The Power of Movement in Plants

When the movements of sleeping leaves are traced during twenty-four hours, the ascending and descend- ing lines do not coincide, except occasionally and by accident for a short space; so that with many plants a single large ellipse is described during each twenty-four hours. Such ellipses are generally narrow and ver- tically directed, for the amount of lateral movement is small. That there is some lateral movement is shown by the ascending and descending lines not coinciding, and occasionally, as with Desmodium gyrans and Thalia dealbata, it was strongly marked. In the case of Meli- lotus the ellipses described by the terminal leaflet during the day are laterally extended, instead of ver- tically, as is usual; and this fact evidently stands in relation with the terminal leaflet moving laterally when it goes to sleep. With the majority of sleeping plants the leaves oscillate more than once up and down in the twenty-four hovrs; so that frequently two cllipses, one of moderate size, and one of very large size which includes the nocturnal movement, are described within the twenty-four hours. For instance, a leaf which stands vertically up during the night will sink in the morning, then rise considerably, again sink in the afternoon, and in the evening reascend and assume its vertical nocturnal position. It will thus describe, in the course of the twenty-four hours, two ellipses of unequal sizes. Other plants describe within the same time, three, four, or five ellipses. Occasionally the longer axes of the several ellipses extend in different directions, of which Acacia Farnesiana offered a good instance. The following cases will give an idea of the rate of movement: Ozalis acetosella completed two ellipses at the rate of 1 h. 25 m. for each; Marsilea quadrifoliata, at the rate of 2h.; Trifolium subterraneum, one in 3h. 80 m.; and Arachis hypogea, in 4h. 50 m. But the number of ellipses described within a given time depends largely on the state of the plant and ou the conditions to which it is exposed. It often hap- pens that a single ellipse may be described during one

day, and two on the next. Hrythrina corallodendron made four ellipses on the first day of observation and only a single one on the third, apparently owing to having been kept not sufficiently illuminated and perhaps not warm enough. But there seems likewise to be an innate tendency in different species of the same genus to make a different number of ellipses in the twenty-four hours: the leaflets of Trifolium repens made only one; those of TL. resupinatum two, and those of YL. subierraneum three in this time. Again, the leaflets of Oxalis Plumierit made a single ellipse; those of O. bupleurifolia, two; those of O. Valdiviana, two or three; and those of O. acetosella, at least five in the twenty-four hours.

The line followed by the apex of a leaf or leaflet, whilst describing one or more ellipses during the day, is often zigzag, either throughout its whole course or only during the morning or evening: Robinia offered an instance of zigzagging confined to the morning, and a similar movement in the evening is shown in the diagram (Fig. 126) given under Sida. The amount of the zigzag movement depends largely on the plant being placed under highly favourable conditions. But even under such favourable conditions, if the dots which mark the position of the apex are made at consider- _ able intervals of time, and the dots are then joined, the course pursued will still appear comparatively simple, although the number of the ellipses will be increased; but if dots are made every two or three minutes and these are joined, the result often is that all the lines are strongly zigzag, many small loops, triangles, and other figures being also formed. This fact is shown in two parts of the diagram (Fig. 150) of the movements of Desmodium gyrans. Strephiwm floribundum, observed under a high temperature,

