HARVARD MEDICAL LIBRARV IN THE Francis A.Countway Library of Medicine BOSTON 4^ Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons and Harvard Medical School http://www.archive.org/details/experimentalmorpdave2 EXPEEIMENTAL MORPHOLOGY 'lY^^^o Experimental Morphology BY CHARLES BENEDICT DAVENPORT, Ph.D. IXSTRUCTOR IX ZOOLOGY IX HAETAED UXIVERSITY PAPvT SECOND EFFECT OF CHEMICAL AXD PHYSICAL AGENTS UPON GROWTH !K"cto Horfe THE MACMILLAN COMPANY LOXDOX: MACinLLAN & CO., Ltd. 1899 All rights reserved Copyright, 1899, By the MACMILLAN COMPANY. Norinooli Press J S. Gushing & Co. - Berwick & Smith Norwood Mass. U.S.A. PEEFACE TO PAET II Develop:mext consists of gro-^tli and differentiation, accom- panied in the larger organisms by nuclear- and cell-division. The present Part deals -u-itli growth. The importance of the study of growth cannot be over- estimated, and it is a cause for wonder that the treatment of the subject has been so much neglected by text-books. Indeed, it is a surprising fact that it has not been thoroughly and svs- tematicallv investigated. For. in last analysis, the maintenance of the human race depends upon that property which protoplasm among all substances alone displays of increasing itself for an indefinite time and to an indefinite amount. And the possibil- ity of increasing the human race beyond limits that are not far off depends upon a better knowledge of the conditions of growth. The reader has only to consider that the world's supply of 2500 million bushels of wheat, 2000 million bushels of maize. 90 million tons of potatoes, and its untold millions of tons of beef. pork, and fish are reproduced each year by growth. The mineral matters of the soil are being washed out into the sea and are largely lost, but the capacity of growth under appropriate conditions is never lost : it redoubles as the amount of the growing substance is increased. The only thing, then, which limits growth is the limitations in the conditions of growth. What are these conditions? This is the important question to which attention has been directed in this Part. Aside from this practical interest, the study of growth is important as bearing on the question of the dependence of xital activities, and especially development, upon external conditions, xa xb ■ PREFACE and the possibility of the control of development by appropri- ately altering those conditions. Growth phenomena show them- selves, indeed, particularly susceptible to this control, and are consequently especially valuable for experimental study. In the preparation of the Second Part, I have been put again under heavy obligations to my friend and colleague, Dr. G. H. Parker, who has read most of the manuscript and made important suggestions. I am also indebted to Dr. H. E. Saw- yer for reading Chapter XI in the manuscript, and to my wife for much painstaking work on the manuscript and proofs and for compiling the index. C. B. D. Cambridge, Mass., Dec. 11, 1898. COXTEXTS CHAPTER X PAGE Introduction : Ox Xormal Growth 281 CHAPTER XI Effect of Chemical Agents upon Growth § 1. Effect of Chemical Agents upon the Rate of Growth . . . 293 1. The Materials of which Organisms are composed . . . 29i a. Analysis of the Entire Organism 296 b. Detailed Account of the Various Elements used as Food 304 Oxygen, 304 ; Hydrogen, 306 ; Carbon, 306 ; Xitro- geu, 307 ; Phosphorus, 313 ; Arsenic, Antimony, and Bismuth, 311; Sulphur, 311; Chlorine, 316; Bromine, 316 ; Iodine, 317 ; Fluorine, 317 ; Lith- ium, 318; Sodium, 318; Potassium, 318; Rubidium and CcBsium. 320; Strontium, 321 ; Manganese, 321 ; Iron, 321 ; Magnesium, 323 ; Silicon, 321 ; Copper, 324. 2. The Organic Food used by Organisms in Growth . . . 324 a. Fungi .......... 324 b. Green Plants 326 c. Animals . • . . . . . . . . . 327 Amoeba, 328 ; Amphibia, 329 ; Mammals, 330. 3. Growth as a Response to Stimuli ...... 331 a. Acceleration of Growth by Chemical Stimulants . . 331 b. The Election of Organic Food 333 § 2. Effect of Chemical Agents upon the Direction of Growth — Chemo- tropism 335 1. Chemotropism in the Tentacles of Insectivorous Plants . 335 2. Chemotropism of Roots ........ 336 3. Chemotropism of Pollen-tubes 337 4. Chemotropism of Hyphge 840 5. Chemotropism of Conjugation Tubes in Spirogyra . . 342 Literature 343 xvi CONTENTS CHAPTER XII The Effect of Water upon Growth PAeE § 1. Effect of Water upon the Rate and Quantity of Growth . . 350 § 2. Effect of Water on the Direction of Growth — Hydrotropism . 355 1. Roots 356 2. Rhizoids of Higher Cryptogams 357 3. Stems 358 4. Pollen-tubes 358 5. Hyphse of Fungi . 358 Literature 360 CHAPTER XIII Effect of the Density of the Medium upon Growth § 1. Effect of Density upon the Rate of Growth 362 Literature 369 CHAPTER XIV Effect of Molar Agents upon Growth § 1. Effect of Molar Agents upon the Rate of Growth .... 370 1. Contact 370 2. Rough Movements 370 3. Deformation . 372 4. Local Removal of Tissue . ' 375 § 2. Effect of Contact upon the Direction of Growth — Thigmotropism 376 L Twining Stems ......... 376 2. Tendrils 377 3. Roots 380 4. Cryptogams . . 381 5. Animals ' . . . .382 6. The Accumulation of Contact-stimulus and Acclimatization to it 382 7. Explanation of Thigmotropism 383 § 3. Effect of Wounding upon the Direction of Growth — Trauma- tropism 384 1. False Traumatropism . . . . . . . . 384 2. True Traumatropism 384 § 4. Effect of Flowing Water upon the Direction of Growth — Rheo- tropism . . . 387 Summary of the Chapter 388 Literature 389 CONTENTS XVll CHAPTER XY Effect of Gravity upon Growth PAGB § 1. Effect of Gravity upon the Rate of Growth 391 § 2. The Effect of Gravity upon the Direction of Growth — Geotropism 391 1. False Geotropism 392 2. True Geotropism 392 a. Roots .... 392 6. Stems .... 397 c. Rhizoma . ' . 398 d. Cryptogams 398 e. Animals .... 398 /. After-effect in Geotropism 401 Summary 402 Literature 403 CHAPTER XVI §1- §2. Effect of Electricity upon Growth Effect of Electricity upon the Rate of Growth . . . . 405 Effect of Electricity upon the Direction of Growth — Electro- 409 tropism 409 1. False and True Electrotropism 2. Electrotropism in Phanerogams 3. Electrotropism in Other Organisms 4. Magnetropism ..... 5. Explanation of Electrotropism and Summary Literature 409 411 412 413 413 414 CHAPTER XVII Effect of Light upon Growth § 1. Effect of Light on the Rate of Growth 416 1. Retarding Effect of Light 416 2. Accelerating Effect of Light 423 3. The Effective Rays 427 a. The Effective Rays in the Retardation of Growth by Light 427 h. TheEffectiveRaysinthe Acceleration of Growth byLight 432 4. The Cause of the Effect of Light on the Rate of Growth . 436 § 2. Effect of Light upon the Direction of Growth — Phototropism . 437 1. Plants 437 2. Animals 442 a. Serpulidae 442 6. Hydroids 443 xvm CONTENTS General Considerations . a. Persistence of Stimulation b. Acclimatization to Light c. Mechanics of Phototropism Literature PAGE 444 444 444 444 445 CHAPTER XVIII Effect of Heat on Growth § 1. Effect of Heat on the Rate of Growth .... 1. Plants 2. Animals . 3. Some General Phenomena accompanying Heat Effects a. Latent Period ....... b. Sudden Change of Temperature ■ c. Cause of Acceleration of Growth by Heat § 2. Effect of Heat on the Direction of Growth — Thermotropism 1. Effect of Radiant Heat . 2. Conducted Heat 3. Causes of Thermotropism Summary of the Chapter Literature ..... 4.50 450 457 460 460 460 461 463 463 464 466 467 467 CHAPTER XIX Effect of Complex Agents upox Growth, and General Conclusions § 1. The Cooperation of Geotropism and Phototropism . . . 470 § 2. Effect of Extent of Medium on Size 473 § 3. General Considerations relating to the Action upon Growth of External Agents 478 1. Modification of Rate of Growth 478 2. Modification of Direction of Growth — Tropism . . . 480 3. Adaptation in Tropisms ......... 484 4. Critical Points in Tropism 484 Literature ■ 486 List of Tables in Parts I and II 489 Index to Parts I and II 493 Part II THE EFFECT OF CHEMICAL AND PHYSICAL AGENTS UPON GROWTH CHAPTER X INTRODUCTION: ON NORMAL GROWTH Oegakic growth is increase in volume.* It is not develop- ment ; it is not differentiation ; it is not increase in mass, altliougli the latter may often serve as a convenient measure of growth. In analyzing the processes of growth in organisms we must recognize at the outset that organisms are composed of living matter and formed substance, and that growth may therefore result from the increase in volume of either of these. The living matter, in turn, is composed of two principal substances : the plasma and the enchylema or cell sap ; so growth may be due to the increase of either of these substances, — may result either from assimilation, or more strictly from the excess of * Growth has been variously defined. Thus Huxley has called growth *' increase in size," which is essentially the same as my definition. Sachs ('87, p. 404) defines growth as an increase in volume intimately bound up with change of form (" eine mit Gestaltveranderung innig verkniipfte Volumen- zunahme") ; and he illustrates the definition by the example of the growth of a sprout from its beginning to its full development. In this case two phenom- ena are distinguishable : first, increase in volume, and, second, the filling out of the details of form. As Sachs says, these phenomena taken together are gen- erally denominated " development " ; and it seems decidedly advantageous to retain this word with its usual signification, and to distinguish the two compo- nent processes by the terms growth and differentiation. Pfeffbr's ('81, p. 46) definition differs still more widely from the one pro- posed above. He defines growth as change in form in the protoplasmic body (" die gestaltliche Aendenmg im Protoplasmakorper " ) ; and he goes on to say that increments of volume and mass are not proper criteria of growth. Pfeffer illustrates this statement by the following example : A plant stem or a cell mem- 281 282 INTRODUCTION [Ch. X. the constructive over the excretory processes of the plasma, or from the taking in of water.* Of the three factors involved in growth — increase of formed substance, of plasma, and of enchylema — the part played by the last seems to me to have been underestimated. Plant physiol- ogists have been in the best position to acquire the facts. brane can be permanently elongated by extension beyond the limits of elasticity without the volume necessarily increasing ; — and he apparently means to include such an artificial defoi-mation in his definition of growth. "And," he continues, "under certain circumstances a diminution of volume of a plant seg- ment can indeed occur as a result of growth, when, for example, the elasticity of the wall is increased by growth and water is pressed from the cell until equi- librium is restored." It may be doubted, however, if Pfeffer would say that in this case the cell, as a whole, had grown ; but if he would, then his definition is a wide departure from ordinary usage. Also, Vines ('86, p. 291) offers a definition, which is intermediate between that of Sachs and that of Pfeffer. "By growth," he says, "we mean per- manent change of form, accompanied usually by increase in bulk." But then he goes on to say, "Nor does this increase even of the organized structures of an organ, that is of the protoplasm and the cell wall, necessarily imply that it is growing. Thus, an increase of the cell wall may take place without any appre- ciable enlargement of the cell, as, for instance, when a cell wall thickens." But since the thickening is a " permanent change of form," it should be consid- ered by the author a growth process were not increase in size of the cell after all, in the author's mind, the most important criterion of its growth. Pinally, Prank ('92, p. 355) finds no other criterion for growth than an increase in vol- ume (dependent, however, upon the increase of a particular substance). Thus, with these different plant physiologists, we see the word growth bearing the ideas of Increase of volume and differentiation, then of differentiation alone, and, finally, of increase of volume alone. * Various analyses of the process of growth have been made by different authors. Let us look at a few of these opinions. Says Huxley ('77, p. 2), " growth is the result of a process of molecular intussusception." According to N. J. C. MiJLLER ('80, p. 100), "all phenomena of growth depend, in last analy- sis, upon this, that the molecule of the solid substance is introduced into the region of growth." Prank ('92, p. 355) understands by growth that increase of volume which consists of the apposition or intussusception of new solid mole- cules of similar matter ("welche auf der An- oder Einlagerung neuer fester Molekule gleichartigen Stoffes beruhen "). According to Verworn ('95, p. 475), growth is due to the excess of assimilation over disassimilation. These defi- nitions include what I regard as only half the process of growth. On the other hand, Driesch ('94, p. 37) distinguishes two kinds of cell growth: (1) passive growth, due to imbibition of water, and (2) active growth, resulting from assimilation. This classification agrees with the one I have pro- posed, but I think the term passive growth very inapt, since the imbibition of water is as truly an active process as any other vital activity. Int.] OX XOEMAL GROTTTH 283 They have recognized in the tip of the plant three growth regions. At the extreme tip of the stem (or radicle) is the region of rapid cell division but comparatively slo^v growth ; next below is the zone exhibiting the Grand Period of growth; and still below is the zone of histological differ- entiation (Fig. 75). In the first zone growth of plasma is occurring ; in the second zone growth of the enchylema is chiefly taking place; in the third zone there is growth of formed substance. The immense preponderance of the growth of the second period (at 7 days) is an index to the preponderat- ing influence in growth of the imbibition of water. Id MM. A 10 /\ MM. 1 \ 0 / \ \ 1 MM. ^ \ Fig. 75. — Curve of daily growth iu length of a disc, originally 1 mm. long, and taken immediately behind the vegetation point of a radicle of Phaseolus. It comes to occupy in successive days the three zones referred to iu the text. From Sachs, Lectures on Plant Physiology. ■ i ■ ' : 1 ' 1 ' 1 ! \ 1 i III! 1 1 /;"\i 1 I ! ./<. \ 1 ' y/\ ^- ' ■ 1 // 1 \ ! z,-'-^-/ \ /T''- / \ \ K 1 // / \ ^ k \ // 1/ \ \ // / \ 1 ]\ I • i /-' / \ \ 1 1 \y / ^^x 1 .^y^ , \. \ \ c^S^^""^ ! i ■--- , Hi ...-\s. ___._._. 0.5 LO 1.5 2.0 2.5 3.0 3.5 1.0 4.5 5.0 5.5 6.0 6.5 7.0 Fig. 76. — Curve representing the intensity of gro"n-th of roots of Pisum sativum, ; Vicia sativa, ; and Lens esculenta, , the time being assumed to be constant. The length of the abscissae in the direction from left to right corresponds to the distance, in millimeters, of the marked spaces on the root from the root apex. The ordinates correspond to the amount of growth, in milli- meters, of the corresponding piece of the root after 20 hours. From Ciesielski ('72.) That which occurs with one and the same piece of the stem on successive days takes place simultaneously at the different zones of the growing organ. Thus in the radicle (Fig. 76) 284 INTRODUCTION [Ch. X. 90?i / / I I I IJ I I V ^ I Fig. 77. — Curve showing the percentage of water in suc- cessive internodes of hothouse plants of Heterocen- tron roseum Hook, et Arm., about 4 decimeters high. The ordinates indicate the percentage of water at eacli internode from the terminal bud (I) to the fifth (VI). (From Kraus, '79.) we find during a period of 20 hours little growth occurring at the root tip, a maximum of growth at 3 or 4 millimeters from the tip, and further up less growth, until a zone of almost no growth is reached. An analysis of the substance of the stem at different levels below the tip reveals the same thing — a sudden increase in the amount of water from 73% at the tip to 88% at the first internode (II), reaching a maxi- mum at 93% in the second internode (III), then falling slightly (92.7) to the fifth internode (VI, Fig. 77). The experiments and ob- servations upon which these conclu- sions rest thus agree in assigning the chief role to water in the growth of plants. While the fact that water constitutes a large proportion of the growing animal was made known by the classical researches of Baudkimont and Martin Saint-Ange ('51), the impor- tance of the part which it plays in the growth of animals seems first to have been appreciated by Loeb ('92, p. 42), who showed how in the withdrawal of water by plasmolytic meth- ods growth was interfered with. Later I made a series of determinations of the relative part played by water and dry substance in the growth of an animal (tadpole). Eggs and embryos at various ages were weighed after removal of super- ficial water. Then they were kept in a desiccator from which air had been pumped and which contained a layer of sulphuric acid to absorb moisture. After repeated weighings a condition was found in which the drying mass lost no more water (con- stant weight). The total diminution in weight indicated the mass or volume of free water contained in the organism at the beginning of the experiment. Numerous weighings were Int.] ON NORMAL GROWTH 285 made during two seasons upon Amblystoma, toads, and frogs. All series showed the same thing ; the most complete series is that given in the following table : — TABLE XXII Embrtos of Frogs. 1895* Date. Days after Hatching. Average Weight, in Mg. Weight of Dry Substance, IN Mg. Weight of Water in Mg. %0F Water. May 2 1 1.83 .80 1.03 56 " 3 2 2.00 .83 1.17 59 " 6 5 3.43 •80 2.63 77 " 8 7 5.05 .54 4.51 89 " 10 9 10.40 .72 9.68 93 " 15 U 23.52 1.16 22.36 96 June 10 41 101.0 9.9 91.1 90 July 23 84 1989.9 247.9 1742.0 88 These results are graphically represented in Fig. 78. The curve and table show that, exactly as in plants, there is a Da.y.i. 10 ZO 30 4<3 SO 60 /O SO 9o Fig. 78. — Graphic represeutation of last column of Table, showing percentage of water in frog embryos from 1 to 84 days after hatching. Compare with Fig. 77. period of slow growth accompanied by abundant cell division — the earliest stages of the Qg^- Then follows, after the first * Compare with, the less complete table of Baudkimont and Martin Saint- Ange, '51, p. 532. 286 INTRODUCTION [Cii. X few hours, a period of rapid growth due ahnost exclusively to imbibed water, during which the percentage of water rises from 56 to 96 ; lastly comes the period of histological differen- tiation and deposition of formed substance, during which the amount of dry substance increases enormously, so that the per- centage of water falls to 88 and below. But the growth is due chiefly to imbibed water. The foregoing facts thus unite in sustaining the conclusion that at the period of most rapid growth of organisms growth is effected by water more than by assimilation. In later development the proportion of water slowly falls. This fact is vs^ell brought out in the following tables : — TABLE XXIII Showing the Percentage of Water in Chick Embryos at Various Stages UP to Hatching, from Potts, '79 Hours op Brooding. Absolute Weight in Gram. % Water. 48 0.06 83 54 0.20 . 90 58 , 0.33 88 91 1.20 83 96 1.30 68 124 2.03 69 264 6.72 59 TABLE XXIV Showing the Percentage of Water in the Human Embryo at Various Stages up to Birth, from Fehling, '77 Age in Weeks. Absolute Weight in Gram. % Water. 6 0.975 97.5 17 36.5 91.8 22 100.0 92.0 24 242.0 89.9 26 569.0 86.4 30 924.0 83.7 35 928.0 82.9 39 1640.0 74.2 IxT.] OX XORMAL GROWTH 287 These results indicate that during later development growth is largely effected by excessive assimilation or by storing up formed substance. From another standpoint we can recognize two kinds of gro^vth : one a transitory growth, after which the enlarged organ may return again to its former size, and the other a pe/-- nnanent or developmental growth, which is a persisting enlarge- ment, and plays an important part in development. As an example of transitory growth may be cited the case of the Sen- sitive Plant, whose leaflets when touched turn upwards as a result of the growth of cells on the convex side ; but this enlargement is only temporary — it is transitory growth. This phenomenon is indeed usually not included in the idea of growth ; yet it is well-nigh impossible to draw a sharp line of distinction between it and permanent growth. For example, when a tendril of the Passion Flower is touched it may curve as a result of growth of the cells on the convex side, and this curvature may later become obliterated, as in the case of the Sensitive Plant ; but the longer the contact is continued the more the cells enlarge, and the more their walls become perma- nently modified. Thus the condition of temporary growth shades insensibly into that of permanent growth. ' So far as possible we shall consider in this book only developmental growth. Still another classification may be made of the phenomena of growth. We may distinguish between diffused and localized growth. In diffused growth the entire individual or many of its parts are involved. In localized growth the process is con- fined to a limited region. Thus in the early development of the frog diffused growth occurs, while in the formation of the appendages we have an example of localized growth. Since localized growth is an important factor in differentiation, many of the data concerning this phenomenon will be first considered in the Part dealing with Differentiation. Normal growth may or may not be accompanied by cell-divi- sion. But usually cell-division occurs sooner or later in the growing mass. The act of cell-division seems to retard the process of growth. This conclusion follows from some experi- ments of Ward ('95, p. 300) on the growth of bacteria, which 288 INTRODUCTION [Ch. X are summarized in the curve, Fig. 79. This shows how the growtli in length of, the bacterial rods is delayed at intervals PERIOD OF MINM. GROWT &. WHEN FIRST CELL DIVISION OCCURRED. ;;:; second period of minm. growth,^ when 2d cell division occurred. Fig. 79. — Curve of growth of a bit of a filament of Bacillus ramosus, 27.30 M long at the beginning and 70.88 /x at the end of the period of observation. The curve shows certain periods of diminished growth (indicated by tlie arrows below the curve), which correspond to cell-division. From Wakd ('95, p. 300). by the nuclear divisions and the accompanying formation of transverse septa.* The course of normal growth may now be studied in 20 MM. ^ ^ -^ / 10 MM. / ' / 20 40 CO _ 80 100 Fig. 80. — Curve of length of shell of Lymnsea stagnalis at intervals from hatching up to 85 days. From Semper, Animal Life, p. 163. * Attention may here be called to a phenomenon which has repeatedly been observed when a single growing mammal has been weighed at regular intervals. This is a sort of alternation of periods of unusually rapid growth with periods of diminished growth, the interval being a day or two. There is an irregu- larity in the length of these periods. See Saint- Lou p, '93 ; compare also MiNOT, '91, Table XIV. Int.] OX XORMAL GROWTH 289 6°/ the case of certain selected, typical organisms. This may be most quickly done by the use of curves whose abscissae represent time intervals and whose ordinates represent size or weight. Figures 80, 81, and 82 are such curves. In all cases excepting that of guinea pig (in which the curve represents the growth of only a comparatively late developmental period, namely, from birth onward) the curves exhibit one characteristic shape. The absolute increments are not, however, shown directly by these curves. To obtain them one must transform the curves into others in which the successive ordinates shall represent the absolute incre- ment of weight over the last preceding. Under these circumstances the absolute increments rapidly reach a maximum from which they decline to zero.* AYhy does the growth de- cline to zero ? The theory has been suggested f that there is a " certain impulse given at the time of im- pregnation which gradually fades out, so that from the beginning of the new growth there occurs a diminution in the rate of growth." The facts of ' ' 1 11 / i >^ / 1 / /u ' N \ ■\. N A- a 5 10 Mos . Fig. 81. — The continuous line (a) represents the weights in fractions of a kilogramme attained by guinea pigs from birth until 12 months old. The broken line (&) rep- resents the daily percentage increments (% 's at the right) of the same guinea pigs up to 7 months. After Minot ('91). * Another method of representing curves of growth has been proposed by Professor Minot ('91, p. 148), who argues that for a given period the rate of growth should be expressed as the fraction of weight added during that period ; for, he says, "the increase in weight depends on two factors: first, upon the amount of body substance, or, in other words, of growing material present at a given time ; second, upon the rapidity with which that amount increases itself." Such a curve of percentage daily increments is given for the guinea pig in Fig. 81. Since, however, the greater part of the " body substance " at its period of greatest growth is not " growing material," as assumed, but water, the peculiar value of the curve of percentage increments is doubtful. t By MixoT, '91, p. 151. u 290 INTRODUCTION [Ch. X > / I I I / I / f 7 ' I f I I / / / / / / / 1 — _ Lk ^ / / / y' growth in the tip of the phxnt do not, however, support this theory, for the protoplasm at this point may go on growing for centuries, as we see in the case of trees. Some of the protoplasm at the tip is, how- ever, constantly falling back to form part of the stalk : this part soon ceases to grow, undergoing histological dif- ferentiation. The reason why the animal ceases at length to grow is the same as the reason why the differ- entiated tissue below the tip of the epicotyl ceases to grow — not because there is a nec- essary limit to growth force at a certain distance from impregnation, but because it is in the nature of the species that the individual should cease to grow at this point. The indefinite growth of this part, the limited growth of that, are as much group characters as any structural quality. To recapitulate briefly : Growth is increase in size, and may result from increment of either the formed substance through secretion, the plasma through assimilation, or the enchylema through imbibition. This increment may be either transitory or permanent ; the latter class chiefly concerns us here. Growth may be either diffused throughout the entire organism, or local, forming a factor of differentiation. In normal growth the increase is at first slow, then rapidly increases to a maxi- mum, and, finally, in most animals, diminishes to zero. This final cessation is a special quality of certain organisms, to be explained like structural qualities, on special grounds. DAYS 2 4 0 8 10 Pig. 82. — Curves of growth of Phaseolus » multiflorus (coutinuous line) and Vicia faba (broken line). The ordinates rep- resent actual lengths attained on the respective days by a bit of stem origi- nally 1 mm. long. After Sachs, Lect- ures on Plant Physiology. Int.] ON NORMAL GROWTH 291 0 1 3 3 1 5 6 7 8 KG. / / 21 20 19 18 17 16 15 14 13 13 11 10 9 8 7 € 5 4 3 / / ,/ / / / / /' y V y v / / / / / /^ A / / f , / / / / / / 1 1 J _y / 01 2 3 4 5 6 7 8 years Fig. 83. — Curve of growth of man. The ordinates represent weight in kilogrammes. The prenatal part of the curve is constructed from the data of Fehling ('77) ; the postnatal part from Quetelet ('71) for the male. The curve of the first twelve months of postnatal life is adapted from Galton ('84). LITERATURE Baudrimont, a., and G. T. Martin Saint-Axge, '51. Recherches anato- miques et physiologiques sur le developpement du foetus et en parti- culier sur revolution embryonnaire des oiseaux et des batrachiens. Mem. presentes par divers savants a I'Acad. des Sci. de I'lnst. Nat. de France. XI, 469-692. 18 pis. 292 INTRODUCTION [Ch. X Dkiesch, H. '94. Analytische Theorie der organischen Entwicklung. Leip- zig. Engelmann, 185 pp. 1894. Fehung, H. '77. Beitriige zur Pliysiologie der placentaren Stoffverkehrs. Arch. f. Gynakologie. XI, 523-557. Frank, A. B. '92. Lehrbuch der Botanik nach dem gegenwartigen Stand der VVissenschaft bearbeitet. I. Band. Leipzig. 1892. Galton, F. '84. Life History Album. 172 pp. London. 1884. Huxley, T. H. '77. A Manual of the Anatomy of Invertebrated Animals. 698 pp. London. 1877. Kraus, G. '79. Ueber die Wasservertheilung in der Pflanze. Festschr. z. Feier des Hundertjahrigen Bestehens d. Natm-f. Ges. in Halle, pp. 187-257. LoEB, J. '92. Untersuchungen zur physiologischen Morphologie der Thiere. II. Organbildung und Wachsthura. 82 pp. 2 Taf. Wurzburg. 1892. MixoT, C. S. '91. Senescence and Rejuvenation. Jour, of Physiol. XII, 97-153. Plates 2-4. MiJLLER, N. J. C. '80. Handbuch der allgemeinen Botanik. I. Theil. Heidelberg. 1880. Pfeffer, W. '81. Pflanzenphysiologie. Engelmann. Leipzig. 2 Bde. 383 + 474 pp. Poll, R. '79. Untersuchungen Uber die chemischen Veranderungen im Hiihnereiwahrendder Bebrutung. Landwirth. Versuchs-Stat. XXIII, 203-247. QuETELET, A. '71. Anthropometrie ou mesure des differentes facultes de riiomrae. 479 pp. 2 pis. Bruxelles and Paris. 1871. Sachs, J. '87. Vorlesungen iiber Pflanzenphysiologie. Leipzig. Engel- mann. 884 pp. 1887. Saint-Loup, R. '93. Sur la vitesse de croissance chez les Souris. Bull. Soc. Zool. de France. XVIII, 242-245. Yerworn, M. '95. Allgemeine Physiologie. 584 pp. Jena : Fischer. 1895. Vines, S. H. '86. Lectures on the Physiology of Plants. Cambridge [Eng.j Univ. Press. 