. 2 27 29 8.. 12 26 14 11.. 1 3 28 13: 13 43 13 Sun fast: — 18.. 14 48 12 16.. 13 27 23.. 15 37 11 21.. 1 20 25 November 2. . 16 20 10 26.. 2 16 24 12.. 15 45 11 May 1.. 3 24 17. . 14 54 12 16.. 3 48 23 22.. 13 44 13 31.. 2 33 24 27. . 12 14 15 June 5.. 1 45 25 December 2. . 10 25 16 10.. 49 26 7.. 8 21 18 Sun slow: + 12.. 6 5 21 15.. 13 27 17. . 3 41 23 20.. 1 18 28 22.. 1 12 26 25. . 2 22 : 29 Sun slow: -\- 30.. 3 22 30 27.. 1 17 28 LIGH.T 59 87. Place. The effect of latitude upon the sun's altitude, and the conse- quent light intensity have been discussed in the pages which precede. Lati- tude has also a profound influence upon the duration of daylight, but the importance of the latter apart from intensity is not altogether clear. The variation of intensity due to altitude has been greatly overestimated; it is practically certain, for example, that the dwarf habit of alpine plants is not to be ascribed to intense illumination, since the latter increases but slightly with the altitude. It has been demonstrated astronomically that about 20 per cent of a vertical ray of sunlight is absorbed by the atmosphere by the time it reaches sea level. At the summit of Pike's Peak, which is 14,000 feet (4,267 meters) high, the barometric pressure is 17 inches, and the absorption is approximately 1 1 per cent. In other words, the .light at sea level is 80 per cent of that which enters the earth's atmosphere; on the summit of Pike's Peak it is 89 per cent. As the effect of the sun's altitude is the same in both places, the table of curves on page 57 will apply to both. Taking into account the difference in absorption, the maximum in- tensity at sea level and at 14,000 feet on the fortieth parallel is .98 and 1.09 respectively. The minimum intensities between 8:00 a.m. and 4:00 p.m. of the growing period are .64 and .71 respectively. The correctness of these figures has been demonstrated by photometer readings, which have given al- most exactly the same results. Such slight variations are quite insufficient to produce an appreciable adjustment, particularly in structure. They are far within the efficient difference, and Reinke'- has found, moreover, that photosynthetic activity in Elodea is not increased beyond the normal in sunlight sixty times concentrated. In consequence, it is entirely unneces- sary to take account of different altitudes in obtaining light values. The slope of a habitat exerts a considerable effect upon the intensity of the incident light. If the angle between the slope and the sun's ray be 90°, a square meter of surface will receive the maximum intensity, i. At an angle of 10°, the same area receives but .17 of the light. This relation be- tween angle and intensity is shown in the table which follows. The influence of the light, however, is felt by the leaf, not by the slope. Since there is no connection between the position of the leaf and the slope of the habitat, the latter may be ignored. In consequence, it is unnecessary to make al- lowances for the direction of a slope, viz., whether north, east, south, or west, in so far as light values are concerned. The angle which a leaf makes with its stem determines the angle of incidence, and hence the amount of light received by the leaf surface. This is relatively unimportant for two reasons. This angle changes hourly and daily with the altitude of the iReinke, J. Bot. Zeit., 41:713. 1883. 6o THE HABITAT sun, and the intensity constantly swings from one extreme to the other. Moreover, the extremes i.oo and 0.17, even if constant, are hardly sufficient to produce a measurable result. When the angle of the leaf approaches 90°, there is the well-known differentiation of leaf surfaces and of chlorenchym, but this has no relation to the angle of incidence. Table of Intensity at Various Angles S'GLE INTENSITY 90 1.00 80 .98 70 .94 60 .87 50 .77 40 .64 30 .50 20 .34 10 .17 In the sunlight, it makes no difference at what height a light reading is taken. In forest and thicket as well as in some herbaceous formations, the intensity of the light, if there is any difference, is greatest just beneath the foliage of the facies. In forests especially, the light is increasingly diffuse toward the ground, particularly where layers intervene. In woodland for- mations, moreover, the exact spot in which a reading is made must be care- fully chosen, unless the foliage is so dense that the shade is uniform. A very satisfactory plan is to take readings in two or more spots where the shade appears to be typical, and to make a check reading in a "sunfleck," a spot where simlight shows through. In forests and thickets, the sunflecks are fleeting, and the light value is practically that of the shade. In pass- ing into open woodland and thicket, the sunflecks increase in size and per- manence, until finally they exceed the shade areas in amount and become typical of the formation. Reflected and Absorbed Light 88. The fate of incident light. The light present in a habitat and incident upon a leaf is not all available for photosynthesis. Part is reflected or screened out by the epidermis, and a certain amount passes through the chlorenchym, except in very thick leaves. The light absorbed is by far the greatest in the majority of species. Many plants with dense coatings of' hairs reflect or withhold more light than they absorb, and the amount of light reflected by a thick cuticule is likewise great.- As light is imponder- able, the actual amount absorbed or reflected by the leaf can not be deter- mined. It is possible, however, to express this in terms of the total amount LIGHT 6l received, by means of readings with solio paper, and the knowledge thus obtained is of great importance in interpreting the modifications of certain types of leaves. For example, a leaf with a densely hairy epidermis may receive light of the full intensity, i ; the amount reflected or screened out by the hairs may be 95 per cent of this, the amount absorbed 5 per cent, and that transmitted, nil. In the majority of cases, however, the absorbed light is considerably more than the reflected amount mitted. or trans- 89. Methods of determina- tion. If results are to be of value, reflected and transmit- ted light must be determined in the habitat of the plant simultaneously with the total light which a leaf receives. An approximation of the light reflected from a leaf surface is secured by placing the photometer so that the , light reflected is thrown upon the solio strip. A much more satisfactory method, however, is to determine it in connec- tion with the amount of light transmitted through the epi- dermis. This is done by stripping a piece of epidermis from the upper surface of the leaf and placing it over the slit in the photometer for an exposure. An exposure in the full light of the habitat is made simultaneously with another photometer, or immediately afterward upon the same strip. When the epidermis is not too dense, both exposures are permitted to reach the same tint, and the relation between them is precisely that of their lengths of exposure. Or- dinarily the two exposures are made absolutely simultaneous by placing the epidermis over half of the opening, leaving the other half to record the full light value, and the results, or epidermis prints, are referred to a multi- Fig. 14, Leaf print: exposed 10 m., 11 a.m. Au- gust 20. The leaves are from sun and shade forms of Bursa bursa-pastoris, J?osa sayii, Thalictrum sparsiflorum, and Machaeranthera aspera. In each the shade leaf prints moi-e deeply. 62 THE HABITAT pie standard. The difference between the two values thus obtained repre- sents the amount of reflected light together with that screened by the epi- dermis. The amount of light transmitted through the leaf may be measured in the same way by using the leaf itself in place of the epidermis alone. The time of exposure is necessarily long, however, and it has been found practicable to obtain leaf prints by exposing the leaf in a printing frame, upon solio paper, at the same time that the epidermis, print is made. In a few species both the upper and lower epidermis can be removed and the amount of light absorbed determined directly by exposing the strip covered with the chlorenchym. Generally, however, this must be computed by subtracting the sum of the per cents of reflected and transmitted light from 100 per cent, which represents the total light. 90. Leaf and epidermis prints. In diphotic leaves the screening effect of the lower epidermis may be ignored. Isophotic sun leaves, i. e., those nearly upright in position or found above light-colored, reflecting soils, are usually strongly illumi- nated on both sides, and the ab- sorbed light can be obtained only by measuring the screening effect of both epiderms. Shade leaves and submerged leaves often contain chloroplasts in the epidermis, and the above method can not be applied to them. In fact, in habitats where the light is quite diffuse, practically all incident light is absorbed. The rare exceptions are those shade leaves with a distinct bloom. In addition to their use in obtaining the amount of light absorbed, both leaf and epidermis prints are extremely interesting for the direct comparison of light relations in the leaves of species belonging to different habitats. The relative screen- ing value of the upper and lower epidermis, or of the corresponding epiderms of two ecads or two species, is readily ascertained by exposing, the two side by side in sunshine, over the slit in the photometer. For leaf prints fresh leaves are desirable, though nearly the same results can be obtained from Fig. 15. Leaf print: exposure as before. Sun and shade leaves of Achillea lanulosa, Capnoides aureum, Antennaria umbri- nella, Galium boreale, and Potentilla propinqua. LIGHT 63 leaves dried under pressure. The leaves are grouped as desired on the glass of a printing frame, and covered with a sheet of solio. They are then ex- posed to full sunlight, preferably at meridian, and the prints evaluated by means of the multiple standard. This method is especially useful in the comparison of ecads of one species. These differences due to transmitted light are very graphic, and can easily be preserved by "toning" the print in the usual way. Expression of Results 91. Light records. The actual photographic records obtained by photo- meter and selagraph can at most be kept but a few months, unless they are "toned" or fixed. "Toning" modifies the color of the exposure materially, and changes its intensity so that it can not be compared with readings not fixed. It would involve a great deal of inconvenience to make all compari- sons by means of toned strips and standard, even if it were not for the fact that it is practically impossible to obtain exactly the same shade in lots toned at different times. The field record, if carefully and neatly made, may well take the place of a permanent one. The form is the following : ^ t u 1 to 1 < a I 1 d 1 1 3 CO Si ■g ■0 ■s 14/9/04 12:C0 M. 12:03 P.M. 12:15 P.M. spruce Spruce Brook b'nk Milky Way- Moss Glen Grotto 2600 m. 2500 m. 2500 m. N.E.£0° Level E.3° Opulaster Streptopus Filix Ifoot Surface 2:10 2:12 2:13 160 s. 210 s. 360 s. 3s 3 s. 3 s. .019 .012 .008 i ,, 92. Light sums, means, and curves. Owing to the fact that the scia- graph has not yet been used in the field, no endeavor has been made to de-' termine the light value for every hour of the day in different habitats. Consequently there has been no attempt to compute light sums and means. Photometer readings have sufficed to interpret the effect of light in the structure of the formation, and of the individual, but they have not been sufficiently frequent for use in ascertaining sums and means. The latter are much less valuable than the extremes, especially when the relative dura- tion of these is indicated. Means, however, are readily obtained from the continuous records. Light sums are probably impracticable, as the factor is not one that can be expressed in absolute terms. The various kinds and combinations of light cvirves are essentially the same as for humidity. The level curve through a series of habitats is the most illuminating, but the day curve of hour variations is of considerable value. The curve of '64 THE HABITAT daily duration, based upon full sunlight, is also of especial importance for plants, and stations which receive both sun and shade during the day. TEMPERATURE 93. In consequence of its indirect action, temperature does not have a strikingf effect upon the form and structure of the plant, as is the case with water and light. Notwithstanding, it is a factor of fundamental im- portance. This is especially evident in the diaracter and distribution of vegetation. It is also seen in the germination and growth of plants, in the .length of season, and in the important influence of temperature upon hu- midity, and hence upon water-content. Because of its intimate relation with the comfort of mankind, the determination of temperature values has re- ceived more attention than that of any other factor, and excellent simple and recording instruments are numerous. For plants, it is also necessary to employ instruments for measuring soil temperatures. The latter un- questionably have much less meaning for the plant than the temperatures of the air, but they have a direct influence upon the imbibition of water, and upon germination. Thermometers 94. Air thermometers. The accurate measurement of temperature re- quires standard thermometers. Reasonably accurate instruments may be standardized by determining their error, but they are extremely unsatis- factory in practice, since they result in a serious waste of time. Accurate thermometers which read to the degree are entirely serviceable as a rule, but instruments which read to a fraction of a degree are often very much to be desired. The writer has found the "cylindrical bulb thermometer, Centigrade scale" of H. J. Green, to be an exceedingly satisfactory instru- ment. The best numbers for general use are 247 and 251, which read from . -15" to 50° C. and are graduated in .2". They are respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are delicate and require careful handling, but even in class work this has proved to be an advantage rather than otherwise. In making readings of air temperatures with such thermometers, constant precautions must be taken to expose the bulb directly to the wind and to keep it away from the hand and person. 95. Soil thermometers. The thermometer described above has been used extensively for soil temperatures. The determination of the latter is con- veniently combined with the taking of soil samples, by using the hole for a temperature reading. When carefully covered, these holes can be used from day to day throughout the season without appreciable error, even in TEMPERATURE 6S gravel soils. Repeated tests of this have been made by simultaneous read- ings in permanent and newly made holes, and the results have always, been the same. It has even been found that the error is usually less than i degree when the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in the fact that the thermometer must be raised. to make the reading. With a -little practice, however, the top of the column of mercury may be raised to the surface and read be- fore the change of temperature can react upon it. This is especially important in very moist or wet soils where the bulb becomes coated with a film of moisture. This evaporates when the bulb is brought into the air, and after a moment or two the rhercury slowly falls. Regular soil thermometers are indispensable when read- ings are desired at depths greater than 12-18 inches. They possess several disadvantages which restrict their use almost wholly to permanent stations. It is scarcely possible to carry them on field trips, and the time required to place them in the soil renders them practically useless for single readings. Moreover, the instruments are expensive, ranging in price from $7 for the two-foot thermometer, to $19 for the eight- foot one. When it is recognized that deep-seated tempera- tures are extremely constant and that the slight fluctuations affect, as a rule, only the relatively stable shrubs and trees, it is evident that such temperatures are of restricted impor- tance. Still, in any habitat, they must be ascertained before they can well be ignored, though it is unwise to spend much time and energy in their determination. Soil thermometers of the form illustrated may be obtained from H. J. Green, Brooklyn. Fig. 16. Soil thermometer 96. Maximum-minimum thermometers. These are used for determining the range of temperature within a given period, usually a day. Since- they are much cheaper than thermographs, they can replace these in part, although they merely indicate the maximum and minimum temperatures for the day, and do not register the time when each occurs. The maximum is a mercurial thermometer with a constriction in the tube just above the bulb; this allows the mercury to pass out as it expands, but prevents it from running back, thus registering the maximum temperature. The minimum thermometer contains alcohol. The column carries a tiny dumbbell-shaped marker which moves down with it, but will not rise as 66 THE HABITAT the liquid expands. This is due to the fact that the fluid expands too slowly to carry the marker upward, while the surface tension causes it to be drawn downward as the fluid contracts. The minimum temperature is indicated by the upper end of the marker. In setting up the thermome- ters, they are attached by special thumbscrews to a support which holds them in an oblique position. The minimum is placed in a special holder above the maxi- mum which rests on a pin that is used also for screwing the piv- ot-screw into po- sition. The sup- Fig. 17. Maximum-minimum thermometer. nort is screwed tightly to the cross-piece of a post, or in forest formations it is fastened di- rectly to a board nailed upon a tree trunk. A shelter has not been used in ecological work, although it is the rule in meteorological observations. The minimum thermometer is set for registering by raising the free end, so that the marker runs to the end of the column. The mercury of the maxi- mum is driven back into the bulb by whirling it rapidly on the pivot-screw after the pin has been taken out. This must be done with care in order that the bulb may not be ■ broken. As soon as the in- strument comes to rest, it is raised and the pin re- placed, great care being taken to lift it no higher than is necessary. When the night maximum is sought, the thermometer should be whirled several times in order to drive the column sufficiently low. Usually, in such cases, a record is made of this point to make sure that the maximum read is the actual one. If the pivot-screw is kept well oiled, less force will be required to drive the mercury back. In practice, the thermometers have been observed at 6:00 a.m. and 6:00 p.m. each day, thus permitting the reading of the maximum-minimum for both day and night. Pairs of maximum-minimum thermometers are to be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Juhen P. Friez, Baltimore, Maryland, at a cost of $8.25. Fig. 18. Terrestrial radiation thermometer. TEMPERATURE 67 97. Radiation thermometers. These are used to determine the radiation in the air, and from the soil, i. e., for solar and terrestrial radiation. The latter alone has been employed in the study of habitats, chiefly for the purpose of ascertaining the difference in the cooling of different soils at night. The terrestrial radiation thermometer is merely a special form of minimum thermometer, so arranged in a support that the bulb can be placed di- rectly above the soil or plant to be studied. It is other- wise operated exactly like the minimum thermometer, and the reading gives the minimum temperature which the air above the plant or soil reaches, not the amount of radiation. As a conse- quence, these instruments are valuable only where read in connection with a pair of maximum-minimum thermometers in the air, or when read in a series of in- struments placed above dif- ferent soils or plants. 98. Thermographs. Two types of standard instru- ments are in general use for obtaining continuous records of air temperatures. These are the Draper thermograph, made by the Draper Manufacturing Company, 152 Front St., New York city ($25 and $30), and the Richard thermograph sold by Julien P. Friez, Baltimore ($50). After careful trial had detjionstrated that they were equally accurate, the matter of cost was considered decisive, and the Draper thermograph has been used ex- clusively in the writer's own work. This instrument closely ■resembles the psychrograph manufactured by the same company. It is made in two sizes, of which the larger one is the more satisfactory on account of the greater distance between the lines of the recording disk. The thermometric part consists of two bimetallic strips, the contraction and expansion of which Fig. 19. Draper thermograph. 68 THE HABITAT are communicated to a hand carrying a pen. The latter traces a line on the record, sheet which is attached to a metal disk made to revolve by ari eight- day clock. In practice the thermograph is set up in the shelter which con- tains the psychrograph, and in exactly the same manner. The clock is wound, the record put in place, and the pen inked in the same way- also. The proper position of the pen is determined by making a careful ther- mometer reading under the shelter, and then regulat- ing the pen-hand by means of the screws at the base of it. A similar test read- ing is also made each week, when the clock is re- wound. A record sheet may be left in position for three weeks, the pen being filled each week with a dif- ferent ink. The fixed or- der of using the inks is red, blue, and green as already indicated. Owing to the fact that they are practically station- ary, soil thermographs are of slight value, except at base stations. Here, the facts that they are expen- sive, that the soil tempera- tures are of relatively lit- tle importance, and that they can be determined as easily, or nearly so, by simple thermom.eters, make the use of such instru- ments altogether unnecessary, if not, indeed, undesirable. In a perfectly equipped research station, they undoubtedly have their use, but at ordinary stations, and in the case of private investigators, their value is in no wise commensurate with their cost. Fig. 20. Shelter for thermograph. TEMPERATURE 69 Readings 99. Time. The hourly and daily fluctations of the temperature of the air render frequent readings desirable. It is this variation, indeed, which makes single readings, or even series of them, inconclusive, and renders the use of a recording instrtiment almost imperative in the base station at least. Undoubtedly, a set of simultaneous readings at different heights in one station, or at the same height in dififerent stations, especially if made at the maximum, have much value for comparison, but their full significance is seen only when they are referred to a 'continuous base record. Such series, %noreover, furnish good results for purposes of instruction. In re- Fig. 21. Richard thermograph. search work, . however, it has been found imperative to have thermc^raphs in habitats of widely dififerent character. With these as bases, it is possible to eke them out with considerable satisfaction by means of maximum- minimum thermometers in less different habitats, or in dififerent parts of the same habitat. Naturally these are less satisfactory, and are used only when expense sets a limit to the number of thermographs. In a careful analysis of a single habitat, more can be gained by one base thermograph supplemented by three pairs of maximum-minimum thermometers in dissimi- lar areas of the habitat than by two thermographs, and the cost is the same. 7° THE HABITAT 100. Place and height. For general air temperatures, thermograph and thermometer readings are made at a height of 3 feet (i meter). Soil tem- peratures are regularly taken at the surface and at a depth of i foot. When a complete series of simultaneous readings is made in one station, the levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, TO, and 15 inches in the soil. When sun and shade occur side by side in the same formation, as is true of many thickets and forests, surface read- ings are regularly made in both. Similarly, valuable results are obtained by making simultaneous readings on the bare soil, on dead cover, and upon a leaf, while the influence of cover is readily ascertained by readings upon it and beneath it. A full series of station readings made at the salne time upon north, east, south, and west slopes is of great importance in studying the effects of exposure. Expression of Results 101. Temperature records. Neither field nor permanent form is re- quired for thermographic records, other than the record sheet itself, which contains all the necessary information in a fairly convenient form. Al- though the temperature of a particular hour and day can not be read at a mere glance, it can be obtained so easily that it is a waste of time to make a tabular copy of each record sheet. For thermometer readings, either sin- gle or in series, 'the following form is used : ^ s « 1^ s .2 m •0 1 < 1 a 53 pos:tion of READING 6 >. ^ •3 k 3 feet Surf. 12 in. S 17/8/04 6:30 A.M Spruce Jack Brook 2350 m. N.E. 5° Mertensiare 90 9° 9.8° 10° Clear '■ " Half gravel Hiawatha 2550 m. N.E. 7° Asterare 11.2° 11.2° 14.8° 10° Clear a " 6:00 PM Spruce Jack Brook 2550 m. N.E. 5° Mertensiare 11.4° 11.4° 9 8° 11° Cloudy '■ " Half gravel Hiawatha 2550 m. N.E. 7° Asterare 12" 13.8° 16.4° 11° Cloudy 102. Temperature sums and means. The amount of heat, i. e., the num- ber of calories received within a given time by a definite area of plant sur- face, can be determined by means of a calorimeter. From this the tempera- ture sum of a particular period may be obtained by simple addition. In the present condition of our knowledge, it is impossible to establish any exact connection between such results and the functional or growth effect that can be traced directly to heat. As a consequence, temperature sums do not at present contribute anything of value to an understanding of the relation between cause and effect. The mean daily temperature is readily TEMPERATURE 71 obtained by averaging twenty-four hour-temperatures recorded by the ther- mograph. The method employed by Meyen^, of deriving the mean directly from the maximum and minimum for the day, is not accurate ; from a large number of computations, the error is always more than two degrees. On the other hand, the mean obtained by averaging the maximum and minimum for the day and night has been found to deviate less than i de- gree from the mean proper. This fact greatly increases the value of maxi- mum-minimum instruments if they are read daily at 6 :oo a.m. and 6 :oo p.m. 1034 Temperature curves. The kinds and combinations of temperature curves are almost without number. The simple curves of most interest are those for a series of stations or habitats, based upon the level of three feet, or the surface, or the daily mean. The curves for each station represent- ing the different heights and depths and the season curve of the daily means for a habitat are also of much importance. One of the most illuminating combinations is that which groups together the various level curves for a series of habitats. Other valuable combinations are obtained by grouping the curves of daily means of different habitats for the season, or the var- ious station curves. 104. Plant temperatures. The direct effects of temperature as seen in nutrition and growth can be ascertained only by determining the tempera- ture of plant tissues. The temperatures of the air and of the soil surface have an important effect upon humidity, and water-content, and through them upon the plant, but heat can influence assimilation, for example, only in so far as it is absorbed by the assimilating tissue. The temperatures of the leaf, as the most active nutritive organ of the plant, are especially im- portant. While it is a well-known fact that internal temperatures follow those of the air and soil closely, though with varying rapidity of response, this holds less for leaves than for stems and roots. Owing to the very obvious difficulties, practically nothing has yet been done 'in this important field. A few preliminary results have been obtained at Minnehaha, which serve to show the need for such readings. Gravel slide rosettes in an air temperature of 24° C. and a surface temperature of 40° C. gave the follow- ing surface readings: Parmelia, 40°, Eriogonum, 38.6°, Arctostaphylus, 35°, Thlaspi, 31.8°, and Senecio, 31°. The leaf of Eriogonum flavum, which is smooth above and densely hairy below, indicated a temperature of 31.8° when rolled closely about the thermometer bulb with the smooth sur- face out; and 28°. when the hairy surface was outside. The surface read- ' Meyen, F. J. F. Grundriss der Pflanzengeographie, 12. 1836. 72 THE HABITAT- ings of the same leaf were .S°-i° higher when made upon the upper smooth surface. This immediately suggests that the lower surface may be modified to protect the leaf from the great heat of the gravel, which often reaches 50° C. (122° F.). PRECIPITATION 105. General relations. As the factor which exerts the most important control upon water-content and humidity, rainfall must be carefully con- sidered by the ecologist. It is such an obvious factor, and is usually spoken of in such general terms that the need of following it accurately is not evi- dent at once. When it is recognized that the fluctuations of water-content are directly traceable to it, it becomes clear that its determination is as important as that of any indirect factor. This does not mean, however, that the amount of yearly rainfall is to be taken from the records of the nearest weather station, and the factor dismissed. Like other instruments, the rain gauge must be kept at the base station of the area under study, and when this is extensive or diverse, additional . instruments should be put into commission. While the different parts of the same general cli^ matic region may receive practically the same amount of precipitation dur- ing the year, it is not, necessarily true that the rainfall of any particular storm is equally distributed, especially in the mountains. Nothing less than an exact knowledge of the amount of rain that falls in the different areas will make it possible to tell how much of the water-content found at any particular time in these represents merely the chance differences of precipitation. The forms of precipitation are rain, dew, hail, snow, and frost. Of these, hail is too infrequent to be taken into account, while frost usually occurs only at the extremes of the growing season, and in its effect is rather to be reckoned with temperature. Snow rarely falls except during the period of rest, and, while it plays an important part as cover, it is merely one of several factors that determine the water-content of the soil at the beginning of spring. The influence of dew is not clearly understood. It is almost always too slight in amount and too fleeting to affect the water- content of the soil. It seems probable that it may serve by its own evapora- tion to decrease in some degree the. water loss from the soil, and from be- dewed plants. If, however, the dew is largely formed by the water of the toil and of the plant, as is thought by some, then it is negligible as a re- inforcement of water-content. From the above, it is evident that rainfall alone exerts a profound effect upon the habitat, and it is with its measure- ment that the ecologist is chiefly concerned. PRECIPITATION 73 106. The rain gauge, as the illustration shows, is a cylindrical vessel with a funnel-shaped receiver at the top, which is 8 inches in diameter. The re- ceiver fits closely upon a narrower brass vessel or measuring tube in which the rain collects. The ratio of surface between receiver and tube is lo to i. For readings covering a general area, the rain 'gauge is placed in the open, away from buildings or other obstructions, and is sunken in the ground suffi- ciently to keep it upright. In localities where winds are strong, it is usually braced at the sides also or supported by a wooden frame. In measuring the amount of rain in the measuring tube, the depth is divided by ten in ffant View. TertieeH StetUm. HorizantcO. SeeUan^r Fig. 22. Rain gauge showing construction. order to ascertain the actual rainfall. The depth is measured by inserting the measuring-rod through the hole in the funnel until it touches the bot- tom. It is left for a second or so, quickly withdrawn, and the limit of the wetted portion noted. In the case of standard rods, the actual rainfall is read directly in hundredths, so that the division by ten is unnecessary. After each reading, the measuring-tube is carefully drained, replaced, and the receiver put in position. No regular time for making readings is neces- sary. During a rainy period, it is customary to make a measurement each day, but it has been found more satisfactory for ecological purposes to measure each shower, and to record its duration. These two facts furnish 74 THE HABITAT a ready due to the relative amount of run-off in each fall of rain. The measurement of snowfall is often made merely by determining its depth. For comparison with rainfall, the rain gauge with receiver and tube with- drawn is used. The snow which falls is melted, poured into the measur- ing tube, and measured in the ordinary way. The U. S. Weather Bureau standard rain gauge, with measuring stick, may be obtained of H. J. Green, or of J. P. Friez for $5.25. 