The Power of Movement in Plants

the midrib being thus di- BO rected towards the stemor / yf axis of the plant. The angle aed which the midrib formed with the horizon was mea- sured in one case at dif- ferent hours: at noon it stood horizontally; late in the even- ing 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 158° circuninutation and nyctitropic movement of a terminal leaflet (1 traced from 8.30 aM. tu same hour on following morning.

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 8 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 roeet 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 Hematoxylon (see former Fig. 153), but more striking from the greater number of the leaflets. Adenanthera, Prosopis, and Neytunia (Tribe 20).—With Ade- nanthera pavonia the leaflets turn edgeways and sink at night. In Prosomis 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 them- selves move downwards, and at the same time backwards or towards the stem ofthe plant. The main petiole rises.

Mimosa pudica (Tribe 20).—This plant has been the subject of innumerable observations; but there are some points in rela- tion 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 be- come neatly imbricated with their upper surfaces protected. The four pinne also approach each other closely, and the whole leaf is thus rendered very compact. The main petiole sinks down- wards during the day till late in the evening, and rises until very early in the morning. The stem is continually cireumnu- tating at a rapid rate, thongh 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 ot 12h. We shall immediately see that the main petiole is like- wise continually circumnutating, as is each separate pinna and each separate leaflet. Therefore, if the movement of the apex of any one leaflet were to he traced, the course described would be compounded of the movements of four separate parts,

A filament had been fixed on the previous eveiing, longi- tudinally 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 ona 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 8rd. 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 down- wards and four times upwards. As the ascending and descend- ing lines do not coincide, the petiole manifestly circumnnu- tates; the great evening fall 7°25%%m.on#\--. and nocturnal rise being an exaggeration of one of the cir- cumnutations. It should, how- ever, be observed that the pe- tiole 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 i found at 12.50 a.m. (ie. after \ midnight) standing almost up- right, and much more highly ° inclined than it was at 10.40 P. p.m. When observed again at Mimosa pudica: circumnutation and s nyctitropic movement of main pe- 4 a.m, it had begun to fall, and tiole, traced during 34 h. 30 m. continued falling till 6.15 a.m., after which hour it zigzagged and again circeumnutated. Similar observations were made on another petiole, with nearly the same result.

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 23 ellipses in 69 m. On the second occasion, when the temperature was 81°—86° F., it made rather more than 8 ellipses in 67 m. Therefore, Fig. 157, though now sufficiently complex, would have been in- comparably 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 abso- lutely straight line is the result.

To show that the movement of the petiole is in all proba- bility 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 statiqnary, 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 415 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 circum- nutating 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 pinnee move independently of the main petiole. The petiole of a leaf was fixed to a cork support, close to the point whence the four pinnew diverge, with a short fine filament cemented longitudinally to one of the twe terminal pinuz, 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 pw. 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 pinnas approach each other, and the one which was observed moved inwards 59° between noon and 6.45 p.m. Ten observations were mace in the course of 2h. 20m. (at average intervals of 14 m.) between 4.25 and 6.45 p.m.; and there was now, when the leat was going to sleep, no swaying from side to side, but a steady inward movement. Here therefore there is in the evening the sume 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 circum- nutates. A pinna was cemented with shellac on the summit ot 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 circum- nutation. On the second day, when more accustomed to its new state, it oscillated twice up tnd twice down during the 24h. This plant was subjected to a rather low temperature, viz., 62°—64° F.; had it been kept warmer, no doubt the move- ments 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 con- tinually circumuutating 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 occa- sionally 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 uo signs of sleeping, and were only slightly sensitive to

stick, and the movements of two leaves were traced on a verticai glass during 72h. The plants were still kept in the dark, ex- cepting that at each observation, which lasted 3 or 4 minutes, Mimosa pudica: circumnutation and nyctitropic movement of 4 leaflet (with pinna secured), traced on a vertical glass, from 8 a.m. Sept. 14th to 9 a.m. 16th. 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 morn- ing) 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.

With plants kept ata high temperature and exposed to the light, the most rapid circumnutating movement of the apex of a leaf which was observed, amounted to 53, of an inch in one second; and this would have equalled 3 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 $ of an inch in a vertical direction in 15 m.; and on another occasion § of an inch in 60 m.; but there was also some lateral movement.