710 pp. 1886. Ward, H. M. '95. (See Chapter XVII, Literature.) CHAPTER XI EFFECT OF CHEMICAL AGENTS UPON GROWTH We shall consider this subject under two heads : (1) Effect upon the rate of growth, and (2) Effect upon the direction of growth. § 1. Effect of Chemical Agents upon the Rate of Growth Organic growth, occurring in a material composed of water, plasma, and formed substance, consists in the increment of each of these components. The means and results of varying the quantity of water in the organism will be discussed in the next chapter ; here we are to consider the results of assimilation, including the production of formed substance. The scope of our work may be more precisely defined as the answer to the question, What role do the various chemical substances (exclud- ing water) play in the metabolic changes involved in growth ? There are two roles played by chemical substances in the body ; and, accordingly, we may distinguish two kinds of chemical agents having diverse effects upon growth. These agents — foods, in the widest sense of the word — must supply the material — the atoms — from which the molecules of the plasma, or of its formed substance, are made up ; and, sec- ondly, they must supply energy for metabolism. Foods, then, yield to the organisms matter and motion ; they are hylogenic (plastic) and thermogenic (respiratory). These two offices of chemical agents in growth are only in certain cases exerted by distinct kinds of food . In the case of animals, sodium chloride and iron compounds are examples of wholly plastic foods, while the free oxygen taken into the body is chiefly thermogenic. In the case of the free oxygen, how- ever, it is quite probable that it is sometimes used in the con- struction of the molecule of active albumen which, according 293 294 EFFECT OF CHEMICAL AGENTS [Ch. XI to LoEW, constitutes the essential living substance. For exam- ple, oxygen performs this office in LoEw's ('96, p. 39) hypoth- esis of the formation of albumen in plants from the nitroge- nous products which result from the action of an enzyme upon the reserve proteicls, i.e. leucine. Thus leucine formaldehyd C6H13NO2 + 70 = 2C02-t- H2O + 4 CH2O + NH3. formaldehyd asparagln 4 CH2O + 2 NH3+ O2 = C4H3NA+ 3 H2O. Now since, as Loew makes probable, asparagin is a stage in the production of albumen, the free oxygen molecule may be essential to the synthesis of the living substance. The facts indicate that the plastic and thermogenic functions of foods- are inextricably intermingled — that both are exhib- ited in the mutations of the living substance as well as in the respiratory processes. Thus, on the one hand, assimilation is accompanied not only by endothermic (energy-storing) but also by exothermic (energy-releasing) processes ; while, on the other hand, the partial oxidation of the food proteids may be a necessary step towards assimilation. It is because the net result is the storing or the release of energy that we may speak of any complex of processes as enthodermic or exothermic, and certain foods as plastic or thermogenic. The source of energy in organisms is not, however, solely food. Energy may likewise be derived directly from energy in the environment. This source is of greatest importance in the case of chlorophyllaceous organisms, but it is probably not of importance for them alone. For the heat and light of the environment aid, as we have seen (pp. 166-171, 222-225), various metabolic processes in all kinds of protoplasm. 1. The Materials of. which Organisms are Composed. — To determine the sorts of materials which plastic food must supply to the body, it will be instructive to consider the proportional composition of the body out of water and dry substance, both organic and inorganic. Such data, gathered from various animals and plants, are given in the following table : — §1] UPON THE RATE OF GROWTH 295 TABLE XXV Animals Species. ^1 % 0 Matter TO : Weight. B .J O 0 Matter to UliSTANOE. a to >■ s AtTTHORITY. ^ S 3 < o < z; ^ S 1 O G < Sponges, average .... 79.4 11.0 9.7 54.1 45.9 Kefkenbeeg, 80 Medusce. : Khizostoma cuvieri. . . 95.4 1.6 3.0 34.8 65.2 (( Actiniaria : Anthea cereus 87.6 10.7 1.6 87.0 13.0 c; Actinia mesembryanth. 83.0 15.4 1.7 90.0 10.0 C( Sagartia troglodytes . . 76.8 20.9 2.3 81.1 18.9 (( Cerianthus membran. . 87.7 11.6 1.7 87.2 12.8 (( Alcyonium palmatum . 84.3 10.8 4.9 68.8 31.2 (( Asteroidea : Astercanthion glacialis . 82.3 14.1 3.6 79.6 20.4 (S Annelida : Lumbricus complanatus 87.8 9.7 2.4 80.2 19.8 (( Crustacea : Oniscus murarius .... 68.1 21.2 10.6 66.6 33.4 Bezold, '57 Squilla mantis 72.0 22.1 5.9 78.9 21.1 Kkukenbeeg, '80 Astacus fluviatilus . . . 74.1 16.8 9.1 64.9 35.1 Bezold, '57 Mollusca : Doris tuberculata .... 88.4 9.0 2.6 77.6 22.4 KRL'EEJfBEEG, 'sa Doriopsis limbata .... 86.5 12.4 1.1 91.9 8.1 U Arion empiricorum . . . 86.8 10.1 3.1 76.5 23.5 Bezold, '57 Limax maximus .... 82.1 16.4 1.5 91.6 8.4 u Ostrea virginiana .... 88.3 10.8 0.9 92.3 7.7 (without shell) Tunicata : "Rnt.Tvllns 93.6 3.1 3.3 48.4 51.6 Keueenbeeg, 'sa JJTJUX V A1U.O ......... Vertebrata : Cyprinus auratus .... 77.8 17.6 4.6 79.1 20.9 Bezold, '57 Triton igneus 80.2 16.1 3.7 81.1 18.9 Triton cristatus 79.6 17.0 3.4 82.9 17.1 Bombinator igneus . . . 77.3 19.4 3.3 85.2 14.8 Bafo cinereus 79.2 14.8 6.0 71.1 28.9 Rana esculenta 82.7 14.2 3.0 82.2 17.8 Angius fragilis 55.0 32.1 12.9 71.5 28.5 Lacerta viridis 71.4 2.3.2 5.4 81.3 18.7 Sparrow 67.0 67.5 27.8 27.4 5.2 5.0 84.3 85.2 15.7 14.8 Bat Mouse 70.8 65.7 25.7 29.6 3.5 4.7 88.0 86.3 12.0 13.7 VOLKilANN, '' Man 4 — 296 EFFECT OF CHEMICAL AGENTS Plants [Ch. XI Oats, in blossom .... 77.0 6.4 16.6 27.8 72.2 Wolff, '65 Wheat, in blossom . . . 69.0 9.3 21.7 30.0 70.0 a Pea vines, green .... 81.5 4.8 13.7 26.0 74.0 li, a. Analysis of the Entire Organism. — We are now ready to consider the atomic composition of the dry substance of organ- isms. VoLKMANN ('74) has contributed data on this subject in the case of man. Thus the dry substance gives : — c 52.9% o 18.5% H 7.7% N 7.4% Ash 13.4% In the case of a plant (stems and leaves of dry clover) we have, according to Johnson : — C 0 H N S P Eemaining ash 47.4% 37.8% 5.0% 2.1% 0.12% 0.30% 2.0% These two determinations, fairly typical of the higher plants and the higher animals respectively, run nearly parallel. The greatest difference is shown by the nitrogen, which is more than three times as abundant in animals as in plants. Oxygen, on the other hand, is more abundant in plants. The ash, in turn, must be further analyzed. The following table gives the percentage composition of the ash of various organisms : — TABLE XXVI Animal. New-bokn Dog. Menhaden (Fish). Oat Plant. Authority. BUNGE, '89. Cook, '68, p. 498. AvENDT (Vines, '86, p. 129). CaO 29.5 40.0 12.1 P2O5 39.4 35.8 8.8 K2O 11.4 7.1 45.9 CI 8.4 3.1 6.1 NazO 10.6 4.7 2.32 MgO 1.82 3.1 4.12 PeaOs 0.72 0.54 SiOz 6.1 17.2 SO3 2.86 §1] UPON THE KATE OF GROWTH 297 It would be valuable to kno^y the relative number of atoms of each of the metalloids and metals named in the preceding table. This may be determined in the following way : Find the proportion by weight of each metallic radical (excluding the oxygen) in the entire ash and divide this percentage by the molecular weight of the metal. The varying weights of the elements are thus eliminated, and a set of numbers which indi- cate relative abundance of atoms is obtained. We multiply these small fractions by 1000 for convenience. The results are as follows : — TABLE XXVII >£w-BOKx Dog. Menhadex (Fish). Oat Plaxt. Ca 528 , 715 216 P 555 506 124 K 243 151 974 CI 24 174 173 Na 343 150 60 Mg 45 77 114 Fe 9 — 6 Si — 101 287 S — — 36 These examples may suffice to show how diverse is the com- position of different organisms and how diverse therefore must be their requirements in the Avay of food to build up the adult bod}-. The examples serve also to indicate what are the important elements for organisms. They appear to be the same for animals and plants, and are : — *Carbon *Calcium Sodium *Sulphur *Oxygen *PotassiuirL Chlorine Silicon *Hydrogen *Phosphorus *]\Iagnesium *Iron *Nitrogen While this list does not pretend to exhaust the elements found in organisms, it contains those wliich are usually present. In the organism the atoms just named are, of course, found in combination. The carbon, oxygen, hydrogen, and nitrogen are contained in the organic matter of the organism — the * Those elements which are starred (*) are essential to all organisms. 298 EFFECT OF CHEMICAL AGENTS [Ch. XI greater part of the entire dry matter. The relations of the remaining elements are largely obscure. Some of them form inorganic acids in the body, such as hydrochloric and sulphuric acids. Others form inorganic compounds deposited in the body as supporting or protective substances, such as the calcic phos- phate and calcic carbonate of bone and spicules and the silicic oxide of plants. But there can be little doubt that a large and highly important part of these elements is built up into organic molecules and in this position plays a weighty and varied part in metabolism and growth. As examples of the way in which the metals and metalloids occur in the organic molecule I may cite a few cases in which the molecular structure has been deter- mined more or less satisfactorily. Thus we have sulphur in albu- men, 0^211^^2^188022 ; iron in hematin, Cg^HggN^FeOg ; phos- phorus in lecithin, 042^^84^-^09' ^^^^ nuclein, 029H49NgP30225 magnesium in chlorophyllan, and various halogens in the urates of sodium, calcium, and lithium. In discussing as we shall immediately the importance of each of these elements for the formation of the body we shall find additional facts con- cerning the importance of the metallic atoms in the organic molecules. There is good reason for believing that the pecul- iar properties of haemoglobin depend upon its iron and that the characteristic properties of nuclein depend largely upon its phosphorus and (as the later investigations indicate) its iron also. As our knowledge develops, the importance to many organic molcules in the living body of metallic or metalloid elements becomes clearer; the "ash" of the body is not some- thing accidental, secondary, or superfluous, but is an essential part of the organism. We must now consider the source of the various elements necessary to nutrition. Botanists early determined that 0, H, and O enter the organism through the carbonic acid and water which green plants absorb. Ooncerning the source of nitrogen there lias been less certainty ; it is generally believed to come from nitrates in the soil, but that matter will be considered later in more detail. Another source of the more characteris- tically organic elements, 0, H, N, and O, is, without doubt', organic compounds of various sorts. Although it has long been known that insectivorous plants absorb the organic mat- § 1] UPON THE RATE OF GROWTH 299 ters they digest, still it is only recently that the fact that green plants in general can make use of organic compounds has been recognized. Bokorny ('97) has lately brought together the evidence which makes this conclusion certain. It appears that in the presence of organic solutions some algre may grow for half a year in the absence of carbon dioxide ; and the potato plant may, even in the dark, store up starch in its tubers in the presence of rich organic food. Nitrogen may also be gained from amido-bodies ; thus, Bassler ('87) found that his maize cultures grew better in asparagin, CO2H . CH(NH3) . CH2 • C0(NH2), than in potassium nitrate. Thus plants may gain their C, H, O, and N from either organic or inorganic sources. Non-chlorophyllaceous organisms, on the other hand, have long been known to gain their C, H, N, and O from organic ■> [ ■i^^i^s'' compounds ; indeed, it has gen- P 1 ^ i '^*'^"'* *■ erally been believed that they can gain those elements from organic compounds only. Cer- tain observations, however, Fig. 84. - a, Nitrosomonas europsea (ni- throw doubt upon the entire trite bacteria from Ziirich) ; 6, Nitro- j_ J- jt • 1 T r J^^ somonas iavensis (nitrite bacteria correctness of this belief ; these ^^^^ j^^.^^ . ^^ ^itrobacter (nitrate are especially the remarkable bacteria from Quito). Magnified lOSO. results gained by Winograd- ^^t^"" winogradsky, from Fischek, . .„ . , Yorlesungen iiber Bakterien, 1897. SKY ( 90) from nitriiymg bacte- ria (Fig. 84). This author found that the bacteria could grow in a mixture of inorganic salts free of organic matters. Solutions free from organic matter were prepared by the following means : The culture vessels were cleaned by boiling in them sulphuric acid and potassium permanganate. The water used in the cultures and in washing the vessels was twice distilled in vessels without joints of organic material — the second time with the addition of sulphuric acid and potassium permanganate. The magnesium sulphate and potassium phosphate, used as food, were calcined ; the calcium carbonate, used in excess, was likewise calcined and saturated with carbon dioxide ; finally, the ammonium sul- phate was especially prepared to avoid organic impurities. The culture flasks were plugged with calcined amianthus, not with cotton. The solu- tions were inoculated with a mere trace of the culture containing nitrifying 300 EFFECT OF CHEMICAL AGENTS [Ch. XI organisms. The cultures were then reared either in complete darkness or in dim light. The precautions taken to eliminate organic matters would seem to be complete ; but those who know the difficulties of such experi- ments seem willing to admit the possibility " that exceedingly minute quan- tities of organic nitrogen and carbon are actually present " (Jordan and Richards, '91, p. 880). On the other hand, " exceedingly minute quanti- ties " of organic matter could hardly account for the vigorous growth of bac- teria ; and, in general, a priori objections cannot be permitted to overthrow results gained by the use of methods which are beyond reproach. The organisms placed in this water, deprived of the last traces of organic matter, developed rapidly, but not quite so rapidly as organisms placed in natural water to which the necessary salts had been added. Analysis showed that not only nitrates but also organic carbon compounds had been formed. Thus the careful experiments of Winogeadsky demonstrate what the less critical experiments of HERiEUS ('86) had already rendered probable, that a complete synthesis of organic matter may take place through the action of living beings and independently of the solar rays. These noteworthy observa- tions, then, obliterate the last sharp line of distinction between the nutritive processes of chlorophyllaceous and non-chloro- phyllaceous organisms. We may now state that the elements C, H, N, and O may be gained from complex organic food materials by all organisms, and from simple compounds, such as carbonic acids, ammonia, and water, by all chlorophyllaceous organisms, and, very probably, by certain non-chlorophylla- ceous ones also. The elements other than C, H, N, and O are probably gained by chlorophyllaceous and non-chlorophyllaceous organ- isms alike, from either inorganic or organic compounds con- taining the necessary elements ; although, possibly, animals make use of the metals more readily when they are in organic compounds. Suitable proportions of the different metals in a nutritive solution for green plants are given in the following tables, showing various standard solutions employed by differ- ent experimenters : — I § 1] UPOX THE RATE OF GROWTH 301 TABLE XXVin Nutritive Solutions for Phanerogams Sachs' Solution Grammes. Distilled i^ater 1,000.0 Potassium nitrate, KXO3 1.0 Calcium sulphate, CaSO^ 0.5 Magnesium sulphate, MgSOi 0.5 Calcium phosphate, CaHPOi 0.5 Ferrous sulphate, FeS04 trace Schimper's Solution ('90) Distilled water 600.0 Calcium nitrate, Ca(X03)2 6.0 Potassium nitrate, KXO3 1.5 Magnesium sulphate, MgS04 1.5 Xeutral potassiu.m phosphate, K3PO4 . . . 1.5 Sodium chloride, XaCl 1.5 Frank's Solution Water, -^j distilled ; |^ Berlin reservoir water . 1,000,000.0 Calcium nitrate, Ca(X03)2 267.4 Potassium chloride, KCl 121.5 Potassium phosphate, K3PO4 101.9 Magnesium sulphate, MgSOi • 7 H2O .... 100.2 Ferric chloride, FcoCle trace Tlae first differs .from the others chiefly in the absence of chlo- rine. How fit ordinary waters may be to provide all these salts is shown by this analysis of the ordinary drinking water of Bos- ton (JORDAX and ElCHAEDS, '91) : — TABLE XXIX Parts by Weight of Inorganic Matters in 1,000,000 Parts of Potable Water Sulphuric acid 4.58 Chlorine 4.00 Alumina and oxide of iron 0.75 Calcium oxide 6.45 Magnesia . 1.60 Potash 0.92 Soda 5.00 Silica 3.04 Nitrates 0.25 302 EFFECT OF CHEMICAL AGENTS [Cii. Xi All of the elements mentioned above, except phosphorus, appear in this list. Thus, ordinary drinking water is clearly well adapted to the nutrition of plants. For algce^ MoLiscH ('95) used the solution given in the fol- lowing table : — TABLE XXX Nutritive Solution for Alg^ Grammes. Water 1,000.0 (NH4)2HP04 0.8 (KH2)P04 0.4 MgSb4 0.4 FeS04 trace (2 drops of a 1% sol.) Here we note an absence of the calcium used in the solutions for phanerogams. Fungi likewise require a mixture of salts, according to Nageli ('80, p. 354) and Benecke ('95), in the following proportions : — TABLE XXXI Nutritive Solutions for Fungi Benecke's Solution Nageli's Solution Grammes. Water 1,000.0 (NH4)HoP04 .... 0.5 MgS04 + 7H20 ... 0.5 KCI2 0.5 reS04 0.05 Organic Compounds Gkammes. Water 1,000.0 KH0PO4 5.0 MgS04 + 7H20 . . . 0.01 K,S04 0.5 NH4CI 10.0 FeS04 trace Glycerine 50.0 The solutions differ principally in the proportion of the salts. Finally, all animals likewise require a certain quantity of salts. What the proportions are can be shown in the case of young mammals, which live during part of their growing period exclusively upon milk. A wonderfully close relation exists, indeed, between the proportions of the mineral con- stituents of milk and of the young mammal before it has begun to suck. This is shown in the analyses made hj Bunge ('89) upon the milk of a dog and the body of its newly-born pup. §1J UPOX THE RATE OF GROWTH 303 TABLE XXXII Comparison of Ash in New-boex Dog and in the Milk of its Mother Ash. In Milk: In New-born Dog : % OF Ash. % OP Ash. P2O5 34.2 39.4 CaO 27.2 29.5 CI 16.9 8.4 K2O 15.0 11.4 NaoO 8.8 10.6 MgU 1.5 1.8 FeoOs 0.12 0.72 The quantities in the two columns are fairly similar. The greatest proportional difference occurs in the case of iron^ and this fact requires a special explanation, which is briefly this, that iron is more important for the rapidly growing embryonic stages than for later life. So likewise in the eggs of birds we find stored up all the mineral matters which are necessary for their development. Thus the yoke of the hen's Qgg contains phosphoric acid, lime, chlorine, potassa, soda, magnesia, iron oxide, and silica in the relative abundance indicated in this descending series. This series closely agrees with that given for milk. In the case of marine animals, also, certain inorganic elements are a necessary food. The following list of such elements is hased upon the results of thorough experiments by Heebst ('97) upon developing eggs of sea-urchins, starfishes, hydroids, ctenophores, and tunicates ; the most favorable proportions of the elements Avere not, however, determined : calcium (in the form of carbonate, sulphate, or chloride), chlorine, iron (trace), magnesium, phosphorus (as CagP.^Og or CaHPO^), potassium, sodium, and sulphur. Now all these elements are found in sea water, which in the Mediterranean Sea near Naples contains in 1000 parts of water * * After Eorchhammer ('61), p. 383. 304 EFFECT OF CHEMICAL AGENTS [Ch. XI 30.292 parts NaCl 3.240 " MgClj 2.638 " MgS04 1.605 " CaS04 0.779 " KCl 0.080 " silicic acid, calcium phosphate, and insolu- Total . 38.634 ble residue, including CaCOs and FcgOa. From the foregoing tables it appears that mineral substances^ and essentially the same mineral substances, are required by all organisms. The differences in this respect between the different organisms are slight. Thus while sodium is not included among the necessary elements of either chlorophyl- laceous plants or fungi, its occurrence in considerable quantity in milk, probably associated with chlorine as common salt, indi- cates that it is important for some animals. The important conclusion seems warranted that all organisnjs. may use as hylogenic food any sort of compound which will furnish the appropriate elements, but that among animals, and to a less degree among fungi, organic combinations have the preference because they fulfil at the same time the thermogenic function. h. Detailed Account of the Various Elements used as Food. — We may now consider the part which each element plays in the growth of the body as a whole, reserving for the Fourth Part a consideration of the specific role which the element plays in organs of the body. We may, in general, consider first the share taken by the element in the constitution of the body, then the form in which the element gains access to the body, and finally what general effect it has upon the growth of the organism. Oxygen. — Excepting carbon, oxygen constitutes a greater part of the body, by weight, than any other element. Between 20% and 25% of the dry substance of the human body and between 35% and 45% of that of green plants is oxygen. The oxygen used as hylogenic food comes to land-animals from the organic compounds and the air consumed by the developing young ; the oxygen of water-animals may come from their food or from the oxygen dissolved in water ; finally. § 1] UPON THE RATE OF GROWTH 305 that of phanerogams is believed to be gained chiefly from the air at all parts of the body, roots as well as stems and leaves. Since oxygen is of great importance for metabolism (p. 2), it is naturally essential to growth. It is well known that the lower the oxygen tension is, the more slowly do seeds germinate and pass through their early stages of growth. Thus in an experiment performed by Bert ('78, pp. 848-853) barley grains which germinated and were reared at various pressures had in six days the following dry weights : — Atmospheric Pkessuke. Resulting Weight. 76 cm. mercury (normal atmosphere) 50 " (0.66 " ) 25 " (0.33 " ) 7 " (0.1 " ) 8.8 mg. 7.1 6.2 No growth On the contrary, as the oxygen pressure increases up to about twice the normal, growth is accelerated, but beyond that point growth is retarded until at about 7 atmospheres growth hardly occurs. In older seedlings, observations upon which have been made by WiELER ('83), Jentys ('88), and Jaccaed ('93), atmos- pheric pressure below the normal, even down to one-fourth or one-eighth of the normal, appears to induce accelerated growth ; likewise in pure oxygen at the atmospheric pressure growth is as rapid as or somewhat more rapid than in the ordinary at- mosphere. When, however, the oxygen tension is above the atmospheric pressure or below one-eighth of the normal, growth is retarded. It thus appears that an abnormal oxygen pressure may accelerate growth, and as we shall see later, the same effect is produced by other abnormal conditions. Among animals, also, the oxygen tension exerts an important effect upon growth. This is shown by the experiments of Raubee ('84, pp. 57-65) upon the eggs of the frog. To get a variable atmospheric pressure Rauber used champagne flasks in which were put the eggs and a little water. Through the air-tight cork of the inverted flask one end of a U-shaped glass tube of proper length and strength was passed. To the other end was fixed a funnel through which 306 EFFECT OF CHEMICAL AGENTS [Ch. XI mercury could be poured into the tube. The column of mercury produced the increased pressure in the flask, and the difference in height of the mercury in the two arms of the tube was a measure of this pressure. To get a pressure below the normal a partial vacuum was produced by a water- pump and the flask was then sealed. We assume (with Bert, 78, p. 1153) that the chief effect of the variation in the atmospheric pressure was the variation in the amount of oxygen absorbed by the water. Pure oxygen was also used in the flask. At a pressure of three atmosplieres no growth occurred. At a pressure of two atmospheres growth was slower than at the normal pressure. At three-fourths of an atmosphere also growth was retarded and at one-half an atmosphere death generally occurred. Thus the optimum condition of oxygen tension is near the normal for the atmosphere. The same thing is indicated in a qualitative way by the experiments of Loeb ('92). The stems of the hydroid Tubu- laria possess in ordinary water a high regenerative capacity, but in water deprived of oxygen by boiling no regeneration takes place, although, after the water has been aerated again by shaking, rapid growth occurs. Hydrogen. — This element forms, in its various compounds, between 5% and 10%, by weight, of the dry substance of organisms. How is it acquired? In the case of plants it is believed that it is taken into the organism as a constituent of water, which combines with carbon or carbonic acid in the plant, forming either starch directly or some other compound from which starch is later derived. Other possible sources of hydrogen are ammonia and its compounds, also the organic compounds absorbed. The hydrogen of fungi and animals has clearly been derived from the latter compounds alone. The effect of hydrogen gas upon growth seems to be merely that due to the replacement of oxygen; it has no active effect. Carbon is the largest constituent of dry organic matter, of which it forms between about 44% and 60%. In green plants it is obtained for the most part from the carbon dioxide or carbonic acid of the air which is absorbed by the leaves. Other sources of carbon for green plants are found in many organic compounds, such as urea, uric acid, hippuric acid, glycocoll, kreatin, guanin, asparagin, lucin, tyrosin, and acetamid. These afford nitrogen also. Certain green plants make use of animal ^1] UPOX THE RATE OF GROWTH 307 matter, e.g. insects, as food. Fungi and animals obtain their carbon from carbon compounds elaborated by plants. The indispensableness of carbon for the life of all organisms as well as for their growth requires no illustration. Nitrogen. — Of the importance of nitrogen as a hylogenic food nothing more need be said than that it is essential to the formation of albumen. The ordinary form in which nitrogen is Pig. 85. — Cultures of Sinapis alba in pure quartz sand, to whicli have been added equal amounts of a nutritive solution, but unequal quantities of nitrogen in the form of calcic nitrate, as follows : A, without nitrogen ; B, 0.1 gramme calcic nitrate in each vessel ; C, 0.6 gramme calcic nitrate in each vessel. After a photo- graph. From Frank ('92) . obtained by the seedling is, as alread}^ stated (p. 298), that of the nitrates or ammonia in the soil (Fig. 85). Growing fungi gain it chiefly from nitrogenous organic compounds, but many fungi can make use of ammonium nitrate for this purpose. Growing animals gain nitrogen chiefly, if not exclusively, from organic compounds, especially albumen and allied substances. Free nitrogen is found wide-spread in nature. It forms about 79% of the volume of the air; penetrates into Avater, though in a smaller proportion than in the air, and even into the soil. Thus for organisms living in any of these media free 308 EFFECT OF CHEMICAL AGENTS [Ch. XI nitrogen is a possible food. Is it actually made use of ? This question has until recently usually been answered in the nega- tive, and this conclusion was the more readily accepted since nitrogen is a notoriously inert gas. Another view, however, has within recent years come to obtain. It developed in this wise. It had long been known that land which has lain fallow or on which clover or other leguminous crops have been reared is in a way strengthened as if fertilized, and it was also known that this strengthening is due to the fact that the soil acquires nitrogen from the air and " fixes " it in the form of nitrates. While the studies of Pasteur on fiermentation, since 1862, had paved the way for the interpretation of the process, the fact that it is due to organ- isms was first proved by Schlosing and MiJNTZ (77, '79). This proof they made by chloroforming a certain mass of nitri- fying earth and finding that the nitrifying process thereupon ceased. Later, they isolated a form of bacteria which had the nitrifying property. Their results were quickly confirmed and extended by others, notably Berthelot ('85, '92, etc.), in a long series of investigations, so that there is now no question that the nitrification of the earth is brought about by the activity of bacteria, perhaps of several species. The results thus gained were extended to some of the higher fungi by Frank ('92, p. 596). Spoi'es of Penicillium cladosporioides were sown in a nutritive solution of pure grape sugar and mineral salts, completely free of nitrogen, and in the presence of air which had been freed from ammonia by passing through sulphuric acid. The fungi grew, but not so rapidly as those in a solution containing nitrogenous compounds, and produced a mass of hyphse. These hyphse, when tested, yielded ammonia. One such culture solution of 65 cc. volume became filled with the fungus mass in ten months and yielded 0.0035 gramme of nitrogen, which must have been derived from the air. As similar results have been gained for other molds by Berthelot ('93) and for Aspergillus and Penicillium glau- cum by Puriewitsch ('95), we seem almost justified in pre- dicting that the capacity for assimilating free, atmospheric nitrogen will prove to be a characteristic of all fungi. Now if it is conceded that some organisms can make use of the nitrogen of the air, it is clear that the a priori objection to § 1] UPON THE RATE OF GROWTH 309 all organisms doing so, the objection, that is, on the ground of the inertness of nitrogen, is disposed of. The question is now merely one of fact. Do the algse, the higher plants, and the animals make nitrogenous compounds out of free atmospheric nitrogen? Of these groups, the algse first claim our attention because of the inherent probability that they will act more like bacteria than any other group, since they pass over into the bacteria through such connecting forms as the OscillariEe and Nostocs. ScHLosrisrG and Laurent ('92), Frank ('93), Koch and KossOAYiTSCH ('93), experimented with various species of Nos- toc, Oscillaria, Lyngbya, Tetraspora, Protococcus, Pleurococ- cus, Cystococcus, Ulothrix, etc., and found that, when supplied with a non-nitrogenous food, these plants produced nitrogenous compounds in the substratum, evidently gaining their nitrogen from the previously purified air. Doubt exists, however, as to whether the free nitrogen is taken in directly by the algse or only after having been assimi- lated by bacteria associated with the algse and by them made into nitrogenous compounds. For the latter alternative speak the experiments of Kossowitsch ('94) and Molisch ('95). KossowiTSCH, who with Koch had previously found, that algse gain nitrogen from the air when reared in impure cultures, now took special pains to get algal cultures free from bacteria. To this end he reared algse on potassium silicate permeated by a nutritive solution. The pure cultures thus gained were then grown in a sterilized flask to which air, freed from ammonia, was admitted. The nutritive solution was made of salts free from nitrogen but containing the other essential elements. The results of this experiment were striking. The pure cult- ures of algse grew for a time, but then ceased. New non- nitrogenous food did not revive them, but the addition of nitrates caused rapid growth. Other evidence was gained from analyses. When the cult- ures of algse were pure there was no increase in the amount of nitrogen in the dry matter of the algse. But when bacteria were mingled with the algse, the quantity of nitrogen was increased. This is shown in the following typical analy- sis:— 310 EFFECT OF CHEMICAL AGENTS [CiT. XI Contents of Cultukb. Milligrammes of N in Citltcrb. At the Beginning. At the End. Cystococcus, pure culture Cystococcus, with bacteria ^ .,, " I. with sugar 2.6 2.6 2.6 2.7 3.1 8.1 These results, abundantly confirmed by Molisch ('95), seem to show that unless bacteria are present algse can build up free N into nitrogenous compounds only slowly, if at all.