The Power of Movement in Plants · Charles Darwin
CLASS III. ACOTYLEDONS.
Marsilea. Marsileaceae (4). Githago segetum (Caryophylleae).—The first leaves produced by young seedlings, rise up and close together at night. On a rather older seedling, two young leaves stood at noon at 55° above the horizon, and at night at 86°, so each had risen 31°. The angle, however, was less in some cases. Similar observations were occasionally made on young leaves (for the older ones moved very little) produced by nearly full-grown plants. Batalin says (‘Flora,’ Oct. 1st, 1873, p. 437) that the young leaves of Stellaria close up so completely at night that they form together great buds. Sida (Malvaceae).—the nyctitropic movements of the leaves in this genus are remarkable in some respects. Batalin informs us (see also ‘Flora,’ Oct. 1st, 1873, p. 437) that those of S. napaea fall at night, but to what angle he cannot remember. The leaves of S. rhombifolia and retusa, on the other hand, rise up vertically, and are pressed against the stem. We have therefore here within the same genus, directly opposite movements. Again, the leaves of S. rhombifolia are furnished with a pulvinus, formed of a mass of small cells destitute of chlorophyll, and with their longer axes perpendicular to the axis of the petiole. As measured along this latter line, these cells are only 1/5th of the length of those of the petiole; but instead of being abruptly separated from them (as is usual with the pulvinus in most plants), they graduate into the larger cells of the petiole. On the other hand, S. napaea, according to Batalin, does not possess a pulvinus; and he informs us that a gradation may be traced in the several species of the genus between these two states of the petiole.
Sida rhombifolia presents another peculiarity, of which we have seen no other instance with leaves that sleep: for those on very young plants, though they rise somewhat in the evening, do not go to sleep, as we observed on several occasions; whilst those on rather older plants sleep in a conspicuous manner. For instance a leaf (.85 of an inch in length) on a very young seedling 2 inches high, stood at noon 9° above the horizon, and at 10 P.M. at 28°, so it had risen only 19°; another leaf (1.4 inch in length) on a seedling of the same height, stood at the same two periods at 7° and 32°, and therefore had risen 25°. These leaves, which moved so little, had a fairly well-developed pulvinus. After an interval of some weeks, when the same seedlings were 2½ and 3 inches in height, some of the young leaves stood up at night quite vertically, and others were highly inclined; and so it was with bushes which were fully grown and were flowering. Fig. 126. Sida rhombifolia: circumnutation and nyctitropic (or sleep) movements of a leaf on a young plant, 9½ inches high; filament fixed to midrib of nearly full-grown leaf, 2 3/8 inches in length; movement traced under a sky-light. Apex of leaf 5 5/8 inches from the vertical glass, so diagram not greatly enlarged.
The movement of a leaf was traced from 9.15 A.M. on May 28th to 8.30 A.M. on the 30th. The temperature was too low (15°–16° C.), and the illumination hardly sufficient; consequently the leaves did not become quite so highly inclined at night, as they had done previously and as they did subsequently in the hot-house: but the movements did not appear otherwise disturbed. On the first day the leaf sank till 5.15 P.M.; it then rose rapidly and greatly till 10.5 P.M., and only a little higher during the rest of the night (Fig. 126). Early on the next day (29th) it fell in a slightly zigzag line rapidly until 9 A.M., by which time it had reached nearly the same place as on the previous morning. During the remainder of the day it fell slowly, and zigzagged laterally. The evening rise began after 4 P.M. in the same manner as before, and on the second morning it again fell rapidly. The ascending and descending lines do not coincide, as may be seen in the diagram. On the 30th a new tracing was made (not here given) on a rather enlarged scale, as the apex of the leaf now stood 9 inches from the vertical glass. In order to observe more carefully the course pursued at the time when the diurnal fall changes into the nocturnal rise, dots were made every half-hour between 4 P.M. and 10.30 P.M. This rendered the lateral zigzagging movement during the evening more conspicuous than in the diagram given, but it was of the same nature as there shown. The impression forced on our minds was that the leaf was expending superfluous movement, so that the great nocturnal rise might not occur at too early an hour.
