a "% @ * WV.G 4 aa Be “ed trpeea oe Kee e ery» ——E AO} Os Glass (opyvight NO ee COPYRIGHT DEPOSIT. i 5 ‘ ‘ “AL q / , 7h SEeANT PHYSIOLOGY AND RGOLOG Y BY FREDERIC EDWARD CLEMENTS, Pa.D. Professor of Botany in the University of Minnesota WITH 125 ILLUSTRATIONS NEW YORK HENNEY HOLT AND COMPANY 1907 ” : » a i is , y 4 QKFo | pony Sr oe ‘ ae LIBRARY of CONGRESS ? . Two Gopies Received AUG 2 .907 Gepyright Entry Cus 2, 7 GLASS A RX; No. 183437 GOPY 5. Copyright, 1907, ~ BY HENRY HOLT AND COMPANY ¥ Cc eve £ ars ; Ze es ce ROBERT DRUMMOND COMPANY, PRINTERS, NEW YORK 4 / ss x n bi “* Ie i ‘ac ss pos or : aad r id eet PREFACE Tue point of view and the methods of study first advanced in “Research Methods in Ecology”? have proved so satisfactory in teaching as to make it desirable to embody them in a text-book. The present text has been based largely upon “‘ Research Methods,” though most of the matter is new'or rewritten. The manner of treatment is essentially the same, but the subject-matter has been rearranged and broken up into a larger number of chapters. The plant is first considered as an individual, with respect to factor, function, and form, and then as a member of a plant group or formation. The reasons for regarding ecology and physiology as essentially the same have been given elsewhere, and need not be repeated here. An endeavor has been made to give the various parts of this vast field their proper importance. Since ecology and physiology are merged, it is manifestly impossible to give to either what would be regarded as a complete treatment by a specialist in either line. No attempt has been made to touch all the points in each, but it is thought that nothing really fundamental has been omitted. The book is intended for use with classes in second-year botany in college and university. In manuscript form, it has been in such use for two years with good results. Although the amount of laboratory and field work is large, it is possible to accomplish all of it in a course requiring 6-8 hours of laboratory time each week. This can be done only by careful planning on the part of both in- structor and student, and for this reason the following suggestions are offered as aids. The instructor will find it imperative to plan in advance for the experiments for the whole year, in order that plants may be ready as needed. Seeds and fruits for the study of migration should be collected in the fall. Shade tents, water-content series, and com- lll lv PREFACE petition cultures must also be prepared early in the year. Types of hydrophytes, xerophytes, etc., should be grown in the plant- house in so far as possible. Students should be instructed to make duplicate plantings of all plants to be used in order to make sure of an adequate supply at all times. It has also been found desira- ble to teach the students the use of the paraffin method of em- bedding tissues, thus saving much time and securing better results. The work on adaptation to water and light is best carried on in the form of joint experiments, in which each student is assigned a definite part. In the experiments much use has been made of the common sunflower. This is on account of the ease with which it may be secured and grown, but when a larger choice is possible other plants may often be substituted to advantage. In just as far as possible, the work of the student should be among plants out-of-doors. This is imperative in the chapters on vegetation, and is very desirable in all cases where it is feasible, even in the study of plant functions. For vegetation work, the knowledge of the more important genera and species of the several formations is indispensable. If the student does not already have this knowledge, the names should be furnished him by any desira- ble method, without taking the time necessary for identification. A bibilography has not seemed necessary and has not been given. A fairly full list of the more important works is found in “ Research Methods.” Apart from the latter, Pfeffer’s ‘‘ Pflanzenphysiologie,”’ Mac Dougal’s ‘“Text-book of Plant Physiology,” Sach’s ‘‘Text-book of Botany,” Vines’ “Lectures on the Physiology of Plants,” and Kerner’s ‘‘Pflanzenleben”’ have been frequently consulted in the preparation of the text. Grateful acknowledgment is made of the kindness of Dr. C. E. Bessey, Dr. D. T. MacDougal, and Dr. Edith Clements in reading and criticizing the text. The author is also indebted to Dr. Edith Clements for many drawings, and for the use of cuts from “‘The Relation of Leaf Structure to Physical Factors,” and to Mr. R. J. Pool and Mr. A. W. Sampson for the loan of several photographs. FREDERIC EDWARD CLEMENTS. THE UNIVERSITY or NEBRASKA, March 1907. NO op we CONTENTS CHAPTER I STIMULUS AND RESPONSE TRICE ECCT ALIOTIS. =o cic! Se sek hele ans Boe a re SE IMMER COERE SCHTIRIELES oS. 8 22 Sere o Doecacn « 14s See et ORD od? INEPEREST OE! SVENITIUIT Sc. os sn 3 ie x nc weer aes w mov a were ae td PENIAETE OE ELCSDORSC: 5 ose bc. s seis ence en dee kb weve mia ea one peeeemettiottih ACaptation. 0... as ceeded cla ais wvainte oa baeede oo SOUS Te 0 a a ce en Sorint and Abnormal Adjustment. .:..-........6 00800 ccs ceee CHAPTER II THE WATER OF THE HABITAT meron of the Plant to Water... 00.066... ccc ence cnccaewes meee mee of Water Stimuli... . 2.55... ee ede cc enue ME ENMEEL Fh Soe. Ecce Ss sNas 2s MO Shc oS oa bw wb oe eS eae ed: « . Influence of other Factors upon Water Content. ................ . Available and Non-available Water Content. ................... ES NEES eye chee Te TY el! Sin d's afew Ninkel E's sw gravto ema e sat meeengnitauion Of Water Content....... 2.2... 2652s cence cence mmc or Water Content Readings ..........5.26.020cecececeae Mitardeion of Readings. .....:........0.60065 Cachet ul 3 Os eee DEE DE SSAC AS A OI eae age ane Ue re nara eee Pe Experiment 1. Measurement of Water Content. ............... . The Determination of Available Water. ...............ccccceee- ere aI ADIL ALS <6. 3s ade leg oka a OS a OO Oe ee es Experiment 2. Determination of Available Water.............. MopiryInGc Factors pacsuence of Soil upon Water Content... .. 2... 5..5.0.0dc ceases Zk: 22. ieee nS EMICEAIE C0 2 oa) 5 “52 ops Skin ate cod w ava ew BS ee lew ogee OE tre ae a eM ere heh ges os ahs Blue wade s wee Oo a eica es ee vi CONTENTS PAGE 23...Chemical: Nature of Soils. 2056... aia ene tii 18 24, Air Comtembs acces see cee te ene 19 25. Determination: of Sols Properties)... «: 45.0 0 eee eee 19 Experiment 3. Porosity and Rate of Evaporation.............. 20 26. Influence of Precipitation upon Water Content................. 21 2t.. Measurement of Ramtally ts <2 eo 4. oo ee ee 21 28:, Physiography; vse nese eke in ae ae ee 22 29. ‘The Influence of Slopeseu.2sieuse ac. soe ae eee 23 30. ‘The Influence of Surfaces25...4 cn. se sie ie oe ee ee 23 3L: the Influence of ‘Climatic: Wactors).-..> .. 73). ae eee 24 32. HUMIGItY 6) <.ca'e als o5os we thes Hetenc eos le bs 24 33. Modifying Influence of Temperature and Wind.................. 25 34. Influence of Pressure and Physiographic Factors. ............... 26 oo. Effect of Climate.and Habitat. . 54.5... < das ra eee se 27 36... Measurement of Humidity... 2.62. 0... .224 5. 0.55 ee 27 37. Sling and Cog. Psychrometer..: 0. 0.5.2. 2... 7s eee 28 38. Making a-Reading:..6..55 cence. ss eee ee oe ee ieee eee 29 39. Use of Humidity Tables... 50... 6.68. «ees oe oe 30 Experiment 4. Measurmeg Humidity. .2..:5.....5. 0... eee 30 40. Method of Habitat Study......... .Meéasurement of Rainfallocs. 0. 8/32 dae ee eee 21 2&7 Physiosraphy. ). cabs ae Paci 2 eee oe oe Oe ee a 22 29:-'The Influence. of Slope: 22 ie... 4: 2a8 eee eee 23 50. The Influence of ‘Surfaces 25.2 .....c. aac eee ee 23 31 The Influence of Climatic Factors. ....... 22 352 eee 24 32. HHMI Ye ae ee Siete Bie ea eRe ee 24 33. Modifying Influence of Temperature and Wind.................. 25 34. Influence of Pressure and Physiographic Factors. ............... 26 30. Effect of ‘Climate and Habitat... 7: 262 .\0 4. he. 2 ee 27 36: Measurement of Humidity.:...<.Jc224.. ¢-. =. ee 27 3(. Sling and-Cog Psychrometer.......%/.. as: <2 16802 eee ee 28 oo. Making a Reading: ic...) cee seca Selec 29 39. Use of Humidity Tables. .. 2.0.2.2. 6 ono. nba tna ie ee 30 Experiment 4. Measurmg Humidity. ...:........ >. 42 350 eee 30 40. Method of Habitat Study: 2... 2.2.22: ooo set a ee 30 41..Choice of Stations.o....2 cen eee en eee 31 42. Constant: Factors: \.c (sce & 20 cis ee ee ae er 31 43. Simultaneous Readings: 2.2%... :$.0 5.6. bs Ve seen ee 31 44. Point.and Hour Readimes.% «25 5. ska nck ae eee 33 45. Records: 3 20 eeu setae ee 33 46; Kinds-of Curves: oc :h4204 4.200 oe aes a i 34 47: Combinations:of ‘Curves. ... 2 ee 77 97. The Amount Absorbed..< 2.0.55 4.200 606 5 Sees ee ah Experiment 22.. Wipidermis and Leaf Prmts. .. 2... 2-- -4eeeeee 78 98. "The Production of Chiorophyll).. 23 22c.-~ . 52695: -0 eee i 99. The: Nature of Chlorophyll... * 2.4) 5-< = See eee 79 100. The Influence of Darkness. ~ <2... 2. 2. fas 6 eee 80 Experiment 23. Influence of Light and Darkness.............. 81 101. Photosynthesis :, ..@ 240 s.-6. os ese ee ee eee 81 102. Absorption and Diffusion of Carbon Dioxide. ................... 81 103. Chemical Changes during Photosynthesis. ...................... 82° Experiment 24. Dependence of Photosynthesis upon Aeration and Thigh: 0.2 90.4 2a oor see ea ee 83 104. Measurement: of Photosynthesis). :.0... 20 5... see see 83 Experiment 25. Relation of Photosynthesis to Sun and Shade.. 83 105. ‘Translocation. . 2.2.05. 6.38 S yeas eee eee 83 Eapermment 26... Translocation..." 4 9 oes ee 84 106. Storage of Food Matenaloe is) sal 2s ase eee eee 84 Haeperiment 27. Storage Tissues...) a-ha.) ae 85 107. Influence of Light upon the Number and Position of Chloroplasts. 85 Experiment 28. Arrangement of Chloroplasts.................. 87 108: Movement of Stems and Jueaves... 2. 220.5. a-ak see oe ee 87 109. “Phototropism, 2.5.85 222) a Gas ea Swe Se ree eee 87 Experiment 29. Phototropre Movements. 252-4 ao- a8 ee 89 110. Nyctotropism...: 2.0.4 22. 836s eee Se ee eee ee eee ee 89 Experment 30. Nyctotropie Movements:= =-.2 2) ee ee 89 CHAPTER V ADJUSTMENT TO TEMPERATURE 111. Relations of Plants to. @emperature.. >. 225. 3 see eee 90 112. The Measurement. of Temperature. 2.252520 fn | ee 91 113. 114. 115. 116. eiy. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. CONTENTS 1x PAGE MINES MEUEOE IS OW gone» Vin agen Va niDig sim One ba EW aed aaa ¥ « 92 EE TOO AGUTES. opens os gaa geminata be wei hc baba es 92 Experiment 31. Temperatures of Plant and Habitat............ 93 nerd CGMDCPrabure. occ. cece wane peas sae ke tlve alwwe vs 94 memeeninenee Of Other Factors. .. 0.5. ce ec ea ee ee ee ee ee 95 Favorable and Unfavorable Temperatures...................... 95 5) a re BP ro te ah ee ara 96 peerment Sc. Lifects of Freezing... 2... 6606.0 eke et ce ae 97 Demers Hemera tures Types Produced: by Adaptation=to Waters sn. 5-5454-cr- oy eee 155 175.) General) Meatures of: Xeropliytes) =). see eer cee ae 156 176. Lypes of Iheak Xerophytes <2 0-3... cee eee ee 156 17%. Normal Beat: XMeropliytesa. 7). aes ae ee ee ee 157 WSs Storage MMCavies: -. ca.5 iS ase eo seeh Sees 5 ole eee rae 158 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. ies 195. 196. £37. 198. 199. 200. 201. 202. 203. 204. 205. CONTENTS xl PAGE CLS oo oa ou.5 x «a, 3:8 bade Rae age IR me nD wha ae be Ww ens 159 Experiment 48. Study of Normal Leaf Xerophytes............ 159 En Fe CTONN VCS, ioe ins sae ease ek area ate Ola emai aa 4 ae 159 Experiment 49. Study of Xerophytic Leaves.................. 162 ESET UGS. ys cS i's gio cm 0 whee hare et SAU ren eae WA Ore a Rashes 162 aE SUCED SCTODUMECS. 5.01. Ui ve hd dete woes eehdnte we oe ce 162 Experiment 50. Form and Structure of Stem Xerophytes..... 164 OME Gi aa ae GL tim bine Gv alm ape Aa Cee a OMe ES Sina 164 Experiment 51. Comparison of Mesophyte and Xerophyte...... 165 RR aro 2 WR crea OLN am Oe bin RSE Rew we PR oie is 165 NRMMMRMERIUPRE ES PRTG 2) re, 4. Wo ada) «uk © RMR Re sees 648s 166 Experiment 52. Structure of Amphibious Plants............... 167 UAL SSL AS RRR IP SRP OS SOR ste eae crcl aba om deta ae 1 Ye acs oe ed ee 167 Haperiment. 53. Structure of Floating Plants.................. 168 eM AT iy. ou wed a Rene eS Oks Mow gta uunn ss Mere ees 168 Experiment 54. Structure of Submerged Plants................ 168 Ee A ie, wichita n ot 28 oe MEPS ce Aly hots eS aati NE Mca 168 Experiment 55. Study of Water Content Types............... 170 CHAPTER VIII ADAPTATION TO LIGHT erection of Organs to Light. 0 36.06 s ol ee ci leek cel 171 MuEeRIeC GH, Ge COIOPOPIASUS. 5. 64 6s ee sa eae She SR as weenie sles 171 Moaimmeations of the Chlorenchym. ..........5.....00cc%ecevere: 172 OPS DDSSTT Ey SENS dN ara pala ne Ne nara ERE ese eer 0k 174 SPELL LISS CS, Ae et ee ae rarer een A Pat eS 176 Bree Ole ne HMIGEENNIS? «5. 2.d-4. 6 5. acts be ds a nbde Mion Mae we ee 177 Pe EMEMIE OID WEANES ety <5 Sere 2 AF eas b's es hu.t 4 eben bees 178 Changes of Outline, Size and Thickness........................ 178 PEMA TIME CL SUCLUISS cy Sk 2s 5 5a ay Se5 esis s00a (elie Me a aceead sae reo Re ee A Experiment 56. The Production of Adaptations to Light........ 181 fiypies of Leaves as Determined by Light... 0.026.005. 50.05.2. 0: 182 RHieerieis ana omade Plarits.. 23% Lscusc slo. 55 <0 wd Acie ee areslones 183 Experiment 57. Sun and Shade Forms in Nature.............. 184 CHAPTER IX THE ORIGIN OF NEW FORMS Sele ive Ob eV OLUUCO INS oa.) o/s 2 cass ose tiene eh ie ad Sle see Bins Seven 185 ee OmIN ee HS EAS GLOLE YS, £.5.- (.') ic awa eacrdce wipers ais Mateideil ORLA SE 185 Porsiing and. Inconstamt Forms: 2 foc. 6.5 ase e be oe Oke ee bee 186 WrieiioayWeseent before Darwit >... i4 sib aes... us coves 187 Waive tie. te Origin of Species. «24 f2c3- 28a os se neko wnt: 188 Poni nametiber WAarwill «s <7 5 0c 4c sed pac eee wh So eee. 189 Xl 206. 207. 208. 209. 210. 211. 212. 2138. 214. 215. 216. 217. 218. 219. 220. 221. 222. 223. 224. 2205. 226. 220. 228. 229. 230. 231. 232. 233. 234. 235. 236. 237. 238. 239. 240. CONTENTS PAGE Fundamental Methods of Eivolution. ./... .22..5:..