The Power of Movement in Plants

by Charles Darwin; Sir Francis 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.

[4] Sachs in ‘Flora,’ 1863, p. 470, etc; Pfeffer, ‘Die Period. Bewegungen,’ etc., 1875, p. 53.

Fig 128. Oxalis acetosella: circumnutation and nyctitropic movements of a nearly full-grown leaf, with filament attached to the midrib of one of the leaflets; traced on vertical glass during 20 h. 45m.

On another occasion the petiole of a leaf was secured to a little stick close beneath the leaflets, and a filament tipped with a bead of sealing-wax was affixed to the mid-rib of one of them, and a mark was placed close behind. At 7 P.M., when the leaflets were asleep, the filament depended vertically down, and the movements of the bead were then traced till 10.40 P.M., as shown in the following diagram (Fig. 129). We here see that the leaflet moved a little from side to side, as well as a little up and down, whilst asleep.

Oxalis Valdiviana.—The leaves resemble those of the last species, and the movements of two leaflets (the main petioles of both having been secured) were traced during two days; but the tracings are not given, as they resembled that of O. acetosella, with the exception that the up and down oscillations were not so frequent during the day, and there was more lateral movement, so that broader ellipses were described. The leaves awoke early in the morning, for by 6.45 A.M. on June 12th and 13th they had not only risen to their full height, but had already begun to fall, that is, they were circumnutating. We have seen in the last chapter that the cotyledons, instead of sinking, rise up vertically at night.

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.

Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during its evening descent, when going to sleep. Temp. 78°–81° F.

In bright sunshine the leaflets assume a highly inclined dependent position. A leaflet in diffused light was observed rising for 25 m. A blind was then pulled up so that the plant was brightly illuminated (BR in Fig. 134), and within a minute it began to fall, and ultimately fell 47°, as shown in the diagram. This descent was performed by six descending steps, precisely similar to those by which the nocturnal fall is effected. The plant was then again shaded (SH), and a long slow rise occurred until another series of falls commenced at BR’, when the sun was again admitted. In this experiment cool air was allowed to enter by the windows being opened at the same time that the blinds were pulled up, so that in spite of the sun shining on the plant the temperature was not raised.

The effect of an increase of temperature in diffused light is shown in Fig. 135. The temperature began to rise at 11.35 A.M. (in consequence of the fire being lighted), but by 12.42 a marked fall had occurred. It may be seen in the diagram that when the temperature was highest there were rapid oscillations of small amplitude, the mean position of the leaflet being at the time nearer the vertical. When the temperature began to fall, the oscillations became slower and larger, and the mean position of the leaf again approached the horizontal. The rate of oscillation was sometimes quicker than is represented in the above diagram. Thus, when the temperature was between 31° and 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.

[8] ‘Systema Veg. Florae Peruvianae et Chilensis,’ tom. i. p. 95, 1798. We cannot understand the account given by the authors of the behaviour of this plant in its native home. There is much about its power of foretelling changes in the weather; and it appears as if the brightness of the sky largely determined the opening and closing of the leaflets.

A bush about 4 feet in height, a native of Chili, which was thickly covered with leaves, behaved very differently, for during the day it never closed its leaflets. On July 6th the earth in the small pot in which it grew appeared extremely dry, and it was given a very little water. After 21 and 22 days (on the 27th and 28th), during the whole of which time the plant did not receive a drop of water, the leaves began to droop, but they showed no signs of closing during the day. It appeared almost incredible that any plant, except a fleshy one, could have kept alive in soil so dry, which resembled the dust on a road. On the 29th, when the bush was shaken, some leaves fell off, and the remaining ones were unable to sleep at night. It was therefore moderately watered, as well as syringed, late in the evening. On the next morning (30th) the bush looked as fresh as ever, and at night the leaves went to sleep. It may be added that a small branch while growing on the bush was enclosed, by means of a curtain of bladder, during 13 days in a large bottle half full of quicklime, so that the air within must have been intensely dry; yet the leaves on this branch did not suffer in the least, and did not close at all during the hottest days. Another trial was made with the same bush on August 2nd and 6th (the soil appearing at this latter date extremely dry), for it was exposed out of doors during the whole day to the wind, but the leaflets showed no signs of closing. The Chilian form therefore differs widely from the one at Würzburg, in not closing its leaflets when suffering from the want of water; and it can live for a surprisingly long time without water.

Tropaeolum majus (?) (cultivated var.) (Tropaeoleae).—Several plants in pots stood in the greenhouse, and the blades of the leaves which faced the front-lights were during the day highly inclined and at night vertical; whilst the leaves on the back of the pots, though of course illuminated through the roof, did not become vertical at night. We thought, at first, that this difference in their positions was in some manner due to heliotropism, for the leaves are highly heliotropic. The true explanation, however, is that unless they are well illuminated during at least a part of the day they do not sleep at night; and a little difference in the degree of illumination determines whether or not they shall become vertical at night. We have observed no other so well-marked a case as this, of the influence of previous illumination on nyctitropic movements. The leaves present also another peculiarity in their habit of rising or awaking in the morning, being more strongly fixed or inherited than that of sinking or sleeping at night. The movements are caused by the bending of an upper part of the petiole, between ½ and 1 inch in length; but the part close to the blade, for about 1/4 of an inch in length, does not bend and always remains at right angles to the blade. The bending portion does not present any external or internal difference in structure from the rest of the petiole. We will now give the experiments on which the above conclusions are founded.

A large pot with several plants was brought on the morning of Sept. 3rd out of the greenhouse and placed before a north-east window, in the same position as before with respect to the light, as far as that was possible. On the front of the plants, 24 leaves were marked with thread, some of which had their blades horizontal, but the greater number were inclined at about 45°, beneath the horizon; at night all these, without exception, became vertical. Early on the following morning (4th) they reassumed their former positions, and at night again became vertical. On the 5th the shutters were opened at 6.15 A.M., and by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m. and had acquired their diurnal position, they were placed in a dark cupboard. They were looked at twice during the day and thrice in the evening, the last time at 10.30 P.M., and not one had become vertical. At 8 A.M. on the following morning (6th) they still retained the same diurnal position, and were now replaced before the north-east window. At night all the leaves which had faced the light had their petioles curved and their blades vertical; whereas none of the leaves on the back of the plants, although they had been moderately illuminated by the diffused light of the room, were vertical. They were now at night placed in the same dark cupboard; at 9 A.M. on the next morning (7th) all those which had been asleep had reassumed their diurnal position. The pot was then placed for 3 h. in the sunshine, so as to stimulate the plants; at noon they were placed before the same north-east window, and at night the leaves slept in the usual manner and awoke on the following morning. At noon on this day (8th) the plants, after having been left before the north-east window for 5 h. 45 m. and thus illuminated (though not brightly, as the sky was cloudy during the whole time), were replaced in the dark cupboard, and at 3 P.M. the position of the leaves was very little, if at all, altered, so that they are not quickly affected by darkness; but by 10.15 P.M. all the leaves which had faced the north-east sky during the 5 h. 45 m. of illumination stood vertical, whereas those on the back of the plant retained their diurnal position. On the following morning (9th) the leaves awoke as on the two former occasions in the dark, and they were kept in the dark during the whole day; at night a very few of them became vertical, and this was the one instance in which we observed any inherited tendency or habit in this plant to sleep at the proper time. That it was real sleep was shown by these same leaves reassuming their diurnal position on the following morning (10th) whilst still kept in the dark.

The pot was then (9.45 A.M. 10th) replaced, after having been kept for 36 h. in darkness, before the north-east window; and at night the blades of all the leaves (excepting a few on the back of the plants) became conspicuously vertical.

At 6.45 A.M. (11th) after the plants had been illuminated on the same side as before during only 25 m., the pot was turned round, so that the leaves which had faced the light now faced the interior of the room, and not one of these went to sleep at night; whilst some, but not many, of those which had formerly stood facing the back of the room and which had never before been well illuminated or gone to sleep, now assumed a vertical position at night. On the next day (12th) the plant was turned round into its original position, so that the same leaves faced the light as formerly, and these now went to sleep in the usual manner. We will only add that with some young seedlings kept in the greenhouse, the blades of the first pair of true leaves (the cotyledons being hypogean) stood during the day almost horizontally and at night almost vertically.

A few observations were subsequently made on the circumnutation of three leaves, whilst facing a north-east window; but the tracings are not given, as the leaves moved somewhat towards the light. It was, however, manifest that they rose and fell more than once during the daytime, the ascending and descending lines being in parts extremely zigzag. The nocturnal fall commenced about 7 P.M., and the leaves had risen considerably by 6.45 A.M. on the following morning.

Leguminosae.—This Family includes many more genera with sleeping species than all the other families put together. The number of the tribes to which each genus belongs, according to Bentham and Hooker’s arrangement, has been added.

Crotolaria (sp.?) (Tribe 2).—This plant is monophyllous, and we are informed by Mr. T. Thiselton Dyer that the leaves rise up vertically at night and press against the stem.

Lupinus (Tribe 2).—The palmate or digitate leaves of the species in this large genus sleep in three different manners. One of the simplest, is that all the leaflets become steeply inclined downwards at night, having been during the day extended horizontally. This is shown in the accompanying figures (Fig. 137), of a leaf of L. pilosus, as seen during the day from vertically above, and of another leaf asleep with the leaflets inclined downwards. As in this position they are crowded together, and as they do not become folded like those in the genus Oxalis, they cannot occupy a vertically dependent position; but they are often inclined at an angle of 50° beneath the horizon. In this species, whilst the leaflets are sinking, the petioles rise up, in two instances when the angles were measured to the extent of 23°. The leaflets of L. sub-carnosus and arboreus, which were horizontal during the day, sank down at night in nearly the same manner; the former to an angle of 38° and the latter of 36°, beneath the horizon; but their petioles did not move in any plainly perceptible degree. It is, however, quite possible, as we shall presently see, that if a large number of plants of the three foregoing and of the following species were to be observed at all seasons, some of the leaves would be found to sleep in a different manner.

Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in daytime; B, leaf asleep, seen laterally at night.

In the two following species the leaflets, instead of moving downwards, rise at night. With L. Hartwegii some stood at noon at a mean angle of 36° above the horizon, and at night at 51°, thus forming together a hollow cone with moderately steep sides. The petiole of one leaf rose 14° and of a second 11° at night. With L. luteus a leaflet rose from 47° at noon to 65° above the horizon at night, and another on a distinct leaf rose from 45° to 69°. The petioles, however, sink at night to a small extent, viz., in three instances by 2°, 6°, and 9° 30 seconds. Owing to this movement of the petioles, the outer and longer leaflets have to bend up a little more than the shorter and inner ones, in order that all should stand symmetrically at night. We shall presently see that some leaves on the same individual plants of L. luteus sleep in a very different manner.