107. Precipitation records. From the periodic character of precipitation, rainfall sums, means, and curves have little importance in the careful study of the habitat. The rainfall curve for the growing season is an aid in ex- plaining the curve of water-content, and the mean rainfall of a region gives some idea of its vegetation, though even here the matter of its distribution is of primary importance. The rain and snow charts published by the U. S. Weather Bureau furnish data of some importance for the general study of vegetation, but it is evident that they can play little part in a system which is founded upon the habitat. Precipitation records, for reasons of brevity and convenience, are united with wind records, and the form will be found under the discussion of this factor. WIND 108. Value of read- ings. On account of its direct effect upon hu- midity, and its conse- quent influence upon water-content, the part which wind plays in a habitat can not be ignored in a thorough investigation. It is an important element in ex- posure, and accordingly has a marked mechani- cal effect upon the 'vege- tation of exposed habi- tats, alpine slopes, sea- coasts, plains, etc. Owing to its inconstancy and its extreme variation in velocity, single wind readings are absolutely without value. When read in series, anemometers give some information upon the comparative air movement in different hab- Fig. 23. Simple anemometer. WIND 75 itats, but the chance of error is great, except when the breeze is steady. Anemographs alone give real satisfaction. Accurate results, however, are not obtainable without a series of two or more in different habitats, and it is still an open question whether the results obtained justify the expense. For a completely eqiiipped base station, anemometer, anemograph, and wind vane are desirable instruments, but the study of the habitat has by no means reached the stage of precision in which their general use is necessary. 109. The anemometer in its simplest form is adapted only to readings made under direct observation, as a sudden change in the direction of the wind reverses the move- ment of the indicator needle. This simple wind gauge, shown in figure 23, has been used for in- structional purposes, and to a slight extent, .also, in ascertaining the effect of cover. In constant winds, successive single readings are found to have value, but, ordinarily, the obser- vations must be simultane- ous. Careful tests of this simple instrument show that it is essentially accurate. It may be obtained from the C. H. Stoelting Com- pany, 31 W. Randolph St., Chicago, for $25. The standard anemometer (Fig. 24) is practically a recording instrument up to 1,000 miles, but as the dials run on without any indication of the total number of revolutions, it must be visited and read each day. This renders its use difficult for habitats which are some distance apart. When exact determinations of wind values become neces- sary, the most successful method is to establish a series of three standard anemometers. One of these should be placed upon the most exposed part of a typically open habitat, the second in the most protected part of the same habitat, while the third is located in the midst of a representative forest formation. If the two habitats are close together, the daily visits Fig. 24. Standard anemometer. 76 THE (HABITAT can be rilade without serious inconvenience. The reading of the registerr jng dials. requires detailed explanation, and for this the reader is referred to the printed directions which accompany the instrument. In setting up ■the anemometer it must be borne in mind that the ecologist desires the wind Velocity for a particular habitat. In consequence, the precautions which the meteorologist takes to place the instrument at a certain height and well away from surrounding obstructions do not hold here. Standard anemo- meters are furnished by H. J. Green, and J. P. Friez for $25 each.. The anemograph is an anemometer electrically connected with an auto- matic register. It is the only instrument adapted to continuous weekly records in different habitats, but the price, $75 ($25 for the anemometer and $50 for the register) is practically prohibitive, at least until a complete series of ecographs for other factors has been obtained. 110. Records. The following form is used as a combined record for precipitation and wind: H — t ^r S a 1-5 a ■s cd tn s 1 s 5« ■ -RAINFALL WIND 'v. ea ■-. Q 1 |l 1. '53 a 1 29/8/04 31/8/04 2/9/04 3/9/04 6:30 P.M. 5:45 p.M 4:00 P.M. 10:00 p.M Half gravel Hiawatha 2350 m. n.b;'i7° Asterare u 1 Trace .2 Trace 8 hours 10 min. 2 hours 5 12 7 18 3 ft. N.W. W. :■ SOIL 111. Soil as a factor. In determining the value of the soil as a factor in a particular habitat, it must be clearly recognized that its importance lies solely in the control which it exerts upon water-content and riutrient-con- tent. The former is directly connected with the texture or fineness of the soil, the latter with its chemicar nature. Accordingly, the structure of the soil and its chemical composition are the fundamental points of attack. •These are not at all of equal value, however. Water is both a food, and a solvent for the nutrient salts of the soil. Furthermore, the per cent of sol- uble salts, as determined in mechanical analyses, is practically the same for "all ordinary soils. Indeed, the variations for the same soil types are as great as for entirely different types. For these reasons, soluble salt-con- tent may be ignored except where it is readily seen to be excessive, as in alkaline- soils; and determinations of chemical composition are necessary only in those soils which contain salts or acids to an injurious degree, e. g., SOIL 77 alkaline soils, peat bogs, humus swamps, etc. The structure of the soil, on the other hand, in the usual absence of excessive amounts of solutes, absolutely controls the fate of the water that enters the ground, in addition to its influence upon the run-off. It determines the amount of gravitation water lost by percolation, as well as the water that can be raised by capil- larity. The resultant of these, the total soil water or holard, is hence an effect of structure, while the size and compactness of the particles are con- clusive factors in controlling the chresard. It must be recognized, how- ever, that these are all factors which enable us to interpret the amount of holard or chresard found in a particular soil. They have no direct impor- tant effect upon the plant, but influence it only in so far as they affect the water present. 112. The value of soil surveys. The full appreciation of the preeminent value of water-content, particularly of the chresard, greatly simplifies the ecological study of soils. The ecologist is primarily concerned with soil water only in its relation to the plant, and while a fair knowledge of soil structure is essential to a proper understanding of this, he has little concern with the detailed study of the problems of soil physics. For the sake of a proper balance of values, he must avoid the tendency noted elsewhere of ignoring the claims of the plant, and of studying the soil simply as the seat of certain physical phenomena. Accordingly, it is felt that mechanical and chemical analyses, determinations of soluble salt-content, etc., have much less value than has been commonly supposed. The usual methods of soil survey, which pay little or no attention to water-content, and none at all to available water, are practically valueless for ecological research. This state- ment does not indicate a failure to appreciate the importance of the usual soil methods for many agricultural problems, such as the use of fertilizers, conservation of moisture, etc., though even here to focus the work upon water-content would give much more fundamental and serviceable results. For these reasons, slight attention will be paid to methods of mechanical and chemical analysis. In their stead is given a brief statement of the origin, structure, and character of soils with especial reference to water-content. 113. The origin of soils. Rocks form soils in consequence of weathering, under the influence of physical and biotic factors. Weathering consists of two processes, disintegration, by which the rock is broken into component particles of various sizes, and decomposition, in which the rock or its frag- ments are resolved into minute particles in consequence of the chemical disaggregation of its minerals, or of some other chemical change. These processes are usually concomitant, although, as a rule, one is more evident than the other. The relation between them is dependent upon the character 78 THE HABITAT of the rock and the forces which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a rule disintegrate more rapidly than they decom- pose; sedimentary rocks, on the other hand, tend to decompose more rapidly than they disintegrate. In many cases the two processes go hand in hand. This difference is the basis for the distinction, first proposed by Thurmann, between those rocks which weather with difficulty and those which weather readily. The former were called dysgeogenous, the latter eugeogenous. Thurmann restricted the application of the first term to those rocks which produce little soil, but it seems more logical to apply dysgeogenous to those in which disintegration is markedly in excess of decomposition, and eugeogenous to those rocks that break down rather readily into fine soils. With respect to the general character of the soil formed, rocks are pelogen- ous, clay-producing, psammogenous, sand-forming, or pelopsammogenous, producing mixed clay and sand. The first two are divided into perpelic, hemipelic, oligopelic, perpsammic, etc., with reference to the readiness with which they are 'weathered, but this distinction is not a very practicable one. The grouping of soils into silicious, calcareous, argillaceous, etc., with reference to the chemical nature of the original rock, is of no value to the ecologist, apart from the general clue to the physical properties which it furnishes. 114. The structure of soils. The water capacity of a soil is a direct result of the fineness of the particles. Since the water is held as a thin surface film by each particle or group of them, it follows that the amount of water increases with the water-holding surface. The latter increases as the particles become finer and more numerous, and thus produce a greater aggregate surface. The upward and downward movements of water in the soil are likewise in immediate connection with the size of particles. The upward or capillary movement increases as the particles become finer, thus making the irregular capillary spaces between them smaller, and magnify- ing the pull exerted. On the contrary, the downward movement of gravita- tion water, i. e., percolation, is retarded by a decrease in the size of the soil grains and hastened by an increase. Hence, the two projyerties, capillarity and porosity, are direct expressions of the structure of the soil, i. e., of its texture or fineness. Capillarity, however, increases the water-content of. the upper layers permeated by the roots of the plant, while porosity decreases it. On the basis of these properties alone, soils would fall into two groups, capillary soils and porous soils, the former fine-grained and of high water- content, the latter coarse-grained and with relatively little water. A third factor, however, of great importance must be taken into account.. This is the pull exerted upon each water film by the soil particle itself. This pull ap- SOIL 79 parently increases in strength as the film grows thinner, and explains why it finally becomes impossible for the root-hairs to draw moisture from the soil. This property, like capillarity, is most pronounced in fine-grained soils, such as clays, and is least evident in the coarser sands and gravels. It seems to furnish the direct explanation of non-available water, and, in consequence, to indicate that the chresard is an immediate result of soil texture. 115. Mechanical analysis. From the above it is evident that, with the same rainfall, coarse soils will be relatively dry, and fine soils correspondingly moist. However, this difference in holard is somewhat counterbalanced by the fact that the chresard is much greater in the former than in the latter. The basis of these relations can be obtained only from a study of the tex- ture of the soil. The usual method of doing this is by mechanical analysis. This is far from satisfactory, since the itse of the sieves often brings about the disaggregation of groups of particles which act as units in the soil. Furthermore, the analysis affords no exact evidence of the compactness of the soil in nature, and tests of capillarity and porosity made with soil sam- ples out of position are open to serious error. Nevertheless, mechanical analyses furnish results of some value by making it possible to compare soils upon the basis of texture. For ecological purposes, mi- nute analyses are undesirable; their value in any work is doubtful. A separation of soil into gravel, sand, and silt-clay is sufficient, since the relative proportion of these will explain the holard and chresard of the soil concerned. The latter are also affected in rich soils, especially of forests, by the organic matter present. If this is in a finely divided condition, the amount is de- termined by calcining. When a definite layer of leaf- Fig. 25. Sieves for mold is present, as in forests and thickets, its water-value s i n y . is found separately, since its power of retaining water is altogether out of proportion to its weight. 116. Kinds of soils. It is very doubtful whether it is worth while to at- tempt to distinguish soils upon the basis of mechanical analysis. Un- questionably, the most satisfactory method is to distinguish them with respect to holard and chresard, and to regard texture as of secondary im- portance. A series of soil classes which comprise various soil types has been proposed by the U. S. Bureau^ 'of Soils as follows: (i) stony loam, (2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine, sand, (7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) silt loam, (12) clay loam, (13) clay, (14) adobe. These are based ' Instructions to Field Parties and Descriptions of Soil Types, 35. 1903. 8o THE HABITAT entirely upon .mechanical analyses, and in some cases are too closely related to be useful. The line between them can nowhere, be sharply drawn. In- deed, the variation within one class is so great that soils have frequently been referred .to the wrong group. Thus, Cassadaga sand (gravel 22 per cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely re- lated to Oxnard sandy loam (26-37-18-12) and to Afton fine sandy loam (28-43-18-8) than to Coral sand (61-29-3-4), Galveston sand (6-91- i-i), or Salt Lake sand (84-15-1-0). Elsinore sandy loam (8-38- 35-10) is much nearer to Hanford fine sandy loam (9-36-33-14) than to Billings sandy loam (1-60-22-11) or to Utuado sandy loam (48-23- 19-8). The soil types are much more confused, and for ecological pur- poses at least are entirely valueless. Lake Charles fine sandy loam has the composition, 1-34-52-9 ; Vernon fine sandy loam, i-37-54-7> while many other so-called types show nearly the same degree of identity. 117. The chemical nature of soils. The effect of alkaline and acid sub- stances in the soil upon water-content and the activities of the plant is far from being well understood. It is generally recognized that salts and acids tend to inhibit the absorptive power of the root-hairs. In the case of saline soils, this inhibitive effect seems to be established, but the action of acids in bogs and swamps is still an open question. It is probable that the influence of organic acid has been overestimated, and that the curious anomaly of a structural xerophyte in a swamp is to be explained by the stability of the ancestral type and by the law of extremes. Apart from the. effect which excessive am.ounts of acids and salts may have in. reducing the chresard, (the chemical character of the soil is powerless to produce structural modification in the plant.) Since Thurmann's researches there has been no real support of the contention that the eheinical properties of the soil, not its physical nature, are the decisive factors' in the distribution and adaptation of plants. It is not sufficient that thg vegetation of a silicious soil differs from that of a calcareous one. . A soil can modify the; plants upon it only though its water-content, or the solutes it contains. Hence, the chemical composition of the original rock is immaterial, except in so far as it modifies these two factors. Humus, moreover, while an important factor in growth, has no formative influence beyond that which it exerts through water-content. PHYSIOGRAPHY 118. Factors. The physiographic factors of a definite habitat are altitude, exposure, slope, and surface. In addition, topography is a general though less tangible factor of regions, while the, dynamic forces of weathering,. PHYSIOGRAPHY 8 [ erosion, and sedimentation play a fundamental role in the change of habi- tats. It is evident, however, that these,- except where they affect the de- struction of. vegetation directly, can operate upon the plant only through more direct factors, such as water, light, and temperature. While they are themselves not susceptible of measurement, they can often be expressed in terms of determinable factors, i. e., slope, exposure, and surface. Fun- damentally, they constitute the forces which change one habitat into another, and, in consequence, are really to be considered as the factors which pro- duce succession. The static features of physiography, altitude, etc., lend themselves readily to determination by means of precise instruments. These- factors, though by no means negligible, are remote, and consequently their mere measurement is insufficient to indicate the nature or extent of their influence upon the plant. It is necessary to determine also the manner and degree in which they affect other factors, a task yet "to be done. Readings of altitude, slope, and exposure are so easily made that the stu- dent must carefully avoid the tendency to let them stand-- at' their own value, which is slight. Instead, they should be made the starting point for ascertaining the difl'erences which they produce in water^ontent, humidity, wind, and temperature. Altitude 119. Analysis into factors. Of all physiographic features, altitude is the most difficult to resolve into simple factors. Because of general geographic relations, it has a certain connection with rainfall, but this is vague and inconstant. Obviously, in its influence upon the plant, altitude is really pressure, and in consequence its effect is exerted upon the climatic and not the edaphic factors Of the habitat. Theoretically, the decrease af air- pressure in the increased altitude directly affects humidity, light, and tem- perature. Actually, while there is unquestionably a decrease in the ab- sorption of the light and heat rays owing to the fact that they traverse less atmosphere, which is at the same time less dense, this seems to be negligible. Photometric readings at elevations of 6,000 and 14,000 feet have so far failed to show more than slight differences, which are alto- gether too small to be efficient. The effect upon humidity is greater, but the degree is uncertain. Continuous psychrographic records at different ele- vations for a full season, at least, will be necessary to determine this, since the psychrometric readings so far made, while referred to a base psychro- graph, are too scattered to be conclusive. Finally, the length of the sea- son, itself a composite, is directly dependent upon the altitude. This rela- tion, though obscure, rests chiefly upon the rarefaction of the air which prevents the accumulation of heat in both the soil and the air. 82 THE HABITAT 120. The barometer. To secure convenience and accuracy in the de- termination of altitude, it is necessary to use both a mercurial and an aneroid barometer. ITie latter is by far the most serviceable for. field work, but it requires frequent standardizing by means of the former. The mer- curial form is much more accurate and should be read daily in the base station. It is practically impossible to carry it in the field, except in the so-called mountain form, which is of great service in establishing the alti- tudes of a series of stations. In use the aneroid barometer may be checked daily by the mercurial standard, or it may be set at the altitude of the base station, thus giving a direct reading. After the normal pressure at the base has once been ascertained, however, the most satisfactory method is to set the aneroid each day by the standard, at the same time noting the pressure deviation in feet of elevation (see p. 46). The absolute elevation of the var- ious stations of a series may be determined either by adding or subtracting this devia- tion from the actual reading at the station, or by noting the change from the base station, and then adding or subtracting this from the normal of the latter. When it is impossible to check the aneroid by means of a mercurial barometer, the average of a series of readings made at different days at one station, especially if taken during settled weather, will practically eliminate the daily fluctuations, and yield a result essentially accurate. Even in this event, the accuracy of the aneroid should be checked as often as possible, since the mechanism may go wrong at any time. The barograph, while a valuable instrument for base stations, is not at all neces- sary. These instruments can be obtained from all makers of meteorological apparatus, such as H. J. Green, and J-. P. Friez. Aneroid barometers reading to 16,000 feet cost about $20 ; the price of the Richards aneroid barograph is $45. Ordinary observatory barometers cost $30-$4o; the standard instru- ment sells at $75-$ioo. The mountain barometer, which is altogether the most serviceable for the ecologist, ranges from $30-$55, depending upon accessories, etc. Fig. 26. Aneroid barometer. PHYSIOGRAPHY 83 Slope 121. Concept. This term is used in the ordinary sense to indicate the relation of the surface of a habitat to the horizon. Although it is a com- plex of factors, or rather influences several factors, these are readily de- terminable. The primary effect of slope is seen in the control of run- off and drainage, and consequently of water-content, although these are likewise affected by soil tex- ture and by surface. Slope, more- over, as a concomitant of exposure, has an important bearing upon light and heat by virtue- of deter- mining the angle of incidence, and also tipon wind, and, through it, upon the distribution of snow. At present, while it can be expressed definitely in degrees, it has not yet been connected quantitatively with more direct factors. This is, how- ever, not a difficult 'task, and it is probable that we shall soon come to express slope principally in amount of run-off, and of incident heat. 122. The clinometer. In the simplest form, this instrument is merely a semicircle of paper, with each half graduated from 1-90". It is mounted on a board and placed base upward, upon a wooden strip, 2 feet long and 2 inches wide, which has a true edge. At the cen- ter of the circle is attached a line and plummet for reading the per- pendicular. A more convenient form is shown in figure 28, which is both clinometer and compass. This also necessitates the use of a Fig. 27. Mountain barometer: (a) in carry- ing case; (t>) set up for use. 84 THE HABITAT basing strip to eliminate the inequalities of the surface. The dial face is graduated to show inches of rise per yard, as 'well as the number of de- grees, but the latter, as the simpler term, is preferable for ecological work. In making a reading, the basing strip is placed upon a representative area of the slope, and pressed down firmly to equalize slight irregularities. The clinometer is moved slightly along the upper edge, causing the marker to swing freely. After the latter comes to rest, the instrument is carefully turned upon "its back, when the angle of the slope in degrees may be read directly. Two or three such readings in different areas will suffice for the entire habitat, unless it be extremely irregular. The clinometer with com- pass may be obtained from the Keuffel and Esser Company, iii Madison St., Chicago, Illinois, for $5. Fig. 28. Combined clinometer and compass. 123. The trechometer. For measuring the effect of slope upon run-off, a simple instrument called the trechometer (rpi-xu), to run off) has been devised. This consists merely of a metal tank, 3 x 4 x 12 inches, hold- ing 144 square inches of water, with an opening J4 x 12 inches at the base in front, closed by a tight-fitting slide. Three metal strips, 2 x 12 inches, are fastened to the front of the tank in such a way as to enclose a square foot of soil into which the strips penetrate an inch. In the front strip is an opening, i inch square, provided with a drip from which the run-off is collected in -a measuring vessel. In use, the instrument is put in position with the metal rim forced down i inch into the soil ; the tank is filled, the graduate put in place, and the slide raised. The run-off for a square foot is the amount of water caught by the graduate, and is represented in cubic inches per square foot. For obtaining results which express slope alone, comparisons must be made upon the same soil, from which all cover, dead and living, has been removed. They must be as closely together in time as possible, at least during the same day, as rain or evaporation will PHYSIOGRAPHY 85 cause considerable error. It is ob-vious that with the same slope or on a level the trechometer may also be used to advantage to determine the ab- sorptive pov^fer of soils of different texture. It serves well a similar pur- pose when used in different habitats to measure the composite action of slope, soil, and cover in dividing the rainfall into run-off and absorbed water. Exposure 124. Exposure refers primarily to the direction toward which a slope faces, i. e., its exposition or insolation with respect to sun and wind. It is not altogether separable from slope, however, inasmuch as the angle of the slope has some effect upon the degree of exposure. The chief influence of exposure is exerted through temperature, since slopes longest exposed to the sun's rays receive the most heat. This is supplemented in an important degree by the fact that a group of rays i foot square will occupy this area only on slopes upon which they fall at right angles. In all other cases the rays are spread over a longer area, with a consequent reduction in the amount of heat received. This effect is felt principally in evaporation from the soil, and in soil temperatures. For the leaf, it is largely if not entirely negligible, since the angle of incidence is determined by the position of the leaf, which is the same for each species whether on the level or upon a slope. On this account, exposure has little or no bearing upon light, except that the total amount of light received by the aggregate vegetation of a slope will be greater than for a level area of the same size. The effect of wind varies with the exposure. It is naturally most pronounced in those directions from which the prevailing dry or cold winds blow, and it is greatly emphasized by the fact that the opposite exposure is corres- pondingly protected. The influence of wind, especially in producing evaporation from the plant and the soil, increases with the slope, since the mutual protection of the plants, or that afforded the soil by the cover, is much reduced. Finally, the distribution of the snow by the wind, a matter of considerable importance for early spring vegetation, is largely determined by exposure. Exposure is expressed directly in terms of direction, to which is added the angle of the slope. A good field compass, reading to twelve points, is sufficient. It should be checked, of course, by the declination of the needle at the place under observation. A convenient instrument is the one already mentioned, in which compass and clinometer are combined, since these are regularly used at the same time. 125. Surface. The most important consideration with respect to surface is the presence or absence of cover, and the character of the latter. With 86 THE HABITAT the exception of snow, cover is, however, a question of vegetation, living and dead, and consequently is to be referred to the discussion of biotic factors. The surface of the soil itself often shows irregularities which must be taken into account. Such are the rocks of boulder and rock fields, the hummocks of meadows and bogs, the mounds of prairie dog towns, the innumerable minute gullies and ridges of bad lands, the raised tufts of sand-hills, etc. The influence of these is not profound, but they do have an appreciable effect upon the run-off, temperature, and wind. In many cases, this is distinctly measurable, but as a rule little more can be done than to indicate that the surface is even or uneven, and to describe the degree and kind of unevenness. 125. Record of physiographic factors. Altitude, slope, exposure, and surface are essentially constant factors, and are determined once for all, after a few check readings have been made, except in those relatively rare habitats in which dynamic forces are very active. The form of record used is the following : DATE FORMATION STATION GROUP ALTITUDE SLOPE EXPOSURE SURFACE 10/7/02 Gravel slide.. Golf Woks... Eriogonare... 2700 m. 230 N.N.W. Even '■ Brook bank.. Jack Brook... Violare 2550 m. 50 E.N.E. " " Half gravel .. Hiawatha .... Achilleare.... 2600 m. 143 E. Uneven " Spruce Milky Way... Opulasterare. 2625 m. 120 N. Even 127. Topography. As heretofore indicated, questions pertaining 'to the form and development of the land concern groups of habitats within which each habitat is the unit of investigation after the manner already laid down. A knowledge of topography is essential to the accurate mapping of a region, for which the simple methods of plane table and contour work are ■ employed, while the geology of the surface is of primary importance in the study of successions. BIOTIC FACTORS 128. Influence and importance. Biotic factors are animals and plants. With respect to influence they are usually remote, rarely direct. Neverthe- less, they often play a decisive part in the vegetation. Their effect is, as a rule, felt directly by the formation rather than the habitat, but in either case the one reacts upon the other. Such factors are not themselves sus- ceptible of exact measurement, but their influence upon the habitat is usually measurable in terms of the physical factors affected. In the case EIOTIC FACTORS 87 of biotic factors, it must be distinctly understood that these -are not properly factors of the habitat as a physical complex, but that they are rather to be considered as reactions exerted by the effect, or formation, upon the cause or habitat. This is most especially true of plants. 129. Animals. The activities of man fall into two classes: (i) those that destroy vegetation, and (2) those that modify it. There are rare in- stances also where the work of man has changed a new or already denuded habitat. In the cases where the vegetation is destroyed, the habitat itself is sufficiently changed to permit the effect to be measured by physical factor instruments. Otherwise, the influence is felt only by the formation, as when man makes possible the migration of weeds, and it can be measured in terms of invasion by the quadrat alone. It becomes especially evident, then, in the case of man's activities, that where they produce a denuded habitat they are to be regarded as factors in the habitat ; when they merely affect the formation, this is not strictly true. The changes wrought by other animals are essentially the same as those produced by man. They are not so marked nor so important, but their relation to habitat and forma- tion is the same. As a rule, however, they affect the habitat much less than they do the formation. 