<'...)e. oo ee 191 Onigin by Adaptation? 2-0: sc.sse. ka eee ee ee 192 Originvby “Variation 52.52/36 ss Fe 195 Ongin-by ‘Mutation. 2.25 5 ya Sa Ss eee 196 Onguntby Ey bridation: i256. 54: 0 bo ee ee co ee 197 Experiment 58. The Occurrence of New Forms in Nature...... 198 Natural Selection sac)... 242 OSs he eee eee 198 PSOVRGIONG o's 2 so Sg Tere 199 Oly BEMESIS!, 2 {4 v's. eS oa es oo, Sie eee ee 199 Expermental. Kvolation.....c.. 024.4220 .0 ee ee 201 CHAPTER X METHODS OF STUDYING VEGETATION The Study of Vegetation: o...0. 4 ese os es eee 202 he Quadrat.. .a2.s0c Sane dec ee ee 202 Kinds of Quadrats: i. .. S052 SA ee eee 203 Marking out Quadtats. .1)4).g2.75..).0 soo gee ee er 204 The jbist Quad@rab. sos) ose Bo ed os ee ee 204 AbundaMee soi: sissies Bel dhlees ox eee ae ape es eer a 205 The Chart: Quadrat. .c2n%s seo conartct eS eee eee 205 Makine Quadrat Charts. "tol: 40 5 2tvies ae ee ele 206 The Permanent: Quadrat: ..asi5. 6 Jed 4 os cies oe eee 208 The Denuded Quadrat. . osu jeu en doe ke wed oe eee 209 WPraMSECES SS syn id we AS ort crenss vc inte Boe Reet ls Oe coe 210 The Line: Dranse€ct. .. 52.06/00. 2 bac oo See eee 210 The Belt: Transect .\.5 32 «dine Sh eee eee 211 The‘*Migration; Circle. . 555400. 4a leew Se reece ee 212 Formation Maps: .. Oye 226 CONTENTS xlil PAGE NE Ae ene Se ne ae Pe RPE rar ne, ery, 5 227 EEE an foo 2 it 2 hk ns a eas eave chien hay ee chy Paes de ee Experiment 60. The Structure of a Formation................. 229 EE sik kes 7G eke acctnok pace oak a a ORE a eR 229 mmerument G1. Layered Formations. ...... 225 0.06sasereuaene 230 235d werd Sin dg goa toe ial ai Ae ee ata eae AR eee 230 ELEN DY TIADIGMES, ooo ooo oc ons wece a ve el ns ap ene wd as 231 IME ORENSEREIOTIS. 22. Fs ars: «aly wo mine xpi abs GEMS E GO Des Ph 232 247. Developmental or Physiographic Classification. .................. 233 RI ERERU GEOR, ooo 5 colon bo Bios xo Gandini ece a WY aie Glee 234 eee and Closed Formations. ... .... 2.26 -.ccseescsenscasccaes 235 ELEM ENCES So Sk gd xd Sia ws Cees taniee sad aa pee eam ee 5 235 Experiment 62. Comparison of Formations.................... 236 CHAPTER XII AGGREGATION AND MIGRATION RM 2 Salah. cid ah ie an nig Ke nah oe Pale dl merwieta wa’ 237 ECO ANON. oo a. os soe ie ek ce ce eee be dbe eb peeces 237 EPR PIGION. oo. cn ecin ks al ee tes ce ed ee wine w be wise 239 Experiment 63. Study of Families and Communities........... 240 PEE een SEE Oho dt ase tbe et oak eee awe eee ee 240 EE cass nc ABE oh 8 SK orc cle bs eR ols woe ee 241 RMN ME EMSSCHMEATION, 2. . 6655 bees chee ses tee bese ences 241 mueemecrentions for Migration. .. ~~... i... ee ee cece cece sees 242 memeeimence Of Seed Production... .......5.....00.00cncceceeetee 243 SEER MRPT aD PUISSCTIANIWICS, 9.55 od a oc cen oe aid os boc vee sD ahoe eae wees 244 meee eenis Of Mieration. 2... 525 eee ect eevee ees 244 fener Work of Migration Agents. ......5.........050 ccc ce ee eeees 248 Experiment 64. Modifications for Migration................... 249 er erircetion Of Migration: . .. 2.6. roe eee cence 249 Experiment 65. Amount and Direction of Migration............ 250 CHAPTER XIII COMPETITION AND ECESIS ARAN ho eo och o-oo Sa 5 ww 8 one in a eye 418s pialolenw mares ae 251 meer spruscle for, Existence. ... .. . 0)... - ice eek ee eee e ae 251 meme bare of Competition... .. 2... Dos ve 2 cine cette nels ne 252 AcE PRVOLVEG. =: 2s. bis SEE Oo Se line tens ete: 253 eee eespetiiion tor Water and Light... 2.2... 56.2.0 e eee eee tees 253 268. Competition between Parents and Offspring.................... 254 269. Competition between Different Species...............02.....05: 255 270. Influence of Vegetation Form and Habitat Form................ 256 2 Dies Be Sg 20S os a 256 XIV 272. 273. 274. 275. 276. PATET 278. Zo 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295. 296. 297. 298. 299. 300. 301. 302. 303. 304. 305. 306. 307. 308. 309. CONTENTS PAGE Vegetation: Pressure... 254.25 65.04.04 hugs. ook ee ee 257 The Results of Competition... -5<4.......... 4.2... 258 The Study of Competition... ...:...:2.2.. 2... ot. eee 259 Competition’ Cultures...) 9....5.002 «ae os. 259 Competition Quadrats. 2.0.5.0... $2... canes. ws sc 261 Experiment 66.. Competition. 2.2.0.2... 5.2)... 2. eee 261 WGESIS$ he thas s Qa a Me Obs aed oe ee a Oe 261 The Wactors in Meesis. «6 of 5.35.66 les deca ee 262 Germination of the Seed... 20... 65. 63¢0501..0.. 263 ithe, ttectvot tlabitatas semen se 5 see prtieet ree 264 Adjustment: to the Habitat..... 0... ..2.5.5...2..5 2 eee 265 Experiment 67. Influence of Habitat Form upon Ecesis........ 265 Barriers: ss 22630. 3 02 6 iws oe ee ee eS 265 Physical and Biological Barriers)j.02 02.50)... ... = ee 266 Influence of Barriers..............s0hee whe es eee 266 Distance. oc okie ses oe ee ee cas ee oe 267 Endemism:. « 20. ose 302 ne Ses 2 eye 2 ee eee 268 Experiment 68. Barners and Endemism.-...2...:..-.2eeeeee 269 CHAPTER XIV INVASION AND SUCCESSION Tnwasion.'. i6)cob.a ooo 83 cei eee lene ale etee 6 ee 270 The Mannerot Imvasiont 4)". 120 ee ee ee MES 270 Invasion at Different Levels: ...... 2.0. 23:22 2.0) ee 272 Kinds of Imvasion. ..2 00.06 0. 2.4 ee oa ve eee 272 Indigenous and Derved Species... ..:.. 052.) 0) ee oe 273 Experiment 69. Vnivasion....0 0629.2) oes |. oe 273 SUCCESSION. 3.6.6 essa Has ne wy 2 ws oe ee aka A Ona 2S Kinds of Sticeession. ... os. 6002.6 a ea eee 274 Primary Suceéessions.... ..).2 ta See $22 oe 275 Succession in Colluvial Soils-. . ....5....5...42. 5 oe 276 Suecession-in Alluvial Soils. ;.2. 5. .05..04...2 5000-00 eee DHF Succession. in Atolian Soils... 2... 28 one bee 2 oe 278 Secondary Successions. . 2.2... kh. sso ee cee 278 Succession in Eroded: Soils.:...: 2.5, 4528 Jo. se eee 279 Suceession in Flooded Soils... o.oo ee eee 280 Successions Due to. Man: 2... .....542 ee ae ee 280 Suceessions aa- Burned Areas:. .-che siete ee ee ee 281 Succession in Lumbered Areas. . 2.5... 24-2..225.2. 05 eee 282 Succession by Cultivation... 9222242 a,c een oe 282 Reactions of Plants upon the Habitat:......%:.-.>-.. - >see 282 The laws of Suecession.. 0.35... Pewee eee ee ee ne 284 ‘Phe Study of Suecessions!050\0: .. 22 Gee eer el 286 Method.of Alternating; Areas: 3... 22 seeasee oe en. 286 The Reliet Methodos oo .2o75)40 Sissies ee ee Oe 287 Experiment 70. The Study of a Secondary Succession........... 288 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320. 321. 322. 323. 324. 325. 326. 327. 328. 329. 330. 331. CONTENTS XV CHAPTER XV ALTERNATION AND ZONATION PAGE The Relation between Alternation and Zonation................. 289 EE res ns ds ag teva ete wie cad es Ck ak a! os Saks ia Bide, nee oe 289 PE SUeTMANON: . Msc Gait Gels doe Pee ga wiels vee eck eweneen 289 Seepermerernic £0 TUCESISS oF fa ies kee we wa eis dene ee oa ele eee wk 8 290 meernaion Due to Competition... ..... 2.00000. ececeseces desea 291 MEE DEAT TSA GLONUS So oek ley lc) pisces seu eoe etn a grea 6 cow ble ae ab we 291 Normal Alternation of Facies, Consocies, etc................000. 292 BeemeweenLeTHALION Of SPCClGS. . 2 6. 6c. es ce cst ese seta saane 293 Ee UMN E PC TIPO oe oa ke Sea aiole lo et ce ee elp oe einen da vewees 293 neem ALCCENALION. 5 05. enh eo aiad ee eee ee eee Ss 294 Haperiment 71. Alternation of Species.................2..--. 294 i a Se CNSR ai oN (ee a de ne ae a 294 ME RAE EEL AC SOW LENE Saco. 5) eh choo. x eretanw arts a mss, ono. s aeidielee ble s 6A be shone 295 mere ne. to Mioration and Wecesis. .... 02.662. e eae ec ete eee ees 296 Zones Due to Reaction....... Pit Gel hE Meo EA NG age, gf re ea ee 296 Reet tty PHYSICAL FP ACLOIS. 3/00 00cu oc ace eee eels sees eee ees 297 Physiographic Symmetry............... i ee Re Aatiae tits PaaS 298 Puen RIT N CHCUAGMON. 5 fois) c)spe nla hie8d cso wie cele tien eo bee ols 298 SMM L HIG 5.6 oc hera ois a gde ox Wine ee sce ye Gee eb we tea ees 299 ee Tear te oo eee evi oaie oe Uae dean oe Used ears 8 299 Ea IR CHBI OREN. cre tT a oh ota big es io Bae w % oh w ESE sucha S woaad 2%, 0% 300 £072, LE GTLGCECTTS de ane oa GeRe Rae Ree ce ete cee Oe 301 EL SEAS TLE GTI SIS oS Sah aR canara ars 302 Experiment 72. Zonation of Pond and Meadow Formations..... 303 PLANT PHYSIOLOGY AND ECOLOGY CHAPTER I STIMULUS AND RESPONSE 1. Fundamental relations. A plant is an organism capable of nourishing itself under the control of external conditions, and of modifying its form and structure in accordance with this fact. Hence the most important matter in the study of plants as living things is to find out how the making and the using of food take place, and how the carrying out of these processes affects the structure of the plant. In seeking to explain the behavior of the living plant, i.e., its activities or functions, the first need is to discover the external forces that control it. We must next determine the effects which these produce. These are first seen in the functions of the plant, and in some cases they become evi- dent here alone. As arule, however, many of them appear sooner or later as a change in the minute structure or form of the plant. The proper task of physiology is the study of the external factors of the environment or habitat in which the plant lives, and of the activities and structures which these factors call forth. The former are causes, the latter effects. The sequence of study is consequently factor, function, and form, and the primary object to discover the nature and amount of this fundamental connec- tion between the causative factors and the resulting functions and forms. Physiology was originally understood to’ be an inquiry into the origin and nature of plants. This is the view that pervades the following pages, and in accordance with this the subject-mat- ter of ecology is merged with that of physiology. 2. The nature of stimuli. Any factor of the habitat that produces a change in the functions of a plant is a stimulus. The 2 PLANT PHYSIOLOGY AND ECOLOGY real test of the latter is therefore furnished by the plant, since the presence of a stimulus can only be ascertained by the re- sponse made by the plant. Moreover, while it is possible for the effect of a stimulus to remain invisible or latent for a time, a factor which works in this way can never be recognized as a stimulus until its effect becomes apparent. Stimulus and re- sponse are consequently not only inseparably connected, but the latter is the only obtainable evidence of the action of habitat factors. Since plants grow constantly under slight fluctuations in the habitat, it has come about that they do not respond to minute differences of factors. Living plants are in constant response to stimuli, and they are stimulated anew only by an in- crease or decrease in the factor sufficient to bring about an appre- ciable change in a function. Sometimes the total withdrawal of a factor acts as a profound stimulus, as in the case of a plant placed in darkness. The nature of the plant itself is of the utmost importance in determining what differences are sufficient to con- stitute stimuli. A species whose characters have been fixed by heredity responds much less readily to external factors than does one in which the structures are variable or plastic. In other words, a difference sufficient to produce a change in the latter has no effect upon the former. Such a difference constitutes a stimulus for the one, but not for the other. Thus, while light acts as a stimulus to all green plants, a certain change in the intensity of the light is a stimulus only to those plants that are plastic enough to show a response to it. It has been the practice to distinguish between the tonic action of external factors, such, for example, as that of light upon the chloroplast, and the stimulatory action of such forces, as seen in the bending of leaves toward the light, or the movement of sensitive leaves in response to a shock. In the one case the energy of the impinging factors results in an immediate and usu- ally proportionate amount of work being done. In the other this factor brings about the release of stored-up energy in the plant, which in many instances results in a disproportionate amount of work. However, a careful analysis of these two proc- esses shows that at the bottom they are essentially the same. Furthermore, they are seen to differ only in degree, and not in kind, when one examines the many processes intermediate between the two. STIMULUS AND RESPONSE 3 3. The kinds of stimuli. The simplest grouping of stimuli is with respect to the force concerned. The factors of a habitat are water, soluble salts, humidity, light, temperature, wind, soil, pressure, physiography, gravity, polarity, and biotic factors. Cer- tain ones of these, namely, soil, physiography, pressure, and biotic factors, can act upon plants only through the action of other factors, as a rule. For example, the wind normally influ- ences the plant only through humidity, and the soil through water content. Since a stimulus can be determined only by the response of the plant to it, only those factors that act imme- diately upon a function can be termed stimuli. These are the universal forces, gravity and polarity, and the physical factors, water, soluble salts, humidity, light, and temperature. With respect to certain mechanical effects, wind may also act as a stimulus, and the same is often true of biotic factors in the case of sensitive, insectivorous, and gall-producing plants. Stimuli are often distinguished as internal and external, but the distinction is of little value. This is due to the fact that internal stimuli are obscure in nature and effect: it is not improbable that they are merely the latent results of external stimuli. In any event, little can be done with them until more is known of the precise action of external stimuli. It is with the latter alone that our present study is concerned. 