We now come to a remarkable position of the leaves when asleep, which is common to several species of Lupines. On the same leaf the shorter leaflets, which generally face the centre of the plant, sink at night, whilst the longer ones on the opposite side rise; the intermediate and lateral ones merely twisting on their own axes. But there is some variability with respect to which leaflets rise or fall. As might have been expected from such diverse and complicated movements, the base of each leaflet is developed (at least in the case of L. luteus) into a pulvinus. The result is that all the leaflets on the same leaf stand at night more or less highly inclined, or even quite vertically, forming in this latter case a vertical star. This occurs with the leaves of a species purchased under the name of L. pubescens; and in the accompanying figures we see at A (Fig. 138) the leaves in their diurnal position; and at B the same plant at night with the two upper leaves having their leaflets almost vertical. At C another leaf, viewed laterally, is shown with the leaflets quite vertical. It is chiefly or exclusively the youngest leaves which form at night vertical stars. But there is much variability in the position of the leaves at night on the same plant; some remaining with their leaflets almost horizontal, others forming more or less highly inclined or vertical stars, and some with all their leaflets sloping downwards, as in our first class of cases. It is also a remarkable fact, that although all the plants produced from the same lot of seeds were identical in appearance, yet some individuals at night had the leaflets of all their leaves arranged so as to form more or less highly inclined stars; others had them all sloping downwards and never forming a star; and others, again, retained them either in a horizontal position or raised them a little.

Fig. 138. Lupinus pubescens: A, leaf viewed laterally during the day; B, same leaf at night; C, another leaf with the leaflet forming a vertical star at night. Figures reduced.

We have as yet referred only to the different positions of the leaflets of L. pubescens at night; but the petioles likewise differ in their movements. That of a young leaf which formed a highly inclined star at night, stood at noon at 42° above the horizon, and during the night at 72°, so had risen 30°. The petiole of another leaf, the leaflets of which occupied a similar position at night, rose only 6°. On the other hand, the petiole of a leaf with all its leaflets sloping down at night, fell at this time 4°. The petioles of two rather older leaves were subsequently observed; both of which stood during the day at exactly the same angle, viz., 50° above the horizon, and one of these rose 7°–8°, and the other fell 3°–4° at night. We meet with cases like that of L. pubescens with some other species. On a single plant of L. mutabilis some leaves, which stood horizontally during the day, formed highly inclined stars at night, and the petiole of one rose 7°. Other leaves which likewise stood horizontally during the day, had at night all their leaflets sloping downwards at 46° beneath the horizon, but their petioles had hardly moved. Again, L. luteus offered a still more remarkable case, for on two leaves, the leaflets which stood at noon at about 45° above the horizon, rose at night to 65° and 69°, so that they formed a hollow cone with steep sides. Four leaves on the same plant, which had their leaflets horizontal at noon, formed vertical stars at night; and three other leaves equally horizontal at noon, had all their leaflets sloping downwards at night. So that the leaves on this one plant assumed at night three different positions. Though we cannot account for this fact, we can see that such a stock might readily give birth to species having widely different nyctitropic habits.

Little more need be said about the sleep of the species of Lupinus; several, namely, L. polyphyllus, nanus, Menziesii, speciosus, and albifrons, though observed out of doors and in the greenhouse, did not change the position of their leaves sufficiently at night to be said to sleep. From observations made on two sleeping species, it appears that, as with Tropaeolum majus, the leaves must be well illuminated during the day in order to sleep at night. For several plants, kept all day in a sitting-room with north-east windows, did not sleep at night; but when the pots were placed on the following day out of doors, and were brought in at night, they slept in the usual manner. the trial was repeated on the following day and night with the same result.

Some observations were made on the circumnutation of the leaves of L. luteus and arboreus. It will suffice to say that the leaflets of the latter exhibited a double oscillation in the course of 24 h.; for they fell from the early morning until 10.15 A.M., then rose and zigzagged greatly till 4 P.M., after which hour the great nocturnal fall commenced. By 8 A.M. on the following morning the leaflets had risen to their proper height. We have seen in the fourth chapter, that the leaves of Lupinus speciosus, which do not sleep, circumnutate to an extraordinary extent, making many ellipses in the course of the day.

Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3).—Only a few observations were made on these three genera. The petioles on a young plant, about a foot in height, of Cytisus fragrans rose at night, on one occasion 23° and on another 33°. The three leaflets also bend upwards, and at the same time approach each other, so that the base of the central leaflet overlaps the bases of the two lateral leaflets. They bend up so much that they press against the stem; and on looking down on one of these young plants from vertically above, the lower surfaces of the leaflets are visible; and thus their upper surfaces, in accordance with the general rule, are best protected from radiation. Whilst the leaves on these young plants were thus behaving, those on an old bush in full flower did not sleep at night.

Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep at night.

Trigonella Cretica resembles a Melilotus in its sleep, which will be immediately described. According to M. Royer,[^[9]] the leaves of Medicago maculata rise up at night, and “se renversent un peu de manière à presenter obliquement au ciel leur face inférieure.” A drawing is here given (Fig. 139) of the leaves of M. marina awake and asleep; and this would almost serve for Cytisus fragrans in the same two states.

[9] ‘Annales des Sc. Nat. Bot.’ (5th series), ix. 1868, p. 368.

Melilotus (Tribe 3).—The species in this genus sleep in a remarkable manner. The three leaflets of each leaf twist through an angle of 90°, so that their blades stand vertically at night with one lateral edge presented to the zenith (Fig. 140). We shall best understand the other and more complicated movements, if we imagine ourselves always to hold the leaf with the tip of the terminal leaflet pointed to the north. The leaflets in becoming vertical at night could of course twist so that their upper surfaces should face to either side; but the two lateral leaflets always twist so that this surface tends to face the north, but as they move at the same time towards the terminal leaflet, the upper surface of the one faces about N.N.W., and that of the other N.N.E. The terminal leaflet behaves differently, for it twists to either side, the upper surface facing sometimes east and sometimes west, but rather more commonly west than east. The terminal leaflet also moves in another and more remarkable manner, for whilst its blade is twisting and becoming vertical, the whole leaflet bends to one side, and invariably to the side towards which the upper surface is directed; so that if this surface faces the west the whole leaflet bends to the west, until it comes into contact with the upper and vertical surface of the western lateral leaflet. Thus the upper surface of the terminal and of one of the two lateral leaflets is well protected.

The fact of the terminal leaflet twisting indifferently to either side and afterwards bending to the same side, seemed to us so remarkable, that we endeavoured to discover the cause. We imagined that at the commencement of the movement it might be determined by one of the two halves of the leaflet being a little heavier than the other. Therefore bits of wood were gummed on one side of several leaflets, but this produced no effect; and they continued to twist in the same direction as they had previously done. In order to discover whether the same leaflet twisted permanently in the same direction, black threads were tied to 20 leaves, the terminal leaflets of which twisted so that their upper surfaces faced west, and 14 white threads to leaflets which twisted to the east. These were observed occasionally during 14 days, and they all continued, with a single exception, to twist and bend in the same direction; for one leaflet, which had originally faced east, was observed after 9 days to face west. The seat of both the twisting and bending movement is in the pulvinus of the sub-petioles.

Fig. 140. Melilotus officinalis: A, leaf during the daytime. B, another leaf asleep. C, a leaf asleep as viewed from vertically above; but in this case the terminal leaflet did not happen to be in such close contact with the lateral one, as is usual.

We believe that the leaflets, especially the two lateral ones, in performing the above described complicated movements generally bend a little downwards; but we are not sure of this, for, as far as the main petiole is concerned, its nocturnal movement is largely determined by the position which the leaf happens to occupy during the day. Thus one main petiole was observed to rise at night 59°, whilst three others rose only 7° and 9°. The petioles and sub-petioles are continually circumnutating during the whole 24 h., as we shall presently see.

The leaves of the following 15 species, M. officinalis, suaveolens, parviflora, alba, infesta, dentata, gracilis, sulcata, elegans, coerulea, petitpierreana, macrorrhiza, Italica, secundiflora, and Taurica, sleep in nearly the same manner as just described; but the bending to one side of the terminal leaflet is apt to fail unless the plants are growing vigorously. With M. petitpierreana and secundiflora the terminal leaflet was rarely seen to bend to one side. In young plants of M. Italica it bent in the usual manner, but with old plants in full flower, growing in the same pot and observed at the same hour, viz., 8.30 P.M., none of the terminal leaflets on several scores of leaves had bent to one side, though they stood vertically; nor had the two lateral leaflets, though standing vertically, moved towards the terminal one. At 10.30 P.M., and again one hour after midnight, the terminal leaflets had become very slightly bent to one side, and the lateral leaflets had moved a very little towards the terminal one, so that the position of the leaflets even at this late hour was far from the ordinary one. Again, with M. Taurica the terminal leaflets were never seen to bend towards either of the two lateral leaflets, though these, whilst becoming vertical, had bent towards the terminal one. The sub-petiole of the terminal leaflet in this species is of unusual length, and if the leaflet had bent to one side, its upper surface could have come into contact only with the apex of either lateral leaflet; and this, perhaps, is the meaning of the loss of the lateral movement.

The cotyledons do not sleep at night. the first leaf consists of a single orbicular leaflet, which twists at night so that the blade stands vertically. It is a remarkable fact that with M. Taurica, and in a somewhat less degree with M. macrorrhiza and petitpierreana, all the many small and young leaves produced during the early spring from shoots on some cut-down plants in the greenhouse, slept in a totally different manner from the normal one; for the three leaflets, instead of twisting on their own axes so as to present their lateral edges to the zenith, turned upwards and stood vertically with their apices pointing to the zenith. They thus assumed nearly the same position as in the allied genus Trifolium; and on the same principle that embryological characters reveal the lines of descent in the animal kingdom, so the movements of the small leaves in the above three species of Melilotus, perhaps indicate that this genus is descended from a form which was closely allied to and slept like a Trifolium. Moreover, there is one species, M. messanensis, the leaves of which, on full-grown plants between 2 and 3 feet in height, sleep like the foregoing small leaves and like those of a Trifolium. We were so much surprised at this latter case that, until the flowers and fruit were examined, we thought that the seeds of some Trifolium had been sown by mistake instead of those of a Melilotus. It appears therefore probable that M. messanensis has either retained or recovered a primordial habit.

