The Power of Movement in Plants

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 Fig. 129. tracings are not given, as they quent during the day, and there Oxulis acetosella: circumnuta- was more lateral movement, so that ee of leaflet when asleep ; : : raced on vertical glass risen to their full height, but had already begun to fall, that is, that the cotyledons, instead of sinking, rise up vertically at

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 18°. 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 ascend- ing 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 here- after 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 autation of foliaceous pe- tiole, filament fixed ob- liquely across end of petiole; movements traced on ver- tical glass from 9. A.M. June 26th to 8.50 a.m. 28th. Apex of leaflet 44 inches trom the glass, so movement not much magnified. Plant 9 inches high, illuminated from above. Temp, 23}°-

Oxalis bupleurifolia.—This species is rendered remarkable by the petioles being foliaceous, like the phyllcdes of many Acacias. ‘The leaflets are small, of a paler green and more tender consistence than the folia- ceous petioles. The leaflet which was observed was °55 inch in length, and was borne by a petiole 2 inches long and °8 inches 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. rusci- Jormis. Nevertheless, in the present species the nyctitropic movements are perfectly performed, The folia- ceous 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 move- ment was not sufficient to be called sleep. The ascending and descend- ing lines did not coincide, so that an ellipse was formed each day. There was but little zigzagging; if the filament had been fixed longitudi- nally, we should probably have seen that. there was more lateral move- ment than appears in the diagram.

A terminal leaflet on another leaf was next observed (the potiole being secured), and its movements are shown in Fig. 131. During the day the leaflets are extended horizon- tally, 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, “aged 4SB] ay} Ul sv oules ayy SUOMpUo "YISZ N'Y CHB OF YING OUNG UO “WV G WoT seL[s [woIjsoA B UO paov.] $ qiIpruL yy Suole paxye yuaweyy YA Gayvay [vutuite; jo yuauaaow oidosyyoku pue TorenUUNLD + YyOfrunajdng synxo

second day it plainly circumnutated between 8 a.m. and 4.30 p.m, Averrhoa bilimbi (Oxalide).—It hag long been. known," firstly, that the leaflets in this genus sleep; secondly, that they move spontaneously during the day; and thirdly, that they are sensi- tive to a touch; but in none cf these respects do they differ essentially from the species of Oxalis. They differ, however, as Mr. R. I. Lynch ¢ has lately shown, in their spontaneous move- ments being strongly marked. In the case of A. bilimbi, it isa 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 gyvans, 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. 182). The main petiole is in con- stant movement during the day, but no careful observations were made on it. The following diagrams are graphic representa- tions 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 pul- vinus, of one of the lateral leaflets was at the centre of a gradu- ated are 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 Averrioa bilimbi: angular movements of a leaflet during its evening descent, when going to sleep. Temp. 78°-81° F. 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 are, In the following diagrams the ordinates represent the angles which the leaflet made with the vertical at successive instants.* It follows that a fall in the curve represents an actual dropping of the leaf, and that the zero line represents a vertically de- pendent position. Fig. 183 represents the nature of the move- iments which occur in the evening, as soon as the leaflets begin to assume their nocturnal position. At 4.55 PM. 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 owr page, the leaflet is represented falling from 75° instead of 85°. Shortly after 6 pM. 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 5m. The complete state of rest of the leaflet which ultimately followed is vot shown in the dia- gram. 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. ‘'he amplitude of the oscillations diminished, while the periods of oscillation became shorter.

In bright sunshine the leaflets assume a highly inclined de- pendent position. A leaflet in diffused light was observed rising for 25m. 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 t» 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 1s * In all the diagrams 1 mm.in ment. In Figs. 133 and 134 the the horizontaldirection represents — temperature is represented (along one minute of time. Each mm. tle ordinates) in the scale of 1 in the vertical direction repre- mm. to each 01°C. In Fig. scnts one degrev of angular move- 135 each mm. equals 0°2° F, ‘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

Averrhoa bilimbi: angular movements of leaflet during a change from bright illumination to shade; temperature (broken line) remaining nearly the same. 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 oscil- lation was sometimes quicker than is represented in the above diagram. Thus, when the temperature was between 31° end