Abutilon Darwinii (Malvaceae).—The leaves on some very young plants stood almost horizontally during the day, and hung down vertically at night. Very fine plants kept in a large hall, lighted only from the roof, did not sleep at night for in order to do so the leaves must be well illuminated during the day. The cotyledons do not sleep. Linnæus says that the leaves of his Sida abutilon sink perpendicularly down at night, though the petioles rise. Prof. Pfeffer informs us that the leaves of a Malva, allied to M. sylvestris, rise greatly at night; and this genus, as well as that of Hibiscus, are included by Linnæus in his list of sleeping plants. Anoda Wrightii (Malvaceae).—The leaves, produced by very young plants, when grown to a moderate size, sink at night either almost vertically down or to an angle of about 45° beneath the horizon; for there is a considerable degree of variability in the amount of sinking at night, which depends in part on the degree to which they have been illuminated during the day. But the leaves, whilst quite young, do not sink down at night, and this is a very unusual circumstance. The summit of the petiole, where it joins the blade, is developed into a pulvinus, and this is present in very young leaves which do not sleep; though it is not so well defined as in older leaves.
Gossypium (var. Nankin cotton, Malvaceae).—Some young leaves, between 1 and 2 inches in length, borne by two seedlings 6 and 7½ inches in height, stood horizontally, or were raised a little above the horizon at noon on July 8th and 9th; but by 10 P.M. they had sunk down to between 68° and 90° beneath the horizon. When the same plants had grown to double the above height, their leaves stood at night almost or quite vertically dependent. The leaves on some large plants of G. maritimum and Brazilense, which were kept in a very badly lighted hot-house, only occasionally sank much downwards at night, and hardly enough to be called sleep. Oxalis (Oxalidæ).—In most of the species in this large genus the three leaflets sink vertically down at night; but as their sub-petioles are short the blades could not assume this position from the want of space, unless they were in some manner rendered narrower; and this is effected by their becoming more or less folded (Fig. 127). The angle formed by the two halves of the same leaflet was found to vary in different individuals of several species between 92° and 150°; in three of the best folded leaflets of O. fragrans it was 76°, 74°, and 54°. The angle is often different in the three leaflets of the same leaf. As the leaflets sink down at night and become folded, their lower surfaces are brought near together (see B), or even into close contact; and from this circumstance it might be thought that the object of the folding was the protection of their lower surfaces. If this had been the case, it would have formed a strongly marked exception to the rule, that when there is any difference in the degree of protection from radiation of the two surfaces of the leaves, it is always the upper surface which is the best protected. But that the folding of the leaflets, and consequent mutual approximation of their lower surfaces, serves merely to allow them to sink down vertically, may be inferred from the fact that when the leaflets do not radiate from the summit of a common petiole, or, again, when there is plenty of room from the sub-petioles not being very short, the leaflets sink down without becoming folded. This occurs with the leaflets of O. sensitiva, Plumierii, and bupleurifolia.
Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B, diagram of leaf asleep, also seen from vertically above. There is no use in giving a long list of the many species which sleep in the above described manner. This holds good with species having rather fleshy leaves, like those of O. carnosa, or large leaves like those of O. Ortegesii, or four leaflets like those of O. variabilis. There are, however, some species which show no signs of sleep, viz., O. pentaphylla, enneaphylla, hirta, and rubella. We will now describe the nature of the movements in some of the species. Oxalis acetosella.—The movement of a leaflet, together with that of the main petiole, are shown in the following diagram (Fig. 128), traced between 11 A.M. on October 4th and 7.45 A.M. on the 5th. After 5.30 P.M. on the 4th the leaflet sank rapidly, and at 7 P.M. depended vertically. for some time before it assumed this latter position, its movements could, of course, no longer be traced on the vertical glass, and the broken line in the diagram ought to be extended much further down in this and all other cases. By 6.45 A.M. on the following morning it had risen considerably, and continued to rise for the next hour; but, judging from other observations, it would soon have begun to fall again. Between 11 A.M. and 5.30 P.M. the leaflet moved at least four times up and four times down before the great nocturnal fall commenced; it reached its highest point at noon. Similar observations were made on two other leaflets, with nearly the same results. Sachs and Pfeffer have also described briefly[4] the autonomous movements of the leaves of this plant.
Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep; traced on vertical glass during 3 h. 40 m. Oxalis Ortegesii.—The large leaves of this plant sleep like those of the previous species. The main petioles are long, and that of a young leaf rose 20° between noon and 10 P.M., whilst the petiole of an older leaf rose only 13°. Owing to this rising of the petioles, and the vertical sinking of the large leaflets, the leaves become crowded together at night, and the whole plant then exposes a much smaller surface to radiation than during the day. Oxalis Plumierii.—In this species the three leaflets do not surround the summit of the petiole, but the terminal leaflet projects in the line of the petiole, with a lateral leaflet on each side. They all sleep by bending vertically downwards, but do not become at all folded. The petiole is rather long, and, one having been secured to a stick, the movement of the terminal leaflet was traced during 45 h. on a vertical glass. It moved in a very simple manner, sinking rapidly after 5 P.M., and rising rapidly early next morning. During the middle of the day it moved slowly and a little laterally. Consequently the ascending and descending lines did not coincide, and a single great ellipse was formed each day. There was no other evidence of circumnutation, and this fact is of interest, as we shall hereafter see.
Oxalis sensitiva.—The leaflets, as in the last species, bend vertically down at night, without becoming folded. The much elongated main petiole rises considerably in the evening, but in some very young plants the rise did not commence until late at night. We have seen that the cotyledons, instead of sinking like the leaflets, rise up vertically at night. Oxalis bupleurifolia.—This species is rendered remarkable by the petioles being foliaceous, like the phyllodes of many Acacias. The leaflets are small, of a paler green and more tender consistence than the foliaceous petioles. The leaflet which was observed was .55 inch in length, and was borne by a petiole 2 inches long and .3 inch broad. It may be suspected that the leaflets are on the road to abortion or obliteration, as has actually occurred with those of another Brazilian species, O. rusciformis. Nevertheless, in the present species the nyctitropic movements are perfectly performed. The foliaceous petiole was first observed during 48 h., and found to be in continued circumnutation, as shown in the accompanying figure (Fig. 130). It rose during the day and early part of the night, and fell during the remainder of the night and early morning; but the movement was not sufficient to be called sleep. The ascending and descending lines did not coincide, so that an ellipse was formed each day. There was but little zigzagging; if the filament had been fixed longitudinally, we should probably have seen that there was more lateral movement than appears in the diagram.
Fig. 130. Oxalis bupleurifolia: circumnutation of foliaceous petiole, filament fixed obliquely across end of petiole; movements traced on vertical glass from 9 A.M. June 26th to 8.50 A.M. 28th. Apex of leaflet 4½ inches from the glass, so movement not much magnified. Plant 9 inches high, illuminated from above. Temp. 23½°–24½° C. A terminal leaflet on another leaf was next observed (the petiole being secured), and its movements are shown in Fig. 131. During the day the leaflets are extended horizontally, and at night depend vertically; and as the petiole rises during the day the leaflets have to bend down in the evening more than 90°, so as to assume at night their vertical position. On the first day the leaflet simply moved up and down; on the second day it plainly circumnutated between 8 A.M. and 4.30 P.M., after which hour the great evening fall commenced. Fig. 131. Oxalis bupleurifolia: circumnutation and nyctitropic movement of terminal leaflet, with filament affixed along the midrib; traced on a vertical glass from 9 A.M. on June 26th to 8.45 A.M. 28th. Conditions the same as in the last case.
