The Power of Movement in Plants

20°, and 19°5° above the inclined positions Teak “econ: Pear which they had occupied during the day. above, reduced For instance, in the first of these four cases to one-half na- i s ral size. the petiole stood in the day at 23°, and at se ase pa night at 693° above the horizon. In the unusually large evening the rising of the petioles is almost completed before the leaflets sink perpendicularly downwards. Cirenmnutation.—The circumnutating movements of four young shoots were observed during 5h. 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 31m. (temp, 91°F.) it changed its course by as much as a rectangle six times, describing a figure which apparently. represented two ellipses.

Desmodium gyrans: A, stem during the day ; B, stem with leaves asleep, Copied from a photograph ; figures reduced. 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* these leaflets move through an angle of 8° in the course of from 10 to 30 seconds. 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 2th

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 con- siderable 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 31m. (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 2.24pm. and 3p.m., when six intermediate dots were made; and again at 4.46 and 4.50. But the result was widely different after 6P.M..—that is, after the great nocturnal descent had commenced; for though nine dots were then made in the course of 32m , 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.

Had the above plant been subjected to a higher temperature than 67°—70° F., the movements of the termiual 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°—98° 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 atten- tion 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 move- ment, 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 after- wards allowed to sink to 70° F., and the sub-petiole of the ter- minal leaflet then curved downwards. The water was afterwardg

D o0B-o6T ‘dutay, ‘asoqe moy payeurumpys yuerg “a[eos [vutSrt0 Jo parry 00 0} paonpet wetSeiq ‘ssvj3 vortea ay) Woy sayour g xode sqr fyayvay [eUulWte} Jo qiaprur oy paxye quatuLyly ‘U 8b SuLtnp (papnyout ajorad *y78u9| ut soyour $e) Joy Jo ywauaaour oidoayryo4u puy uovynuumo.dso ¢ sunuhb wrpousay raised to 12U° F., and the sub-petiole straightened itself. Similar experluents 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 Pm. (ie. after 5 h, 23 m.), though the leaflets had sunk considerably, they had py no means ac- quired their nocturnal vertically dependent position. Pfetfer, on the other hand, says * that this occurred with him in from $ h. to2h.; perhaps the difference in our results may be due to the plant on which we experimented being a very young and vigorous secdling.

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 some- times 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 re- main stationary for a period. In these several respects there is no difference, except in rapidity and extent, between their move- ments 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 tempersture. 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 40m. 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. Cn 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 1m. 30s. to lm. 45s. There was, however, no jerking, and this fact may perhaps be attributed to the resistance of the water.

Sachs status that the leaflets do not move until the surround- ing 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 11P.m. and at 1 a.m. the leaftet was still jerking rapidly; at 3.30 4 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 lam. At 6.30 a.m. (temp. 61° F.) its inclination was still less than before, and again less at 6.45 a.m.; by 7.404 mM. it had risen, and at 830 a.m. was again seen to jerk. This leaflet, therefore, was moving during the whole night, and the move- ment was by jerks up to 1 a.m. (and possibly later) and again at 8.30 am., 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 former Fig. 148) must be considered as almost rudi- mentary. They are also rudimentary in function, if this ex- pression 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 after- wards (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 1ateral 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 scid 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° be- neath 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.

Asin 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 scedling 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 Jast 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 sub-

sequent appearance, may be attributed to reversion to more ct less distant progenitors.* No one supposes that the rapid movements of the lateral leaflets of 7. 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 soma relation with their rudimentary condition, and therefore ob- served 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, how- ever, this remarkable difference in the two cases; in Desmo- dium 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 Desmo- dium is 8mm., 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 9°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 rudi- mentary leaflets of. Desmodium, we apparently have the proxi- mate 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 pre- vented 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

* Desmodium vespertilionis is rudimentary lateral leaflets. Du- closely allied to D. gyrans, and chartre, ‘ Elémentsde Botanique, it seems only occasionally to bear —- 1867, p. 353, 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 Desmo- dium, 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 layopus 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 163°. Amphicarpeea monoica (Tribe 8).—The leaflets sink down ver- tically at night, and the petioles likewise fall considerably. Amphicarpea 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,

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 tcrminal leaflet of a fine young leaf (2} 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 pre- vented. The movement of the leaf was traced during 48 h., frum 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.