4. The nature of response. Plants seem to have no special sense- organs for perceiving stimuli, and no definite sensory tracts for transmitting them. Consequently an external stimulus acting upon a plant is ordinarily converted into a response at once. The latter, as a rule, becomes evident immediately. In many cases some time elapses before the final response becomes visible, and in rare instances the response remains latent or impercepti- ble. A marked decrease in humidity calls forth an immediate increase in the amount of water evaporated from the leaf, but a final response is seen in the closing of the stomata. The re- sponse to decreased light, on the contrary, is much less rapid and obvious. This difference in behavior is largely due to the func- tional response being more marked and more easily perceptible in the first case. The first response of a plant to a stimulus is always functional. The nature and intensity of the stimulus determine whether this is followed by a structural response also. The amount of re- 4 PLANT PHYSIOLOGY AND ECOLOGY sponse is dependent upon the intensity of the stimulus, and it is In many cases proportional to it. The same stimulus may not produce the same response in two different species, or neces- sarily in two plants of the same species. It does have this effect in individuals and species that are equally plastic. The study of response is facilitated by distinguishing two kinds, viz., func- tional and structural. Many reactions to stimuli are functional alone. In a large number of cases a structural change also occurs, and this is the rule when the functional change is pronounced. Consequently, it becomes convenient to distinguish functional response as adjustment, and structural response as adaptation. 5. Adjustment and adaptation. The adjustment of a plant to the stimuli of its habitat is taking place constantly. It is seen daily in the processes of nutrition and growth. As long as the stimuli are normal for the habitat, the adjustment of the plant is restricted to its ordinary activities. But when the stimuli become unusual in amount or in kind, either by a change of habitat or by a modification within it, the consequent adjustment becomes more evident, and is then usually recorded in the plant’s struc- ture. Adjustment may be expressed in the movement of parts or organs, such as the closing of stomata or a change in the posi- tion of leaves, or in growth or modification of structure. Slight or periodic adjustment usually concerns function alone. Adjust- ment is profoundly affected by the nature of the factor, and is in direct relation to the intensity of the latter. Adaptation com- prises all structural changes resulting from adjustment. It in- cludes both growth and modification. The latter is really growth in response to unusual stimuli, a fact that furnishes the clue to all evolution. Growth is periodic and quantitative: it is the result of the normal and continuous adjustment of the plant to the stimuli of its own habitat. On the contrary, modification is relatively permanent and qualitative: it is the response to stimuli of an unusual kind or intensity. A good knowledge of the way in which growth occurs is indispensable to the under- standing of modification. In endeavoring to find the connection between habitat and plant, however, it is in the modification of the plant and not so much in its growth that the significant responses to stimuli are to be sought. In the following survey of the relation between the stimuli, functions, and structures of the plant, the physical factors of the STIMULUS AND RESPONSE 5 habitat and the functional responses to them are considered under adjustment. Growth is also placed here both for con- venience and for the reason that it leads logically to the study of modifications. In consequence, the treatment of adantation is practically confined to modifications of structure. 6. Kinds of adjustment. With respect to the factor con- cerned, the functional responses of the plant are distinguished primarily as adjustment to water, light, or temperature. Re- sponses to soluble salts are properly considered under water, while the direct changes due to wind usually affect the form of the plant alone. The response to gravity is so universal, final, and absolute that it hardly falls within adjustment proper. Indi- rect factors, i.e., such as soil, wind, rainfall, pressure, and physi- ography, which can affect a function only by acting upon another or direct factor, do not properly produce response, but the change resulting from their influence is to be ascribed to the direct factor ° concerned. For example, the effects of soil, physiography, wind, and pressure are chiefly to be sought under adjustment to water, because of their action upon water content or upon humidity. 7. Normal and abnormal adjustment. The unusual stimuli resulting from a greatly changed habitat or from a new one pro- duce an unusual or abnormal response in function and often in form. Adjustment is consequently to be regarded as normal or abnormal. Normal adjustment is characteristic of a plant that passes from youth to maturity in its own habitat. The functions are carried on in the manner usual to the species, and there is in consequence no modification of structure. Abnormal adjustment occurs in those plants that migrate into a new or different habitat, or those whose habitat is seriously changed. It is characterized, as arule, by profound disturbance of function, though the latter clearly depends upon the intensity of the change. The most familiar cases of abnormal response are due to biotic factors, particularly parasitic fungi and insects. In most in- stances of this sort, the disturbance is merely functional, but often also the change in function is followed by a modification in growth or structure, as in the “‘cedar apples”’ and “witches, brooms”’ produced by rusts, and in the galls due to insects. A plant acted upon by a parasitic fungus or insect is said to be in a pathological condition. The study of the effect of the parasite upon the host-plant is called pathology, and it is regarded 6 PLANT PHYSIOLOGY AND ECOLOGY as a subdivision of physiology. A plant that is more or less hin- dered in carrying out its usual functions by the presence of a fungus exhibits abnormal adjustment due to a biotic factor of the habitat. A sun plant that finds itself placed in the shade has likewise to adjust itself to light stimuli that are abnormal to it. During the period of adjustment it also is in a pathological condition. In both cases the adjustment must be successfully carried out or the plant dies. Consequently normal functioning is physiological and abnormal functioning is pathological. There is clearly no hard- and-fast line between the two, since any plant is acted upon by abnormal stimuli while it is getting established in a new habitat, but these same stimuli become entirely normal when the plant has become adapted to them. In studying the behavior of plants, it is both illogical and inconvenient to separate the normal and the abnormal. In the practical study of specific plant diseases, such separation is a matter of convenience, but in an elementary treat- ment it is undesirable to distinguish pathology from physiology. CHAPTER II PRE. WATER .OF THE HABITAT 8. Relation of the plant to water. The responses of the plant to the water of its habitat are so numerous and so essential that water must be regarded as the most important of all factors which affect the plant. This is emphasized by the fact that practically all indirect factors, i.e., soil, wind, etc., can influence the plant only through their action upon water. Water is no more indispensable to ordinary plants than is light or temperature, since a green plant can not live and function if any of these is lacking. It is proper to speak of it as more important, for the reason that water is the immediate cause of a larger number of vital functions. Perhaps the greatest value of water to the plant lies in its use as food. In addition it is the vehicle by which solid foods, 1.e., soluble salts, are taken from the soil, and gases, carbon dioxide and oxygen, from the air, and by which the foods made by the leaves are carried to all parts of the plant. It is water that causes the stretching of the cell wall by which growth is made possible, and it also gives the rigidity so essential to stems of herbaceous plants. As a factor of the habitat, though not as a stimulus, water is an important agency in the reproduction of mosses and ferns and in the distribution of the plant body, or seeds of water plants. Inthe form of humidity, water regulates the loss of water from leaves. Finally, as is to be expected from the above summary, water exerts a much greater influence upon the form and structure of the plant than any other factor. 9. The nature of water stimuli. A terrestrial plant is con- stantly subjected to the simultaneous action of water stimuli, the water content of the soil acting upon the roots, and the humidity of the air upon the leaves. Water content regulates the water supply, humidity the water loss. The two are compensatory, and 7 8 PLANT PHYSIOLOGY AND ECOLOGY the final response to a stimulus of either sort can only be deter- mined by reference to the action of the other. An increase or decrease in water loss produces a corresponding change in the amount of water absorbed, and a change in water supply tends to produce a consequent change in water loss. This is strictly true only when the stimuli are normal. For example, a decrease in humidity causes increased water loss, which is compensated, as a rule, by increased activity at the root surface. Frequently the water supply is insufficient to compensate for heavy or rapid water loss, and the proper balance can be reached only by closing the stomata. In the case of excessive supply or loss, neither compen- sation suffices, and the plant dies. A change of structure, ie., adaptation to water stimuli, results when the compensation of supply for loss or the reverse is more or less inadequate, but not to a degree sufficient to cause death. In addition to this funda- mental compensating action of water stimuli upon the plant as a whole, water content affects the growth of roots in such a way that the direction of growth is determined by the distribution of the moisture of the soil. The rule is that roots turn and grow toward the area of greatest moisture. This phenomenon is known as hydrotropism: it will be discussed under absorption. 1o. Water content. The water content of a habitat is the total amount of water found in the layer of soil occupied by the roots. The water of lower strata may be raised, and ultimately used by the plant, but it is not properly water content until it reaches the roots. The water is found in the form of thin films surrounding the soil particles. The amount depends upon the thickness of these films. In soils that are saturated the films run together, forming drops and masses of water. In air-dry soils there is still a very thin film about the smallest particles. The amount of water content varies most widely in different habitats. Impervious rocks contain practically none, until cracks and rifts are formed by weathering. Marshes, ponds, streams, etc., repre- sent habitats with maximum water. Dry habitats, such as prairies, plains, gravel slides, sand-hills, etc., have a low water content, varying as arule from 38-15%. Wet habitats vary from saturation, as in stream banks, wet meadows, bog hummocks, etc., where the percentage ranges from 20-80% in accordance with the soil, to the maximum found in bodies of water. Moist habitats, meadows, forests, and cultivated fields usually contain from 15-30%. THE WATER OF THE HABITAT 9 11. Influence of other factors upon water content. The most important differences between habitats are due to differences of water content. The latter arise largely from the effect of the other factors of the habitat. All of these have an influence either direct or indirect upon the amount of water present, but soil, rainfall, physiography, and humidity are the most important. A sandy soil contains less water than a clay soil, even when both receive the same amount of rain. It goes without saying that a habitat in a desert region has a lower water content than one in a humid region, because of differences in precipitation and humidity. Two habi- tats with the same soil and atmospheric conditions may owe their difference to slope, which tends to decrease water content. The latter is likewise affected by cover, which prevents rainfall from running away before it can be absorbed, and also decreases the loss by evaporation from the surface of the soil. Heat and wind decrease water content indirectly by decreasing the humidity, par- ticularly upon exposed slopes. The plant covering itself has the effect of a dead cover in reducing evaporation from the soil, but on the other hand it tends to decrease water content, owing to the use of water by the plants. 12. Available and non-available water content. If a rooted plant is allowed to wilt and die, a careful examination of the soil shows that some water still remains. The amount depends upon the kind of soil, but all soils agree in the ability to retain some portion of the water content. This is due to the fact that the attraction of soil particles for the water films increases as the film grows thinner, until finally it is greater than the attraction exerted by the root-hairs. At this point the plant is unable to obtain water, and it rapidly dies by wilting. The water thus held by the soil can not be used by the plant, and it is hence called the non- available water, or echard. It is usually but a small part of the water commonly present, particularly in moist or saturated soils. In a fine-grained compact soil, like clay, the amount of non-avail- able water is large; in coarse-grained soils, e.g., sand and gravel, it is very small. Of the total water content, or holard, the larger portion can be absorbed by the plant, and is consequently termed available water. The response of the plant to water content is determined by the amount available for absorption and not by the total amount present. This available water, or chresard, differs for the different soils, and for dissimilar species of plants. It is 10 PLANT PHYSIOLOGY AND ECOLOGY diminished by the presence of excessive amounts of salts in the soil, and also by unusually low temperatures. Hence in measuring the water content of habitats the best practice is to determine both holard and chresard, bearing in mind that it is the latter alone that results in adjustment and adaptation. 