made several little triangles at the rate of 43 m. for each. Mimosa pudica, similarly observed, de- scribed three little ellipses in 67 m.; and the apex of a leaflet crossed 54, of an inch in a second, or 0-12 inch in a minute. ‘The leaflets of Averrhoa made a countless number of little oscillations when the temperature was high and the sun shining. The zigzag movement may in all cases be considered as an attempt to form small loops, which are drawn out by a prevailing movement in some one direction. The rapid gyrations of the little lateral leaflets of Des- modium belong to the same class of movements, somewhat exaggerated in rapidity and amplitude. The jerking movements, with a small advance and still smaller retreat, apparently not exactly in the same line, of the hypocotyl of the cabbage and of the leaves of Dionza, as seen under the microscope, all probably come under this same head. We may suspect that we here see the energy which is freed during the incessant chemical changes in progress in the tissues, converted into motion. Finally, it should be noted that leaflets and probably some leaves, whilst describing their ellipses, often rotate slightly on their axes; so that the plane of the leaf is directed first to one and then to another side. This was plainly seen to be the case with the large terminal leaflets of Des- modium, Erythrina and Amphicarpza, and is probably common to all leaflets provided with a pulvinus. With respect to the periodicity of the movements of sleeping leaves, Pfeffer* has so clearly shown that this depends on the daily alternations of light and darkness, that nothing farther need be said on this

head. But we may recall the behaviour of Mimosa in the North, where the sun does not set, and the complete inversion of the daily movements by artificial light and darkness. It has also been shown by us, that although leaves subjected to darkness for a mode- rately long time continue to circumnutate, yet the periodicity of their movements is soon greatly dis- turbed, or quite annulled. The presence of light or its absence cannot be supposed to be the direct cause of the movements, for these are wonderfully diversified even with the leaflets of the same leaf, although ~all have of course been similarly exposed. The move- ments depend on innate causes, and are of an adaptive nature. The alternations of light and darkness merely give notice to the leaves that the period has arrived for them to move in a certain manner. We may inter from the fact of several plants (Tropzolum, Lupinus, &c.) not sleeping unless they have been well illuminated during the day, that it is not the actual decrease of light in the evening, but the contrast between the amount at this hour and during the early part of the day, which excites the leaves to modify their ordinary mode of circumnutation.

As the leaves of most plants assume their proper. diurnal position in the morning, although light be excluded, and as the leaves of some plants continue to move in the normal manner in darkness during at least a whole day, we may conclude that the periodi- city of their movements is to a certain extent in- herited.* The strength of such inheritance differs * Pfeffer denies such inherit- ‘ Nachwirkung,” or the after- ance; he attributes (‘ Die Period. Bewegungen,’ pp. 30-56) the periodicity when prolonged for a day or two in d.rkness, to

effects of light and darkness. But we are unable to follow his train of reasoning. There does not seem to be any more reason for much in different species, and seems never to be rigid ; for plants have been introduced from all parts of the world into our gardens and greenhouses; and if their movements had been at all strictly fixed in relation to the alternations of day and night, they would have slept in this country at very different hours, which is not the case. Moreover, it has been observed that sleeping plants in their native homes change their times of sleep with the changing seasons. *

We may now turn to the systematic list (p. 320). This contains the names of all the sleeping plants known to us, though the list undoubtedly is very imperfect. It may be premised that, as a general tule, all the species in the same genus sleep in nearly the same manner. But there are some ex- ceptions; in several large genera including many sleeping species (for instance, Oxalis), some do not sleep. One species of Melilotus sleeps like a Tri- folium, and therefore very differently from its con- geners; so does one species of Cassia. In the genus Sida, the leaves either rise or fall at night; and with Lupinus they sleep in three different methods. Re- turning to the list, the first point which strikes us, is that there are many more genera amongst the Legu- minose (and in almost every one of the Leguminous tribes) than in all the other families put together; and we are tempted to connect this fact with the great

atlributing such mov: ments to this eause than, for instance, the in- herited habit of winter and summer wheat to grow best at different seasons; fir this habit is lost after a few yeurs, like the movements of leaves in darkness after a few days. No doubt some eff.et must be produced on the seeds by the long-continu d culti- vation of the parent-plants under difterent climates, but no one pro- bably would call this the ‘“‘ Nach- wirkung ” of the climates,

mobility of the stems and leaves in this family, as shown by the large number of climbing species which it contains. Next to the Leguminose come the Mal- vacee, together with some closely allied families. But by far the most important point in the list, is that we meet with sleeping plants in 28 families, in all the great divisions of the Phanerogamic series, and in one Cryptogam. Now, although it is probable that with the Leguminose the tendency to sleep may have been inherited from one or a few progenitors and possibly so in the cohorts of the Malvales and Chenopodiales, yet it is manifest that the tendency must have been acquired by the several genera in the other families, quite independently of one another. Hence the ques- tion naturally arises, how has this been possible ? and the answer, we cannot doubt, is that leaves owe their nyctitropic movements to their habit of cir- cumnutating,—a habit common to all plants, and everywhere ready for any beneficial development or modification.