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 & caffra and with a second unnamed species, the petioles at the same time rose slightly. The movements of the terminal leaflet of /-. 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 com- menced, 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.

a terminal leaflet, as we wished to observe its movements when asleep. The plant was placed in the morning of June lUth 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 posi- tion, describing a figure which represented two irregular -el- lipses. On the next day it circumnutated in a greater degree, describing four irre- gular ellipses, and by 3 p.m. had risen into a horizontal po- sition. By 7.15 p.m. it was asleep and vertically depen- dent, but continued to circum- nutate as long as observed, until 11 p.m.

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 am. and 4 P.M., after which hour the great nocturnal fall commenced. On the third day the movement was equally great in ampli- tude, 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.

Arythrina crists-galli: circumnuta- tion and nyctitropic movement of terminal leaflet, 33 inches in length, traced during 25h.; apex of leaf 33 inches from the vertical glass. Figure reduced to one-half of original scale. Plant illumi- nated frora above; temp. 174°- Apios tuberosa (Tribe 8).—The leaflets sink vertically down at night. Phaseolus vulyaris (Tribe 8).—The leaflets likewise sink verti- cally 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 verti- cally dependent and asleep. With Ph. caracalla and Hernan- desit, 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. Rox- burghii, 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 Pha- seoleze observed by us (with the exception of the primary leaves of Phaseolus Roaburylit) 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 Amphicarpea. 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,

Cesalpinia, Hemutorylon, Gleditschia, Poinciana.—The leaflets of two species of Cxsalpinia (Tribe 13) rose at night. With Hematoxylon Campechianum (Tribe 18) the leaflets move for- wards at night, so that their midribs stand parallel to the petiole, and their now vertical lower surfaces are turned out- wards (Fig. 153). The petiole sinks a little. In Gleditschia, if we understand correctly Duchartre’s description, and in Poin- ciana Gillicsti (both belonging to Tribe 13), the leaves behave in the same manner.

Hematoxylon 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 Linnzeus, and since by Duchartre. Our observations were made chiefly on C. flori- bunda* and corymbosa, but several other species were casually observed. The horizontally extended leaflets sink down verti- cally 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 move- ments are effected by means of a well-developed pulvinus at the base of each leaflet, as could be plainly seen when a straiglit 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

* Tam informed by Mr. Dyer near to C. laevigata. It is no douht that Mr. Bentham believes that — the same as the form described by C. floribunds (a common green- Jiindley (‘ Bot. Reg.,’ Tub. 1422)" house bush) is a hybrid raised in as C. Herbertiana, 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. Cassia corymbcsa: A, plant durmg day; B, same lant at night. Both figures copied from yhotographs,

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°.

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: A, upper part of plant during the day; B, same p ant at night. Figures reduced from photographs. 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 newly or quite vertically. This, together with the dependent position of the leaflets, makes the whole plant won- derfully 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.

Cassia mimosvides.— 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 alittle, 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°, diverged 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 consider- able 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 (. floribunda, ealliantha, and jubescens were observed, each during three or four days; they were essentially alike, those of the last-named specics 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 18th to 8.80 a.m. 17th; but those during the last 2 h. are alone given in Fig. 156. From 8 a.m. or each day (by which hour the leaf had assumed its diurnal posi- tion) to 2 or 3 p.m. it either zigzageed or cireumnutated over nearly the same small space; at between 2 and 8 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 Icaflet was asleep at 6 p.at., and whilst the glass filament hung

perpenjicularly 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. Bauhinia (Tribe 15).— Fig. 156 The nyctitropic movements aes of four species were alike, (h bo and were highly peculiar. “i A plant raised from seed \ oe sent us from South Brazil 3 by Fritz Miiller, was more aie especially observed. The Iraves are large and deeply i netched at their ends. At A night the two halves rise ; up and close completely i together, like the opposite ; leaflets of many Legumi- nose. With very young plants the petioles rise con- siderably at the same time; one, which was inclined at noon 45° above the hori- zon, at night stood at 75°; } it thus rose 30°; another i Hf rose 34°. Whilst the two i halves of the leaf are closing, i the midrib at first sinks i vertically downwards and i afterwards bends back- ; wards, so as to pass close A along one side of its own ; upwardly inclined petiole; if