The Power of Movement in Plants

each observation whicli lasted for only a minute or two; but the result was that their hypocotyls bowed themselves to fis side, whence some light occasionally entered, in lines which were only slightly zigzag. Although not a single ellipse was cven approximately formed, we inferred from the zigzag lines—and, as it proved, correctly—that their hypocotyls were circumnuta- ting, for on the following day these same seedlings were placed in a completely darkened room, and were observed each time by the aid of a small wax taper held almost directly above them, and their movements were traced on a horizontal glass above ; and now their hypocotyls clearly circum- nutated (Fig. 168, and Fig. 39, formerly given, p. 52); yet they moved a short distance towards the side where the taper was held up. Ifwe look at these diagrams, and suppose that the taper had been held more on one side, and that the hypocotyls, still circumnutating, had bent themselves within the same time much more towards the light, long zigzag lines would ob- viously have been the result.

Again, two seedlings of Solanwm lyco- persicum were illuminated from above, but accidentally a little more light entered on one than on any other side, and their hypocotyls became slightly bowed towards the brighter side; they moved in a zigzag line and described in their course two little triangles, as seen in Fig. 37 (p. 50), and in another tracing not given. Thesheath- like cotyledons of Zea mays behaved, under nearly similar circumstances, in a nearly ; ' . similar manner, as described in our first 4% sativa : heliotropic

movement and circum- chapter (p. 64), for they bowed themselves _putation of sheath-like during the whole day towards one side, cotyledon (14 inch in making, however, in their course some height) traced on hori- 2 zontal glass from 8 A.M. conspicuous flexures. Before we knew 4, 10.93 p.m. Oct. 1th. how greatly ordinary circumnutation was modified by a lateral light, some seedling oats, with rather old and therefore not highly sensitive cotyledons, were placed in front of a north-east window, towards which they bent all day in a strongly zigzag course. On the following day they continned to bend in the same direction (Fig. 169), but zigzagged much less. The sky, however, became between 12.40 and 2.86 v.u.

overcast with extraordinarily dark thunder-clouds, and it was interesting to note how plainly the cotyledons circumnutated during this interval. The foregoing observations are of some value, from having ben made when we were not attending to heliotropism; and they led us to experiment on several kinds of seed- lings, by exposing them to a dim lateral light, so as to observe the gradations between ordinary circumnutation and heliotropism, Seedlings in pots were placed in front of, and about a yard from, a north-east window ; on each side and over the puts black hoards were placed; in the rear the pots were open to the diffused light of the room, which had a second north-east and a north-west window. By hanging up one or more blinds before the window where the seedlings stood, it was easy to dim the light, so that very little more entered on this side than on the opposite one, which received the diffused light of the room. Late in the evening the blinds were successively removed, and as the plants had heen subjected during the day to a very obscure light, they continued to bend towards the window later in the evening than would otherwise haveoccurred. Most of the seedlings were selected because they were known to be highly sensitive to light, and some because they were but little sensitive, or had become so from having grown old.. The movements were traced in the usual manner on a horizontal glass cover; a fine glass filament with little triangles of paper having been cemented in an upright position to the hypocotyls. Whenever the stem or hypocotyl became much buwed towards the light, the latter part of its course had to be traced on a vertical glass, parallel to the window, and at right angles to the horizontal glass cover.

gularly towards a bright lateral light. In order to ascertain how straight a course it would pursue when fairly well illumi- nated on one side, seedlings were first placed before a south-west window on a cloudy and rainy morning; and the movement of two hypocotyls were traced for 3h., during which time they became greatly bowed towards the light. One of these tracings is given on p. 422 (Fig. 170), and the course may be seen to be almost straight. But the amount of light on this occasion was superfluous, for two seedlings were placed before a north-east window, protected by an ordinary linen and two muslin blinds, yet their hypocotyls moved towards this rather dim light in only slightly zigzag lines; but after 4P.m., as the light waned, the lines became distinctly zigzag. One of these seedlings, moreover, described in the afternoon an ellipse of considerable size, with its longer axis directed towards the window.