Mimosa albida.*—The leaves of this plant, one of which is here figured (Fig. 159) reduced to 2 of the natural size, present some interesting peculiarities. It consists of a long petiole bearing only two pinnz (here represented as rather more divergent than is usual), each with: two pairs of leaflets. But the inner * Mr. Thistleton Dyer informs Linn. Soc.,’ vol. xsx. p. 390) te us that this Peruvian plant (which be “the species or variety which was sent to us from Kew) is con- most commonly represents the sidered by Mr. Bentham (‘Traus. —sensitiva of our gardens.”

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. Neverthe- less, 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 pinne are likewise much shortened and obliquely truncated; this fact was well seen in some seedlings of M. pwica, in which the third leaf above the cotyledons bore only two pinnee, 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 Jf, albida. In this latter species the main petiole termi- nates in a little point, and on each side of this there is a pair of minute, flattened, lancct-shaped projections, hairy on their margins, which drop off and disappear soon after the leaf is fully developed. There can hardly be w doubt that these little projections are the last and fugacious representatives of an additional pair of lcaflets to each pinna; for the outer one is twice as broad as the inner one, and a little longer, viz. +4, of an inch, whilst the inner one is only $58 long. Now if the basal pair of leaflets of the existing leaves were to become rudimcn- tary, we should expect that the rudiments would still exhibit some trace of their present great inequality of size. The con- clusion that the pinne of the parent-form of M.

albida possessed at least three pairs of leaflets, instcad 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.

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 pinne 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 pinne at the same time that they approach each other sink downwards, and thus instead of extending hori- zontally 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 days, and was considerable in extent, but less complex than that of MZ. 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 tho microscope to travel J; 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 pinne, each bearing many small leaflets. These, when the plant is asleep, are directed forwards and become imbricated. The angle between the two terminal pinnz was diminished at night, in one case by 15°; and they sank almost vertically down- wards. The hinder pairs of pinne likewise sink downwards, but do not convorge, 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 yinne, 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 pinne of Schrankia aculeata also sink at night.

Acacia Farnesiana (Tribe 22).—The different appearance pre- sented by a bush of this plant when asleep and awake is won- derful. 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 pinne then look like bits of dangling string. The following remarks and measurements do not fully apply to the small leaf here figured. The pinne 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 pinne of one specimen formed together an angle of 100° during the day, and at night of only 88°, so each had moved 31° forwards. The penultimate pinne 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 pinnez were directed

Acacia Farnesiana: A, leaf during the day; B, the same leaf at night. during the day, each about 21° backwards, and at night 38° forwards, so each had moved 59° forwards. But the pinne at the same time sink greatly, and sometimes hang almost perpen- dicularly 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 move- ment 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. a Albizzia lophantha (Tribe 23).—-The leaflets at night come into contact with one another, and are directed towards the apex of the pinna, The pinne 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 vetiole rises but little. The first-formed leaf above the coty- ledons 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 ericeefolia (Myrtacese).—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. Gnothera mollissima (Onagrariez).—According to Linneus (Somnus Plantarum’), the leaves rise up vertically at night. Passiflora gracilis (Passifloracee).—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 verti- cally 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 18th 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. Passiflora gracilis: circumnutation and nyctitropic movement of Jeaf traced on vertical glass, from 8.20 a.m. Oct. 13th to 10 a.m. 14th Figure reduced to two-thirds of original scale.

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. Siegesbeckia orientalis (Compositw#).—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 consi:

derable angle beneath the horizon at 10 p.m. In the case of four youngish leaves, which were from 2 to 2} 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 Nicotiana glaca: shoots with leaves expanded during the day, and asleep at night. Figures copied from photographs, and reduced.

of the species which inust 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. Ip omea cerulea and purpurea (Convolvulacesz). —The leaves on very young plants, a foot or two in height, are depressed at night

Nicotiana tabacum :; circumnutation and nyc- titropic movement of a leat’ (5} inches in length), traced on a vertical glass, from 3 pM. July 10th to 8.10 a.m. 13th. Apex of leaf 4 inches from glass. Temp. 17$°- 185° C. Figure reduced to one-half original scale. to between 68° and 80° beneath the horizon; and some hang quite vertically downwards. On the following morn- ing they again rise into w 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 (Solaner).— Tho young leaves of both these species sleep by bendinh vertically up- wards. Figures of two shoots of N. glauca, awake and asleep (Fig. 162), are given on p 385 : one of the shoots, from which the photo- graphs were taken, was accidentally bent to oue side. At the base of the petiole of NV. 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. 886 (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 alittle. The diurnal sinking move- ment 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.