The Power of Movement in Plants

organs—yct the tendency differs greatly different species, and is variable in degree in the individuals of the same species, as may be seen in almost any pot of seedlings of a long cultivated plant.* There 1s therefore a basis for the modification of this tendency to almost any beneficial extent. That it has been modified, we see in many cases: thus, it is of more importance for insectivorous plants to place their leaves in the best position for catching insects than to turn their leaves to the light, and they have no such power. If the stems of twining plants were to bend towards the light, they would often be drawn away from their supports; and as we have seen they do not thus bend. As the stems of most other plants are heliotropic, we may feel almost sure that twining plants, which are distributed throughout the whole vascular series, have lost a power that their non- climbing progenitors possessed. Moreover, with Ipo- mea, and probably all other twiners, the stem of the young plant, before it begins to twine, is highly helio- tropic, evidently in order to expose the cotyledons or the first true leaves fully to the light. With the Ivy the stems of seedlings are moderately heliotropic, whilst those of the same plants when grown a little older

* Strisburger has shown in hig interesting work (‘ Wirkune des Lichtes . . . auf Schwarmsporen,’ 1878), that the movement of the swarm-spores of various lowly organised plants to a lateral light iz influenced by their stage of development, by the temperature to which they are subjected, by the degree of illumination under which they have been raised, and , by other unknown causes; so that the swarm-spores of the same species may move across the field of the microscope either to or from

the light. Some individuals, more- over, appear to be indifferent to the light; and those of different species behave very differently. The brighter the light, the straighter is their course. They exhibit also for a short time the after-effects of light. In all theso resp: cts they re-ewble the higher plants. See, also, Stahl, ‘ Ueber den einfluss der Lichts auf die Bewegungs - erscheinungen der Schwarmsporen’? Verh. d. phys.- med. Geselsshalft in Wiirzburg, B. xii. 1878,

are apheliotropic. Some tendrils which consist of modified leaves—organs in all ordinary cases strongly diaheliotropic—have been rendered apheliotropic, and their tips crawl into any dark crevice. Even in the case of ordinary heliotropic movements, it is hardly credible that they result directly from the action of the light, without any special adaptation. We may illustrate what we mean by the hygroscopic movements of plants: if the tissues on one sideof an organ permit of rapid evaporation, they will dry quickly and contract, causing the part to bend to this side. Now the wonderfully complex movements of the pollinia of Orchis pyramidalis, by which they clasp the proboscis of a moth and afterwards change their position for the sake of depositing the pollen-masses on the double stigma—or again the twisting move- ments, by which certain seeds bury themselves in the ground *—follow from the manner of drying of the parts in question; yet no one will suppose that these results have been gained without special adapta- tion. Similarly, we are led to believe in adaptation when we see the hypocoty] of a seedling, which contains chlorophyll, bending to the light ; for althuugh it thus receives less light, being now shaded by its own coty- ledons, it places them—the more important organs—in the best position to be fully illuminated. The hypo- cotyl may therefore be said to sacrifice itself for the good of the cotyledons, or rather of the whole plant. But if it be prevented from bending, as must some- times occur with seedlings springing up in an en- tangled mass of vegetation, the cotyledons themselves bend so as to face the light ; the one farthest off rising

up, and that nearest to the light sinking down, o1 both twisting laterally.* We may, also, suspect that the extreme sensitiveness to light of the upper part of the sheath-like cotyledons of the Graminez, and their power of transmitting its effects to the lower part, are specialised arrangements for finding the shortest path to the light. With plants growing on a bank, or thrown prostrate by the wind, the manner in which the leaves move, even rotating on their own axes, so that their upper surfaces may be again directed to the light, is a striking phenomenon. Such facts are rendered more striking when we remember that too intense a light injures the chlorophyll, and that the leaflets of several Leguminose when thus exposed bend upwards and present their edges to the sun, thus escaping injury. On the other hand, the leaflets of Averrhoa and Oxalis, when similarly exposed, bend downwards.

