The Power of Movement in Plants

Mirabilis longiflora and jalapa (Nyctaginee)—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 (Polygones).—Professor Batalin informs us that the young leaves rise up vertically at night. This is likewise the case, according to Linneus, with several species of Amaranthus (Amaranthacez); and we observed asleep move- ment of this kind in one member of the genus. Again, with Chenopodium album (Chenopodiez), the upper young leaves ot 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 Pim:lia linvides and spectabdilis (Thymelez) slecp at night.

Euphorbia jacquinieflora (Buphorbiacee). — 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% inches in length), and its movements were traced on a vertical glass during 72h., 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 sc 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, accord- ing 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 move- ments 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 (Euphorbiacew).— The leaflets of this plant sleep, as described by Pfeffer,* 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, Pinus Nordmanniana (Coniferee).—M. Chatin states * that the Isaves, which are horizontal during the day, rise up at night, so os to assuine a position almost perpendicular to the branch frora which they arise; we presume that he here refers to a horizontal branch. He adds: “En méme temps, ce mouvement d’érection est accompagné d’un mouvement de torsion imprimé & la partie basilaire de la feuille, et pouvant souvent parcourir un arc da 90 degrés.” As the lower surfaces of the leaves are white, whilst the upper are dark green, the tree presents a widely different appearance by day and night. The leaves on a small tree in a pot did not exhibit with us any nyctitropic move- ments. We have seen in a former chapter that the leaves of Pinus pinaster and Austriuca are continually circumnutating.

Thalia dealbata (Cannacee).—The leaves of this plant sleep by turning vertically upwards; they are furnished with a well- developed pulvinus. It is the only instance known to us of a very large leaf sleeping. The blade of a young leaf, which was as yet only 13} inches in length and 63 in breadth, formed at noon an angle with its tall petiole of 121°, and at night stood vertically in a line with it, and so had risen 59°. The actual distance travelled by the apex (as measured by an orthogouic tracing) of another large leaf, between 7.30 a.m. and 10 P.m., was 104 inches. The circumnutation of two young and dwarfed leaves, arising amongst the taller leaves at the base of the plant, was traced on a vertical glass during two days. .On the first day the apex of one, and on the second day the apex of the other leaf, described between 6.40 a.m. and 4PM. two ellipses, the longer axes of which were extended in very different directions from the lines representing the great diurnal sinking and nocturnal rising movement. Maranta arundinacea (Cannaces).—The blades of the leaves, which are furnished with a pulvinus, stand horizontally during

the day or between 10° and 20° above the horizon, and at night vertically upwards. They therefore rise between 70° and 90° at night. The plant was placed at noon in the dark in the hot- house, aud on the following day the movements of the leaves were traced. Between 8.40 and 1030 a.m. they rose, and then fell greatly till 1.87pm. But by 3p.m. they had again risen a little, and continued to rise during the rest of the afternoon and night; on the following morning they stood at the same level as on the previous day. Darkness, therefore, during a day and a half does not interfere with the periodicity of their movements. On a warm but stormy evening, the plant whilst being brought into the house, had its leaves violently shaken, and at night not one went to sleep. On the next morning the plant was taken back to the hot-house, and again at night the leaves did not sleep; but on the ensuing night they rose in the usual manner between 70° and 80°. This fact is analogous with what we have observed with climbing plants, namely, that much agitation checks for a time their power of circumnutation ; but the effect in this instance was much more strongly marked and prolonged.

Colocasia antiquorum (Caladium esculentum, Hort.) (Aroidee), —The leaves of this plant sleep by their blades sinking in the evening, so as to stand highly inclined, or even quite vertical'y with their tips pointing to the ground. They are not provided with a pulvinus. The blade of one stood at noon 1° beneath the horizon; at 4.20 p.m., 20°; at 6p.m., 43°; at 7.20 p.m., 69°; and at 8 30 p.m., 68°; so it had now begun torise; at 10.15 p.m. it stood at 65°, and on the following early morning at 11° beneath the horizon. Tbe circumnutation of another young leaf (with its petiole only 3 inches, and the blade 4 inches in length), was traced on a vertical glass during 48 h.; it was dimly illuminated through a skylight, and this seemed to disturb the proper perio- dicity of its movements. Nevertheless, the leaf fell greatly during both afternoons, till either 7.10 pm. or 9 P.M., When it rose a little and moved laterally. By an early-hour on both mornings, it had assumed its diurnal position. The well-marked lateral movement for a short time in the early part of the night, was the only interesting fact which it presented, as this caused the ascending and descending lines not to coincide, in accord- ance with the general rule with circumnutating organs, The movements of the leaves of this plant are thus of the most simple kind; and the tracing is not worth giving. We havo seen that in another genus of the Aroidew, namely,-Pistia, the

leaves rise so much at night that they may almost be said to sleep. Strephium jloribundum™ (Graminee).— The oval leaves are provided with a pulvinus, and are extended horizontally or declined a little beneath the horizon during the day. Thoso on tho upright culms simply rise up vertically at night, so that their tips are directed towards the zenith. (Fig. 164.) Strephium floribundum: culms with leaves during the day, and when asleep atnight. Figures reduced.