13. Soil samples. In obtaining samples of soil for finding the water content, the usual practice is to remove the air-dried sur- Fic. 1.—Geotomes and soil can, showing at the left the plunger for remoy- ing the soil core. face, noting its depth, and then to sink the soil-cutter or geotome, with a slow, boring movement, in order to avoid packing the soil. This tendency is further reduced by cutting a long core a decimeter at a time. As soon as the sample is dug, the plunger is used to press the core from the geotome directly into an air-tight soil can. The lid is screwed on as quickly as possible, and the number recorded immediately with such notes as are desirable. The cans should —_— THE WATER OF THE HABITAT 1 be numbered with paint on both lid and side in such a way that the number may be read at a glance. The rule is to weigh the cans as quickly as possible after the sample is taken, though when necessary they can be kept for several days without appreciable error. For weighing, delicate balances are to be preferred, but when these are not available, coarser balances which weigh accu- rately to one centigram give satisfactory results. The best method is to weigh the soil sample in the can. Turning the soil out upon the pan or upon paper saves one weighing, but there is always some slight loss, and the chances of serious mishap are many. After weighing, the sample is dried in a water-bath or oven. At a tem- perature of 100° C. this is ordinarily done in twenty-four hours; stiff clays require a longer time. High temperatures must be avoided with soils that contain much leaf-mold or other organic matter, in order that this may not be volatilized, and hence lead to an error in the result. When a drying-oven is not accessible, soil samples are dried in the air, preferably in sunshine. This usually takes several days, and a test weighing is generally necessary to determine that the drying-out is completed. The weighing of the dried soil is made as before. The can is carefully brushed out and weighed also. 14. Computation of water content. To find the percent of water content, the second weight, i.e., of the dried sample and can, w’, is subtracted from the first weight, w, of the original sample andecan. The weight of the can, w?, is taken from the dried weight, w’. The first result is then divided by the second, and the result is the percent of total water content, i.e., holard, figured upon the eae / dry soil as a basis. The formula is — = =W, in which W is the —w? holard in percent. Water content has generally been computed upon the moist soil as a basis.1_ This method leads to inexactness in the comparison of habitats, however, and should be abandoned. Since most of the results so far obtained have been made in this way, it is necessary at present that the basis be taken into account in comparing the results of different workers. The most satisfac- tory method for the present is to express the results in grams per hundred grams of moist soil. For example, 20/100 indicates a water content that is 20% of the moist weight or 25% of the dry weight. 1 Research Methods, 28. 12 PLANT PHYSIOLOGY AND ECOLOGY 15. Time of water content readings. Since conditions in a plant-house are nearly constant, a single reading of water con- tent made at any time is fairly representative. This is not the case in the field, owing to the influence of rainfall, evaporation, and gravity in changing the amount of soil water. An isolated reading has very slight value, and it is therefore necessary to obtain a basis for comparison by making either a series of read- ings in one place at different times or in different places at the same time. Whenever the proper amount of time can be given to field work, the best method is to select a series of stations or habitats, and to take readings throughout the entire series at different times. Under ordinary conditions the time of day at which a particular sample is taken is of little importance, since the variation during a day is usually slight. This does not apply to exposed wet soils or to soils which have just been wetted by rain. When a series of readings is made in different places, how- ever, it is better that the readings be made in rapid succession. Simultaneous readings are necessary only when it is desired to determine whether there is a difference in the rate of loss in the various habitats. 16. Location of readings. In determining the location of readings in the field, it is desirable to obtain as great a range as possible. Where the topography is much broken, as in mountain regions, a series of stations a mile long will include a number of different habitats. In general, fewer habitats are accessible, and it is then necessary to locate a station in each of the more or less diverse areas of each habitat. Grassland, woodland, and marsh show striking differences of water content as well as of other factors. A hilly prairie that drops through meadowland into forest offers an unusually good opportunity for a series of stations that will show the effect of soil, slope, cover, etc., upon the water content. On account of the small differences always present, each station should be definitely located where differ- ences of soil, slope, etc., are the most striking. For the sake of future readings, the exact location of each station is perma- nently marked and recorded. Successive readings are made as near to the preceding ones as possible, though new samples should not be taken too near the old holes. A difference of a few feet produces no appreciable error, if the station is uniform in character. THE WATER OF THE HABITAT 13 17. Depth of samples. The general rule is that the depth of soil samples is determined by the distance to which the roots penetrate. The practice is to remove the air-dried surface in which no roots are found, and to take a sample to the proper depth. When the actively absorbing root surfaces are localized, as in deep-rooted plants, it 1s necessary to confine samples to the area in which absorption occurs. This is especially true when the water supply of a particular species of plant is to be determined, but, in the case of vegetation in general, roots pene- trate to such different depths that a sample which includes the greater part of the distance concerned is satisfactory. The usual depth of a sample is 3 decimeters. In shallow or wet soils, cores to a depth of 1 or 2 decimeters suffice, while in very dry soils, and in the case of shrubs and trees, deeper samples are required. Experiment 1. Measurement of water content. Take a 2-dm. sample from a dry soil and another from a moist soil; take one also from a sand and a loam. Weigh, dry, and compute percentages upon the basis of the dry soil. 18. The determination of available water. The amount of water that a plant can absorb from the soil can be readily deter- mined only by finding the amount left when the plant wilts com- pletely. This determination can easily be made in the labora- tory or plant-house, though in the field it is attended with some difficulty. A thrifty plant growing in a pot of medium size is the best for the purpose. It is necessary that the pot be glazed or covered with sheet rubber in order to prevent too rapid drying of the soil. At the beginning of the experiment, three soil samples are taken in such a manner that they will indicate any variation in water content in different parts of the soil. The depth of the core is regulated by the size of the pot and the position of the roots. The holard is found in the usual way, and is expressed upon the basis of 100 grams of dry soil, e.g., 25/100. . The aver- age of the three samples is taken as representative: this average is most readily arrived at by weighing and drying the samples in one can. The soil is then permitted to dry out slowly. Sud- den drouth often impairs the power of absorption, and the plant wilts even though considerable available water is present. The proper time to take the second reading is indicated by the thor- ough wilting of the leaves. It is undesirable to wait for com- 14 PLANT PHYSIOLOGY AND ECOLOGY plete wilting, since the younger parts are able to draw upon the watery tissues of stem and root for some time after the plant is unable to obtain water from the soil. Three samples are again taken from the soil, and the average water content determined as before. This is the non-available water or echard. This is likewise computed for 100 grams of dry soil, and the result is subtracted from the holard. The final result is the available water, or chresard, expressed in the number of grams to 100 grams of dry soil. 19. Chresard of habitats. In order to find the amount of water available for a plant in its own habitat, it is necessary to produce wilting by cutting off the water-supply. This is accom- plished by digging up a plant in its own soil and transferring it to a pot of good size. The pot is placed in the hole from which the plant is taken, and a canvas awning arranged to prevent wetting by the rain. Where the distance of the habitat makes this method difficult or impracticable, the plant is transferred to the plant-house. In either case the soil samples are taken as indicated above, and the chresard is arrived at in exactly the same way. The available and non-available water of six rep- resentative soils, together with the amount necessary for satu- ration, are indicated in the following table. The figures serve equally well to indicate percentages and the number of grams of water for each 100 grams of dry soil. It must be clearly recog- nized that these figures will not be exactly the same for every kind of sand, clay, etc.: Soil Holard Echard Chresard SAG wes 0) ces ct cca ti Lage aS 14.3 a 14 Clays eee: otic hee een ital aon 47.4 9.3 38.1 TOES hs See are a eee ee 59.3 LOM 49.2 1 B62) 0 WAM eee oR rade rate stents Aches 64.1 10.9 Sane EUS he eo ee a ee eee 6573 11.9 53.4 Seat bora ote ga na Jo oe eta ieee 68.5 Nez ike Experiment 2. Determination of available water. Germinate sun- flower seeds in sand and in rich garden soil. Transplant the seedlings from time to time so that they will be from 1 to 2 feet in height when they reach a 6-inch pot. Find the holard and echard in the way in- dicated, and from these compute the available water content of each soil. THE WATER OF THE HABITAT 15 MODIFYING FACTORS 20. Influence of soil upon water content. The soil of a habitat is of the greatest importance in determining the amount of water content, and also the kind of water, i.e., the chemical substances found in solution. The amount of water present is directly de- pendent upon the texture or fineness of the soil, that is, upon its physical properties. The kind and amount of nutrient mate- rial dissolved in the water are determined by the chemical nature of the soil. In studying the influence of the latter, it is conse- Fic. 2.—Glacial boulders at Lake Moraine, below Pike’s Peak, in which the disintegrating action of weather is aided by the roots of trees. quently necessary to examine the physical structure and to deter- mine the chemical composition. These are not of equal value, however. The amounts and kinds of soluble materials in all ordi- nary habitats are so nearly alike that differences in chemical com- position are of little importance. They play a large part only where soluble chemical compounds are present 1n excessive amounts, as in alkaline soils, or when the amount of humus is unusually large or deficient. An excess of soluble salts hinders absorption 16 PLANT PHYSIOLOGY AND ECOLOGY and reduces the chresard, while an excess of acids has the oppo- site effect. The structure of the soil, on the contrary, has an almost absolute control upon the fate of the water that enters the ground, in addition to its influence upon the water that runs off. It determines the amount of water drained away in response to gravity, and also the amount that can be raised from the lower layers by means of capillary action. The total water content is dependent in the first place upon the amount of water that soaks into the soil. Of this, the holard is that part which the soil holds in spite of the action of gravity, together with that which may be raised from time to time by capillarity. The chresard, how- ever, is only that part which the root-hairs are able to take up in opposition to the pull of the soil particles. 21. Origin and structure. Soils are formed from rock by the action of weathering. The latter is due to the influence of Fic. 3.—Decomposition of a granite boulder into gravel and sand, and the further breaking down of these by the roots of herbaceous plants. both physical and biological factors, acting separately or together. Weathering consists of two processes. The one is disintegration, by which the rock is broken into fragments of various sizes; the THE WATER OF THE HABITAT 17 other is decomposition, by means of which the original rock, or its fragments, is broken down into minute particles. These two processes usually go hand in hand, although as a rule one is more marked than the other. Their relative importance is deter- mined by the character of the rock, and by the forces that act upon it. Hard rocks, e.g. granite, as a rule, disintegrate more rapidly than they decompose, while sedimentary rocks, such as sandstone, limestone, etc., tend to decompose more rapidly than they disintegrate. In many cases of weathering, the two pro- cesses are of equal importance. 22. Water capacity. A soil owes its capacity for water to the fineness of its particles. Since the water is in the form of PSS UTS). ox Z pkita MOC Se ce il oo nut if ee — 0; lena NM s oD = i NN “aN Sub ce A) Sas HAN ea SS \ fi NY ay ne Ap LZ) Os TD, I _A > UY, KS \ A NS hi IN: ave eo pia ae yy A yt : ul? VL ors a =" iy Dy; a zi Ze er 4 1 € Oy & Benth / pl Soh lb . RE ue AWS Sma x). Fic. 4.