It was shown in the last chapter that heliotropism is a modified form of circumnutation; and as every growing part of every plant cireumnutates more or less, we can understand how it is that the power of bending to the light has been acquired by such a multitude of plants throughout the vegetable kingdom. The manner in which a circumnutating movement—that is, one consisting of a succession of irregular ellipses or loops—is gradually converted into a rectilinear course towards the light, has been already explained. First, we have a succession of ellipses with their longer axes directed towards the light, each of which

* Wiosner has made remarksto tracted from B. Ixxvii, (1878) nearly the same effect with respect Sith. der k. Akud. der Wisseusch, to leaves: ‘Die undulirende Nu- Wien. tation der Internodien,’ p. 6, ex- is described nearer and nearer to its source; then the loops are drawn out into a strongly pronounced zigzag line, with here and there a small loop still formed. At the same time that the movement towards the light is increased in extent and accelerated, that in the opposite direction is lessened and retarded, and at last stopped. The zigzag movement to either side is likewise gradually lessened, so that finally the course becomes rectilinear. Thus under the stimulus of a fairly bright light there is no useless expenditure of force.

As with plants every character is more or less variable, there seems to be no great difficulty in be- lieving that their circumnutating movements may have been increased or modified in any beneficial manner by the preservation of varying individuals. The inheritance of habitual movements is a necessary contingent for this process of selection, or the survival of the fittest; and we have seen good reason to believe that habitual movements are inherited by plants. In the case of twining species the circumnutating move- ments have been increased in amplitude and rendered more circular; the stimulus being here an internal or innate one. With sleeping plants the movements . have been increased in amplitude and often changed in direction; and here the stimulus is the alternation of light and darkness, aided, however, by inheritance. In the case of heliotropism, the stimulus is the unequal illumination of the two sides of the plant, and this determines, as in the foregoing cases, the modifica- tiqn of the circumnutating movement in such a manner that the organ bends to the hght. A plant which has been rendered heliotropic by the above means, might readily lose this tendeney, judging from the cases already given, as soon as it became useless ot

injurious. A species which has ceased to be helio- tropic might also be rendered apheliotropic by the preservation of the individuals which tended to cir- cumnutate (though the cause of this and most other variations is unknown) in a direction more or less opposed to that whence the light proceeded. In like manner a plant might be rendered diaheliotropic. Means of observation --Apogeotropism — Cytisus—Verbena—Beta— Gradual conversion of tle movement of circumnutation into apogeo- tropism in Rubus, Lilium, Phalaris, Avena, and Bra:sica—A pogeo- tropism retarded by heliotropism—Effected by the aid of juints or pulvini—Movements of flower-peduncles of Oxalis—Geneial remarks on apogeotropism—Geotropism—Movements of radicles— Burying of seed-capsules—Use of process—Trifolium subterraneum —Arachis— Amphicarpea— Di. geotropism—Conclusion.

dified forms of circumnutation. Extremely fine fila- ments of glass, bearing two minute triangles of paper, were fixed to the summits of young stems, frequently to the hypocotyls of seedlings, to flower-peduncles, radicles, &c., and the movements of the parts were then traced in the manner already described on vertical and horizontal glass-plates. It should be remembered that as the stems or other parts become more and more oblique with respect to the glasses, the figures traced on them necessarily become more and more magnified. The plants were protected from light, excepting whilst each observation was being made, and then the light, which was always a dim one, was allowed to enter so as to interfere as little as possible with the movement in progress; and we did not detect any evidence of such interference.

tation and heliotropism, we had the great advantage of being able to lessen the light; but with geotropism analogous experiments were of course impossible. We could, however, observe the movements of stems placed at first only a little from the perpendicular, in which case geotropism did not act with nearly so much power, as when the stems were horizontal and at right angles to the force. Plants, also, were selected which were but feebly geotropic or apogeotropic, or had become so from having grown rather old. Another plan was to place the stems at first so that they pointed 30 or 40 degrees beneath the horizon, and then apo- geotropism had a great amount of work to do before the stem was rendered upright; and in this case ordinary circumnutation was often not wholly oblite- rated. Another plan was to observe in the evening plants which during the day had become greatly curved heliotropically ; for their stems under the gra- dually waning light veryslowly became upright through the action of apogeotropism ; and in this case modified circumnutation was sometimes well displayed.