Horizontally extended leaves arising from much inclined or almost horizontal culms, move at night so that their tips point towards the apex of the culm, with one lateral margin directed towards the zenith; and in order to assume this position the leaves have to twist on their own axes through an angle of nearly 90°. Thus the surface of the blade always stands vertically, whatever may be the position of the midrib or of the leaf as a whole. The circumnutation of a young leaf (2°3 inches in length) was traced during 48 h. (Fig. 165). The movement was remarkably simple; the leaf descended from before 640 a.m. until 2 or 2.50 p.m., and then rose so as to stand vertically at about 6 P.m., descending again late in the night or in the very early morning.

* A. Brongniart first observed a Soc. Bot. de France,’ tom. vii that the leaves of this plant and 1860, p. 470. of Marsilea sleep: see ‘ Bull. de On the second day the descending line zigzagged slightly. As Strephium floribundum : ciyeumnu- tation and nyctitropic m>vement of a leaf, traced from 9 a.m. June 26th to 8.45 a.m. 27th; filament fixed along the midrib. Apex of leaf 84 inches from the vertical glass; plant illuminated from above. Temp. 23$°-243° ©,

usual, the ascending and de- scending lines did not coincide. On another occasion, when the temperature was a little higher, viz., 24°-263° C., a leaf was -observed 17 times between 8.50 Am. and 12.16 p.m.; it changed its course by as much as a rectangle six times in this in- terval of 3 h. 26 m., and de- scribed two irregular triangles and a half. The leaf, therefore, on this occasion circumnutated rapidly and in a complex manner. Marsilea quadrifoliata (Mar- sileaceze).—The shape of a leaf, expanded horizontally during the day, is shown at A (Fig. 166). Each leaflet is provided with a well-developed _pulvinus. When the leaves sleep, the two terminal leaflets rise up, twist half round and come into con- tact with one another (B), and are afterwards embraced by the two lower leaflets (C); so that the four leaflets with their lower surfaces turned outwards form a vertical packet. The curva- ture of the summit of the petiolo of the leaf figured asleep, is merely accidental. The plant was brought into a room, where the temperature was only a little above 60° F., and the movement of one of the leaflets (the petiole having been secured) was traced

during 24h. (Fig. 167). The leaf fell from the early morning till 1.50 p.m, and then rose till 6 p.m., when it was asleep. A Marsilea quadrifoliaia: A, leaf during the day, seen from vertically above Bb, leaf beginning to go to sleep, seen laterally; C, the same asleep. Figures reduced to one-half of natural scale. vertically dependent glass filament was now fixed to one of the terminal and inner leaflets; and part of the tracing in Fig. 167, after 6 p.m., shows that it continued to sink, making one zigzag, until 1040p.m. At 6.45 a.m. on the following morning, the leaf was awaking, and the filament pointed above the vertical glass,

L5O'pm Marsilea quadrifoliata: circumnutation and nyctitropic movement of leaflet traced on vertical glass, during nearly 24h. Figure reduced to two- thirds of original scale. Plant kept at rather too low a temperature. but by 8.25 am. it occupied the position shown in the figure. The diagram differs greatly in appearance from most of those previously given; and this is due to the leaflet twisting and moving laterally as it approaches and comes into contact with

its fellow. The movement of another leaflet, when asleep, was traced between 6 p.m. and 10.385 p.m., and it clearly cir- cumnutated, for it continued for two hours to sink, then rose, and then sank still lower than it was at 6 p.m. It may be seen in ihe preceding figure (167) that the leaflet, when the plant was subjected to a rather low temperature in the house descended and ascended during the middle of the day in a somewhat zigzag line; but when kept in the hot-house from 9am. to 3 P.w. at ahigh but varying temperature (viz., between 72° and 83° F.) a leaflet (with the petiole secured) circumnutated rapidly, for it made three large vertical ellipses in the course of the six hours. According to Brongniart, Marstlea pubescens sleeps like the present species. These plants are the sole cryptogamic ones known to sleep.