—Diagrammatic sketch showing the structure of a fine soil and the position of root-hairs in it. The root epidermis (e) gives rise to root- hairs (h) which push their way between the angular soil particles sur- rounded by thin water films. The air spaces are white. (After Sachs.) <= thin films upon the soil particles, the amount necessarily increases with an increase in the water-holding surface. The latter is in- creased as the particles become finer and more numerous, and thus produce a greater aggregate surface. The ‘movement of water upward and downward in the soil is likewise dependent upon the size of the particles. As the latter become finer, the 18 PLANT PHYSIOLOGY AND ECOLOGY irregular capillary spaces between them grow smaller, and the upward or capillary movement is increased. On the contrary, the downward movement of water, i.e., percolation, which is caused by gravity, is retarded by a decreasee in the size of the soil grains, and hastened by an increase. The properties of the soil which regulate the upward and downward movement of water are respectively capillarity and porosity. Both are de- pendent upon the structure or fineness of the soil, though in a manner directly opposite to each other. Capillarity increases with the fineness of the soil, porosity with its coarseness. Capil- larity augments the water content of the upper layers, while porosity decreases it. Upon this basis alone, soils 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. However, a third factor of great impor- tance must be taken into account. This is the pull exerted upon each water film by the soil particle itself. The pull seems to in- crease in strength as the film grows thinner, and this explains why it finally becomes impossible for the root-hairs to draw mois- ture from the soil. This property, like capillarity, is most pro- nounced in fine-grained soils, such as clays, and is least evident in the coarser sands and gravels. It furnishes the explanation of non-available water, and indicates that the chresard is directly connected with soil texture. 23. Chemical nature of soils. Apart from the effect of exces- sive amounts of acids or salts, the chemical nature of the soil is of slight importance, except in the case of soils exhausted by intensive cultivation. In nature the necessary nutrient salts are so uniformly distributed that the chemical composition of the original rock is immaterial. A soil can modify the plants upon it only through its water content, or the soluble salts, or solutes, that it contains. Hence, when differences of structure or distribution occur between habitats with different soils, the cause is not to be sought in the fact that the soil is silicious, cal- careous, or argillaceous, but in the effect of the texture upon water content. It now appears entirely incorrect to ascribe the pres- ence or absence of certain species on limestone soils to the chem- ical nature of the latter. The most important chemical elements in the soil appearing in the form of salts and connected with the growth of green plants are nitrogen, sulphur, phosphorus, iron, THE WATER OF THE HABITAT 19 potassium, calcium, and magnesium. These occur usually as nitrates, sulphates, phosphates, chlorides, carbonates, or oxides. The effect of alkalies and acids in the soil upon water con- tent and absorption is not altogether understood. Experiments indicate that alkalies hinder absorption, and acids promote it. In other words, alkalies reduce the amount of available water, while acids increase it. Alkaline soils are really dry soils, a fact clearly established by the character of the plants that grow upon them. On the contrary, acid soils are wet; usually indeed they show maximum water content. They contain plants which are adjusted to an excessive water supply. The majority of such plants exhibit adaptations to water, but some of them have the appearance of dry land plants, or xerophytes. A careful exam- ination of the structure of the latter reveals modifications due to water. Consequently, it seems almost certain that such “‘bog xerophytes’’ are dry land plants, which in coming to grow in water have retained certain superficial ear-marks of the original habitats. 24. Air content. In all soils, but particularly in acid ones, the air content is a factor of considerable importance, owing to the constant use of oxygen by the roots. The amount of air present depends upon the water content and the compactness of the soil. Air content varies inversely as the water content: it is large in dry soils and very small in wet ones, especially those covered with water. Water plants, i.e., hydrophytes, show characteristic modi- fications called forth in response to a low air content. When a soil becomes packed, the movement of the air into and through the soil is impeded, and a very important task of cultivation is to keep the soil so stirred that the air content does not fall toolow. “Sour ”’ soils, including “sour ”’ bogs, owe their nature to the production of organic acids in the presence of a low supply of oxygen. When stirring the soil is possible, ‘‘sourness ” is easily remedied, since complete decomposition prevents the formation of acids. 25. Determination of soil properties. Since the soil is prima- rily important because of its influence upon water content, the determination of soil texture is the principal task in this connection, In an elementary study of habitats it is sufficient to know that a soil, or the soil water, is acid or alkaline, without further reference to amount. Such a test is readily made in the field by means of fresh strips of litmus paper. Air content is in large degree a result 20 PLANT PHYSIOLOGY AND ECOLOGY of water content, and has but an indirect or obscure effect upon the water present. The texture of the soil is chiefly a matter of size of grains or fineness. ‘The latter can best be ascertained by the use of sieves. Two sieves of 1 mm. and of .05 mm. mesh will make it possible to separate the soil into gravel, sand, and silt-clay. The structure of the soil is then expressed in percents, e.g., gravel 10%, sand 15%, silt-clay 75%. A mechanical analysis of a soil throws little additional light upon its behavior with respect to water. It is much more helpful to know its porosity and capillarity under conditions as nearly natural as possible. Both of these are determined by using a cylinder of the soil concerned and noting the rate with which water moves downward or upward through it. For general purposes, however, a measurement of porosity suffices for both, since capil- larity varies inversely as the porosity. Thus sand is extremely porous, but possesses very little capillarity. Clay, at the other extreme, exhibits strong capillary movement, but is very slightly porous, while intermediate soils approach the one or the other in behavior in direct proportion to the amount of the predominant constituent. The use of soil-cores from typical habitats yields the most satisfactory data concerning porosity and capillarity in rela- tion to actual water content. When these are not obtainable, instructive results can be obtained by using loose soils in the plant- house, provided that the soils are well packed before the experi- ments are made. The presence of decaying vegetable matter increases the porosity of a soil, and correspondingly decreases its capillarity, but after complete decomposition humus tends to increase capillary action, especially in coarse soils. The amount of humus may be readily ascertained by weighing a soil before and after it is thoroughly burned. Experiment 3. Porosity and rate of evaporation. Fill three 2-inch flower-pots respectively with sand, loam, and clay that are nearly dry but not powdery. Pack each soil firmly until its surface is a half-inch below the edge of the pot. Place each pot in a tall Stender dish, and slowly pour water upon the clay until the former runs into the dish through the hole in the pot. Pour the same amount upon both sand and loam, and measure the amount of water that percolates through each. Weigh each pot of soil, and, together with a graduate of water having an equal surface, put them in a sunny place. Weigh each every day or two for a week or more. As soon as the sand shows no further THE WATER OF THE HABITAT 21 loss, place the three soils in a water-bath and dry them out completely. Make a final weighing. Compute the percentage of water in the satu- rated soils at the beginning, the rate of loss from each as compared with the water surface, and the amount of water in each at the time the sand showed no further loss in the air. 26. Influence of precipitation upon water content. In all habitats except those where the supply of water is constant, owing to the presence of springs, streams, ponds, or other bodies of water, the dependence of water content upon rainfall is absolute. Soil and slope determine how much of the latter finds its way into the ground, but their action is secondary. Daily rains are able to keep practically any soil saturated, regardless of its character or the slope. All habitats not covered with water reach their maximum water content immediately after a heavy rain or during the rainy season. The water decreases gradually throughout a dry period or season, only to again approach the maximum when precipitation takes place. The latter occurs in various forms, such as rain, hail, dew, frost, and snow. Of all these, rain is by far the most important. In spite of local exceptions to the rule, hail is too infrequent to be taken into account. Frosts have at best only a slight and fleeting effect upon water content, especially in view of the fact that they usually fall outside the growing season. Snow- fall is often of great importance. It not only acts as a cover to prevent evaporation, but upon thawing it also enters the soil directly just as rain does. The loss by run-off from slopes is much greater, owing to the frozen condition of the ground. The exact importance of dew is not easily determined. Dew is almost always too small in amount and too fleeting to add directly to the water content of the soil. By its own evaporation it doubtless decreases in a slight degree the amount of water lost by the soil and by bedewed plants. In studying the water content of habitats a knowledge of the amount of rain usually suffices, though in the study of habitats in spring, the amount and distribution of snows must also be taken into account. 27. Measurement of rainfall. Rainfall is measured by means of a rain-gauge, an instrument which collects in a narrow vessel the rain falling upon a large surface. In the standard instrument the ratio of surface between receiver and tube is 10 to 1. A direct measurement of the water in the tube must be divided by 10 to give the rainfall, or a standard measuring-rod, upon which this 22 PLANT PHYSIOLOGY AND ECOLOGY compensation is already made, may be used. In elementary work it is impracticable to measure the rainfall in different habitats. It is fairly satisfactory to use the reports of rainfall obtained from a neighboring weather station when the latter is not more than a few miles distant. The effect of rainfall upon water content is best ascertained by taking soil samples in different habitats immediately after a rain, and then determining the increase in water content. In open, hilly regions there sometimes occur in spring differences in water content which can only be explained by a knowledge of the distribution of the late snows of winter. 28. Physiography. Physiographic factors are altitude, expos- ure, slope, and surface. There are in addition certain physio- Fic. 5.—Mountain sides showing differences of slope, exposure, and cover. graphic processes, such as weathering, erosion, and sedimen- tation, which play a fundamental part in changing habitats, but these do not act directly upon water content. The latter is di- rectly influenced by slope and surface, while altitude and exposure operate only through humidity. Cover, either dead or living, while not exactly a physiographic feature, affects water content in much the manner of surface, under which it may well be considered. THE WATER OF THE HABITAT 23 29. The influence of slope. By slope is meant the inclination of the surface of a habitat with respect to the horizon. The principal effect of slope is in controlling run-off and drainage, and through them water content, although they are at the same time affected by surface and soil texture. Slope has also a less direct influence through its action upon heat and wind, which in turn modify humidity and thus affect the water content. Slope is expressed in degrees of the angle made by the intersection of a line bounding the surface and the base line. It is measured by means of a clinometer, a simple instrument in which a line and plummet indicate the angle of slope upon a_ semicircle graduated in degrees. In making a reading, it is desirable to use a basing strip, a piece of wood 1 meter long and 5 centimeters wide, with a true edge. The basing strip is placed upon an area typical of the slope, and is pressed down firmly to equalize irregu- larities. The clinometer is moved gently along the upper edge, causing the marker to swing freely. When the latter comes to rest, the instrument is carefully turned upon its back, and the angle of slope read directly in degrees. Two or three readings in different parts give a very satisfactory mean for the entire habitat. The angle of slope can not be directly connected with the amount of water content, because of the other factors con- cerned. The rule is that the rainfall lost by run-off increases with the angle, and the water absorbed correspondingly decreases. In two or more areas essentially alike in soil, cover, and rainfall, differences in water content are directly determined by differ- ences in slope. 