; saanqesod uty Jo aduvyo ayy saoys OUI] Tayxorq syy, ‘atuvs oy} Surarewat yYory fainquiadaiay Jo asuvyo BV SULINpP yayvay jo quawtaAoM AL[NSuUB : tquupg DoYy.Liaap 32° C., 14 oscillations of a few degrees occurred 1n 19m. On the other hand, an oscillation may be much slower ; thus a leaflot was observed (temperature 25° C.) to rise during 40 m. before it fell and completed its oscillation. Porlieria hygrometrica (Zygophylles) —The leaves of this plant (Chilian form) are from 1 to 14 inches 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 ona bush from Chili, which was sent to us from Kew, bore many leaflets, whilst those on plants in the Botanic Garden at Wirzburg bore only 8 or 9 pairs; and the whole character of the bushes appeared some- what different. We shall also see that they differ in a remarkable physio- logical 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 up- wards. On horizontal branches the younger petioles likewise move at night in the same direction as before, that is,

Policria hygrometrica: cir- cumnutation and nycti- tropic movements of pe- tiole of leaf, traced from 9.35 a.m, July 7th to about midnight on the 8th. Apex of leaf 74 inches from tho vertical glass. Temp. 194°-203°C. towards the branch, and are consequently then extended heri- contally; 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 posi- tion, 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, except- ing the basal ones, have the whole of their upper surfaces and half of their lower surfaces well protected. Those on the oppo- site sides of the same petiole do not come into close contact at night, as occurs with the leaflets of so many Leguminosz, 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 { of an inch in length, on an upright branch, was observed during 46 h., and is shown in the preceding diagram (Fig. 186). On the first morning, the leaf fell a little and then rose until lpm, 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, indi- cates that in its native arid home it is affected in some manner by the dryness or dampness of the atmosphere.* In the Botanic Garden at Wiirzburg, 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,

* «Systema Veg. Flore Peru- vianz et Chilensis,’ tom. i. p. 95, 1798. We cannot understand the account given by the authors of the behaviow: of this plant in its native home ‘There is much about its power of foretell’ ng changes in the weather; and it sky largely di termined the open ing and closing of the leatleta. or even quite, closed during ghe 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 wate.ed, and on the following morning the leaflets were fully ex- panded. The other plant growing in a pot, after having been exposed to heavy rain, was placed before » window in the Labo- ratory, with its leaflets open, and they remained so during the daytime for 48h.; 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 svon 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.

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 ir 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 alinost incredible that any plant, except a fleshy one, could have kept alive in soil so dry, which resembled the dust on aroad. On the 29th, when the bush was shaken, some lcaves fell off, and the remaining ones were unable to sleep at night. It was therefore moderately watered, as well as syringed, late in theevening. On the next morning (80th) the bush looked as fresh as ever, and at night the leaves went to sleep. 1t may be added thut 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 intenscly 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 ieaflets showed no signs of closing. The Chilian form therefore differs widely from the one at Wiirzburg, in not closing its leaflets when suffering from the want of water; and it can live for a surprisingly long time without water.

Tropeolum mijus (?) (cultivated var.) (Tropsoles).—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 illu- minated during at least a part of the day they do not sleep at nivyht; and a little difference in the degree of illumination deter- mines 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 move- ments are caused by the bending of an upper part of the petiole, between 4 and 1 inch in length; but the part close to the blade, for about 3 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. 8rd 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 hori- zontal, 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 3h. 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 5h. 45m. 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 in- stance 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 posi- tion 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 86 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 25m., 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 posi- tion 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 circum- nutation 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 consider- ably by 6.45 a.m. on the following morning.

Leguminose.—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. ?) (Lribe 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 in- clined downwards at night, having been during the day ex- tended horizontally. This is shown in the accompanying figures (Fig. 187), 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 peticles

Cuar. VII. SLEEP OF LEAVES. 341 did not move in any plainly perceptible degree. It is, however, quite possible, as we shall presently sce, that if a large number of plants of the three foregoing and of the following species Lupinus pilosus; A, leaf seen from vertically above in daytime; B, leaf asleep, seen laterally at night. were to be observed at all seasons, some of the leaves would be found to sleep in a different manner. In the two following species the leaflets, instead of moving downwards, rise at night. With DL. Hartwegii some stood at noon at a mean angle of 86° 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° 380’. 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 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 pight. Figures reduced.

I. pubescens ; and in the accompanying figures we sce 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 quita 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 leatlets 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 horizontai position or raised them a little.