130. Plants. As a dead cover, vegetation is a factor of the habitat proper, but it has relatively little importance, since it occurs regularly dur- ing the resting period. Its chief effects are in modifying soil temperature, and in holding snow and rain, and thereby increasing the water-content. By its gradual decay, moreover, it not only adds humus to the soil, but it thereby increases the water-retaining capacity of the latter also. The cover of living vegetation rfeacts upon the habitat in a much more vital fashion, exerting a powerful effect upon every physical factor of the habitat. The factors thus affected are distinctly measurable though it is often impossible to determine just how much of the factor is directly trace- able to the ' vegetation. This is a simple problem in the case of most aerial factors, especially light, blit it is extremely difficult for soil factors, such as water-content and soil texture. In the case of all habitats covered with formations, by far the great majority, it is impossible as well as unneces- sary to separate the physical factors of the habitat proper from the re- action upon them which the plant covering exerts. Indeed, the great differ- entiation of habitats is largely due to the universal principle that in vegetation the effect or formation always reacts upon the cause or habitat in such a way as to modify it. As fundamental causes of succession, the discus- sion of the various reactions of vegetation is reserved for another place. .88 THE habitat- Methods OF Habitat Investigation 131. The use of the various instruments previously described depends largely upon the preponderance of simple instruments or recording ones. The former necessitate a number of well-trained assistants; the latter re- quire only a part of the time of one investigator. For the most satis- factory results, however, an assistant is all but indispensable. Since sim- ple instruments are most easily obtained because of their cheapness, and are especially adapted to purposes of instruction, the method of using them will be described first, and then that of ecograph batteries. THE METHOD OF SIMPLE INSTRUMENTS 132. Choice of stations. This method is based upon simultaneous read- ings by means of simple instruments in a series of habitats, or of stations Fig. 29. Series of stations; I, at Minnehaha; II, at Lincoln in the prairie formation. in a single habitat. Such readings are necessary for 'the variable atmos- 'pheric factors, humidity, light, temperature, and wind. . Frequent read- ings suffice for water-content and precipitation, while only two or three determinations, enough to check out the error, are necessary for the con- stant factors, altitude, slope, exposure, and surface. An account of the exact procedure employed in class study at Lincoln and Minnehaha will best serve to illustrate the use of this method. The series of stations chosen at Lincoln' were primarily within a single formation, for the purpose of determining the physical factor variation in different areas. One series was located in the prairie-grass formation (Koelera-Andropogon-psilium) , and consisted of the following stations: (i) low prairie, (2) crest of ridge I, METHOD OF SIMPLE INSTRUMENTS S^ (3) northeast slope of ridge I, (4) grassy -ravine, (5) southwest slope of ridge II, (6) bare crest of ridge II, (7) thicket ravine. The other series was established in the bur-oak-hickory forest (Quercus-Hicoria-hylium) at the following stations: (i) thicket, (2) woodland, (3) knoll in forest, (4) depression in forest, (5) level forest floor, (6) nettle thicket, (7) brook bank. At Minnehaha the series was primarily one of different formations: (i) the pine formation (PimtS'xerohylium), (2) the gravel-slide formation {Pseudocymopterus-Mentzelia-chalicium), (3) east slope of spruce forest (Picea-Pseudotstiga-hylium) , (4) • ridge in the spruce forest, (5) north slope of spruce forest, (6) brook bank in forest, (7) the thicket formation (Quercus-Cercocqrpus-lochmodium), (8) the aspen formation (Populus hylium). When permanent or temporary quadrats are established, they are ordinarily used as regular stations, • since this enables one to refer the physical factor readings to a few definite individual plants, as well as to the entire formation. The transects in figure 29 illustrate two of the above series of stations. 133. Time of readings. The frequency of simple readings and the times at which they are made must be regulated largely by opportunity and convenience. In addition to making readings once or twice a week through- out the season, the series should be read at least once every day for a rep- resentative week or two. It is also very desirable to have a series for each hour of a typical day, or of two days, one of which is clear, the other cloudy. When a single daily reading is made, it should be taken at or as near me- ridian as possible. The usual series is the one obtained by simultaneous observations at the same level in different stations. An important series is also secured by simultaneous readings at the various levels of the same station, though it is not necessary to take this series frequently. 134. Details of tlie mettiod. After the stations have been selected by a careful preliminary survey of the habitat or series of habitats, their location is indicated by a small flag bearing a number, in case there is no danger of these being disturbed. Otherwise, less conspicuous stakes are used. The ordinary practice is to visit each station of the series, and to take readings of water-content, altitude, slope, and exposure. On the first trip these are all made by the instructor, but after a short time the determination of each factor may be assigned in rotation to each of the students. After these constant factors have been read and recorded, one student is equipped with photometer, thermometer, and psychrometer, and, if desirable, anemometer, and left at the first station. At each succeeding station the same plan is followed, so that at the end of the series the constant factors have all been 90 THE HABITAT read, and there is an observer at each station prepared to make readings of the variable ones. The task of acquainting the students with the operation of photometer, psychrometer, etc., can best be done in class or at a previous field period, as it is evident that they must be familiar with the instruments before they can use them accurately in the field series. The details of oper- ation have already been given and need not be repeated here. The task of obtaining readings at the same moment may be met by supplying each observer with a watch, which runs exactly with all the others, or by making observations upon signal. The second means has been found most success- ful in practice, since the signal fixes the attention at the exact moment. Fig. 30. A denuded station in the aspen formation. The best plan is for the instructor to occupy a commanding position some- where near the middle of the series, and to give the signals by shout or whistle at the proper interval. Considerable care and experience are neces- sary to do the last satisfactorily. Sufficient time must be given for the operation of the instrument and the making of the record. In addition, a period must be perm.itted to elapse which is long enough for every instru- ment to reach the proper reading. For example, in a series which contains a gravel slide and a forest, the thermometer which has just been used for an air reading will require four or five times as long an interval to respond to the temperature of the gravel as to that of the cool forest floor. In such series, the instructor should regularly take his place in the station where METHOD OF SIMPLE INSTRUMENTS gi the response is slowest or greatest. He must record the exact time of each signal, and note any general changes of sky or wind that produce temporary fluctuations at the time of reading. When the readings extend over 'a whole day, the usual plan is to begin at the last station and take a second series of water-content samples, noting the exact time in order that the rate of water loss may be determined. A check series of physiographic factors may be made at this time also, or this may be left for future visits. While it is un- necessary to take soil samples oftener than once a day, it is important to inake at least one series at each visit. Sometimes it becomes desirable to know the rate of water loss in different stations during the day, and in this event, samples are taken at one or two hour intervals for the entire day. In making simultaneous readings at the different levels of one station, the observers are grouped in one spot in such a way that they do not interfere with the correct reading of each instrument. Readings of this sort are most valuable in the case of temperature, which shows greater differences at the various levels. Important differences of humidity and wind also, are readily obtained, and, in layered formations, marked variations in the amount of light. In the open, the ordinary levels for temperature are 6 feet, 3 feet, surface, 5, 10, and 15 inches in the ground, and for wind and humidity, 6 feet, 3 feet, and surface. In forests the same levels are used for comparison with formations in the open, but a more desirable series for light especially is secured by making readings at the height of, or better, just below the var- ious layers. Series of this sort are likewise made on signal. The best time of day is that of a period in which the middle station is read near meridian, since the variation due to time is sufficiently small to permit fairly accurate comparisons between the readings for the different stations. 135. Records. The form used for recording the observations made by means of simple instruments is shown below. It is hardly necessary to state that it may be readily modified to suit the needs of different investigators. Ordinarily, each sheet is used for the records of one habitat or series alone, but for convenience sake, the records of two different series are here com- bined. The figures given are taken from records for the prairie and forest formations at Lincoln. 92 THE HABITAT Koelera-Andropogpn-psilium April 25, 1901. Clear. South wind. TEMPERATURE LIGHT HUMIDITY WATER- CONTENT WIND TIME P.M. 3:20 3:24 3:30 3:35 3:40 3:45 3:55 % 2:40 3:05 STATION. l^m. Surf. 5 10 15 l%m. Surf. IMm. Surf. 5 10 15 IKm. Surf. 1 27.8 26.5 26.9 26.2 28 28 26.4 29.6 31.3 28.5 30 32.4 40.8 30.8 17.8 18.3 18 ..2 16.8 18.6 23.8 16 15.8 16.6 14.2 13.2 14.6 16 13 10.9 12.8 13.5 11.6 14.2 15 11 57 59 58 63 59 51 68 59 59 59 66 60 51 70 17.7 17.9 16.5 24.4 10.7 5 27 14.6 12.2 16.9 21.3 17 8.3 24.3 17.4 14.3 19 24.8 17.2 10.3 21.4 740 1100 980 920 1080 1010 680 280 2. 510 3: 520 4 460 5 490 6; 410 7 52 Q n ercus-Hicpria-hy Hum April 20, 1901. Clear. Southeast wind. TEMPERATURE LIGHT HUMIDITY WATER- CONTENT -WIND TIME A.M. 10:40 10:46 10:50 10:55 11:00 12:00 12:05 11:10 11:20 % 11:30 11:45 STA- TION l^m. Surf. 5 10 15 l^m. Surf. IJ^m. Surf. 5 10 15 IJ^m. Surf. 1 2 3,.... 4 5..... 6 7 16 16.2 16.2 15,6 17.6 16.2 15.8 25 30.5 17.8 26.2 25.4 20.2 17.2 9.6 8.5 7.6 10.6 7.6 8.4 6.4 8.4 8.4 7.8 8.4 7.4 7 6.4 7.Si 7.8 8 8.2 7.2 6.2 6.1 .08 .11 .08 .06 .03 .02 .05 .06 .09 .06 .03 .02 .01 .04 73 73 73 81 90 32 82 81 86 95 95 95 90 90 24.2 22 22.1 25.4 27.2 27.6 23.8 19.2 22.5 20.4 23.1 19.8 20.8 19 19.5 19.4 21.6 22.4 18.8 18.8 19.3 298 . 375 640 275 178 115 60 2 6 12 2 4 METHOD OF ECOGRAPH BATTERIES 136. A battery of recording instruments, consists of a sciagraph, a psy- chrograpli, and a thermograph, to which an anemograph is added when pos- sible. As stated before, the determination of water-content by the geotome method is more satisfactory than by any automatic instrument yet devised. When the base station is located where the sunlight is unobstructed, which should be the case whenever possible, it is unnecessary to include a sciagraph in those batteries placed in similarly exposed stations, since the light values will be the same. As a rule, batteries are established within different zones or different habitats, except where a highly diversified habitat is made the subject of special inquiry. Such a restriction arises from the fact that ex- pense, care of operation, etc., place a limit upon the number of batteries, METHOD OF ECOGRAPH BATTERIES 93 and, in such case, the task of primary importance is to establish the physical character of representative habitats. For these reasons, the first series of thermographs established in 1903 was located with respect to altitude, the instruments being placed at Manitou 2,000 m., Minnehaha, 2,600 m., and Mount Garfield 3,800 m. In 1904, the stations established for the record of temperature and humidity were situated with respect to habitats represent- ing the four formations : gravel slide, half gravel slide, spruce forest, and brook bank. The batteries are located and set up according to the directions already given. A 2-meter quadrat with the battery as the middle is staked and rnapped. Within this, all readings of water-content, soil temperature, and physiographic factors are made. Altitude, slope, exposure, and cover are recorded when each battery is located, and a soil sample is taken for me- chanical analysis; When the position of the batteries permits it, water-con- tent readings should be made frequently, once or twice a week at least. In addition, a complete series of samples should be taken daily for a period sufficient, to indicate the ordinary extremes of water-content. , The ecograph battery of each habitat constitutes a standard to which the results obtained by simple instruments may be referred with accuracy. It not only does this, but it also serves as a basis for interpreting the readings of simple instruments in distant habitats of the same character. In this way a few batteries judiciously placed make possible the exact physical investiga- tion of a large number of habitats, covering a considerable area. The only limit, indeed, upon this method is that placed by time. The proportionate use of batteries and of simple instruments must be largely determined by the conditions which confront the investigator. It is obvious that, where expense is not a decisive factor, the gain in time and in completeness of results is enormously in favor of the battery. There is an additional value in the automatic and continuous record which can not be overlooked. When the use of instruments in the study of habitat and formation becomes uni- versal, the importance of the ecograph will be immeasurably enhanced. It v^fill be possible to secure duplicate records of batteries located in the most remote and diverse regions, from the equator to the poles, and comparative phytogeography upon a scientific basis will for the first time be possible. This opens an alluring vista of the future when ecologists the world over will cooperate in such a way that the. results obtained by ecograph batteries anywhere on the globe will permit of exact comparison. 94 THE HABITAT THE EXPRESSION OF PHYSICAL FACTOR RESULTS 137. The form of results. It is almost inevitable that the general adop- tion of precise methods of measuring the habitat will result in a common form for expressing the physical character of the latter. An actual diag- nosis of each habitat is not a difficult matter, after the factors are carefully measured, and will unquestionably lead to very desirable definiteness and precision. The accurate investigation of the physical factors of a number of habitats for one growing season furnishes the necessary material for a diagnosis based upon the mean for the growing season. Similar results for two or three seasons will yield a diagnosis as accurate and as final as that of a formation, or, indeed, as that of many species. The author's investi- gations have not yet gone far enough to warrant proposing a final form for this, but the following diagnosis is offered as a suggestion: • Elymus-Muhlenbergia-chalicium. Habitat: holard 9 per cent, chresard 8 per cent, relative humidity 40 per cent, light 0.6, soil colluvial gravel (gravel 70 per cent, sand 27 per cent, silt 3 per cent), air temperature 65", surface 82°, soil 59°, wind 10 miles, rainfall 8 inches, altitude 2,800 m., slope 23°, exposure south, surface even, cover open, no active biotic agencies. The detailed comparison of habitats is made most readily by the graphic method of curves, which constitute the most desirable form of expression in connection with the original record upon which they are based. Factor means are particularly desirable for diagnostic purposes, and they furnish valuable curves also. Factor sums are impracticable at present, and it seems doubtful that they will ever be of much value. It is by no means impos- sible, however, that a more detailed and exact knowledge of the physiology of adaptation, coupled with methods of precision in the habitat, will render them necessary. Factor Records 138. Experience has shown that the practice of making hasty and often formless records in the field is unwise and is apt to be inaccurate as well. The time saved in the field is more than counterbalanced by that consumed in copying the results into the permanent form. The danger of error in field notes rapidly taken is very grave, and the chance of confusion and the waste of time in deciphering them are great. Moreover, the task of checking a copy with the original, which is absolutely necessary for accuracy, involves a further expenditure of time and energy. For these reasons the field record should be made in permanent form. Definite record sheets are used, and the invariable rule is made that all readings are to be noted in ink at the time and spot where they are taken. On a long journey, or in the face of PHYSICAL FACTOR RESULTS 95 many observations, the tendency to take notes or to record observations rapidly is very great, but this will correct itself after a few attempts to use such notes. The record forms for various factors have been indicated in the proper place, as well as the one for simultaneous readings. Ecograph sheets are carefully filed, and constitute permanent records. With a little practice they may be read almost as easily as tables, and any attempt to put them into tabular form is a mere waste of time. For purposes of study and of publication, it often becomes necessary to bring together all the results obtained for _a particular habitat, both by simple instruments and by ecc^raphs. The form of record used for this is essentially that already indicated for simultaneous readings on page 92, since general features and constant factors can not well be included in the table. Record sheets of this type have been printed at a cost of $5 per thousand, and the various factor records can be obtained at about the same rate. The size of sheet used is gyi x yj4 inches. The record book is the usual note-book cover, which has been found neither too large nor too small. It is protected from dirt and rain by a covering of oilcloth which overlaps the edges. Record books should be carefully labeled, and each one should contain a single year's records. Factor Curves 139. Plotting. The paper employed is divided into centimeter squares which are subdivided into 2-millimeter units. For ordinary curves the size of sheet is 93^ x 7^ inches, which makes it possible for curve sheets to be filed in the record book. Tablets containing 60 of these sheets can be obtamed for 20 cents each from the Central School Supply House, Chicago. For curves longer than 9 inches special sizes of sheets must be used. On account of their inconvenience large sheets are avoided whenever possible. This can usually be accomplished by increasing the numerical value of the intervals. The inks employed in plotting are the waterproof inks of Chas. Higgins & Co., Brooklyn, New York. These are made in ten or more colors, black, violet, indigo, blue, green, yellow, orange, brown, brick red, carmine, and scarlet, and cost 25 cents per bottle. In addition to being waterproof, they make it possible to combine curves in all conceivable ways without destroying their identity. Furthermore, it is a great advan- tage to use the same color invariably for the same kind of curve : thus, it has been the practice to indicate the 3-foot, surface, 5, 10, and 15-inch temperature curves by violet, green, yellow, blue, and carmine respectively. A fine-pointed pen, such as the Spencerian No. i, is most satisfactory for inking; drawing pens, such as Gillott's Crowquill, are too finely pointed for ordinary use. 96 THE HABITAT In plotting a curve, it is first necessary to determine the value of the interval, and the extreme range of the curve or combination. For example, in the case of temperature, it is most convenient to assign a value of i° Centigrade to each centimeter, since the thermometers used read to one- fifth of a degree, which corresponds exactly to the 2-millimeter units of each square. The length of the sheet permits a range of 22 degrees Centigrade, and the actual limits must be deterniined for the particular results to be employed. For the same region, it is very desirable that the unit interval and the range be the same, in order that all curve sheets may admit of direct comparison. Indeed, it is greatly to be hoped that in the future ecologists will agree to a uniform system of curve-plotting, cartography, etc., as the geographers are beginning to do in the construction of maps. Tlie major intervals are written, or, better, typewritten, at both sides of the sheet, and the time or space intervals are indicated at the top. Each curve sheet is properly labeled, and essential data indicated. The readings are taken from the field record, and their proper positions indicated by a dot. These are connected first by a pencil line, the curves being made abrupt rather than flowing; and the line, after having been carefully checked, is traced in ink. 140. Kinds of curves. Curves are named both with reference to the factor concerned and the position or sequence of the readings. The factors which lend themselves most readily to this method of representation are the variable ones, water-content, humidity, light, temperature, and wind, and corresponding curves are distinguished. Altitude and slope may likewise be shown by means of curves, but the use of cross section or con- tour lines serves the same purpose and is more natural. With regard to time and position, curves are distinguished as level, station, and point curves. A level curve is one based upon readings made at the same level through a series of stations or of habitats, e. g., the level curve of surface temperature. The station curve represents the various levels or points at which readings are made in a single station. The point curve has for a basis the hourly or daily variation of a factor at a particular point or level in a station. All of these may be simple curves, Vv'hen established upon a single reading for a series, or mean curves when they are based upon the mean of a number of readings. Curves which show the extremes of a factor, i. e., the maximum and minimum, are also extremely- valuable, though a combination of the two for comparison is preferable. 141. Combinations of curves are invaluable for bringing similar curves together, and permitting ready comparison of them. For this, and also because they save space, they are regularly employed to the almost complete PHYSICAL FACTOR RESULTS 97 exclusion of single curves. Combinations are made simply by tracing the curves to be compared upon the same sheet, it being understood that dis-, similar curves, e. g., level and station, can not be combined. Colored inks are an absolute necessity in ^combining; the primary principle underlying their use is that curves that approach closely or cross each other must be traced in inks that contrast sharply. As elsewhere stated, it has been made the invariable rule to use the same color for the same level or point. This applies especially to temperature, but holds also for humidity, light, wind, and water-content, so that the color always indicates the level. For the same reason, it is applied to a combination of point curves for one station, though it is inapplicable to a series of point curves when these lie in the same level. Light readings above 6 feet and water-content readings below 15 inches necessitate the use of additional colors. Combinations may be made of the curves of a single factor for purposes of comparison, or they may consist of curves of different factors in order to aid in interpreting or indicating their relation to each other. Curves of the same factor may be combined to form various series. The level series consists of all the level curves for the stations under observation, e. g., the six levels for temperature, three levels for wind, etc. Similarly, the station series is a combination of all the station curves, and a correspond- ing arrangement may be made for point curves with reference either to station or to level. An extremely valuable combination of curves is that of the holard and chresard for a series of stations. The most important com- binations of the curves of different factors are naturally those based upon factors intimately related to each other or to the plant. The grouping of water-content and humidity curves is of great value, especially when the transpiration curve is added. Light and temperature curves make an in- teresting combination, while a humidity, temperature, and wind series is of much aid in tracing the connection between these factors. Finally, it is altogether feasible to arrange the curves of water-content, humidity, light, temperature, and wind upon the same sheet in such fashion as to give a graphic representation of the whole physical nature of a single habitat or a series. In all combinations of curves representing different factors, it must be borne in mind that the position of a curve does not represent a definite value with reference to the others, since some are based upon per cents, others upon degrees, etc. The comparison must be based upon the character of the curves, but even then it is an important aid. An instruc- tive grouping has been employed where series of readings on the same day, or on two successive days in forest and in prairie have yielded the usual level series of curves. The series for the two habitats are arranged on the same page, one at the right and the other at the left, and permit direct 98 THE HABITAT comparison of corresponding level or factor curves, both with respect to position and character. 142. The amplitude of all the curves described above is determined by the unit values of the factors concerned, while the length is dependent upon the number of stations, points, or times. The value assigned the latter upon the plotting paper is purely arbitrary, but it is most convenient to fix this at the centimeter square. The unit value for temperature is i" Centi- grade per square, each subdivision of the latter representing 0.2,. and the range being 22 degrees. For water-content curves, each square represents a value of 2 per cent, the smaller square being 0.4 per cent, and the range 2-48 per cent. The unit value for humidity is taken as 5 per cent, making each small square i per cent, and giving room on the sheet for the entire range from i-ioo per cent. Owing to the anemometer used, curves of wind velocity have been based upon the number of feet per minute. One hundred feet is taken as the unit value, and the range is from 0-2200 feet. The unit value for the curve of light intensity is .005. Each small square is .001, which permits a range from .001 to .01 on one sheet. Consequently, when it is desired to plot the curve of a series of habitats with a range in intensity greater than this it is necessary to use a double sheet. This is the usual device when the range of curves is too great, except where the excess is slight. In this case the curve is left open at the top, and the value which the crest attains is indicated. All curves in combination are labeled at the beginning or left to indicate the level, station, or point, and at the end or right to show the time, or day, if this is not the basis of the curve or series. The discussion that precedes deals exclusively with curves representing factors determined in the field. It applies with equal force to results obtained by instruments in control houses. In these, however, all factors except those directly experimented with, usually water-content and light, are practically equalized, and the curves based upon them are used chiefly to show how nearly equal they have become. The important curves are those 6t the water-content series, both holard and chresard, and of the shade tents. Where several houses are differentiated with respect to tem- perature or humidity, curve series of both these factors are necessary. Factor Means and Sums 143. It has been shown elsewhere that the daily mean of temperature can be closely approximated from the maximum and minimum of both day and night. Maximum-minimum instruments for the other factors are lacking, however, and for light, humidity, and wind these values can only be ob- PHYSICAL FACTOR RESULTS 99 tained from the ecograph which makes it possible to get the exact mean from the sum of all the hour readings. When it conies to the seasonal mean, the ecograph is even more necessary, exception being made for water-content, in which case a number of readings on various days through the season will suffice. The value of factor means for diagnosis and for curves has already been sufficiently commented upon, and the feasibility of factor sums already indicated. CHAPTER III. THE PLANT Stimulus and Response general relations 144, The nature of stimuli. Whatever produces a change in the func- tions of a plant is a stimulus. The latter may be a force or a material ; it may be imponderable or ponderable; effect, not character, determines a stimulus. Consequently, reaction or response decides what constitutes a stimulus. The presence of the latter can be recognized only through an appreciable or visible response, since it is impossible to discriminate between an impact which produces no reaction and one which produces a merely latent one. From this it is evident that quantity is decisive in determining whether the impact becomes a stimulus. Plants grow constantly under the influence of many stirnuli, all varying from time to time in amount. Small changes in these are so frequent that, in many cases at least, the plant no longer appreciably reacts to them. Such • changes, though usually measurable, are not stimuli. Futhermore, it must be clearly recognized that plants which are in constant response to stimuli are stimulated anew by an efficient in- crease or decrease in the amount of any one of these. As is well known, however, such increase or decrease is a stimulus only within certain limits, and the degree of change necessary to produce a response depends upon the amount of the factor normally present. The entire absence of a force usually present, moreover, often constitutes a stimulus, as is evident in the case of light. The nature of the plant itself has a profound bearing upon the factors that act as stitnuli. Many species are extremely labile, and react strongly to relatively slight stimuli ; others are correspondingly stable, and respond only to stimuli of much greater force. Some light is thrown upon the nature of this difference by the behavior of ecads. A form which has grown under comparatively imiform conditions for a long time seems to respond less readily, and is therefore less labile than one which is sub- ject to constant fluctuation. In many cases this is not true, however, and the degree of stability, i. e., of response, can only be connected in a general way with taxonomic position. 145. The kinds of stimuli. The factors of a habitat are external to the plant, and consequently are termed external stimuli. Properly speak- ing, all stimuli are external, but since the response is often delayed or can GENERAL RELATIONS lOI not be clearly traced, it may be permissible to speak of internal stimuli, i. e., those which appear to originate within the plant. These, however, are extremely obscure, and it is hardly possible to deal with them until much more is known of the action of external stimuli. Of the latter, certain forces, gravity and polarity, act in a way not at all understood, and as they are essentially alike for all plants and all habitats, they can here be ignored. Stimuli are imponderable when, like light and heat, they are measured with reference to intensity, and ponderable, when, as in the case of water-content, humidity, and salt-content, they can be expressed in mass or weight. It is undesirable to insist upon this distinction, however, since the real character of a stimulus is determined by its effect, and the latter is not necessarily dependent upon whether the stimulus is one of force or one of material. There is, however, a fundamental difference between factors with respect to their relation to the plant. Direct factors alone are stimuli, since indirect factors must always act through them. For example, the wind, its mechanical influence excepted, can affect the plant only in so far as it is converted into the stimulus of increased or decreased humidity. Con- sequently, the normal stimuli of the plants of a formation are: (i) water- content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. Soil, pressure, physiography, and biotic factors influence plants only through these, and are not stimuli, though exceptions must be made of biotic factors in the case of sensitive, insectivorous, and gall-producing plants. 146. The nature of response. Since plants have no special organs for the perception of stimuli, nor sensory tracts for their transmission, an ex- ternal stimulus acting upon a plant organ is ordinarily converted into a re- sponse at once. The latter as a rule becomes evident immediately; in some cases it is latent or imperceptible, or some time elapses before the chain of re- sponses finds visible expression. A marked decrease in humidity calls forth an immediate increase of transpiration, but the ultimate response is seen in the closing of the stomata. A response to decreased light intensity, on the other hand, is much less rapid and obvious. This difference is largely due to the fact that the functional response is more marked, or at least more perceptible in one case than in the other. Response is the reaction of the plant to a stimulus ; it begins with the impact of an efficient factor, and ends only with the consequent final read- justment. The immediate reaction is always functional. The nature and intensity of the stimulus determine whether this functional response is followed by a corresponding change in structure. The consideration of this theme consequently gains in clearness if a functional and a structural 102 THE PLANT response be distinguished. The chief value of this distinction lies in the fact that many reactions are functional alone; it serves also to emphasize the absolute interdependence of structure and function, and the imperative need of considering both in connection with the common stimulus. For these reasons, the logical treatment is to connect each stimulus with its proper functional change, and, through this, with the corresponding modi- fication of structure. For the sake of convenience, the term adjustment is used to denote response in function, and adaptation, to indicate the re- sponse in structure. 