We now determined that the light should be made dim enough, so we began by exposing several seedlings before a north-east window, protected by one linen blind, three muslin blinds, and a towel. But so little light entered that a pencil cast no perceptible shadow on a white card, and the hypocotyls did not bend at all towards the window. During this time, from 8.15 to 10.50 a.m., the hypocotyls zigzagged or circum- nutated near the same spot, as may be seen at A, in Fig. 171. The towel, therefore, was removed at 10,50 a.m., and replaced by two muslin blinds, and now the light passed through one ordinary linen and four muslin blinds. When a pencil was held upright on a card close to the seedlings, it cast a shadow (pointing from the window) which could only just be distinguished. Yet this very slight excess of light on one side sufficed to cause the hypocotyls of all the seedlings immediately to begin bending in zigzag lines towards the window. The course of one is shown at A (Fig..171): after moving towards the window from 10.50 a.m, to 12.48 p.m. it bent from the window, and then returned in a nearly parallel line; that is, it almost completed between 12.48 and 2 Pm. a narrow ellipse. Late in the evening, as the light waned, the hypocotyl ceased to bend towards the window, and circum- nutated on a small scale round the same spot; during the night it moved considerably backwards, that is, became more upright, through the action of apogeotropism. At B, we have a tracing of the movements of another seedling from the hour (10.50 a.m.)

“when the towel was removed; und it is in all essential respects similar to the previous one. In these two cases there could be no doubt that the ordinary circumnutating movement of the hypocotyl was modified and rendered heliotropic. 10°50'1.m, Apios graveolens : heliotropic movement and circumnutation of the hypo cotyls of two seedlings towards a dim lateral light, traced on a horizontal glass during the day. The broken lines show their return nocturnal

courses. Height of hypocotyl of A ‘5, and of B55 inch. Ficure teduced to one-half of original scale. ~ Brassica oleracea.—The hypocotyl of the cabbage, when not disturbed by a lateral light, circumnutates in a complicated * manner over nearly the same space, and a figure formerly given is here reproduced (Fig. 172). If the hypocotyl is exposed to a moderately strong lateral light it moves quickly towards this side, travelling in a straight, or nearly straight, line. But when the lateral light is very dim its course is extremely tortuous, and evidently consists of modified circumnutation. Seedlings were placed before a north-east window, protected by a linen and muslin blind and by a towel. The sky was cloudy, and when- ever the clouds grew a little lighter an additional muslin blind was temporarily suspended. The light from the window was

Brassica oleracea ordinary circumnutating mcvement of the hrpocoty! of a seedling plant. thus so much obscured that, judging by the unassisted eye, the seedlings appeared to receive more light from the interior of the room than from the window; but this was not really the case, as was shown by a very faint shadow cast by a pencil on a card. Nevertheless, this extremely small excess of light on one side caused the hypocotyls, which in the morning had stood upright, to bend at right angles towards the window, so that in the evening (after 4.23 p.m.) their course had to be traced on a vertical glass parallel to the window. It should be stated that at 3.30 p.m., by which time the sky had become darker, the towel was removed and replaced by an additional muslin blind, which itself was removed at 4 p.m., the other two

blinds bemg left suspended. In Fig. 173 the course pursued, between 8.9 A.M. and 7.10 p.m., by one cf the hypocotyls thna Brassica oleracea : heliotropic movement and circumnutation of a hypocoty} towards a very dim lateral light, traced during 11 hours, ona horizontal glass in the morning, and on a vertical glass in the evening. Figure reduced to one-third of the original scale. exposcd is shown. It may be observed that during the first 16 m. the hypocotyl moved obliquely from the light, and this,

no doubt, was due to its then circumnutating in this direction. Similar cases were repeatedly observed, and a dim light rarely or never produced any effect until from a quarter to three- quarters of an hour had elapsed. After 5.15 p.m., by which time the light had become obscure, the hypocotyl Fig. 174, began to circumnutate 6°30" about the same spot. The contrast between the two figures (172 and 173) would have been more striking, if they had been originally drawn on the same scale, and had been equally reduced. But the movements shown in Fig. 172 were at first more mag- nified, and have been re- duced to only one-half of the original scale; whereas those in Fig. 173 were at first less magnified, and have been reduced to a one-third scale. A tracing made at the same time with the last of the movements of a second hypocotyl, presented a closely analogous appear- ance; but it did not bend quite so much towards the ager th light, and it circumnu- 93 aa iniinee tated rather more plainly. Pialiis Comrines: blotone meremen