* While ScHLOSiKG and Muntz, Berthelot, and others were gaining an explanation of the enrichment of fallow ground, Hellriegel ('86) and Wilfarth were making their investi- gations upon the cause of the enriching action of leguminous crops, which led them to the conclusion that it was due to the fixation of nitrogen in plants with root-nodosities containing bacteria, the compounds thus formed by the symbiotic bacteria being directly assimilated by the plant. This conclusion has been repeatedly sustained for leguminous plants (Fig. 86). The question whether green phanerogamous plants other than legumes can make use of free atmospheric nitrogen is one which is still in hot debate, and it is not an easy one to answer. The calm conviction, based chiefly upon the excellent work of BoussiNGAULT ('60), and Lawes, Gilbert, and Pugh ('61), that nitrogen is not thus obtained was rudely shaken by the paper of Frank ('93) in which that author stated that he finds that nitrogen is removed from the air by non-leguminous plants — plants, moreover, which are not known to have bacteria living in their roots. Consequently he is of opinion that perhaps the fixation of free nitrogen may be carried on by any living plant cell. The difficulties in the solution of the problem may thus be set forth. It is recognized that all growing plants make use * One "crucial test" of Frank still requires an explanation. If the free nitrogen is "fixed" by the aid of bacteria, the process should go on in the dark. Experiment shows that it does not do so. This difficulty Kossowitsch over- comes by the assumption that the activities of the bacteria are dependent upon certain carbohydrates which the plant can affOrd them only in the light. §1] UPON THE RATE OF GROWTH 311 of nitrogen, but the nitrogen is usually obtained from the soil in the form of nitrates. It is feasible to determine by analysis the amount of nitrogen in the soil at the beginning of the ex- periment and the amount in the seed planted, and then after the experiment to determine the quantity in the soil and in the plant. But the difficulty comes in interpreting the results. Fig. 86. — Parallel cultures of peas in the symbiotic and the non-symhiotic conditions. Each series comprises three culture vessels: B, the symbiotic plants in soil with- out nitrogen; C, the non-symbiotic plants in like soil; A, for comparison, non- symbiotic plants after addition of nitrate to the soil. After a photograph. (From Frank, '92.) of the analyses. Thus the fact that the sum of nitrogen in the plant and the soil is greater at the end than at the beginning of the experiment does not prove that the plant has taken in free nitrogen ; for, as we have seen, the soil contains nitrifying bacteria, which intermediate between the free nitrogen of the air and the nitrates absorbed by the green plants. This diffi- 312 EFFECT OF CHEMICAL AGENTS [Ch. XI culty may be partly met by sterilizing the soil, but this prob- ably produces also other changes than the death of the bacteria. The most careful observations of the last five years have not supported Frank's generalization. Here and there there have been observers who, like Liebscher ('92) and Stoklasa ('96), believe they have evidence for the direct assimilation of free nitrogen by the cells of phanerogams. But the evidence for the contrary opinion is predominant. The experiments which speak for the theory that green plants cannot directly make use of free nitrogen are not in unison. Thus, some indicate that in non-leguminous as well as leguminous plants nitrogen of the air is indirectly made use of through the action of the bacteria of the soil, while accord- ing to others it would seem not to be made use of at all. To the first class belong the experiments of Petermann ('91, '92, and '93) with barley, of Nobbe and Hiltner ('95) with mustard, oats, and buckwheat, and of Pfeiffer and Franke ('96) with mustard. To the second class belong the experi- ments of SCHLOSING and Laurent ('92 and '92"^) with various plants, Day ('94) with barley, and Aeby ('96) with mustard. In the second class, however, the experimental con- ditions did not favor the development of the bacteria of the soil. The experiments of PETERMAisrisr were, however, carried out upon a very large scale and under practically normal con- ditions and showed a marked difference between the acquisi- tion of nitrogen by barley growing in an unsterilized soil and in a sterilized one. Likewise Nobbe and Hiltner, and Pfeiffer and Franke, were careful to rear plants under normal conditions, so that their results are worthy of especial consideration. They agree that there is an acquisition of- nitrogen by the plant growing in normal soil and that this occurs only when the soil is unsterilized. We conclude then that probably phanerogams, like algse, can use the free nitro- gen of the air as food only after it has been converted into nitrates by the action of the nitrifying organisms — the bac- teria of the soil. Turning now to animals, whose nutrition is often compared with that of fungi, we find an absence of knowledge on the subject of the nutritiveness of free nitrogen. It is clear that §1] UPON TPIE RATE OF GROWTH 313 land animals are in a favorable position for making use of it, .since it penetrates with the oxygen to all parts of the body. It is found in the blood of mammals, but still as free nitrogen. Whether it eventually becomes fixed in the body is entirely unknown. The a priori argument against such fixation — the argument of inertness — has lost much of its force since the discovery of the nitrifying organisms. Phosphorus. — This element is of constant occurrence in organisms. It has been found in yeast, mucors of the most diverse kinds, seeds, plant tissues, and animal tissues of all kinds. It is indeed one of the first among the mineral ele- ments of most organisms, as shown on pages 296 and 297o Of the dry substance of a fish, about 7 % is phosphoric acid, and the dry substance of many seeds yields 15%. Phosphorus occurs in organisms as phosphoric acid compounds. Of these the most important organic compounds are niichin, which has albuminoid properties, and occurs chiefly in all nuclei and in deutoplasm ; lecithin, of a fatty nature, occurring in yeast, Plasmodium of ^thalium, seeds, milk, yolk of eggs, and nervous tissue ; and glyceri7i-phosphoric acid, a product of ■decomposition of lecithin and found wherever the latter occurs. As examples of inorganic salts we have the sodium and potas- sium phosphates of the blood and tissues and the calcium phos- phate deposited in bone. So important an element as phosphorus would naturally form an essential part of the food of all growing organisms. It is supplied at first in the germ, — seed or Q^^, — but later must •come from without. Plants gain phosphorus from the disinte- grating rocks. Animals derive it chiefly from plants, directly or indirectly, or from the calcic phosphate of the sea ; in mam- mals it is supplied to the developing young through the milk, which, as we have seen on page 303, is rich in phosphates. The abundance of phosphorus in the body indicates that its •office in the organism is an important one, and its peculiar abundance in seeds, yolk, and milk indicates that it is especially important in growth. Experiments have been directed towards this point. The fact has long been established that plant growth cannot occur in the absence of phosphates, and this is true not only for green plants, but also for molds and yeast 314 EFFECT OF CHEMICAL AGENTS [Ch. XI (Raulin, '69). Embryos of variouc marine animals also will not develop in the absence of phosphorns (Herbst, '97). The peculiar importance of phosphorus for growth is also indicated by the fact that Haetig and Weber ('88) found more phos- phoric acid in the ash of the growing ends of the plant than in its fully differentiated parts. Again, LoEW ('91^^) found that Spirogyra kept in a nutritive solution of salts in which phos- phorus alone was lacking, continued to live and, indeed, to form starch and albumen, but its cells did not grow or divide ;, so that LoEW concludes that an important use of phosphorus is to nourish the cell-nucleus, and this fact is easily understood from the known importance of the phosphorus-containing nuclein of chromatin in cell-division. All these facts go tO' show that phosphorus is of prime importance in the growth of organisms. Arsenic, Antimony, Boron, and Bismuth, and their compounds,, are apparently all injurious to organisms, so that sublethal solutions strong enough to be active, interfere with or even inhibit growth. Sulphur. — This element is, without doubt, of constant oc- currence in organisms of all sorts, for it constitutes between 0.3% and 2% of all proteids, out of which organic bodies are largely composed. Sulphur forms between 0.6% and 1% of various (dry) organs of man, and nearly 1% of the dry sub- stance of a month-old seedling of Sinapis alba. In the growth of a plant (Sinapis alba) the amount of its sulphur increases from 0.02 mg. in the seed to 84.4 mg. in the adult plant, and, indeed, it has been shown in one case (Aeendt, '59, for the oat plant) that the percentage of sul- phuric acid in the ash increases from 2.9 to 4.2 as the plant develops from a seedling to maturity. Since the sulphur goes chiefly into the composition of the living substance, its hylo- genic importance for growth is evident. The form in which sulphur may be taken into the body is very varied. It is well known that green plants usually absorb it in the form of sulphates, especially sulphates of potassium, calcium, and magnesium ; marine animals take it chiefly from the calcic sulphate of sea water, and land animals gain it largely from organic sulphur compounds produced in §1] UPOX THE RATE OF GRO^YTH 315 plants. "Whetlier non-clilorophTllaceous plants can make use of it has been much discussed, and is worthy of further inves- tigation. WixOGEADSKY ('88, "89) has, indeed, shown that the sulpho-bacteria store up pure sulphur from sulphuretted hydrogen (HgS), and Peesch ('90) has concluded, as a result of feeding himself with pure sulphur and analyzing the sulphur of the urine, that about one-fourth of the srdphur taken into the body in an elementar\" form becomes built up into organic molecules. Recently Heebst ("97) has shown that embryos iVoS S days Fig. 87. — Right, Larva of Echinus reared for 72 hours in -^-ater containing all the necessary salts ; the sulphur being in the form of 0.26%. magnesium sulphate and 0.19J- calcic sulphate, and the phosphate in the form of CaHP04. The larva is normal. Left, Larva reared for 68 hours in a solution containing no sulphur nor CaCl.2. The typical larva -without sulphur, but with CaCL, differs from this chiefly in the presence of rudimentary spicules ; kr. spicule-forming cells. (From Herbst, '97.) of sea-urchins and other marine animals do not develop in the absence of sulphur (Fig. 87). These facts indicate that not green plants merely, but all organisms, can use elementary sulphur as a hylogenic food. TJie Halogens, chlorine, bromine, iodine, and fluorine, are elements which are closely similar in their chemical reactions 316 EFFECT OF CHEMICAL AGENTS [Cii. XI outside of organisms, and carry a part of tliat peculiarity with them into organisms. All of them are of physiological interest ; but so far as we know chlorine and iodine are most important. Clilorine. — This element is probably of constant occurrence in organisms, and indeed in rather large quantities, as Table XXVII, p. 297, shows. It is not strange that it should be so, since this element is very widely distributed in all waters. This very fact, however, renders it possible that chlorine is merely an accidental constituent of organisms, being unessential to growth, and precisely this conclusion has been maintained. Of green plants it was early asserted that growth occurs as readily in solutions containing no chlorine as in ordinary potable water, but of late years evidence opjDosed to this view has been accumulating. Thus, while it appears that growth may occur in the absence of chlorine, Aschofp ('90) and others have found that growth is not so vigorous as in solutions containing this element. It has been thought that chlorine makes an advantageous combination with the potassium neces- sary for the plant, but the true significance of the favorable properties of chlorine remains undetermined. Turning to animals, we find that chlorine occurs in the milk of mammals, and is therefore probably necessary to them. According to Herbst ('97, p. 709) it is a necessarj^ food for young echinoids. Certainly in the form of sodium chloride it is an essential food of the higher animals, and some of them, especially the herbivora, require large quantities of it, as " salt- licks " testify. The function of chlorine is not altogether plain. It must be used in the production of the hydrochloric acid of the digestive juices. In addition, sodium chloride is found widely distributed in the tissues. It has often been asserted that it goes through the tissues unchanged ; but Nencki and Schoumow-Simanowsky ('94) believe that it is probably disintegrated and the chlorine built up into organic molecules. Bromine. — The normal occurrence of traces of this element has lately been demonstrated by Hotter ('90) for a great variety of plants. It occurs most abundantly in various fruits, apple, pear, peach, and also in the leaves and twigs of many plants, as well as in various berries. Its normal occur- § 1] UPON THE RATE OF GROWTH 317 rence permits us to believe that it has an importance, if not for growth, at least for development ; there is, however, no direct evidence that it is generally necessary to organisms. Since it is closely allied to chlorine, the question whether it may replace chlorine in growth has been tested. NEoSTCKI and Schoumow- SiMANO\ySKY ('94) have found that in the higher animals the bromides can replace the chlorides to a limited extent, but are clearly less advantageous. Altogether the importance of bromine for growth is slight. Iodine. — This element is of Avide-spread occurrence- in organ- isms, probably as a constituent of organic molecules, for it is found in plants, especially in some species of Fucus and Lami- naria (cf. Eschle, '97); in invertebrates, esj^ecially in sponges and the stem of Gorgonia; and in vertebrates, especially in mammals, where it has recently been shown to be most im- portant for growth. It has long been known that mammals which have been deprived of the thyroid gland acquire a weak condition of body known as myxoedema. This effect has been accounted for by the loss, to the organism, of a substance elaborated in the thyroid gland ; for when the thyroid gland, or a docoction of it, is fed to the animal it recovers to a certain extent its normal condition. The nature of this substance has been investigated by Batjmaxn" and Eoos ('96), who find that it is a compound of iodine — thyroiodine ; for when thyro- iodine is fed to the myxoedema patient, the same favorable result ensues as follows feeding upon the gland. Fluorine is found rather widely distributed among verte- brates in very minute quantities. It forms about 1.3% of the total ash of bone. It is present also in the hen's Qgg^ being more abundant in the yolk than in the albumen (Tammann, '88). Since it is chiefly found in the body as a constituent of bone, possibly in the form of the mineral apatite, CajQFg (P04)g, it may very well be that, its chief importance is in the consti- tution of this formed substance. According to Brandl and TappeijSI^er ('92) the normal amount in the body may be greatly increased by feeding small quantities of sodium fluoride during a long period. Altogether, we have no reason for thinking that fluorine is essential to the growth of organisms in general. 318 EFFECT OF CHEMICAL AGENTS [Cii. XI Salts of Alkalis. — This group contains one or two of the most important elements of which organisms are composed. The elements are only slightly replaceable by one another. Lithium seems normally to have little importance for growth, although slight traces of it have been found spectroscopically in the blood (Hoppe-Seylee, '81, p. 453). Sodium is probably of constant occurrence in organisms. The quantity of it in the body is widely different in different species ; and as our table on page 297 shows, it may vary in its position from one of the most abundant of the elements to one of the least. Whereas sodium is not essential to the nutrition of plants, it is necessary in the form of sodium chloride to certain higher animals. Sodium is also found in all the tissues of the body, and perhaps enters into the albuminoid molecule (Nencki,' '94). The fact that, as we have seen on page 303, soda is a prominent constituent of milk, indicates its impor- tance in the growth of mammals. Finally, Heebst ('97) has been able to demonstrate its indispensability for the growth of marine animals. Potassium constitutes an important part of all organisms. It forms the largest part of the ash of nearly all phanerogams (see page 297) and is markedly abundant in yeast and in many invertebrates. It occurs in the body as chloride and as sul- phate, and probably also in combination with albumen and various organic acids (Vines, p. 134). That it is an essential and unreplaceable food for all organ- isms is indicated by trustworthy experiments upon fungi, algse, phanerogams, invertebrates, and vertebrates. Raulhst ('69) first showed that only culture-solutions containing this metal permit the growth of fungi. The experiments of Benecke ('96) upon the growth of certain molds, e.g. Aspergillus, are worth citing in detail as an illustration of the method. He sowed spores in culture- vessels made of a glass which analysis had shown to be free from potassium. Two of these vessels contained a nutri- tive aqueous solution consisting of 3% cane sugar and 0.25% magnesium sulphate. To the one of these solutions was added 1.2% potassium nitrate, and 0.26% potassium i^hosphate; and to the other 1% sodium nitrate and 0.5% sodium phosphate. After four days the first culture was covered by a sheet of fungi and in five weeks the whole surface was black with spores ; in §1] upo^nt the eate of growth 319 the second culture, on the other hand, little growth had occurred even at the end of five weeks. The crop from each of the cultures was then harvested and its dry weight determined. That of the first culture was 0.3 gramme, that of the second 0.03 gramme. By a somewhat similar procedure Molisch ('95) has been able to show that potassium is essential to the growth of algse ; and NoBBE and others ('71) that it is necessary for the growth of phanerogams. The experiments upon animals have been rather less satisfac- tory on account of the greater ditjiiculties in experimentation. 'MUli/// Fig. 88. — Two embryos of Sphserechinus from parallel cultm-es. a, reared in a solu- tion containing all the necessary salts ; embryo normal, b, reared in the same solution, but without potassium; blastula wall abnormally dense, and embryo of small size. (From ELeebst, '97.) Nevertheless we have some trustworthy data upon this matter. On the side of the invertebrates we have the experiments of LoEB ('92), who placed a hydroid, Tubularia, in fresh w^ater to ■which solutions of various combinations of the salts found in sea water were added so as to give approximatel}'" the nor- mal osmotic effect. Under these circumstances regeneration of the hydroids occurred only in the solutions containing potassium. Again, Herbst ('97) finds potassium essential to the groAvth of embryos of echinoids (Fig. 88). Thus the potassium compounds seem necessary to the processes upon w^hich growth depends. On the side of vertebrates we have the somewhat inconclusive results of Kemmeeich ('69), who fed two young dogs of the same age and of nearly the same size upon meat boiled until a large portion of its salts was extracted. To the food of one dog he added only sodium chlo- 320 EFFECT OF CHEMICAL AGENTS [Cii. XT ride ; to tliat of the other, both sodium chloride and potassium salts. After 26 days the dog fed on the potassium salts as well as the sodium chloride was about 30% heavier than the other, and this difference was reasonably ascribed to the beneficial effects of the potassium. The particular part which potassium takes in growth is still somewhat doubtful. The recent observations of Copeland ('97) with seedlings reared in water cultures in which sodium replaces potassium, lead him to the conclusion that potassium is necessary to turgescence. The potassium salts become lodged in the cell-sap, as analysis shows, and are therefore, perhaps, one of the principal causes of imbibition. Ruhidium and Ocesium. — These rather rare metals are important only because of the fact that they may replace potassium in the growth of some fungi. Winogradsky ('84) recognized this to be the case with rubidium in yeast cultures. Nageli ('80) found that in molds rubidium and Ccesium gave even greater growth of dry substance than potassium cultures^ a conclusion abundantly confirmed by the studies of Benecke ('95). Whatever, therefore, is the significance of potassium for growth, rubidium and caesium seem to have the same significance. Earthy Metals. — Under this head are included the elements calcium, strontium, and barium, which form compounds having closely similar molecular structure and properties. We might therefore expect them to be in some degree mutually replace- able. Calcium shares with potassium and phosphorus the position of one of the most abundant elements of organisms. It is found in every tissue and seems to be a constant accomjjaniment of protoplasm. Its constant occurrence is indicative of its importance as food for both plants and animals. It is apparently indispensa- ble and unreplaceable by any other element in phanerogams ; but MOLISCH ('94) finds that growth can take place in certain molds (Penicillium, Aspergillus) as well in its absence as in its presence, and in some algfe, but not in all (Molisch, '95), calcium is apparently of little importance. In animals, calcium can be replaced by other elements only to a very slight extent. § 1] UPON THE RATE OF GROAVTH 321 Its absence from the water in which echinoid larvae are developing produces dwarfs. In vertebrates, owing to the need of this metal for the skeleton, large quantities of calcium are taken in by the growing organism. In normal growth, then, the food of animals and phanerogams must contain calcium. Strontium, although closely allied to calcium, is rather rarely found in considerable amount in organisms. In certain plants, as e.g. Fucus, it is constantly present. It can be stored up in the animal organism when supplied abundantly in the iood, but in general is believed to be of little importance for growth. 3Iangane§e, likewise, is not of general importance, although it is found abundantl}^ in certain plants, e.g. Trapa natans, Quercus robur, and Castanea vesca, and in the excretory organ of the mollusc Pinna squamosa (Kkukexberg, '78). Iroji. — This most abundant of the heavy metals occurs so frequently in organic substances, especially those related to the compounds found in protoplasm, that it is little wonder that iron has been found to be an essential ingredient of all proto- plasm, hardly less important than oxygen itself. Although its occurrence has been demonstrated, especially by Schneider ('89), in air the large groups of animals, its amount in any individual or organ is always very small. Iron oxide (FcgOg) forms between 1% and 2% of the ash of muscle, about 5% of the ash of blood, and rarely rises to 5% in plants. It is found in the body, for the most part, as in yolk (BuxGE, '85), in organic union. It occurs thus in the chro- matin of the nucleus of all cells (2^Iacallum, '92, '94 ; SCHXEIDEE, '95). That chromatin contains iron has been demonstrated by Macallum ('91) bj'' means of a microchemical method whose general validity has never, so far as I know, been qviestioned. It was shown by Bunge, in 1885, that when tissue is put into ammonium sulphide the iron, even in an organic molecule, is separated from its compound, and smiting with the sulphur forms ferrous sulphide (FeS). This ferrous sulphide appears in the proto- plasm as green granules (black in large quantity) ; and the fact that these granules appear abundantly in the nucleus shows that iron is especially abundant there. Additional evidence is given if after several weeks the chromatin loses its stained appearance and becomes rusty. This result is interpreted as due to the formation of ferric oxide, Fe203; for when the 322 EFFECT OF CliEMICAL AGENTS [Ch. XI nuclei are subjected to hydrochloric acid and potassium ferricyanide they immediately assume a deep azure-blue color, clearly due to the formation of "Prussian blue" or Fe^(Fe C,;lSrg)3. This series of reactions can only be explained on the ground that there is iron in the nucleus. Iron has shown itself to be essential to the growth of all organisms. Its importance for growth is indicated by its rela- tive abundance in the yolk of birds' eggs, and by the fact of its occurrence in larger proportion' in a mammal just born than Pig. 89. — Echinus larvse from parallel cultures, all 51 days old. a, reared in a solu- tion containing all salts, including iron as FeClg ; b and c, from solutions contain- ing all salts excepting iron. (From Herbst, '97.) in later stages. When excluded from a nutritive solution which is otherwise complete, growth is imperfect (Fig. 89). This may be associated with the facts that in plants the chloro- phyll- granules are not developed in its absence, that in the higher animals hsemoglobin cannot be formed, and that the chromatic substance of all cells requires it. The question in what form iron is absorbed b};^ the organism has been the subject of an extensive discussion. Although ■d-oses of metallic iron have long been used with favorable § 1] UPOX THE RATE OF GROWTH 323 results in medical practice, Buxge ('85) concluded that only organic iron compounds are assimilable. Tlie studies of KuxKEL ('91 and '95) and Woltekixg ('95) have, however, shown in the clearest manner that inorganic iron is assimilated, stored in the liver, and made use of in the construction of such organic compounds as the haemoglobin of the blood. At the same time, as Socix ('91) and others have shown, iron may be gained from organic compounds. Apparentl}% iron com- pounds of any sort may be made use of by the organism. Magnesium. — This metal is closely associated with calcium, the two usually occurring together in organisms just as they do in the inorganic world. As the table on I3age 297 shows, magnesium is of constant occurrence among organisms, although never present in great quantity. The magnesium is gained by green plants at the same time with the calcium from mineral salts (chiefly magnesium carbonate and sulphate) derived from disintegrating rocks. Fungi can also make direct use of the salts of magnesium (excepting always the chloride) ; and ani- mals, although no doubt chiefly gaining their magnesium from plants or the waters in which they live, may make use of min- eral salts, especially in the construction of the mineral parts of various formed substances, such as those of bone. ^ So constant an element is presumably necessary to the organ- ism, and numerous observations make it quite certain that this is true for green plants in general. Indeed, the fact that mag- nesium occurs in chlorophyllan of chlorophyll makes it prob- able that it functions in assimilation. Concerning its indis- pensableness for fungi there can likewise be no doubt, since Bexecke's ('95, p. 519) experiments show it to be replaceable neither by calcium, barium, or strontium. While some investi- gators, like Hoppe-Seylek, believed it to have little signifi- cance for the animal body, — to be an accidental accompaniment of calcium, — later study has shown that it is of importance for regeneration of hydroids (Loeb, '92), and, according to Herbst ('97), a constituent of the sea water which is necessary to the normal growth of various marine larvae. Also its occur- rence, although slight, in milk, as well as its very abundant occurrence in seeds, indicate that it plays an important, if an unknown, part in growth. 324 EFFECT OF CHEMICAL AGENTS [Cii. XI Silicon. — This element is of wide occurrence among organ- isms. In many plants, especially the grains and grasses, it is exceeded in abundance only by potassium. Among the lower organisms whole groups, e.g. diatoms, Radiolaria, and glass sponges, are characterized by the great amount of silica made use of. Even in vertebrates it is found wide-spread in the blood, gall, bones, feathers, and hair. The silicon required for the body is gained by plants and lower organisms from the soluble silicates or silicic acid of the soil and waters ; by verte- brates, probably from plants. Although Sachs ('87, p. 271) was able in 1862 to show that growth even of maize (about one -third of whose ash is silica) continues in the absence of silicon, yet some grains do better, according to Wolff ('81), when that element is abundant. It is significant, likewise, that, as Poleck ('50) found, 7% of the ash in the albumen of the hen's egg is silicon. Copper. — Brief mention may be made of the fact that this metal occurs in a great variety of organisms, but usually in minute quantity. It occurs as a physiologically important con- stituent of the hsemocyanine of the blood of the squid (Feed- ERICQ, '78, p. 721), of crabs and lobsters, and of certain gas- tropods and lamellibranchs (Fredektcq, '79). 2. The Organic Food used by Organisms in Growth. — All organisms may use organic compounds as food ; all organisms which contain no chlorophyll, certain bacteria excepted, must do so. This organic food may consist of solutions of definite composition or it may consist of solid masses of plant and animal tissue composed of varied and indeterminate kinds of organic molecules. The former are made use of chiefly by plants ; the latter, by the higher animals. This distinction is, however, not a necessary and constant one, for on the one hand insectivorous plants and many fungi live upon solid masses which they digest, and on the other hand some of the Protozoa can be fed upon known solutions, and doubtless some of the higher animals could be likewise fed, although few experiments seem to have been made in this direction. Even in methods of nutrition there is no sharp line to be drawn between the different groups of organisms. a. Fungi. — Our knowledge of the effect of known chemi- § 1] UPON THE RATE OF GROWTH 325 cal compounds used as food has been mucli more advanced by studies upon this group tlian upon any other. Yeast and bacteria have been especially investigated, but valuable results have been obtained upon the higher fungi also. Con- sidering all these results together, it appears that the nutritive value of an organic compound is perhaps chiefly determined by its assimilability; that is, the less the energy required to attack and transform the compound by the various chemical means at hand, the more favorable it is as a food. This assimilability depends in turn upon the molecular structure — upon a certain molecular instability or lability — upon the possession of that quality which is found in its extreme expression in many organic poisons. As Loew ('91, p. 761) has expressed it: Poison action, like nutritive action, is a relative conception. An indifferent body can become, by entrance of one atom-group, a nutritive substance ; by entrance of an additional atom group, a poison. While a certain lability — that is, a certain degree of ease of decomposition — is a condition of the nutritive quality of a substance, a slight increase of this lability can give it a poisonous character, especially when the loosely arranged atoms can link into that atom-grouping upon which the vital movement in the protoplasm depends. Thus methan is indifferent for bacteria, methylalcohol is a nutritive sub- stance, formaldehyd is a poison, and its combination with sodic sulphate again a nutritive material.