Averrhoa bilimbi (Oxalidæ).—It has long been known,[5] firstly, that the leaflets in this genus sleep; secondly, that they move spontaneously during the day; and thirdly, that they are sensitive to a touch; but in none of these respects do they differ essentially from the species of Oxalis. They differ, however, as Mr. R. I. Lynch[6] has lately shown, in their spontaneous movements being strongly marked. In the case of A. bilimbi, it is a wonderful spectacle to behold on a warm sunny day the leaflets one after the other sinking rapidly downwards, and again ascending slowly. Their movements rival those of Desmodium gyrans. At night the leaflets hang vertically down; and now they are motionless, but this may be due to the opposite ones being pressed together (Fig. 132). The main petiole is in constant movement during the day, but no careful observations were made on it. The following diagrams are graphic representations of the variations in the angle, which a given leaflet makes with the vertical. The observations were made as follows. The plant growing in a pot was kept in a high temperature, the petiole of the leaf to be observed pointing straight at the observer, being separated from him by a vertical pane of glass.
The petiole was secured so that the basal joint, or pulvinus, of one of the lateral leaflets was at the centre of a graduated arc placed close behind the leaflet. A fine glass filament was fixed to the leaf, so as to project like a continuation of the midrib. This filament acted as an index; and as the leaf rose and fell, rotating about its basal joint, its angular movement could be recorded by reading off at short intervals of time the position of the glass filament on the graduated arc. In order to avoid errors of parallax, all readings were made by looking through a small ring painted on the vertical glass, in a line with the joint of the leaflet and the centre of the graduated arc. In the following diagrams the ordinates represent the angles which the leaflet made with the vertical at successive instants.[7] It follows that a fall in the curve represents an actual dropping of the leaf, and that the zero line represents a vertically dependent position. Fig. 133 represents the nature of the movements which occur in the evening, as soon as the leaflets begin to assume their nocturnal position.
At 4.55 P.M. the leaflet formed an angle of 85° with the vertical, or was only 5° below the horizontal; but in order that the diagram might get into our page, the leaflet is represented falling from 75° instead of 85°. Shortly after 6 P.M. it hung vertically down, and had attained its nocturnal position. Between 6.10 and 6.35 P.M. it performed a number of minute oscillations of about 2° each, occupying periods of 4 or 5 m. The complete state of rest of the leaflet which ultimately followed is not shown in the diagram. It is manifest that each oscillation consists of a gradual rise, followed by a sudden fall. Each time the leaflet fell, it approached nearer to the nocturnal position than it did on the previous fall. The amplitude of the oscillations diminished, while the periods of oscillation became shorter. [5] Dr. Bruce, ‘Philosophical Trans.,’ 1785, p. 356. [6] ‘Journal Linn. Soc.,’ vol. xvi. 1877, p. 231. [7] In all the diagrams 1 mm. in the horizontal direction represents one minute of time. Each mm. in the vertical direction represents one degree of angular movement. In Figs. 133 and 134 the temperature is represented (along the ordinates) in the scale of 1 mm. to each 0.1 degree C. In Fig. 135 each mm. equals 0.2° F. Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced.
32° C., 14 oscillations of a few degrees occurred in 19 m. On the other hand, an oscillation may be much slower; thus a leaflet was observed (temperature 25° C.) to rise during 40 m. before it fell and completed its oscillation. Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a change from bright illumination to shade; temperature (broken line) remaining nearly the same. Fig. 135. Averrhoa bilimbi: angular movement of leaflet during a change of temperature; light remaining the same. The broken line shows the change of temperature. Fig. 136. Porlieria hygrometrica: circumnutation and nyctitropic movements of petiole of leaf, traced from 9.35 A.M. July 7th to about midnight on the 8th. Apex of leaf 7½ inches from the vertical glass. Temp. 19½°–20½° C.