147. Adjustment and adaptation. The adjustment of a plant to the stimuli of its habitat is a constant process. It is the daily task, seen in nutrition and growth. So long as these take place under stimulation by factors which fall within the normal variation of the habitat, the problems belong to what has long been called physiology. When the stimuli become imusual in degree or in kind, by a change of habitat or a modification in it, adjustment is of much greater moment and is recorded in the plant's struc- ture. These structural records are the foundation of proper ecological study. Since they are the direct result of adjustment, however, this affords further evidence that a division of the field into ecology and physiology is illogical and superficial. Slight or periodical adjustment may concern function alone; it may be expressed in the movement of parts or organs, such as the closing of stomata or changes in the position of leaves, in growth, or in modifications of structure. This expression is fundamentally affected by the nature of the factor and is in direct relation to the intensity of the latter. Adaptation comprises all structural changes resulting from adjustment. It includes both growth and modification. The latter is merely growth in response to unusual stimuli, but this fact is the real clue to all evolution. Growth is periodic, and in a sense quantitative; it results from the normal continuous adjustment of the plant to the stimuli of its proper habitat. In contrast, modification is relatively permanent and qualitative; it is the response to stimuli unusual in kind or intensity. A definite knowledge of the processes of growth is indispensable to an under- standing of modification. In the fundamental task of connecting plant and habitat, it is the modification of the plant, and not its growth, which records the significant responses to stimuli. For this reason the discussion of adaptation in the pages that follow is practically confined to modification of structure. This is particularly desirable, since growth has long been the theme of physiological study, while modification has too often been con- sidered from the structural standpoint alone. The comparatively few studies that have taken function into account have been largely empirical; GENERAL RELATIONS IO3 in them neither stimulus nor adaptation has received anything approaching adequate treatment. 148. The measurement of response. The amount of response to a stimulus is proportional to the intensity of the factor concerned. This does not mean that the same stimulus produces the same response in two distinct species, or necessarily in two plants of one species. In these cases the rule holds only when the plants or species are equally plastic. For each in- dividual, however, this quantitative correspondence of stirriulus and response is fundamental. It is uncertain whether an exact or constant ratio can be established between factor and function; the answer to this must await the general use of quantitative methods. There can be no doubt, however, that within certain limits the adjustment is proportional to the amount of stimulus, whereas reaction is well known to be abnormal or inhibited beyond certain extremes. It is quite erroneous to think that reaction is independent of quantity of stimulus, or to liken the stimulating factor to "the smallest spark (which) by igniting a mass of powder, produces an enormous mechanical effect."^ Such a statement is only apparently true of the action of mechanical stimuli upon the few plants that may properly be said to possess irritability, such as sensitive plants and certain insectivorous ones. Of the normal relation of response to direct factors, water, light, etc., it is entirely tmtrue. Axiomatically, there is ordinarily an essential correspond- ence, also, between the amount of adjustment and of adaptation. This correspondence is profoundly affected, however, by the structural stability of the plant. From the preceding it follows that the measurement of response and the relating it to definite amounts of direct factors as stimuli are two of the most fundamental tasks of ecology. The exact determination of physical factors has no value apart from its use for this purpose. It is perfectly clear that precise methods of measuring stim.uli call for. similar methods in determining the amount of adjustment and of adaptation. The problem is a difficult one, and it is possible at present only to indicate the direction which its development should take, and to describe a few methods which will at least serve as a beginning. To cover the ground adequately it is necessary to measure response by adjustment and by adaptation separately, and in the latter to find a measure for the individual and one for the species. The one is furnished by the methods of morphology and the other by biometry. iPFEFFER-EwART. Plivsiology of Plants, 1:13. lOCO. 104 THE PLANT A primary requisite for any method for measuring adjustment is that it be applicable to field conditions. Many instruments for measuring trans- piration, for example, are valueless, not because they are inaccurate, but because the plant studied is under abnormal conditions. To avoid the latter is absolutely necessary, a fact which makes it peculiarly difficult to devise a satisfactory field method. After the latter has been found and applied, it becomes possible to check other methods by it, and to give them real value. The final test of a field method is three-fold: (i) the plant must be studied while functioning" normally in its own habitat; (2) the method must give accurate results; and (3) it must permit of extensive and fairly convenient application in the field. Until methods of this character, some of which are described later, have been employed for some time, it is impossible to connect definite intensities of factor stimuli with measured amounts of adjustment. Ultimately, it seems certain that researches will regularly take this form. Adaptation is primarily indicated by changes in the arrangement and character of the cells of the plant. Since these determine the form of each organ, morphology also furnishes important evidence in regard to the course of adaptation, but form can be connected certainly with a'djustment only through the study of cellular adaptation. In tracing the modifications of cell and of tissue, the usual methods of histology, viz., sectioning and drawing, suffice for the individual. It is merely necessary to select plants and organs which are as nearly typical as can be determined. The ques- tion of quantity becomes paramount, however, since it often gives the clue to qualitative changes, and hence it is imperative that complete and accurate measurements of cells, tissues, and organs be made. These measurements, when extended to a sufficiently large number of plants, serve to indicate the direction of adaptation in the species. They constitute the materials for determining biometrically the mean of adaptation for the species and the probable evolution of the latter. In its present development, biometry con- tains too much mathematics, and too little biology. This has perhaps been unavoidable, but it is to be hoped that the future will bring about a wise sifting of methods, which will make biometry the ready and invaluable servant of all serious students of experimental evolution. This condition does not obtain at present, and in consequence it seems unwise to consider the subject of biometry in this treatise. 149. Plasticity and fixity. As the product of accumulated responses, each species is characterized by a certain ability or inability to react to stimuli. Many facts seem to indicate that the degree of stability is con- nected with the length of time during which the species is acted upon by GENERAL RELATIONS I05 the same stimuli. It seems probable that plants which have reacted to sun- light for hundreds of years will respond less readily to shade than those which have grown in the sun for a much shorter period. This hypothesis is not susceptible of proof in nature because it is ordinarily impossible to distinguish species upon the basis of the time .during which they have occupied one habitat. Evidence and ultimate proof, perhaps, can be obtained only by field and control experiments, in which the time of occupation of any habitat is definitely known. Even in this case, however, it is clear that antecedent habitats will have left effects which can neither be traced nor ignored. Additional support is given this view by the fact that extreme) types, both ecological and taxonomic, are the most stable. Intense xero- phytes and hydrophytes are much more fixed than mesophytes, though the intensity of the stimulus has doubtless as great an influence as its duration. Composites, labiates, grasses, orchids, etc., are less plastic than ranals, rosals, etc., but there are many exceptions to the apparent rule that fixity increases with taxonomic complexity. At present it seems quite impossible to suggest an explanation of the rule. Recent experiments indicate that there may be ancestral fixity of function, as well as of structure. It has been found, for example, that the flowers of certain species always react normally to the stimuli which produce opening and closing, while others make extremely erratic response. If further work confirms this result and extends it to other functions, the necessity of arriving at a better under- standing of fixity will be greatly emphasized. It is impossible to make progress in the study of adaptation without recognizing the fundamental importance of ancestral fixity as a factor. E. S. Clements^ has shown that a number of species undergo pronounced changes in habitat without showing appreciable modification. Consequently, it is incorrect to assume that each habitat puts a structural impress upon every plant that enters it. For this reason, the writer feels that the current explanation of xerophytic bog plants, etc., is probably wrong, and that the discrepancy between the nature of the habitat and the structure of the plant is to be explained by the persistence of a fixed ancestral type. The anomaly is scarcely greater than in cases that have proved capable of being explained. 150. The law of extremes. When a stimulus approaches either the maximum or minimum of the factor for the species concerned, response becomes abnormal. The resulting modifications approach each other and in some respects at least become similar. Such effects are found chiefly in 1 The Relation o£ Leaf Structure to Physical Factors. 1905. I06 THE PLANT growth, but they occur to some degree in structure also. It is imperative that they be recognized in nature as well as in field and control experiment, since they directly affect the ratio between response and stimulus. The data which bear upon the similarity of response to extremes of different factors are too meager to permit the formulation of a rule. It is permissible, however, to suggest the general principle that extreme stimuli produce similar growth responses, and to emphasize the need of testing its appli-- cation to adaptation proper. 151. The method of working hypotheses. In the study of stimulus and response, where the unimpeachable facts are relatively few, and their present correlation slight, the working hypothesis is an indispensable aid. "The true course of inductive procedure . . . consists in anticipating nature, in the sense of forming hj'potheses as to the laws which are prob- ably in operation, and then observing whether the combinations of phenomena are such as would follow from the laws supposed. The investi- gator begins with facts and ends with them. He uses such facts as are in the first place known to him in suggesting probable hypotheses ; deducing other facts which would happen if a particular hypothesis is true, he proceeds to test the truth of his notion by fresh observations or experi- ments. If any result prove different from what he expects, it leads him either to abandon or to modify his hypothesis ; but every new fact may give some new suggestion as to the laws in action. Even" if the result in any case agrees with his anticipations, he does not regard it as finally confirma- tory of his theory, but proceeds to test the truth of the theory by new de- ductions and new trials."^ Iri the treatment of adjustment and adaptation which follows, the method of multiple working hypotheses is uniformly em- ployed. No apology is felt to be necessary for this, since the whole endeavor is to indicate the proper points of attack, and not to distinguish between that which is conjectural and that which is known. If an hypothesis occa- sionally seem to be stated too strongly, it is merely that it appears, after a survey of the problem from all sides, to explain the facts most satisfactorily. The final proof of any hypothesis, however, rests not only upon its ability to explain all the facts, but also upon the inability of other hypotheses to meet the same test. The discovery and examination of all possible hypoth- eses, and the elimination of those that prove inadequate are the essential steps in the method of working hypotheses. ijEVONS, W. A. The Principles of Science, 2:137. 1874. GENERAL RELATIONS I07 HYDROHARMOSE ADJUSTMENT 152. Water as a stimulus. Plants are continually subjected to the action of the water of the soil and of the air; exception must naturally be made of submerged plants. The stimulus of soil water acts upon the absorbing organ, the root, while that of humidity affects the part most exposed to the air, viz., the assimilative organ, which is normally the leaf. But since both are simultaneous water stimuli, a clearer conception is gained of this operation if they are viewed as two phases of the same stimulus. This point of view receives further warrant from the essential and intimate relation of humidity and water-content as determined by the plant. They are in fact largely compensatory, as is shown at some length later. In determining the intensity of the two, a significant difference between them must be recognized. The total humidity of the air at any one time consti- tutes a stimulus to the leaf which it touches. This is not true of the total soil water. Part of the latter is not available imder any circumstances, and can not affect the plant, at least directly. The chresard alone can act as a stimulus, but even this is potential in the great majority of cases, since the actual stimulus is not the water available but the water absorbed. Thej latter, moreover, contains many nutrient salts which are in themselves/ stimuli, but as they normally have little bearing upon the action of waterV as a stimulus they are to be considered only when present in excessivej amounts. 153. The influence of other factors upon water. The amount of hu- midity is modified directly by temperature, wind, precipitation, and pressure,, and, through these, it is affected by altitude, slope, exposure, and cover. Naturally, also, the evaporation of soil water has a marked influence. In determining water-content, atmospheric factors, with the exception of pre- cipitation, are usually subordinate to edaphic ones. Soil texture, slope, and precipitation act directly in determining soil water, while temperature, wind, and pressure can operate only through humidity. This is likewise true of altitude, exposure, and cover, though the latter has in addition a profound effect upon run-off. Biotic factors can affect humidity or water-content only through the medium of another factor. Light in itself has no action upon either, but through its conversion into heat within the chloroplast, it has a profound effect upon transpiration. The following table indicates the general relation between water and the other physical factors of 'the habitat. 108 THE PLANT The order of the signs, ±, denotes that the water increases and decreases with an increase and decrease of the 'factor, or the reverse, h=. Humidity ± Water-content Temperature =F Temperature Wind=F Wind=F Precipitation ± Precipitation Pressure ± Pressure =F Soil texture Soil texture Altitude =F Porosity =F Capillarity Slope q= Slope =F Exposure =F Exposure q= Cover ± Cover ± 154. Response. The normal functional responses to water stimuli are absorption, diffusion, transport, and transpiration. Of these, absorption and transpiration alone are the immediate response to soil water and humidity, respectively. Consequently they are the critical points of attack in study- ing the fundamental relation of the plant to the water of its habitat. In determining the pathway of the response, it is necessary to trace the steps in diffusion and transport, but, as these are essentially alike for all vascular plants, this task lies outside the scope of the work in hand. As previously suggested, the relation between absorption and transpiration is strictly com- pensatory, though, for obvious reasons, the amount of water transpired is usually somewhat less than the amount absorbed. Absorption falls below transpiration when extreme conditions cause temporary or permanent wilt- ing; the two activities are essentially equal after a growing plant reaches maturity. In all cases, however, the rule is that an increase or decrease in 'water loss produces a corresponding change in the amount of water absorbed, and, conversely, variation in absorption produces a consequent change in transpiration. This is strictly true only when the stimuli are normal. For example, a decrease in humidity causes increased water loss, which, through diffusion and transport, is compensated by increased activity of the root surface. Frequently the water supply is insufficient to compen- sate for a greater stimulus, and the proper balance can be attained only by the closing of the stomata. In the case of excessive stimuli, neither com- pensation suffices, and the plant dies. Many mesophytes and all xerophytes have probably resulted from stimuli which regularly approached the limit of compensation for each, and often overstepped, but never permanently exceeded it. For hydrophytes, the danger arises from excessive water supply, not water loss. There is a limit to the compensation afforded by transpiration, which is naturally dependent upon the amount of plant sur- HYDROHARMOSE I09 face exposed to the air. No compensation occurs in the case of submerged plants; floating hydrophytes possess a single transpiring leaf surface, while the leaves of amphibious plants behave as do those of mesophytes. The whole question of response to water stimuli thus turns upon the compensa- tion for water loss afforded by 'water supply where the latter is moderate or precarious, and upon the compensation for water supply furnished by water loss where the supply is excessive, submerged plants excepted. 155. The measurement of absorption. As responses to measured stimuli of water-content and humidity, it is imperative that the amount of absorption and of transpiration be determined quantitatively. It is also extremely desirable that this be done in the normal habitat of the plant. A careful examination of the problems to be met quickly discloses the great difficulty of obtaining a direct and accurate measure of absorption under normal conditions, especially in the field. For this purpose, the ordinary potometric experiments by means of cut stems . are valueless. The use of the entire plant in a potometer yields much more trustworthy results, though the fact that the root is under abnormal conditions can not be overlooked, especially in the case of mesophytes and xerophytes. While potometric conditions are less abnormal for amphibious plants, the error is not wholly eliminated, since the roots normally grow in the soil. The potometer can be made of value for quantitative work only by checking the results it gives by means of an instrument or a method in which the plant functions nor- mally. In consequence, the potometer can not at present be usecj to measure absorption directly, .though, as is further indicated in the discussion of transpiration, it is a valuable supplementary instrument, after the check mentioned has been applied to its use with a particular species. An estimate of the amount of absorption may be obtained either in the field or in the control house by taking samples from the protected soil at different times. Since it is impossible to determine the weight of the area in which the roots lie, and since the soil water is often unequally distributed, this method can not yield exact results. An accurate method of measuring absorption under essentially normal conditions has been devised and tested in the control house. The essential feature of the process is the placing a plant in a soil containing a known quantity of water, and removing it after it has absorbed water from the soil for a certain period. In carrying out the experiment, a soil consisting of two parts of sod and one of sand was used, since the aeration is more perfect and the particles are more easily removed from the roots. The soil was completely dried out in a water bath and then placed in a five-inch battery jar. The latter, together with the rubber cloth used later to prevent evaporation, was weighed to the decigram. A weighed quantity of water was added, and the whole again weighed as a no THE PLANT check. Two plants of Helianthus annuus were taken from the pots in which they had grown, and the soil was carefully washed from the roots. Each plant was weighed with its roots in a dish of water to prevent wilting, and then carefully potted, one in each battery jar. A thistle tube was placed in the soil of each jar to facilitate aeration, as well as the addition of weighed amounts of water, when necessary, and the rubber cloth attached in the usual manner to prevent evaporation. The entire outfit was weighed again, and the weighing repeated at 8:00 a.m. and 5:00 p.m. for five days, in order to determine the amount of transpiration and its relation to the water absorbed. The plants were kept in diffuse light to prevent excessive water loss while the roots were becoming established. At the close of the experiment, the jar and its contents were weighed finally. The plants were removed and weighed, the soil particles being shaken from the roots into the jar, which was also weighed. The results obtained were as follows : Wt. of pot and dry soil Wt. of pot and wet soil Total MO HiO left mo ab- sorbed /fzO tran- spired I II 1846.0 g. 1886.7 g. 2218.0 g. 2253.2 g II 2174.3 g. 2221.6 g. 372.0 g. 366.5 g. 32S.3 g. 334.9 g. 43.7 g. 31.6 g. 43.7 g. 31.6 g. The amount of water absorbed may be obtained directly by subtracting the final weight of the jar and moist soil from their first weight, but a desirable check is obtained by taking the dry weight of jar and soil from the first, and the final weight of these, and subtracting the one from the other as indicated in the table. A second check is afforded by daily weighings, from which the amount of water transpired is determined. Since the two sunflower plants made practically no growth during the period of experi- ment, the exact correspondence between water absorbed and water lost is not startling, though it can not be expected that the results will always coincide. This method has certain slight sources of error, all of which, it is thought, have been corrected in a new and more complete series of experiments now being carried on. The aeration of the soil is not entirely normal, as is also true of the capillary movements of the water, on account of the non- porous glass jar and the rubber cloth. Since the latter are necessary condi- tions of All accurate methods for measuring absorption and transpiration, the resulting error must be ignored. It can be reduced, however, by forcing air through the thistle lube from time to time. Sturdy plants, such as the sunflower, are the most satisfactory, since they recover more quickly from the shock of transplanting. Almost any plant can be used, however, if HYDROHARIIOSE III repotted in a loose sandy soil often enough. This permits the root system to develop normally, and also makes it possible to wash the soil away with- out injury to the root. The method is so recent that there has been no opportunity to test it in the field. It would seem that it can be applied without essential change to plants in their normal habitats. Very large herbs or plants with extensive root system.s could not be used to advantage, and to be practicable the experiments would need to be carried on near the base station. The great value of the method, however, lies in its use as a check in determining the accuracy of other methods, and in practice it will Fig. 31. Absorption and transpiration of Helianthus annuus. I and II, plants repotted in soil of known weight and water-content; III, plant undistured in the original soil; IV, potometer containing plant with cut stem; V, potometer with entire plant. often be found convenient and time-saving to use the latter, 'after they have once been carefully checked for different groups of species. This matter is further considered under measures of transpiration. 156. The quantitative relation of absorption and transpiration. Bur- gerstein^ has summarized the results of various investigators in the state- ment "that between the quantitative absorption of water on the one hand and emission on the other there exists no constant parallelism or proportion," 1 Die Transpiration der Pflanzen, 14. 1904, 112 THE PLANT and he has cited the work of Krober, and of Eberdt in proof. This state- ment holds, however, only for short periods of a few hours, or more rarely, a day, and even here its truth still remains to be conclusively demonstrated. The discrepancy between absorption and transpiration for a short period is often greater than for a longer time, but it is evident that a transient change in behavior or a small error in the method would inevitably produce this result. Eberdt found the discrepancy for a few hours to be 1-2 ccm. in an entire plant of Heliqnthus anniiiis, while for a whole day the water absorbed was 33.57 ccm. and the water lost 33.98 ccm. Krober's experiments with cut branches of Asclepias incarnata showed a maximum difference for 12 hours of 2.5 ccm., but the discrepancy for the first 24 hours was i ccm. and for the second 1.9 ccm. In both cases, the potometer was employed. Consequently, as will be shown later, Eberdt's results are not entirely trust- worthy, while those of Krober, made with cut stems, are altogether unre- liable. Hence, it is clear that the discrepancy is slight for a period of several days or weeks, and that it may be ignored without serious error, except in a few plants, that retain considerable water as cell sap, in consequence of extremely rapid growth. Accordingly, the amount of transpiration, which may be readily and accurately determined, can be employed as a measure of absorption that is sufficiently accurate for nearly all purposes. The truth of this statement may be easily confirmed. It is evident that the amount of water absorbed equals the amount transpired plus that retained by the plant as cell-sap, or used in the manufacture of organic compounds. In plants not actively growing, the amount lost equals that absorbed, as already shown in the experiment with HeUanthus. According to Gain', Deherain has found that a plant rooted in ordinary soil transpired 680 kg. of water for each kilogram of dry substance elaborated. In HeUanthus annuus, the dry matter is' 10 per cent of the weight of the green plant. A well-grown plant weighing 1,000 grams, therefore, consists of 100 grams of dry matter and 900 of water. The length of the growing period for such a plant is approximately 100 days, during which it transpires 68 kilo- grams of water. Assuming the rate of transpiration 'and of growth to be constant, the plant transpires 680 grams daily, adds 9 grams to its cell-sap, and I gram to its dry weight. The amount of water in a gram of cellulose and its isomers is about 3/5. Consequently, the total water absorbed daily by the plant is 689.6 grams. The 680 grams transpired are 98.6 per cent of the amount absorbed; in other words, only 1.4 per cent of the water absorbed is retained by the plant. From this it is evident that the simplest 1 Recherches sur le R61e Physiologique de I'Eau dans la V^gdtation. Ann. Nat. Sci., 7:20:65. 1895. HYDROHARMOSE II3 and most convenient measure of absorption under normal conditions can be obtained through transpiration, since the discrepancy between absorption and transpiration is scarcely larger than the error of any method applicable to the field. Conversely, the measure of absorption obtained by the process described in the preceding section serves also as a measure of transpiration. The determination of the latter in the field is so much simpler, however, that it is rarely desirable to apply the absorption method. 157. Measurement of transpiration. The water loss of a plant may be determined absolutely or relatively. Absolute or quantitative determinations are by (i) weighing, (2) collecting, or (3) measuring the water absorbed; relative values are indicated by hygroscopic substances. A number of methods have been employed more or less generally for measuring trans- piration. The great majority of these can be used to advantage only in the laboratory, and practically all fail to meet the fundamental requirement for successful field work, namely, that the plant be studied under normal con- ditions in its own habitat. The following is a summary of the various methods, the details of which may be found in Burgerstein. 1. Weighing. This is the most satisfactory of all methods for deter- mining water loss. It is more accurate than any other, and is unique in that it does not place the plant under abnormal conditions. On the score of convenience, moreover, it excels every other method capable of yielding quantitative results. Various modifications of weighing are employed, but none of these have all the advantages of a direct, simple weighing of the plant in its own soil. 2. Collecting the water transpired. This may be done by collecting and weighing the water vapor exhaled by a plant placed within a bell jar, or by weighing a deliquescent salt, such as calcium chloride, which is used to absorb the water of transpiration. Tlie decisive disadvantage of these methods is that transpiration is carried on in an atmosphere far more humid than normal. If an excessive amount of salt is used, the air is abnormally dry. In both cases, the water loss decreases until it reaches a point much below the usual amount. Finally, all methods of this kind are open to con- siderable error, and are inconvenient, especially in field work. They aie ■ of relatively slight value in comparison with weighing. 3. Potometers. It has already been shown that the amount of water absorbed is a close measure of the amount transpired. In consequence, the potometer can be used to determine the am.ount of transpiration provided the absorption is not abnormal. It is rarely and only with much difficulty that this condition can be met. The use of cut stems and branches does not meet it, and even in the case of plants with roots, the results must be 114 THE PLANT compared with those obtained from absorption experiments made with plants rooted in soil before they can be relied upon. This necessity practi- cally puts the potometer out of commission for accurate work, unless future study may show a somewhat constant ratio between the absorption of a plant in its own soil and that of a plant placed in a potometer. 4. Measuring absolute humidity. The cog psychrometer makes it possible to determine the increased relative humidity produced within a glass cylinder or special tin chamber by a transpiring plant. From this result the absolute humidity is readily obtained, and by means of the latter the actual amount of water given off. The evident drawback to this method is that the increasing humidity within the chamber gives results entirely abnormal for the plant concerned. 5.. Self-registering instruments. There are various methods for regis- tering the amount of transpiration, based upon weighing, or upon the poto- meter. The Richard recording evaporimeter has all the advantages of weighing, inasmuch as the water loss is measured in this way, and in addi- tion the amount is recorded upon a revolving drum, obviating the necessity of repeated attention in case it is desirable to know the exact course of transpiration. On the other hand, methods which depend upon the poto- meter, while graphic, are not sufficiently accurate to be of value.. 6. The use of hygroscopic materials. Hygroscopic substances change their form or color in response to moisture. As they indicate comparative water loss alone, they are of value chiefly in the study of the stomatic surfaces of leaves. F. Darwin^ has used strips of horn, awns of Stipa, and epidermis of Yucca to construct small hygroscopes for this purpose. In these instruments the error is large, but as no endeavor is made to obtain exact results, it is negligible. Filter paper impregnated with a 3-5 per cent aqueous solution of cobalt chloride is deep blue when dry. If a strip of cobalt paper is placed upon a leaf and covered with a glass slip it turns bright rose color, the rapidity of the change affording a clue to the amount of transpiration. 158. Field methods. The conditions which a satisfactory field method of measuring transpiration must fulfill have already been discussed; they are accuracy, simplicity, and normality. These conditions are met only by weighing the plant in its own soil and habitat. This has been accomplished by means of the sheet-iron soil box, already described under the determina- tion of the chresard. The method is merely the familiar one of pot and balance, slightly modified for field use. The soil block, which contains the 'Observations on Stomata by a New Method. Proc. Camb. Phil. Soc, 9:303. 