* Additional laws of nutritive value which hold true in many cases are as follows : The assimilable carbon compounds con- tain the group CHg or at least CH. Under otherwise similar conditions, compounds with one carbon atom are assimilated with great difficulty (methylamin) or not at all (formic acid, chloral) ; and, in general, but with important exceptions in the case of certain classes of substances, the assimilability increases as the number of carbon atoms in the molecule increases. The * The graphic formulas of these substances are : — OH OH / / CH4 CH3-OH CH2 CH2 OH SOsNa. methan. methylalcohol. formaldehyd. formaldehyd-sodic sulphate. 326 EFFECT OF CPIEMICAL AGENTS [Cii. XI radical HO- is usually easily released, consequently we find compounds containing this radical in general more assimilable than allied compounds in which the HO- is replaced by H. Especially is this true when the HO- is joined with radicals containing carbon and hydrogen atoms. For example, foods with the radical -CHg • OH are more nutritive than those with -CHg. Hydroxylized acids are better food for bacteria than non-hydroxylized — lactic acid, CgHgOg, is better than pro- pionic acid, C3HQO2. It is, perhaps, a special case under this rule that multivalent alcohols — i.e. those containing several HO groups — are better foods than the univalent ones; for instance, glycerine, CHgOH-CHOH-CHgOH, is better than propylalcohol, CHg • CHg • CHgOH. Finally, the entrance of the extremely unstable aldehyd (-CH:0) and keton (-CO-) groups increases the nutritive capacity of the food ; for example, glucose, CHgOH ■ (CH • 0H)4CH0, or fructose, CH^OH. (CH.0H)3 -CO-CHaOH, is better than mannit, CH2OH . (CH • OH)^- CH2OH. But all substances containing the group CHOH are good foods, since this compound can be used directly in the construction of carbohydrates, and eventu- ally of albumen. As an example of the application of these general principles may be given this series of substances arranged in the order of their decreasing nutritive value for yeast and molds (Nageli and LoEW, '80) : 1, sugars ; 2, mannit, glycerine, the carbon groups in leucin ; 3, tartaric acid, citric acid, succinic acid, the carbon groups in asparagin ; 4, acetic acid, ethylalcohol, kinic acid ; 5, benzoic acid, salicylic acid, the carbon groups in pro- pylamine; 6, the carbon groups in methylamin, phenol. In conclusion may be mentioned a food for bacteria which, although inorganic, resembles organic compounds in that it may serve as a source of energy. This is the hydrogen disul- phide employed by the sulphur bacteria and allied forms (WmOGRADSKY, '87, '89). h. Green Plants. — It has already been shown that organic compounds can be assimilated by green plants. The experi- ments which have shown this have been made upon both algse and phanerogams, especially by Bokoeny, Loew, A. Meyer, and Laurent. It appears that, in the absence of carbon § 1] UPON THE RATE OF GROWTH 327 dioxide, Spirogyra may form starch from methylalcohol, for- maldehyd, glycol, methylal, acetyletliylesteracetat, acetic acid, lactic acid, butyric acid, succinic acid, phenol, asparaginic acid,, citric acid, acid calcium tartrate, ammonium tartrate, calcium bimalate, glycocoll, tyrosin, leucin, urea, hydantoin, kreatin, and peptones.* Phanerogams form starch from glycerine, cane sugar, levulose, dextrose, lactose, maltose, mannit, dulcit, and lecithin. While daylight favors assimilation it can be in some cases dispensed with. Thus the potato plant can accumulate starch in the dark when glycerine is used as food. An attempt to gain a deeper insight into the conditions of formation of albumen from organic matter has been made by HajStsteen ('96), who reared Lemna in solutions of grape sugar or cane sugar, on the one hand, and amids, like asparagin, urea, glycocoll, leucin, alanin, or kreatin, on the other; also on grape sugar and inorganic nitrogenous bodies such as potassium nitrate, sodium nitrate, ammonium sulphide, and ammonium sulphate. As a result of feeding on grape sugar alone at 20° C. during 24 hours, much starch was stored in the cells ; when grape sugar and inorganic nitrogenous bodies were combined little starch and much albumen were produced. Much albumen was also gained when the nutritive solution contained cane sugar and urea, or asparagin and ammonium chloride, or asparagin and ammonium sulphate. The production of albu- men in green plants is favored by nitrogenous organic com- pounds. e. Animals. — These organisms are distinguished from the foregoing by an immense requirement of energy for their muscular processes, much of which is continually lost to the organism in the form of motion communicated to the environ- ment. On the other hand, growth is generally slower than in plants. Consequently in animal nutrition thermogenic foods are the more important and the nutritive processes are prevail- ingly exothermic. The plastic processes are the less striking ; nevertheless, it is they which chiefly concern us in our study of growth. Among the organic compounds ingested, carbohydrates are * The reagents were employed in about 0.1% solution, and whenever acid were neutralized with lime-water. 328 EFFECT OF CHEMICAL AGENTS [Cii. XI believed to have little importance as plastic foods — we have chiefly to consider the use of the complex fats, albuminoids, and other organic compounds. We can make little use of the extensive tables of calorific properties of foods which, however important for determining capacity for supplying energy, afford little insight into the plastic properties of the food — its importance for the growth of dry substance or the imbi- bition of water. The difficulties in the way of feeding animals upon known nutritive solutions are great, both because solutions are not their normal food, and because, in the case of water organisms, the bacteria introduced with the animals thrive better than they. Consequently the observations on nutritive compounds for animals are meagre. It will be best to consider them under the types upon which they have been made. Amceba. — We owe important studies on the foods of Amoeba to the fact that some species have a pathogenic importance. Cultures of them have therefore been made by bacterio- logical methods. It is found that various, even innocuous kinds, will grow upon egg albumen in distilled or phenylated water kept at about 15° C. (Ceivelli et Maggi, '70, '71 ; Monti, '95), upon agar-agar sheets from which the soluble substances have been removed by repeated washing in distilled water (Beyerinck, '96), upon"Fucus [Chondrus?] crispus" (Celli, '96), or upon slices of potato (Gorini, '96). It is thus clear that, in addition to salts, Amoeba needs only a very simple nitrogenous diet. It is, however, uncertain whether the amoeba feeds directly from the organic food stuff, or indi- rectly upon the bacteria which grow upon the food supplied. Infusoria. — A beginning has been made in the study of this group by Fig. 90. — Poiytoma uveUa, Ogata ('93), who reared pure cultures a flagellate infusorian. ^f ^j^^ flagellate Polytoma uvella (Fig. (From Verworn, 95.) j. • ^ i j.- 90) on plates of nutrient gelatine (which is extremely rich in protein) and also upon a medium composed of 500 ccm: meat bouillon, 12.5 grammes grape sugar, and 250 grammes of a Japanese mixture of algse called " nori," derived mostly from the species Porphyra vulgaris. §1] UPON THE RATE OF GROWTH 329 The mixture was cooked, neutralized, filtered, sterilized, and infected with the flagellates, which had been isolated by means of capillary tubes. Here again growth was supported by simple, definite foods. Amphibia. — It is a great leap from Protozoa to vertebrates, but precise feeding experiments are almost lacking in the invertebrate Metazoa. In the present group come the pioneer experiments of YuxG ('83). This author fed a number of tadpoles derived from the same batch of eggs upon foods of various kinds in unlimited quantity and under otherwise similar conditions. After forty-two days the size of the tad- poles in each lot was measured and the following results were obtained : — TABLE XXXIII Results of Feeding Tadpoles on Various Substances No. OF Vessel. 1 3 3 4 5 6 Kinds of J Food. 1 1 I Aquatic Plants : Anacharis AND Spirogyra. Albuminous Egg Enve- lope OF Frog. Pieces of Yolk of Hen's Egg. Albumen of Hen's Egg. Pieces OF Fish Flesh. . Pieces OF Beef Flesh. Length of tadpole Breadth of tadpole 18.3 4.2 23.2 5.0 26.0 5.8 33.0 Q.Q 38.0 8.8 43.5 9.2 It will be observed that the importance for growth was not proportional to the calorific properties of the respective foods, for the yolk of the hen's Qgg has about 40% higher fuel value than dry Qg^ albumen or dry flesh. The least growth occurred with a plant food which is relatively rich in carbohydrates, and has some protein and little fat ; the third greatest growth occurred with the yolk, which has more fat than protein ; next comes Qgg albumen, with more protein than fat ; and, finally, fish and beef flesh, characterized by their high percentage of nitrogenous matter. The tadpole grows fastest on a highly nitrogenous diet. 330 EFFECT OF CHEMICAL AGENTS [Ch. XI Additional experiments upon the frog's egg have been made by Danilewsky ('95), who found that such eggs placed in water containing -^-^^-qq of lecithin gained in 54 days 300% greater weight than those reared in pure water. The author believes, however, that this small quantity cannot act in a directly nutritive manner but, rather, that it favors in some way the assimilation of food. Mammals. — The requirements of agriculture have led to numberless experiments upon the feeding of domesticated mam- mals. Yet they are for the most part of little value for our purpose. A few of the better class of experiments from our point of view may be given, together with such conclusions as they permit. Years ago, Lawes and Gilbert ('53), from extensive feed- ings of sheep and pigs, upon diverse foods, reached the conclu- sion that those "apparently grew more where, with no defi- ciency of other matters, the nitrogenous constituents were very liberally supplied. Hence, the gross increase obtained might be somewhat more nitrogenous with the large supply of nitrog- enous food ; but it would in that case, according to some experiments of our own, contain a larger proportion of water, and less of solid matter, than where more fat had been pro- duced." More recent studies made on various domesticated animals tend to confirm these results. A mixed diet with an abundance of nitrogenous food permits of greater growth than an equal quantity of food of one kind, or mixed food in which the nitrogenous constituent is scant. Growth tends to increase with the quantity of nitrogenous food rather more closely than with that of the non-nitrogenous food devoured. This conclusion is sustained by the striking results gained by Prosher ('97) in investigating the cause of the varying percentage rate of growth of different mammals. The rela- tion between growth and the percentage of the different organic and certain mineral constituents of milk is given in the following table : — §1] UPON THE RATE OF GROWTH 331 TABLE XXXIV Showing for Vaeious Mammals the Time eequieed to double the Birth- weight, THE Percentage of Different Organic Constituents in the Milk, and the Relative Quantity of Albumen, Calcium Oxide, and Phosphoric Acid in the Milk of the Different Species — the Quan- tity in Man being taken as the Standard 1 3 3 4 5 6 7 8 Species. Relative Time to Double Weight. % Fat. % SUGAK. % Albumen. Relative Quantity Albumen. Relative Quantity CaO. Relative Quantity P2O6. Man . Horse Ox . Pig . Sheep Dog . Cat . 1 1 3 1 4 I^ 1 1 8 1 22 3.5 1.1 4.5 6.9 10.4 10.6 3.3 6.1 4.5 2.0 4.2 3.1 4.9 1.9 2.3 4.0 6.9 7.0 8.3 9.5 1.0 1.2 2.2 3.7 3.8 4.45 5.1 1 4 5 8 14 1 3 4 9 10 From this table it is clear that there is a close relation between rate of growth and the percentage of albumen only among the organic substances of milk. This relation is best brought out by comparing columns 2 and 5. The last two col- umns show a close relation between growth and the quantity of calcium and phosphorus in the milk. But of the organic substances the quantity of the nitrogenous compound deter- mines the rate of growth. 3. Growth as a Response to Stimuli. — Hitherto we have regarded the process of growth in too mechanical a way, as though certain nutritive compounds, passing into a chemical mill, were inevitably transformed, at a certain rate, into proto- plasm or formed substance. We have now to recognize that the growth processes are essentially vital processes, and, as such, characterized by all that complexity which we find in such a vital process as response to stimuli. a. Acceleration of G-roioth hy Chemical Stimulants. — Many chemical agents which are not themselves food may stimulate the growth processes. We have already seen (p. 51) how certain poisons cause, in dilute solutions, accelerated move- 332 EFFECT OF CHEMICAL AGENTS [Ch. XI nients and lieiglitenecl metabolism. To the cases previously given may be added the experiments of Schulz ("88), who found that various poisons, such as corrosive sublimate, iodine, bromine, and arsenious acid, increase the activities of yeast in fermentation. It is not strange, therefore, to find that poisons may, at a certain concentration, accelerate growth. That they do so follows from the experiments of Richards ('97), who reared the molds Aspergillus, Penicillium, and Botrytis in nutritive solutions to which had been added small quantities of zinc sulphate, other metallic salts, cocaine, morphine, and other alkaloids. After five to seven days Aspergillus, reared in nutritive solutions in which sugar was the organic compound, had gained the following dry weights (in milligrammes). In all the . experiments, except those in the column headed " Control," the solution contained certain non-nutritious sub- stances in from 0.002% to 0.033% concentration. TABLE XXXV Showing the Total Dry Weight in Milligrammes of a Crop of Asper- gillus REARED in THE ABSENCE AND IN THE PRESENCE OF VARYING Quantities of Irritating Substances Substance. Control. 0.003% 0.004% 0.008% 0.016% 0.033% ZnSOi 335 730 760 765 770 715 NaFl 250 565 405 340 270 245 NaaSiOs 350 520 575 450 435 380 C0SO4 245 405 350 235 170 75 Cocaine 280 410 320 350 390 540 Morphine 160 155 170 140 210 215 It is clear from this table that the addition of even small quan- tities of innutritions and poisonous substances may so excite the hylogenic processes as to cause twice or even far more than twice the normal formation of dry substance in a given time, and that this excessive growth increases with the concentration of the salt up to a certain optimum, beyond which growth declines again to below the normal. Similarly Townsend ('97) has observed that a seedling living under a bell jar whose atmosphere contains a small quantity of ether grows § 1] UPON THE RATE OF GROWTH 333 faster than one under similar conditions but without ether. If the plant is subjected to an increased quantity of ether, growth is retarded. The effect of these poisons is thus very different from that of nutritive substances ; it is due to the irritating properties of the poison. h. The Election of Organic Food. — Not all of the food-stuff presented to the organism is utilized by it — neither, on the one hand, all of the kinds of food, nor, on the other, all of the food of the most acceptable kind if offered beyond a certain amount. There is an election of kind and of amount. A study of an election of kind has been made by DucLAUx ('89), and, especially, Pfeffee, ('95). The method employed was this : To the organisms (various molds, Aspergillus, Penicil- lium, etc.) were offered two compounds ; one more nutritious, the other less so. Under these circumstances, the more nutri- tious compound was usually taken by the organism, while the less nutritious was often left entirely alone ; thus, dextrose was preferred to glycerine, peptone to glycerine, and dextrose to lactic acid, even when, in each case, the quantity of the latter substance was in excess of the former. The election was not, however, always of the more nutritious material, so far as we can judge of relative nutritiveness. Thus, when Aspergillus was sown on a nutritive fluid, containing 8% dex- trose and \(fo acetic acid, proportionally far more of the latter was assimilated than of the former, although the latter is of less value as food, as was shown by the fact that the plant had also to devour a considerable quantity of the dextrose. This extensive assimilation of a slightly nutritive substance is not,, so far as we can see, an adaptive process. The character of the election may change with age, so that what is favorable for growth at one time is not at another. Thus DiTCLAUX ('89) found that alcohol restrains or arrests the germination of the spores of molds, whereas it is made use of almost as abundantly as sugar by the adult plant ; so, like- wise, lactose and mannit cannot nourish young plants when they replace sugar in the nutritive solution, whereas they are a good food for the older plants. So, also, among vertebrates, the food of the young, supplied in the Q^g or in the milk of the parent, is very unlike that 334 EFFECT OF CHEMICAL AGENTS [Cir. XI wliich is most favorable for growth in later stages. So impor- tant is this difference of food at different ages that agriculturists persistently change the ratio of the different foods supplied as their animals increase in age. The reason for this change in food required lies doubtless in this, that the chemical processes of growth change with the age of the animal ; at first imbi- bition of water predominates, then comes the secretion of the various formed substances of the organism and the constant maintenance and increase of the plasma. The most favorable food of an organism at any time is dependent upon the metabolic processes going on at that time. The election of quantity is not less striking. It is well known that an increase above a certain limit in the amount of food presented to an organism or even actually taken into its body does not result in any increase in growth. There is a certain amount, fixed within broad limits, which corresponds to a maximum of nutritiveness. This amount, this feeding capacity, is not, hoAvever, necessarily constant at all stages of the adult growth of the organism. For it has been observed in certain animals, e.g. pigs, that as they grow older there is a steady increase in the amount of food required to produce a pound of gain in weight. Such facts serve to indicate that the rate of growth is largely determined by internal factors. Let us now summarize the results of this study of the effect of chemical agents upon the rate of growth. Of foods, those used in the plastic processes are chiefly to be considered. The substances serving as plastic food must contain all the elements normally occurring in the organism. These are found in di- verse proportions in different organisms, and hence the neces- sity of dissimilar foods. Not only carbon, oxygen, hydrogen, and nitrogen are necessary, but a whole series of other elements, such as phosphorus, sulphur, chlorine, iodine, sodium, potas- sium, calcium, iron, and magnesium are more or less essential. The organic plastic food varies with the group of organisms. Of relatively little importance for green plants, it becomes essential to fungi. In this group we find the most important character of a nutritive substance to be a certain degree of lability. Among animals a mixed diet is especially beneficial, but nitrogenous food favors growth more than non-nitrogenous §2] UPON THE DIRECTION OF GROWTH 385 food. Finally, growth must be studied as a response to chem- ical agents which may stimulate the protoplasm to absorb and assimilate them to the degree required by the organism, or which may stimulate the protoplasm to absorb some other sub- stances, as we have seen in the case of zinc sulphate. The quantity and quality of food needed will, moreover, vary with the age and other qualities of the organism. The consumption of food both in quantity and in quality will be closely deter- mined by the demand. All these complexities in the process of nutrition indicate that it, like other processes in~ organisms, can only be explained on the assumption of a vastly complex molecular organization of the protoplasm. § 2. Effect of Chemical Agents upon the Direction OF Growth — Chemotropism One of the most common processes in the early development of organisms is the turning or bending of a filament, tubule, or lamella. The cause of this turning is clearly an unequal growth of the two sides of the organ. When the bending organs are internal, their movements are largely removed from experimental study ; when external, as in plants,- they more easily lend themselves to our investigation. The object of this investigation is always to find in how far the direction of these* tropic movements is determined or is determinable by external agents. In the present section it is proposed to consider in how far tropic movements are determined by chemical agents. At the outset it must be said that the growth which gives rise to these hendings is frequently due to imbibition of water ; and in such cases it may be only temporary. Yet these temporary bendings pass by such insensible gradations into permanent ones that a sharp distinction between the two is impracticable and unim- portant. Rejecting such a classification of the subject, we may adopt one based on the tropic organ. 1. Chemotropism in the Tentacles of Insectivorous Plants. — This case of chemotropism was the earliest to be observed : it was DAKWiisr ('75, p. 76) who first called attention to it. He found that when drops of w^ater or solutions of non-nitrogenous 336 EFFECT OF CHEMICAL AGENTS [Cii. XI compounds are placed upon the leaves of the sundew, Drosera^ the tentacles remain uninflected; but when a drop of a nitroge- nous fluid, such as milk, urine, albumen, infusion of raw meat,, saliva, or isinglass, is placed on the leaf, the tentacles quickly bend inwards over the drop. Darwik now set to work sys- tematically to determine which salts and acids cause and which do not cause inflection. Of nine salts of ammonia tried, all caused inflection of the tentacles, and of these the phosphate of ammonia was the most powerful. Sodium salts in general cause inflection while potassium salts do not. The earthy salts are in general inoperative, as are likewise those of lead, manganese, and cobalt. The more or less poisonous salts of silver, mercury, gold, copper, nickel, platinum, and chromic and arsenious acids produce great inflection with extreme quick- ness. Other substances which caused inflection were nitric,, hydrochloric, iodic, sulphuric, phosphoric, boracic, and many organic acids ; gallic, tannic, tartaric, citric, and uric acids alone being inoperative. In all these cases, where a bending of the tentacles over the drop occurs, the turning must be regarded as a response to the stimulus of the chemical sub- stance. An excitation proceeds from the irritated region to the protoplasm upon whose imbibitory activity the turning of the tentacles depends. 2. Chemotropism of Roots. — Attention was directed to the fact that roots turn towards or from chemical substances by MoLisCH ('84), who experimented with gases. When grains of maize or peas are sprouted in water, their roots will turn Fig. 91. — Seedling: of Zea, whose radicle originally was just touching the water obliquely with its apex and thereafter nutated in characteristic fashion, keeping close to the air. (From Molisch, '84.) § 2] UPON THE DIRECTION OF GROWTH 337 upwards towards tlie surface of the water — in response to the more abundant oxygen supply there (Fig. 91), and will grow along the surface of the water. Molisch undertook a sys- tematic investigation of the action of various gases in con- trolling this growth. The method employed was as follows : The gases were enclosed in glass vessels whose mouth was closed by a plate of hard rubber perforated by slits, 2 cm. long by 2 mm. broad. The vessel being laid on its side so that the slits were vertical, the rootlet of a germinating grain was placed in front of it. As the gas diffused from the vessel, it was for some time in excess upon one side of the rootlet. The gases experimented with were pure oxygen, pyrogallic acid, nitrogen, carbon dioxide, chlorine, hydrochloric acid gas, illuminating gas, ammonia, nitrous oxide, ether, chloroform, and oil of turpentine. In all cases there occurred, generally after about an hour, a turning of the root towards the gas (positive aerotropism, Molisch), followed by a marked curvature from the slit (neg- ative aerotropism). Since decapitated roots respond in the same way as intact ones, but in less degree, Molisch con- cluded that the gases affect the growing region directly and do not require the intervention of the root-tip. 3. Chemotropism of Pollen-tubes. — The suggestion was early made by Pfeffer ('83), as a consequence of his discovery of chemotaxis in swarm-spores, that perhaps the bending of the antheridium-tube of Saprolegnia towards the oogonium was a case of response to a chemical agent. Steasbueger ('86) offered a similar suggestion for phanerogams. Pfeffer ('88) then made experiments, but was unable to control the direction of growth of pollen-tubes. Molisch ('89 and '93) was next led to undertake further study in this direction by the obser- vation that, when various pollen-grains are germinating in a nutritive drop and a cover-glass is placed over them, the pollen- tubes, after approaching near to the edge of the cover-glass, turn away towards the centre again (Fig. 92, a, 5). The move- ment from the margin of the cover-glass cannot be ascribed to a difference of density produced by evaporation at the margin, for it occurred in a saturated atmosphere ; nor can it be due to surface tension of the bounding film of water, for the turning occurred before the surface film was reached. These results 338 EFFECT OF CHEMICAL AGENTS [Ch. XI were abundantly confirmed by Miyoshi ('94^), so we must conclude that the pollen tube is negatively aerotropic to oxy- gen. However, this negative aerotropism does not occur in all pollen, for that of Orobus vernus and various other legumes, of Primula acaulis, Viola odorata, V. hirta, etc., were indifferent. Fig. 92. — Illustrates chemotropisra of pollen-tubes, a. Negative chemotropism with reference to the air (aerotropism) of pollen-tubes of Narcissus tazetta ; the tubes are growing under a cover-glass in a 7% sugar solution and turn at the edge, a, b, from the air; magnified about 20. 6. Negative aerotropism of pollen-tubes of Cephalanthera pallens, after 20 hours; a, b, edge of cover-glass. c. Stigma of Narcissus tazetta in 7% sugar solution ; pollen-tubes grow towards the stigma ; magnified about 10. (From Molisch, '93) . A second class of chemotropisms is seen in the turning of pollen-tubes towards the stigma of a flower.* When pollen is sown upon a plate of agar -agar or gelatine on which the upper end of a ripe pistil has been placed, the tubes are sent out in * Molisch accounts for the failure of some of the earlier experiments with pollen-tubes on the ground that certain pollen-tubes do not exhibit this class of chemotropism. Among these are Viola odorata, V. hirta, Orobus vernus, etc. — species which are likewise not aerotropic. §2] UPON THE DIRECTION OF GROWTH 339 all directions at first, but quickly grow towards the pistil (Fig. 92, c). MiYOSHi ('94'') found that the top of the pistil was most attractive and that lower sections were less so until the ovary is nearly reached, when the attraction is high again. If the ovules of Scilla are placed on the plate of agar with its own pollen, the pollen-tube will even grow into the micropyle of the ovule. Pollen-tubes of a different species or even genus, e.g. Diervilla rosea. Ranunculus acer, etc., may likewise enter the Scilla ovule, and even hyphse of Mucor stolonifer will turn towards the ovules and penetrate into them. Thus the attract- ing stuff is not a specific stimulant confined in its activity to one kind of pollen-tube nor even to pollen-tubes in general. The nature of the attracting substance has been studied by MiYOSHi. Glucose is certainly present in the fluid excreted' by the ripe stigma of many phanerogams ; and, if the agar-agar substratum contain a 2% solution of cane sugar, there is no longer chemotropism with reference to the ovule, since now the attraction is not confined to a particular point. So it may be concluded that it is the sugar of the ovule or stigma which attracts the pollen-tube and that the excreted fluid contains about a 2% solution. Consequently, the chemotropism of pollen-tubes may be only a special case of chemotropism to sugar. Further experimentation confirmed Miyoshi ('94) in this conclusion. Using the method employed by him in the case of hyphce (p. 340), he injected Tradescantia leaves with various solutions and sowed pollen of Digitalis purpurea upon the leaves. When pure water was injected there was no effect, but the substances named in the foUoAving list attracted the pollen-tubes so that they grew down through the stomata into the leaf : — cane sugar . 2-8 <% levulose . 1 % (slight action) grape sugar, 4-8% lactose . 1-2% ("slight") dextrin . . 1-2% The following solutions were neutral : — maltose . . . . . 1% asparagin ... 2% meat extract . . . glycerine . . . 2-5% peptone gum arable . . 2% 340 EFFECT OF CHEMICAL AGENTS [Ch. XI The following were repellent : — alcohol .... 0.1% potassium sulphate . 1% ammonium sulphate, 1% sodium malate . 