The circumnutation of a leaf of M. officinalis was traced, the stem being left free; and the apex of the terminal leaflet described three laterally extended ellipses, between 8 A.M. and 4 P.M.; after the latter hour the nocturnal twisting movement commenced. It was afterwards ascertained that the above movement was compounded of the circumnutation of the stem on a small scale, of the main petiole which moved most, and of the sub-petiole of the terminal leaflet. The main petiole of a leaf having been secured to a stick, close to the base of the sub-petiole of the terminal leaflet, the latter described two small ellipses between 10.30 A.M., and 2 P.M. At 7.15 P.M., after this same leaflet (as well as another) had twisted themselves into their vertical nocturnal position, they began to rise slowly, and continued to do so until 10.35 P.M., after which hour they were no longer observed.

As M. messanensis sleeps in an anomalous manner, unlike that of any other species in the genus, the circumnutation of a terminal leaflet, with the stem secured, was traced during two days. On each morning the leaflet fell, until about noon, and then began to rise very slowly; but on the first day the rising movement was interrupted between 1 and 3 P.M. by the formation of a laterally extended ellipse, and on the second day, at the same time, by two smaller ellipses. The rising movement then recommenced, and became rapid late in the evening, when the leaflet was beginning to go to sleep. The awaking or sinking movement had already commenced by 6.45 A.M. on both mornings.

Trifolium (Tribe 3).—The nyctitropic movements of 11 species were observed, and were found to be closely similar. If we select a leaf of T. repens having an upright petiole, and with the three leaflets expanded horizontally, the two lateral leaflets will be seen in the evening to twist and approach each other, until their upper surfaces come into contact. At the same time they bend downwards in a plane at right angles to that of their former position, until their midribs form an angle of about 45° with the upper part of the petiole. This peculiar change of position requires a considerable amount of torsion in the pulvinus. The terminal leaflet merely rises up without any twisting and bends over until it rests on and forms a roof over the edges of the now vertical and united lateral leaflets. Thus the terminal leaflet always passes through an angle of at least 90°, generally of 130° or 140°, and not rarely—as was often observed with T. subterraneum—of 180°. In this latter case the terminal leaflet stands at night horizontally (as in Fig. 141), with its lower surface fully exposed to the zenith. Besides the difference in the angles, at which the terminal leaflets stand at night in the individuals of the same species, the degree to which the lateral leaflets approach each other often likewise differs.

Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at night.

We have seen that the cotyledons of some species and not of others rise up vertically at night. The first true leaf is generally unifoliate and orbicular; it always rises, and either stands vertically at night or more commonly bends a little over so as to expose the lower surface obliquely to the zenith, in the same manner as does the terminal leaflet of the mature leaf. But it does not twist itself like the corresponding first simple leaf of Melilotus. With T. Pannonicum the first true leaf was generally unifoliate, but sometimes trifoliate, or again partially lobed and in an intermediate condition.

Circumnutation.—Sachs described in 1863[^[10]] the spontaneous up and down movements of the leaflets of T. incarnatum, when kept in darkness. Pfeffer made many observations on the similar movements in T. pratense.[^[11]] He states that the terminal leaflet of this species, observed at different times, passed through angles of from 30° to 120° in the course of from 1½ to 4 h. We observed the movements of T. subterraneum, resupinatum, and repens.

[10] ‘Flora,’ 1863, p. 497.

[11] ‘Die Period. Bewegungen,’ 1875, pp. 35, 52.

Trifolium subterraneum.—A petiole was secured close to the base of the three leaflets, and the movement of the terminal leaflet was traced during 26½ h., as shown in the figure on the next page.

Between 6.45 A.M. and 6 P.M. the apex moved 3 times up and 3 times down, completing 3 ellipses in 11 h. 15 m. The ascending and descending lines stand nearer to one another than is usual with most plants, yet there was some lateral motion. At 6 P.M. the great nocturnal rise commenced, and on the next morning the sinking of the leaflet was continued until 8.30 A.M., after which hour it circumnutated in the manner just described. In the figure the great nocturnal rise and the morning fall are greatly abbreviated, from the want of space, and are merely represented by a short curved line. The leaflet stood horizontally when at a point a little beneath the middle of the diagram; so that during the daytime it oscillated almost equally above and beneath a horizontal position. At 8.30 A.M. it stood 48° beneath the horizon, and by 11.30 A.M. it had risen 50° above the horizon; so that it passed through 98° in 3 h. By the aid of the tracing we ascertained that the distance travelled in the 3 h. by the apex of this leaflet was 1.03 inch. If we look at the figure, and prolong upwards in our mind’s eye the short curved broken line, which represents the nocturnal course, we see that the latter movement is merely an exaggeration or prolongation of one of the diurnal ellipses. The same leaflet had been observed on the previous day, and the course then pursued was almost identically the same as that here described.

Fig. 142. Trifolium subterraneum: circumnutation and nyctitropic movement of terminal leaflet (.68 inch in length), traced from 6.45 A.M. July 4th to 9.15 A.M. 5th. Apex of leaf 3 7/8 inches from the vertical glass, and movement, as here shown, magnified 5 1/4 times, reduced to one-half of original scale. Plant illuminated from above; temp. 16°–17° C.

Trifolium resupinatum.—A plant left entirely free was placed before a north-east window, in such a position that a terminal leaflet projected at right angles to the source of the light, the sky being uniformly clouded all day. The movements of this leaflet were traced during two days, and on both were closely similar. Those executed on the second day are shown in Fig. 143. The obliquity of the several lines is due partly to the manner in which the leaflet was viewed, and partly to its having moved a little towards the light. From 7.50 A.M. to 8.40 A.M. the leaflet fell, that is, the awakening movement was continued. It then rose and moved a little laterally towards the light. At 12.30 it retrograded, and at 2.30 resumed its original course, having thus completed a small ellipse during the middle of the day. In the evening it rose rapidly, and by 8 A.M. on the following morning had returned to exactly the same spot as on the previous morning. The line representing the nocturnal course ought to be extended much higher up, and is here abbreviated into a short, curved, broken line. The terminal leaflet, therefore, of this species described during the daytime only a single additional ellipse, instead of two additional ones, as in the case of T. subterraneum. But we should remember that it was shown in the fourth chapter that the stem circumnutates, as no doubt does the main petiole and the sub-petioles; so that the movement represented in Fig. 143 is a compounded one. We tried to observe the movements of a leaf kept during the day in darkness, but it began to go to sleep after 2 h. 15 m., and this was well pronounced after 4 h. 30 m.

Fig 143. Trifolium resupinatum: circumnutation and nyctitropic movements of the terminal leaflet during 24 hours.

Trifolium repens.—A stem was secured close to the base of a moderately old leaf, and the movement of the terminal leaflet was observed during two days. This case is interesting solely from the simplicity of the movements, in contrast with those of the two preceding species. On the first day the leaflet fell between 8 A.M. and 3 P.M., and on the second between 7 A.M. and 1 P.M. On both days the descending course was somewhat zigzag, and this evidently represents the circumnutating movement of the two previous species during the middle of the day. After 1 P.M., Oct. 1st (Fig. 144), the leaflet began to rise, but the movement was slow on both days, both before and after this hour, until 4 P.M. The rapid evening and nocturnal rise then commenced. Thus in this species the course during 24 h. consists of a single great ellipse; in T. resupinatum of two ellipses, one of which includes the nocturnal movement and is much elongated; and in T. subterraneum of three ellipses, of which the nocturnal one is likewise of great length.

Securigera coronilla (Tribe 4).—The leaflets, which stand opposite one another and are numerous, rise up at night, come into close contact, and bend backwards at a moderate angle towards the base of the petiole.

Fig. 144. Trifolium repens: circumnutation and nyctitropic movements of a nearly full-grown terminal leaflet, traced on a vertical glass from 7 A.M. Sept. 30th to 8 A.M. Oct. 1st. Nocturnal course, represented by curved broken line, much abbreviated.

Lotus (Tribe 4).—The nyctitropic movements of 10 species in this genus were observed, and found to be the same. The main petiole rises a little at night, and the three leaflets rise till they become vertical, and at the same time approach each other. This was conspicuous with L. Jacoboeus, in which the leaflets are almost linear. In most of the species the leaflets rise so much as to press against the stem, and not rarely they become inclined a little inwards with their lower surfaces exposed obliquely to the zenith. This was clearly the case with L. major, as its petioles are unusually long, and the leaflets are thus enabled to bend further inwards. The young leaves on the summits of the stems close up at night so much, as often to resemble large buds. The stipule-like leaflets, which are often of large size, rise up like the other leaflets, and press against the stem (Fig. 145). All the leaflets of L. Gebelii, and probably of the other species, are provided at their bases with distinct pulvini, of a yellowish colour, and formed of very small cells. The circumnutation of a terminal leaflet of L. peregrinus (with the stem secured) was traced during two days, but the movement was so simple that it is not worth while to give the diagram. The leaflet fell slowly from the early morning till about 1 P.M. It then rose gradually at first, but rapidly late in the evening. It occasionally stood still for about 20 m. during the day, and sometimes zigzagged a little. The movement of one of the basal, stipule-like leaflets was likewise traced in the same manner and at the same time, and its course was closely similar to that of the terminal leaflet.

Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B, with leaves asleep at night. SS, stipule-like leaflets.

In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12 genera have been observed by ourselves and others, but only in Robinia with any care. Psoralea acaulis raises its three leaflets at night; whilst Amorpha fruticosa,[^[12]] Dalea alopecuroides, and Indigofera tinctoria depress them. Ducharte[^[13]] states that Tephrosia caribaea is the sole example of “folioles couchées le long du pétiole et vers la base;” but a similar movement occurs, as we have already seen, and shall again see in other cases. Wistaria Sinensis, according to Royer,[^[14]] “abaisse les folioles qui par une disposition bizarre sont inclinées dans la même feuille, les supérieures vers le sommet, les inférieures vers la base du petiole commun;” but the leaflets on a young plant observed by us in the greenhouse merely sank vertically downwards at night. The leaflets are raised in Sphaerophysa salsola, Colutea arborea, and Astragalus uliginosus, but are depressed, according to Linnæus, in Glycyrrhiza. The leaflets of Robinia pseudo-acacia likewise sink vertically down at night, but the petioles rise a little, viz., in one case 3°, and in another 4°. The circumnutating movements of a terminal leaflet on a rather old leaf were traced during two days, and were simple. The leaflet fell slowly, in a slightly zigzag line, from 8 A.M. to 5 P.M., and then more rapidly; by 7 A.M. on the following morning it had risen to its diurnal position. There was only one peculiarity in the movement, namely, that on both days there was a distinct though small oscillation up and down between 8.30 and 10 A.M., and this would probably have been more strongly pronounced if the leaf had been younger.