Summary and Concluding Remarks on the Nyctitropie or Sleep-movements of Leaves—That these movements are in some manner of high importance to the plants which exhibit them, few will dispute who have ob- served how complex they sometimes are. Thus with Cassia, the leaflets which are horizontal during the day not only bend at night vertically downwards with the terminal pair directed considerably backwards, but they also rotate on their own axes, so that their lower surfaces are turned outwards. The terminal leaflet of Melilotus likewise rotates, by which movement one of its lateral edges is directed upwards, and at the sume time it moves either to the left or to the right, until its upper surface comes into contact with that ot the lateral leaflet on the same side, which has like- wise rotated on its own axis. With Arachis, all four leaflets form together during the night a single vertical packet; and to effect this the two anterior leaflets have to move upwards and the two posterior ones forwards, besides all twisting on their own axes. In the genus Sida the leaves of some species move at night through. an angle of 90° upwards, and of others

through the same angle downwards. We have seen a similar difference in the nyctitropic movements of the cotyledons in the genus Oxalis. In Lupinus, again, the leaflets move either upwards or downwards; and in some species, for instance L. luteus, those on one” side of the star-shaped leaf move up, and those on the opposite side move down; the intermediate ones rota- ting on their axes ; and by these varied movements, the whole leaf forms at night a vertical star instead of a horizontal one, as during the day. Some leaves and leaflets, besides moving either upwards or downwards, become more or less folded at night, as in Bauhinia and in some species of Oxalis. The positions, indeed, which leaves occupy when asleep are almost infinitely diversified; they may point either vertically upwards or downwards, or, in the case of leaflets, towards the apex or towards the base of the leaf, or in any inter- mediate position. They often rotate at least as much as 90° on their own axes. The leaves which arise from upright and from horizontal or much inclined branches on the same plant, move in some few cases in a different manner, as with Porlieria and Strephium. The whole appearance of many plants is wonderfully changed at night, as may be seen with Oxalis, and still more plainly with Mimosa. A bush of Acacia Farnesiana appears at night as if covered with little dangling bits of string instead of leaves. Excluding a few genera not seen by ourselves, about which we axe in doubt, and excluding a few others the leaflets of which rotate at night, and do not rise or sink much, there are 37 genera in which the leaves or leaflets rise, often moving at the same time towards the apex or towards the base of the leaf, and 32 genera in which they sink at night. The nyctitropic movements of leaves, leaflets, and

petioles are effected in two different ways ; firstly, by alternately increased growth on their opposite sides, preceded by increased turgescence of the cells; and secondly by means of a pulvinus or aggregate of small ‘cells, generally destitute of chlorophyll, which become alternately more turgescent on nearly opposite sides ; and this turgescence is not followed by growth except during the early age of the plant. A pulvinus seems to be formed (as formerly shown) by a group of cells ceasing to grow at a very early age, and therefore does not differ essentially from the surrounding tissues. The cotyledons of some species of Trifolium are pro- vided with a pulvinus, and others are destitute of one, and so it is with the leaves in the genus Sida. We see also in this same genus gradations in the state of the development of the pulvinus; and in Nicotiana we have what may probably be considered as the commencing development of one. The nature of the movement is closely similar, whether a pulvinus is absent or present, as is evident from many of the diagrams given in this chapter. It deserves notice that when a pulvinus is present, the ascending and descending lines hardly ever coincide, so that ellipses are habitually described by the leaves thus provided, whether they are young or so old as to have quite ceased growing. This fact of ellipses being described, shows that the alternately increased turgescence of the cells does not occur on exactly opposite sides of the pulvinus, any more than the increased growth which causes the movements of leaves not furnished with pulvini. When a pulvinus is present, the nyctitropic movements are continued for a very much longer period than when such do not exist. This has been amply proved in the case of cotyledons, and Pfeffer has given observations to the same effect with respect

to leaves. We have seen that a leaf of Ifimosa pudica continued to move in the ordinary manner, though somewhat more simply, until it withered and died. It may be added that some leaflets of Trifolium pratense were pinned open during 10 days, and on the first evening after being released they rose up and slept in the usual manner. Besides the long con- tinuance of the movements when effected by the aid of a pulvinus (and this appears to be the final cause of its development), a twisting movement at night, as Pfeffer has remarked, is almost confined to leaves thus provided.