0.5-2% It will be observed that all the attracting substances are sugars, 4. Chemotropism of Hyphae. — While various authors had noticed an apparent movement of hyphss towards certain chemical agents or towards their hosts, the earliest systematic observations upon this subject are those of Wortmann ('87). He placed fly-legs and other nutritive substances in Saprolegnia cultures and noticed that the hyphye left their original direc- FiG. 93. — Negative chemotropism of a hypha of Peziza trifoliorum from the secre- tions of a mycelium of Aspergillus niger. (From Reinhardt, '92.) tions to grow straight towards the food substance. Later Reinhardt ('92) showed that hyphse of Peziza may be lured out of their straight direction by spores of Mucor placed near them ; or, again, if a plate of gelatine rich in sugar be placed above a plate of pure gelatine upon which hyphse of Peziza are growing, all hyphse will send up branches to meet the more nutritive surface ; or, again, if Aspergillus niger, whose secre- tions are fatal to Peziza, is placed near the latter, the hyphse cease to grow at a distance of about 2 mm. from the Aspergillus and then send out shoots which grow away from the injurious substance (Fig. 93). The most exhaustive studies on this subject are, however, those of MiYOSHi ('94), who worked with germinating spores of Mucor mucedo and M. stolonifer, Phycomyces nitens, Peni- cillium glaucum, Aspergillus niger, and Saprolegnia ferox. Perforated membranes were employed, either in the form of plant epidermis with stomata or of collodion films perforated §2] UPON THE DIRECTION OF GROWTH 341 by a fine needle point. Tlie solutions were injected into the leaf of Tradescantia, spores were sown upon its stoma-bearing surface, and the whole was kept in a moist chamber. If the solution was attractive, the growing hyphse penetrated into the stomata, whereas in the absence of the solution they showed no tendency to do so. Similarly, spores sown on the perforated plate sent hyphge downwards through the holes when the plate was floating on attractive solutions, but not otherwise. Mole- FiG. 94. — Upper figure : Piece of the under side of a leaf of Tradescantia discolor injected with 2% ammonium chloride, and sown with spores of Mucor stolonifer. The young hyphse show chemotropic turnings, and have eventually penetrated into the stomata. Drawn 27 hours after sowing the spores ; magnified 100. Loiver figure : Penicillium glaucum growing on a leaf of Tradescantia, which has heen injected with a 2% solution of cane sugar. The hyphae have branched, and the branches have penetrated into the stomata. Drawn 25 hours after sowing the spores; magnified 70. (From Miyoshi, '94.) cules diffusing out from the solution through the openings determine the direction of the growing hyphse, so that from all directions hyphse grew radially towards the openings in the membranes (Fig. 94). To prove that the result gained was truly chemotropism, and not something else, a series of experiments was made. That it was not a response to gravity was shown by sowing the spores below as well as on top of a leaf ; that it was not a response to light or moisture was shown by keeping the cult- 342 EFFECT OF CHEMICAL AGENTS [Ch. XI ures in a dark, moist chamber ; in both cases, the chemotropic responses occurred. That contact was not the determining factor was shown by placing above and below sheets of perfo- rated collodion a layer of 5% gelatine, deprived of calcium salts, which are attractive. One of these layers was fertilized with grape sugar, the other remained sterile. The spores were sown sometimes in the sterile, sometimes in the fertile layer of gelatine ; in the former case, the hyphse always grew (up or down) into the sterile layer ; in the other case, they remained in the fertile layer. If both gelatine layers were sterile, or if both were equally fertile, the hyphse did not grow through the holes in the membranes. Not the holes, but the stuff diffusing from them, determined the direction of growth of the hyphse. There is a close relation between the chemical constitution of the agents and their effects. The following substances are attractive : compounds of ammonium (ammonium nitrate, chloride, malate, tartrate), phosphates (of potassium, sodium, ammonium), meat extract, peptone, sugar, asparagin, lecithin, etc. The following are neutral : glycerine and gum arable (1 to 2%). The following are repellent : all free inorganic as well as organic acids, alkalis, alcohol, and certain salts, e.g. potassium-sodium tartrate, potassium nitrate, calcium nitrate, potassium chloride (2%), potassium chlorate (8%), magne- sium sulphate, sodium chloride (2%), ferric chloride (0.1%), phosphoric acid, etc. Comparing this list with that which Pfeffer and Stange tried on swarm-spores (Pt. I, pp. 36- 38), we find that there is a rather close correlation. In both cases, glycerine (a good food) is neutral, alcohol repellent, phosphates attractive. As in the reaction of swarm-spores, so in those of hyphas, Weber's law is followed. 5. Chemotropism of Conjugation Tubes in Spirogyra. — This case is closely allied to chemotropism of pollen-tubes. Attention was first called to it by Overton ('88), who observed that at the point where the tubes were about to arise bacteria accumulated from the surrounding water. From this phenomenon, and on other grounds, he was led to conclude that a substance is excreted at this point which exercises a directive influence upon the conjugation tubes, insuring their meeting. This conclusion has been confirmed by Haberland LITERATURE • 343 ('90), who finds additional evidence for it in the character of the turnings which the two tubes from the opposite cells undergo in order successfully to impinge upon each other. The results of experimentation upon chemotropism show that various substances may direct the growth of such elon- gated organs as the tendrils, roots, and hyphse of plants ; so that greater growth takes place on the side turned from the region of greatest concentration or towards it, as the case may be. In many instances it can be shown that the direction of growth is on the whole an advantageous one for the organism — so that the directed growth may be considered an adaptive one. In other cases, however, the response seems to have nO' relation to adaptation. If that which controls direction is the unequal concentration, of chemical agents in the medium, the immediate cause is excessive growth on one side, due to excessive imbibition or to excessive assimilative activity. The relation between these causes is doubtless complicated. The chemical agent acts upon the protoplasm, changing its molecular structure ; the changed protoplasm exhibits changed growth activities. Thus we say the chemical agents act as stimuli. LITERATURE Aeby, J. H. '96. Beitrag zur Frage der Stickstoffernahrung der Pflanzen.. Landwirthsch. Versuchs-Stat. XL VI, 409-439. AscHOFF, C. '90. Ueber die Bedeutung des CMors in der Pflanze. Landw.. Jahrbiicher. XVIII, 113-141. Bassler, p. '87. Die Assimilation des Asparagins durcli die Pflanze. Landwirthsch. Versuclis-Stat. XXXIII, 231-240. Baumann, E. '95. Ueber das normale Vorkommen von Jod im Thier- korper. (I. Mitth.) Zeitschr. f. physioL Chemie. XXI, 319-330. 28 Dec. 1895. Baumann, E. and Roos, E. '96. Idem (IL Mitth.), ibidem. XXL 481-493. 2 Apr. 1896. Baumann, E. '96. Idem (III. Mitth.), ibidem. XXII, 1-15. 16 May, 1896. Benecke, W. '95. Die zur Ernahrung der Schimmelpilze nothwendigen Metalle. Jahrb. f. wiss. Bot. XXVIIL 487-530. '96. Die Bedeutung des Kaliums und des Magnesiums fiir Entwickehmg und Wachsthum des Aspergillus niger v. Th., sowie Einiger anderer Pilzformen. Bot. Ztg. LIV. 344 EFFECT OF CHEMICAL AGENTS [Cir. XI Berthelot, '85. Fixation directe de I'azote atmospherique libre par cer- tains terrains argilenx. Comp. Rend. CI, 775-784. 26 Oct. 1885. '92. Nouvelles recherches sur la fixation de I'azote atmospherique par les microbes. Comp. Rend. CXV, 569-574. 24 Oct. 1892. '93. Reclierches nouvelles sur les microorganismes fixateurs de I'azote. Comp. Rend. CXVI, 842-849. 24 Apr. 1893. Bert, P. '78. La pression barometrique. Recherches de physiologic ex- perimentale. 1168 pp. Paris, 1878. Beyerinck, M. W. '93. Bericht iiber meine Kulturen niederer Algen aiif Nahrgelatine. Centralbl. f. Bakteriol. u. Parasitenkunde. XIII, 368- 373. 23 Mar. 1893. '96. Kulturversuche mit Amoben auf festem Substrate. Centralbl. f. Bakteriol. u. Parasitenk. XIX, 257-267. 28 Feb. 1896. Bezold, a. von '57. (See Chapter I, Literature.) BoKORNY, T. '97. Ueber die organische Ernahrung griiner Pflanzen und ihre Bedeutung in der Xatur. Biol. Centralbl. XVII, 1-20; 33-48. Jan. 1897. BoussiNGAULT, J. B. J. D. '60. Agronomie, chemie agricole et physiologie. 2 tomes. Paris, Mallet-Machelier, 1860. Brandl, J., and Tappeimer, H. '92. Ueber die Ablagerung von Fluor- verbindung im Organisraus nach Fiitterung mit Fluornatrium. Ztschr. f. Biol. XXVIII, 518-539. BuNGE, G. '73. LTeber die Bedeutung des Kochsalzes und des Verhalten der Kalisalze im menschlichen Organismus. Ztschr. f. Biol. IX, 104-143. '74. Der Kali-, Natron-, und Chlorgehalt der Milch, vergiichen mit dem anderer Nahrungsmittel und das Gesammt-Organismus der Sauge- thiere. Ztschr. f. Biol. X, 295-335. '85. Ueber die Assimilation des Eisens. Ztschr. f. physiol. Chem. IX, 49-59. '89. Ueber die Aufnahme des Eisens in den Organismus des Sauglings. Ztschr. f. physiol. Chem. XIIT, 399-406. Celli, a. '96. Die Kultur der Amoben auf festem Substrate. Centralbl. f. Bakteriol. XIX, 536-538. 25 Apr. 1896. CoPELAND, E. B. '97. The Relation of Nutrient Salts to Turgor. Bot. Gazette. XXIV, 399-416. Dec. 1897. Cook, G. H. '68. Geology of New Jersey. Newark, 1868. Crivelli, G. B. and Maggi, L. '70. Sulla produzione delle Amibe. Rend. R. Inst. Lombardo. (2), III, 367-375. Plate. '91. Ancora sulla produzione delle Amibe. Ibidem. (2), IV, 198-203. Danilewsky, B. '95. De I'influence de la lecithine sur la croissance et la multiplication des organismes. Comp. Rend. CXXI, 1167-1170. 30 Dec. 1895. '96. De I'influence de la lecithine sur la croissance des animaux a sang chaud. Comp. Rend. CXXIII, 195-198. 20 July, 1896. Darwin, C. '75. Insectivorous Plants. London : Murray. 462 pp. 1875. Day, T. C. '94. The Non- Assimilation of Atmospheric Nitrogen by Germi- nating Barley. Trans, and Proc. Bot. Soc. Edinburgh. XX, 29-34. LITERATURE 345 DuCLAUx, E. '89. Sur la nutrition intracellulaire. Ann. de I'lnst. Pasteur. Ill, 97-112. March, 1889. EscHLE '97. Ueber den Jodgehalt einiger Algenarten. Zeitschr. f. physiol. Chemie. XXIII, 30-37. 27 Mar. 1897. EoRCHHAMMER, G. '61. Resultater af sine Undersogelser, saavel over Salt- msengden i Middelhavets Vand. Overs. K. danske Vidensk Selskabs Forhangl. Aaret. 1861. 379-391. Prank, A. B. '88. Die Ernabrung der Pflanzen mit Stickstoff. Berlin, 1888. '92. Lehrbuch der Botanik, I. 669 pp. Leipzig : Engelmann. '93. Die Assimilation des freien Stickstoffs durch die Pflanzenwelt. Bot. Ztg. LI, 139-156. Eredericq, L. '78. Sur I'organisation et la physiologie du Poulpe. Bull. de I'Acad. roy. de Belg. XLVT, 710-765. '79. Note sur le sang du Ilomard. Ibid. XLVII, 409-413. ■GoRiNi, C. '96. Die Kultur der Amoben auf festem Substrate. Centralbl. f. Bakteriol. XIX, 785. Haberland, G. '90. Zur Kenntnis der Conjugation bei Spirogyra. Sitz- ungsber. d. Akad. d. wiss. Wien. Math.-Xat. CI. XCIX^, 390-400. 1 Tafel. Hansteen, B. '96. Beitrage zur Kenntniss der Eiweissbildung und der Bedingungen der Realisirung dieses Processes im phanerogamen Pflan- zenkorper. Ber. deutsch bot. Ges. XIV, 362-371. 28 Dec. 1896. Hartig, R. and AVeber, R. '88. Das Holz der Rotbuche. Berlin, 1888. [Quoted from Loew, '91, p. 270.] Hellriegel, H. '86. Welche Stickstoffquellen stehen der Pflanze zu Gebote? Tageblatt des 59. Versammelung, Deutsclier Xaturforscher und Aerzte. Berlin, 1886. p. 290. Hensen, V. and Apsteen, C. '97. Die Xordsee-Expedition, 1895, des Deutschen Seefischerei-Vereins. Ueber die Eimenge der im Winter laichenden Fisclie. Wiss. Meei-esuntersuchungen, herausgegeben von der Kommission zur wiss. Unters. d. deutschen Meere in Keil, X. F. Band II, Heft. 2, pp. 1-98. Herbst, C. '97. Ueber die zur Entwickelung der Seeigellarven nothwendigen anorganischen Stoffe, ihre RoUe und ihre Vertretbarkeit, I. Theil. Die zur Entwickelung nothwendigen anorganischen Stoffe. Arch. f. Entwickelungsmechanik d. Organismen. V. 649-793. Taf. XII-XIV. 9 Xov. 1897. Heraeus '86. Zeitschr. f. Hygiene. I. Hoppe-Seyler, F. '81. Physiologische Chemie. Berlin, 1881. Hotter, E. '90. Ueber das Vorkomraen des Bor im Pflanzenreich und dessen physiologische Bedeutung. Landw. Versuchs-Station. XXXVII, 437-458. 1890. Jaccard, p. '93. Influence de la pression des gaz sur le developpement des vegetaux. Conip. Rend. CXVI, 830-833. 17 April, 1893. Jentys, S. '88. Ueber den Einfluss hoher Sauerstoffpressungen auf das Wachsthum der Pflanzen. Unters. a. d. bot. Inst. Tiibingen. II. 419-464. 346 EFFECT OF CHEMICAL AGENTS [Ch. XI Johnson, S. W. '68. How Crops grow. 394 pp. New York. Jordan, E. O. and Richards, E. '91. Investigations upon Nitrification and the Nitrifying Organism. From Report, Mass. State Board of Health, Water Suj)ply, and Sewage. Part II. pp. 865-881. [Dated 1890.] Kemmerich, E. '69. Untersuchungen iiber die physiologische Wirkung der Fleischbriihe, des Fleischextracts und der Kalisalze des Fleisches. Arch. f. d. ges. Physiol. II, 49-93. Koch, A. and Kossowitsch, P. '93. Ueber die Assimilation von freiem Stickstoff durch Algen. Bot. Ztg. LI, 321-325. Nov. 1, 1893. Kossowitsch, P. '94. Untersuchungen iiber die Frage, ob die Algen freien Stickstoff fixiren. Bot. Ztg. LII, 97-116. 16 May, 1894. Krukenberg, C. F. W. '80. (See Chapter II, Literature.) '78. Mangan ohne nachweisbare Mengen von Eisen in der concretionen aus dem Bojanus'schen Organe im Pinna squamosa Gm. Unters. Physiol. Inst. Univ. Heidelberg. II, 287-289. KuNKEL, A. J. '91. Zur Frage der Eisenresorption. Arch. f. d. ges.. Physiol. L, 1-24. 25 June, 1891. '95. Blutbilduug aus anorganischen Eisen. Arch. f. d. ges. PhysioL LXI, 595-606. 25 Sept. 1895. Laurent, E. '88. Sur la formation d'amidon dans les plantes. Bruxelles, 1885. Lawes, J. B. and Gilbert, J. H. '53. On the Composition of Foods, in Relation to Respiration and the Feeding of Animals. Rept. 22d Meet. British Ass. Adv. Sci. for 1852, 323-353. Lawes, J. B., Gilbert, J. H. and Pugh, E. '61. On the Sources of the Nitrogen of Vegetation, wdth Special Reference to the Question whether Plants assimilate Free or Uncombined Nitrogen. Philos. Trans. Roy- Soc. London. CLI, 431-577. Liebscher '93. Beitrag zur Stickstofffrage. Jour. f. Landwirtschaft. XLT. [not seen.] LoEB, J. '92. (See Chapter X, Literature.) LoEW, O. '91. Die chemische Verhaltuisse des Bakterienlebens. CentralbL f. Bakteriol. IX, 659-663. '91^ Ueber die physiologischen Functionen der Phosphorsaure. Biol. CentralbL XI, 269-281. 1 June, 1891. '96. The Energy of the Living Substance. 116 pp. London. 1896. LoEW, O. and Bokorny, T. '87. Chemisch-physiologische Studien iiber Algen. Jour. f. prakt. Chem. XXXVI, 272-291. LiJPKE, R. '89. Ueber die Bedeutung des Kaliums in der Pflanze. Landw^ Jahrb. XVII, 887-913. Macallum, a. B. '91. On the Demonstration of the Presence of Iron in Chromatin by Micro-chemical Methods (Abstract). Proc. Roy. Soc. XLIX, 488-489. July 10, 1891. '92. Idem. Proc. Roy. Soc. XL, 277-286. Jan. 20, 1892. '94. On the Absorption of Iron in the Animal Body. Jour, of PhysioL XVI, 268-297. 17 April, 1894. LITERATURE 347 Meyer, A. '85. Ueber die Assirailations-producte der Laubblatter angio- spermer Pflanzen. Bofc. Ztg. XLIII, 417 et seq. MiYOSHi, C. '94. Ueber Chemotropismus der Pilze. Bot. Ztg. LIT, 1-27. 1 Taf. '94a. Ueber Reizbewegungen der PoUenschlauche. Flora. LXXVIII, 76-93. 24 Jan. 1894. MOLISCH, H. '84. Ueber die Ablenkung der Wurzeln von ihrer normalen Wachsthnmsrichtung durch Gase ( Aerotropismus). Sb. k. Akad. d. Wiss. Wien. XC^, 111-191. '89. Ueber die Ursachen der Wachsthumsrichtungen bei PoUen- schlauclien. Sitzungsanzeiger k. Akad. Wiss. Wien. 1889, No. II. '92. Die Pflanze in ihren Bezieliungen zum Eisen. Jena, 1892. '93. Zur Pliysiologie des Pollens, mit besonderer Riicksiclit auf die chemotropischen Bewegiingeu der PoUenschlauche. Sb. k. Akad. d. Wiss. Wien. CIli, 423-448. '94^. Ueber Chemotropismus der Pilze. Bot. Ztg. LII^, 1-27. '94^ Ueber Reizbewegung der PoUenschlauche. Flora. LXXVIII, 76-93. 24 Feb. 1894. '94=. Die mineralische Niihrung der niederen Pilze. Sb. Wien. Akad. CIIIi, 554-574. '95. Die Ernahrung der Algen. Siisswasser Algen. I. Abhandl. Sb. Akad. Wiss. Wien. CIVi, 783-800. '96. The same. IL Abhandlung. Sb. Akad. Wiss. Wien. CVi, pp. 633-648. Monti, R. '95. Sur les cultures des amibes. Arch. Ital. de Biol. XXIV, 174-176. Nageli, K. v. [with Loew, O.]. '80. Ernahrung der niederen Pilze durch Kohlenstoff- und Stickstoffverbindungen. Sb. Miinchen Akad. X, 277-367. Nencki, M. '94. Note sur les pretendues cendres des corps albumen o'ides. Arch, des sci. biol. St. Petersburg. Ill, 212-215. Nencki, M. and Schoumow-Simanowsky, E. O. '94. Etudes sur le chlore et les halogenes dans I'organisme animale. Arch, des sci. biol. St. Petersburg. Ill, 191-211. Nobbe, F., Schroder, J. and Erdmann, R. '71. Ueber die organische Leistung der Kaliums in der Pflanze. Landw. Versuchs-Stat. XIII, 321-399; 401-423. Nobbe, F. and Hiltjster, L. '95. Vermogen auch Nichtlegumenosen freier Stickstoff aufzunehmen ? Landw. Versuchs-Stat. XLV. 155-159. Ogata, M. '93. Ueber die Reinkultur gewisser Protozoen (Infusorien). Centralbl. f. Bakteriol. XIV, 165-169. 7 Aug. 1893. Overton, C. E. '88. Ueber den Conjugationsvorgang bei Spirogyra. Ber. D. bot. Ges. VI, 68-72. Petermann, a. '91. Contribution a la question de I'azote. Premiere note. Mem. cour. et autres mem. de I'acad. Roy. de Belg. Collection in 8vo. XLIV. 23 pp. 1 pi. Jan. 1891. '92. Seconde note. Same Memoires. XL VII, 1-37, 1 pi. 348 EFFECT OF CHEMICAL AGENTS [Cii. XI Pfeffer, W. '83. (See Chapter I, Literature.) '88. Ueber chemotactische Bewegungen von Bakterien, Flagellaten iind Volvocineen. Uiiters. a. d. bot. Inst. Tubingen, II. 582-661, '95. Ueber Election organischer Nahrstoffe. Jalirb. f. wiss. Bot. XXVIII, 205-268. Pfeiffer, T. and Franke, E. '96. Beitrag zur Frage der Vervtrertung ele- mentaren Stickstoft's durch den Senf. Landw. Versuchs-Stat. XL VI, 117-151. Taf. I. PoLECK, T. '50. Analyse der Asche von Eiweiss und Eigelb der Iluhnereier. Ann. de Physik und Chemie. LXXVI, 155-161. 7 Feb. 1850. Presch, W. '90. Ueber das Verhalten des Schwefels iin Organismus und den Nachweis der unterschwefligen Saure im Menschenharn. Arch. f. path. Anat. u. Physiol. CXIX, 148-167. 2 Jan. 1890. Proscher '97. Die Beziehiingen der Wachsthumsgeschwindigkeit des Siiuglings zur Zusammensetzung der Milch bei verscheidenen Sauge- thieren. Zeitschr. f. physiol. Chem. XXIV, 285-302. 22 Dec. 1897. Puriewitsch, K. '95. Ueber die Stickstoffassimilation bei den Schiuniiel- pilzen. Ber. D. bot. Ges. XIII, 312-345. 27 Xov. 1895. Rauber, a. '84. Ueber den Einfluss der Temperatur, des atmospharischen Druckes und verschiedener Stoffe auf die Entwicklung thierischer Eier. Sitzungsber. d. naturf . Gesell. Leipzig. X, 55-70. Kaulin, J. '69. Etudes chimiques sur la vegetation. Ann. des Sci. Nat. (Bot.). (5), XI, 93-299. E.EINHARDT, M. O. '92. Das Wachsthum der Pilzhyphen. Jahrb. f. wiss. Bot. XXIII, 479-599. Richards, H. M. '97. Die Beeinflussung des Wachsthums einiger Pilze durch chemische Reize. Jahrb. f. wiss. Bot. XXX, 665-688. Roos, E. '96. Ueber die Wirkung der Thyrojodins. Zeitschr, f. physiol. Chem. XXII, 16-61. 16 May, 1896. Sachs, J. v. '87. Vorlesungen liber Pflanzenphysiologie. Leipzig, Engelmann. ScHiMPER, A. F. W. '90. Zur Frage der Assimilation der Mineralsalze durch die grune Pflanze. Flora. LXXIII, 207-261. ScHLOSiNG, T. fils et Laurext, E. '92. Recherches sm- la fixation de I'azote libre par les plantes. Ann. de I'lnst. Pasteur. VI, 65-115. Feb. 1892. '92. Sur la fixation de I'azote libre par les plantes. Ann. de I'lnst. Pasteur. VI, 824-840. Dec. 1892. ScHLCESiNG, T., and Muntz, A. '77. Sur la nitrification par les ferments organises. Comp. Rend. LXXXIV, 301-303. 12 Feb. 1877. '79. Recherches sur la nitrification. Comp. Rend. LXXXIX, 891-894. 24 Nov. 1879. Schneider, R. '89. Verbreitung und Bedeutung des Eisens im animalischen Organismus. Humbolt. VIII, 337-345. Sept. 1889. '95. Die neuesten Beobachtungen iiber naturliche Eisenresorption in thierischen Zellkernen und einige charakteristische Fiille der Eisen- verwerthung im Korper von Gephyreen. Mitth. a. d. Zool. Stat, zu Neapel. XII, 208-215. 6 July, 1895. I LITERATURE 349 ScHULZ, H. '88. Ueber Hefegifte. Arch. f. ges. Physiol. XLII, 517-541. 20 March, 1888. '93. Ueber den Schwefelgehalt menschlicher und thierischer Gewebe. Arch. f. d. ges. Physiol. LIV, 555-573. 7 July, 1893. SociN, C. A. '91. Ill welclier Form wird das Eiseii resorbirt? Zeitschr. f. physiol. Chem. XV, 93-193. 10 Jan. 1891. Stoklasa, J. '96. Studien iiber die Assimilation elementaren Stickstolfs durch die Pflanze. Landw. Jahrb. XXIV, 827-863. Strasburger, E. '86. Ueber fremdartige Bestaubung. Jahrb. f. wiss. Bot. XVII, 50-98. Tammann, G. '88. Ueber das Vorkommen des Fluors in Organismen. Zeitschr. f. physiol. Chem. XII, 322-326. Tappeiner, H. '93. Ueber Ablagerung von Fluorsalzen im Organismus nach Futterung mit Fluornatrium. Sb. Ges. Morph. u. Physiol. Munchen. VIII, 22-26. Townsend, C. O. '97. The Correlation of Growth under the Influence of Injuries. Ann. of Bot. XI, 509-532. Vines, S. H. '86. (See Chapter VIII, Literature.) VoLKMANN, A. W. '74. Untersuchungen iiber das Mengenverhaltniss des Wassers und der Grundstoffe des menschlichen Korpers. Ber. d. Sachs. Ges. d. Wiss., Leipzig. XXVI, 202-247. Wieler, a. '83. Die Beeinflussung des Wachsens durch verminderte Par- tiarpressung des Sauerstoffs. Unters. a. d. bot. Inst. Tubingen. I, 189-232. WiNOGRADSKY, S. '84. Ueber die Wirkung ausserer Einflusse auf die Entwicklung yon Mycoderma viiii. Arb. St. Petersburger Naturf. Ges. XIV, 132-135 [Russian] . Abstr. in Bot. Centralbl. XX, 165-167. '87. Ueber Schwefelbacterien. Bot. Ztg. XLV, 493 et seq. '89. Recherches physiologiques sur les sulphobacteries. Ann. I'lnst. Pas- teur. Ill, 49-60. Feb. 1887. "90. Recherches sur les organismes de la nitrification. Ann. I'Inst. Pasteur. IV, 257-275. May, 1890. '95. Recherches sur I'assimilation de I'azote libre de I'atmosphere par les microbes. Arch, des sci. biol. de St. Petersb. Ill, 297-352. Wolff, E. v. '81. Ueber die Bedeutung der Kieselsaure fiir die Haferpflanze Landw. Versuchs-Stat. XXVI, 415-417. '65. Mittlere Zusamniensetzung der Asche, aller land- und forstwirth- schaftlichen wichtigen Stoffe. Stuttgart, 1865. Woi.TERiNG, H. W. F. C. '95. Ueber die Resorbbarkeit der Eisensalze. Zeitschr. f. physiol. Chem. XXI, 186-233. 26 Nov. 1895. WoRTMANN, J. '87. Zur Kenntniss der Reizbewegung. Bot. Ztg. 812. Yung, E. '83. Contributions a I'histoire de I'influence des milieux physico- chimiques sur les etres vivants. Arch, de Zool. (2). I, 31-52. CHAPTER XII THE EFFECT OF WATER UPON GROWTH We have already, in Chapter X, laid stress upon the im23or- tance of the imbibition of water for the growth of both plants and animals. Here we may consider more in detail the rela- tion between growth and water, both as concerns the rate or quantity of growth and the direction of growth, or hydro- tropism. § 1. Effect of Water upon the Rate and Quantity OP Growth It naturally follows from what we know of the importance of water for growth, that the rate of growth will be closely dependent upon water supply. And as all growth-phenomena have been better studied in plants than in animals, our further illustration of this fact Avill be drawn chiefly from the former. First, plant germination demands a certain minimal quantity of water. What this quantity is may be determined either by finding the least amount which will permit of normal germina- tion, or by measuring the amount absorbed by different seeds before protruding tlieir radicles. This latter quantity has been shown by the careful determinations of Hoffmann ('65, p. 52) to vary from between 40% and 60% of the original dry weight, in the case of various cultivated grains, to over 100% in the case of various Leguminosse. In fungi, likewise, Lesage ('95, p. 311) has found that there is a hygrometric limit below which Penicillium spores will not germinate ; and that the interval elapsing before ger- mination begins is the shorter the moister the atmosphere. The method employed by Lesage is of wide applicability. The tension of the water-vapor formed above a saline solution is less than that formed above distilled water ; and it diminishes in proportion to the concentration 350 §1] RATE AND QUANTITY OF GROWTH 351 of the solution. Indeed, to a given solution of a particular salt, e.g. sodium chloride, there corresponds a constant hygrometric state, however much the temperature may vary. This hygrometric condition may be calculated by the formula : 1 — na, where saturation is taken as unity, n equals the number of grammes of sodium chloride dissolved in 100 grammes of water, and a is a constant factor, varying with the salt, and equal to 0.00601 in the case of sodium chloride. The s]3ores were reared in a moist chamber, whose bottom was made by a plate full of the solution. The results of Les age's experiments are given in the fol- lowing table : — TABLE XXXVI Showing Interval in Days elapsing before Germination when Spores OF Pf.nicillium are kept in Moist Chambers over Various Solutions OF Sodium Chloride n 0 21.5 23.5 26.5 30-33.5 Interval . . . 1 6 9 11 No germination after 171 days From these results it follows by calculation that the spores ger- minate at the hygroscopic state of 0.82-0.84 or over, but not below this limit. The foregoing cases, taken from the two principal groups of plants, agree in showing that a certain amount of water is essential to the revival of the metabolic activities which are preliminary to germination. The large quantity of water absorbed by the seed or spore affords the mechanism or the stimulus to growth. Also, in the later stages of plant growth, the water both of the atmosphere and of the soil is essential. The importance of atmospheric moisture was shown by Reinke ('76), who com- pared the size of the leaf-stem in different hygrometric condi- tions. The method of measuring was as follows : a young potted Datura (one of the Solanacese) was placed so that the lower half of one of its stems was horizontal. A fine platinum wire, suspended from a standard above, hung vertically near the stem, and made one turn around it. The lower end of the wire carried a 2-gramme weight. That part of the wire which sur- rounded the stem was protected by a layer of tinfoil. As the stem swelled or grew thinner, the weight rose or fell. The vertical oscillation of the weight measured the variation in circumference of the stem. 352 THE EFFECT OF WATER [Cii. XII It appeared that as the atmospheric moisture increased, a considerable increase in the cross-section of the stem followed ; and as it diminished, the size of the stem diminished likewise. The results are graphically given in Fig. 95. 1 1 N" ^ ' / >i' If (^ 1 A- — / > A \ / 1 r r ' / 1 ( J 1 J \, 1 H \ rti' 1 (D \ u T' J U) ^ 1 \ , \ / H K i V y / \ 1 1 ' . , -' ' . 1 i ' ' \ \, i ' ^ + - J — - — ' / • s ■^ '-1 _^^- — 1 - -i ~ -1 0- -la- -•^ '■- — ^. -e —{ -10- -1 2- -5 - -^ - -e - - -10- -t _!s - ~ - — — - T- - -1- -^ ■•- -» ^ ■^ -^ — — — — -^ — -Jl -^ a.J x; — -f. -i. -J "1 -H-- — — — — - - TZ -^ — \ " ' i 1 Fig. 95. — Curve of growth in thickness of a Datnra stem, M-M, correlated with variations in relative humidity, H-H, the temperature T-T remaining nearly uni- form. The part of the curve falling helow 0-0 indicates loss of thickness below the normal. The ahscissBe represent hours. (From Reinke, 76.) These experiments have been repeated by Francis Darwin ('93) upon the fruit of a gourd, Cucurbita, whose growth is little influenced by variations in temperature. He determined, by means of a delicate micrometer apparatus, the average incre- ment in microns per minute ; and, at the same time, by means of a dewpoint thermometer, the moisture of the air. The relation between rate of growth and psychometric readings is best shown graphically, as in the curves of Fig. 96. The 10 / \ K J \ -^-- / / ^'•' \: \^^ ^N^ /•' ^^"^ ^-^ ^s/ \ ^ / 0 12«ti pm Fig. 96. — Curve of diameter of fruit of Cucurbita (full line) correlated with varia- tion in humidity (broken line). The abscissae represent hours; the ordinates represent growth in ^ per minute (numbers on the left) and per cents of humidity (numbers on the right) . (From Darwin, '93). § 1] UPON THE RATE AND QUANTITY OF GROWTH 353 figure shows plainly how, in general, an increment or a decre- ment in one of these quantities is accompanied by a corre- sponding change in the other. The cause of this relation between changes in volume and in moisture is partially explained by considering the quickness with which increased growth follows increased moisture. It is undoubtedly due, as Tschaplowitz ('86) has suggested, to the diminution in the transpiration of the plant in moist air as compared with dry air. The change in the rate of transpira- tion is, however, not to be conceived as an immediate physical result of the change in moisture, but as a response to the stim- ulus of greater or less water in the atmosphere. The amount of water in the soil also has an important influ- ence on the rate of growth. Quantitative studies on this well- known fact were afforded by Helleiegel, who reared barley in soils which contained various fractional parts of the satura- tion quantity. Giving his results in the form- of a table, we have the following relation between the humidity of the soil and the amount of dry matter produced, after a certain number of days, in the grain and in the chaff : — TABLE XXXVII Production, in Dry Matter. Grains. Chaflf. 80% 8.77 9.47 60 9.96 11.00 (Max.) 40 10.51 (Max.) 9.64 30 9.73 8.20 20 7.75 5.50 10 0.72 1.80 5 0.12 The principal conclusion that one can draw from this table is that there is an optimum humidity of the soil for growth, which is not, however, the same for all organs. More extensive researches upon this subject have been made by JuMELLE ('89), who studied chiefly the effect of water upon the growth of the various organs of the plant, and by 2 a 554 THE EFFECT OF WATER UPON GROWTH [Cir. XH Gain ('92, '95), who studied the effect upon the entire plant, .so that his results are of especial interest here. Gain planted seeds of various species in sand to which a little garden loam had been added. He was careful either to select seeds of equal size or, after sprouting had occurred, to weed out all but the normal, medium-sized •ones. In one set of experiments, the soil contained from 3% to 6% of water; in the other, from 12% to 16%. Gain found that the entire plant grew faster in the humid than in the dry soil, as the accompanying diagram, Fig. 97, Pig. 97. — Curves of fresh weight of two similar seedliligs of flax, one growing in moist, the other in dry, soil. The maximum weight (M) gained by the plant differs in the two cases, and also the time of gaining that weight. F, time of flowering; fin, time of fructification. (From Gain, '95.) indicates. The aerial parts of the plant are more affected than the subterranean. The ratio of growth of plants in moist soil to those in dry varied from 1.12 (radish) to 2.33 (bean). § 2] HYDROTROPISM 355 Among animals the importance of moisture for the growing young is indicated by the fact that even in species living on the land or in the air the eggs and larvae are frequently con- fined to moist situations, as in pulmonate gasteropods, in many insects, and in reptiles. When this is not the case, the eggs are provided with thick, water-containing envelopes, as in birds, or are placed on succulent leaves, or in special fluid- filled receptacles, as in many insects. Only rarely, as for example in the case of the meal worm, Tenebrio, and the Der- mestidse, are the young found growing in a very dry medium. Doubtless in such cases the amount of water required for growth is less than in the cases where the larvse develop in moist situations. To summarize, a minimal quantity of water is essential to germination and growth ; and above this limit growth pro- ceeds more rapidly with the increase in water up to a maxi- mum which varies with the species. § 2. Effect of Water on the Direction of Growth — Hydrotropism A growing organ, such as a leaf, root, or stolon, is normally in a condition of turgescence, as a consequence of the imbibi- tion of water. So long as the turgidity is equal on the two sides of the organ the latter retains its normal position. If the turgidity is diminished on one side, the organ bends towards that side ; if it is increased on one side, the organ bends from that side. Thus, variations in cell turgidity cause changes in the position of organs. This inequality of turgescence on the two sides of an organ may arise in a homogeneous atmosphere ; for certain organs have the capacity in a dry atmosphere of losing water on one side faster than on the other, and in a moist atmosphere of becoming more turgescent on one side than on the other. Con- sequently, the organ assumes a characteristic position accord- ing as the hygroscopic condition of the atmosphere is high or low. Such hygroscopic movements are of wide occurrence among plants, and are often highly adaptive. We see them, for example, in the folding of vegetative parts of the so-called Resurrection Plant of California (Selaginella lepidophylla), by 356 THE EFFECT OF WATER UPON GROWTH [Ch. XII which the whole plant is rolled into a ball capable of being transported by the wind, perchance to a moister region. These hygroscopic movements occurring in a homogeneous medium are to be distinguished from true hydrotropism, for they are not properly growth phenomena. True hydrotropism occurs in growing elongated organs, such as roots or stolons, which grow from or toward a region of greater or less moisture. The observations on this phenom- enon have not been numerous, and are difficult to bring under one point of view ; consequently, we shall do best to classify the cases studied on the basis of the organs considered. 