[12] Ducharte, ‘Eléments de Botanique’, 1867, p. 349.

[13] Ibid., p. 347.

[14] ‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868.

Coronilla rosea (Tribe 6).—the leaves bear 9 or 10 pairs of opposite leaflets, which during the day stand horizontally, with their midribs at right angles to the petiole. At night they rise up so that the opposite leaflets come nearly into contact, and those on the younger leaves into close contact. At the same time they bend back towards the base of the petiole, until their midribs form with it angles of from 40° to 50° in a vertical plane, as here figured (Fig. 146). The leaflets, however, sometimes bend so much back that their midribs become parallel to and lie on the petiole. They thus occupy a reversed position to what they do in several Leguminosae, for instance, in Mimosa pudica; but, from standing further apart, they do not overlap one another nearly so much as in this latter plant. The main petiole is curved slightly downwards during the day, but straightens itself at night. In three cases it rose from 3° above the horizon at noon, to 9° at 10 P.M.; from 11° to 33°; and from 5° to 33°—the amount of angular movement in this latter case amounting to 28°. In several other species of Coronilla the leaflets showed only feeble movements of a similar kind.

Fig. 146. Coronilla rosea: leaf asleep.

Hedysarum coronarium (Tribe 6).—The small lateral leaflets on plants growing out of doors rose up vertically at night, but the large terminal one became only moderately inclined. The petioles apparently did not rise at all.

Smithia Pfundii (Tribe 6).—The leaflets rise up vertically, and the main petiole also rises considerably.

Arachis hypogoea (Tribe 6).—The shape of a leaf, with its two pairs of leaflets, is shown at A (Fig. 147); and a leaf asleep, traced from a photograph (made by the aid of aluminium light), is given at B. The two terminal leaflets twist round at night until their blades stand vertically, and approach each other until they meet, at the same time moving a little upwards and backwards. The two lateral leaflets meet each other in this same manner, but move to a greater extent forwards, that is, in a contrary direction to the two terminal leaflets, which they partially embrace. Thus all four leaflets form together a single packet, with their edges directed to the zenith, and with their lower surfaces turned outwards. On a plant which was not growing vigorously the closed leaflets seemed too heavy for the petioles to support them in a vertical position, so that each night the main petiole became twisted, and all the packets were extended horizontally, with the lower surfaces of the leaflets on one side directed to the zenith in a most anomalous manner. This fact is mentioned solely as a caution, as it surprised us greatly, until we discovered that it was an anomaly. The petioles are inclined upwards during the day, but sink at night, so as to stand at about right angles with the stem. The amount of sinking was measured only on one occasion, and found to be 39°. A petiole was secured to a stick at the base of the two terminal leaflets, and the circumnutating movement of one of these leaflets was traced from 6.40 A.M. to 10.40 P.M., the plant being illuminated from above. The temperature was 17°–17½° C., and therefore rather too low. During the 16 h. the leaflet moved thrice up and thrice down, and as the ascending and descending lines did not coincide, three ellipses were formed.

Fig. 147. Arachis hypogoea: A, leaf during the day, seen from vertically above; B, leaf asleep, seen laterally, copied from a photograph. Figures much reduced.

Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-half natural size. The minute stipules unusually large.

Desmodium gyrans (Tribe 6).—A large and full-grown leaf of this plant, so famous for the spontaneous movements of the two little lateral leaflets, is here represented (Fig. 148). The large terminal leaflet sleeps by sinking vertically down, whilst the petiole rises up. The cotyledons do not sleep, but the first-formed leaf sleeps equally well as the older ones. The appearance presented by a sleeping branch and one in the day-time, copied from two photographs, are shown at A and B (Fig. 149), and we see how at night the leaves are crowded together, as if for mutual protection, by the rising of the petioles. The petioles of the younger leaves near the summits of the shoots rise up at night, so as to stand vertical and parallel to the stem; whilst those on the sides were found in four cases to have risen respectively 46½°, 36°, 20°, and 19.5° above the inclined positions which they had occupied during the day. For instance, in the first of these four cases the petiole stood in the day at 23°, and at night at 69½° above the horizon. In the evening the rising of the petioles is almost completed before the leaflets sink perpendicularly downwards.

Circumnutation.—The circumnutating movements of four young shoots were observed during 5 h. 15 m.; and in this time each completed an oval figure of small size. The main petiole also circumnutates rapidly, for in the course of 31 m. (temp. 91° F.) it changed its course by as much as a rectangle six times, describing a figure which apparently represented two ellipses. The movement of the terminal leaflet by means of its sub-petiole or pulvinus is quite as rapid, or even more so, than that of the main petiole, and has much greater amplitude. Pfeffer has seen[^[15]] these leaflets move through an angle of 8° in the course of from 10 to 30 seconds.

[15] ‘Die Period. Beweg.,’ p. 35.

Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves asleep. Figures reduced.

A fine, nearly full-grown leaf on a young plant, 8 inches in height, with the stem secured to a stick at the base of the leaf, was observed from 8.30 A.M. June 22nd to 8 A.M. June 24th. In the diagram given on the next page (Fig. 150), the two curved broken lines at the base, which represent the nocturnal courses, ought to be prolonged far downwards. On the first day the leaflet moved thrice down and thrice up, and to a considerable distance laterally; the course was also remarkably crooked. The dots were generally made every hour; if they had been made every few minutes all the lines would have been zigzag to an extraordinary degree, with here and there a loop formed. We may infer that this would have been the case, because five dots were made in the course of 31 m. (between 12.34 and 1.5 P.M.), and we see in the upper part of the diagram how crooked the course here is; if only the first and last dots had been joined we should have had a straight line. Exactly the same fact may be seen in the lines representing the course between 2.24 P.M. and 3 P.M., when six intermediate dots were made; and again at 4.46 and 4.50. But the result was widely different after 6 P.M.,—that is, after the great nocturnal descent had commenced; for though nine dots were then made in the course of 32 m., when these were joined (see Figure) the line thus formed was almost straight. The leaflets, therefore, begin to descend in the afternoon by zigzag lines, but as soon as the descent becomes rapid their whole energy is expended in thus moving, and their course becomes rectilinear. After the leaflets are completely asleep they move very little or not at all.

Fig. 150. Desmodium gyrans: circumnutation and nyctitropic movement of leaf (3 3/4 inches in length, petiole included) during 48 h. Filament affixed to midrib of terminal leaflet; its apex 6 inches from the vertical glass. Diagram reduced to one-third of original scale. Plant illuminated from above. Temp. 19°–20° C.

Had the above plant been subjected to a higher temperature than 67°–70° F., the movements of the terminal leaflet would probably have been even more rapid and wider in extent than those shown in the diagram; for a plant was kept for some time in the hot-house at from 92°–93° F., and in the course of 35 m. the apex of a leaflet twice descended and once ascended, travelling over a space of 1.2 inch in a vertical direction and of .82 inch in a horizontal direction. Whilst thus moving the leaflet also rotated on its own axis (and this was a point to which no attention had been before paid), for the plane of the blade differed by 41° after an interval of only a few minutes. Occasionally the leaflet stood still for a short time. There was no jerking movement, which is so characteristic of the little lateral leaflets. A sudden and considerable fall of temperature causes the terminal leaflet to sink downwards; thus a cut-off leaf was immersed in water at 95° F., which was slowly raised to 103° F., and afterwards allowed to sink to 70° F., and the sub-petiole of the terminal leaflet then curved downwards. The water was afterwards raised to 120° F., and the sub-petiole straightened itself. Similar experiments with leaves in water were twice repeated, with nearly the same result. It should be added, that water raised to even 122° F. does not soon kill a leaf. A plant was placed in darkness at 8.37 A.M., and at 2 P.M. (i.e. after 5 h. 23 m.), though the leaflets had sunk considerably, they had by no means acquired their nocturnal vertically dependent position. Pfeffer, on the other hand, says[^[16]] that this occurred with him in from 3/4 h. to 2 h.; perhaps the difference in our results may be due to the plant on which we experimented being a very young and vigorous seedling.

[16] ‘Die Period. Beweg.,’ p. 39.

The Movements of the little Lateral Leaflets .—These have been so often described, that we will endeavour to be as brief as possible in giving a few new facts and conclusions. The leaflets sometimes quickly change their position by as much as nearly 180°; and their sub-petioles can then be seen to become greatly curved. They rotate on their own axes, so that their upper surfaces are directed to all points of the compass. The figure described by the apex is an irregular oval or ellipse. They sometimes remain stationary for a period. In these several respects there is no difference, except in rapidity and extent, between their movements and the lesser ones performed by the large terminal leaflet whilst making its great oscillations. The movements of the little leaflets are much influenced, as is well known, by temperature. This was clearly shown by immersing leaves with motionless leaflets in cold water, which was slowly raised to 103° F., and the leaflets then moved quickly, describing about a dozen little irregular circles in 40 m. By this time the water had become much cooler, and the movements became slower or almost ceased; it was then raised to 100° F., and the leaflets again began to move quickly. On another occasion a tuft of fine leaves was immersed in water at 53° F., and the leaflets were of course motionless. The water was raised to 99°, and the leaflets soon began to move; it was raised to 105°, and the movements became much more rapid; each little circle or oval being completed in from 1 m. 30 s. to 1 m. 45 s. There was, however, no jerking, and this fact may perhaps be attributed to the resistance of the water.

Sachs states that the leaflets do not move until the surrounding air is as high as 71°–72° F., and this agrees with our experience on full-grown, or nearly full-grown, plants. But the leaflets of young seedlings exhibit a jerking movement at much lower temperatures. A seedling was kept (April 16th) in a room for half the day where the temperature was steady at 64° F., and the one leaflet which it bore was continually jerking, but not so rapidly as in the hot-house. The pot was taken in the evening into a bed-room where the temperature remained at 62° during nearly the whole night; at 10 and 11 P.M. and at 1 A.M. the leaflet was still jerking rapidly; at 3.30 A.M. it was not seen to jerk, but was observed during only a short time. It was, however, now inclined at a much lower angle than that occupied at 1 A.M. At 6.30 A.M. (temp. 61° F.) its inclination was still less than before, and again less at 6.45 A.M.; by 7.40 A.M. it had risen, and at 8.30 A.M. was again seen to jerk. This leaflet, therefore, was moving during the whole night, and the movement was by jerks up to 1 A.M. (and possibly later) and again at 8.30 A.M., though the temperature was only 61° to 62° F. We must therefore conclude that the lateral leaflets produced by young plants differ somewhat in constitution from those on older plants.