1897. HYDROI-IARMOSE I 1 5 plant to be studied, is cut out, and the metal plates put in position as indi- cated in section 53. Indeed, it is a great saving of time and effort to deter- mine transpiration and chresard in the same experiment; this is particu- larly desirable in view of the close connection between them. In this event, the soil block must be small enough not to exceed the load of a field balance. After the block is cut and encased, all the plants are removed, except the one to be studied. If several individuals of the same species are present, it is an advantage to leave all of them, since the error arising from individual variations of water loss may, in this way, be almost completely eliminated. A sheet of rubber or rubber cloth is carefully tied over the box to prevent evaporation from the soil. A broad band is passed under the box to aid in lifting it upon the scales. The latter must be of the platform type, and should have a capacity as great as consistent with the need for moving it about in the field. Weighings are made in the usual way, care being taken to free the surface of the box from soil. The aeration of the soil block is kept normal by removing the rubber for a few minutes from time to time, or by forcing air through a thistle tube. Water is also added through the latter, when it is desired to continue the experiment for a considerable period. After the study of transpiration is concluded, the rubber cloth is removed, soil samples taken, and the soil allowed to dry out until the plant becomes thoroughly wilted. If the box is weighed again, the difference represents the amount of available water. The per cent of chresard is also obtained in the usual way by taking samples for ascertaining the echard, and subtracting this from the holard. Field determinations of water loss yield the most valuable results when different habitat forms, or ecads, of the same species are used. There is little profit in comparing the transpira- tion of a typical sun plant, such as Touterea mnltHiora, with that of a shade plant, such as Washingtonia obtusa. But the simultaneous study of plants like Chamaeneriiini, angustifolium, Gentiana acuta, Scutellaria brittonii etc., which grow in several different habitats, furnishes direct and funda- mental evidence of the course of adjustment and adaptation. Hesselmann^, in his study of open woodlands in Sweden, has employed a method essentially similar to the preceding. Young plants of various species were transferred to pots in the field, where they were allowed to grow for several months before a series of weighings was made to determine the amount of transpiration. Since weighing is the measure used in each, both methods are equally accurate. The one has a certain advantage in that the pots are, perhaps, more easily handled, while the other has the advantage ' Zur Kenntnis des Pflahzenlebens schwedischer Laubwiesen. Beih. Bdt. Cent., 18:311. 1904. ii6 THE PLANT of maintaining the normal relation of soil and roots, a condition more or less impossible in a pot. In both instances the weighing should be done in the habitat, which was not the case in Hesselmann's researches. The slight value of the potometer, which has had a vogue far beyond its merits, is indicated by the following table. These results were obtained from three plants of Helianthus annmis; III was left undisturbed in the pot where it had been growing, IV was placed in a potometer, after the root had been cut off, and V was an entire plant placed in a potometer. The amount of transpiration is indicated in grams per square decimeter of leaf surface. The plants were kept in diffuse light, except for a period of two hours (8:00 to 10:00 A.M.) on the last day, when they were in full sunshine at a temperature of 75° F. Plant IV wilted so promptly in the sunshine that it was found necessary to conclude the experiment in diffuse light. 8 a.m. 5 p.m. 8 a.m. 5r.M. 8 a.m. 5 p.m. 8 a.m. 10a.m. 5 p.m. 8 a.m. Total Ill IV V 2.9 4.7 3.7 7.3 7.2 5.3 2.4 2.9 3.2 6.0 2.3 4.8 1.7 1.0 2.5 1.6 0.6 1.6 2.0 0.9 3.0 3.4 0.5 2.6 2.0 0.5 1.6 1.8 0.4 2.6 31.1 21.0 30.9 The cut plant, IV, lost more water the first day than either of the others, Imt the water loss soon decreased, and at the end of the period was almost nil. The total transpiration for III and V is much the same, but the range of variation for periods of 12 hours is from -\-2 to — i gram. This ex- periment is taken as a fair warrant that the use of cut stems in potometers can not give accurate results. It is inconclusive, however, as to the merits of potometric values obtained by means of the entire plant, and further studies are now being made with reference to this point. 159. Expression of results. From the previous discussion of the relation between them, it follows that an expression of the amount of transpiration likewise constitutes an expression of absorption. It is very desirable also that the latter be based upon root surface and chre.sard, but the difficulty of determining the former accurately and readily is at present too great to make such a basis practicable. In expressing transpiration in exact terms, the fact that plants of the same species or form are somewhat individual in their behavior must be constantly reckoned with. In consequence, experi- ments should be made upon two or three individuals whenever possible, in order to avoid the error arising from this source. HYDROHARMOSE 117 Water loss may be expressed either in terms of transpiring surface or of dry weight. Since there is no constant relation between surface and weight, the terms are not interchangeable or comparable, and in practice it is necessary to use one to the exclusion of the other. Obviously, surface furnishes by far the best basis, on account of its intimate connection with stomata and air-spaces, a conclusion which Burgerstein (/. c, p. 6) has shown by experiment to be true. For the best results, the whole transpiring surface should be determined. This is especially necessary in making com- parisons of different species. In those studies which are of the greatest valu^ viz.. ecads of the same species, it is scarcely desirable to measure stem and petiole surfaces, unless these organs show unusual modification. The actual transpiring surface is constituted by the walls of the cells bordering the intercellular spaces, but, since it is impossible to determine the aggregate area of these, or the humidity of the air-spaces themselves, the leaf surface must be taken as a basis. Since the transpiration through the stomata is much greater than that through the epidermal walls, the number of stomata must be taken into account. Since they are usually less abundant on the upper surface, their number should be jletermined for both sides of the leaf. The errors arising from more or less irregular distribution are elimi- nated by making counts^ near the tip, base, and middle of two or three ma- ture leaves. The most convenient unit of leaf surface is the square deci- meter. The simplest way 'to determine the total leaf area of a plant is to outline the leaves upon a homogeneous paper, or to print them upon a photographic paper. The outlines are then cut out and weighed, and the leaf area obtained in square decimeters by dividing the total weight by the weight of a square decimeter of the paper used. The area may also be readily determined by means of a planimeter. 160. Coefficient of transpiration. At present it does not seem feasible to express the transpiration of a plant in the form of a definite coefficient, but it is probable that the application of exact methods to each part of the problem will finally bring about this result. Meanwhile the following formula is suggested as a step toward this goal: i^g—. LHT, in which t, the transpiration relation of a plant, is expressed by the number of grams of water lost per hour, on a day of sunshine, by one square decimeter of leaf, considered with reference to the stomata of "the two surfaces, and the amount of the controlling physical factors, light, humidity, and temperature, at the time of determination. For Helianthus animus, this formula would appear as follows: t=2 : 1:50:75". To avoid the large figures arising Il8 THE PLANT from the extent of surface considered, the number of stomata per square decimeter is divided by 10,000. This amounts to the number per square millimeter, and time may consequently be saved by using this figure directly. While this, formula obviously leaves much to be desired, it has the great advantage of making it possible to compare ecads of one species, or species of the same habitat or of different habitats, upon an exact basis of factor, function, and structure. ADAPTA7I0N 161. Modifications due to water stimuli. In adaptation, the great desideratum is to connect each modification quantitatively with the corres- ponding adjustment. This is even more difficult than to ascertain the quantitative relation between stimulus and functional response, a task still beset with serious obstacles. At the present time, little more can be done than to indicate the relation of marked adaptations of organs and tissues to the direct factors operating upon them, and to attempt to point out among the functions possibly concerned the one which seems to be the most prob- able connection between the probable stimulus and the structure under investigation. In the pages that follow, no more than this is attempted. The general changes of organs and tissues produced by water are first dis- cussed, and after this is given a summary of the structural features of the plant types based upon water-content. 162. Modifications due to a small water supply. A water supply which may become deficient at any time is compensated either by changes which decrease transpiration, or by those that increase the amount of water absorbed or stored. These operate upon the form and size of the organs concerned, as well as upon their structure. Modifications of the form of leaf and stem are alike in that they lessen transpiration by a reduction of the amount of surface exposed to the air. Structural adaptations, on the other hand, bring about the protection of epidermal cells and stomata, and often intcTial cells also, from the factors which cause transpiration, or they anticipate periods of excessive transpiration by the storage of water in specialized cells or tissues. In certain extreme types the epidermis is itself modified for the absorption of water vapor from the air. 163. The decrease of water loss. The following is a summary of the contrivances for reducing transpiration. I. Position of the leaf. Since the energy of a ray of sunlight is greatest at the sun's highest altitudes, those leaves transpire least which are in such a HYDROHARMOSE II9 position during midday that the rays strilte them as obliquely as possible.- A leaf at right angles to the noonday sun receives ten times as much light and heat upon a square decimeter of surface as does one placed at an angle of 10 degrees. Thi.'j device for reducing the intensity of insolation is best developed in the erect or hanging leaves of many tropical trees. In tem- perate zones, it is' found in such plants as Silphium laciniatum and Lactuca scariola, and in species with equitant leaves. In such plants as Helianthus anniius, the effect is just the opposite, since the turning of the crown keeps the leaves for a long time at a high angle to the incident rays. In the case of mats, it is the aggregation of plants which brings about the mutual pro- tection of the leaves from insolation and wind. 2. Rolling of the leaf. Many grasses and ericaceous plants possess leaves capable of rolling or folding themselves together when drouth threatens. In other cases, the leaves are permanently rolled or folded. The advantage of this device arises not only from the reduction of surface, but also from the fact that the stomata come to lie in a chamber more or less completely closed. In the case of those mosses whose leaves roll or twist, a reduction of surface alone is effected. 3. Reduction of leaf. The transpiring surface of a plant is reduced by decreasing the number of leaves, by reducing the size of each leaf, or by a change in its form. In so far as the stem is a leaf, a decrease in size or a change in shape brings about the same result. The final outcome of reduc- tion in size or number is the complete loss of leaves, and more rarely, of the stem. Such marked decrease of leaf area is found only in intense xerophytes, though it occurs in all deciduous trees as a temporary adapta- tion. Changes in leaf form are nearly always accompanied by a decrease in size. Of the forms which result, the scale, the linear or cylindrical leaf, and the succulent leaf are the most common. Leaves which show a tendency to divide often increase the number of lobes or make them smaller. 4. Epidermal modifications. Excretions of wax and lime by the epidermis have a pronounced effect by increasing the impermeability of the cuticle, and, hence, decreasing epidermal transpiration. It seems improbable that a coating of wax on the lower surface of a diphotic leaf can have this purpose. The thickening of the outer wall of epidermal cells to form a cuticle is the most perfect of all contrivances for decreasing permeability and reducing transpiration. In many desert plants, the greatly thickened cuticle effectually prevents epidermal transpiration. In these also the cuticle is regularly developed in such a way as to protect the guard cells, and even to close the opening partially. An epidermis consisting of two or more layers of cells is an effective, though less frequent device against water loss. When combined with a cuticle, as is usually the case, the imperme- ability is almost complete. Hairs decrease transpiration by screening the I20 THE PLANT epidermis so that the amount of Hght and heat is diminished, and the access and movement of dry air impeded. While hairs assume the most various forms, all hairy coverings serve the same purpose, even when, as in the case of Mesembryanthemum, they are primarily for water-storage. Hairs protect stomata as well as epidermal cells : the greater number of the former on the lower surface readily explains the occurrence of a hairy covering on this surface, even though absent on the more exposed upper side. In some cases, hairs are developed only where they serve to screen the stomata. The modifications of the stomata with respect to transpiration are numer- ous, yet all may be classed with reference to changes of number or level. With the exception 'Of aquatic and some shade plants, the number of stomata is normally greater on the less exposed, i. e., lower surface. The number on both surfaces decreases regularly as the danger of excessive water loss increases, but the decrease is usually more rapid on the upper surface, which finally loses its stomata entirely. It has been shown by many observers that species growing in dry places have fewer stomata to the same area than do those found in moist habitats. This result has been verified experimentally by the writer in the case of Ranunculus sceleratus, in which, however, the upper surface possesses the larger number of stomata. Plants of this species, which normally grow on wet banks, were grown in water so that the leaves floated, and in soils containing approximately lo, 15, 3°' and 40 per cent of water. The averages for the respective forms were: upper 20, lower o; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8; upper 10, lower 6. Reduction of number is effective, however, only under moderate conditions of dryness. As the latter becomes intense, the guard cells are sunken below the epidermis, either singly or in groups. In both cases, the protection is the same, the guard cells and the opening between them being withdrawn from the intense insolation and the dry air. The sun rays penetrate the chimney-shaped chambers of sunken stomata only for a few minutes each day, and they are practically excluded from the stomatal hollows which are filled with hairs. The influence of dry winds is very greatly diminished, as is also true, though to a less degree, for leaves in which the stomata are arranged in furrows. Sunken stomata often have valve-like projections of cuticle which reduce the opening also. Finally, in a few plants, water loss in times of drouth is almost completely prevented by closing the opening with a wax excretion. ^.Modifications in the chlorenchym. A decrease in the size and number of the air passages in the leaf renders the movement of water-laden air to the stomata more difficult, and effects a corresponding decrease in transpi- ration. The increase of palisade tissue, though primarily dependent upon light, reduces the air-spaces, and consequently the amount of water lost; HYDRO HARMOSE I 2 1 The development of sclereids below the epidermis likewise hinders the escape of water. Finally, the character of the cell sap often plays an im- portant part, since cells with high salt-content or those containing mucila- ginous substances give up their water with reluctance. 164. The increase of water supply. Plants of dry habitats can increase their absorption only by modifying the root system so that the absorbing surfaces are carried into the deep-seated layers of soil, and the surfaces in contact with the dry soil are protected by means of a cortex. Exception must be made for epiphytes and a few other plants that absorb rain water and dew through their leaves, and for those desert plants that seem to con- dense the moisture of the air by means of hygroscopic salts, and absorb it through the epidermis of the leaf. The storage of water in the leaf is a very important device; it increases the water supply by storing the surplus of absorbed water against the time of need. Modifications for water storage are occasionally found in roots and stems, but their chief develop- ment takes place in the leaf. The epidermis frequently serves as a reser- voir for water, either by the use of the epidermal cells themselves, by the formation of hypodermal water layers, or by means of superficial bulliform cells. The water cells of the chlorenchym regularly appear in the form of large clear cells, scattered singly or arranged in groups. In this event, they occur either as transverse bands, or as horizontal layers, lying between the palisade and sponge areas, and connecting the bundles. A few plants possess tracheid-like cells which also serve to store water. In the case of succulent leaves, practically the whole chlorenchym is used for storing water, though they owe their ability to withstand transpiration to a combination of factors. 165. Modifications due to an excessive water supply. Water plants with aerial leaf surfaces are modified in such manner as to increase water loss and to decrease water supply, but the resulting modifications are rarely striking. There is a marked tendency to increase the exposed surface'. This is indicated by the fact that, while the leaves of mud and floating forms become larger, they change little or not at all in. thickness. The lobing of leaves is also greatly reduced, or the lobes come to overlap. Leaves of water plants are practically destitute of all modifications of epidermis and stomata, which could serve to hinder transpiration. The stomata are usually more numerous on the upper surface, and in the same species their number is greater in the forms grown in wet places. These facts explain in part the extreme development of air-passages in water plants, though this is, in large measure, a response to the increasing difficulty of aeration. The 122 THE PLANT increase of air-spaces is correlated with reduction of the pahsade, and a decided increase in the sponge. An increase in water supply is indicated by the absence of storage tissues, and the reduction of the vascular system, which, however, is more closely connected with a diminished need for mechanical support. 166. Plant types. The necessity for decreasing or increasing water loss in compensation of the water supply has made it possible to distinguish two fundamental groups of plants upon the twofold basis of habitat and struc- ture. These familiar groups, xerophytes and hydrophytes, represent two extremes of habitat and structure, between which lies a more or less vague, intermediate condition represented by mesophytes. These show no char- acteristic modifications, and it is consequently impossible to arrange them in subgroups. Xerophytes and hydrophytes, on the other hand, exhibit marked diversity among themselves, a fact that makes it desirable to recognize subgroups, which correspond to fundamental differences of habitat or adaptation. It is hardly neces- sary to point out that these types are not sharply defined, or that a single plastic species may be so modified as to ex- hibit several of them. The extremes are always clearly defined, however, and they Fig. 32. Mesophyll of Pedicularis procera (chresard, 15^, light, 1). X 130. indicate the specific tendency of the adaptation shown by other members of the same group. 167. Xerophytic types. With the exception of dissophytes, all xero- phytes agree in the possession of a deep-seated root system, adapted to withdraw water from the lower moist layers, and to conserve from loss from the upper dry layers. Reservoirs are developed in the root, however, in relatively few cases. The stem follows the leaf more or less closely in its modification, except when the leaf is greatly reduced or disappears, in which event the stem exhibits peculiar adaptations. While the leaf is by far the most strikingly modified, it is a difficult task to employ it satisfactorily as the basis, for distinguishing types. Several adaptations are often com- bined in the same leaf, and it is only where one of these is preeminently developed, as in the case of succulence, that the plant can be referred to a definite type. The latter does not happen in many species of the less HYDROHARMOSE 123 intensely xerophytic habitats, and, consequently, it is difficult, if not unde- sirable, to place such xerophytes under a particular group. The best that can be done is to recognize the types arising from extreme or characteristic modification, and to connect the less marked forms as closely as possible with these. Halophytes differ from xerophytes only in the fact that the chresard is determined by the salt-content of the habitat, and not by the tex- ture of the soil. In consequence, they should not be treated as a distinct group. 168. Types of leaf xerophytes. In these, adaptation has acted primarily upon the leaf, while the stem has remained normal for the most part. Even when the leaves have become scale-like, they persist throughout the grow- ing season, and continue to play the primary part in photosyn- thesis. The following types may be distinguished: I. The normal form. The leaf is of the usual dorsiventral character. In place of a reduc- tion in size, structural modifica- tions are used to decrease transpiration. With respect to the protective feature that is predominant, three subtypes may be recognized. The cutinized leaf compensates for a low water-con- tent by means of a thick cuticle, often reinforced by a high de- velopm.ent of palisade tissue. Such leaves are more or less leathery, and they are often evergreen also. Arctostaphylus and many species of Pentstemon are good examples. Lanate leaves, i. e., those with dense hairy coverings on one or both surfaces, as Artemisia, Antennaria, etc., regularly lack both cuticle and 'palisade tissue. The protection against water loss, however, is so perfect that the chlorenchym often assumes the loose structure of a shade leaf. Storage leaves usually have a well- developed cuticle and several rows of palisade cells, but their characteristic feature is the water-storage tissue, which maintains a reserve supply of Fig. 33. Staurophyll of Bahia disseda, showing extreme development of palisade (chresard, 3-9^; light, 1). X 130- 124 THE PLANT water for the time of extreme drouth. Xerophytic species of Helianthus furnish exarnples of transverse bundles of storage cells, while those of Mertensia illustrate the more frequent arrangement in which the water tissue forms horizontal layers. 2. The succulent form. Many succulent leaves are normal in shape and size, though always thicker than ordinary leaves. Usually, however, they are reduced in size and are more or less cylindrical in form. The necessary decrease in transpiration is effected by the reduction in surface, the general storage of water, a waxy coating, and, often also, by a very thick cuticle. Agave, Mesembryanthemum, Sednm, and Senecio furnish excellent ex- amples of this type. 3. The dissected form. The reduction in surface is brought about by the division of the leaf blade into narrow linear or thread-like lobes which are widely separated. The latter are themselves protected by a hairy covering or a thick cuticle, which is often sup- plemented by many rows of palisade, or by storage tissue. Artemisia, Senecio, and Gilia contain species which serve as good examples of this type. 4. The grass form. Xero- phytic grasses and sedges have narrow filamentous leaves with longitudinal furrows which serve to protect the stomata. The furrows are sometimes filled with hairs which are an additional protection, and the leaves often protect themselves further by rolling up into a thread-like shape. The elongated subulate leaves of /uncus and certain Cyperaceae are essentially of this type, although they are usually not furrowed. 5. The needle form. This is the typical leaf of conifers, in which a sweeping reduction of the leaf surface is an absolute necessity. The rela- tively small water loss of the needle leaf is still further decreased by a thick cuticle, and usually also by hypodermal layers of sclerenchyma. 6. The roll form-. Roll leaves are frequently small and linear. Their characteristic feature is produced by the rolling in of the margin on the under side, by which an almost completely closed chamber is formed for the protection of the stomata which are regularly confined to the lower surface of the leaf. The upper epidermis is heavily cutinized and the lower Fig. 34. Diplophyll of Mertensia linearis, showing water cells (chresard, 3-9^, light, 1). X 130. H-iT)ROHARMOSE 1 25 one often protected by hairs. This type is found especially among the genera of the Ericales, but it also occurs in a large number of related families. 7. The scale form. Reduction of leaf surface for preventing excessive water loss reaches its logical culmination in the scale leaf characteristic of m_any trees and shrubs, e. g., Cupressus, Tmnarlx, etc. Scale leaves are leathery in texture, short and broad, and closely appressed to the stem, as wnll as often overlapping. 169. Types of stem xerophytes. In these types the leaves are deciduous early in the growing period, reduced to functionless scales, or entirely absent. The functions of the leaf have been assumed by the stem, which exhibits many of the structural adaptations of the former. Warming^ has distin- guished the following groups : 1. The phyllode form. The petiole is broadened and takes the place of the leaf blade which is lacking. In other cases, the stem is flattened or winged, and it replaces the entire leaf. This type occurs in Acacia, Baccharis, Genista, etc. 2. The virgate form. The leaves either fall off early or they are reduced to functionless scales. The stems are thin, erect, and rod-like, and are often greatly branched. They are heavily cutinized and palisaded, and the stomata are frequently in longitudinal furrows. This type is characteristic of the Genisteae; it is also found in Ephedra, many species of Polygonum, Lygodesmia, etc. 3. The rush form. In Heleocharis, many species of Juncus, Scirpus, and other Cyperaceae, the stem, which is nearly or completely leafless, is cylin- drical and unbranched. It usually possesses also a thick cuticle, and several rows of dense palisade tissue. 4. The cladophyll form. In Asparagus the leaves are reduced to mere functionless scales, and their function is assumed by the small needle-shaped branches. 5. The flattened form. As in the preceding type, the place of the scale- like leaves is taken by cladophylls, which are more or less flattened and leaf- like. Rusciis is a familiar illustration of this form. 6. The thorn form. This is typical of many spiny desert shrubs, in which the leaves are lost very early, or, when present, are mere functionless scales. The stems have an extremely thick cuticle, and the stomata are deeply sunken, as a rule. Colletia and Holacantha are good examples of the type. ' Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902. 126 THE PLANT 7. The succulent form. Plants with succulent stems such as the Cac- taceae, Stapelia, and Euphorbia have not only decreased water loss by ex- treme reduction or loss of the leaves, and the reduction of stem surface, but they also offset transpiration by means of storage tissues containing a mu- cilaginous sap. The cuticle is usually highly developed and the stomata sunken. Thorns and spines are also more or less characteristic features. 170 Bog plants. Many of the xerophytic types just described are found in ponds, bogs, and swamps, where the water supply is excessive, and hydro- phytes would be expected. The explanation that "swamp xerophytes" are due to the presence of humic acids which inhibit absorp- tion and aeration in the roots has been generally accepted. As Schimper has ex- pressed it, bogs and swamps are "physiologi- cally dry", i. e., the available water is small in amount, in spite of the great total water- content. Burgerstein (/. c, 142) has shown, however, that maize plants transpire, i. e., absorb, three times as much water in a solu- tion of 0.5 per cent of oxalic acid as they do in distilled water, and that branches of Taxus in a solution containing i per cent of tartaric acid absorb more than twice as much as in distilled water. Consequently, it seems im- probable that small quantities of humic acids should decrease absorption to the extent necessary for the production of xerophytes in ponds and bogs. Indeed, in many ponds and streams, where Heleocharis, Scirpus, Juncus, etc., grow, not a trace of acid is discoverable. Furthermore, plants with a characteristic hydrophytic structure throughout, such as Ranunculus, Caltha, Ludwigia, Sagittaria, etc., are regularly found growing alongside of apparent xerophytes. Many of the latter, furthermore, show a striking contrast in size and vigor of growth in places where they grow both upon dry gravel banks and in the water, indicating that the available water-content is much greater in the latter. Finally, many so-called "swamp xerophytes" possess typically hydrophytic structures, such as air-passages, diaphragms, etc. In spite of a growing feeling that the xerophytic features of certain amphibious plants can not be ascribed to a low chresard in ponds and swamps, a satisfactory explana- Fig. 35. Polygonum bistor- toides, a stable type: 1, meso- phyll (chresard, 25^); 2, xerophyll (chresard, 3-5^). X 130. HYDROHARMOSE 127 tion of them has been found but recently. This explanation has come from the work of E. S. Clements already cited, in which it was found that certain sun plants underwent no material structural change when grown in the shade, and that the same was true also of a few species which grew in two or more habitats of very different water-content. In accordance with this, it is felt that the xerophytic features found in amphibious plants are due to the persistence of stable structures, which were developed when these species were growing in xerophytic situations. When it is called to mind that monocotyledons, and especially the grasses, sedges, and rushes, are peculiarly stable, it may be readily understood how certain ancestral characters have persisted in spite of a striking change of habitat. Such a hypothesis, can only be confirmed by the methods of experimental evolution, and a critical study of this sort is now under way. 171. Hydrophytic types. Hydrophytes permit a fairly sharp division into three groups, based primarily upon the relation of the leaf surface to the two media, air and water. In submerged plants, the leaves are con- Fig. 36. Hippuris vulgaris: 1, submerged leaf; 2, aerial leaf. X 130. 128 THE PLANT stantly below the water ; in amphibious ones, they grow normally in the air. Floating plants have leaves in which the upper surface is in contact with the air, and the lower in contact with the water. Transpiration is at a maximum in the amphibious plant; it is reduced by half in the floating type, and is altogether absent in submerged plants. Aeration reaches a high develop- ment in amphibious and floating forms, but air-passages are normally absent from submerged forms except as vestiges. Photosynthesis is marked in the former, but considerably weakened in the latter. The vascular system, which attains a moderate developrhent in the amphibious type, is considerably reduced in floating forms, and it is little more than vestigiate in submerged ones. I. The amphibious type. Plants of this type grow in wet soil or in shallow water. The leaves are usually large and en- tire, the stem well developed, and the roots numerous and spread- ing. In the majority of cases the leaves are constantly above the water, but in some species the lower leaves are often covered, normally, or by a rise in level, and they take the form or struc- ture of submerged leaves. This is illustrated by Callitriche auttim- nalis, Hippiiris vulgaris, Ranun- culus delphinif alius, Proserpinaca palustris, Roripa americana, etc. The epidermis has a thin cuticle, or none at all, and is destitute of hairs. The stomata are numer- ous and usually more abundant The palisade tissue is represented Fig. 37. Floating leaf of Spar^anium angus- tifolium. X 130. on the upper than on the lower surface by one or more well-developed rows, but this portion of the leaf is regularly thinner than that of the sponge part. The latter contains . large air-pas- sages, or, in the majority of cases, numerous air-chambers, usually provided with diaphragms. i"he stems are often palisaded, and are characterized by longitudinal air-chambers crossed by frequent diaphragms, which extend downward through the roots. 