30. The influence of surface. The surface of a habitat often shows irregularities which retard the movement of run-off and cause more of the rainfall to soak into the soil. The soil itself often shows such irregularities, e.g., the rocks of boulder and rock fields, the hummocks of meadows and bogs, the mounds of prairie-dog towns, the raised tufts of prairies and sand-hills, ‘the minute gullies and ridges due to erosion, etc. The influence of these is usually not great, but it is always appreciable, and in some cases of considerable importance. Their effects are often measurable by means of soil samples, but it is impossible to ex- press the character of the surface in definite terms. It must suffice to describe the surface as even or uneven, and to indi- cate the kind and amount of unevenness. 24 PLANT PHYSIOLOGY AND ECOLOGY The same is true of cover, which is usually of far greater and more universal importance. Dead and living cover retard run- off and reduce the amount of water lost from the surface of the soil. By their decay, plants add humus to the soil, thereby in- creasing its capacity for absorbing and retaining water. Dead cover is of less importance, since it is found chiefly during the resting period alone, but it plays a part of some value by catch- ing and holding drifting snow. The cover of living vegetation reacts upon the habitat in a much more vital fashion. While it has a powerful effect in increasing water content, it reduces it also by reason of the water lost through evaporation from the plants. Cover can be expressed only in general terms of density and height at present, and it suffices, as a rule, to indicate the character of the plant covering. In this connection it should be noted that other biological factors, viz., man and other ani- mals, often exert an influence upon water content. Except in special cases, such as irrigation and drainage, this effect is exerted through other factors, and does not need further consideration here. 31. The influence of climatic factors. All the atmospheric or climatic factors of a habitat have an effect upon water con- tent, either immediate or otherwise. Such factors are humidity, temperature, wind, pressure, and light. The influence of the last is slight and very indirect. Temperature, wind, and pressure can change the amount of water in the soil only through humidity, and hence they will be further considered under the latter. Humidity acts upon the plant and upon soil water in the same way, i.e., by controlling evaporation. It affects water content directly through water loss from the soil, and indirectly inas- much as the water lost by the plant is first drawn from the soil. Since it is a direct factor, it will be more fully considered in the following sections. It is mentioned here merely to complete the list of factors that play a larger or smaller part in modifying or controlling the water content of the habitat. 32. Humidity. The moisture of the air, which is in the form of vapor, is termed humidity. Owing to the nature of the medium in which it occurs, humidity is much more uniformly distributed than the water content of the soil. For the same reason it fluctu- ates to a much greater degree. It differs from water content also in that a part of the latter, i.e., the echard, is always non- THE WATER OF THE HABITAT 20 available for the purposes of the plant, while the whole humidity of the air is the stimulus that controls the water loss of a leaf. The actual amount of water present in the air is called the abso- lute humidity, and is expressed in milligrams per cubic centi- meter. The relative humidity is the relation between the amount of moisture in the air and the amount necessary to saturate the air under given conditions. It is expressed in percents, and is in common use as the expression of humidity. 33. Modifying influence of temperature and wind. Humidity is affected by temperature, wind, pressure, altitude, exposure, Fic. 6.—Dwarf spruces at timber line on Pike’s Peak, produced by the drying and the mechanical action of almost constant winds. cover, and watercontent. High temperatures increase the capacity of the air for moisture and low temperatures diminish it; the former lower the percent of relative humidity, the latter raise it. Of two regions, or two habitats with the same rainfall, the warmer is the drier. During the day the relative humidity falls as the temperature rises, and rises in the evening as the air grows cooler. Wind has also a powerful effect upon humidity in that dry winds lower the amount of air moisture by mixture or removal, while 26 PLANT PHYSIOLOGY AND ECOLOGY moist winds exert an opposite influence. The most important action of wind is the removal of the more humid air ordinarily occurring above the plants of a habitat, and its replacement by a drier air. This has the effect of keeping the immediate humidity low. This may be readily verified by taking readings of humidity in a sheltered area and in one exposed to the full effect of a strong wind. The moist winds that blow across a large body of water or those that precede a rain either do not have this effect or possess it in but a slight degree. The exact influence of wind upon humidity is best shown in a hilly habitat, such as an undulating prairie. If the velocity of the wind is determined by means of a hand ane- mometer for ravine, slope, and ridge, and simultaneous readings of humidity are taken, the relative humidity will be found to de- crease with the increase in wind velocity. 34. Influence of pressure and physiographic factors. Pressure modifies humidity by varying the density of the air, and hence its Fig. 7.—Station on the summit of Mount Garfield (8800 m.) for deter- mining the effects of altitude by means of plants and instruments. power to hold moisture. The daily fluctuations which constitute weather are slight and are of little importance, except in their rela- tion torainfall. The effect upon humidity is much more pronounced when differences in altitude bring about permanent differences in THE WATER OF THE HABITAT 27 pressure. Altitude has been thought to influence vegetation chiefly by virtue of increased light and decreased heat. Recent studies of the author seem to prove conclusively that light is of practically no importance, and that the marked dwarfing of alpine plants is due largely to the great rarefaction of air by which evaporation is increased. Exposure, i.e., the position of a slope with respect to the sun affects humidity through the action of sun and wind. Slopes longest exposed to the sun’s rays receive the most heat; consequently slopes with a southern exposure regularly show some- what lower humidities than those with northern exposures. The effect of wind is most pronounced upon those slopes exposed to prevailing dry winds. Asarule, these are southern or southwestern, and for reasons both of temperature and wind these are usually the driest slopes of hills and mountains. Cover increases humidity by reducing the influence of temperature and wind. In addition a living cover supplies moisture to the air in consequence of evapora- tion from the plants that compose it. A similar effect is produced by the water content of moist soils, particularly in forests and thickets where the air is sheltered from sun and wind. 35- Effect of climate and habitat. The general humidity of a habitat depends upon climate and location with respect to bodies of water. In comparison with each other forested regions show high humidities, while deserts have low humidities. Coast regions are moist, inland regions relatively dry, lowlands are more humid, table-lands and mountains less humid as a rule. In a particular habitat the relative humidity approaches or reaches saturation during rain or fog, and then gradually decreases to a minimum just before the next rain-storm. There is also a daily maximum and minimum. The highest relative humidity, except when dis- turbed by rain, usually falls at 3or4 a.m. It decreases slowly until 7 or 8 a.m. and then falls more rapidly to a minimum at about 4 p.m., from which point it rises slowly tothe maximum. Variations within the habitat arise chiefly through differences in protection from sun and wind. For somewhat similar reasons the relative humidity is greatest Just above the surface of the soil; it is less at the level of the vegetation, and still less a meter or more above the latter. 36. Measurement of humidity. Humidity is measured by means of a psychrometer. Of the latter there are three types: the sling, the cog, and the stationary psychrometer. All consist of a wet-bulb and a dry-bulb thermometer set in a case. The first 28 PLANT PHYSIOLOGY AND ECOLOGY two are designed to be moved or whirled in the air. The same principle is applied in each, viz., that evaporation produces a decrease in temperature proportional to the amount of moisture in the air. The dry-bulb thermometer is an ordinary thermometer, while the wet-bulb one is covered with a cloth that can be moistened. The former indicates the normal temperature of the air, the latter gives the reduced 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 reduc- tion shown by the wet bulb. 37. Sling and cog psychrometers. For field work the sling and cog psychrometers are much more convenient than the station- Fic. 8.—The cog psychrometer. The wet bulb is the one covered with cloth. ary form. They are generally considered to be more accurate also, since the movement prevents the accumulation of moisture about the wet bulb. Of the two, the cog psychrometer is the more con- THE WATER OF THE HABITAT 29 venient and satisfactory. It is smaller, more compact, and the danger of breakage in use or in carriage is extremely small. It has the further advantage of making it possible to take readings in a layer of air less than 3 centimeters in thickness, and in any posi- tion. The use of the sling psychrometer is attended with grave danger to the instrument in a free space less than two yards across. The cog psychrometer has a single disadvantage owing to the neces- sary use of short thermometers. To secure the proper range, Centi- grade thermometers must be used, and the readings thus obtained must be converted into Fahrenheit temperatures before the humid- ity can be determined from the usual tables.! 38. Making a reading. In general, observations should be taken facing the wind. It is also a wise precaution to shift the position of the instrument a foot or more during the reading, except when the humidity of a definite layer is desired. The cloth of the wet bulb is first moistened with water carried in a small 50-ce. bottle for this purpose. Distilled water is preferable, but tap- water and the water of streams may be used without appreciable error, if the cloth about the wet bulb is changed occasionally to prevent the accumulation of dissolved material. The water is poured slowly upon the cloth of the bulb until it is completely wetted, care being taken not to wet the dry bulb. As the cloth absorbs water reluctantly when perfectly dry, a pipette or a brush is usually a valuable aid in wetting it quickly. The temperature of the water used is of slight consequence, though readings can be made more quickly when the temperature is not too far from that of the air. The psychrometer is held in the proper position, 1.e., the bulbs are placed in the layer of air to be studied, unless a gen- eral reading is to be made, and are then rotated at an even rate and at a moderate rapidity. As the reading must be made when the mercury of the wet bulb reaches the lowest point, the instrument is usually stopped after 100 revolutions, and the position of the column is noted. The lowest point is often indicated by the ten- dency of the mercury to remain stationary. As a rule, the lowest point can be known with certainty only when the next glance shows arise in the column. Check readings of this nature must be made every 25 or 50 revolutions in order to make sure that the mercury has not reached the minimum and then begun to rise while the instrument is in motion. In noting the final reading, care must be 1 Research Methods, 39. 30 PLANT PHYSIOLOGY AND ECOLOGY taken to secure it before the mercury begins to rise in consequence of stopping the movement. For this reason it is desirable to shade the psychrometer with the body when looking at it in the sunshine, and to take pains not to breathe upon the bulbs or to bring them too near the body. At the moment when the wet bulb registers the lowest point, the dry bulb should also be read and the results recorded. 39. Use of humidity tables. To ascertain the relative humid- ity, the difference between the wet- and dry-bulb figures is obtained. This difference, together with the dry-bulb temperature, is referred to the tables. A variation in temperature has less effect than a variation in the difference. In consequence, the dry-bulb reading is expressed in the nearest unit, and the difference is reckoned to the nearest .5. Since the humidity varies with the air pressure, it is necessary to use the table computed for the normal barometric pressure of the place under consideration. Humidity tables are usually computed for pressures of 30, 29, 27, 25, and 23 inches (76, 73.5, 68.5, 63.5, 58.5 cm.). For mountain regions over 2100 meters (7000 feet) additional tables are desirable, but the table for 23 inches will meet all ordinary requirements, since the effect of pressure is small within the usual range of growing-period tem- - peratures. Experiment 4. Measuring humidity. Use a cog psychrometer to determine the range of humidity at 8 a.m., 12 m., and 4 pm. Make readings in quick succession in the plant-house, and in sun and shade out-of-doors, and find the relative humidity for each. 40. Method of habitat study. A real knowledge of physical factors, and of the habitats which are constituted by them, can be obtained only by the use of factor instruments in the field. Such knowledge is of the most fundamental importance in discovering the causes which control the functions and structures of plants, and their grouping into plant formations. All these objects can be obtained by establishing a series of stations, and using the members of the class to take simultaneous readings in them at different times of the year. The ideal method is to begin such a series Just before the opening of spring, and to continue it at proper intervals through- out the entire growing period. This is scarcely feasible in the great majority of cases, and the most practicable method is to take a set or two of readings in the fall, and the same number in winter. Then, THE WATER OF THE HABITAT ol in the spring, readings should be taken every week or two until the work closes. 