1. Roots. — The first studies upon hydrotropism in roots were made in the middle of the eighteenth century ; but they were crude and uncritical. The first adequate experiments were made by Knight ('11). He half-buried some beans in a flower-pot filled with earth, inverted the pot (in which the earth and seeds were retained by a grating), and kept the earth moist by adding water through the hole in the bottom of the pot. The radicles, instead of growing vertically down- wards as radicles normally do, ran horizontally along the sur- face of the moist earth. The same results were got by John- son ('29), who found in addition that if the mouth of the inverted flower-pot, or other seed receptacle, be placed in a moist atmosphere, the roots grow vertically downwards ; they are no longer turned aside by dry air. Then Duchaetre ('56) discovered that when a seedling was grown in relatively dry earth, with its aerial part in a close, moist chamber, the roots did not penetrate vertically into the soil, but grew out horizontally, and even upwards. Sachs ('72) varied this experiment by planting his seeds in a basket made of netting, fixed to a metallic frame, and hung with its sides inclined at an angle of 45° with the horizontal plane. When the appa- ratus was placed in a damp chamber the radicles grew verti- cally downwards ; but in dry air they turned back towards the bottom of the sieve containing the damp earth, and ran along its under surface. Sachs called especial attention to the fact that it is the damper side which becomes concave ; and this shows that the turning is not due to direct physical causes. The second epoch in the study of hydrotropism now began. § 2] HYDKOTROPISM 357 The fact of its existence being granted, tlie conditions of its occurrence were carefully studied. Thus Darwust ('80, Chap- ter III) investigated the locus of the irritable protoplasm. Some of the young bean-radicles were coated for the distance of a millimetre or two from the apex with a mixture of olive oil and lamp black in order to exclude the moist air. Such showed almost no hydrotropic movements. Killing the tip by caustic produced the same result. Thus the terminal two millimetres or so include the irritable protoplasm. This conclusion was disputed by Wiesnee. ('81, p. 133) and Detlefsen ('82) on the grounds that on the one hand coating or killing the tip introduced abnormal conditions to which the failure of hydrotropism might be ascribed, and, on the other hand, after the tip of the root was cut off a curving might still occur. However, a very careful review of the subject with new experiments by Molisch ('84), a pupil of WiESiSrER, con- firmed Darwin's conclusion. Thus Molisch covered all of the radicle excepting the terminal 1 to 1.5 mm. with wet paper. This upper part could then hardly be irritated by an unequal distribution of moisture in the environment. Never- theless, when a strip of moist filter-paper was placed opposite the tip the hydrotropic response occurred. The response must then have been due to a stimulus received exclusively at the tip, and it may be concluded that the tip alone is stimulated by moisture. Now, although only a millimetre or two of the tip is irritable, the response of bending takes place some distance, 7 to 28 mm., behind the tip, nearly in the region of maximum growth. Thus sensitive and responsive regions do not coincide — there is a transmission of stimuli. The facts that the hydrotropic response occurs in the region of rapid growth and that at the minimum temperature of growth response no longer occurs, indicate clearly that the hydrotropism of roots is not the result of mechanical loss of turgescence on one side, but that it is on the contrary a growth phenomenon — a localized growth which is a response to a stimulus. 2. Rhizoides of Higher Cryptogams. — While it is a priori probable that the rhizoids of hepatics should react like the roots of phanerogams, Molisch desired to demonstrate the fact 358 THE EFFECT OF WATER UPON GROWTH [Cii. XII by experiment. Upon a glass disc was placed a piece of moist filter-paper so large that its edges hung vertically downwards as a flap beyond the margin of the disc. Thalli of various Marchantiacese were placed in sand at the margin of the disc in such a way that the young growing edge projected half a centimetre beyond. The disc and the object on it were ex- posed to • daylight, but slowly rotated in a horizontal plane in order to eliminate phototropic action. The young, positively geotropic, rhizoids which developed beyond the margin of the disc did not. grow vertically downwards, but turned towards the flap of moist filter-paper, thus proving that they are posi- tively hydrotropic. The same is doubtless true of the rhizoids of ferns. 3. Stems. — Very few studies seem to have been made upon the hydrotropism of the stems of seedlings; the most impor- tant are those of Molisch. Several sets of experiments were carried out upon seedlings of flax, pepper-grass (Lepidium sativum), bean, Nicotiana camelina, etc. The method employed was nearly that of Wortmann (see below). Of these plants the hypocotyls of the flax alone showed any hydrotropism ; it may accordingly be concluded that stems are markedly hydro- tropic in but few seedlings. 4. Pollen-Tubes. — The reactions to moisture of these organs have been studied by Miyoshi ('94). He placed pollen-grains on the stigma of the same species and found that whereas in a dark, moist chamber the pollen-tubes grew in all directions, when dry air was admitted the pollen-tubes turned towards the centre of the stigma. This turning is best explained as a response to the greater moisture surrounding the mouth of the stigma. It is clearly also an advantageous result, since it tends to direct the pollen-tube to the ovary. 5. Hyphae of Fungi. — While Sachs ('79) early suggested that the sporangium bearer of Phycomyces nitens is negatively hydrotropic, the first experimental evidence on this point was offered by Wortmann ('81). Spores of Phycomyces were sown on bread kept in a moist chamber whose walls were made opaque to prevent phototropism. When, after three or four days, some of the sporangium-bearers had gained a height of one or two centimetres all were bent to one side excepting one which protruded § 2] HYDROTROPISM 359 through a small hole in the glass disc. Parallel to this hypha and close to it was placed a piece of soaked card. After 4 to 6 hours the hypha turned from the damp card ; but when the card was dry no such turning occurred. The extraordinary sensitiveness of the sporangium bearing filaments of Phycomyces has been shown by the experiments of Errara ('93). He found that these organs turned toward rusting iron, china-clay, agate (but not rock crystal), and sul- phuric acid. He explained this result on the ground that these substances absorb the moisture in their vicinity. Con- sequently the hydrotropic filaments turn towards" this rela- tively dryer region. So sensitive, indeed, is this plant that it may be used to detect a very slight difference in the hygro- scopic properties of chemically related substances. Not only do the hyphse of Phycomyces turn from moisture, but, as MOLISCH ('83) shoAved, those of Mucor stolonifer and the relatively great trunk of the toadstool Coprinus velaris respond in the same way. The spores are thereby carried away from the moist situation. In conclusion a word may be said concerning the cause of hydrotropism. It is probable that two diverse phenomena are confused under the term. One of these is seen when a multi- cellular organ like the root of phanerogams is unequally moistened on opposite sides ; the moister side will lose water less quickly than the other, or it may actually imbibe some. Its cells will accordingly become more turgescent and the whole moister side more convex. This result is due to a relatively direct, almost mechanical, cause ; it simulates nega- tive hydrotropism, but it is so different in kind from the true phenomenon that it may be called false hydrotropism. In the second class of cases we see multicellular organs, such as roots, becoming concave towards the slightly moister region, or unicellular organs, such as rhizoids, pollen-tubes and hyphse — organs which cannot be supposed to become unequally tur- gescent on the two sides — exhibiting a -F or — turning. These cases cannot be explained on direct mechanical grounds ; they are responses to stimuli, and, as such, examples of true hydrotropism. These two kinds of hydrotropism may occur in the same organ under different conditions and thus cause turnings in 360 THE EFFECT OF WATER UPON GROWTH [Ch. XH opposite directions. Thus Woetmann ('81, p. 374) finds that a mycelium growing downwards towards water turns horizon- tally before touching it and branches profusely. Similarly a root growing towards water will not penetrate into it, but will turn to one side. The greatly increased moisture causes the reversal of the tropism, but this is probably due to the fact that a false hydrotropism replaces the true response ; however, as true hydrotaxis may take place in both directions, so there may be a true negative as well as positive hydrotropism. I now summarize our conclusions concerning the effect of water upon growth. Water plays a part in growth second in importance to no other agent, so that in its absence growth cannot occur. As the quantity is increased, growth is increased until an optimum is reached. The amount imbibed does not, however, depend directly upon the amount available, but rather upon the needs or the habits of the species. Growth of elon- gated organs may take place from or towards moisture, and the turning may be a true response to the stimulus of higher or lower aqueous tension, — a response which may show itself in a bending at some distance behind the irritable tip. This response is, moreover, often of an advantageous kind, directing the rootlets towards water and the pollen-tube towards the moist stigma or keeping the sporangium in the dry atmosphere necessary for the production of dry spores. In a word, imbi- bition of water and growth with reference to the source of moisture are regulated to the advantage of the species. LITERATURE Darwin, C. '80. The Power of Movement in Plants. London, 1880. Darwin, F. '93. On the Growth of the Fruit of Cucurbita. Ann. of Bot. VII, 459-487. Pis. XXII, XXIII. Dec. 1893. Detlefsen, E. '82. Ueber die von Ch. Darwin behauptete Gehirnfunction der Wurzelspitze. Arb. a. d. bot. Inst. Wiirzburg. II, 627-647. DuCHARTRE, P. '56. Influence de I'humidite sur la direction des racines. Bull. Soe. bot. France. Ill, 583-691. Errara, L. '93. On the Cause of Physiological Action at a Distance. Rept. Brit. Ass. Adv. Sci. for 1892, 746, 747. LITERATURE 361 Gain, E. '92. Influence de I'humidite sur la vegetation. Compt. Rend. CXV, 890-892. 21 Nov. 1892. '95. Recherches sur la role physiologique de I'eau dans la vegetation. Ann. Sci. Nat., Bot. (7), XX, 63-215. Pis. I-IV. Hoffmann, '65. Beitrage zum Keimungsprocess. Landwirthscli. Versuchs- Stat. VII, 47-54. Johnson, H. '29. The Unsatisfactory Nature of the Theories proposed to account for the Descent of the Radicles in the Germination of Seeds, shewn by Experiments. Edinb. New Philos. Mag. VI, 312-317. JuMELLE, H. '89. Recherches physiologiques sur le developpement des plantes aiinuelles. Revue Generale de Bot. I, 101 et seq. Knight, F. A. '11. On the Causes which influence the Direction of the Growth of Roots. Phil. Trans. Roy. Soc. London. Pt. I, 209-219. Lesage, p. '95. Recherches experimentales sur la germination des spores die Penicillium glaucum. Ann. Sci. Nat., Bot. (8), I, 309-322. Nov. 1895. MiYOSHi, M. '94. Ueber Reizbewegungen der PoUenschlauche. Flora. LXXVIII, 76-93. MoLiscH, H. '84. Untersuchungen iiber den Hydrotropismus. Sb. Wien. Akad. LXXXVIIIi, 897-942. Taf. L Reinke, J. '76. Untersuchungen liber AVachsthum. Bot. Ztg. XXXIV, 65-69, 91-95, 106-111, 113-134, 136-160, 169-171. Pis. II, IIL Feb., Mar. 1876. Sachs, J. '72. Ablenkung der Wiirzel von ihrer normalen Wachsthumsricht- ung durch feuchte Korper. Arb. bot. Inst. Wiirzburg. I, 209-222. '79. Ueber Ausschliessung der geotropischen und heliotropischen Krtim- mungen wiihrend des Wachsens. Arb. bot. Inst, zu "Wiirzburg. II, 209-225. TscHAPLOWiTZ, F. C. '86. Untersuchungen iiber die Wirkung der klimat- ischen Factoren auf das Wachsthum der Culturpflanzen. Forsch. Agr. IX, 117-145. [Abstr. in Bot. Jahresber.] XIV, 57, 58. WiESNER, J. '81. Das Bewegungsvermogen der Pflanzen. 212 pp. Wien, 1881. WoRTMANN, J. '81. Ein Beitrag zur Biologic der JMucorineen. Bot. Ztg. XXXIX, 368-374, 383-387. June, 1881. CHAPTER XIII EFFECT OF THE DENSITY OF THE MEDIUM UPON GROWTH In this chapter we cannot, as hitherto, consider the effect of density upon both the rate or quantity and the direction of growth, for no studies seem to have been made upon the latter subject. § 1. Effect of Density upoisr the Rate of Growth We have seen in Chapter III (p. 77) that the increase or decrease in the concentration of a solution produces, by osmosis, changes in the structure of protoplasm, in its locomotion, and in its excretory activity. It remains to be seen to what extent change in density can affect the metabolic activities concerned in growth. The relation between the rate of growth of plants and con- centration has been the subject of much study, e.g. by Wielee, ('83), DeVeies ('77), Jaeius ('86), Jentys ('88), Eschen- hagen ('89), and Stange ('92). It is agreed that, in general, as the solution containing the plant becomes more concentrated the seedling or the fungus (Jentys, p. 455) grows more slowly. The question now arises whether there is any maximum concentration at which growth is completely inhibited. Data on this subject are given by Eschenhagen, who finds that various fungi will not grow at a concentration above the followins: limits : — Aspergillus . . Penicillium . . Botrytis .... Starch. CJI„0„. 53% 55 51 Glycerine. C.,H,0,. 43% 43 37 SoDiTTM Nitrate. NaNO,. 21% 21 16 Common Salt. NaCl. 17% 18 12 362 § 1] EFFECT OF DENSITY UPON KATE OF GROWTH 363 Also Raciboeski ('96) found that Basicliobolus cultivated in a nutritive solution containing 10% peptone, 1% glucose, and the necessary salts, ceased to grow when the concentration of the salts reached the following percents : — sodium chloride, NaCl, 6%. glycerine, CgHgOg, 20%. potassium chloride, KNO3, 11%. glucose, CgHiaOG, 25%. The foregoing maximum concentrations vary with the molec- ular weights of the dissolved substances, indicating that their effect is purely an osmotic one. Germination is likewise affected by concentration, as a valu- able series of experiments by Vandevelde ('97) clearly shows. Seeds of the pea, Pisum sativum, were soaked for 24 hours in solutions of common salt varying from 1% to 35%, then removed, planted, and the percentage of seeds which germinated (G%) and the mean interval elapsing before germination (I) determined. (I) was, more precisely, the time elaps- ing (in days) before one half of the seeds had germinated. The results are given as follows : — TABLE XXXVIII Showing the Relation between the Concentration of the Solution in AVHiCH Peas have been soaked and their Germination % G% I % G% I % G% I % G% I 1 98.00 2.1 10 12.46 5.9 19 4.00 5.7 28 6.83 5.7 2 97.17 3.7 11 7.00 6.6 20 5.67 5.7 29 3.83 6.7 3 85.17 4.0 12 10.00 6.3 21 3.33 7.0 30 10.33 6.5 4 34.50 4.6 13 8.50 6.8 22 1.50 5.6 31 8.83 6.8 5 32.66 4.8 14 7.19 6.3 23 1.83 6.2 32 10.67 7.2 6 16.83 5.0 15 4.50 6.7 24 4.00 5.8 33 23.00 6.2 7 15.. S3 5.2 16 6.83 7.1 25 0.83 5.8 34 31.33 6.5 8 14.83 5.3 17 4.76 6.6 26 4.50 6.3 35 56.83 7.0 9 12.66 5.4 18 3.33 6.2 27 8.17 6.8 This table yields some remarkable results. As the concen- tration increases from 1% to 15%, the percentage of germina- tions diminishes from 98 to 4.5, and the mean germination interval increases from 2.1 days to nearly 7, near which point it remains at all higher concentrations. From 15% to 29% the percentage of germinations fluctuates so irregularly between 6.8 and 0.83 that within these limits it may be considered con- 364 EFFECT OF DENSITY OF THE MEDIUM [Cir. XIII stant. As the concentration increases from 29%, instead of the grains all being killed or failing to germinate, we have the interesting result that the percentage of germinating individ- uals rises rapidly from 4 to 56. Vandevelde hazards the following explanation of this result: "Dilute solutions are easily absorbed ; the more concentrated the solution, the smaller the power of diffusion ; in a saturated solution the seeds do not swell and the action of the surrounding solution is less injurious." But we need to know more of the condi- tions under which this result occurs before we can accept any interpretation. With animals we find growth similarly affected as with plants. LoEB ('92) first showed this in his experiments upon the regenerative growth of decapitated tubularian hydroids. The regenerating hydroids were kept in water more and less dense than, and equally dense with, sea water. At the end of eight days the length of the regenerated piece was measured in seven to nine individuals at each concentration. The results may be given in the form of a curve (Fig. 98) by laying off as abscissae the percents of sodium chloride in the water and as ordinates the corresponding average growth in millimetres : — DILUTE SEA WATER CONCENTRATED SEA WATER Fig. 98. — Curve showing the relation between the density of the medium and the proportional rate of growth of regenerating Tubularia. The maximum- ordinate indicates 10.5 mm. growth in 8 days. The numbers at the base of the curve are per cents of density in excess of distilled water; thus "3" signifies a specific gravity of 1.03. (From Loeb, '92.) This curve shows that the optimum concentration for growth is not, as might have been expected, the normal concentration, but one considerably below the normal, namely 2.5% instead §1] UPON THE RATE OF GROWTH 365 of 3.8%. Anything which favors enclosmosis seems, within certain limits, to favor growth. Regenerating annelids have also been studied at my labora- tory by Mr. J. L. Frazeuk. A large number of worms of a species of Nais, all of approximately the same size, were cut into two parts. Of these the anterior, consisting of twelve seg- ments, was alone preserved for experimentation, and was placed in water either pure or containing a variable amount of com- mon salt in solution. At the end of ten days the anterior piece had regenerated at its tail end a certain number of seg- ments varying with the strength of the solution as shown in the following: — TABLE XXXIX Showing the Average Number of Segments of Nais regenerated per Day IN Various Solutions of Sodium Chloride Solution. No. op Individuals. AVG. No. OF Segments regenerated PER Day. Solution. No. OF Individuals. AvG. No. OP Segjients regenerated PER Day. Water 0.125% 0.188 15 5 16 2.13 1.72 1.42 0.250% 0.375 0.500 7 - 5 5 1.19 1.18 1.14 The decrease in the number of regenerated segments was thus, with increasing concentration, at first rapid, then slow.- Fission, which is so closely bound up with growth that we may treat it as an index of growth, is also controlled by the concentration of the medium. Mr. P. E. Sargent has, at my suggestion, studied this subject in the naid Dero vaga. This species divides so rapidly that ordinarily it doubles its numbers every ten days. The worms were kept in solutions of varying concentration of various salts. They were reared in similar jars, supplied with similar food,* and kept under otherwise similar conditions. A definite number of worms having been put in each jar, the increment at the end of ten days was deter- * 1 to 2 cc. of corn meal extract was added every day to the 200 cc. of water in which the worms were livins;. 366 EFFECT OF DENSITY OF THE MEDIUM [Cii. XIII mined by counting. The results of some of these countings are given below for a number of salts : — TABLE XL Average Increase Per Cent of Individuals of Dero vaga reproducing, DURING Ten Days, in Solutions of Different Salts at Varying Con- centrations. Minus Quantities indicate Diminution in Number of Individuals > 5 a > 3 5 5 Strength of Solu- a tion. o o 'A c! o d o o d d 3 bJO fa o d \4 MoLEC. Weights, 58.5 130 111 95 74.6 Control : water 1^,1 113.0o/„ 75 134.0% 0.05% 100 85.2 50 22.0 50 40% 25 12% 50 12% 0.10 350 77.0 75 -10.6. 50 -12 50 -64 0.15 100 58.0 75 - 2.6 25 -36 25 -28 25 -92 0.20 350 40.8 75 -35.3 50 -58 25 -68 0.25 100 21.5 75 -65.3 25 -72 0.30 350 3.2 75 -84.0 25 -84 0.35 25 -24.0 25 -92 0.40 375 -18.1 25 -100 0.50 150 -35.0 These columns, and especially the first one, show a close relation between concentration and growth (as tested by mul- tiplication of individuals). They show also that the diminished growth falls off rapidly at first with slight increments of con- centration, then less slowly at the higher grades (Fig. 99). Finally they show that different salts have diverse osmotic effects, for sodium chloride is less retarding than any other salt at the same percentage of concentration. The effect of the remaining salts is seen to increase as the molecular weight diminishes, and therefore the osmotic effect increases (Fig. 99). The fact that magnesium sulphate dissociates at these weak concentrations only about two-thirds as much as calcium chlo- ride does, would lead us to expect even a relatively smaller effect as compared with calcium chloride than we find (see p. 74, note); perhaps further experimentation would give facts §1] UPON THE RATE OF GROWTH 367 agreeing closer with theory. On the whole, Table XL indi- cates that it is the osmotic effect which retards growth. In tadpoles a similar retarding effect of solutions has been observed by Yung ('85), who made solutions of 0, 2, 4, 6, and 8 grammes of sea salt in 1000 grammes of water, ^^o and reared frog's embryos in them. Other conditions excepting concentration were believed to be alike in all experiments. In the 0.2% solution the tadpoles developed at nearly the same rate as in pure water. In the denser solutions there was a retardation in development which in- creased with the density ■of the solution, so that in the 0.8% solution the lar- vae hatched out seventeen days behind the normal time. The effect of a sudden ■change in the density of the solution has been es- pecially studied by True <('95). Beans, Vicia faba, which had radicles from 17 to 35 nnn. long, were placed directly in the solu- tions and held there so that the cotyledons alone were free. The cultures were kept in the dark. When the transfer was made sud- denly to a 1% solution of potassium nitrate, it was observed that a mechanical contraction ■occurred, followed by a more or less prolonged period of retar- i N, 1 s \ 1 i \ 1 \ \ \ \ -2; \ 1 \0 ,\ \ \ i\V \ \^ 1 \ N \i . -^ \ V ?\ ^ V o, A\ 1- sv m. \ i^~ \ \ ^^ X ^. no 100 80 70 60 50 40 30 20 10 0 -10 -30 -30 -40 -50 -60 -70 -90 -100 0 .05 .10 .15 .30 ,25 .30 .35 .40 .45 .50 Fig. 99. — Curves of average increase per cent of individuals (ordinates) of Dero vaga re- producing, during 10 days, in solutions of different salts, whose strengths are laid off as abscissae. The data are taken from Table XL. 368 EFFECT OF DENSITY OF THE MEDIUM [Ch. XIII dation in tlie rate of growth. When, on the other hand, the plant was transferred from a salt solution to which it had become accustomed to pure water, a mechanical elongation quickly occurred, but this too was followed by a retardation in the rate of growth. Thus the reduction in the rate of growth is not a mechanical result of change of medium, but is a response to the stimulus of changed environment. We have hitherto considered almost exclusively the effect of aqueous solutions. We must now consider how a variation in the pressure of the atmosphere affects the growth of plants. Upon this subject experiments have been made by Jaccard ('93), who found that when the pressure of the air was reduced from 78 cm. to between 10 cm. and 40 cm. of mercury, growth is two or three or even six times as rapid as in ordinary air. Likewise when the air is compressed to between three and six atmospheres acceleration in growth occurs, although not to the same extent as in the depressed air. If, however, the rarefac- tion is very great (below 10 cm.) or the pressure excessive (over 8 atmospheres), growth is retarded. Experiments indi- cate that this result is not wholly due to the concentration of the oxygen in the air. We may therefore conclude that a change in pressure from the normal accelerates growth by irritating the growing plasma up to a certain limit, beyond, which its injurious effects counterbalance its favorable ones. Summing up this account of the effect of concentrated solu- tions upon the growth of organisms, we find that in general as the density increases beyond the normal the rate of growth diminishes until, at a certain concentration, it ceases. In the particular case of marine organisms a reduction in concentration to a certain point causes excess of growth ; below that point, diminution. It is probable that the diminution in growth is proportional to the osmotic action of the medium (Chapter III). An explanation of the foregoing phenomena may be attained by reference to the principles laid down in preceding chapters. In Chapter X it has been shown that growth depends very largely upon the specific imbibition of water. We do not know the cause of the difference in imbibitory properties at different times ; but if, as has been suggested, it is purely an endosmotic phenomenon resulting from the secretion of plant. § 1] UPON THE RATE OF GROWTH 369 acids or salts in the cell sap, then Ave can understand how a denser medium should diminish or destroy the imbibitory tendency. On the other hand, a change in concentration may act in a more indirect fashion to cause increased growth ; namely, by calling forth a response to the irritation of changed conditions of pressure. LITERATURE EscHENHAGEN, F. '89. Ueber den Einfluss von Lbsungen verschiedener Concentration auf das Wachsthum von Schimmelpilze. Stolp, 1889. Jaccard, p. '93. Influence de la pression des gaz snr le developpement»des vegetaux. Comp. Rend. CXVI, 830-833. 17 April, 1893. Jarius, M. '86. Ueber die Einwirkung an Salzlosnngen auf den Keimungs- process der Samen einiger einheimischer CulturgewJichse. Landwirths- schaft. Versuchs-Stat. XXXII, 149-178. Taf. II. Jentys, S. '88. Ueber den Einfluss hoher Sauerstoffpressungen auf das Wachsthum der Pflanzen. Unters. a. d. bot. Inst. Tubingen. II, 419-464. LoEB, J. '92. (See Chapter X, Literature.) Raciborski, M. '96. Ueber den Einfluss ausserer Bedingungen auf die Wachsthumsweise des Basidiobolus ranarum. Flora. LXXXIII, 110-115. Stange, B. '92. Beziehungen zwischen Substratconcentration, Turgor und Wachsthum bei einigen phanerogamen Pflanzen. Bot. Ztg. L, 253 et seq. True, R. H. '95. On the Influence of Sudden Changes of Turgor and of Temperature on Growth. Ann. of Bot. IX, 365-402. Sept. 1895. Vandevelde, a. J. J. '97. Ueber den Einfluss des chemischen Reagentien und des Lichtes auf die Keimung der Samen. Bot. Centralbl. LXIX, 337-342. 11 March, 1897. DE Vries, H. '77. Ueber die Ausdehung wachsenden Pflanzen-zellen durch ihren Turgor. Bot. Ztg. XXXV, 1-10. Wieler, a. '83. Die Beeinfliissung des Wachsens durch verminderte Par- tiarpressung des Sauerstoffs. Unters. a. d. Bot. Inst. Tubingen. I, 189-232. Yung, E. '85. De I'influence des variations du milieu physico-chimique sur le developpement des animaux. Arch, des Sci. Phys. et Nat. XIV, 502-522. 15 Dec. 1885. 2 b CHAPTER XIV EFFECT OF MOLAR AGENTS UPON GROWTH It is proposed in this chapter to consider, first, tlie effect of contact, rough movements, deformations, and associated molar agents upon tlie rate of growth, and, secondly, the effect of such agents upon the direction of growth. § 1. Effect of Molar Agents upox the Rate of Growth We have already (Chapter IV) seen that profound changes in metabolism and the motion of protoplasm are induced by various sorts of contact. We shall here consider how those changes result in modifications of growth. 1. Contact. — In the case of organs which normally grow upon a solid substratum the growth may take place more rapidly after coming in contact than before. Thus Loeb ('91, p. 29) says of the stolons of the hydroid Aglaophenia pluma, that when they reach a solid surface they begin to grow more rapidly than stolons which develop free in the water. Evidently the stimulus of contact excites to extraordinary growth. 