2. The floating type. With respect to form and the structure of the upper part of the leaf, floating leaves are essentially similar to those of amphibious plants. They are usually lacquered or coated with wax to prevent the PHOTOHARMOSE 1 29 stoppage of the stomata by water. Stomata, except as vestiges, are found only on the upper surface, and the paHsade tissue is much less developed than the sponge, which is uniformly characterized by large air-chambers. The stems are elongated, the aerating system is enormously developed, and the supportive tissues are reduced. In the Lemnaceae, the leaf and the stem are represented by & mere frond or thallus, and the roots are in the process of disappearance, e. g., Spirodela has several, Lemna one, and WoMa none. 3. The submerged type. Both stem and root have been greatly reduced in submerged plants, owing to the generalization of absorption and the density of the water. The leaves are greatly reduced in size and thickness, chiefly, it would seem, for the purpose of insuring readier aeration and great illumination. The leaf may be ribbon-like, linear, cylindrical, or finely dissected. Stomata are sometimes present, but they are functionless and vestigial. A distinction into palisade and sponge tissues, when present, must also be regarded as a vestige; the chlorenchym is essentially that of a shade leaf. The air chambers are much reduced, and sometimes lacking; they function doubtless as reservoirs for air obtained from the water. PHOTOHARMOSE ADJUSTMENT 172. Light as a stimulus. In nature, light stimuli are determined by intensity and not by quality. A single exception is afforded by those aquatic habitats where the depth of water is great, and in consequence of which certain rays disappear by absorption more quickly than others. In forests and thickets, where the leaves transmit only the green and yellow rays, it would appear that the light which reaches the herbaceous layers is deficient in red and violet rays. The amount of light transmitted by an ordinary sun leaf is so small, however, that it has no appreciable effect upon the quality of the light beneath the facies, which is diffuse white light that has passed between the leaves. Indeed, it is only in the densest forests that distinct sunflecks do not appear. Coniferous forests, with a light value less than .005, which suffices only for mosses, lichens, and a few flowering plants, show frequent sunflecks. This is convincing evidence that the light of such habitats is normal in quality. It warrants the conclusion that in all habitats with an intensity capable of supporting vascular plants the light, no matter how diffuse, is white light. The direction of the light ray is of slight importance in the field, apart from the difference in intensity which m.ay result from it. In habitats with diffuse light, the latter comes normally and constantly from above. Likewise, in sunny situations, direction can 130 THE PLANT have little influence, since both the direction and the angle of the incident rays change continually throughout the day, and the position of the leaf itself is more or less constantly changed by the wind. The influence of duration upon the character of light stimuli is difficult to determine. There can be no question that the time during which a stimulus acts has a pro- found bearing upon the response that is made to it. In nature the problem is complicated by the fact that light stimuli are both continuous and periodic. The duration of sunlight is determined by the periodic return of night as well as by the irregular occurrence of clouds. Since one is a regular, and the other at least a normal happening, it is necessary to con- sider duration only with respect to the time of actual sunlight on sunny days, except in the case of formations belonging to regions widely different in the amount of normal sunshine, i. e., the number of cloudy days. In consequence, duration is really a question of the intensities which succeed each other during the day. The differences between these have already been shown to fall within the efficient difference for light, and for this reason ihe ratio between the light intensity of a meadow and of a forest is essen- tially the ratio between the sums of light intensity for the two habitats, i. e., the duration. The larter is of importance only where there is a daily alterna- tion between sunshine and shadow, as at the edge of forest and thicket, in open woodland, etc. In stich places duration determines the actual stimulus by virtue of the sum of preponderant intensities. The periodicity of day- light is a stimulus to the guard cells of stomata, but its relation to intensity in this connection is not clear. The amount of change in light intensity necessary to constitute an efficient stimulus seems to depend upon the existing intensity as well as upon the plant concerned. Apparently, a certain relative decrease is more efficient for sun plants than for shade plants. At least, many species sooner or later reach a' point where a difference larger than that which has been efficient no longer produces a structural response. This has been observed by E. S. Clements {I. c.) in a number of shade ecads. For example, a form of Galium horcale, which grew with difficulty in a light value of .002, showed essentially the leaf structure of the form growing in light of .03, while the form in full sunlight showed a striking difference in the leaf struc- ture. In considering the light stimuli of habitats, it is unnecessary • to discuss the stimulus of total darlaiess upon chlorophyllous plants, although this is of great importance in experimental evolution and in control experi- ment. The normal extremes of light intensity, i. e., those within which chlorenchym can function, are full sunshine represented by i, and a diffuse- ness of .002, though small flowering plants have once or twice been found in an intensity of .001. The maximum light value, even on high mountains, never exceeds i by more than an inconsiderable amount, except for the PHOTOHARMOSE 13I temporary concentration due to drops of dew, rain, etc. It seems improba- ble that the concentrating effect of epidermal papillae can do much more than compensate for the reflection and absorption of the epidermis. Ex- perimental study has shown that the maximum intensity in nature may be increased several, if not many times, without injurious results and without an appreciable increase in the photosynthetic response, thus indicat- ing that the efficient difference increases toward the maximum as well as toward the minimum. 173. The reception of light stimuli. Rays of light are received by the epidermis, by which they are more or less modified. Part of the light is reflected by the outer wall or by the cuticle, particularly when these present a shining surface. Hairs diffract the light rays, and hairy coverings con- sequently have a profound influence in determining stimuli. The walls and contents of epidermal cells furthermore absorb some of the light, especially when the cell sap is colored. In consequence of these effects, the amount of light that reaches the chlorenchym is always less than that inci- dent upon the leaf, and in many plants, the difference is very great. According to Haberlandt-, the epidermal cells of some shade plants show modifications designed to concentrate the light rays. Of such devices, he distinguishes two types : one in which -the outer epidermal wall is arched, another in which the inner wall is deeply concave. Although there can be no qviestion of the effect of lens-shaped epidermal cells, their occurrence does not altogether support Haberlandt's view. Arched and papillate epidermal cells are found in sun plants where they are unnecessary for in- creasing illumination, to say the least. A large number of shade plants show cells of this character, but in many the outer wall is practically a plane. Shade forms of a species usually have the outer wall more arched or papillate, but this is not always true, and, in a few cases, it is the lower epidermis alone that shows this feature. Finally, a localization of this function in certain two-celled papillae, such as Haberlandt indicates for Fittonia verschaffelii, does not appear to be plausible. The epidermis merely receives the light; the perception of the stimulus normally occurs in those cells that contain chloroplasts. The cytoplasm of the epidermal cells, as well as that of the chlorenchym cells, is sensitive to light, but the response produced by the latter is hardly discernible in the absence of plastids, except in those plants which possess streaming pro- toplasm. The daily opening and closing of the stomata, which is due_to_ light, is evidently connected with the presence of chloroplasts in the guard cells. Naturally, the perception of light and the corresponding response occur in the epidermis of many shade and submerged plants which have ' Physiologische Pflanzenanatomie. 3d ed., 537. 1904. 132: THE PLANT chloroplasts in the epidermal cells. Such cases merely serve to confirm the view that the perception of light stimuli is localized in the chloroplast. In conformity with this view, the initial response to such stimuli must be sought in the chloroplast, and the explanation of all adaptations due to light must be found in the adjustment shown by the chloroplasts. 174. Response of the chloroplast. The fundamental response of a plastid to light is the manufacture of chlorophyll. In the presence of carbon dioxide and water, leucoplasts invariably make chlorophyll, and chloroplasts replace that lost by decomposition, in response to the stimulus exerted by light. The latter is normally the efficient factor, since water is always present in the living plant, and carbon dioxide absent only locally at most. Sun plants which possess a distinct cuticle, however, produce leucoplasts, not chloroplasts, in the epidermal cells, although these are as strongly illuminated as the guard cells, which contain numerous chloroplasts. This is evidently explained by the lack of carbon dioxide in the epidermis. This gas is practically unable to penetrate the compact cuticle, at least in the small quantity present in the air. The supply obtained through the stomata is first levied upon by the guard cells and then by the cells of the chlorenchym, with the result that the carbon dioxide is all used before it can reach the epidermal cells. This view is also supported by the presence of chloroplasts along the sides and lower wall of palisade cells, where there is normally a narrow air-passage, and their absence along the upper wall when this is closely pressed against the epidermis, as is usually the case. Furthermore, the leaves of some mesophytes when grown in the sun develop a cuticle and contain leucoplasts. Under glass and in the humid air of the greenhouse, the same plants develop epidermal chloroplasts but no cuticle. This is in entire harmony with the well-known fact that shade plants and submerged plants often possess chloroplasts in the epidermis. Although growing in different media, their leaves agree in the absence of a^^cuti^Cj and consequent absorption of gases through the epidermis. The size, shape, number, and position of the chloroplasts are largely determined by light, though a number of factors enter in. No accurate studies of changes in size and shape have yet been made, though casual measurements have indicated that the chloroplasts in the shade form of certain species are nearly hemispherical, while those of the sun form are plane. In the same plants, the number of chloroplasts is strikingly smaller in the shade form, but exact comparisons are yet to be made. The position and movement of chloroplasts have been the subject of repeated study, but the factors which control them are still to be conclusively indicated. Light is clearly the principal cause, although there are many cases where a marked change in the light intensity fails to call forth any readjustment of the PHOTOHARMOSE I33 plastids. The positidn of air-spaces as reservoirs of carboh dioxide and the movement of crude and elaborated materials from cell to cell frequently have much to do with this problem. Finally, it must be constantly kept in mind that the chloroplasts lie in the cytoplasm, which is in constant contact with a cell wall. Hence, any force that affects the shape of the cell will have a corresponding influence upon the position of the chloroplasts. When it is considered that in many leaves these four factors play some part ill determining the arrangement of the plastids, it is not difficult to under- stand that anomalies frequently appear. It may be laid down as a general principle that chloroplasts tend to place themselves at right angles to rays of diffuse light and parallel to rays of sunlight. This statement is borne out by an examination of the leaves of typical sun and shade species, or of sun and shade forms of the same species. Cells which receive diffuse light, i. e., sponge cells, normally have their rows of plastids parallel with the leaf surface, while those in full sunlight place the rows at right angles to the surface. This disposition at once suggests the generally accepted view that chloroplasts in diffuse light are placed in such a way as to receive all the light possible, while those in sunlight are so arranged as to be protected from the intense illumination. Many facts support this statement with respect to shade leaves, but the need of protection in the sun leaf is not clearly indicated. The regular occurrence of normal chloroplasts in the guard cells seems conclusive proof that full sunlight is not injurious to them. Although the upper wall of the outer row of palisade cells is usually free from chloroplasts, yet it is not at all un- common to find it covered by them. These two conditions are often found in cells side by side, indicating that the difference is due to the presence of carbon dioxide and not to light. In certain species of monocotyledons, the arrangement of the chloroplasts is the same in both halves of the leaf, and there is no difference between the sun and shade leaves of the same species. The experimental results obtained with concentrated sunlight, though otherwise conflicting, seem to show conclusively that full sunlight does not injure the chloroplasts of sun plants, and that the position of plastids in palisade cells is not for the purpose of protection. This arrange- ment, which is known as apostrophe, is furthermore often found in shade forms of heliophytes. In typical shade species, and in submerged plants, the disposition of plastids on the wall parallel with the leaf surface, viz., epistrophe, is more regular, but. even here there are numerous exceptions to the rule. The absorption of the light stimulus by the green plastid results, under normal conditions, in the immediate production of carbohydrates, which in the vast majority of cases soon become visible as grains of starch. The appearance of starch in the chloroplasts of flowering 'plants is such a 134 THE PLANT SMz regular response to the action of light that it is regarded as the normal in- dication of photosynthetic activity. The mere presence of chlorophyll is not an indication of the latter, since chlorophyll sometimes persists in light too diffuse for photosynthesis. The amount of starch formed is directly connected with the light intensity, and in consequence it affords a basis for the quantitative estimation of the response to light. Two responses to light stimuli have a direct effect upon the amount of transpiration. Of the light energy absorbed by the chloro- plast, only 2.5 per cent is used in photosjTi- thesis, while 95-98 per cent is converted into heat, and brings about marked increase in trans- piration. Furthermore, in normal turgid plants, the direct action of light, as is well known, opens the stomata in the morn- ing and closes them at night. 175. Aeration and translocation. The movements of gases and of solutions through the tissues of the leaf are intimately connected with photosynthesis, and hence with responses to light stimuli. Aeration depends primarily upon the periodic opening of the stomata, for, while the carbon dioxide and oxygen of the air are able to pass through epidermal walls not highly cutinized, the amount obtainable in this manner is altogether inadequate, if not negligible. " The development of sponge tissue or aerenchym is intimately connected with the stomata. The position and amount of aerenchym and the relative extent of sponge cells and air-spaces are in part determined by the number and position of the Fig. 38. Ecads of Allionia linearis, showing position of chloroplasts. The palisade shows apostrophe, the sponge epistrophe: 1, sun leaf (chresard, 2-5^, light, 1); 2, shade leaf (chresard, ll%\ light, .012); 3, shade leaf (chresard, 11^; light, .003). X 250. PHOTOHARMOSE 135 breathing pores. The disposition of air spaces has much to do with the arrangement of chloroplasts in both palisade and sponge tissues. Starch formation is also dependent upon the presence of air spaces, but, con- trary to what would be expected, it seems to be independent of their size, since sun leaves, which assimilate much more actively than shade leaves, have the smallest air spaces. From this fact, it appears that the rapidity of aeration depends very largely upon the rapidity with which the gases are used. Translocation likewise affects the arrangement of the chloroplasts and the formation of starch. According to Haberlandt, it also plays the principal part in determining the form and arrangement of the palisade cells. Chloroplasts are regularly absent at those points of contact where the transfer of materials is made from cell to cell, though this is not invariably true. Since air pas- sages are necessarily absent where cell walls touch, it is possible that this disposi- tion of the plastids is likewise due to the lack of aeration. Translocation is directly connected with the appearance of starch. As long as all the sugar made by the chloroplasts is transferred, no starch ap- pears, but when assimilation begins to exceed translocation, the increasing con- centration of the sugar solution results in the production of starch grains. The latter is normally the case in all flowering ' plants, with the exception of those that form sugar or oil, but no starch. The constant action of translocation is practi- cally indispensable to starch formation, since an over-accumulation of carbohy- drates decreases assimilation, and finally inhibits it altogether. In consequence, translocation occurs throughout the day and night, and by this means the accumulated carbohydrates of one day are largely or entirely removed before the next. Fig. 39. Position of chloroplasts in aerial leaf (1) and submerged leaf (2) of Callitriche bifida. X 250. 176. The measurement of responses to light. Responses, such as the periodic opening and closing of stomata, which are practically the same for all leaves, are naturally not susceptible of measurement. This is also true of the transpiration produced by light, but the difficulty in this case is due 136 THE PLANT to the impossibility of distinguishing between the water loss due to light and that caused by humidity and other factors. If it were possible to de- termine the amount of chlorophyll or glucose produced, these could be used as satisfactory measures of response. As it is, they can only be determined approximately by counting the chloroplasts or starch grains. The arrange- ment of the chloroplasts can not furnish the measure sought, since it does not lend itself to quantitative methods, and since the relation to light in- tensity is too inconstant. Hesselmann (/. c, 400) has determined the amount of carbon dioxide respired, by means of a eudiometer, and has based comparisons of sun and shade plants upon the results. As he points out, however, light has no direct connection with respiration. Although the latter increases necessarily with increased nutrition, the relation between them is so obscure, and so far from exact, that the amount of respiration can in no wise serve as a measure of the response to light. As a result of the foregoing, it is clear that no functional response is able to fulrnish a satisfactory measure of adjustment to light, though one or two have per- haps sufficient value to warrant their use. Indeed, structural adaptations offer a much better basis for the quantitative determination of the effects of light stimuli, as will be shown later. In attempting to use the number of chloroplasts or starch grains as a measure of response, the study should be confined to the sun and shade forms of the same species, or, in some cases, to the forms of closely related species. The margin of error is so great and the connection with light sufficiently remote that comparisons between unrelated forms or species are almost wholly without value. It has already been stated that starch is- merely the surplus carbohydrate not removed by translocation; the amount' of starch, even if accurately determined, can furnish no real clue to -the amount of glucose manufactured. In like manner, the number of chloro- plasts can furnish little more than an approximation of the amount of chlorophyll, unless size and color are taken into account. In sun and shade ecads of the same species, the general functional relations are essentially' the same, and whatever differences appear may properly be ascribed to different light intensities for the two habitats. The actual counting of chloroplasts and starch grains is a simple task. Pieces of the leaves of the two or more forms to be compared are killed and imbedded in paraffin in the usual way. To save time, the staining is done in toto. Methyl green is used for the chloroplasts and a strong solution of iodine for the starch grains. When counts are to be made of both, the leaves are first treated with iodin and then stained with the methyl green. The thickness of the microtorrie sections should be less than that of the palisade cells in order that the chloroplasts may appear in profile, thus facilitating the counting. The count is made for a segment 100 n. in width across the entire leaf. PHOTOHARMOSE 1 37 Two segments in different parts of the section are counted, and the result multiplied by five to give the number for a segment i millimeter in width. Although sun and shade leaves regularly differ in size and thickness, no correction is necessary for these. Size and thickness stand in reciprocal relation to each other in ecads, and thickness is largely an expression of the absorption of light, and hence of its intensity. In the gravel, forest, and thicket ecads of Galium boreale, counts of the chloroplasts gave the follow- ing results. The gravel form (light i) showed 3,500 plastids in the i-mm. segment, the forest form (light .03) possessed 1,350, and the thicket form (light .002), i,Q00. In these no attention was paid to the size and form of the plastids in the different leaves, since the differences were inappre- ciable. When this is not the case, both factors should be taken into account. Starch grains are counted in exactly the same way. Indeed, if care is taken to collect leaves of forms to be compared, at approximately the same time on sunshiny days, a count of the chloroplasts is equivalent to a count of the starch grains in the vast majority of cases. Measurements of the size of starch grains can be made with accuracy only when the leaves are killed in' the field at the same time, preferably in the afternoon. Counts of chloro- plasts alone can be used as measures of response in plants that produce sugar or oil, while either chloroplasts or starch grains or both may be made the basis in starch-forming leaves. Hesselmann {I. c, 379) has em.ployed Sachs's iodine test as a measure of photos}'nthesis. This has the advantage of permitting macroscopic ex- amination, but the comparison of the stained leaves can give only a very general idea of the relative photosynthetic activity of two or more ecads. The iodine test is made as follows ■} fresh leaves are placed for a few minutes in boiling water, and then in 95 per cent alcohol for 2-5 minutes, in order to remove the chlorophyll and other soluble substances. The leaves are placed in the iodine solution for J'2-3 hours, or until no further change in color takes place. The strength of the solution is not clearly ihdicated by Sachs, who says: "I used an alcoholic solution of iodin v/hich is best made by dissolving a large quantity of iodin in strong alcohol and adding to this sufficient distilled water to give the liquid the color of dark beer." This solution may be approximated by dissolving 1/3 gram of iodin in 100 grams of 30 per cent alcohol. The stained leaves are put in a white porce- lain dish filled with distilled water, and the dish placed in the strong diffuse light of a window. The colored leaf stands out sharply against the porcelain, and the degree of coloration, and hence of starch content, is determined by the following table : ^ Sachs, J. Ein Beitrag zur Kenntniss der Ernahrungsthatigkeit der Blatter. Gesammelte Abhandlungen iiber Pflanzenphysiologie. 1:355. 1892. 138 THE PLANT 1. bright yellow or leather yellow (no starch in the chlorenchym) 2. blackish (very little starch in the chlorenchym) 3. dull black (starch abundant) 4. coal black (starch very abundant) 5. black, with metallic luster (maximum starch-content) ADAPTATION 177. Influence of chloroplasts upon form and structure. The begin ning of all modifications produced by light stimuli must be sought in the chloroplast as the sensitized unit of the protoplasm. Hence, it seems a truism to say that the number and arrangement of the chloroplasts de- termine the form of the cell, the tissue, and the leaf, although it has not yet been possible to demonstrate this connection conclusively by means of experiment. In spite of the lack of experimental proof, this principle is by far the best guide through the subject of adaptations to light, and in the discussion that follows, it is the fundamental hypothesis upon which all others rest. The three propositions upon which this main hypothesis is grounded are: (i) that the number of chloroplasts increases with the in- tensity of the light; (2) that in shaded habitats chloroplasts arrange them- selves so as to increase the surface for receiving light; (3) that chloroplasts in sunny habitats place themselves in such fashion as to decrease the surface, and consequently the transpiration due to light. In these, there can be little doubt concerning the facts of number and arrangement, since they have been repeatedly verified. The purpose of epistrophe and apostrophe, however, can not yet be stated with complete certainty. The stimulus of sunlight and of diffuse light is the same in one respect, namely, the chloroplasts respond by arranging themselves in rows or lines on the cell wall. The direct consequence- of this is to polarize the cell, and its form changes from globoid to oblong. This effect is felt more or less equally by both palisade and sponge cells, but the disturbing influence of aeration has caused the polarity of the cells to be much less conspicuous in the sponge than in the palisade tissue. While the cells of both are typically polarized, however, they assume very different positions with reference to incident light. This position is directly dependent upon the arrangement of the plastids as determined by the light intensity. In consequence, palisade cells stand at right angles to the surface and parallel with the impinging rays; the sponge cells, conversely, are parallel with the epidermis and at right angles to the light ray. Some plants, especially monocotyledons, exhibit little or no polarity in the chlorench5'm. As a result the leaf does not show a differentiation into sponge and palisade, and the leaves of sun and shade ecads are essentially alike in form and structure. The form of PHOTOHARMOSE 139 the leaf is largely determined by the chloroplasts acting through the cells that contain them. A preponderance of sponge tissue produces an- extension of leaf in the direction determined by the arrangement of the plastids and the shape of the sponge cells, viz., at right angles to the light. Shade leaves are in consequence broader and thinner, and sometimes larger, than sun leaves of the same species. A preponderance of palisade likewise results in the extension of the leaf in the line of the plastids and the palisade cells, i. e., in a direction parallel with the incident ray. In accordance, sun leaves are thicker, narrower, and often smaller than shade leaves. 178. Form of leaves and stems. In outline, shade leaves are more nearly entire than sun leaves. This statement is readily verified by the comparison of sun and shade ecads, though the rule is by no means without exceptions. In the leaf prints shown in figures 14 and 15, the modification of form is well shown in Bursa and Thalictrum ; in Capnoides the change if. less evident, while in AchiUeia and Machaeranthera lobing is more pro- nounced in the shade form, a fact which is, however, readily explained when other factors are taken into account. The leaf prints cited serve as moie satisfactory examples of the increase of size in consequence of an increase in the surface of the shade leaf, although the leaves printed were selected solely with reference to thickness and size or outline. In all comparisons of this kind, however, the relative size and vigor of the two plants must be taken into account. This precaution is likewise necessary in the case of thickness, which should always be considered in connection with amount of surface. The relation between surface and thickness is shown by the follow- ing species, in all of which the size of the leaf is greater in the shade than in the sun. In Capnoides aureum, the thickness of the shade leaf is J4 (6:12) that of the sun leaf; in' Galium boreale the ratio is 5:12, and in Allionia linearis it is 3 :I2. The ratio in Thalictrum sparsiftorum is 9 :i2, and in Machaeranthera aspera 11:12. The thiclmess of ^sun and shade leaves of Bursa bursa-pastoris is as 14:12, but this anomaly is readily explained by the size of the plants; the shade form is ten times larger than the sun form. Certain species, e. g., Erigeron speciosus, Potentilla bipinnatifida, etc., show no change in thickness and but little modification in size or out- line. They furnish additional evidence of a fundamental principle in adaptation, namely that the amount of structural response is profoundly affected by the stability of the ancestral type. The effect of diffuse light in causing stems to elongate, though known for a long time, is still unexplained. The old explanation that the plant stretches up to obtain more light seems to be based upon nothing more than the co- incidence that the light comes from the direction toward which the stem grows. Later researches have shown that the stretching of the stem is due \140 -THE PLANT to the excessive elongation of the parenchyma cells, but the cause of the latter is far from apparent. It is generally assumed to be due to a lack of the tonic action of sunlight, which brings about a retardation of growth in sun plants. The evidence in favor of this view is not con- clusive, and it seems probable at least that the elongation of the parenchyma cells takes place under conditions which favor the mechanical stretch- ing of the cell wall, but inhibit the proper growth of the wall by intussusception. It is hardly necessary to state that the reduced photosynthetic ac- tivity of shade plants favors such an explanation. What- ever the cause, the advantage that results from the elongation of the internodes is apparent. Leaves interfere less with the illumination of those below them, and the leaves of the branches are carried away from the stem in such a way as to give the plant the best possi- ble exposure for its aggregate leaf surface. 179. Modification of the epidermis. The development of epidermal chloroplasts in diffuse light is the only change which is due to the direct effect of light. This does not often occur in the shade ecads of stin species, but chloroplasts are regularly present in the epidermis of woodland ferns and of submerged plants. The slight development of hairs in sciophilous plants is an advantage, but it must be referred to the factors that determine water loss. The significance' Fig. 40. Isophotophyll of Allionia linearis, showing diphoUc ecads: 1, light 1; 2, light .012; 3, light .003. X 130. PHOTOHARMOSE 141 of epidermal papillae in increasing the absorption of light by shade plants has already been discussed. The questions as to what factor has called forth these papillae and what purpose they serve must still be regarded as un- settled. The increased size of the epidermal cells, which is a fairly constant feature of shade ecads, seems to be for the purpose of increasing transloca- tion and transpiration, and to bear no relation to light. The extreme development of the cells of the epidermis in' Streptopus and Limnorchis, which grow at the edge of mountain brooks, has been plausibly explained by E. S. Clements as a contrivance to increase water loss. The presence of a waxy coating, such as that found upon the leaves of Im- patiens aurea and /. pallida, is clearly to prevent the wetting of the leaf and the consequent stoppage of the stomata. In regard to the latter, different observers have noted that the number of the stomata is greater in sun than in shade leaves. This holds generally for sun and shade species, but it is most clearly indicated by different ecads of the same species. In Scutellaria brit- tonii, the sun form possesses 100 stomata per square milli- meter, but in the shade these are reduced to 40 per square millimeter; the sun leaf of Al- lionia linearis has 180 stomata to the square millimeter, the shade leaf 90; In the stable leaf of Erigeron speciosus, however, the number of stomata is the same, 180 per square millimeter, for sunlight and for diffuse light. The presence of the larger number of stomata in the plant exposed to greater loss, which at first thought seems startling, is readily explained by the more intense photosynthetic activity in the sun. Since the absorption of gases is the primary function of the stomata, and transpiration merely secondary, it is evident that sun plants must have more stomata than shade plants. This is further explained by the fact that the small air passages of sun leaves necessitate frequent inlets, which are less necessary in shade leaves with their larger air spaces. In Fig. 41. Isophotophyll of Helianthus pumilus, showing isophotic ecad: 1, sun leaf; 2, shade leaf (light .012). X 130. 