Apoyeotropism.—Plants were selected for observation almost by chance, excepting that they were taken from widely different families. If the stem of a plant which is even moderately sensitive to apogeotropism be placed horizontally, the upper growing part bends quickly upwards, so as to become perpen- dicular; and the line traced by joining the dots successively made on a glass-plate, is generally almost straight. For in- stance, a young Cytisus fragrans, 12 inches in height, was placed so that the stem projected 10° beneath the horizon, and its course was traced during 72 h. At first it bent a very little downwards (Fig. 182), owing no doubt to the weight of the stem, as this occurred with most of the other plants observed, though, as they were of course circumnutating, the short down- ward lines were often oblique. After three-quarters of an hour the stem began to curve upwards, quickly during the first two hours, but much more slowly during the afternoon and night,

a little, and ‘circumnutated during the following day; but it also moved a short distance to the right, which was caused by a little ight having been ac- cidentally admitted on this side. The stem was now inclined 61)? above the horizon, and had therefore risen 70°. With time allowed it would probably have become upright, and no doubt would have continued circum- nutating. The sole remarkable feature in the figure here given is the straightness of the course pursued. The stem, however, did not move upwards at an equable rate, and it sometimes stood almost or quite still. Such periods probably represent attempts to circumnutate in a direction opposite to apogeo- tropism.

The herbaceous stem of a Verbena melindres (?) laid hori- zontally, rose in 7 h. so much that it could no longer be observed on the vertical glass which stood in front of the plant. The long line which was traced was almost absolutely straight. After the 7 h. it still continued to rise, but now circumnutated slightly. On the following day it stood upright, and circum- nutated regularly, as shown in Fig. 82, given in the fourth chapter. The stems of several other plants which were highly sensitive to apogeotropism rose up in almost straight lines, and

Cytisus fragrans : apogeotropic move- ment of stem from 10° beneath te 66° above horizon. traced on ver tical glass, from 8.30 A.M. Marct 12th to 10.30 p.w. 13th. The sub- sequent circumnutating movement is likewise shown up to 6.45 A.M. on the 15th. Nocturnal course represented, as usual, by a broken line. Movement not greatly mag- nified, and tracing reduced to two- thirds of original scale. scen how gradually circumnutation becomes changed into apogco-

Rubus ideus (hybrid): apogeotropic movement ot stem, iraced on a vertical glass during 3 days and 3 ni tropism, under circumstances to be specified in cach instance. Rubus ideus (hybrid).—A young plant, 11 inches in height, growing in a pot, was placcd horizontally; and the upward movemeut was traced during nearly 70 h.; but the plant, though growing vigorously, was not highly sensitive to apogeotropism, or it was not capable of quick movement, for during the above time it rose only 67°. We may see in the diagram (Tig. 184) that during the first day of 12 h. it rose in a nearly straight line. When placed horizontally, it was evidently circumnutating, for it rose at first a little, notwithstanding the weight of the stem, and then sauk down; so that it did not start on its permanently upward course until 1 h. 25 m. had elapsed. On the second day, by which time it had risen considerably, and when apogeotropism acted on it with somewhat less power, its course during 15} h. was clearly zigzag, aud the rate of the upward movement was not equable. During the third day, also of 153 h., when apogeotropism acted on it with still less power, the stem plainly circum- nutated, for it moved during this day 8 times up and 8 times down, 4 times to the left and 4 to the right. But the course was so complex that it could hardly be traced on the glass, We can, however, see that the successively formed irregular ellipses rose higher and higher. Apogeotropism continued to act on the fourth morning, as the stem was still rising, though it now stood only 23° from the perpendicular. In this diagram the several stages may be followed by which an almost rectilinear, upward, apogeotropic course first becomes zigzag, and then changes into a circumnutating movement, with most of the successively formed, irregular ellipses directed upwards,

detected in the accuracy of their bending, whether they stood with their broad or narrow sides facing the light, or in any intermediate position; and so it was with the cotyledons or Avena sativa, which are likewise oval in section. Now, a little reflection will show that in whatever position the cotyledons may stand, there will be a line of greatest illumination, exactly fronting the light, and on each side of this line an equal amount of light will be received; but if the oval stands obliquely with respect to the light, this will be diffused over a wider surface on one side of the central line than on the other. We may there- fore infer that the same amount of light, whether diffused over a wider surface or concentrated on a smaller surface, produces exactly the same effect; for the cotyledons in the long narrow box stood in all sorts of positions with reference to the light, yet all pointed truly towards it.