2. Rough Movements. — Experiments with this agent have been almost confined to bacteria, and have been made by shak- ing. The first experimenter in this direction was Horvath ('78), whose method of work is worth giving in some detail. Glass tubes about 20 cm. loug and 2 cm. wide wei-e half filled by a nutri- tive fluid, inoculated with an infusion full of various bacteria, and sealed. The tubes were then fixed on a board which was made to swing horizontally to and fro by means of a motor through an arc of about 25 cm. length at the rate of 100 to 110 times per minute. At the end of each excursion the board received, by means of a special device, an extra blow. The resulting agitation was like that made in shaking a test-tube. 370 § 1] EFFECT OF MOLAR AGENTS UPON GROWTH 371 The results of Hoevath's experiments were these. Tubes which were shaken for 24 hours were clear at the end of that period, while similar tubes kept at rest had become muddy with bacteria in the same time. If, now, the shaken tubes were kept at rest during a day in a warm oven, they, too, produced a rich growth of bacteria. When, however, the tubes had been shaken for 48 hours they not only were found clear, but they did not become cloudy upon subsequent warming (I, p. 99). The shaking during the briefer period had thus merely interfered with the normal growth processes, but the more prolonged shaking had resulted in the death of the Ibacteria. Subsequent attempts by other workers to reproduce these results, by the use, however, of other methods, partially or completely failed. Reinke ('80), indeed, found that in water agitated for 24 hours bacteria had ceased to grow, but had not been killed; in so far a confirmation of Horvath. Other investigators, however (Buchner, '80 ; Tumas, '81 ; Leone, '85 ; Schmidt, '91 ; Russell, '92 ; and others), have either obtained no effect upon the growth of bacteria, or have found it increased by motion. The results seemed hopelessly dis- cordant. The more recent work of Meltzer ('94) has, however, brought these conflicting observations under one general law, and has thus offered an interpretation of the varied results. He pointed out first that the diverse results of previous investi- gators were due to the use of different species of bacteria and of different kinds and degrees of shaking ; in a word, to dis- similar methods. Meltzer emploj^ed pure cultures of bacteria of ascertained species, and counted the colonies by the well- known bacteriological methods. Shaking was either violent, being done in a machine somewhat like Horvath's ; or slight, being done by hand. The media used were neutral salt solu- tions, Koch's bouillon, or pure water. The results showed that a slight shaking is advantageous to the metabolism of Bacterium ruber in water. Thus, while a culture containing 950 colonies, left quiet, was reduced after 8 days to 259 colonies; shaken, it had at the end of the same period 1366 colonies ; and shaken with glass drops rolling loose in the tube, 372 EFFECT OF MOLAR AGENTS [Ch. XIV it had attained 16,200 colonies. When, however, the shaking was longer continued the number of colonies began to fall off, so that after 21 daj^s the medium shaken with glass drops exhibited only 5 colonies. These facts enable us to distinguish a minimum degree of movement which will permit of growth (shaking for 6 days without glass drops); an optimum (shaking for 8 days with glass drops) ; and a maximum (shak- ing for 21 days with glass drops). The optimum is very diverse in different species. Thus in Bacterium megaterium it is so low that shaking for a little over 1.5 days results fatally. It is probable that the growth of every race of bacteria is attuned to a particular optimum of movement.* 3. Deformation. — Under this head we may consider the effects of pulling and bending upon the growth of an elongated body. Data upon this subject have been obtained only from plants. The effect of pulling upon the growth in length of a plant stem has been studied by Baeanetzky ('79), Scholtz ('87), and Heglee ('93). The results obtained are concordant and important. It appears that when a weight is attached, by means of a cord running over a pulley, to the epicotyl of various seedlings — such as Helianthus, Tropseolum, Cannabis, Linum, etc., — the growth in length of the stem is, under appropriate conditions, not accelerated but retarded. When, for example, a pull of 13 grammes is exerted upon the epicotyl * Concerning the cause of the increase of growth accompanying a slight molar disturbance and the diminished growth and death accompanying a violent one, Meltzee has something to say. He finds in those cultures in which no growth of bacteria occurs, no fragment of cells but on the contrary nothing except a fine "dust" — the bacteria have experienced a molecular disintegration.. In order to explain this disintegration, Meltzek accepts Nageli's conception of the structure of protoplasm, — micellse enveloped by water, — and supposes that a molar disturbance modifies the normal movements of these micellge. A very violent movement causes the micellse to separate completely ; a much less tur- bulent movement causes an increase in the vibrations of the micellse by which they are brought into more intimate contact with food material including oxygen, and more easily get rid of the metabolic products. "Without accepting Nageli's micellar hypothesis, we may account for the beneficial effects of slight movement upon the ground of an increased supply of oxygen afforded by it; and we may regard the fatal effect as the result of disturbed metabolism or of protoplasmic disintegration. § 1] UPON THE RATE OF GROWTH 373 of a Cannabis seedling there is a momentary elongation followed for six hours by no growth whatever ; then periods of growth alternating with periods of retardation occur until, after perhaps 24 hours, the hourly growth is nearly equal to that of the unweighted plant. The least weight which will cause a retardation is different for different plants. In the case of Cannabis and Linum it is below 1.3 grammes, a result which indicates that the weight of the index used in self-recording auxanometers is sufficient to retard the growth of the plant and thus to give an abnormal growth curve. The diminution of growth is not the same at all periods of the plant's development. Thus the retarding effect is greatest at the commencement of the grand period of growth, but begins quickly to diminish until at the maximum of growth there is no retarding effect. Then, as the rate of growth decreases, the retardation becomes more evident. The effect of the pull is best seen in comparing the curves of daily fluctuation of growth of the weighted and unweighted plants. The early morning is the period of rapid growth, and at this time the growth of the stretched plant is quite equal to or exceeds that of the un- n \ 1 \/ " r\""'' r""""' v 12 345678 010111212. 34 56789 101112 12 3 4 5 6 7 8 S) lu Ulli 1 :i a 4 5 0 7 8 9 lull 1^12 3 4 5 6 7 , MIGHT Fig. 100. — Curves of hourly growth, measured in hundredths of a millimetre, — ver- tical scale at the left. The full line gives the course of growth of a Cannabis seed- ling subjected to a pull of 13 grammes. The dotted line gives, for comparison, the course of nearly normal growth, the plant being subjected to a pull of only 3 grammes. The numbers at the bottom indicate hours of the day; the broken line at the bottom indicates night-time. (From Hegler, '93.) weighted one. During the early night, on the contrary, when growth is feeble, the stretched stem grows slowly (Fig. 100). If an etiolated plant is used, the daily growth fluctuations are eliminated and the effect of the larger growth cycle becomes 374 EFFECT OF MOLAR AGENTS [Ch. XIV evident. Thus, in general, the more rapid the growth, the less is the retardation provoked by pulling. The direction in which we must look for an explanation of these facts is indicated by the circumstance that, in the most rapid period, growth is chiefly due to imbibition of water, while in the preceding and succeeding periods it is due more to an increase in the plasma. Thus not all kinds of growth are equally affected by the irritation of pulling, but principally that growth which is due to assimilation. Further insight into this matter is gained from the circumstance that despite the fact that growth is slower in the plant under tension the tur- gescence in the stretching zone is greater in such a plant than in a normal one. This indicates again that it is not the imbib- itory process which is interfered with but rather the assimila- tive one. All these facts thus lead to one conclusion, that, under tension, the plasma, especially that of the cell-wall, grows in length less rapidly than under normal circumstances. This diminution of growth can hardly be explained in a direct mechanical way ; we must consider it a response to the stimulus of pulling. A confirmation of this conclusion is found in the fact that the effect of the pulling gradually wears off. Thus when one of a pair of seedlings of Cannabis sativa is subjected to a pull of 20 grammes, there is a retardation during the first day of 61% in the stretched plant as compared with the control plant; during the second day of 51% ; during the third day of only 9%. (Hegler, '93, p. 389.) In order that the retardation should continue, additional weight must be imposed ; then an increased retardation occurs. Thus in one case Heglee, sub- jected one of two Helianthus seedlings to a pull of 50 grammes. During the first day the retardation of the pulled plant was 20% ; during the second there was an excess of growth over the control of 17%; then 150 grammes were added; on the third day the retardation was 18% ; on the fourth there was an acceleration of 2%. This series of phenomena is clearly like that which we have observed in locomotion — there is an accommodation of the growing protoplasm to the stimulus. There can be little doubt that in the cases of diminished growth in length there is a thickening of the cell-walls and § 1] UPON THE RATE OF GEOWTH 375. probably an increase in cross-section of the whole stem. There is indeed considerable experimental evidence for this con- clusion. Thus ScHENCK ('93) found that when stems are irritated by twisting or bending, an excessive growth both of cell-walls and of the wood as a whole follows. So, too, Newcombe ('95) finds that roots become strengthened by attaching weights to them. In fact, if stems are deprived of their normal swaying movements, for instance by enclosing internodes in plaster casts which inhibit lateral movements and partly support the weight of the superior part of the plant, their walls remain abnormally thin. These effects of deformation are of especial importance because they are so clearly not at all directly mechanical but adaptive ; they are, indeed, rather opposed to the: direct mechanical effects which would tend to stretch the cells, and thus to diminish the thickness of their walls. Here again the organism shows itself a highly irritable thing, capable of responding in an adaptive fashion. 4. Local Removal of Tissue. — When a protist, for instance a Stentor, is transsected, certain changes take place along the cut surface. First, there is a warping of the edges towards each other ; secondly, rapid growth (differential growth, page 287) occurs. Similarly, if a Hydra be cut lengthwise, the free edges may fold towards each other so as to form a smaller cyl- inder, and the seam, by growth, will be healed over. So, too,, in the higher animals, the removal of a bit of tissue results usu- ally in the closure of the wound and growth to fill the gap. We may call these two processes warping and regenerative growth. The causes of these two processes are probably different. The warping seems to result from the presence of tensions and pressures in the tissues whose equilibrium is disturbed by the cut. This process is probably grossly mechanical. The regen- erative growth, however, must have a less direct explanation. It is apparently a typical response to the stimulus of cutting, or of the new environment presented at the cut edge. Here, too, may be mentioned an experiment by Loeb ('92, p. 51), which throws light upon the cause of these internal tensions. Below the crown of tentacles of a Cerianthus a 37G EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIV horizontal slit was made in the side of the body-wall. The tentacles over the slit contracted — there was a sort of negative growth. The shortening was doubtless due, as LoEB says, to the loss of water and con- sequently of turgescence in this part of the body- wall (Fig. 101). I have cut the body- wall of a hydroid immediately below an incipient bud, whereupon the bud at once flattened out. These experiments show how important water pressure is for the maintenance of the size of the body and for growth, and in so far explain the mechanical effect of a cut or other similar wound. V_y § 2. Effect of Contact upon the Direc- Tioisr of Growth — Thigmotropism Pig. 101. — Cerianthus, from wliicli a piece, a,b,c, has been cut, causing a loss of tur- gescence and conse- quent shrinking of tentacles on the cut side. (From Loeb, '92.) Having seen that molar agents can affect the rate of growth, we are in a position to understand how a molar agent, acting upon one side only of an elongated organ or plate of tissue, may induce a less or greater growth upon that side, and, con- sequently, a bending towards or from it. This turning phenomenon may now be considered. Before taking up the permanent growth turnings, however, we may consider a case of transitory growth, which throws valuable light upon the true nature of thigmotropism, and serves to connect it with thigrao- taxis. This is the case of the pseudopodia of Orbitolites, which, according to Verworn ('95, p. 429), float at first free in the water after being protruded through holes in the shell; but as soon as they grow longer and heavier they sink in the water, until their distal ends touch the substratum. To this they become attached by a delicate secretion, and grow out along it by the streaming of the protoplasm. The persistent clinging to the substratum is a thigmotropic reaction, and one which belongs clearly to the category of response. 1. Twining Stems. — The characteristic form of twining plants, like the bean, has long excited the interest of natural- § 2] THIGMOTROPISM 877 ists. A study of the cause of this form was made by Palm ('27), by MoHL ('27), and by Dutrochet ('43, '44) ; and from this early period to the present there have existed on this matter great differences of opinion. On the one hand, there has been maintained a view that had great inherent probability, the view, namely, that the twining of plants is a response to the contact-stimulus of the object about which they are coiled. Against the general validity of this view certain experiments of Darwin ('82, p. 16) seem conclusive. For he found that €ven the hard rubbing of the stalk caused no modification of the normal spiral growth. Accordingly, the conclusion seems generally accepted to-day that the peculiar form of growth of most twining plants is the combined result of geotropism, by which the stem grows upwards, and a special form of nutation, hj which it impinges against the supporting stick and bends round it. The twining is thus mechanical, — depending upon the structure of the stem, — rather than responsive. An exception among twining plants is found in the dodder, Cuscuta. This plant is a parasite, belonging to the Convol- vulus family, and lives upon the flax and other plants. It is leafless, and twines closely about its host, into which it sends the feeding organs — the haustoria. According to the careful studies of Peirce ('94), the stem, when not in contact with any solid body, makes long, steep turns about the axis of the spiral, as is the case with other twiners. If now the free stem, during a period of slow growth, be brought into contact with a wooden or glass rod or a thread at about 3 cm. from the tip, the stem bends sharply, and in the course of 15 hours makes two or three close turns around the vertical support. If the con- tact be made at a distance of only about 1 cm. below the tip, there will be little or no change in the character of the twining. If the point of contact be too far below the tip, — 6 to 7 cm., — there will also be no effect. The result is thus clearly depend- ent upon a stimulus applied at a definite sensitive point ; it is a typical response. 2. Tendrils. — Very similar to the phenomena of close twin- ing in Cuscuta is that of coiling in the tendrils of phanero- gams— those most marvellously sensitive of all plant struct- ures. Darwin ('82) and Pfeffer ('85), particularly, have 378 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIV studied by experimental methods the movements of these organs. The conditions whicli induce the twining of tendrils are peculiar. It appears that tendrils are not irritated by mechanical shaking, such as is produced by a powerful current of air issuing from a narrow tube ; nor by a drop or strong cur- rent of water," provided it contains no hard particles ; nor even by a stream of mercury, or the surface of pure mercury or oiL All solid bodies, however, with the exception of moist gelatine^ when rubbed against the stem stimulate it so as to produce a bending at the irritated point. The behavior of moist gelatine is instructive. Ppeffer first observed that it was exceptional, and Peirce has found the same exception to hold for Cuscuta. Pfeffer's experi- ments showed that, no matter how great the pressure or strokes made by drops of 5 to 14% gelatine, or by glass rods covered by a layer of moist gelatine, no response occurred ; and Peirce ('94, p. 66} found that when the Cuscuta stem came in contact with a rod covered with wet gelatine it made only the long, steep turns characteristic of the free-growing stem. Later, when it reached the part of the rod not covered with gelatine, it formed at once a close, tight coil. That it is not the chemical composition of gelatine which prevents the twining is shown by the fact that dry gelatine, even to 25%, irritates ; also, if some sand is mingled with the moist gelatine, tropism occurs. The fact that wet gelatine does not irritate is. explained by Pfeffer upon the ground that the stimulus is. not given by mere contact, for a blow does not produce it. A certain degree of adhesion between the tendrils and the irritat- ing substance is necessary. The moist surface of the gelatin© prevents such adhesion taking place. The effective irritation is a sort of tickling. The degree of this tickling may, however, be extremely slight. Thus Pfeffer ('85, p. 506) placed a rider of cotton thread, weighing 0.00025 milligramme, on a tendril, and found that no effect was produced so long as it was quiet ; whereas, when the rider was put in motion by a current of air,, a considerable bending occurred. Under similar conditions, a thread weighing 0.00012 milligramme produced no reaction. MacDougal ('96, p. 376) finds that a spider's web, 43 cm. §2] THIGMOTROPISM 379 long, suspended above a tendril of Echinocystis, caused such a reaction that the tendril coiled around and fastened to it. Concerning the irritable region, it appears that in most ten- drils there is an especially sensitive side and an especially sensitive zone. The presence of a more irritable side is not a constant character of all tendrils, however. Thus, Cobsea scandens, Cissus discolor, and others, are irritable on all sides. When a differentiation in this respect occurs, however, a bend- ing takes place towards the more irritable side. The sensitive zone lies either at the tip or immediately below.* The period of develojjment at which the tendril is most irri- table must also be considered. Thus, Darwin ('82,- p. 174) says : " Tendrils which are only three-fourths grown, and, per- haps, even at an earlier age, but not whilst extremely young, have the power of revolving and grasping any object which they may touch. These two capacities . . . both fail when the tendril is full grown." Pfeffer ('85, p. 485) and Mul- LER ('86, p. 104) likewise find tendrils irritable only in the latter part of their growth-period, f There are four periods in the response to a momentary stimulus : (1) a latent period elapsing before the curvature begins to take place ; (2) a period of bending ; (3) a period of quiescence in the bent condition ; and, finally, (4) a period of / ^^ ^^ / •\ r\ / / \J V ^ ^ X "^-^ Fig. 102. — Curve of contraction of a tendril. The distance of the curve from the base represents the amount of displacement of the tip ; one unit on the base-line represents five minutes of time ; 1 to 2, latent period and period of contraction ; 2 to 3, period of maintenance; 3 to 4, period of relaxation. (From MacDougal, '95.) * In Cuscuta, according to Peirce ('94, p. 64), the tip is non-irritable ; the most sensitive zone is 3 cm. below the tip. t In Cuscuta, Peikce ('94, pp. 63, 64) found that neither the stalk of seed- lings nor of older plants, during the period of rapid growth which follows the formation of haustoria, are sensitive. Irritability shows itself only when growth becomes less rapid, as it does some time after forming haustoria. 380 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIV straightening again. These periods are represented graphically in Fig. 102. The latent period is of variable length in differ- ent species. Perhaps the shortest period is that of Cyclan- thera, which Mullee, ('86, p. 103) found to vary in the most sensitive condition of the plant from 5 to 9 seconds. Daewin" {'82, p. 172) found a latent period in Passifiora tendrils of 25 to 30 seconds, while in other cases (Dicentra smilax, Ampe- loj)sis) this period may be 30 to 90 minutes or more in length. The entire time required for the plant to straighten completely again is, according to Mullee, ('86, p. 104), for Cyclanthera about 20 minutes. These times depend, however, to a large extent upon various external agents. Thus a quick response is favored by a temperature between 17° and 33° C, sunlight rather than shade, considerable humidity, and small size of ten- dril. Thus this growth response resembles, in its dependence upon various conditions, other responses, e.g. those of muscle, to stimuli. 3. Roots. — Thigmotropism in roots was apparently first investigated by Sachs ('73, pp. 437-439). He fastened ger- minating seeds of various species in a moist chamber so that their radicles, of about 10 to 30 mm. length, were horizontal. A pin was now placed against the root near its tip so as to exert considerable pressure. Usually within eight or ten hours a turning of the root in the growing region occurred so that it became concave towards the pin and finally made a complete loop about it, or, if the needle was vertical, descended along it in a spiral — a result of the double action of thigmo- tropism and geotropism. This response was, however, rather inconstant and appeared not to be very powerful. The sense of the thigmotropic response was, in the foregoing example, positive ; that is, the turning was towards the solid body. Daewin ('81, p. 131) believed that he had evidence that the response is sometimes negative. Thus he says that the radicle of a germinating bean, coming in contact with a sheet of extremely thin tinfoil (0.003 to 0.02 mm. in thick- ness), was in one experiment deflected without making a groove upon it ; also, when a piece of paper was gummed to the root near the tip, the root usually turned so that it became convex to the gummed paper. But Daewin's conclusion has not been § 2] THIGMOTROPISM 381 sustained by later work. Thus Detlefsex ('82) found that bean radicles plunged right through tinfoil 0.0074 mm. in thickness, and Wiesner ('84) and Spaulding ('94) have shown that the turning from the gummed paper is the result of the injurious action of the gum. Indeed, it is difficult to see how a root, which must force its way through the earth, should turn from a solid surface. Experience shows rather that it turns towards it and tends to grow along it or through it. 4. Cryptogams. — Among Fungi a thigmotropic response has been observed and studied in the mold Phycomyces nitens by Ekeara ('84) and Wortmani^ ('87). They find that rub- bing the sporangiferous hypha lightly with a bristle or glass thread for from three to six minutes will produce a response more powerful even than that to light. As with tendrils, 7% to 14% gelatine, almond oil, and water drops provoke no bend- ing ; but, on the other hand, even the mutual rubbing of two adjacent sporangiferous hyphse may incite a response. The response will, however, appear only when the growing region of the hypha is irritated ; if the hypha is fully grown, no thig- motropism will take place. The sharpest part of the bend will occur at the region of greatest growth, not necessaiily at the point of contact. However, Woetmanx finds that the turn- ing commences at the point of contact, but becomes more and more pronounced as the bend approaches the most rapidly growing part of the hypha. The response to a brief contact is only temporary. Thus when a sporangiferous hypha 2.3 cm. long was touched for one minute with a glass thread a bending began to appear after 15 minutes and disappeared 10 minutes later. Thus Phycomyces follows closely the laws of response of tendrils. The rhizoids of the he]3atic Marchantia and its allies have been found by Molisch ('84, p. 933) to behave in a similar way to the foregoing. When the hepatics are placed on damp filter-paper, over a watch-glass, in a moist chamber and in the light, one finds after 48 hours that rhizoids have penetrated through the filter-paper. Since the paper reveals, even to the microscope, no pores, and since grains of corn starch, of from 2 to 24 microns diameter, will not pass through it, we must con- clude that the root hairs of 10 to 25 microns diameter, although 5^89 EFFECT OF MOLAR AGENTS UPON GROWTH [Cii. XIV consisting of only a single cell, have the power of forcibly boring their way between the fibres of the paper. This phe- nomenon is most easily accounted for on the ground of response to contact. 5. Animals. — The phenomenon of thigmotropism is exhib- ited among animals in the power of growing along an irregular substratum. Thus the stolons of hydroids, Bryozoa,.and some compound ascidians, upon once gaining the surface of an appro- priate substratum, cling closely to it without reference to the direction, up or down, which the substratum may take. It is not necessary that the surface should be a rough one ; the sur- face film of water may incite the response. More striking is the direction of growth of regenerating hydroid stems, accord- ing to the observations of Loeb ('91, p. 18). This experi- menter found that when pieces are cut from a stem of Tubularia and are so placed in a cylindrical glass vessel that the lower end lies buried in the sand while the upper end is in contact with the side of the vessel, then, regeneration of the hydranth occurring, the new growth is perpendicular to the surface of the vessel, i.e. horizontal. This direction is independent of the direction of light rays, since it occurs at all parts of the circum- ference of the vessel ; nor is it a response to gravity, since the stems normally assume a vertical position. The phenomena are, in so far, exactly like those in plants. The experiments made upon plants suggest a series which might be made upon animals. Do stolons exhibit nutations to aid in the finding of the solid substratum ? What substances call forth the thigmotactic response? Will a momentary irri- tation cause turning? The field of thigmotropism, like that of the other tropic phenomena of sessile animals, is an imperfectly worked one. 6. The Accumulation of Contact-Stimulus and Acclimatization to it. — These two phenomena accompanying contact-stimu- lation must be alluded to. Pfeffer ('85, p. 506) pointed out that a soft blow which calls forth no response when given once, will produce bending when given repeatedly for several minutes. The basis for an explanation of this phenomenon is given by an observation of de Vries ('73, p. 307), v/ho with- drew the support from a tendril which had already turned § 2] THIGMOTROPISM 383 through 45° as a result of contact irritation. The tendril con- tinued to bend at the previously irritated point even after the irritant had been removed. Thus a stimulated condition per- sists after the removal of its inciting cause, and this gives a chance for the building up of a powerfully stimulated condition by the accumulation of slight stimuli. The persistence of the stimulated condition is revealed by another set of changes. Pfeffer found that the tendrils of Sicyos when repeatedly rubbed at short and regular intervals first coil and then gradually straighten out. Darwix ('82, p. 155), who had previously shown this same thing, found also that after a tendril of Passiflora gracilis had been stimulated 21 times in 51 hours it finally hardly responded at all. Similar results have been obtained with Drosera hairs (Pfeffek, '85, p. 514). Thus the constantly repeated stimulation produces such a modification of the protoplasm that it eventually fails to respond. On the one hand, this phenomenon is the same as that of fatigue ; on the other, it resembles closely the condition seen in stimulated Protista referred to in. Chapter IV, § 3, and there designated as acclimatization. 7. Explanation of Thigmotropism. — Concerning the cause of the bending of plants as a result of contact, I know of no better explanation than that offered by Sachs ('87, pp. 697-699). He ascribes the bending to a ' difference in the rate of growth on the two sides of the bending cylindrical organ. This is indicated by the fact that it is chiefly in the region of *' stretching" that the response occurs ; i.e. shortly behind the apex. At the very tip, growth by assimilation is chiefly occur- ring ; below the region of stretching, growth is occurring only slowly. The measurements of de Vries ('73) have, however, shown directly that the convex side of the tendril grows faster than the straight tendril and the concave side less rapidly. The convex side thus pushes still farther over the concave, pro- ducing the coiling. As the region of stretching is one of imbibition of water, we conclude that more water is taken in on the convex side than on the concave, where even a loss of Avater may occur ; or perhaps there is a movement of water from the irritated towards the opposite side. The irritant then produces a chemical chanofe on one side of the organism such as to cause 384 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIY a movement towards that side or from tliat side according as the organ is positively or negatively thigmotropic. X y § 3. Effect of Wounding upon the Dikection of Growth — Traumatkopism * Under this head may be considered the effect of a special class of molar agents, namely, those which produce a wound, upon the direction of growth of roots. We may consider first false and then true traumatropism. 1. False Traumatropism. — When the growing tissue of the radicle of a seedling is destroyed on one side by caustic or heat, the radicle turns towards that side (Cie- siELSKi, '72 ; Darwin, '81 ; Spaulding, '94). This result is satisfactorily ex- plained on the ground that the killing of the growing tissue on one side causes a retardation or cessation of growth on that side ; so that, while the points re, y (Fig. 103), remain nearly stationary, the points a;l, ?/^, opposite to them, grow widely apart. It will be seen at once that this bending belongs to a wholly different category from the thigmotropic curva- tures described in the last section. It is, as Darwin called it, a mechanical bending ; it is a false traumatropism. f 2. True Traumatropism. — Of widely different cause from the foregoing is the turning due to slight wounding which has been investigated by Ciesielski ('72), Darwin ('81), Det- LEFSEN ('82), WiESNER ('84), and Spaulding ('94). Dar- win irritated the radicle of a bean by touching it near the apex with dry caustic (nitrate of silver). The point of wound- * From Greek, Tpav/na, a wound. t A phenomenon allied to this is seen when at an early stage one half of a frog embryo is killed and, consequently, remains of small size ; the larger, growing, side soon comes to bend around the smaller, dead, side. A fuller account of these experiments, made by Eoux and others, upon the frog's egg, must be postponed to the next Part of this work, since it is complicated by the phenomenon of regeneration. y Fig. 103. — Diagram of a root tip, illustrating false traumatropism : a;?