It is a very general rule that the first true leaf, though it may differ somewhat in shape from the leaves on the mature plant, yet sleeps like them ; and this occurs quite independently of the fact whether or not the cotyledons themselves sleep, or whether they sleep in the same manner. But with Phaseolus Rew- burghi the first unifoliate leaves rise at night almost sufficiently to be said to sleep, whilst the leaflets of the secondary trifoliate leaves sink vertically at night. On young plants of Sida rhombefolia, only a few inches in height, the leaves did not sleep, though on rather older plants they rose up vertically at night. On the other hand, the leaves on very young plants of Cytisus fragrans slept in a conspicuous manner, whilst on old and vigorous bushes kept in the greenhouse, the leaves did not exhibit any plain nyctitropie move- ment. In the genus Lotus the basal stipule-like leaflets rise up vertically at night, and are provided with pulvini.

As already remarked, when leaves or leafiets change their position greatly at night and by complicated movements, it can hardly be doubted that these must be in some manner beneficial to the plant. If so, we must extend the same conclusion to a large number of sleeping plants; for the most complicated and the simplest nyctitropic movements are connected together by the finest gradations. But owing to the causes spe- cified in the beginning of this chapter, it is impossible in some few cases to determine whether or not certain movements should be called nyctitropic. Generally, the position which the leaves occupy at night indi- cates with sufficient clearness, that the benefit thus derived, is the protection of their upper surfaces from radiation into the open sky, and in many cases the mutual protection of all the parts from cold by their being brought into close approximation. It should be remembered that it was proved in the last chapter, that leaves compelled to remain extended horizontally at night, suffered much more from radiation than those which were allowed to assume their normal vertical position.

The fact of the leaves of several plants not sleeping unless they have been well illuminated during the day, made us for a time doubt whether the pro- tection of their upper surfaces from radiation was in all cases the final cause of their well-pronounced nyctitropic movements. But we have no reason to suppose that the illumination from the open sky, during even the most clouded day, is insufficient for this purpose; and we should bear in mind that leaves which are shaded from being seated low down on the plant, and which sometimes do not sleep, are likewise protected at night from full radiation. Nevertheless, we do not wish to deny that there may exist cases in which leaves change their position considerably at night, without their deriving any benefit from such movements,

always assume at night a vertical, or nearly vertical position, it is a point of complete indifference whether the apex, or the base, or one of the lateral edges, is directed to the zenith. It is a rule of wide generality, than whenever there is any difference in the degree of exposure to radiation between the upper and the lower surfaces of leaves and leaflets, it is the upper which is the least exposed, as may be seen in Lotus, Cytisus, ‘Trifolium, and other genera. In several species of Lupinus the leaflets do not, and apparently from their structure cannot, place themselves vertically at night, and consequently their upper surfaces, though highly inclined, are more exposed than the lower; and here we have an exception to our rule. But in other species of this genus the leaflets succeed in placing themselves vertically ; this, however, is effected by a very unusual movement, namely, by the leaflets on the opposite sides of the same leaf moving in opposite directions.

It is again a very common rule that when leaflets come into close contact with one another, they do so by their upper surfaces, which are thus best protected. In some cases this may be the direct result of their rising vertically ; but it is obviously for the pro- tection of the upper surfaces that the leaflets of Cassia rotate in so wonderful a manner whilst sinking downwards; and that the terminal leaflet of Melilotus rotates and moves to one side until it meets the lateral leaflet on the same side. When opposite leaves or leaflets sink vertically down without any twisting, their lower surfaces approach each other and some- times come into contact; but this is the direct and inevitable result of their position. With many species of Oxalis the lower surfaces of the adjoining lcaflets are pressed together, and are thus better protected

than the upper surfaces; but this depends merely ou each leaflet becoming folded at night so as to be able to sink vertically downwards. ‘he torsion or rotation of leaves and leaflets, which occurs in so many cases, apparently always serves to bring their upper surfaces into close approximation with one another, or with other parts of the plant, for their mutual protection. We see this best in such cases as those of Arachis, Mimosa albida, and Marsilea, in which all the leaflets form together at night a single vertical packet. If with Mimosa pudica the opposite leaflets had merely moved upwards, their upper surfaces would have come into contact and been well protected; but as it is, they all successively move towards the apex of the leaf; and thus not only their upper surfaces are pro- tected, but the successive pairs become imbricated and mutually protect one another as well as the petioles. This imbrication of the leaflets of sleeping plants is a common phenomenon.