41. Choice of stations. The stations of a series should be chosen with care, and as a result of considerable previous knowledge of the locality to be studied. The stations must not be too numer- ous nor too far apart. Within these restrictions, however, they should represent several distinct formations, and as many dissimi- lar areas in each as possible. A series can not well be more than two miles long, and one of a mile, or even a half mile, is to be pre- ferred. A good series will contain at least a dry, a wet, and a shady habitat, e.g., a prairie, a swamp, and a forest. In class work of this sort at Lincoln, two series were first established, one in prairie, the other in woodland. For reasons of convenience and time- saving, these are now replaced by a prairie series consisting of the following stations: (1) meadow, (2) crest of ridge, (8) northeast slope, (4) ravine, (5) southwest slope, (6) sandy ridge, (7) willow thicket, (8) high prairie, (9) bog, (10) south slope. 42. Constant factors. After the stations have been chosen, their location should be permanently indicated in such a way that they can be readily found from time to time. In order to avoid mistakes, the instructor should first take the entire class through all the stations, pointing out the general differences and illustrating the use of instruments not already familiar. If this does not take too long, readings of the more constant factors, water content, slope, exposure, surface, and cover are made at the last station, and a student equipped with thermometer, psychrometer, and pho- tometer is left in charge. At each succeeding station the same plan is followed, so that upon reaching the first station the con- stant factors have all been read, and there is an observer at each station prepared to make readings of the variable factors. When the preliminary survey has occupied all the time available, the same method is employed upon the second visit, but the beginning is naturally made at the first station. The observers are shifted upon successive visits so that each student has an opportunity to become acquainted with every station. When the class is large, two or more students may be left at a station, and the work divided between them. 43. Simultaneous readings. The task of obtaining readings at the same moment is met by taking observations upon signal. The instructor places himself at a commanding station, prefera- 32 PLANT PHYSIOLOGY AND ECOLOGY bly near the middle of the series, and gives the signals by a shout or whistle at the proper interval. Considerable care and prac- tice are required in order to do the last satisfactorily. Suffi- cient time must be given for the operation of the instrument and the making of the record. In addition, a period which is long enough for each instrument to reach the proper reading must be permitted to elapse. For example, in a series which contains a gravel slide and a forest, a thermoineter which has Fic. 9.—Observers making simultaneous readings of humidity in a series of stations in the prairie formation at Lincoln. 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 a series, the place where the response is slowest or greatest often makes the best signal-station. The instructor records the exact time of each signal, and notes any general changes of sky or wind that pro- duce fluctuations at the time of reading. Temperature, humidity, and wind are read usually at 14 meters, and at the surface of the soil. Soil temperatures are obtained from the holes left in making soil samples. These holes are closed with corks to pre- THE WATER OF THE HABITAT 33 vent a change in temperature, and are used only on the day they are made. Light readings are of course necessary only when one or more stations are shaded. For the sake of convenience and accuracy, factors are always determined in the same order, viz., temperature, humidity, wind, light; and the same is true of the various points or levels, e.g., 14 meters, surface, and soil. 44. Point and hour readings. Readings taken as above at the same point in the stations of a series are said to be made at the same level, as the 14-meter level, the surface level, etc. Readings may also be taken simultaneously through the different points of a single station. In this work, the observers are grouped in each station in such fashion that they do not interfere with the correct reading of each instrument. Such determinations are most valuable in the case of temperature, which shows greater differences at the various levels. Important differences of humidity and wind are also discovered, and, in layered formations, marked variations in the amount of light. Series of this sort are likewise read upon signal. Hour series are indispensable for obtaining the variation of each factor during the day. They are read for each level upon signal in the manner already indi- cated, but the series is repeated every hour throughout the day, The number and position of the levels in all of the different series are properly determined by the character of the vegetation. In general, there should be levels corresponding to the surface, to the height of the herbaceous vegetation, and an air level above the latter. For temperature, one or two soil levels are necessary also. 45. Records. A definite form of field record saves much time and prevents many mistakes. Printed blanks of the form indicated below, 7293 inches in size, have been found to be the most satisfactory. Each blank suffices for recording two full sets of readings through ten stations. The details may of course be modified as seems desirable. The blanks are carried in a cover protected with oil-cloth. The field readings are entered directly in ink in the case of temperature and wind, while light, humidity, and water content are recorded only when the final results are obtained, field memoranda being employed for the direct readings. 34 PLANT PHYSIOLOGY AND ECOLOGY Oak-hickory forest. April 20, 1901. Clear. Southeast wind. Temperature. Light. Humidity. | Water content. Wind. Ti : a a aaa Oa a a | he 10:40110:46 10:50)10:55/11:00)12:00]12:05)11:10)11:20 Percent. 11:30'11:45 Sta. |14m.| Surf.|1 dm.|/2 dm.|4 dm.|14 m.| Surf./14 m.| Surf.|/1 dm. 2dm.|4dm. 14m.| Surf. i altGe|25 9.6| 8.4) 7.8} .08| .06) 73 | 81 |24.2/19.2)19.5) 298) O ee, 2\30.5| 8.5) 8.4) 7.8). 1) 209) 73°86 22> 12275 NOMA ono eee 3 6.2/17.8| 7.6) 7.8) 8 -08) O06) 73 | 95° 122.1'20 .4)\21 1619640 a6 4 eee 2\10.6| 8.4) S.2) 206) .03) Sl |_ 95) 125. 4/23 21)/22. 4 eae 5 117:6(25°4) 7.6) 7.4) 7.2) .03) 202) 90 | 95) 127-219 2S8i1Sss ees eee 6 |16.2/20.2| 8.4) 7 6.2) .02} .01) 82 | 90 |27.6/20.8)/18.8) 115) 4 TW. ise 2} 6.41 6.4) 6.1) .05) .04) 82 | 90 |23.8]/19 |19.3) 60) O 46. Kinds of curves. The most graphic way of bringing out the factor differences between habitats or stations is by means of curves. The factors that lend themselves most readily to this method are the variable ones, water content, humidity, light, temperature, and wind. Curves representing these are spoken of respectively as water-content curves, humidity curves, etc. With regard to the time and position of the readings upon which they are based, they are divided into level, station, and point curves. A level curve is one based upon readings made at the same level through a series of stations, e.g., the level curve of surface temperature. The station curve shows the variation of a factor through the different points at which readings are made in a single station. The point curve has for its basis the hourly or daily variation of a factor at a single point in a station, such as the variation in humidity during the day at the surface of a barren ridge. All of these may be simple curves when based upon a single reading through a level, station or day, or mean curves when they are based upon the average of a number of such readings. 47. Combinations of curves. Curves are often combined in order to permit of a ready comparison between them. Com- bination is brought about by tracing upon the same sheet the curves to be compared. Dissimilar curves, e.g., level and sta- tion, can not be combined. Colored inks are an absolute necessity in making combinations. The principle underlying their use is that curves which approach closely or cross each other must ‘be traced in inks that contrast sharply. It is important to use the same color invariably for the same level or point. The variety Prairie: 4 25°00 Water Content Curves Prairie: 5,409 aL! ai fs U Ae PCA oN eae Fae nt | in pee | ee Eger cat — pores iii wan = HI eed EEE fail edition FEE wae ws oneal ead | L.|-| helen} Hinaa REBER vet rst 4 i eT | | inn ei — — 22 = 2 oe = == roe Bs: D4 FIG. 10.—Combinations of water content curves, showing the holard in per cent at depths of 5, 10 and 15 inches through seven stations in the prairie formation. The two sets of readings were separated by an interval during which rain fell. THE WATER OF THE HABITAT Bt) of ways in which curves can be combined is almost endless. For the beginner, however, the most satisfactory are those in which but one factor is taken into account. The most useful are those in which the curves of temperature, or of any other factor, for the different levels are drawn together. Similarly, the curves showing the variations of temperature for each station may be combined. A combination of the greatest value is obtained by contrasting the curves of holard and chresard for a series of sta- tions. A very interesting combination may be obtained by arranging the series of curves for two distinct habitats, such as prairie and forest, side by side upon the same sheet, thus per- mitting the direct comparison of the curves for various factors. 48. Plotting curves. The plotting-paper employed is ruled in centimeter squares which are divided into 2-millimeter units. The sheet is 24X18 cm. in size, thus making it possible to file the curves in the record book. A fine-pointed pen, such as the Spencerian No. 1, is used for plotting. The inks used are the Higgins Waterproof Inks, which are made in the following colors: black, violet, indigo, blue, green, yellow, orange, brown, brick red, carmine, and scarlet. In addition to being waterproof, they make it possible to combine curves readily without destroying their identity. It is also a great advantage to use the same color invariably for the same kind of curve. In plotting a curve, it is first necessary to fix the value of the centimeter square or interval, as well as the extreme range of the curve itself. For example, in the case of temperature, a value of 1° Centigrade is assigned to each centimeter, since the thermometers used read to one-fifth of a degree, which thus cor- responds with the value of the 2-millimeter units of each square. The length of the sheet permits a range of 22 degrees Centigrade, within which the greater number of temperature curves for a particular season will fall. It is very desirable that the unit interval and the range be the same for each factor, in order that all curve sheets for the same region may admit of direct com- parison. The major intervals are indicated at both sides of the sheet, and the time or the space intervals at the top. The read- ‘ings upon which the curve is based are taken from the field record, and the proper position of each is indicated by a dot. The dots are first connected by a pencil line, the curves being made angular rather than flowing. After being carefully checked, the line is 36 PLANT PHYSIOLOGY AND ECOLOGY traced in ink. Hach curve sheet is properly labeled, and such explanatory notes as are desirable are written upon the back. 49. Intervals for the different factors. In practice, the in- tervals and ranges of the curves of the other factors have been arbitrarily fixed, as in the case of temperature. For water-con- tent curves each square represents a value of 2%, the smaller squares being 0.4%, and the range 2-48%. The unit value for humidity curves is taken as 5 percent, thus giving room on the sheet for the entire range from 1-100 percent. When a hand anemometer is used, curves of wind velocity are 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 .05.. Each small square is .01, which permits a range of .01 to 1. on a sheet. Consequently, in plotting the curve of a series of habitats with a range in in- tensity greater than this, it is necessary to paste two sheets to- gether end to end. This is the usual device when the range of curves is too great, except when the excess is slight. In this case, the curve is left open at the top, and the maximum value is indicated at one side. 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, when this is not the basis of the curve or series. Experiment 5. Determining the physical factors of habitats. The in- structor first chooses a series of stations comprising as many different habitats as can be conveniently studied. Tach station, and especially those that fall within the same habitat, is located with respect to strik- ing differences of vegetation as well as physical factors. The position of each is ‘fixed permanently by means of a stake. The number of stations is necessarily determined by the size of the class and the number of instruments available. Each observer is furnished with thermometer and psychrometer, and, when the variation in light warrants, with a photometer also. Geotomes, clinometer, compass, barometer, and usually also an anemometer, are carried through upon the preliminary survey by the class and the readings made in common. Returning through the series, an observer is left in each station, and the instructor then places himself at the proper point for signaling. Readings are always made in a fixed sequence. Temperature is taken first, in the order of air, surface, and soil; humidity follows, and finally light. To familiarize the student with methods, and to have a slight check upon THE WATER OF THE HABITAT o7 results, it is advisable to take two or three complete sets of readings in rapid succession. A complete set of habitat readings should be taken at least once during the fall and winter respectively. SAeS koe fy Pong keto, Xo : THA, & 0 Waray a 3 oy Ci GS : » j pe a Fie. 14.