It has been shown in the previous chapters that the leaves and cotyledons of all plants are continually moving up and down, generally to a slight but some- times to a considerable extent, and that they describe either one or several ellipses in the course of twenty- four hours; they are also so far affected by the alter- nations of day and night that they generally, or at least often, move periodically to a small extent ; and here we have a basis for the development of the greater nyctitropic movements. That the movements of leaves and cotyledons which do not sleep come within the class of circumnutating movements cannot be doubted, for they are closely similar to those of hypocotyls, epicotyls, the stems of mature plants, and of various other organs. Now, if we take the simplest

case of a sleeping leaf, we see that it makes a single ellipse in the twenty-four hours, which resembles one described by a non-sleeping leaf in every respect, except that itis much larger. In both cases the course pursued is often zigzag. As all non-sleeping leaves are inces- santly circumnutating, we must conclude that a part at least of the upward and downward movement of one that sleeps, is due to ordinary circumnutation ; and it seems altogether gratuitous to rank the remainder of the movement under a wholly different head. With a multitude of climbing plants the ellipses which they describe have been greatly increased for another pur- pose, namely, catching hold of a support. With these climbing plants, the various circumnutating organs have been so far modified in relation to light that, differently from all ordinary plants, they do not bend towards it. With sleeping plants the rate and amplitude of the movements of the leaves have been so far modified in relation to light, that they move in a certain direction with the waning light of the evening and with the increasing light of the morning more rapidly, and to a greater extent, than at other hours

But the leaves and cotyledons of many non-sleeping plants. move in a much more complex manner than in the cases just alluded to, for they describe two, three, or more ellipses in the course of a day. Now, if a plant of this kind were converted into one that slept, one side of one of the several ellipses which each leaf daily describes, would have to be greatly increased in length in the evening, until the leaf stood ver- tically, when it would go on circumnutating about the same spot. On the following morning, the side of another ellipse would have to be similarly increased in length, so as to bring the leaf back again into its diurnal position, when it would again circumnutate

until the evening. If the reader will lock, for in- stance, at the diagram (Fig. 142, p. 351), representing the nyctitropic movements of the terminal leaflet of Trifolium subterraneum, remembering that the curved broken lines at the top ought to be prolonged much higher up, he will see that the great rise in the evening and the great fall in the morning together form a large ellipse like one of those described during the daytime, differing only in size. Or, he may look at the diagram (Fig. 108, p. 236) of the 34 ellipses described in the course of 6 h. 35 m. by a leaf of Lupinus spectosus, which is one of the species in this genus that does not sleep; and he will see that by merely prolonging upwards the line which was already rising late in the evening, and bringing it down again next morning, the diagram would represent the movements of a sleeping plant.

With those sleeping plants which describe several ellipses in the daytime, and which travel in a strongly zigzag line, often making in their course minute loops, triangles, &c., if as soon as one of the ellipses begins in the evening to be greatly increased in size, dots are made every 2 or 3 minutes and these are joined, the line then described is almost strictly rectilinear, in strong contrast with the lines made during the day- time. This was observed with Desmodium gyrans and Mimosa pudica. With this latter plant, moreover, the pinne converge in the evening by a steady move- ment, whereas during the day they are continually converging and diverging to a slight extent. In all such cases it was scarcely possible to observe the difference in the movement during the day and even- ing, without being convinced that in the evening the plant saves the expenditure of force by not moving laterally, and that its whole energy is now expeaded