In the large genus Desmodium by far the greater number of the species are trifoliate; but some are unifoliate, and even the same plant may bear uni- and trifoliate leaves. In most of the species the lateral leaflets are only a little smaller than the terminal one. Therefore the lateral leaflets of D. gyrans (see Fig. 148) must be considered as almost rudimentary. They are also rudimentary in function, if this expression may be used; for they certainly do not sleep like the full-sized terminal leaflets. It is, however, possible that the sinking down of the leaflets between 1 A.M. and 6.45 A.M., as above described, may represent sleep. It is well known that the leaflets go on jerking during the early part of the night; but my gardener observed (Oct. 13th) a plant in the hot-house between 5 and 5.30 A.M., the temperature having been kept up to 82° F., and found that all the leaflets were inclined, but he saw no jerking movement until 6.55 A.M., by which time the terminal leaflet had risen and was awake. Two days afterwards (Oct. 15th) the same plant was observed by him at 4.47 A.M. (temp. 77° F.), and he found that the large terminal leaflets were awake, though not quite horizontal; and the only cause which we could assign for this anomalous wakefulness was that the plant had been kept for experimental purposes during the previous day at an unusually high temperature; the little lateral leaflets were also jerking at this hour, but whether there was any connection between this latter fact and the sub-horizontal position of the terminal leaflets we do not know. Anyhow, it is certain that the lateral leaflets do not sleep like the terminal leaflets; and in so far they may be said to be in a functionally rudimentary condition. They are in a similar condition in relation to irritability; for if a plant be shaken or syringed, the terminal leaflets sink down to about 45° beneath the horizon; but we could never detect any effect thus produced on the lateral leaflets; yet we are not prepared to assert positively that rubbing or pricking the pulvinus produces no effect.

As in the case of most rudimentary organs, the leaflets are variable in size; they often depart from their normal position and do not stand opposite one another; and one of the two is frequently absent. This absence appeared in some, but not in all the cases, to be due to the leaflet having become completely confluent with the main petiole, as might be inferred from the presence of a slight ridge along its upper margin, and from the course of the vessels. In one instance there was a vestige of the leaflet, in the shape of a minute point, at the further end of the ridge. The frequent, sudden and complete disappearance of one or both of the rudimentary leaflets is a rather singular fact; but it is a much more surprising one that the leaves which are first developed on seedling plants are not provided with them. Thus, on one seedling the seventh leaf above the cotyledons was the first which bore any lateral leaflets, and then only a single one. On another seedling, the eleventh leaf first bore a leaflet; of the nine succeeding leaves five bore a single lateral leaflet, and four bore none at all; at last a leaf, the twenty-first above the cotyledons, was provided with two rudimentary lateral leaflets. From a widespread analogy in the animal kingdom, it might have been expected that these rudimentary leaflets would have been better developed and more regularly present on very young than on older plants. But bearing in mind, firstly, that long-lost characters sometimes reappear late in life, and secondly, that the species of Desmodium are generally trifoliate, but that some are unifoliate, the suspicion arises that D. gyrans is descended from a unifoliate species, and that this was descended from a trifoliate one; for in this case both the absence of the little lateral leaflets on very young seedlings, and their subsequent appearance, may be attributed to reversion to more or less distant progenitors.[^[17]]

[17] Desmodium vespertilionis is closely allied to D. gyrans, and it seems only occasionally to bear rudimentary lateral leaflets. Duchartre, ‘Eléments de Botanique,’ 1867, p. 353.

No one supposes that the rapid movements of the lateral leaflets of ‘D. gyrans’ are of any use to the plant; and why they should behave in this manner is quite unknown. We imagined that their power of movement might stand in some relation with their rudimentary condition, and therefore observed the almost rudimentary leaflets of Mimosa albida vel sensitiva (of which a drawing will hereafter be given, Fig. 159); but they exhibited no extraordinary movements, and at night they went to sleep like the full-sized leaflets. There is, however, this remarkable difference in the two cases; in Desmodium the pulvinus of the rudimentary leaflets has not been reduced in length, in correspondence with the reduction of the blade, to the same extent as has occurred in the Mimosa; and it is on the length and degree of curvature of the pulvinus that the amount of movement of the blade depends. Thus the average length of the pulvinus in the large terminal leaflets of Desmodium is 3 mm., whilst that of the rudimentary leaflets is 2.86 mm.; so that they differ only a little in length. But in diameter they differ much, that of the pulvinus of the little leaflets being only 0.3 mm. to 0.4 mm.; whilst that of the terminal leaflets is 1.33 mm. If we now turn to the Mimosa, we find that the average length of the pulvinus of the almost rudimentary leaflets is only 0.466 mm., or rather more than a quarter of the length of the pulvinus of the full-sized leaflets, namely, 1.66 mm. In this small reduction in length of the pulvinus of the rudimentary leaflets of Desmodium, we apparently have the proximate cause of their great and rapid circumnutating movement, in contrast with that of the almost rudimentary leaflets of the Mimosa. The small size and weight of the blade, and the little resistance opposed by the air to its movement, no doubt also come into play; for we have seen that these leaflets if immersed in water, when the resistance would be much greater, were prevented from jerking forwards. Why, during the reduction of the lateral leaflets of Desmodium, or during their reappearance—if they owe their origin to reversion—the pulvinus should have been so much less affected than the blade, whilst with the Mimosa the pulvinus has been greatly reduced, we do not know. Nevertheless, it deserves notice that the reduction of the leaflets in these two genera has apparently been effected by a different process and for a different end; for with the Mimosa the reduction of the inner and basal leaflets was necessary from the want of space; but no such necessity exists with Desmodium, and the reduction of its lateral leaflets seems to have been due to the principle of compensation, in consequence of the great size of the terminal leaflet. Uraria (Tribe 6) and Centrosema (Tribe 8).—The leaflets of Uraria lagopus and the leaves of a Centrosema from Brazil both sink vertically down at night. In the latter plant the petiole at the same time rose 16½°.

Amphicarpoea monoica (Tribe 8).—The leaflets sink down vertically at night, and the petioles likewise fall considerably. A petiole, which was carefully observed, stood during the day 25° above the horizon and at night 32° below it; it therefore fell 57°. A filament was fixed transversely across the terminal leaflet of a fine young leaf (2 1/4 inches in length including the petiole), and the movement of the whole leaf was traced on a vertical glass. This was a bad plan in some respects, because the rotation of the leaflet, independently of its rising or falling, raised and depressed the filament; but it was the best plan for our special purpose of observing whether the leaf moved much after it had gone to sleep. The plant had twined closely round a thin stick, so that the circumnutation of the stem was prevented. The movement of the leaf was traced during 48 h., from 9 A.M. July 10th to 9 A.M. July 12th. In the figure given (Fig. 151) we see how complicated its course was on both days: during the second day it changed its course greatly 13 times. The leaflets began to go to sleep a little after 6 P.M., and by 7.15 P.M. hung vertically down and were completely asleep; but on both nights they continued to move from 7.15 P.M. to 10.40 and 10.50 P.M., quite as much as during the day; and this was the point which we wished to ascertain. We see in the figure that the great sinking movement late in the evening does not differ essentially from the circumnutation during the day.

Fig. 151. Amphicarpoea monoica: circumnutation and nyctitropic movement of leaf during 48 h.; its apex 9 inches from the vertical glass. Figure reduced to one-third of original scale. Plant illuminated from above; temp 17½°–18½° C.

Glycine hispida (Tribe 8).—The three leaflets sink vertically down at night.

Erythrina (Tribe 8).—Five species were observed, and the leaflets of all sank vertically down at night; with E. caffra and with a second unnamed species, the petioles at the same time rose slightly. The movements of the terminal leaflet of E. crista-galli (with the main petiole secured to a stick) were traced from 6.40 A.M. June 8th, to 8 A.M. on the 10th. In order to observe the nyctitropic movements of this plant, it is necessary that it should have grown in a warm greenhouse, for out of doors in our climate it does not sleep. We see in the tracing (Fig. 152) that the leaflet oscillated twice up and down between early morning and noon; it then fell greatly, afterwards rising till 3 P.M. At this latter hour the great nocturnal fall commenced. On the second day (of which the tracing is not given) there was exactly the same double oscillation before noon, but only a very small one in the afternoon. On the third morning the leaflet moved laterally, which was due to its beginning to assume an oblique position, as seems invariably to occur with the leaflets of this species as they grow old. On both nights after the leaflets were asleep and hung vertically down, they continued to move a little both up and down, and from side to side.

Erythrina caffra.—A filament was fixed transversely across a terminal leaflet, as we wished to observe its movements when asleep. The plant was placed in the morning of June 10th under a skylight, where the light was not bright; and we do not know whether it was owing to this cause or to the plant having been disturbed, but the leaflet hung vertically down all day; nevertheless it circumnutated in this position, describing a figure which represented two irregular ellipses. On the next day it circumnutated in a greater degree, describing four irregular ellipses, and by 3 P.M. had risen into a horizontal position. By 7.15 P.M. it was asleep and vertically dependent, but continued to circumnutate as long as observed, until 11 P.M.

Fig. 152. Erythrina crista-galli: circumnutation and nyctitropic movement of terminal leaflet, 3 3/4 inches in length, traced during 25 h.; apex of leaf 3½ inches from the vertical glass. Figure reduced to one-half of original scale. Plant illuminated from above; temp. 17½°–18½° C.

Erythrina corallodendron.—The movements of a terminal leaflet were traced. During the second day it oscillated four times up and four times down between 8 A.M. and 4 P.M., after which hour the great nocturnal fall commenced. On the third day the movement was equally great in amplitude, but was remarkably simple, for the leaflet rose in an almost perfectly straight line from 6.50 A.M. to 3 P.M., and then sank down in an equally straight line until vertically dependent and asleep.

Apios tuberosa (Tribe 8).—The leaflets sink vertically down at night.