Phaluris Canartensis,— ledon, towards a dull lateral light, traced Thesheath-likecotyledous ona horizontal glass from 8.15 a.m. Sept. of this monocotyledonous 16th to 7.45 a.m. 17th. Figure reduced plant were selected for to one-third of original scale. trial, because they are very sensitive to light and circumnutate well, as formerly shown (see Fig. 49, p. 63). Although we felt no doubt about the result, some seedlings were first placed before a south-west window on a moderately bright morning, and the movements of one were traced. As is so common, it moved

for the first 45 m. in a zigzag line; it then felt the full influence of the light, and travelled towards it for the next 2h. 30m. in an almost straight line. The tracing has not been given, as it was almost identical with that of Apios under similar circum- stances (Fig. 170). By noon it had bowed itself to its full extent; it then circumnutated about the same spot and described two ellipses; by 5 p.m. it had retreated considerably from the light, through the action of apogeotropism. After some pre- liminary trials for ascertaining the right degree of obscurity, some seedlings were placed (Sept. 16th) before a north-east window, and light was admitted through an ordinary linen and three muslin blinds. A pencil held close by the pot now cast a very faint shadow on a white card, pointing from the window. In the evening, at 4.80, and again at 6 p.m., some of the blinds were removed. In Fig. 174 we see the course pursued under these circumstances by a rather old and not very sensitive cotyledon, 1°9 inch in height, which became much bowed, but was never rectangularly bent towards the light. From 11 a.m., when the sky became rather duller, until 6.30 p.m., the zigzageging was conspicuous, and evidently consisted of drawn- out ellipses. After 6.30 p.m. and during the night, it retreated in a crooked line from the window. Another and younger seed- ling moved during the same time much more quickly and to a much greater distance, in an only slightly zigzag line towards the light; by 11 a.m. it was bent almost rectangularly in this direction, and now circumnutated about the same place.

Tropeolum majus.—Some very young see lings, bearing only two leaves, and therefore not as yet arrived at the climbing stage of growth, were first tried before a north-east window without any blind. The epicotyls bowed themselves towards the light so rapidly that in little more than 8 h. their tips pointed rectangularly towards it. The lines traced were either nearly straight or slightly zigzag; and in this latter case we see that a trace of circumnutation was retained even under the influence of a moderately bright light. Twice whilst these epicotyls were bending towards the window, dots were made every 5 or 6 minutes, in order to detect any trace of lateral movement, but there was hardly any; and the lines formed by their janction were nearly straight, or only very slightly zigzag, as in the other parts of the fgures. After the epicotyls had bowed themselves to the full extent towards the light, ellipses of considerable size were described in the usual manner.

After having seen how the epicotyls moved towards a mode rately bright light, seedlings were placed at 7.48 a.m. (Sept. 7th) before a north-east window, covered by a towel, and shortly afterwards by an ordinary linen blind, but the epicotyls still moved towards the window, At 9.13 a.m. two additional muslin blinds were suspended, so that the seedlings received very little more light from the window than from the interior of the room The sky varied in brightness, and the seedlings occasionally

748'a.m Tropaolum majus : heliotropic movement and circumnutation of the epicotyl of a young seedling towards a dull lateral light, traced on a horizontal glass from 7.48 a.M. to 10.40 p.m. Figure reduced to one-half of the original scale. received for a short time less light from the window than from the opposite side (as ascertained by the shadow cast), and then one of the blinds was temporarily removed. In the evening tho blinds were taken away, one by one. The course pursued by an epicotyl under these circumstances is shown in Fig. 175. During the whole day, until 6.45 pm., it plainly bowed itself towards the light; and the tip moved over a considerable space. After 6.45 p.m. it moved backwards, or from the window, till