That the bending of the cotyledons to the light depends on the illumination of one whole side or on the obscuration of the whole opposite side, and not on a narrow longitudinal zone in the line of the light being affected, was shown by the effects of painting longitudinally with Indian ink one side of five coty- ledons of Phalaris. These were then placed on a table near to a south-west window, and the painted half was directed either to the right or left. The result was that instead of bending ina direct line towards the window, they were deflected from the window and towards the unpainted side, by the following angles, 35°, 83°, 31°, 43°, and 39°. It should be remarked that it was hardly possible to paint one-half accurately, or to place all the seedlings which are oval in section in quite the same position relatively to the light; and this will account for the differences in the angles. Five coty-

ledons of Avena were also painted in the same manner, but with greater care; and they were laterally de- flected from the line of the window, towards the unpainted side, by the following angles, 44°, 44°, 55°, 51°, and 57°. This deflection of the cotyledons from the window is intelligible, for the whole unpainted side must have received some light, whereas the oppo- site and painted side received none; but a narrow zone on the unpainted side directly in front of the window will have received most light, and all the hinder parts (half an oval in section) less and less light in varying degrees; and we may conclude that the angle of deflection is the resultant of the action of the light over the whole of the unpainted side.

It should have been premised that painting with Indian ink does not injure plants, at least within several hours; and it could injure them only by stop- ping respiration. To ascertain whether injury was thus soon caused, the upper halves of 8 cotyledons of Avena were thickly coated with transparent matter,—4 with gum, and 4 with gelatine; they were placed in the morning before a window, and by the evening they were normally bowed towards the light, although the coatings now consisted of dry crusts of gum and gelatine. Moreover, if the seedlings which were painted longitudinally with Indian ink had been injured on the painted side, the opposite side would have gone on growing, and they would consequently have become bowed towards the painted side; whereas the curvature was always, as we have seen, in the opposite direction, or towards the unpainted side which was exposed to the light. We witnessed the effects of injuring longi- tudinally one side of the cotyledons of Avena and Phalaris; for before we knew that grease was highly injurious to them, several were painted down one side

horizontally, aud the upper part of the stem rose 58° in 46 h., in the manner shown in the accom- panying diagram (Tig. 185). We here see that during the whole of the second day of 153 h., the stem plainly circumnutated whilst bending upwards throngh apogeotropism. It had still to rise considerably, for when the last dot in the figure was made, it stood 82° from an upright position. Phalaris Canariensis—A cotyledon of this plant (1°3 inch in height) has already been decribed as rising in 4h. 30 m. from 40° beneath the hori- zon into a vertical position, passing through an angle of 180° in a nearly straight line, and then abruptly be- ginning to circumnutate. Another somewhat old cotyledon of the same height (but from which a true leaf had not yet protruded), was similarly placed at 40° beneath the horizon, For the first 4h. it rose in a nearly straight course (Fig. 186), so that by 1.10 p.m. it was highly inclined, and now apo- geotropism acted on it with much less power than before, and it began to zigzag. At 4.15 p.m. (i.e. in 7 h. from the commencement) it stood vertically, and afterwards continued to circum- nutate in the usual manner about the same spot. Here then we have a graduated change from a straight up-

nutation, instead of an abrupt change, as in the former case. Avena sativa.—The sheath-like coty- ledons, whilst young, are strongly apo- geotropic; and some which were placed at 45° beneath the horizon rose 90° in 7 or 8 h. in lines almost absolutely apoges- tropic movement of stein, traced on a vertical glass during 2 days and 2 nights, from 10.40 a.m, March 18th to 8 AM 20th. Figure reduced to one-half of the original scale, straight. An oldish cotyledon, from which the first leaf began ta

Phalants Canariensis: spogeotropic move- ment of cotyledon, traced on a vertical and horizontal glass, from 9.10 A.M. Sept. 19th to9 aM. 20th. Figure here re- duced to one-fifth of original scale. protrude whilst the fol- lowing observations were being made, was placed at 10° beneath the horizon, and it rose only 59° in 94h. It behaved rather differently from any other plant, observed by us, for during the first 43 h. it rose in a line not far from straight; during the next 63 h. it cireumnutated, that is, it descended and again ascended in a strongly marked zigzag course; it then resumed its upward movement in a moderately straight line, and, with time allowed, no doubt would have be- come upright. In this case, after the first 43 h., ordinary circumnutation almost completely con- quered for a time apogeo- tropism.