142 THE PLANT shade plants, moreover, the decrease in the number is compensated in some measure by the ability of the epidermal cells to absorb gases directly from the air. 180. The differentiation of the chlorenchym. The division of the chlorenchym into two tissues, sponge and palisade, is the normal conse- quence of the unequal illumination of the leaf surfaces. Exceptions to this rule occur only in certain monocotyledons, in which the leaf tissue consists of sponge-like cells throughout, and in those stable species that retain more or less palisade in spite of their change to diffuse light. The difference in the illumination of the two surfaces is determined by the position of the leaf. Leaves that are erect or nearly so usually have both sides about equally illuminated, and they may be termed isophotic. Leaves that stand more or less at right angles to the stem receive much more light upon the upper surface than upon the lower, and may ac- cordingly be termed diphotic. Certain dorsiventral leaves, however, absorb practically ■ as much light on the lower side as upon the upper. This is true of sun leaves with a dense hairy covering, which screens •out the greater part of the light incident upon the upper surface. It occurs also in xerophytes which grow in light-colored sands and gravels that serve to reflect the sun's rays upon the lower surface. In deep shade, moreover, there is no essential difference in the intensity of the light received by the two surfaces, and shade leaves are often isophotic in consequence. From these examples it is evident that isophotic and diphotic leaves occur in both sun and shade, and that the intensity of the light is secondary to direction, in so far as the modification of the leaf is concerned. The essential connection of sponge tissue with diffuse light is conclu- sively shown by the behavior of shade ecads, but further evidence of great value is furnished by diphotic leaves, and those with hairy coverings. The sponge tissue, whiph in the shade leaf is due to the diffuse light of the habitat, is prodiiced in the hairy leaf as a consequence of the absorption and diffraction of the light by the covering. In ordinary diphotic leaves, the Fig. 42. Diphotophylls of Quercus novi- mexicana: 1, sun leaf; 2, shade leaf of the same tree (light .06). X 130. PHOTOHARMOSE 143 absorption of light in the palisade reduces the intensity to such a degree that the cells of the lower half of the leaf are in diffuse light, and are in consequence modified to form sponge tissue. The sponge tissue of the diphotic leaf is just as clearly an adaptation to diffuse light as it is in those plants where the whole chlorenchym is in the shade of other plants or of a covering of hairs. As is indicated later, all these relations permit of ready confirmation by experiment, either by changing the position of the leaf or by modifying the intensity or direction of the light. The preceding discussion makes it fairly clear that sponge tissue is developed primarily to increase the light- absorbing surface. Because of its direct connection with photosynthesis, the sponge tis- sue is the especial organ of aeration, also, and since it shows a high development of air spaces for this purpose, it is inevitably concerned in transpiration. It seems to be partly a coincidence, however, that the sponge is found next to the lower surface upon which the stomata are most numerous. This is indicated by artificial ecads of Ranun- culus sceleratus, in which sponge tissue is unusually de- veloped, although the -stomata are much more numerous upon the upper surface. Palisade tissue is apparently developed primarily as a protection against water loss, particularly that due to the absorption of light by the chloroplast. The small size of the intercellular passages between palisade cells likewise aids in decreasing transpiration. The fact that leaves with much palisade tissue transpire twice a much as shade leaves is hardly an objection to this view, as Hesselmann {I, c, 442) would think. It is readily explained by the intense photosyn- thesis of sun plants, which makes necessary an increase, usually a doubling, in the number of stomata, in consequence of which the transpiration is increased. Fig. 43. A plastic species, Mertensia poly- phylla, showing the effect of water upon the sponge: 1, chresard 25jg; 2, chresard V2.%. X130. 144 THE PLANT 181. Types of leaves. Isophotic leaves are equally illuminated and possess more or less uniform chlorenchym. Diphotic leaves are unequally illuminated, and exhibit a differentiation into palisade and sponge tissues. They may be distinguished as isophotophylls and diphotophylls respec- tively.^ Isophotic leaves fall into three types based upon the intensity of the light. The staurophyll, or palisade leaf, is a sun type in which the equal illumination is due to the upright position or to the reflection from a light soil, and in which the chlor- enchym consists wholly of rows of palisade cells. The diplophyll is a special form of this type in which the intense light does not penetrate to the middle of the leaf, thus re- sulting in a central sponge tissue, or water- storage tissue. The spongophyll, or sponge leaf, is regularly a shade type; the chlor- enchym consists of sponge cells alone. For the present at least it is also necessary to refer to this group those monocotyledons which grow in the sun but contain no pali- sade tissue. Diphotic leaves always contain both palisade and sponge, though the ratio between .them varies considerably. Diphot- ophylls are characteristic of sunny mesophytic habitats. They are frequent in xerophytic habi- tats as well as in woodlands where the light is not too diffuse. In the case of stable species, this type of structure sometimes persists in the dif- fuse light of coniferous forests. Floating leaves, in which the light is almost completely cut off 'frorn the lower surface, are also members of this group. Submerged leaves, on the other hand, are spongophylls. 182. Heliophytes and sciophytes. The great majority of sun plants possess diphotophylls. This type is represented by Pedicularis procera 'Clements, E. S. The Relation of Leaf Structure to Physical Factors. 1905. Fig. 44. A stable species Erigeron speciosus: 1, sun leaf; 2 shade leaf (light .03). X 130. Fig. 45. Spongophyll of Gyrostachys stricta (light 1). X130. EXPERIMENTAL EVOLUTION 145 (fig. 32). Plants with isophotophylls are found chiefly in xerophytic places, though erect leaves of this type occur in most sunny habitats. The staurophyll, in which the protection is due to the extreme development of palisade tissue, is illustrated by Allionia linearis (fig. 40) and Bahia dissecta (fig. 33). The diplophyll, which is characterized by a central band of sponge tissue or storage cells, is found in Mertcnsia linearis (fig. 34). The form of the ■ spongophyll that is found in certain monocotyledons is shown by Gyrostachys stricta ,(^S- 45)- The spongophyll (fig. 38:3, 39:2) is frequent among plants of deep shade, but as the leaf sections of Allionia (figs. 38, 40) and Quercns (fig. 42) show, the diphotophyll . is the rule in shade ecads. Experimental Evolution 183. Scope. The primary task of experimental evolution is the de- tailed study, under measured conditions, of the origin of new forms in nature. As a department of botanical research that is as yet unformed, it has little concern with the host of hypotheses and theories which rest merely upon general observation and conjecture. A few of these constitute , good working hypotheses or serve to indicate possible points of attack, but the vast majority are worthless impedimenta which should be thrown away at the start. It is the general practice to speak of evolution as founded upon a solid basis of incontestible facts, but a cursory examination of the evidence shows that it is drawn, almost without exception, from observaton alone, and has in consequence suffered severely from interpretation. With the exception of DeVries's work on mutation, sustained and accurate investiga- tion of the evolution of plants has been lacking. As a result, botanical research has been built high upon an insecure foundation, nearly every stone of which must be carefully tested before it can be left permanently in place. In a field so vast and important as evolution, experiment should far outrun induction, and deduction should enter only when it can show the way to a working hypothesis of real merit. The great value of DeVries's study of mutation as an example of the proper experimental study of evolution has been seriously reduced by the fact that the "mutation theory" has carried induction far beyond the warrant afforded by experiment. The investigator who plans to make a serious study by experiment of the origin of new plant forms should rest secure in the conviction that the most rapid and certain progress can be made only by the accumulation of a large number of un- impeachable facts, obtained by the most exact methods of experimental study. The general application of field experiment to evolution will render the current methods of recognizing species quite useless. It will become im- perative to establish an experimental test for forms and species, and to 146- THE PLANT apply this test critically to every "new species." Descriptive botany, as- practiced at present, will fall into disuse, as scientific standards come to prevail, and in its place will appear a real science of taxonomy. In the latter the criteria upon which species are based will be obtained solely by experiment. 184. Fundamental lines of inquiry. There are two primary and sharply defined fields of research in experimental evolution, namely, adapta- tion in consequence of variation (and mutation), and hybridization. The latter constitutes a particular field of inquiry, which is not intimately con- nected with the problems of evolution in nature. In the study of specific adaptation, two questions of profound importance appear. One deals with the effects of ancestral fixity or plasticity in determining the amount of modification produced by the habitat. These are fundamental problems, and a solution of them can not be hoped for until exact and trustworthy data have been provided by numerous experimental researches. It thus becomes clear that the principal, if not the sole task of experimental evolution for years to come is the diligent prosecution of accurate and prolonged experi- ment in the modification of plant forms. It seems inevitable that this will be carried on along the lines that have already been indicated. Plants will be grown in habitats of measured value, or in different intensities of the same factor. The relation between stimulus and adjustment will form the basis of careful quantitative study, and the final expression of this relation in structural modifications will find an exact record in drawings, photo- graphs, exsiccati, and biometrical measures. The making of an accurate and complete record of the whole course of each experiment of this sort is an obligation that rests upon every investigator. Studies in experimental evolution will prove time-consuming beyond all other lines of botanical research, and the work of one generation should appear in a record so perfect that it can be used without doubt or hesitation as a basis for the studies of the succeeding generation. 185. Ancestral form and structure. The significance of the fact that some species have been found to remain unaltered structurally under changes of habitats that produced striking modifications in others has already been commented upon. It is hardly necessary to indicate the important bearing which this has upon evolution. The very ability of a plant to undergo modification, and hence to give rise to new forms, depends upon the degfee of fixity of the characters which it has inherited. Stable plants are less susceptible of evolution than plastic ones. The latter adapt themselves to new habitats with ease, and in each produce a new form, which may serve EXPERIMENTAL EVOLUTION I47 as the starting point of a phylum. There is at present no clue wliatever as to what calls forth this essential difference in behavior. This is not surpris- ing in view of the fact that there have been no comparative experimental studies of stable and plastic species. Until these have been made, it is im- possible to do more than to formulate a working hypothesis as to the effect of stability, and an explanation of the forces which cause or control it is altogether out of the question. 186. Variation and mutation. New forms of plants are known to arise by three methods, viz., variation, niutation, a daptatio n. The evidence in support of these is almost whplLy observational, and consequently more or less inexact, but for each there exist a few accurate experiments which are conclusive. Origin by variation and subsequent selection is the essence of the Darwinian theory of the origin of species. According to this the appearance of a new form is due to the accumulation, and selection, through a long period, of minute differences which prove advantageous to the plant in its competition with others in nature, or are desirable under cultivation. Slight variations appear indiscriminately in every species. Their cause is hot known, but since they are found even in the most uniform habitats, it is. impossible to find any direct connection between them and the physical factors. In the case of origin by mutation, the new form appears suddenly, with definite characteristics fully developed. Selection, in the usual sense of the term, does not enter into mutation at all, though the persistence of the new form is still to be determined by competition. Muta- tions are known at present for only a few species, and their actual appearance has been studied in a very few cases. Like variations, they are indiscrimi- nate in character. The chief difference between them is apparently one of degree. Indeed, mutation lends itself readily to the hypothesis that it is simply the sudden appearance of latent variations which have accumulated within the plant. DeVries regards constancy as an essential feature of mutation, but the evidence from the mutants of Onagra is not convincing. Indeed, while there can be no question of the occurrence of mutation in plants, a fact known for many years, the facts so far brought forward in support of the "mutation theory" fall far short of proving "the lack of significance of individual variability, and the high value of mutability for the origin of species."^ Mutations do not show any direct connection with the habitat, but their sudden appearance suggests that they may be latent or delayed responses to the ordinary stimuli. Origin by adaptation is the immediate consequence of the stimuli exerted by the physical factors of a 'DeVries, H. Die Mutationstheorie, 1:6. 1901. 148 THE PLANT habitat. This fact distinguishes it from origin by variation, or by mutation. The new form may appear suddenly, often in a single generation, or grad- ually, but in either case it is the result of adaptation that is necessarily advantageous, because it is the result of adjustment to controlling physical factors. Origin by adaptation is perhaps only a special kind of -origin by variation, but this might be said with equal truth of mutation. New forms resulting from adaptation are like those produced from mutation, in that they appear suddenly as a rule and without the agency of selection. They are essentially different, inasmuch as their cause may be found at once in the habitat, and since a reversal of stimuli produces, in many cases at least, a reversion in form and structure to the ancestral type. A valid distinction between forms or species upon the basis of constancy is impracticable at the present time. It is doubtful that such a distinction can ever be made in anything like an absolute sense, since all degrees of fluctua- tion may be observed between constancy and inconstancy. In all events, it is gratuitous to make constancy the essential criterion in the present state of our knowledge. So little is certainly known of it that it is equally un- scientific to affirm or to deny its value, and even a tentative statement can not be ventured until a vast amount of evidence has been obtained from ex- periment. Accordingly, there is absolutely no warrant, other than tradition, for limiting the term species to a constant group. In the evolutionary sense, a species is the aggregate ancestral group and the new forms which have sprung from it by variation, mutation, or adaptation. It should not be regarded as an isolated unit for purposes of descriptive botany; indeed, its use in this connection is purely secondary. It is properly the unit to be used in indicating the primary relationships which are the result of evolution. On the basis of their actual behavior in the production of new forms, species may be distinguished as variable, mutable, or adaptable. The new form which results from variation is a rariant; the product of mutation is a mutant, and that of adaptation, an ecad. The following examples serve to illustrate these distinctions. Machaer anther a canescens, judging from the numerous minute intergrades betv/een its many forms, is a variable species, i. e., one in which forms are arising by the gradual selection of small variations. It apparently comprises a large number of variants, M. canescens aspera, superba, ramosa, viscosa, etc. Onagra lamarckiana is a mutable species : it comprises many mutants, e. g.,' Onagra lamarckiana gigas, O. I. nanella, O. I. lata, etc. Galium boreale is an adaptable species: it possesses one distinct ecad, Galium boreale hylocolum, which is the shade form of the species. EXPERIMENTAL EVOLUTION I49 187. Methods. The best of all experiments in evolution are those that are constantly being made in nature. Such experiments are readily dis- covered and studied in the case of origin by adaptation; variants present much greater difficulties,, while mutants are very rare under natural condi- tions. The method which makes use of these experiments may be termed the method of natural experiment. The number of ecads which appear naturally in vegetation is limited, howevfr, and it is consequently very desirable to produce them artificially, by the method of habitat culture. This method, while involving more labor than the preceding, yields results that are equally conclusive, and permits the study of practically every species. The method of control culture, which is carried on in the plant- house, naturally does not possess the fundamental value of the field methods. It is an invaluable aid to the latter, however, since it permits the physical factors to be readily modified and controlled. All these methods are based on the indispensable use of instruments for the measurement of physical factors. METHOD OF NATURAL EXPERIMENT 188. Selection ol species. Species that are producing variants or ecads are found everywhere in nature; those which give rise to mutants seem, however, to be extremely rare. Consequently, mutants can not be counted upon for experimental work, and their study scarcely needs to be con- sidered. When a mutant is discovered by some fortunate chance, the mutable species from which it has sprung, and related species as well, should be subjected to the most critical surveillance, in the hope that new mutants will occur or the original one reappear. On account of the sud- denness with which they appear, mutants do not lend themselves readily to natural experiment, and after they have once been discovered, inquiry into the causes and course of mutation is practicable only by means of habitat and control cultures. Among variable species, those are most promising that show a wide range of variation and are found in abundance over extensivje areas. A species which occurs in widely separated, or more or less isolated areas, furnishes especially favorable material for investigation, since distance or physical barriers partly eliminate the leveling due to con- stant cross-fertilization. The individuals or groups which show appreciable departure from the type are marked and observed critically from year to year. The direction of the variation and the rapidity with which small changes are accumulated can best be determined by biometrical methods. Representative individuals of the species and each of its variants should likewise be selected from year to year. After being photographed, these are preserved as exsiccati, and with the photographs constitute a complete 15° THE PLANT graphic record of the course of variation. When the latter is made evident in structural feature also, histological slides are an invaluable part of the record. Polydemic species are by far the best and most frequent of all natural experiments. In addition to plants that are strictly polydemic, i. e., grow in two or more distinct habitats, there are a large number which occur in physically different parts of the sfime habitat. The recognition of polydemics is the simplest of tasks. As a rule, it requires merely a careful examination of contiguous formations in order to ascertain the species common to two or more of' them. The latter are naturally most abundant along the eco- tones between the habitats, and, as a result, transition areas and mixed formations are almost inexhaustible sources of ecads. Many adaptable species are found throughout several formations, however, and such are experiments of the greatest possible value. Not infrequently species of thfe manuals are seen to be ecads, in spite of their systematic treatment, and to constitute natural experiments that can be readily followed. Finally, it must be kept in mind that some polydemics are stable, and, do not give rise to ecads by structural adaptation. They not only constitute extremely in- teresting experiments in themselves, but they should also be very carefully followed year by year, since it seems probable that the responses are merely latent, and that they will appear suddenly in the form of mutants. In natural experiments it is sometimes difficult to distinguish which form is the ecad and which the original form of the species. As a rule, however, this point can be determined by the relative abundance and the distribution, but m cases of serious doubt, it is necessary to appeal to experimental cultures. Although habitats diiJer more or less with respect to all their factors, the study of polydemics needs to take into account only the direct factors, water-content, humidity, and light. Humidity as a highly variable factor plays a secondary part, and in consequence the search for ecads may be entirely confined to those habitats that show efficient differences in the amount of water-content or of light. Temperature, wind, etc., do not pro- duce ecads, and may be ignored, except in so far as they affect the direct factors. Complexes of factors, such as altitude, slope, and exposure, are likewise effective only through the action of the component simple factors upon water and light. The influence of biotic factors is so remote as to be negligible, especially in view of the fact that ecads are necessarily favorable adaptations, and are in consequence little subject to selective agencies. The essential test of a habitat is the production of a distinguish- able ecad, byt a knowledge of the water-content and light values of the habitats under examination is a material aid, since a minute search of each formation is necessary to reveal all the ecads. It is evident that habitats or EXPERIMENTAL EVOLUTION TSI areas that do not show efficient differences of water or light will contain no ecads of their common species, and also that extreme differences in- the amount of either of these two factors will preclude origin by adaptation to a large degree, on account of the need for profound readjustment. The general rule followed by most polydemics is that sun species will give rise to shade forms, and vice versa, and that xerophytes will produce forms of hydrophytic tendency, or the converse, when the areas concerned are not too remote, and the water or light differences are efficient, but not inhibitive. Some species are capable of developing naturally two series of ecads, one in response to light, the other to water-content, but they, unfortunately, have been found to be rare. Greatly diversified regions, such as the Rocky mountains, in which alternation is a peculiarly striking feature of the vegeta- tion, are especially favorable to the production of ecads, and hence for the study of natural experiments in origin by adaptation. 189. Determination of factors. For the critical investigation of the ■origin of new forms, an exact knowledge of the factors of the habitat, both physical and biotic, is imperative. In the case of variable species, these factors determine what variations are of advantage, and thereby the direc- tion in which the species can develop. They are the agents of selection. .With mutants, the factors of the habitat are apparently neither causative nor selective, though it seems probable that further study of mutants will show an essential connection between mutant and factor. In any event, the persistence of a mutant in nature, and its corresponding ability to initiate new lines of development, is as much dependent upon the selection exerted by physical and biotic factors as is the origin of variants. Physical factors are causative agents in the production of ecads, as has been shown at length elsewhere. The form and structure of the ecad are the ultimate responses to the stimuli of light or water-content, and the quantitative determination of the latter is accordingly of the most fundamental importance. The meas- urement of factors has been treated so fully in the preceding chapters that it is only necessary to point out that the thorough investigation of habitats by instruments is as indispensable for the study of experimental evolution as. for that of the development and structure of the formation. Furthermore, it is evident that a knowledge of physical factors is as imperative for habitat and control cultures as for the method of natural experiment. In the latter, however, the biotic factors demand unusual attention, since pollination, iso- lation, etc., are often decisive factors in origin by variation and in 'uie per- sistence of mutants. Measurements of adjustment, i. e., functional response to the direct factor concerned, are extremely valuable, but not altogether indispensable 152 THE PLANT to research in experimental evolution. This is due to the fact that a knowlr edge of adjustment is important in tracing the origin of new forms only when adjustment is followed by adaptation, and in all such cases the ratio between the two processes seems to be more or less constant. In the present rudimentary development of the subject, however, it is very desirable to make use of all methods of measuring functional responses to water and light that are practicable in the field. Certain methods that are difficult of application in nature may be used to advantage in control cultures, and the results thus secured can be used to interpret those obtained from natural experiments and field cultures. 190. Method of record. As suggested elsewhere, there are four im- portant kinds of records, which should be made for natural experiments, and likewise for habitat and control cultures. These are exsiccati, photo- graphs, biometrical formulae and curves, and histological sections. These serve not merely as records of what has taken place, but they also make it possible to trace the course of evolution through a long period with an accuracy otherwise impossible, and even to foreshadow the changes which will occtir in ihe future. The possibility of doing this depends primarily upon the completeness of the record, and for this reason the four methods indicated should be used conjointly. In the case of ecads and mutants, exsiccati, photographs, and sections are the most valuable, and in the ma- jority of cases are sufficient, since both ecads and mutants bear a more distinctive impress than variants do. On the other hand, since variations are more minute, the determination of the mean and extreme of variation by biometrical methods is almost a prerequisite to the use of the other three methods, which must necessarily be applied to representative individuals. Exsiccati and photographs are made in the usual way for plants, but it is an advantage to photograph each ancestral form alongside of its proper ecads, mutants, or variants, in addition to making detail pictures of each form and of the organs which show modification. In the collection of material for histological sections, which deal primarily with the leaf or with stems in the case of plants with reduced leaves, a few simple precautions have been found necessary. Whenever possible, material should be killed where it is collected, since in this way the chloroplasts are fixed in their normal position. In case. leaves that can not be replaced easily have become wilted, an immersion of 5-6 hours in water will make it possible to kill them without shrinkage. In selecting leaves, great pains must be taken to collect only mature leaves. When the plants have a basal rosette, or distinct radical leaves, mature leaves are taken from both stem and base. In all cases where the two surfaces of the leaf can not be readily, distinguished, the upper one is clearly marked. EXPERIMENTAL EVOLUTION 153 METHOD OF HABITAT CULTURES 191. Scope and advantages. By means of experiments actually made in the field, practically every species that is capable of modification can be made to produce new forms, the origin of which can be traced in the manner already indicated. Field experiments of this sort are especially favorable to the production of ecads from adaptable species. No attempt has yet been made to apply it to mutable or variable species, but its ultimate application ' to these does not sean at all impossible. The chief advantage of the method of habitat cultures is seen in the great range of choice in selecting the plant for experiment, and the habitat or area in which the experiment is carried out. A polydemic species which already has one or more ecads can be ex- tended to a number of different habitats of known value, and a complete series of ecads obtained, based either upon water-content, or light, or upon both. On the other hand, an endemic species, or one brought from a remote flora, can be placed in as many habitats as desired, and the appearance of ecads followed in each. Frequently, results of much value are obtained in a diversified habitat by growing its mos± plastic species in those areas which show the greatest differences in water-content or light intensity. Habitat cultures give results which are practically as perfect as those obtained from natural experiments, since the course of adaptation in no wise depends upon whether the agent by which the seed or propagule is carried into the new habitat is natural or artificial. Cultures of this kind further possess the distinct advantage of permitting more or less modification of the physical factors themselves. However, when it is desirable to have the factors under as complete control as possible, it is necessary to use the method of control cultures in the planthouse. 192. Methods. All field experiments in evolution are based upon a change of habitat. The latter is accomplished by the modification of the habitat itself, or by the transfer of the species to one or more different hab- itats, or to different areas of the same habitat. In both cases the choice of habitats is made upon the basis of efficient differences of water-content or light. Saline situations do not constitute an exception, since the chresard is really the effective stimulus. Cultures at different altitudes, which afford striking results, appear to concern several factors, but in the final analysis, water-content and humidity are alone found to be really formative. Cul- tures may furthermore be distinguished as simple or reciprocal. Simple cultures are those in which a species is transferred to one or more habitats, or in which a habitat is modified in one or more ways. Reciprocal cultures are possible only with polydemic species, or with endemics after ecads have -1 54 . . THE PLANT been produced by experiment. Modification or transfer is made in the usual way, but reciprocally, i. e., the original form is transferred to the habitat of ■the e,cad, and the latter to the habitat of the former ; or the shade in which some individuals of the ecad are growing may be destroyed, and at the same •time individuals of the type may be shaded. Bpth transfer and modification may be applied to the same species, but since the same measured change of iactor can be obtained in either way, the use of both is undesirable, with the exception of the rare cases where they serve as checks upon each other. The transfer of a .seed or plant is so much simpler and more convenient that this method is the one regularly used. It sometimes happens, however, that a change of water-content or light intensity is readily and conveniently made, and is desirable for other rea.sons. It is evident that both transfer, and modification require that the factor •records of the various habitats or areas be as full as possible, at least so far as water -content, humidity, and light are concerned. In the case of the areas that are to be modified, these factors are determined before the change is made. Afterward they are read from time to time during the growing season, and are also checked by readings made near at hand in the unmodi- fied formation. The readings made in the beginning should correspond closely to the check readings, but in case of disagreement the latter are to be taken as conclusive. 