/, region of wounding. The line y-y' ran orig- inally parallel to the line x-x', but by growth has been brought to make an angle of 90° with it. 1 i §3] TKAUMATROPISM 385 ing appeared as a small, dark spot on one side of the radicle. The seedlings were then suspended in a moist chamber, over water, and at a temperature of 14.4° C. After 24 hours almost all of the radicles showed a marked curvature from the wounded side. Other means can be used to produce this result : a thin slice cut off obliquely from tlie tip of the radicle, a drop of shellac, of copper sulphate, of potassium hydrate, or steam at about 95° C. (Spaulding) — in general, any agent which irritates strongly without killing. Concerning the place on the root where the injury must be made in order that a response should appear, Spaulding finds that if the branding is made further from the apex than 1.5 mm. no traumatic curva- ture will occur ; or if an oblique cut at the apex should not involve the growing root tip, as at ah, Fig. 104, it is without effect. So it seems that the pro- liferating root tip is the sensitive part. The point of maxi- mum curvature is, how- ever, not at the root tip, but lies in the region of most rapid growth. The length of time elapsing between injury and response varies with the species and with the tem- perature. Thus, at a temperature of 18° C. the curvature begins in from 45 to b^ minutes, and reaches a maximum within 24 hours (Wibsner). The long, latent period and the slowness of complete response give an insight into the reason for this separation of the per- ceiving and responding zones. During the latent period the irritated tissue is be- coming stretching tissue through the im- bibition of water, while the root tip is becoming generated in advance, leaving b Fig. 104. — Diagram showing tlie rela- tions of the root tip, -r.i, and the root cap, r.c. ; a, b, line of a cut which involves only the root cap. Fig. 105.— Median longi- tudinal section through a gypsum cast, b, sur- rounding and repress- ing a root of Vicia f aba. Natural size. (From Pfeffer, '93.) 2c :386 EFFECT OF MOLAR AGENTS UPON GROWTH [Cii. XIV the irritated protoplasm behind. This can be demonstrated if (following Pfeffer, '93) the freshly irritated radicle is con- lined in a plaster cast (Fig. 105). The growth of the tip is .slackened so that it does not leave the irritated tissue far behind ; at the same time, the zone of stretching encroaches upon the root tip, so that, if the radicle is released after several days, the curvature, although taking place, occurs abnormally near the tip. If the response were transmitted a fixed dis- tance backward from the tip, we should expect that in the confined plant either the turning would take place at the normal position or would altogether fail to occur. Since neither of these effects follows, the conclusion is strengthened that the identical plasm which is irritated responds, producing the traumatic curvature. Temperature, controlling as it does the rate of growth, con- trols the length of the latent period. According to the obser- vations of WiESNEE, this is at — Temperature. In Damp Chamber. In Water. 8.0° C. 17.5° C. 25.0° C. 33.0° C. 438 min. 87 37 58 393 min. 52 28 70 Thus, 25° C. seems to be about the optimum temperature for the traumatroj3ic response ; and this is a temperature 10° to 12° below the optimum for protoplasmic movements (Chap. VIII, § 2). The true explanation of traumatropism remains, after pro- longed discussion, uncertain. Daewust regarded it as a response to stimulation, but Detlefsen and Wiesistee. sought to show that it is the immediate mechanical result of the injury. Thus, Detlefsejst assumed tensions in the root cap, which caused the root when cut on one side to shorten on the ■other ; but this conclusion was disproved by Spaulding, who found that the usual results do not follow when the root cap is injured without injuring the root tip. Weisner's explanation was based on the observation that decapitated roots grow more § 4] RHEOTROPISM 387 rapidly in water than normal ones. In like manner, if the root is, as it were, half decapitated, by cutting the tissue on one side, an abnormally rapid growth will take place on that side, proximal to the injury. The reason for this more rapid growth, continues Wiesner, is that the nutritive fluids intended for the tip are prevented by the injury from attaining the tip, and, consequently, go to build up the cell-walls above the wound. While the possibility of this explanation cannot be denied, the facts are not opposed to another interpretation, such as is offered by Spaulding, and which is more in accord with the explanations of other tropic phenomena. This expla- nation is that the wounding produces a chemical change in the protoplasm of one side of the root, such that growth occurs more rapidly upon that side, either as a result of increased upbuilding of cell-walls or of increased imbibition of water, or of both. § 4. Effect of Flowhstg Water upon the DiEECTioisr OF Growth — Rheotropism When the radicle of a bean is suspended by the cotyledons above a stream of flowing water, so that it lies in the axis of the current and points down stream, the free end, as it grows, gradually turns, and becomes directed up stream. It turns against the current. This remarkable phenomenon, named rheotropism by its dis- coverer, Jonsso:n" ('83, p. 518), has been carefully studied by him by the following method : The stream, whose rate could be controlled at will, flowed in a trough. Over the current, seedlings of maize and other grains, with well-developed rad- icles, were suspended so that the radicles lay in the axis of the current, and were directed either up stream or down as desired. When the rootlets were directed down stream, a turning began, after a latent period of several hours, and reached its final posi- tion in the current in about twenty hours. If the rootlet was originally directed up stream, it simply grew straight ahead until mechanically bent out of position by the impact of the water. By directing a rootlet alternately down stream and up stream after each rheotropism has occurred, the whole root may become very zigzag. These facts show clearly that cer- 388 EFFECT OF MOLAR AGENTS UPON GROWTH [Cii. XIV tain radicles are sensitive to the impact of water, so that they turn in such a way that the irritant shall affect their two sides equally, and shall be directed against the tip. While the pre- cise part of the radicle which is sensitive to the current has not been determined, the responding part is in the region of greatest growth, and the response is such that the root becomes concave towards the irritant. Among animals, a response which probably belongs to this category has been observed by Loeb ('91, p. 36) in the grow- ing stem of the hydroid Eudendrium. In a vessel of water in which all other hydranths turned towards the light, there was one which rose near the cloacal opening of an ascidian, from which strong currents of water passed. This individual was turned with its concave side towards the impinging current. It is probable that the current had induced a rheotropic response. Summary of the Chapter The rate of growth of the entire organism or of its organs is accelerated by contact, as in the stolons of hydroids ; by rough movement, as in bacteria ; by cutting — when accelerated growth occurs along the cut surface. Growth in length may be retarded by pulling, as in stems and roots. IMany organisms show themselves very sensitive in their growth to mechanical irritation. The direction of growth is determined by external agents, acting as irritants, in the cases of the twining dodder, tendrils in general, roots, the hyphse of Phycomyces, the rhizoids of hepatics, and the hydranths of various hydroids. Wounding may cause a turning from the wounded side in the radicle of a seedling, and radicles and hydroids may even grow so as to face the current. The sensitiveness to contact may be excessively great, a swinging cotton rider weighing 0.00025 mg. causing a tendril to bend. This sensitive region varies from near the tip in roots to some distance behind the tip in the case of tendrils. The response may occur after seconds, as in Cuscuta, or after hours, as in traumatropism of radicles. The bending usually reaches a maximum at the region of greatest growth. The LITERATURE 389 stimulated condition persists for some time after irritation, so that a summation of effects may occur. An acclimatization to contact may appear, when no response is invoked by the irrita- tion. The thigmotropic response may be explained, in general, as due to a chemical change, wrought by the one-sided impact, such as to cause a disturbance in the equality of the growth processes — assimilation, secretion, or imbibition — on the two sides of the organ. LITERATURE Baranetzky, J. 79. Die tagiiche Periodizitat im Langenwachsthixm der Stengel. Mem. de I'Acad. de St. Petersb. XXVII. 91 pp. BucHNER, H. '80. Ueber die experimentelle Erzeugung des Milzbrand- coiitagiums aus den Heupilzen. Sb. k. bayer. Akad. Miinchen. X, 368-413. CiESiELSKi, T. '72. Untersuchungen iiber die Abwartskriimnuing der Wurzel. Beitrage zur Biologie der Pflanzen. Band I, Heft II. pp. 1-30. Taf. I. Darwin, C. '65. On the Movements and Habits of Climbing Plants. Jour. Linn. Soc. (Bot.). IX, 1-118. '82. The Movements and Habits of Climbing Plants. 208 pp. 3d thousand. London, 1882. [Date of first edition, 1875.] Darwin, C. and F. '81. The Power of Movement in Plants. 592 pp. New York, 1881. Detlefsen, E. '82. (See Chapter XIT, Literature.) DuTROCHET, H. '43. Des mouvements revolutifs spontanes qui s'observent Chez les vege'taux. Ann. Sci. Nat. XX (Bot.), 306-329. '44. Recherches sur la volubilite des tiges de certains vegetaux et sur la cause de ce phenomene. Ann. Sci. Nat. II (Bot.), 156-167. Errara, L. '84. Die grosse Wachsthumsperiode beiden Fruchttragern von Phycomyces. Bot. Ztg. XLII, Nos. 32 to 36. Aug., Sept. 1884. Hegler, R. '93. Ueber den Einfluss des mechanischen Zugs auf das Wachsthum der Pflanze. Beitrage z. Biol. d. Pflanzen. VI, 383-432. Horvath, a. '78. (See Chapter IV, Literature.) JoNSSON, B. '83. (See Chapter IV, Literature.) Leone, T. '85. Sui microorganismi delle acque potabili. Atti della R. Accad. Lincei (4) Rencond. I. LoEB, J. '91. Untersuchungen zur physiologischen Morphologie der Thiere. I. Ueber Heteromorphose. Wtirzburg, 1891. '92. (See Chapter X, Literature.) MacDougal, D. T. '96. The Mechanism of Curvature of Tendrils. Ann. of Bot. X, 373-402. April, 1896. Meltzer, S. J. '94. (See Chapter IV, Literature.) 390 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch/XIV MoHL, H. V. '27. Ueber den Bau uiid das Windeii der Ranken und Schling- pflanzen. Tubingen, 1827. MoLiscH, H. '84. (See Chapter XII, Literature.) MiJLLER, O. '86. Untersuchungen iiber die Ranken der Cucurbitaceen. Beitriige z. Biol. d. Pflanzen. IV, 97-144. Newcombe, F. C. '95. The Regulatory Formation of Mechanical Tissue. Bot. Gazette. XX, 441-448. Oct. 1895. Palm, L. H. '27. Ueber das Winden der Pflanzen. 101 pp. 3 Tab. Stutt- gart, 1827. Peirce, G. J. '94. A Contribution to the Physiology of the Genus Cuscuta. Ann. of Bot. VIIT, 53-117. PI. VIII. March, 1894. Pfeffer, W. '85. Zur Kenntniss der Kontaktreize. Unters. a. d. bot. Inst. Tubingen. I, 483-535. '93. Druck und Arbeitsleistung durch wachsende Pflanzen. Abh. sachs. Ges. d. Wiss., Leipzig. XX, 235-474. Reinke, J. '80. Ueber den Einfluss mechanischer Erschiitterung auf die Entwickelung der Spaltpilze. Arch. f. d. ges. Physiol. XXIII,, 434-446. Russell, H. S. '92. The Effect of Mechanical Movement upon the Growth of Certain Lower Organisms. Bot. Gazette. XVII, 8-15. Jan. 1892. Sachs, J. '73. Ueber das Wachsthum der Haupt- und Nebenwurzeln. Arb. Bot. Inst. Wurzburg. I, 385-474. '87. (See Chaj^ter X, Literature.) ScHENCK, H. '93. Ueber den Einfluss von Torsionen und Biegungen auf das Dickenwachsthum einiger Lianen-Stamme. Flora. LXXVII, 313-326. Schmidt, B. '91. Ueber den Einfluss der Bewegung auf das Wachsthum und die Virulenz der Mikroben. Arch. f. Hygiene. XIII, 247. ScHOLTZ, M. '87. Ueber den Einfluss von Dehnung auf das Langenwachs- thuin der Pflanzen. Beitr. z. Biol. d. Pflanzen. IV, 365-408. Spaulding, V. M. '94. The Traumatropic Curvature of Roots. Ann. of Bot. 423-452. PI. XXIL Dec. 1894. Tumas, L. '82. Ueber die Bedeutung der Bewegung fur das Leben der niederen Organismen. Medic. Wochenschr. 1882. No. 18. St. Peters- burg. Verworn, M. '95. AUgemeine Physiologie [1st Aufl.]. 584 pp. Jena, 1895. DE Vries, H. '73. Langenwachsthum der Ober- und Unterseite sich kriitn- mender Ranken. Arb.. Bot. Inst. Wurzburg. I, 302-316. WiESNER, J. '84. Untersuchungen tiber die Wachsthumsbewegungen der Wurzeln. Sb. k. Akad. Wiss. Wien. LXXXIX^, 223-302. Wortmann, J. '87. Zur Kenntniss der Reizbewegungen. Bot. Ztg. XLV;. 785 et seq. Dec. 1887. CHAPTER XV EFFECT OF GRAVITY UPON GROWTH § 1. Effect of Gkavity upoisr the Rate of Gkowth It is a result of the sessility of the higher plants that they and their parts are acted upon continuously in one direction by gravity. It might consequently be suspected that the rate of their growth would be affected if they were placed in an abnormal position with respect to this agent. In the experi- ments which have been made to test the correctness of this suspicion various methods have been employed. When the growth of Penicillium which is removed from gravity's action by being slowly revolved on a klinostat is coinpared with that of a plant under normal conditions, the former is found to be more rapid. The plant seems to reap an advantage from not hav- ing to sustain its own weight (Ray, '97). When Phycomyces is inverted, its growth is slower than in the normal position (Elfving, '80). These two experiments upon fungi indicate a considerable sensitiveness on their part to gravity. Experi- ments made by Elfving ('80) and Schwarz ('81), upon the growth of inverted phanerogams and those from which the action of gravity had been eliminated by the slowly rotating klinostat, as well as those which had been subjected to exces- sive pressure by the centrifugal machine, yielded for the most part only negative results. Upon the rate of growth of seed- lings gravity has little effect. § 2. The Effect of Gravity upon the Direction of Growth — Geotropism As with contact so with gravity two classes of effects may be distinguished, which, while often producing similar results, bring them about through very dissimilar processes. The first of these is a mechanical effect due to gravity acting upon the 391 392 EFFECT OF GRAVITY UPON GROWTH [Cii. XV growing organ as it might upon any other heavy body. The second is a vital effect, having no immediate, direct, pliysical relation to the cause. The first may be called false geotro- pism ; the second, true geotropism. 1. False geotropism may be treated very briefly, since it is not a true growth phenomenon. As an example of such geotro- pism may be cited the downward bending of the top of a stem heavily laden with a head of seed or with fruit, or the upward growth of the long fronds of kelp in the sea on account of the buoyant effect of the dense sea water. In such cases the direction of the growth can be accounted for upon well-known principles of hydrostatics. 2. True Geotropism. — The biological effect, on the other hand, which is seen in true geotropism is often opposed, to the gross physical one. It shows itself especially in various groups of sessile plants and animals. Since, unfortunately, com- paratively few experiments have been made upon geotropism in animals, the great mass of our knowledge on this subject is derived from studies on plants. The general fact of geotropism strongly impresses one who stands on the shore of a lake in our northern country and looks across to the dense pine forest on the opposite side. The landscape is composed of vertical and horizontal lines — vertical lines below formed of close-set trunks of trees, hori- zontal lines above formed of the great branches. If at one side a steep slope ascends, its outline is obscured by the grill-work of perpendicular lines formed by the vegetation which clothes it. Here the effect — the dissimilar effect — of gravity in determining the direction of growth of two organs, trunk and branch, is seen. There are, however, still other organs which may respond to the same stimulus. Among these are the roots, flower stalks, and leaves of phanerogams ; the vertical parts of many vascular cryptogams ; the sporiferous hyphse of fungi and some vertical algse, e.g. Chara. There is no need to examine all these cases of geotropism, but only such of them as will help us to get at the principles of the action of gravity in determining the direction of growth. a. Roots. — When a seedling is so placed that its radicle is horizontal, the radicle does not continue to grow out in the §2] GEOTROPISM 393 same direction ; but all additional growth is vertical so that the radicle bends sharply downwards — it is positively geotropic. The curvature does not take place at the very tip of the root, — in the region of growth by assimilation, — but immediately behind in the region of stretching or of growth by imbibition. The precise region at which the curvature occurs was deter- mined by CiESiELSKi ('72), by means of a method illustrated in Fig. 106. It is here seen that the maximum bending occurs in this radicle at between 3 and 4 mm. from the tip. It is also plain that the geotropic curvature is in some way connected with growth — could not occur without growth. To show that the normal vertical growth of a root is due to the press- ure of gravity, Kxight, in 1806, determined experimentally that the direction of its growth can be deter- mined by other pressures replacing gravity, such as centrifugal force. Thus when seedlings were attached to the rim of a wheel and this was made to rotate rapidly about a hori- zontal axis the radicles grew straight outward from the axis ; thus in the sense in which the centrifugal press- ure acted, as before they had grown in the sense of the pull of gravity. A second method of proof, employed by Sachs ('79), consisted in eliminating the effect of gravity by means of the klinostat (Chapter V, § 1), under which circumstances the root grew irregularly. Such results leave no room for doubt that it is gravity which determines the verticality of the root. The question now arises, in what way does gravity control the direction of growth of this organ ? An early suggestion was that the action was direct, due to the relatively great specific gravity of the root tip ; but this idea was easily refuted by a mass of evidence. Thus it is not, a priori, easy to see Fig. 106. — An originally straight radicle of the pea, graduated in 0.5 mra. spaces and placed nearly vertically with its apex directed u^jwards in the direc- tion of a, has turned down- wards in the zone of greatest growth at between 3.5 and 4 mm. from the tip. (From Cie- SIELSKI, '72.) 394 EFFECT OF GRAVITY UPON GROWTH [Cn. XV how the mere weight of the tip of the plant, supported as it is by the firm soil, should enable it to find its way down. Experiments, also, have shown that the radicle will curve downwards against considerable resistance such as is afforded by the surface of a cup of mercury, by a weight which is lifted by means of a thread passing over a pulley (Johnson, '29), or by a delicate spring which is compressed by the down-curv- ing radicle (Wachtel, '95). By these various methods it has been demonstrated that the down-curving of the geotropic radicle is an active process capable of overcoming resistance amounting, in one case, to 150 milligrammes. Consequently gravity does not act in a grossly mechanical way, but as a stimulus inciting a growth response. If now the down-curving of the root is a response the question arises, where is the stimulus received ; at the region of curvature or at some other point ? This is a question which has excited much discussion, owing to apparent contradictions in the experimental evidence. The first attempt to answer this question was made by Ciesielski * ('T2), who cut off the terminal one-half millimeter of the radicle, placed the rootlet in a horizontal position, and found that, although growth con- tinued, no down-curving occurred until, after several days, the root tip had regenerated. He concluded that the root tip is useful or necessary in geotropism. Sachs ('73, p. 433) repeated Ciesielski's experiments and likewise found an. incomplete exhibition of geotropism. But this he attributed to the excessive irritation of the cutting, which led to exagger- ated movements of nutation, tending to obscure geotropism. Dae-WIN ('80) confirmed Ciesielski's results after both cutting off the root apex and cauterizing it, and explained Sachs' partial failure to get like results on the ground that he did not amputate the radicles in a strictly transverse direction. Dab, WIN concluded that the "tip of the radicle is alone sensi- tive to geotropism, and that when thus excited it causes the ad- joining parts to bend." The exact length of the sensitive part seems to vary with certain conditions, but it is generally less * Hartig ('66, p. 53) had already stated that the decapitated root was not geotropic. §2] GEOTROPISM 395 than 1 to 1.5 mm. This terminal millimeter or so, then, acts like a sense organ in a vertebrate, which receives the sensation at some distance, it may be, from the responding organ. From this time on, two schools may be distinguished, of which the first, following Darwiist, regarded the stimulus as received at the root tip and transmitted to the region of growth ; while the second conceived the stimulus to act di- rectly upon the root in the bending region and to arise from the difference in the pressures on the upper and lower surfaces of the radicle. The second school persistently denied the validity of the decapitation experiments, Wiesner ('81) in particular maintaining that decapitation inhibited growth and consequently growth curvatures, but did not necessarily remove the sensitive organ. The first school was forced to new experi- ments. Francis Darwin ('82) maintained on the basis of such experiments that it is not the cutting per se which inhibits geotropism ; for the cutting of the root tip, as for example, lengthwise, without its removal, permits the normal response. But it was easy to reply to this that a transverse cut might well affect growth more seriously than a longitudinal one. The satisfactory solution of this difficulty required a method by which, without mutilating the root, gravity should act horizontally upon the chief growing part of the root without so acting upon the root tip. A method for accomplishing this was invented by Pfeffer ('94). A radicle of a bean or other species, fixed to a klinostat, was made to grow into a small glass tube, closed at its further end and bent so as to form two arms, at right angles to each other, and each about 1.5 to 2.0 mm. long. The preparation was now turned until the root tip was directed ver- tically downwards, while the rest of the root, and especially its chief growing re- gion, was horizontal (Fig. 107). This region Fig. io7. — Diagram ii- was then subiected to the full transverse lustratmg the method '' _ , , employed in Pfbf- action of gravity, while the tip was not so fer's experiment, acted upon. Meanwhile the normal growth processes were not interfered with, for as the root lengthened it backed out, so to speak, from the bent tube, the apex remain- 396 EFFECT OF GRAVITY UPON GROWTH [Cii. XV ing at the blind end. Under these conditions no geotropic cur- vature occurred ; but such curvature always took place when the root tip was placed horizontally or at any acute angle with the horizon. This experiment seems, then, better fitted than any previous one to prove that the geotropic sensitiveness of the root resides in the apex.* Consequently, since, as an in- spection of Fig. 106 will show, the curving part of the root can contain none of the originally irritated cells, there must be a transmission of stimuli from the root tip to the curving region. Two associated phenomena remain to be considered. First, geotropic response is more powerful, the bending becomes stronger, as the angle made by the root with the vertical in- creases, reaching a maximum when the root is horizontal (Sachs, '73, p. 454). Secondly, as already indicated, response does not take place immediately after the root is placed hori- zontally. In one experiment of Sachs' ('73, p. 440) a bean radicle placed horizontally and growing in loose earth at 20° C. began to turn downwards in the second hour. There is thus a considerable latent period. The cause of geotropism in roots is indicated by the con- ditions of its occurrence already mentioned. It is intimately associated with growth, yet, as Kirchner ('82) has shown, it may occur at a temperature (2° to 3.5° C.) at which growth is extremely slow. The turning is clearly due to unequal growth upon the convex and concave sides of the root. But is it due to acceleration on the convex side over the normal, to retarda- tion on the concave side, or to both? Measurements have been made upon plants to decide this question. In one set of such measurements made upon the bean radicle by Sachs ('73, p. 463) the growth was hastened about 3% on the convex side and retarded 42% on the concave side, so that both accelera- tion and retardation occur. Finally, lateral roots which run more or less obliquely down- wards have been shown by Sachs ('74) and others to be influenced by gravity; for, if the plant be inverted, the roots will turn until they assume their normal inclination to the horizontal plane. * But species may differ in this respect as in phototropism (see page 441). §2] GEOTROPISM 397 h. Stems. — The central fact that the upward growth of stems is determined by gravity is established by the observa- tions that on the klinostat no definitely directed growth occurs, and that on the centrifugal machine the stem turns in the opposite sense to that of the cen- trifugal pressure. The stem is nega- tively geotropic. As with roots, so with stems, a number of questions now arise : Where is the sensitive region and where the response? At what inclination of the stem is the strongest geotropic cur- vature called forth? What is the im- mediate cause of the curvature? As Fig. 108 shows, the response of the stem of a seedling is fundamentally different from that of the radicle. In- stead of the tropism beginning at one point, and continuing there as in the root (Fig. 106), it begins close below the cotyledons of the seedling and passes downwards towards the base as far as growth is still occurring. Response, consequently, takes place along the whole "stretching" region. The sensitive re- gion also, unlike that of the root, is not confined to the tip, but extends along the entire bending stem. The position in which the strongest response is incited was believed by Sachs ('79% p. 240) to be the horizontal one, and Bateson and DAiiWiisr ('88) have confirmed this conclusion. Their method depends upon the fact that a stem placed horizontally and restrained for several hours from taking the verti- cal position will, upon being released, suddenly spring upwards. In using this method it was found that the stem springs through a greater arc after having been Fig. 108. — Course of geotro- pism in a plumule. The successive figures 1 to 16 indicate successive stages in the geotropic turning of a seedling growing in half darkness. Placed at first horizontal as at 1, the plant has become completely erect at 16. The most rapid growth is just behind the cotyle- dons and diminishes toward the base. The temporary bending beyond the vertical is to be noted. (FromSTRAS- BURGER, Noll, Schenck, and ScHiMPER, Textbook of Botany, Macmillan.) S98 EFFECT OF GRAVITY UPON GROWTH [Cii. XV restrained for two hours in a horizontal position than if re- strained in any oblique position, whether the tip be directed up or down. Concerning the cause of the curvature there is little to add to what was said under "Roots." The remote cause is appar- ently the dissimilar action of gravity on the upper and the under sides of the stem ; the immediate cause is the difference in growth on the two sides of the stem. In the special case of stems with knots, the knots show themselves especially respon- sive to the geotropic stimulus. c. Rhizoma. — These horizontally running, root-like, subter- ranean stems are strikingly responsive to gravity, as Elfving ('80), especially, has shown. He has reared various rushes in a glass box with their axes making various angles with the vertical. In their subsequent growth all the rhizomes of the plants extended in a strictly horizontal direction. In this case any component of gravity, however small, running in the direc- tion of the axis of the rhizome seems to irritate. The curving into a horizontal plane may be called transverse geotropism (diageotropism, Fkank). d. Cryptogams. — We have already seen (p. 391) that the sporangiferous hyphse of Phycomyces nitens are negatively geotropic. The same is true of certain algte. Thus Richter ('94) has found that when the stem of Chara is inverted the youngest two or three internodes curve upwards in their further growth so that the apex of the stem is now directed zenithwards. On the other hand the rhizoids of this species are positively geotropic. Rotation experiments show that in the absence of the directive pressure of gravity there is no definite orientation. Finally, some mosses are slightly geo- tropic. Thus, Bastit ('91) reared Polytrichum juniperinum in the dark in both air and water, some plants being placed right side up, others inverted. In both cases new branches budded from the roots and, although etiolated, grew irregularly up- wards— there was a feeble negative geotropism. e. Animals. — Since only sessile organisms can be expected to show marked geotropism, this phenomenon among animals must be confined to rather few groups. It has been studied hitherto exclusively in the group of hyclroids. Many repre- §2] GEOTROPISM 399 sentatives of this group sliow themselves, however, markedly responsive. The first observations upon geotropism in hydroids seem to have been made by Loeb ('91% pp. 27, 28). He says : "When a stolon was formed at the cut end of a vertical stem (of Ag- laophenia) it grew (in case it did not come in contact with a solid body) first a short distance horizontally, and then down- wards. In horizontal stems the stolon grew directly down- Wi- \^ - b 110 109 Pig. 109. — Positive geotropism of regenerated stolons {Wi, W^) and negative geotro- pism of regenerated hydranths of Aglaophenia pluma. The original piece of tlie stem is included between b and c. This piece was placed vertically, right end up, in the aquarium. At the cut end, b, the stolon (TF^) has arisen, but has soon begun to grow downwards. It has produced a vertical hydranth at s. W