Phaseolus vulgaris (Tribe 8).—The leaflets likewise sink vertically down at night. In the greenhouse the petiole of a young leaf rose 16°, and that of an older leaf 10° at night. With plants growing out of doors the leaflets apparently do not sleep until somewhat late in the season, for on the nights of July 11th and 12th none of them were asleep; whereas on the night of August 15th the same plants had most of their leaflets vertically dependent and asleep. With Ph. caracalla and Hernandesii, the primary unifoliate leaves and the leaflets of the secondary trifoliate leaves sink vertically down at night. This holds good with the secondary trifoliate leaves of Ph. Roxburghii, but it is remarkable that the primary unifoliate leaves which are much elongated, rise at night from about 20° to about 60° above the horizon. With older seedlings, however, having the secondary leaves just developed, the primary leaves stand in the middle of the day horizontally, or are deflected a little beneath the horizon. In one such case the primary leaves rose from 26° beneath the horizon at noon, to 20° above it at 10 P.M.; whilst at this same hour the leaflets of the secondary leaves were vertically dependent. Here, then, we have the extraordinary case of the primary and secondary leaves on the same plant moving at the same time in opposite directions.

We have now seen that the leaflets in the six genera of Phaseoleae observed by us (with the exception of the primary leaves of Phaseolus Roxburghii) all sleep in the same manner, namely, by sinking vertically down. The movements of the petioles were observed in only three of these genera. They rose in Centrosema and Phaseolus, and sunk in Amphicarpæa.

Sophora chrysophylla (Tribe 10).—The leaflets rise at night, and are at the same time directed towards the apex of the leaf, as in Mimosa pudica.

Caesalpinia, Hoematoxylon, Gleditschia, Poinciana.—The leaflets of two species of Caesalpinia (Tribe 13) rose at night. With Haematoxylon Campechianum (Tribe 13) the leaflets move forwards at night, so that their midribs stand parallel to the petiole, and their now vertical lower surfaces are turned outwards (Fig. 153). The petiole sinks a little. In Gleditschia, if we understand correctly Duchartre’s description, and in Poinciana Gilliesii (both belonging to Tribe 13), the leaves behave in the same manner.

Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B, branch with leaves asleep, reduced to two-thirds of natural scale.

Cassia (Tribe 14).—The nyctitropic movements of the leaves in many species in this genus are closely alike, and are highly complex. They were first briefly described by Linnæus, and since by Duchartre. Our observations were made chiefly on C. floribunda[^[18]] and corymbosa, but several other species were casually observed. The horizontally extended leaflets sink down vertically at night; but not simply, as in so many other genera, for each leaflet rotates on its own axis, so that its lower surface faces outwards. The upper surfaces of the opposite leaflets are thus brought into contact with one another beneath the petiole, and are well protected (Fig. 154). The rotation and other movements are effected by means of a well-developed pulvinus at the base of each leaflet, as could be plainly seen when a straight narrow black line had been painted along it during the day. The two terminal leaflets in the daytime include rather less than a right angle; but their divergence increases greatly whilst they sink downwards and rotate, so that they stand laterally at night, as may be seen in the figure. Moreover, they move somewhat backwards, so as to point towards the base of the petiole. In one instance we found that the midrib of a terminal leaflet formed at night an angle of 36°, with a line dropped perpendicularly from the end of the petiole. The second pair of leaflets likewise moves a little backwards, but less than the terminal pair; and the third pair moves vertically downwards, or even a little forwards. Thus all the leaflets, in those species which bear only 3 or 4 pairs, tend to form a single packet, with their upper surfaces in contact, and their lower surfaces turned outwards. Lastly, the main petiole rises at night, but with leaves of different ages to very different degrees, namely some rose through an angle of only 12°, and others as much as 41°.

[18] I am informed by Mr. Dyer that Mr. Bentham believes that C. floribunda (a common greenhouse bush) is a hybrid raised in France, and that it comes very near to C. laevigata. It is no doubt the same as the form described by Lindley (‘Bot. Reg.,’ Tab. 1422) as C. Herbertiana.

Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at night. Both figures copied from photographs.

Cassia calliantha.—The leaves bear a large number of leaflets, which move at night in nearly the same manner as just described; but the petioles apparently do not rise, and one which was carefully observed certainly fell 3°. Cassia pubescens.—The chief difference in the nyctitropic movements of this species, compared with those of the former species, consists in the leaflets not rotating nearly so much; therefore their lower surfaces face but little outwards at night. The petioles, which during the day are inclined only a little above the horizon, rise at night in a remarkable manner, and stand nearly or quite vertically. This, together with the dependent position of the leaflets, makes the whole plant wonderfully compact at night. In the two foregoing figures, copied from photographs, the same plant is represented awake and asleep (Fig. 155), and we see how different is its appearance.

Fig. 155. Cassia pubescens: A, upper part of plant during the day; B, same plant at night. Figures reduced from photographs.

Cassia mimosoides.—At night the numerous leaflets on each leaf rotate on their axes, and their tips move towards the apex of the leaf; they thus become imbricated with their lower surfaces directed upwards, and with their midribs almost parallel to the petiole. Consequently, this species differs from all the others seen by us, with the exception of the following one, in the leaflets not sinking down at night. A petiole, the movement of which was measured, rose 8° at night.

Cassia Barclayana.—The leaflets of this Australian species are numerous, very narrow, and almost linear. At night they rise up a little, and also move towards the apex of the leaf. For instance, two opposite leaflets which diverged from one another during the day at an angle of 104°, diverted at night only 72°; so that each had risen 16° above its diurnal position. The petiole of a young leaf rose at night 34°, and that of an older leaf 19°. Owing to the slight movement of the leaflets and the considerable movement of the petiole, the bush presents a different appearance at night to what it does by day; yet the leaves can hardly be said to sleep.

The circumnutating movements of the leaves of C. floribunda, calliantha, and pubescens were observed, each during three or four days; they were essentially alike, those of the last-named species being the simplest. The petiole of C. floribunda was secured to a stick at the base of the two terminal leaflets, and a filament was fixed along the midrib of one of them. Its movements were traced from 1 P.M. on August 13th to 8.30 A.M. 17th; but those during the last 2 h. are alone given in Fig. 156. From 8 A.M. on each day (by which hour the leaf had assumed its diurnal position) to 2 or 3 P.M., it either zigzagged or circumnutated over nearly the same small space; at between 2 and 3 P.M. the great evening fall commenced. The lines representing this fall and the early morning rise are oblique, owing to the peculiar manner in which the leaflets sleep, as already described. After the leaflet was asleep at 6 P.M., and whilst the glass filament hung perpendicularly down, the movement of its apex was traced until 10.30 P.M.; and during this whole time it swayed from side to side, completing more than one ellipse.

Fig 156. Cassia floribunda: circumnutation and nyctitropic movement of a terminal leaflet (1 5/6 inch in length) traced from 8.30 A.M. to same hour on following morning. Apex of leaflet 5½ inches from the vertical glass. Main petiole 3 3/4 inches long. Temp. 16°–17½° C. Figure reduced to one-half of the original scale.

Bauhinia (Tribe 15).—The nyctitropic movements of four species were alike, and were highly peculiar. A plant raised from seed sent us from South Brazil by Fritz Müller, was more especially observed. The leaves are large and deeply notched at their ends. At night the two halves rise up and close completely together, like the opposite leaflets of many Leguminosae. With very young plants the petioles rise considerably at the same time; one, which was inclined at noon 45° above the horizon, at night stood at 75°; it thus rose 30°; another rose 34°. Whilst the two halves of the leaf are closing, the midrib at first sinks vertically downwards and afterwards bends backwards, so as to pass close along one side of its own upwardly inclined petiole; the midrib being thus directed towards the stem or axis of the plant. The angle which the midrib formed with the horizon was measured in one case at different hours: at noon it stood horizontally; late in the evening it depended vertically; then rose to the opposite side, and at 10.15 P.M. stood at only 27° beneath the horizon, being directed towards the stem. It had thus travelled through 153°. Owing to this movement—to the leaves being folded—and to the petioles rising, the whole plant is as much more compact at night than during the day, as a fastigiate Lombardy poplar is compared with any other species of poplar. It is remarkable that when our plants had grown a little older, viz., to a height of 2 or 3 feet, the petioles did not rise at night, and the midribs of the folded leaves were no longer bent back along one side of the petiole. We have noticed in some other genera that the petioles of very young plants rise much more at night than do those of older plants.

Tamarindus Indica (Tribe 16).—The leaflets approach or meet each other at night, and are all directed towards the apex of the leaf. They thus become imbricated with their midribs parallel to the petiole. The movement is closely similar to that of Haematoxylon (see Fig. 153), but more striking from the greater number of the leaflets.

Adenanthera, Prosopis, and Neptunia (Tribe 20).—With Adenanthera pavonia the leaflets turn edgeways and sink at night. In Prosopis they turn upwards. With Neptunia oleracea the leaflets on the opposite sides of the same pinna come into contact at night and are directed forwards. The pinnae themselves move downwards, and at the same time backwards or towards the stem of the plant. The main petiole rises.

Mimosa pudica (Tribe 20).—This plant has been the subject of innumerable observations; but there are some points in relation to our subject which have not been sufficiently attended to. At night, as is well known, the opposite leaflets come into contact and point towards the apex of the leaf; they thus become neatly imbricated with their upper surfaces protected. The four pinnae also approach each other closely, and the whole leaf is thus rendered very compact. The main petiole sinks downwards during the day till late in the evening, and rises until very early in the morning. The stem is continually circumnutating at a rapid rate, though not to a wide extent. Some very young plants, kept in darkness, were observed during two days, and although subjected to a rather low temperature of 57°–59° F., the stem of one described four small ellipses in the course of 12 h. We shall immediately see that the main petiole is likewise continually circumnutating, as is each separate pinna and each separate leaflet. Therefore, if the movement of the apex of any one leaflet were to be traced, the course described would be compounded of the movements of four separate parts.

A filament had been fixed on the previous evening, longitudinally to the main petiole of a nearly full-grown, highly-sensitive leaf (four inches in length), the stem having been secured to a stick at its base; and a tracing was made on a vertical glass in the hot-house under a high temperature. In the figure given (Fig. 157), the first dot was made at 8.30 A.M. August 2nd, and the last at 7 P.M. on the 3rd. During 12 h. on the first day the petiole moved thrice downwards and twice upwards. Within the same length of time on the second day, it moved five times downwards and four times upwards. As the ascending and descending lines do not coincide, the petiole manifestly circumnutates; the great evening fall and nocturnal rise being an exaggeration of one of the circumnutations. It should, however, be observed that the petiole fell much lower down in the evenings than could be seen on the vertical glass or is represented in the diagram. After 7 P.M. on the 3rd (when the last dot in Fig. 157 was made) the pot was carried into a bed-room, and the petiole was found at 12.50 A.M. (i.e. after midnight) standing almost upright, and much more highly inclined than it was at 10.40 P.M. When observed again at 4 A.M. it had begun to fall, and continued falling till 6.15 A.M., after which hour it zigzagged and again circumnutated. Similar observations were made on another petiole, with nearly the same result.