Brassica oleracea.— The hypocotyls of several young seedlings placed horizontally, rose up ver- tically in the course of 6 or 7h. in nearly straight lines. A seedling which had grown in darkness to a height of 24 inches, and was therefore rather old and not highly sensitive, was placed so that the hypocotyl projected at be- tween 80° and 40° beneath the horizon. The upper part alone became curved upwards, and rose during the first 3h. 10 m. in a nearly straight

line (Fig. 187); but it was not possible to trace the upward move- mext on the vertical glass for the first 1h. 10 m., so that the nearly straight line in the diagram ought to have been much longer. During the next 11h. the hypocotyl cireum- nutated, describing irregular figures, each of which rose a little above the one previously formed. During the night and following early morn- ing it continued to rise in a zigzag course, so that apogeotropism was still acting. At the close of our ob- servations, after 23 h. (represented by the highest dot in the diagram) the hypocotyl was still 32° from the perpendicular. There can be little doubt that it would ulti- mately have become upright by describing an additional number of irregular ellipses, one above the other.

Apogeotropism retarded by Helio- tropism. — When the stem of any plant bends during the day towards a lateral light, the movement is opposed by apogeotropism; but as the light gradually wanes in the evening the latter power slowly gains the upper hand, and draws the stem back into a vertical position. Here then we have a good opportunity for observing how apogeotropism acts when very nearly balanced by an opposing force. For instance, the plumule of Tropeolum majus (see former Fig. 175) moved towards the dim evening light in a slightly zigzag

Brassica oleracea: apogeotropic movement of hypocotyl, traced on vertical glass, from 9.20 AM. Sept. 12th to 8.30 a.m. 18th. The upper part of the figure is more magnified than the lower part. If the whole course had been traced, the straight upright line would have been much longer. Figure here reduced to one-third of the original scale. 10.40 p.m., during which time it zigzageed and described an ellipse of considerable size. The hypocotyl of Brassica oleracea (see former Fig. 173) moved in a straight line to the light until 5.15 p.m., and then from the light, making in its backward course a great rectangular bend, and then returned for a short distance towards the former source of the light; no observa- tions were made after 7.10 p.m., but during the night it re- covered its vertical position. A hypocotyl of Cassia tora moved in the evening in 4 somewhat zigzag line towards the failing light until 6 p.m., and was new bowed 20° from the perpendi- cular; it then returned on its course, making before 10.30 P.m. four great, nearly rectangular bends and almost completing an ellipse. Several other analogous cases were casually observed, and in all of them the apogeotropic movement could be seen to consist of modified circumnutation.

Apogeotropic Movements effected by the aid of joints or pulvini, —Movements of this kind are well known to occur in the Gramines, and are effected by means of the thickened bases of their sheathing leaves; the stem within being in this part thinner than elsewhere.* According to the analogy of all other pulvini, such joints ought to continue circumnutating for a long period, after the adjoining parts have ceased to grow. We therefore wished to ascertain whether this was the case with the Graminez; for if so, the apward curvature of their stems, when extended horizontally or laid prostrate, would be explained in accordance with our view—namely, that apogeotropism results from modified circumnutation. After these joints have curved upwards, they are fixcd in their new position by increased growth along their lower sides.

Lolium perenne.—A young stem, 7 inches in height, consist- ing of 3 internodes, with the flower-head not yet protruded, was selected for observation. A long and very thin glass fila- ment was cemented horizontally to the stem close above the second joint, 3 inches above the ground. This joint was subse- quently proved to be in an active condition, as its lower side swelled much throngh the action of apogeotropism (in the manner described by De Vries) after the haulm had been fastened down for 24]. ia a horizontal position. The pot was

* This structure has been re- die Aufrichtung des gelagerten cently discribed by De Vries in Getreides” in ‘ Landwirthschaft- an interesting article, ‘Ueber — liche Juhrbiicher,’ 1880, p. 473, so placed that the end of the filament stood beneath the 2-inch object glass of a microscope with an eye-piece micrometer, each division of which equalled =35 of an inch. The end of the fila- ment was repeatedly observed during 6 h., and was seen to be in constant movement; and it crossed 5 divisions of the micro- weter (z}g inch) in 2h. Occasionally it moved forwards by jerks, some of which were z;'55 inch in length, and then slowly retreated a little, afterwards again jerking forwards. These oscillations were exactly like those described under Brassica and Dionza, but they occurred only occasionally. We may therefore conclude that this moderately old joint was continually circumnutating on a small scale.