193. Transfer. After the species to be used for experiment has been chosen, the various habitats or areas selected, and the direct factors meas- ured by instruments, the actual transfer of the individuals is made by means of seeds, preferably in autumn, though the results are, practically the same if seeds are kept over the winter and planted at the opening of spring. The natural method is to scatter the seeds in the place selected, as though they had been carried by the usual agents of migration. The mortality is usu- ally great in such case, however, and the chances of success are increased by actually planting the seeds. This is the method which has been used in making cultures of species of the European Alps on the summit of Mount Garfield in the Rocky mountains. The number of seeds used is recorded in order to obtain some estimate of germination and competition. While the use of the seed or disseminule possesses the great advantage of making the experiment essentially a natural one, the transfer of rosettes, seedlings, or young plants makes the results more certain, and consequently saves time, even though the actual transfer is somewhat more difficult. It is hardly necessary to point out that the removal of the plant should be made with the greatest care. The best success is obtained by making the transfer on cloudy .or rainy days, and when shade plants are to be placed in sunny situations, they should be transplanted late in the afternoon. When the task of carry- EXPERIMENTAL EVOLUTION 155 ing them is not too great, it is a distinct advantage to move a number of individuals in the same block of earth. The transfer of mature plants is inadvisable, except for those perennials which can not readily be secured in an early stage. This naturally does not apply to woody plants, evergreen herbs, mosses and lichens ; the last two may be transferred at any time with satisfactory results. Each culture is carefully marked with stakes, and definitely located by means of landmarks. ■-A^....V. Fig. 46. Series for producing hydrophytic forms under control: 1, amphibious; 2, floating; 3, competition; 4, submerged. Reciprocal transfers may be made by means of seed or plant. Since the experiment is a complex one, all the care possible should be taken to make sure that the plants become established in the reciprocal situations, and con- sequently, it is often advisable to transfer both seeds and plants. Reciprocal transfer is of paramount value in solving the problem which bog plants present. A slight modification of the method makes it possible to obtain experimental evidence of the polyph3'letic origin of species in consequence of adaptation. In an experiment mentioned elsewhere, the transfer of Kuhnistera purpurea to the area occupied by K. Candida, and vice versa, is designed to show whether one has been derived from the other. If the two species are moved into an area which contains more water than that usuallj occupied by K. purpurea, and less water than is found where K. Candida habitually grows, the resulting modifications will throw much light upoii the 156 THE PLANT origin of polyphyletic species. In this connection, it hardly needs to be pointed out that this simple transfer of a species to several separated areas of a new habitat may often furnish complete proof that a new form may arise at different -times, and at different places. 194. Modification of the habitat. Efficient changes in the habitat are brought about by increasing or decreasing the water-content, or by varying the light intensity between sunshine and the diffuse light of deep forests. Humidity can not well be regulated except in so far as it is connected with water-content. Since its effects merge with those of the latter, its modifi- cation is unnecessary. An increase in water-content is readily brought about by irrigation. A stream may be dammed and its water allowed to spread over the area to be studied, or the water may be carried to the proper place by deflecting the stream or by digging a canal. The construction of earth reser- itojjS^ makes it possible to ibtain almost any per cent of soil water by varying the size of the reservoir or the height of th,e wall or bank. Near:,a base station, such as Minnehaha, vvhere there is a sirnple system of water- iworks, the experimental area may be watered whenever desirable by means of a hose. Water-content may be read- ily decreased by drainage, or by the deflection of a stream. When such means are not available, as in the case of extensive marshes, hum- mocks may be used or con- structed, and the soil blocks containing plants placed up- on them. By the use of sand or gravel, the water-content of mesopEytic areas can be reduced in a similar manner, or by surrounding the plant in situ with either of these soils which hold little water. In meadows, especially, the addition of a large quantity of alkaline salts decreases the amount of available water, while the holard may be reduced by denuding the soil about the plants concerned. Fig. 47. Control ecad of Ranunculus sceleraitts, holard W% (50 cc). EXPERIMENTAL EVOLUTION 157 In sunny habitats, the light intensity is most easily reduced by means of cloth awnings, which can be put in place conveniently. It is not a difficult matter to produce effective shade by using shrubs or small trees for this pur- pose. This plan is especially advantageous in habitats too remote to make frequent visits feasible. When a shrub or tree is used, the experiment nec- essarily requires a longer time, though this disadvantage is partly compen- sated by the fact that the shelter requires practically no attention after the shrub is once established. Forest plantations furnish ex- cellent examples of this kind of experiment. On the other hand, clear- ings afford the only ex- amples of habitats mod- ified in such manner as to increase the light. In nature, the diffuse light in. which shade plants grow is due to the pres- ence of tall plants, chiefly shrubs and trees, and an increase in the light intensity is possi- ble only through the thinning-out or removal of the plant screen. This is a task of consid- erable magnitude in for- ests, but it can be readily accomplished in thickets and at the edges of wood- lands. It is quite practicable to establish a series of awriings or clearings of various light values, but the labor required is hardly worth while when it is recalled that the method of transfer makes it possible to take advantage of the various intensities already found in nature. METHOD OF CONTROL CULTURES \ ' V-' C'' KF _^¥ ^■H^ w^^mlk^ ^?^«P.^'.^:' ■/:^'ftj5.:.:'..-.- ■ . J Fig. 48. Control ecad of Ranunculus sceleratus, holard40^(200cc.). 195. Scope and procedure. Control experiments are necessarily carried on in the planthouse, since factors can be controlled in the field only with great difficulty. Their greatest value is in connection with 15? THE PLANT experiments that are being carried on in the habitat, but they also constitute an invaluable means of independent research, since it is not at all difficult to approximate the conditions of a habitat, especially with reference to water-content and light. The essential feature of the method is that the less important factors are equalized as far as possible, while the direct factors, water-content and light, are under the complete control of the investi-gator. By the equalization of humidity and temperature is meant experimentation in which all the plants of each experiment are subjected to the same amounts of these factors. It is a matter of no importance what- ever whether the humidity and temperature are constant or variable. In the case of soil, which is not a variable, it naturally happens that the plants are placed once for all in the same soil mixture. Batteries consisting of thermograph and psychrograph have been kept in the different control houses, but although used at first to give some idea of the hourly and daily fluctuations of temperature and humidity, they have slight bearing upon the evolution of new forais under control. For use in con^iection with supple- mentary experiments in adjustment and adaptation, the batteries have proved to be indispensable. Control experiments are regularly made in series which are planned with reference to as many modifications as the efficient difference of the factor and the plasticity of the species con- cerned permit. 196. Water-content series. An account of the experiments which have been carried on for four generations with Ranunculus sceleratus will serve to show the application of culture methods to the origin of new forms in response to varying water-content. This species was chosen because it grows readily in the planthouse, is plastic, and, Since it is naturally am- phibious, permits of much modification in both directions. The smallest amount of water per day under which the seedlings would igrow was found to be 25 cc. This was taken as one extreme for the series, and deep water in which the plant could be submerged as the other. An arbitrary series was tentatively made as follows: 25 cc, 50 cc, 100 cc, 150 cc, 200 cc, mud, shallow water, and deep water. Further study justified these divisions, since the first six gave efficient differences in water-content, and the resulting forms all showed differences of structure as well as of growth and form. .Seedlings of the same age, and as nearly alike as possible, were transplanted to large pots of which there were four for each of the first six; they were placed in half-barrels for mud and floating forms, and in a barrel for submerged forms. After a few days, when they had become well established, the plants in the pots were watered in the amounts indicated, as often as was necessary to keep the most xerophytic form alive; the soil for EXPERIMENTAL EVOLUTION 159 the mud form was kept covered with a thin film of water ; the leaves of the form in shallow water were kept floating on the surface, and those of the last form submerged just below the surface. The water in which the sub- merged form grew was aerated by means of a spigot near the bottom of the barrel. From time to time water-content de- terminations were made of the soil in the pots until it was definitely as- certained that the holard was practically constant. The nine new forms ob- tained by adaptation showed striking differ- ences in vigor and growth, as may be seen from the figures. In all cases, these were ac- companied by distinct and often striking dif- ferences in the number and position of the sto- mata, the amount of sponge and palisade tis- sues, and the develop- ment of air passages. Photographs were made of a typical plant of each form, and the dif- ferent leaf structures were preserved in permanent mounts. The xerophytic and the submerged form were unable to produce flowers, and it was necessary to develop them anew in each generation. The other forms fruited abundantly, and the succeeding generations of each form were produced from plants which had grown the year before in the same conditions. In addition to the develop- ment of a series of new water-content forms, this experiment was begun in the hope of determining whether the modifications of a plastic species tend to become fixed if each new form is grown constantly under the same conditions. A period of four years is too short, however, to throw much light upon this problem. Fig. 49. Floating form o£ Ranunculus sceleraius grown under control. l60 THE PLANT Helianthus annmts has been used for other series of experiments, in ■which alkaHne salts or different soils are employed to vary the water-content. These are more complex and hence are not as satisfactory as the series described above, but they are valuable for the light they throw upon the behavior of plants in similar conditions in nature. In the case of soil, however, the adaptation may be referred to water-content alone, if thoroughly leached sands and gravels are used, so that the difference is solely one of water-retaining power. 197. Light series. Cloth tents have been found the most satisfactory means of obtaining different light intensities in the planthouse. The cloth permits the air to circulate to a considerable degree, and in consequence the equalization of humidity and temperature is much more complete than in the glass houses first employed. The cloth tents, or shade tents as they are called, are cubical, each dimension being i meter. The series which has been most used consists of three tents: the first is made of cheese- cloth and has a light value of .i ; the second is of thin muslin, and has a value of .04, while the third is made of dark cambric and the light is reduced to .01. A more desirable series is one with five tents, which have approxi- mately the following light intensities: .1, .05, .01, .007, .003. Plants grown in shade tents should be repotted as often as they will permit in order to increase the aeration of the soil. The amount of water given them must also be decreased as the shade increases. Mesophytic species give the best results in shade tents, xerophytes thrive less well, and amphibious plants do not grow at all except in the brightest light. Excellent results have been obtained with Helianthus, Taraxacum, Gaura, and Onagra, while Ran- unculus sceleratus is unalDle to produce flowers and seeds in a light intensity of .01. A number of important supplementary experiments have been made in connection with light tents. These do not result in the production of new forms, but they throw much light upon it. Plants have been placed in the shade tents so that certain leaves would be in the sun and others in the shade. Young leaves have been fixed at various angles with the stem, and they have been revolved 90° or 180° in order to change the rela- tion of their surfaces. Soils of different colors, e. g., loam and sand, have been used to determine the effect of light reflected from their surfaces. Shade tents make it possible to illuminate plants from the top, bottom, or side, and to carry on a large number of fundamental experiments in adjust- ment and adaptation. CHAPTER IV. THE PLANT FORMATION Methods of Investigation and Record 198. The need of exact methods. The use of instruments in the study of the habitat has made it evident that the loose methods of descriptive ecology were altogether inadequate to the accurate investigation of the formation. This feeling has been heightened by the recognition of the fact that vegeta- tion exhibits both development and structure, and is, in consequence, open to exact methods of inquiry. In the search for feasible methods, it was quickly seen that the quadrat, first^ used for determining the abundance of species, furnished the key to the problem. Accordingly, the principle un- derlying it, viz., that of intimate detailed study and record, was developed and extended in such a way as to give rise to a number of methods of precision. These have been applied in the field for several years with signal success, and they are here described in the conviction that they con- stitute a satisfactorv svstem, if not, indeed, the only one for the exact study of formations. There has been a growing appreciation of the fact that the superficial methods of descriptive ecology made it impossible to build upon such a foundation, and they, indeed, were making actual progress in the field of ecology more and more difficult. Ecologists have now begun to see clearly that precise methods are as indispensable in the habitat as they are to the study of the structure and modification of the plant. For some reason, however, they have been slow to perceive that accuracy in the investigation of the cause, the habitat, is a fruitless task unless it be followed by corres- ponding exactness in the study of the effect, the formation. After having uT-ged the fundamental necessity of instrumental methods, for six or seven years, both in season and out of season, the writer does not feel called upon to further plead the cause of the quadrat. The final acceptance of the instrument was inevitable if progress were to be made in the habitat, and it is just as obvious that the quadrat must be accepted if the study of the habitat is to bear fruit in the interpretation of the formation. The use of the quadrat does not mean that the general methods of descriptive ecology are all to be discarded, whether they have value or not. The statement that quadrat methods are indispensable signifies merely that they must be used for research work in the development and structure of vegetation. They are ^ Pound and Clements. A Method o£ Determining the Abundance of Secondary- Species. Minn. Bot. Studies, 2:19. 1898. l62 THE FOEMATION not necessary in reconnaissance, nor do they displace general methods of real value. The use of the latter in even a supplementary vfay will grad- ually be discontinued, however, as fields become smaller by reason of in- crease in the number of workers, and as the need for precise methods becomes more universally felt. The quadrat constitutes the initial concept from which all the methods have grown. In itself, it has given rise to a variety of quadrats applicable to the most fundamental problems of vegetation. From it have come, on the one hand, the migration circle, and on the other, the transect. The latter in turn has yielded the ecotone chart, and the layer chart. All of these are based upon direct and detailed contact with vegetation itself, and permit accurate recording of all. the results obtained. QUADRATS 199. Uses. In its simplest form, the quadrat, as the name implies, is merely a square area of varying size marked off in a formation for the pur- pose of obtaining accurate information as to the number and grouping of the plants present. As indicated above, it was first used for determining the abundance of the various species of a formation. This made it possible to ascertain the relative rank of the species of layers and formations, and enabled one for the first time to gain some idea of the minute structure of a bit, of vegetation. The results were at once applied to the task of establish- ing a numerical basis for abundance, and of working out a new system of abundance to correspond. The quadrat method was also used to determine the character of seasonal aspects, and to yield a knowledge of the exact differences in diverse areas of the same formation. Incidentally, the deter- minations of abundance were made the basis of an actual census pf certain alpine formations. This, while it was extremely interesting to find that a square mile of alpine meadow contained approximately 1,500,000,000 plants, was confessedly destitute of ecological value. The most important applica- tions of the quadrat idea were made by Clements^ in the chart, the perma- nent and the denuded quadrats. The development of these was due to the fact that zones or formations permit of comparison upon floristic as well as physical grounds, and that a detailed record of their structure is necessary for this purpose. Similar comparisons are necessary for the consocies, zones, and patches of the same formation, and the quadrat becomes an in- dispensable means for studying alternation and zonation. For the investi- ^The Development and Structure o£ Vegetation, 84. 1904. Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Neb., 5:29. 1901. QUADRATS I 63 gation of invasion year by year, and especially for succession, the method of permanent quadrats is imperative, and the denuded quadrat an invaluable aid. Changes, which would otherwise be incompletely observed and im- perfectly recorded, are followed in the minutest detail and recorded with perfect accuracy. 200. Possible objections. The use of the quadrat has led to the criticism that it is needlessly detailed and thorough, and that, after all, the space covered is but a minute part of the entire formation. The first objection is one that has also been urged against the use of instruments of precision in the habitat. It is always brought forward by those who have not used in- struments, and as witnesses they are of necessity incompetent. No one who is familiar with the instrument or the quadrat by actual practice has felt that the methods based upon them were too thorough. In no case has the writer ever listed or mapped a quadrat without discovering some new fact or relation, or clearing up an old question. It can not be denied that quadrat methods require both time and patience, but this is true of any kind of re- search work that is at all worth while. Every ecologist, moreover, that has the interests of his field at heart and deprecates the present slipshod work, will appreciate the necessity of methods which seem like drudgery to the mere dabbler. The second objection, that the quadrat is at best but a small bit of the area under investigation, seems at first to be a valid one. It can not be gainsaid that the actual space studied is insignificant as compared with the whole formation ; still, it must be obvious that even a single quadrat can add at least some facts of value, which can never be obtained by the best of general methods. Furthermore, if the formation be an actual and not an imaginary one, a single quadrat will be in some measure repre- sentative. In the more homogeneous ones, it will have much the same value that a type specimen bears to the species established upon it. In formations which are less uniform, its value is correspondingly reduced, so that in formations which show marked zones, consocies, or patches, it becomes necessary to locate a quadrat in each. In the matter of representa- tion alone, the graphic method of the quadrat map with its close-focus detail photograph, is far superior to anything that can be obtained by the ordinary description and photograph. Finally, the scientific study and recording of succession, and particularly of competition, is an impossibility without the aid of the permanent and denuded quadrat. The stoutest champion of the practice of walking through a formation, and jotting down impressions, can not avoid their use if he would attack these problems, and, once familiar with the quadrat, his objections to the drudgery of thoroughness will soon vanish. 164 THE FORMATION Kinds of Quadrats and Their Use 201. Size and kinds. The unit size of quadrat is the meter, and when the term is used without qualificatien, it refers to the meter quadrat. To make them strictly comparable, and exactly divisible, unit quadrats are always grouped in squares; thus a major quadrat is a square of four units, and a perquadrat one of sixteen units, or four meters square. Quadrats of greater size are necessary in woodland and forest, where the rule, however, is that the woody plants alone are recorded for the whole quadrat, the her- baceous growth being listed or mapped for but one or two representative units. For special purposes, quadrats of 3, 5, 6, etc., meters may be used, but they are much less convenient. Quadrats are further distinguished with respect to their use. A list quadrat is one in which the plants are merely listed and the number of individuals of each species indicated. Chart quadrats are those in which the area concerned is accurately mapped on plotting paper. Both list and chart quadrats are rendered permanent by careful labeling, so that their changes can be followed frdm year to year. The greater value of the chart causes practically all permanent quadrats to be of this type, and for the same reason only permanent chart quadrats are converted into denuded ones. 202. Tapes and stakes. The lines for marking out quadrats are made of strong white tape, 5/8 inches wide. This is doubled and sewed firmly at both edges. Under moderate stretching, the tape is carefully marked off into decimeters, and eyelets 5 mm. in diameter are set in at each end and at the marks. This can readily be done by any shoemaker at slight ex- pense. The usual lengths are one and two meters, as these are most frequently used, and they can also be easily combined to make larger quadrats. The tapes are slightly longer than one meter in order that the distance between the end eyelets may be exact. The tapes of the larger forest quadrats should be divided into lengths of one meter, as these permit ready plotting and also make it possible to interpolate a meter quadrat for the study of the undergrowth at any point. The intervals of the tape are numbered from left to right, as conspicuously and clearly as possible. For this a waterproof ink or paint is very desirable. For holding the tapes in position, hatpins, nails, and meat-skewers have been used with more or less satisfaction. The ideal stake, however, is one which holds the tape close to the ground, and can be readily moved. It is merely a stout wire, 3 mm. in diameter and 8 inches long, looped at the top, sharpened at the tip, and with a small ring of solder 3 inches from the tip. QUADRATS 165 203. Locating quadrats. In staking a quadrat, the end tapes are in- variably placed so that the numbers read from left to right, and the side tapes so that they read down. In mapping, a fifth tape is stretched parallel to the top, and as each decimeter strip is marked, the outer tape is shifted to delimit the new strip. Indeed, the side tapes can be placed alone, and the plotting tapes moved down one at a time as the mapping proceeds, but it is usually more satisfactory to locate the quadrat exactly and to square it first, a task most easily done by enclosing the whole quadrat, and then using a fifth tape. In the case of list quadrats in open vegetation, the measuring strip is unnecessary, but as a rule it facilitates counting, as well as mapping. tl^ -L^ ■»■ -j,vr*v. i..-*- AiJ^ J? jp^uaafc.. "HtwC>Ttu«^-ri t..^,. ■^'%>^::"- Fig. 50. Mapping a major quadrat on Mount Garfield at 3,600 m. The List Quadrat 204. Description. This, as the simplest form of quadrat, is employed primarily to ascertain the abundance of species in a formation or during a- particular aspect of it. Since this can be obtained readily from the chart, the list quadrat has fallen more and more into disuse, except where it is desired to determine abundance alone, or to aid in deciding whether a chart is really representative. The size depends almost wholly upon the nature of the vegetation. When the number of trees is to be determined, a quadrat of 10 or 50 meters is necessary. In ordinary herbaceous forma- tions", the usual size is 2 meters, while the meter quadrat is used when the l66 THE FORMATION plants are especially small and crowded, as in alpine meadows. The loca- tion of the quadrat is based upon the general rule, but since its especial task is the determination of the greatest variable in vegetation, viz., number, it is necessary to use more quadrats, and to place them in areas which show the greatest differences in the mixture of species. For example, it was found that a half dozen list quadrats, when carefully located in the prairie formation, gave results almost identical with those obtainable from a larger number. With a little experience, the various degrees of mixture can be picked out superficially, and the corresponding number of quadrats es- tablished. If a single list quadrat is to be made, for a formation or station, such a time should be selected as will make it possible to cover the greatest number of plants. Fortunately, this usually falls near the middle of the summer, when the remains of spring plants are still in evidence, and the ■autumn ones are sufficiently developed to be recognizable. In taking the ■census of different aspects, the quadrat should be made as near the middle •of the period as is possible. 205. Manner of use. In listing a quadrat, i. e., counting the individuals of each species, the plan followed is to list the smaller, less conspicuous plants first, since they are apt to be tramped down. As a rule, the outside tapes and the taller species afford sufficient landmarks. When this is not the case, the measure tape is used, and the individuals of all species are checked as they are found, while in the first method one species, rarely two, is taken at a time. In cases of peculiar difficulty, it may be permissible to pull or break plants as they are counted, but ordinarily this can and should be avoided. Clusters, and bunches of stems from the same root are counted as single plants, and the number of stems indicated by an exponent. In the case of bunch grasses, each bunch counts as one plant. 206. Table of abundance. The species are arranged in the final list in the order of their numerical importance, and are divided into groups which correspond to the dift'erent degrees of abundance. The latter are arranged in two series, based upon the fact that association is by groups or by individuals. The table of abundance, based upon a 2-meter quadrat rather than upon the 5-meter one, by means of which the earlier results were obtained, is as follows : Social exclusive, no other species of vascular plants present social inclusive, above 100 gri gregarious^ 100-50 copious^ cop^ gr^ gregarious^ 50-25 copious^ cop^ gfS gregarious" 25-10 copious' cop' sg subgregarious 10- 5 subcopious sc vg vixgregarious 5- I sparse sp QUADRATS 167 It is obvious that the above outline is fauhy inasmuch as it takes no account of the height and width of the individuals. This is a serious defect, and it constitutes one of the many reasons why the list quadrat should be replaced by the chart quadrat. The prairie formation affords an un- usually striking illustration of this. A single quadrat may be filled by ten plants of Psoralea iiorihunda, and at the same time contain 22,000 plants of Festuca octoflora. Yet the former is conspicuous and controlling; the latter plays an altogether insignificant role. This difference is readily shown by comparing a plant of each. The one is 3 x 3 feet, the other 3 x ^ inch. Such figures furnish a valuable check upon mere number, but make the brief, graphic designation of abundance difficult. An attempt has been made to solve this problem by roughly determining the space occupied by the plant, by means of the formula, height {r Ji^) X abundance. This would give Psoralea a value of 210, and Festuca one of 1.6, which much more nearly represents their real importance in the formation. Abundance or numerical value is a iloristic concept entirely, and has little place in ecology unless checked in the way indicated. The whole problem, ecologi- cally, depends upon an intimate knowledge of competition, and its solution in consequence is at present impossible. The Chart Quadrat 207. Description and use. The detailed labor required in mapping makes it advisable to use the meter quadrat. An additional reason of much impor- tance is furnished by the desirability of securing a detail photograph of the quadrat. This is impossible with field cameras, which should not exceed 6^2 X 8^2 inches, and are indeed most serviceable in the 4x5 size, if the area be larger. In open formations, the major quadrat of 2 meters can be used if necessary, but this is very rarely the case. Forest quadrats of ten meters square are easily charted, but detail photographs can not be made of them. Larger quadrats are impracticable; they can be counted but not mapped to advantage. The location of the chart quadrat must be decided by the structure to be studied. Its greatest service is in connection with zones and societies of the same formation, which can be easily compared in the chart form. In fact, the chart quadrat may well be regarded as the fundarnental method for inquiry into zonation and alternation. It is an important aid in delimiting areas from the contiguous formations, and in determining the relationships of mixed formations. It is also used to record the character of the different aspects, but this is done more satisfactorily by the permanent quadrat. i68 THE FORMATION 208. The chart used is a decimeter square, and the scale is consequently 10 :i. It is outlined on centimeter plotting paper, and the centimeter squares are numbered at the edges to correspond to the intervals of the quadrat, i. e., the top and bottom lines are numbered from left to right, and the side lines from top to bottom. These outlines are ruled in quantity and used as needed, or the forms can be furnished by the printer. In practice, a special quadrat book the size of the chart has been used. The need of a second book may be avoided lay outlining two charts on the plotting sheet, and filing the latter in the field record book. In the few cases where 2-meter quadrats are desir- able, four charts are used, care being taken to label them so that they can be combined whenever necessary. Ten-meter quadrats are recorded on the deci- meter chart also, each meter interval corresponding to a centimeter, i. e., the scale is ioo:i. Fig. r