10.40 p.m., when the last dot was made. Here, then, we have a distinct heliotropic movement, effected by means of six elongated figures (which if dots had been made every few ininutes would have been more or less elliptic) directed towards the light, with the apex of each suc- Fig. 176. cessive ellipse nearer to the window than the previous one. Now, if the light had been only a littie brighter, the epicotyl would have bowed itself more to the light, as we may safely conclude from the previous trials; there would also have been less lateral movement, and the ellipses or other figures would have been drawn out into a strongly marked zigzag line, with probably one or two small loops still formed. If the light had been much brighter, we should have had a slightly zigzag line, or one quite straight, for there would have been more movement in the direc- tion of the light, and much less from side to side.

Sachs states that the older inter- nodes of this Tropzolum are aphe- liotopic; we therefore placed a plant, 11% inches high, in a box, blackened within, but open on one side in front of a north-east window Tropeolum majus: heliotropic without any blind. A filament was tion of an old internode to- the summit on one. plant ae wards a lateral light, traced 3 plant, and to on a horizontal glass from 8 the fourth internode of another. ficiently bright, to induce aphelio- tropism, for both plants bent slowly towards, instead of from the window during four days. The course, during two days of the first-mentioned internode, is given in Fig. 176; and we see that it either circumnutated on a small scale, or travelled in a zigzag line towards the light. We have thought this case of feeble heliotropism in one of the older internodes of a plant,

which, whilst young, is so extremely sensitive to light, worth sensitive to light, whilst the hypocotyls are much less sensitive than those of most other seedlings, as we had often observed with surprise. It seemed therefore worth while to trace their move- ments. They were exposed to a lateral light before a north-east window, which was at first covered merely by a muslin blind, but as the sky grew brighter about 11 am., an additional linen olind was suspended. After 4 p.m. one blind and then the other was removed. The seedlings were protected on each side and above, but were open to the diffused light of the room in the rear. Up- right filaments were fixed to the hypocotyls of two seed- lings, which stood vertically inthe morning. Theaccom- panying figure (Fig. 177) shows the course pursued by one of them during two days; but it should be particularly noticed that during the second day the seedlings were kept in darkness, and they then circumuutated round nearly the same small space. On the first day (Oct. 7th) the hypocoty] moved from 8 am. to 12.23 p.m, toward

Cassia tora: heliotropic movement and circumnutation of a hypocotyl (13 inch in height) traced on a horizontal glass from 8 a.m. to 10.10 p.m. Oct, 7th. Also its circumnutation in darkness from 7 a.m. Oct. 8th to 7.45 AM Oct. 9th. the light in a zigzag line, then turned abruptly to the left and afterwards described a small ellipse. Another irregular 432 MODIFIED CIRCUMNUTATION. Cuar. VIIL eliipse was completed between 3 p.m. and about 5.30 P.M, the hypocotyl still bending towards the light. The hypocotyl

Biqnonia capreolata: aphe- liotropic movement of a tendril, traced on a hori- zontal glass from 6.45 A.M. July 19th to 10 a.m. 20th. Movements as originally traced, little magnified, here reduced to two-thirds of the original scale. was straight and upright in the morn- ing, but by 6 p.m. its upper half was bowed towards the light, so that the chord of the are thus formed stood at an angle of 20° with the perpendicular. After 6 p.m. its course was reversed through the action of apogeotropism, and it continued to bend from the window during the night, as shown by the broken line. On the next day it was kept in the dark (excepting when each observation was made by the aid of a taper), and the course followed from 7 4,M on the 8th to 7.45 a.m. on the 9th is here likewise shown. The difference between the two parts of the figure (177), namely, that described during the daytime on the 7th, when exposed toa rather dim lateral light, and that on the 8th in darkness, is striking. The difference consists in the lines during the first day having been drawn out in the direction of the light. The movements of the other seedling, traced under the same circumstances, were closely similar.