Fig. 157 Mimosa pudica: circumnutation and nyctitropic movement of main petiole, traced during 34 h. 30 m.

On two other occasions the movement of the main petiole was observed every two or three minutes, the plants being kept at a rather high temperature, viz., on the first occasion at 77°–81° F., and the filament then described 2½ ellipses in 69 m. On the second occasion, when the temperature was 81°–86° F., it made rather more than 3 ellipses in 67 m. therefore, Fig. 157, though now sufficiently complex, would have been incomparably more so, if dots had been made on the glass every 2 or 3 minutes, instead of every hour or half-hour. Although the main petiole is continually and rapidly describing small ellipses during the day, yet after the great nocturnal rising movement has commenced, if dots are made every 2 or 3 minutes, as was done for an hour between 9.30 and 10.30 P.M. (temp. 84° F.), and the dots are then joined, an almost absolutely straight line is the result.

To show that the movement of the petiole is in all probability due to the varying turgescence of the pulvinus, and not to growth (in accordance with the conclusions of Pfeffer), a very old leaf, with some of its leaflets yellowish and hardly at all sensitive, was selected for observation, and the plant was kept at the highly favourable temp. of 80° F. The petiole fell from 8 A.M. till 10.15 A.M., it then rose a little in a somewhat zigzag line, often remaining stationary, till 5 P.M., when the great evening fall commenced, which was continued till at least 10 P.M. By 7 A.M. on the following morning it had risen to the same level as on the previous morning, and then descended in a zigzag line. But from 10.30 A.M. till 4.15 P.M. it remained almost motionless, all power of movement being now lost. The petiole, therefore, of this very old leaf, which must have long ceased growing, moved periodically; but instead of circumnutating several times during the day, it moved only twice down and twice up in the course of 24 h., with the ascending and descending lines not coincident.

It has already been stated that the pinnae move independently of the main petiole. The petiole of a leaf was fixed to a cork support, close to the point whence the four pinnae diverge, with a short fine filament cemented longitudinally to one of the two terminal pinnae, and a graduated semicircle was placed close beneath it. By looking vertically down, its angular or lateral movements could be measured with accuracy. Between noon and 4.15 P.M. the pinna changed its position to one side by only 7°; but not continuously in the same direction, as it moved four times to one side, and three times to the opposite side, in one instance to the extent of 16°. This pinna, therefore circumnutated. Later in the evening the four pinnae approach each other, and the one which was observed moved inwards 59° between noon and 6.45 P.M. Ten observations were made in the course of 2 h. 20 m. (at average intervals of 14 m.), between 4.25 and 6.45 P.M.; and there was now, when the leaf was going to sleep, no swaying from side to side, but a steady inward movement. Here therefore there is in the evening the same conversion of a circumnutating into a steady movement in one direction, as in the case of the main petiole.

It has also been stated that each separate leaflet circumnutates. A pinna was cemented with shellac on the summit of a little stick driven firmly into the ground, immediately beneath a pair of leaflets, to the midribs of both of which excessively fine glass filaments were attached. This treatment did not injure the leaflets, for they went to sleep in the usual manner, and long retained their sensitiveness. the movements of one of them were traced during 49 h., as shown in Fig. 158. On the first day the leaflet sank down till 11.30 A.M., and then rose till late in the evening in a zigzag line, indicating circumnutation. On the second day, when more accustomed to its new state, it oscillated twice up and twice down during the 24 h. This plant was subjected to a rather low temperature, viz., 62°–64° F.; had it been kept warmer, no doubt the movements of the leaflet would have been much more rapid and complicated. It may be seen in the diagram that the ascending and descending lines do not coincide; but the large amount of lateral movement in the evening is the result of the leaflets bending towards the apex of the leaf when going to sleep. Another leaflet was casually observed, and found to be continually circumnutating during the same length of time.

The circumnutation of the leaves is not destroyed by their being subjected to moderately long continued darkness; but the proper periodicity of their movements is lost. Some very young seedlings were kept during two days in the dark (temp. 57°–59° F.) except when the circumnutation of their stems was occasionally observed; and on the evening of the second day the leaflets did not fully and properly go to sleep. The pot was then placed for three days in a dark cupboard, under nearly the same temperature, and at the close of this period the leaflets showed no signs of sleeping, and were only slightly sensitive to a touch. On the following day the stem was cemented to a stick, and the movements of two leaves were traced on a vertical glass during 72 h. The plants were still kept in the dark, excepting that at each observation, which lasted 3 or 4 minutes, they were illuminated by two candles. On the third day the leaflets still exhibited a vestige of sensitiveness when forcibly pressed, but in the evening they showed no signs of sleep. Nevertheless, their petioles continued to circumnutate distinctly, although the proper order of their movements in relation to the day and night was wholly lost. Thus, one leaf descended during the first two nights (i.e. between 10 P.M. and 7 A.M. next morning) instead of ascending, and on the third night it moved chiefly in a lateral direction. The second leaf behaved in an equally abnormal manner, moving laterally during the first night, descending greatly during the second, and ascending to an unusual height during the third night.

Fig 158. Mimosa pudica: circumnutation and nyctitropic movement of a leaflet (with pinna secured), traced on a vertical glass, from 8 A.M. Sept. 14th to 9 A.M. 16th.

With plants kept at a high temperature and exposed to the light, the most rapid circumnutating movement of the apex of a leaf which was observed, amounted to 1/500 of an inch in one second; and this would have equalled 1/8 of an inch in a minute, had not the leaf occasionally stood still. The actual distance travelled by the apex (as ascertained by a measure placed close to the leaf) was on one occasion nearly 3/4 of an inch in a vertical direction in 15 m.; and on another occasion 5/8 of an inch in 60 m.; but there was also some lateral movement.

Mimosa albida.[^[19]]—The leaves of this plant, one of which is here figured (Fig. 159) reduced to 2/3 of the natural size, present some interesting peculiarities. It consists of a long petiole bearing only two pinnae (here represented as rather more divergent than is usual), each with two pairs of leaflets. But the inner basal leaflets are greatly reduced in size, owing probably to the want of space for their full development, so that they may be considered as almost rudimentary. They vary somewhat in size, and both occasionally disappear, or only one. Nevertheless, they are not in the least rudimentary in function, for they are sensitive, extremely heliotropic, circumnutate at nearly the same rate as the fully developed leaflets, and assume when asleep exactly the same position. With M. pudica the inner leaflets at the base and between the pinnae are likewise much shortened and obliquely truncated; this fact was well seen in some seedlings of M. pudica, in which the third leaf above the cotyledons bore only two pinnae, each with only 3 or 4 pairs of leaflets, of which the inner basal one was less than half as long as its fellow; so that the whole leaf resembled pretty closely that of M. albida. In this latter species the main petiole terminates in a little point, and on each side of this there is a pair of minute, flattened, lancet-shaped projections, hairy on their margins, which drop off and disappear soon after the leaf is fully developed. There can hardly be a doubt that these little projections are the last and fugacious representatives of an additional pair of leaflets to each pinna; for the outer one is twice as broad as the inner one, and a little longer, viz. 7/100 of an inch, whilst the inner one is only 5/100–6/100 long. Now if the basal pair of leaflets of the existing leaves were to become rudimentary, we should expect that the rudiments would still exhibit some trace of their present great inequality of size. The conclusion that the pinnae of the parent-form of M. albida possessed at least three pairs of leaflets, instead of, as at present, only two, is supported by the structure of the first true leaf; for this consists of a simple petiole, often bearing three pairs of leaflets. This latter fact, as well as the presence of the rudiments, both lead to the conclusion that M. albida is descended from a form the leaves of which bore more than two pairs of leaflets. The second leaf above the cotyledons resembles in all respects the leaves on fully developed plants.

[19] Mr. Thiselton Dyer informs us that this Peruvian plant (which was sent to us from Kew) is considered by Mr. Bentham (‘Trans. Linn. Soc.,’ vol. xxx. p. 390) to be “the species or variety which most commonly represents the M. sensitiva of our gardens.”

Fig. 159. Mimosa albida: leaf seen from vertically above.

When the leaves go to sleep, each leaflet twists half round, so as to present its edge to the zenith, and comes into close contact with its fellow. The pinnae also approach each other closely, so that the four terminal leaflets come together. The large basal leaflets (with the little rudimentary ones in contact with them) move inwards and forwards, so as to embrace the outside of the united terminal leaflets, and thus all eight leaflets (the rudimentary ones included) form together a single vertical packet. The two pinnae at the same time that they approach each other sink downwards, and thus instead of extending horizontally in the same line with the main petiole, as during the day, they depend at night at about 45°, or even at a greater angle, beneath the horizon. The movement of the main petiole seems to be variable; we have seen it in the evening 27° lower than during the day; but sometimes in nearly the same position. Nevertheless, a sinking movement in the evening and a rising one during the night is probably the normal course, for this was well-marked in the petiole of the first-formed true leaf.

The circumnutation of the main petiole of a young leaf was traced during 2 3/4 days, and was considerable in extent, but less complex than that of M. pudica. The movement was much more lateral than is usual with circumnutating leaves, and this was the sole peculiarity which it presented. The apex of one of the terminal leaflets was seen under the microscope to travel 1/50 of an inch in 3 minutes.

Mimosa marginata.—The opposite leaflets rise up and approach each other at night, but do not come into close contact, except in the case of very young leaflets on vigorous shoots. Full-grown leaflets circumnutate during the day slowly and on a small scale.

Schrankia uncinata (Tribe 20).—A leaf consists of two or three pairs of pinnae, each bearing many small leaflets. These, when the plant is asleep, are directed forwards and become imbricated. The angle between the two terminal pinnae was diminished at night, in one case by 15°; and they sank almost vertically downwards. The hinder pairs of pinnae likewise sink downwards, but do not converge, that is, move towards the apex of the leaf. The main petiole does not become depressed, at least during the evening. In this latter respect, as well as in the sinking of the pinnae, there is a marked difference between the nyctitropic movements of the present plant and of Mimosa pudica. It should, however, be added that our specimen was not in a very vigorous condition. The pinnae of Schrankia aculeata also sink at night.