in gaining quickly its proper nocturnal position by a direct course. In several other cases, for instance, when a leaf after describing during the day one or more fairly regular ellipses, zigzags much in the evening, it appears as if energy was being expended, so that the great evening rise or fall might coin- eide with the period of the day proper for this movement. The most complex of all the movements performed by sleeping plants, is that when leaves or leaflets, after describing in the daytime several vertically directed ellipses, rotate greatly on their axes in the evening, by which twisting movement they occupy a wholly different position at night to what they do during the day. For instance, the terminal leaflets of Cassia not only move vertically downwards in the evening, but twist round, so that their lower surfaces face outwards. Such movements are wholly, or almost wholly, confined to leaflets provided with a pulvinus. But this torsion is not a new kind of movement introduced solely for the purpose of sleep; for it has been shown that some leaflets whilst describing their ordinary ellipses during the daytime rotate slightly, causing their blades to face first to one side and then to another. Although we can see how the slight periodical movements of leaves in a vertical plane could be easily converted into the greater yet simple nyctitropic movements, we do not at present know by what graduated steps the more complex movements, effected by the torsion of the pulvini, have been acquired. A probable explanation could be given in each case only after a close investigation of the movements in all the allied forms.

From the facts and considerations now advanced we may conclude that nyctitropism, or the sleep of leaves and cotyledons, is merely a modification of their ordi- hary circumnutating movement, regulated in its period and amplitude by the alternations of light and dark- ness. ‘The object gained is the protection of the upper surfaces of the leaves from radiation at night, often combined with the mutual protection of the several parts by their close approximation. In such cases as those of the leaflets of Cassia—of the terminal leaflets of Melilotus—of all the leaflets of Arachis, Marsilea, &c.—we have ordinary circumnutation modified to the oxtreme extent known to us in any of the several great classes of modified circumnutation. On this view of the origin of nyctitropism we can understand how it is that a few plants, widely distributed throughout the Vascular series, have been able to acquire the habit of placing the blades of their leaves vertically at night, that is, of sleeping,—a fact otherwise inexplicable.

The leaves of some plants move during the day in a manner, which has improperly been called diurnal sleep; for when the sun shines brightly on them, they direct their edges towards it. To such cases we shall recur in the following chapter on Heliotropism. It has been shown that the leaflets of one form of Porlieria hygrometrica keep closed during the day, as long as the plant is scantily supplied with water, in the same manner as when asleep; and this apparently serves to check evaporation. There is only one other analogous case known to us, namely, that of certain Gramineze, which fold inwards the sides of their narrow leaves, when these are exposed to the sun and to a dry atmosphere, as described by Duval-Jouve.* We have also observed the same phenomenon in Elymus arenareus,

‘There is another movement, which since the time of Linnzus has generally been called sleep, namely, that of the petals of the many flowers which close at night. ‘These movements have been ably investigated by Pfeffer, who has shown (as was first observed by Hofmeister) that they are caused or regulated more by temperature than by the alternations of light and darkness. Although they cannot fail to protect the organs of reproduction from radiation at night, this does not seem to be their chief function, but rather the protection of the organs from cold winds, and especially from rain, during the day. The latter seems probable, as Kerner * has shown that a widely different kind of movement, namely, the bending down of the upper part of the peduncle, serves in many cases the same end. The closure of the flowers will also exclude nocturnal insects which may be ill-adapted for their fertilisation, and the well-adapted kinds at periods when the tempcrature ig not favourable for fertilisation. Whether these movements of the petals consist, as is probable, of modified circumnutation we do not know.