Apheliotropism—We succeeded in observing only two cases of aphelio- tropism, for these are somewhat rare ; and the movements are generally so slow that they would have been very troublesome to trace. Bignonia capreolata—No organ of any plant, as far as we have seen, bends away so quickly from the light as do the tendrils of this Bignonia. They are also remarkable from circum- nutating much less regularly than most other tendrils, often remaining unar. VIL APHELIOTROPISM. 433 contact with the trunks of trees.** The stem of a young plant was tied to a stick at the base of a pair of tine tondrils, which projected almost vertically upwards; and it was placed in front of a north-east window, being protected on all other sides from the light. The first dot was made at 6.45 a.m., and by 7.35 A.M. both tendrils felt the full influence of the light, for they moved straight away from it until 9.20 a.m., when they circumuutated for a time, still moving, but only a little, from the light (see Fig. 178 of the left-hand tendril), After 3 p.m. they again moved rapidly away from the light in zigzag lines. By a late hour in the evening both had moved so far, that they pointed in a direct line from the light. During the nigit they returned a little in a nearly opposite direction. On the following morning they again moved from the light and con- verged, so that by the evening they had become interlocked, still pointing from the light. The right-hand tendril, whilst converging, zigzagged much more than the one figured. Both tracings showed that the apheliotropic movement was a modi- fied form of circumnutation.

Cyclamen Persicum.— Whilst this plantis in flower the peduncles stand upright, but their uppermost part is hooked so that the flower itself hangs downwards. As soon as the pods begin to swell, the peduncles increase much in length and slowly curve downwards, but the short, upper, hooked part straightens itself. Ultimately the pods reach the ground, and if this is covered with moss or dead leaves, they bury themselves. We have often seen saucer-like depressions formed by the pods in damp sand or sawdust; and one pod (°3 of inch in diameter) buried itself in sawdust for three-quarters of its length.t We shall have occasion hereafter to consider the object gained by this burying process. The peduncles can change the direction of their cur- vature, for if a pot, with plants having their peduncles already bowed downwards, be placed horizontally, they slowly bend at right angles to their former direction towards the centre of the earth. We therefore at first attributed the movement to geotropism ; but a pot which had lain horizontally with the pods

of Climbing Plants,’ 1875, p. 97. t The peduncles of several other species of Cyclamen twist themselves into a spire, and av- cording to Erasinus Darwin ,* Bo- tauic Garden,’ Canto., iii. p. 126), the pods forcibly penctrate the earth. See also Grenier and Godron, ‘Fl sre de France,’ tom ii p. #59. all pointing to the ground, was reversed, being still kept hori- zontal, so that the pods now pointed directly upwards ; it was then placed in a dark cupboard, but the pods still pointed up- wards after four days and nights. The pot, in the same position, was next brought back into the light, and after two days there was some bendinz downwards of the peduncles, and on the fourth day two of them pointed to the centre of the earth, as did the others after an additional day or two. Another plant, in a pot which had always stood upright, was left in the dark cupboard for six days; it bore 3 peduncles, and only one became within this

Cyclamen Persicum: downward apheliotropic movement of a flower-peduncle, greatly magnified (about 47 times ?), traced on a horizontal glass from 1 p.m. Feb, 18th to 8 a.m. 21st. time at all bowed downwards, and that doubtfully. The weight, therefore, of the pods is not the cause of the bending down. This pot was then brought back into the light, and after three days the peduncles were considerably bowed downwards. We are thus Jed to infer that the downward curvature is due to apheliotropism ; though more trials ought to have been made. In order to observe the nature of this movement, a peduncle bearing a large pod which had reached and rested on the ground, was lifted a little up and secured to a stick. A filament was fixed across the pod with a mark beneath, and its moves

ment, greatly magnified, was traced on a horizontal glass during 67h. The plant was illuminated during the day from above. A copy of the tracing is given on p. 484 (Fig. 179); and there can be no doubt that the descending movement is one of modified circumnutation, but on an extremely small scale. The observa- tion was repeated on another pod, which had partially buried itself in sawdust, and which was lifted up a quarter of an inch above the surface; it described three very small circles in 24h. Considering the great length and thinness of the peduncles and the lightness of the pods, we may conclude that they would not be able to excavate saucer-like depressions in sand or sawdust, or bury themselves in moss, &c., unless they were aided by their continued rocking or circumnutating move- ment.