The nyctitropic movement of the blade is gene- rally effected by the curvature of the uppermost part of the petiole, which has often been modified into a pulvinus; or the whole petiole, when short, may be thus modified. But the blade itself sometimes curves or moves, of which fact Bauhinia offers a striking instance, as the two halves rise up and come intc close contact at night. Or the blade and the upper part of the petiole may both move. Moreover, the petiole as a whole commonly either rises or sinks at night. ‘This movement is sometimes large: thus the petioles of Cassia pubescens stand only a little above the horizon during the day, and at night rise up almost, or quite, perpendicularly. The petioles of the younger leaves of Desmodium gyrans also rise up ver- tically at night. On the other hand, with Amphi-

carpea, the petioles of some leaves sank down as much as 57° at night; with Arachis they sank 39°, and then stood at right angles to the stem. Genc- rally, when the rising or sinking of several petioles on the same plant was measured, the amount differed greatly. This is largely determined by the age of the leaf: for instance, the petiole of a moderately old leaf of Desmodium gyrans rose only 46°, whilst the young ones rose up vertically; that of a young leaf of Cassia . floribunda rose 41°, whilst that of an older leaf rose only 12°. It is a more singular fact that the age of the plant sometimes influences greatly the amount of movement; thus with some young seedlings of a Bau- hinia the petioles rose at night 30° and 34°, whereas those on these same plants, when grown to a height of 2 or 3 feet, hardly moved at all. The position of the leaves on the plant as determined by the lighi, seems also to influence the amount of movement of the petiole; for no other cause was apparent why the petioles of some leaves of Melilotus officinalis rose as much as 59°, and others only 7° and 9° at night.

In the case of many plants, the petioles move at night in one direction and the leaflets in a directly opposite one. Thus, in three genera of Phaseolez the leaflets moved vertically downwards at night, and the petioles rose in two of them, whilst in the third they sank. Species in the same genus often differ widely in the movements of their petioles. Even on the same plant of Lupinus pubescens some of the petioles rose 30°, others only 6°, and others sank 4° at night. ‘Lhe leaflets of Cassia Barclayana moved so little at night that they could not be said to sleep, yet the petioles of some young leaves rose as much as 34°. These several facts ay parently indicate that the movements

of the petioles are not performed for any special pur- pose; though a conclusion of this kind is generally rash. When the leaflets sink vertically down at night and the petioles rise, as often occurs, it is certain that the upward movement of the latter does not aid the leaflets in placing themselves in their proper posi- tion at night, for they have to move through a greater angular space than would otherwise have been necessary. Notwithstanding what has just been said, it may be strongly suspected that in some cases the rising of the petioles, when considerable, does beneficially serve the plant by greatly reducing the surface exposed to radiation at night. Jf the reader will compare the two drawings (Fig. 155, p. 871) of Cassia pubescens, copied from photographs, he will see that the dia- meter of the plant at night is about one-third of what it is by day, and therefore the surface exposed to radiation is nearly nine times less. A similar conclusion may be deduced from the drawings (Fig. 149, p. 358) of a branch awake and asleep of Des- modium gyrans. So it was in a very striking manner with young plants of Bauhinia, and with Ovzalis Oriegesit.

We are led to an analogous conclusion with respect to the movements of the secondary petioles of certain pinnate leaves. The pinne of Mimosa pudica con- verge at night; and thus the imbricated and closed leaflets on each separate pinna are all brought close together into a single bundle, and mutually protect one another, with a somewhat smaller surface exposed to radiation. With Albizziu lophantha the pinne close together in the same manner. Although the pinne of Acacia Farnesiana do not converge much, they sink downwards. Those of Neptunia oleracea likewise

move downwards, as well as backwards, towards the base of the leaf, whilst the main petiole rises. With Schrankia, again, the pinne are depressed at night. Now in these three latter cases, though the pinne do not mutually protect one another at night, yet after having sunk down they expose, as does a dependent sleeping leaf, much less surface to the zenith and to radiation than if they had remained horizontal. Any one who had never observed continuously a sleeping plant, would naturally suppose that the leaves moved only in the evening when going to sleep, and in the morning when awaking; but he would be quite mistaken, for we have found no exception to the rule that leaves which sleep continue to move during the whole twenty-four hours; they move, however, more quickly when going to sleep and when awaking than at other times. That they are not stationary during the day is shown by all the diagrams given, and by the many more which were traced. It is troublesome to observe the movements of leaves in the middle of the night, but this was done in a few cases; and tracings were made during the early part of the night of the movements, in the case of Oxalis, Amphicarpza, two species of Erythrina, a Cassia, Passiflora, Euphorbia and Marsilea; and the leaves after they had gone to sleep, were found to be in constant movement. When, however, opposite-leaflets come into close contact with one another or with the stem at night, they are, as we believe, mechanically prevented from moving, but this point was not sufficiently investigated.