Embryology of Leaves.—A few facts have been in- cidentally given in this chapter on what may be called the embryology of leaves. With most plants the first leaf which is developed after the cotyledons, resembles closely the leaves produced by the mature plant, but this is not always the case. The first leaves produced by some species of Drosera, for instance by D. Capensis, differ widely in shape from those borne by the mature plant, and resemble closely the eaves of D. rotundifolia, as was shown to us by Prof. Williamson of Manchester. The first true leaf of

the gorse, or Ulex, is not narrow and spinose like the older leaves. On the other hand, with many Leguni- nous plants, for instance, Cassia, Acacia lophantha, &c., the first leaf has essentially the same character as the older leaves, excepting that it bears fewer leaflets. In Trifolium the first leaf generally bears only a single leaflet instead of three, and this differs somewhat in shape from the corresponding leaflet on the older leaves. Now, with Trifolium Pannoniewm the first true leaf on some seedlings was unifoliate, and on others completely trifoliate; and between these two extreme states there were all sorts of gradations, some seedlings bearing a single leaflet more or less deeply notched on one or both sides, and some bearing a single additional and perfeet lateral leaflet. Here, then, we have the rare opportunity of seeing a structure proper to a more advanced age, in the act of gradually encroaching on and replacing an earlier or embryological condition. The genus Melilotus is closely allied to Trifolium, and the first leaf bears only a single leaflet, which at night rotates on its axis so as to present one lateral edge to the zenith. Hence it sleeps like the terminal leaflet of a mature plant, as was observed in 15 species, and wholly unlike the corresponding leaflet of Trifolium, which simply bends upwards. It is therefore a curious fact that in one of these 15 species, viz., M. Tawrica (and in a lesser degree in two others), leaves arising from young shoots, produced on plants which had been cut down and kept in pots during the winter in the green- house, slept like the leaves of a Trifolium, whilst the leaves on the fully-grown branches on these same plants afterwards slept normally like tl ose of a Meli- lotus.

If young shoots rising from the ground may be considered as new individuals, partaking to a certain extent of the nature of seedlings, then the peculiar manner in which their leaves slept may be considered as an embryological habit, probably the result of Meli- lotus being descended from some form which slept like a Trifolium. This view is partially supported by the leaves on old and young branches of another species, M. Messanensis (not included in the above 15 species), always sleeping like those of a Trifolium.

The first true leaf of Mimosa albida consists of a simple petiole, often bearing three pairs of leaflets, ali of which are of nearly equal size and of the same shape: the second leaf differs widely from the first, and resembles that on a mature plant (see Fig. 159, p- 379), for it consists of two pinne, each of which bears two pairs of leaflets, of which the inner basal one is very small. But at the base of each pinna there is a pair of minute points, evidently rudiments of leaflets, for they are of unequal sizes, like the two succeeding leaflets. These rudiments are in one sense embryological, for they exist only during the youth of the leaf, falling off and disappearing as soon as it is fully grown.

With Desmodium gyrans the two lateral leaflets are very much smaller than the corresponding leaflets in most of the species in this large genus; they vary also in position and size; one or both are sometimes absent; and they do not sleep like the fully-developed leaflets. They may therefore be considered as almost rudimentary ; and in accordance with the general prin- ciples of embryology, they ought to be more constantly aud fully developed on very young than on old plants. But this is not the case, for they were quite absent on some young seedlings, and did not appear until from 10 to 20 leaves had been formed. This fact Jeads to the suspicion that D. gyrans is descended through a unifoliate form (of which some exist) from a trifoliate species ; and that the little lateral leaflets reappear through reversion. However this may be,

the interesting fact of the pulvini or organs of move- ment of these little leaflets, not having been reduced nearly so much as their blades—taking the large terminal leaflet as the standard of comparison—gives us probably the proximate cause of their extrao:dinary power of gyration. Distinction between heliotropism and the effects of light on the perio~ dicity of the movements of leaves—Heliotropic movements of Beta, Solanum, Zea, and Avena—Heliotropic movements towards an obscure light in Apios, Brassica, Phalaris, Tropeolum, and Cassia —AplLeliotropic movements of tendrils of Bignonia—Of flower- peduncles of Cyclamen—Burying of the pods—Heliotropism and apheliotropisin modified forms of circumnutation—Stcps by which one movement is converted into the other—Transversal- heliotropismus or diahelictropism, influenced by epinasty, the weight of the part and apogeotropism—Apogeotropism overcome during the middle of the day by diahelictropism—Eftects of the weight of the blades of cotyledons—So-called diurnal sleep—Chiloro- phyll injured by intense light—Movements to avoid intense light.