Porlieria hygrometrica (Zygophylleæ).—The leaves of this plant (Chilian form) are from 1 to 1½ inch in length, and bear as many as 16 or 17 small leaflets on each side, which do not stand opposite one another. They are articulated to the petiole, and the petiole to the branch by a pulvinus. We must premise that apparently two forms are confounded under the same name: the leaves on a bush from Chili, which was sent to us from Kew, bore many leaflets, whilst those on plants in the Botanic Garden at Würzburg bore only 8 or 9 pairs; and the whole character of the bushes appeared somewhat different. We shall also see that they differ in a remarkable physiological peculiarity. On the Chilian plant the petioles of the younger leaves on upright branches, stood horizontally during the day, and at night sank down vertically so as to depend parallel and close to the branch beneath. The petioles of rather older leaves did not become at night vertically depressed, but only highly inclined. In one instance we found a branch which had grown perpendicularly downwards, and the petioles on it moved in the same direction relatively to the branch as just stated, and therefore moved upwards.
On horizontal branches the younger petioles likewise move at night in the same direction as before, that is, towards the branch, and are consequently then extended horizontally; but it is remarkable that the older petioles on the same branch, though moving a little in the same direction, also bend downwards; they thus occupy a somewhat different position, relatively to the centre of the earth and to the branch, from that of the petioles on the upright branches. With respect to the leaflets, they move at night towards the apex of the petiole until their midribs stand nearly parallel to it; and they then lie neatly imbricated one over the other. Thus half of the upper surface of each leaflet is in close contact with half of the lower surface of the one next in advance; and all the leaflets, excepting the basal ones, have the whole of their upper surfaces and half of their lower surfaces well protected. Those on the opposite sides of the same petiole do not come into close contact at night, as occurs with the leaflets of so many Leguminosae but are separated by an open furrow; nor could they exactly coincide, as they stand alternately with respect to one another.
The circumnutation of the petiole of a leaf 3/4 of an inch in length, on an upright branch, was observed during 36h., and is shown in the preceding diagram (Fig. 136). On the first morning, the leaf fell a little and then rose until 1 P.M., and this was probably due to its being now illuminated through a skylight from above; it then circumnutated on a very small scale round the same spot until about 4 P.M., when the great evening fall commenced. During the latter part of the night or very early on the next morning the leaf rose again. On the second day it fell during the morning till 1 P.M., and this no doubt is its normal habit. From 1 to 4 P.M. it rose in a zigzag line, and soon afterwards the great evening fall commenced. It thus completed a double oscillation during the 24 h.
The specific name given to this plant by Ruiz and Pavon, indicates that in its native arid home it is affected in some manner by the dryness or dampness of the atmosphere.[8] In the Botanic Garden at Würzburg, there was a plant in a pot out of doors which was daily watered, and another in the open ground which was never watered. After some hot and dry weather there was a great difference in the state of the leaflets on these two plants; those on the unwatered plant in the open ground remaining half, or even quite, closed during the day. But twigs cut from this bush, with their ends standing in water, or wholly immersed in it, or kept in damp air under a bell-glass, opened their leaves though exposed to a blazing sun; whilst those on the plant in the ground remained closed. The leaves on this same plant, after some heavy rain, remained open for two days; they then became half closed during two days, and after an additional day were quite closed. This plant was now copiously watered, and on the following morning the leaflets were fully expanded. The other plant growing in a pot, after having been exposed to heavy rain, was placed before a window in the Laboratory, with its leaflets open, and they remained so during the daytime for 48 h.; but after an additional day were half closed. The plant was then watered, and the leaflets on the two following days remained open. On the third day they were again half closed, but on being again watered remained open during the two next days. From these several facts we may conclude that the plant soon feels the want of water; and that as soon as this occurs, it partially or quite closes its leaflets, which in their then imbricated condition expose a small surface to evaporation. It is therefore probable that this sleep-like movement, which occurs only when the ground is dry, is an adaptation against the loss of moisture.
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