Relation vetween Cireumnutation and Heliotropism.— Any one who will look at the foregoing diagrams, showing the movements of the stems of various plants towards a lateral and more or less dimmed light, will be forced to admit that ordinary circumnutation and heliotropism graduate into one another. When a plant is exposed to a dim lateral light and continues during the whole day bending towards it, receding late in the evening, the movement unquestionably is one of heliotropism. Now, in the case of Tropzeolum (Fig. 175) the stem or epicotyl obviously cii:cumnu- tated during the whole day, and yet it continued at the same time to move heliotropically; this latter movement being effected by the apex of each succes- sive elongated figure or ellipse standing nearer to the light than the previous one. In the case ot Cassia (Fig. 177) the comparison of the movement ot the hypocotyl, when exposed to adim lateral light and to darkness, is very instructive; as is that between the ordinary circumnutating movement of a seedling Brassica (Figs. 172, 173), or that of Phalaris (Figs. 49, 174), and their heliotropic movement towards a window protected by blinds. In both these cases

and in many others, it was interesting to notice how gradually the stems began to circumnutate as the light waned in the evening. We have therefore many kinds of gradations from a movement towards the light, which must be considered as one of circumnutation. very slightly modified and still consisting of ellipses or circles,—though a movement more or less strongly zigzag, with loops or ellipses occasionally formed,—to a nearly straight, or even quite straight, heliotropie course.

A plant, when exposed to a lateral light, though this may be bright, commonly moves at first in a zigzag line, or even directly from the light; and this no doubt is due to its circumnutating at the time in a direction either opposite to the source of the light, or more or less transversely to it. As soon, however, as the direction of the circumnutating move- ment nearly coincides with that of the entering light, the plant bends in a straight course towards the light, if this is bright. The course appears to be rendered more and more rapid and rectilinear, in accordance with the degree of brightness of the light—firstly, by the longer axes of the elliptical figures, which the plant continues to describe as long as the light remains very dim, being directed more or less accurately towards its source, and by each successive ellipse being de- scribed nearer to the light. Secondly, if the light is only somewhat dimmed, by the acceleration and increase of the movement towards it, and by the retardation or azrestment of that from the light, some lateral movement being still retained, for the light will interfere less with a movement at right angles to its direction, than with one in its own direction.*

* In his paper, ‘U+ber oitho- — tleile’ (« Arbeiten des Bot. Inst trope und plagiotrope Pfanzen- in Wiirzburg,’ Band ii. Heft ii The result is that the course is rendered more or less zigzag and unequal in rate. Lastly, when the light ‘is very bright all lateral movement is lost; and the whole energy of the plant is expended in rendering the circumnutating movement rectilinear and rapid in one direction alone, namely, towards the light. The common view seems to be that heliotropism is a quite distinct kind of movement from circumnuta- tion; and it may be urged that in the foregoing diagrams we see heliotropism merely combined with, or superimposed on, circumnutation. But if so, it must be assumed that a bright lateral light completely stops circumnutation, for a plant thus exposed moves in a straight line towards it, without describing any ellipses or circles. If the light be somewhat obscured, though amply sufficient to cause the plant to bend towards it, we have more or less plain evidence of still- continued circumnutation. It must further be assumed that it is only a lateral light which has this extraor- dinary power of stopping circumnutation, for we know that the several plants above experimented on, and all the others which were observed by us whilst grow- ing, continue to circumnutate, however bright the light may be, if it comes from above. Nor should it be forgotten that in the life of each plant, circumnuta- tion precedes heliotropism, for hypocotyls, epicotyls, and petioles circumnutate before they have broken through the ground and have ever felt the influence of light.