Sacus first clearly pointed out the important dif- ference between the action of light in modifying the periodic movements of leaves, and in causing them to bend towards its source.* The latter, or heliotropic movements are determined by the direction of the light, whilst periodic movements are affected by changes in its intensity and not by its direction. The periodicity of the circumnutating movement often continues for some time in darkness, as we have seen in the last chapter ; whilst heliotropic bending ceases very quickly when the light fails. Nevertheless, plants which have ceased through long-continued darkness to move pe- riodically, if re-exposed to the light are still, according to Sachs, heliotropic.

heliotropism, implies that a plant, when unequally illuminated on the two sides, bends from the light, instead of, as in the last sub-class of cases, towards it; but apheliotropism is comparatively rare, at least in a well-marked degree. There is a third and large sub- cluss of cases, namely, those of “Transversal-Helio- tropismus” of Frank, which we will here call diahelio- tropism. Parts of plants, under this influence, place themselves more or less transversely to the direction whence the light proceeds, and are thus fully illumi- nated. There is a fourth sub-class, as far as the final cause of the movement is concerned; for the leaves of some plants when exposed to an intense and injurious amount of light direct themselves, by rising or sinking or twisting, so as to be less intensely illuminated. Such movements have sometimes been called diurnal sleep. If thought advisable, they might be called parabeliotropic, and this term would correspond. with our other terms. ;

It will be shown in the present chapter that all the movements included in these four sub-classes, con- sist of modified circumnutation. We do not pretend to say that if a part of a plant, whilst still growing, did not .circumnutate—though such a supposition is most im- probable—it could not bend towards the light; but, as a matter of fact, heliotropism seems always to consist of modified circumnutation. Any kind of movement in relation to light will obviously be much facilitated by each part circumnutating or bending successively in all directions, so that an already existing movement has only to be increased in some one direction, and to be lessened or stopped in the other directions, in order that it should become heliotropic, apheliotropic, &c., as the case may be. In the next chapter some obser- vations on the sensitiveness of plants to light, their

420 MODIFIED CIRCUMNUTATION. Cuap VIII rate of bending towards it, and the accuracy with which they point towards its source, &c., will be given. Afterwards it will be shown—and this seems to us a point of much interest—that sensitiveness to light is sometimes confined to a small part of the plant; and that this part when stimulated by light, transmits an influence to distant parts, exciting them to bend. Heliotropism.— When a plant which is strongly heliotropic (and species differ much in this respect) is exposed to a bright lateral light, it bends quickly towards it, and the course pursued by the stem is quite or nearly straight. But if the light is much dimmed, or occasionally interrupted, or admitted in only a slightly oblique direction, the course pursued is more or less zigzag ; and as we have seen and shall again see, such zigzag move- ment results from the elongation or drawing out of the ellipses, loops, &e., which the plant would have de- scribed, if it had been illuminated from above. On several occasions we were much struck with this fact,

flected by the light being slightly lateral, traced on a horizontal glass from 8.30 A.M. to 5.30 p.m. Direction of the lighted taper by which it was illuminated, shown by a line joining the first and penultimate dots. Figure reduced to one-third of the original scale, whilst observing the circumnuta- tion of highly sensitive seedlings, which were unintentionally illu- minated rather obliquely, or only at successive intervals of time. For instance, two young seedlings of Beta vulgaris were placed in the middle of a room with north-east windows, and were kept covered up, except during