—Cross-section of the leaf of the monks-hood, Aconitum colum- bianum, showing the palisade tissue above and the sponge tissue below. hairs, and that of the lower to develop stomata. Thus while hairs and stomata often occur on both surfaces of a leaf, hairs are often more numerous upon the upper surface than upon the lower, while stomata are regularly more numerous upon the lower. In many leaves, hairs are found only upon the upper epidermis, while in others, stomata occur only upon the lower. The epi- dermal cells proper have their outer walls cutinized and usually thickened also. In these likewise the outer wall or cuticle is generally thicker on the upper surface than upon the lower. 73. The chlorenchym. As a rule, the cortical region of the leaf consists wholly of parenchyma cells filled with chloroplasts. From its nature. this tissue is called chlorenchym. The latter comprises two distinct parts, viz., the palisade parenchyma and the sponge parenchyma. In the normal leaf, the palisade tissue occurs in the upper half, and the sponge tissue in the lower half. The probable causes of this differentiation are discussed under Light. At present it is sufficient to point out that the position and development of these two tissues are directly connected with differences in the degree of exposure to light and humidity 58 PLANT PHYSIOLOGY AND ECOLOGY shown by the two surfaces. The palisade tissue consists of rec- tangular cells elongated at right angles to the surface, and packed so closely in rows that the intercellular air-passages are scarcely visible. The sponge cells, though usually irregular in outline, are more or less elongated in the direction of the surface. They are loosely connected, and their irregular forms permit the pres- ence of numerous large air-spaces. The relative amounts of palisade and sponge tissue in the leaf are determined by water and light, and a further discussion of this matter will be found under Adaptation. 74. The reduced bundles. The repeated division of the fibro- vascular bundles that enter the leaf is adapted to meet the in- creased need for support caused by its form. At the same time it serves to carry water to all parts of the leaf, which is the organ that needs water most. A close examination of a reduced bundle in section shows its intimate relation to the cells of the chlorenchym. The supportive elements of the bundle are greatly reduced. In many instances spiral vessels alone remain, thereby greatly facili- tating the passage of water from the bundles into the cells of the chlorenchym. Experiment 15. Structure of a leaf. Cut thin cross-sections of a sunflower leaf, preferably by the paraffin method. Under the high power draw in full detail a segment across the leaf at a point where a small bundle occurs. Pay especial attention to the air-spaces and the number and position of chloroplasts in both palisade and sponge tissue. Strip a bit of epidermis from each surface. Count the stomata in two or three fields, and compute the number for a square centimeter of each surface. Estimate the surface of a leaf by first weighing the entire blade, and then two or three pieces of it a centimeter square taken from different portions, and dividing the first weight by the average weight of a piece. The quotient is the surface in square centi- meters. Estimate the number of stomata on each surface of the entire leaf. 75. Diffusion in the leaf. The water absorbed by the roots is carried throughout the leaf by the reduced bundles. The water passes from the vessels into the cells of the chlorenchym by rea- son of the osmotic pressure of the latter reinforced by the attrac- tion of the protoplasmic membrane. The latter effect is due to the fact that the vessel has no protoplasm to counteract this pull. The water passes from cell to cell by diffusion, exactly as in the ADJUSTMENT TO WATER 59 root. Diffusion in the leaf is due to osmotic pressure, arising in part from the active production of organic acids and salts in the cells, but chiefly, it would appear, from the increased density of the sap caused by evaporation. The latter reason doubtless causes diffusion to set most strongly toward those areas in which evaporation is greatest. Here, as elsewhere in the plant, diffusion currents are always in the direction of greatest use. 76. Transpiring surface. In ordinary leaves, especially those found in the sunshine, the cutinized wall of the epidermal cells either entirely prevents transpiration from them or reduces it to an insignificant amount. The transpiring surface, therefore, is not the epidermis of the leaf, but it is formed by the cells that lose water rapidly and in relatively large amounts. It is composed of the aggregate cell surfaces that border on air-spaces, both in the sponge and the palisade tissue. At these places, the cell-sap, which fills the cell walls, passes into vapor whenever the air in the passages is not completely saturated. The moist air that fills the spaces gives up some of its moisture through the stomata to the drier air outside. It seems probable, however, that a more important factor in water loss is the passing of the moist air itself through the stomata, owing to the constant movement of leaves in the wind. In this case, drier air at once passes in to take its place. Consequently, while the number, size, and position of the stomata determine the ease and rapidity with which air and moisture pass out, the stomata do not form part of the trans- piring surface. On the outside, the guard-cells are protected against evaporation in exactly the manner of epidermal cells, and the surfaces next the opening are also cutinized. The inner surface of the guard-cell next the air-chamber is usually exposed to the air of the latter, and consequently contributes very slightly to the transpiring surface. Experiment 16. Measurement of the actual transpiring surface. Find the linear extent of the air-spaces shown in the drawing of a sunflower leaf by attaching a needle-point to a thread, and allowing the thread to run out as each side of an air-space is measured. If the result thus obtained is squared, it will represent roughly the area of transpiring sur- face for a square segment of the leaf of the width of the area studied. The two surfaces of this segment would represent the corresponding leaf surface. Determine the ratio between the actual leaf surface and the transpiring surface. 60 PLANT PHYSIOLOGY AND ECOLOGY 77. Structure and position of stomata. The simplest form of stoma is a hole, which remains constantly open, and is surrounded by peculiar epidermal cells. In all vascular plants, however, the opening is regulated by two guard-cells. Below the opening regu- larly occurs an air-space of variable size, the air-chamber. The guard-cells vary considerably in different plants, but in all they agree in being oblong or cylindrical cells, bent in such fashion that they join broadly at the ends, but are free at the middle. The outer wall of each guard-cell is thickened, and in many cases the inner wall also. The wall next the opening is likewise more or less thickened, while that opposite the opening, i.e., touching the epidermal cell, is the thinnest. Guard-cells are almost invariably - filled with chloroplasts which contain starch. Stomata usually occur singly, scattered more or less uniformly over the epidermis, but in some plants they are found in groups. SN ARAYA Rae OH a Fic. 15.—Distribution of stomata in the epidermis of an orchid, Calypso borealis. The lower epidermis (1) has 36 stomata per square milli- meter of surface, the upper (2) but 2 per square millimeter. Though commonly on a level with the epidermal cells, stomata are often sunken for protection, either singly, as in the century plant, or in groups, as in the oleander. Sunken stomata usually possess an outer chamber or court, formed by the over-arching of the epidermal cells. Their guard-cells usually exhibit one or ADJUSTMENT TO WATER 61 two pairs of valves, though these are often found in other stomata also. As already indicated, stomata are ordinarily more abundant upon the lower epidermis of horizontal leaves, and in some species are restricted to this surface. In many aquatic and rosette plants, the stomata are more abundant upon the upper surface, and in floating plants they occur on this surface alone. The leaves of sub- merged plants normally lack stomata. In forms more recently submerged, the latter sometimes persist, but are functionless. 78. The functions of stomata. In their simplest form, stomata are for the purpose of permitting the ingress and egress of carbon dioxide and oxygen, though moisture must also pass out through them. In the thalloid liverworts, growing closely pressed upon moist earth, the danger of drying out through the openings alone would seem small. In leafy stemmed plants, this danger is greatly increased, and has necessitated the development of guard-cells. The latter consequently have charge of the secondary function of stomata, which is to regulate the amount of transpiration. The movements of the guard-cells are regulated by light and by the interaction of humidity and water content. Stomata open in strong light and close in weak light; consequently they show a periodic movement, opening in the morning and closing at night. When transpiration tends to exceed absorption, as in the case of a great decrease in humidity, the guard-cells close. This checks transpiration, and usually enables the roots to meet the deficiency. When this occurs, or when the water supply is renewed, as by a rain, the guard-cells open. 79. Movements of guard-cells. The movements of the guard- cells are brought about by changes in their turgidity. Stomata close when the plant becomes flaccid, i.e., when the plant is losing more water than it absorbs. They open again when the plant becomes turgid in response to increased absorption or decreased transpiration. Generally speaking, stomata are open for the exchange of gases, when the danger from excessive water loss is slight, and they remain closed when the danger is great. As a matter of fact, the closure is rarely quite perfect, so that some moisture escapes even when the stoma is closed. The mechanism by which the stomata open and close is a simple one. It is most readily understood by comparing the top of a stoma with a cross-section of one. The thinnest wall of each guard-cell is the one next the epidermal cell, the others, particularly 62 PLANT PHYSIOLOGY AND ECOLOGY the upper and outer, being more or less thickened. Since the guard- cell is active, it has a relatively high osmotic pressure, and draws water readily from the adjoin- ing epidermal cells. The result is to cause the inner walls of each guard-cell to become more and more convex. Since the guard- cells are firmly joined to each other at the ends, the increased Fic. 16.—Diagram of the stoma of turgidity forces them apart in Helleborus. The shape of the guard- the center, as though each were cells when the stoma is open is shown : by the heavy lines. ‘The thick outer pulled by astring attached to the aN Aten Sthooada) | taiddle of the immer maleimee long as the plant remains fully turgid, the stomata stay open, except of course for the regular closing at night. When the water loss tends to become excessive, or the water supply deficient, the osmotic pressure of the epidermal cells exceeds that of the guard-cells. Water is withdrawn from the latter, the inner wall becomes less convex, and releases the strain upon the two guard-cells, which close in consequence. Closure in its turn is maintained until the usual turgidity is re- stored. In many cases it is probable also that the epidermal cells adjoining the stomata aid in this process by their shape and move- ment. _Experiment 17. Movement of guard-cells. Strip a small piece of _ epidermis from the leaf of a turgid and of a wilted sunflower. Immerse the strips for a few minutes in a killing solution containing osmic acid (preferably Flemming’s solution), wash and examine the stomata under the high power. Remove fresh strips from the same sunflowers. Place the turgid epidermis in a 5% solution of common salt and the flaccid one in distilled water. Examine under the high power and explain the results. Make a schematic drawing of a stoma when open and when. closed. Experiment 18. Position of stomata and water loss. Select four similar leaves of sunflower or, better, of a plant which has stomata only on the lower surface. Cover the upper side of one leaf with wax melting at a low temperature. Wax the lower surface of the second and both surfaces of the third. The fourth leaf is not coated, in order to serve as a check. Fix each leaf in a small vial filled with a known amount of water, in such fashion that the water lost is supplied through the petiole. Determine the loss from each leaf by weighing or measuring the water ADJUSTMENT TO WATER 63 in the vial. Remove the leaves from the vials and premit them to dry out, noting the time required in each case. 80. The influence of physical factors upon transpiration. Three factors, humidity, water content, and light, affect trans- piration directly. Their influence is seen not only in their ability to cause the stomata to open and close, but also in determining the rapidity of transpiration when the stomata are open, and, indeed, though in much smaller measure, when the latter are closed. We have already seen that light causes stomata to open in the morning and close at night, thus resulting regularly in less transpiration at night than during the day. In addition the greater part of the light energy absorbed by the chloroplast, usually more than 95%, is converted into heat, and produces water loss or, as it is sometimes called, chlorovaporization.