Acacia Farnesiana (Tribe 22).—The different appearance presented by a bush of this plant when asleep and awake is wonderful. The same leaf in the two states is shown in the following figure (Fig. 160). The leaflets move towards the apex of the pinna and become imbricated, and the pinnae then look like bits of dangling string. The following remarks and measurements do not fully apply to the small leaf here figured. The pinnae move forwards and at the same time sink downwards, whilst the main petiole rises considerably. With respect to the degree of movement: the two terminal pinnae of one specimen formed together an angle of 100° during the day, and at night of only 38°, so each had moved 31° forwards. The penultimate pinnae during the day formed together an angle of 180°, that is, they stood in a straight line opposite one another, and at night each had moved 65° forwards. The basal pair of pinnae were directed during the day, each about 21° backwards, and at night 38° forwards, so each had moved 59° forwards. But the pinnae at the same time sink greatly, and sometimes hang almost perpendicularly downwards. The main petiole, on the other hand, rises much: by 8.30 P.M. one stood 34° higher than at noon, and by 6.40 A.M. on the following morning it was still higher by 10°; shortly after this hour the diurnal sinking movement commenced. The course of a nearly full-grown leaf was traced during 14 h.; it was strongly zigzag, and apparently represented five ellipses, with their longer axes differently directed.

Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf at night.

Albizzia lophantha (Tribe 23).—The leaflets at night come into contact with one another, and are directed towards the apex of the pinna. The pinnae approach one another, but remain in the same plane as during the day; and in this respect they differ much from those of the above Schrankia and Acacia. The main petiole rises but little. The first-formed leaf above the cotyledons bore 11 leaflets on each side, and these slept like those on the subsequently formed leaves; but the petiole of this first leaf was curved downwards during the day and at night straightened itself, so that the chord of its arc then stood 16° higher than in the day-time.

Melaleuca ericaefolia (Myrtaceae).—According to Bouché (‘Bot. Zeit.,’ 1874, p. 359) the leaves sleep at night, in nearly the same manner as those of certain species of Pimelia.

Œnothera mollissima (Onagrarieae).—According to Linnæus (‘Somnus Plantarum’), the leaves rise up vertically at night.

Passiflora gracilis (Passifloracae).—The young leaves sleep by their blades hanging vertically downwards, and the whole length of the petiole then becomes somewhat curved downwards. Externally no trace of a pulvinus can be seen. The petiole of the uppermost leaf on a young shoot stood at 10.45 A.M. at 33° above the horizon; and at 10.30 P.M., when the blade was vertically dependent, at only 15°, so the petiole had fallen 18°. That of the next older leaf fell only 7°. From some unknown cause the leaves do not always sleep properly. The stem of a plant, which had stood for some time before a north-east window, was secured to a stick at the base of a young leaf, the blade of which was inclined at 40° below the horizon. From its position the leaf had to be viewed obliquely, consequently the vertically ascending and descending movements appeared when traced oblique. On the first day (Oct. 12th) the leaf descended in a zigzag line until late in the evening; and by 8.15 A.M. on the 13th had risen to nearly the same level as on the previous morning. A new tracing was now begun (Fig. 161). The leaf continued to rise until 8.50 A.M., then moved a little to the right, and afterwards descended. Between 11 A.M. and 5 P.M. it circumnutated, and after the latter hour the great nocturnal fall commenced. At 7.15 P.M. it depended vertically. The dotted line ought to have been prolonged much lower down in the figure. By 6.50 A.M. on the following morning (14th) the leaf had risen greatly, and continued to rise till 7.50 A.M., after which hour it redescended. It should be observed that the lines traced on this second morning would have coincided with and confused those previously traced, had not the pot been slided a very little to the left. In the evening (14th) a mark was placed behind the filament attached to the apex of the leaf, and its movement was carefully traced from 5 P.M. to 10.15 P.M. Between 5 and 7.15 P.M. the leaf descended in a straight line, and at the latter hour it appeared vertically dependent. But between 7.15 and 10.15 P.M. the line consisted of a succession of steps, the cause of which we could not understand; it was, however, manifest that the movement was no longer a simple descending one.

Fig. 161. Passiflora gracilis: circumnutation and nyctitropic movement of leaf, traced on vertical glass, from 8.20 A.M. Oct. 13th to 10 A.M. 14th. Figure reduced to two-thirds of original scale.

Siegesbeckia orientalis (Compositæ).—Some seedlings were raised in the middle of winter and kept in the hot-house; they flowered, but did not grow well, and their leaves never showed any signs of sleep. The leaves on other seedlings raised in May were horizontal at noon (June 22nd), and depended at a considerable angle beneath the horizon at 10 P.M. In the case of four youngish leaves which were from 2 to 2½ inches in length, these angles were found to be 50°, 56°, 60°, and 65°. At the end of August when the plants had grown to a height of 10 to 11 inches, the younger leaves were so much curved downwards at night that they might truly be said to be asleep. This is one of the species which must be well illuminated during the day in order to sleep, for on two occasions when plants were kept all day in a room with north-east windows, the leaves did not sleep at night. The same cause probably accounts for the leaves on our seedlings raised in the dead of the winter not sleeping. Professor Pfeffer informs us that the leaves of another species (S. Jorullensis ?) hang vertically down at night.

Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day, and asleep at night. Figures copied from photographs, and reduced.

Ipomœa caerulea and purpurea (Convolvulaceae).—The leaves on very young plants, a foot or two in height, are depressed at night to between 68° and 80° beneath the horizon; and some hang quite vertically downwards. On the following morning they again rise into a horizontal position. The petioles become at night downwardly curved, either through their entire length or in the upper part alone; and this apparently causes the depression of the blade. It seems necessary that the leaves should be well illuminated during the day in order to sleep, for those which stood on the back of a plant before a north-east window did not sleep.

Nicotiana tabacum (var. Virginian) and glauca (Solaneae).—The young leaves of both these species sleep by bending vertically upwards. Figures of two shoots of N. glauca, awake and asleep (Fig. 162), are given on p. 385: one of the shoots, from which the photographs were taken, was accidentally bent to one side.

Fig. 163. Nicotiana tabacum: circumnutation and nyctitropic movement of a leaf (5 inches in length), traced on a vertical glass, from 3 P.M. July 10th to 8.10 A.M. 13th. Apex of leaf 4 inches from glass. Temp. 17½°–18½° C. Figure reduced to one-half original scale.

At the base of the petiole of N. tabacum, on the outside, there is a mass of cells, which are rather smaller than elsewhere, and have their longer axes differently directed from the cells of the parenchyma, and may therefore be considered as forming a sort of pulvinus. A young plant of N. tabacum was selected, and the circumnutation of the fifth leaf above the cotyledons was observed during three days. On the first morning (July 10th) the leaf fell from 9 to 10 A.M., which is its normal course, but rose during the remainder of the day; and this no doubt was due to its being illuminated exclusively from above; for properly the evening rise does not commence until 3 or 4 P.M. In the figure as given on p. 386 (Fig. 163) the first dot was made at 3 P.M.; and the tracing was continued for the following 65 h. When the leaf pointed to the dot next above that marked 3 P.M. it stood horizontally. The tracing is remarkable only from its simplicity and the straightness of the lines. The leaf each day described a single great ellipse; for it should be observed that the ascending and descending lines do not coincide. On the evening of the 11th the leaf did not descend quite so low as usual, and it now zigzagged a little. The diurnal sinking movement had already commenced each morning by 7 A.M. The broken lines at the top of the figure, representing the nocturnal vertical position of the leaf, ought to be prolonged much higher up.

Mirabilis longiflora and jalapa (Nyctagineae).—The first pair of leaves above the cotyledons, produced by seedlings of both these species, were considerably divergent during the day, and at night stood up vertically in close contact with one another. The two upper leaves on an older seedling were almost horizontal by day, and at night stood up vertically, but were not in close contact, owing to the resistance offered by the central bud.

Polygonum aviculare (Polygoneae).—Professor Batalin informs us that the young leaves rise up vertically at night. This is likewise the case, according to Linnæus, with several species of Amaranthus (Amaranthaceae); and we observed a sleep movement of this kind in one member of the genus. Again, with Chenopodium album (Chenopodieae), the upper young leaves of some seedlings, about 4 inches in height, were horizontal or sub-horizontal during the day, and at 10 P.M. on March 7th were quite, or almost quite, vertical. Other seedlings raised in the greenhouse during the winter (Jan. 28th) were observed day and night, and no difference could be perceived in the position of their leaves. According to Bouché (‘Bot. Zeitung,’ 1874, p. 359) the leaves of Pimelia linoides and spectabilis (Thymeleae) sleep at night.

Euphorbia jacquiniaeflora (Euphorbiaceae).—Mr. Lynch called our attention to the fact that the young leaves of this plant sleep by depending vertically. The third leaf from the summit (March 11th) was inclined during the day 30° beneath the horizon, and at night hung vertically down, as did some of the still younger leaves. It rose up to its former level on the following morning. The fourth and fifth leaves from the summit stood horizontally during the day, and sank down at night only 38°. The sixth leaf did not sensibly alter its position. The sinking movement is due to the downward curvature of the petiole, no part of which exhibits any structure like that of a pulvinus. Early on the morning of June 7th a filament was fixed longitudinally to a young leaf (the third from the summit, and 2 5/8 inches in length), and its movements were traced on a vertical glass during 72 h., the plant being illuminated from above through a skylight. Each day the leaf fell in a nearly straight line from 7 A.M. to 5 P.M., after which hour it was so much inclined downwards that the movement could no longer be traced; and during the latter part of each night, or early in the morning, the leaf rose. It therefore circumnutated in a very simple manner, making a single large ellipse every 24 h., for the ascending and descending lines did not coincide. On each successive morning it stood at a less height than on the previous one, and this was probably due partly to the increasing age of the leaf, and partly to the illumination being insufficient; for although the leaves are very slightly heliotropic, yet, according to Mr. Lynch’s and our own observations, their inclination during the day is determined by the intensity of the light. On the third day, by which time the extent of the descending movement had much decreased, the line traced was plainly much more zigzag than on any previous day, and it appeared as if some of its powers of movement were thus expended. At 10 P.M. on June 7th, when the leaf depended vertically, its movements were observed by a mark being placed behind it, and the end of the attached filament was seen to oscillate slowly and slightly from side to side, as well as upwards and downwards.

Phyllanthus Niruri (Euphorbiaceae).—The leaflets of this plant sleep, as described by Pfeffer,[^[20]] in a remarkable manner, apparently like those of Cassia, for they sink downwards at night and twist round, so that their lower surfaces are turned outwards. They are furnished as might have been expected from this complex kind of movement, with a pulvinus.

[20] ‘Die Period. Beweg.,’ p. 159.