The Power of Movement in Plants

We have as yet referred only to the different positions of tne leaflets of L. pubescens at night; but the petioles likewise differ in their movements. That of a young leaf which formed a highly inclined star at night, stood at noon at 42° above the horizon, and during the night at 72°,so had risen 380°. The petiole of another leaf, the leaflets of which occupied » similar position at night, rose only 6°. On the other hand, the petiole of a leaf with all its leaflets sloping down at night, fell at this time 4°. The petioles of two rather older leaves were subse- quently observed ; both of which stood during the day at exactly the saine angle, viz., 50° above the horizon, and one of these rose 7°—8°, and the other fell 8°—4° at night.

We meet with cases like that of L. pubescens with some other species. On a single plant of L. mutabilis some leaves, which stood horizontally during the day, formed highly inclined stars at night, and the petiole of one rose 7°. Other leaves which likewise stood horizontally during the day, had at night all theiz leaflets sloping downwards at 46° beneath the horizon, but their petioles had hardly moved. Again, L.lut us offered a still more remarkable case, for on two leaves, the leaflets which stood at noon at about 45° above the horizon, rose at night to 65° and 69°, so that they formed a hollow cone with steep sides. Four leaves on the same plant, which had their leaflets horizontal at noon, formed vertical stars at night; and three other leaves equally horizontal at noon, had all their leaflets sloping down- wards at night. So that the leaves on this one plant assumed at night three different positions. Though we cannot account for this fact, we can see that such a stock might readily give birth to species having widely different nyctitropic habits.

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

Some observations were made on the circumnutation of the leaves of L. luteus and arbureus. It will suffice to say that the leaflets of the latter exhibited a double oscillation in the course of 24h.; for they fell from the early morning until 1015 a.m., then rose and zigzagged greatly till 4p.m., after which hour the great nocturnal fall commenced. By 8 a.m. on the following morning the leaflets had risen to their proper height. We have seen in the fourth chapter, that the leaves of Lupinus speciosus, which do not sleep, circumnutate to an extraordinary extent, making many ellipses in the course of the day.

Medicago marina: A. leaves during the day ; B, Icaves asleep at night. a few observations were made on these three genera. The petioles on a young p'ant, about a foot in height, of Cytisus fragrans vose at night, on one occasion 23° and on another 33°. The three leaflets als) lend upwards, and at the same time approach each other, so that the base of the central leaflet overlaps the bases of the two lateral leaflets. They bend up so much that they press against the stem; and on looking down on one of these }oung plants from vertically above, the lower surfaces of the leaflets are visible; and thus their upper surfaces, in accordance with the general rule, are best protected from radiation. Whilst the leaves on these young plants were thus behaving, those on an old bush in full flower did not sleep at night.

Trigonella Oretica resembles a Melilotus in its sleep, which will be immediately described. According to M. Royer,* the leaves of Medicago maculata rise up at night, and “se renversent un peu de maniére 4 presenter obliquement au ciel leur face in- ferieure.” A drawing is here given (Fig. 139) of the leaves of M. marina awake and asleep; and this would almost serve for Cytisus fragrans in the same two states. Melilotus (Tribe 3).—The species in this genus sleep in a remarkable manner. The three leaflets of each leaf twist through an angle of 90°, so that their blades stand vertically at night with one lateral elge presented to the zenith (Fig. 140). We shall best understand the other and more complicated move- ments, if we imagine ourselves always to hold the leaf with the tip of the terminal leaflet pointed to the north. The leaflets in becoming vertical at night could of course twist so that their upper surfaces should face to either side; but the two lateral leaflets always twist so that this surface tends to face the north, but as they move at the same time towards the terminal leaflet, the upper surface of the one faces about N.N.W., and that of the other N.N.E. The terminal leaflet behaves differently, for it twists to either side, the upper surface facing sometimes east and sometimes west, but rather more commonly west than east The terminal leaflet also moves in another and more remarkable manner, for whilst its blade is twisting and becoming vertical, the whole leaflet bends to one side, and invariably to the side towards which the upper surface is directed; so that if this surface faces the west the whole leaflet bends to the west, until it comes into contact with the upper and vertical surface of the western lateral leaflet. Thus the upper surface of the terminal and of one of the two lateral leaflets is well protected.

The fact of the terminal leaflet twisting indifferently to either side and afterwards bending to the same side, seemed to us 80 remarkable, that we endeavoured to discover the cause. We imagined that at the commencement of the movement it might be determined by one of the two halves of the leaflet being a little heavier than the other. Therefore bits of wood were gummed on one side of several leaflets, but this produced no effect; and they continued to twist in the same direction as

Uelilotus officinalis: A, leaf during the daytime. B, another leaf asleep. C, a leaf asleep as viewed from vertically above; but in this case the terminal leaflet did not happen to be in such close contact with the Jateral one, as is usual. they had previously done. In order to discover whether the same leaflet twisted permanently in the same direction, black threads were tied to 20 leaves, the terminal Jeaflits of which twisted so that their upper surfaces faced west, and 14 white threads to leaflets which twisted to the east. These were ob- served occasionally during 14 days, and they all continued, with a single exception, to twist and bend in ihe same direction; for

oue leaflet, which had originally faced east, was observed after 9 days to face west. The seat of both the twisting and bending movement is in the pulvinus of the sub-petioles. We believe that the leafluts, especially the two lateral ones, in performing the above described complicated movements generally bend a little downwards; but we are not sure of this, for, as far as the main petiole is concerned, its nocturnal move- ment is largely determined by the position which the leaf happens to occupy during the day. Thus one main petiole was observed to rise at night 59°, whilst three others rose only 7° and 9°. The petioles and sub-petioles are continually circum- nutating during the whole 24 h., as we shall presently see.

The leaves of the following 15 species, M. officinalis, suaveolens, parviflora, alba, tufestu, dentutu, gracilis, sulcata, elegans, ceruleu, petitpicrreana, macrorrhiza, Itulict, secundiflura, and Taurica, sleep in nearly the same manner as just described; but the bending to one side of the terminal leaflet is apt to fail unless the plants are growing vigorously. With WM. petitpierrea.a and secundiflora the terminal leaflet was rarely seen to bend to one side. In young plants of .I/. /talica it bent in the usual manner, but with old plants in full flower, growing in the same pot and observed at the same hour, viz., 8.30 p.m., none of the terminal leaflets on several scores of leavcs had bent to one side, though they stood vertically; nor nad the two lateral leaflets, though standing vertically, moved towards the terminal one. At 1030 p.m, and again one hour after midnight, the terminal leaflets had become very slightly bent to one side, and the lateral leaflets had moved a very little towards the terminal one, so that the posit.on of the leaflets even at this late hour was far from the ordinary one. Again, with M. 7aurica the terminal leaflets were never seen to bend towards either of the two lateral leaflets, though these, whilst becoming vertical, had bent towards the terminal one. The sub-petiole of the terminal leaflet in this species is of unusual length, and if the leaflet had bent to one side, its upper surface could have come into contact only with the apex of either lateral leaflet; and this, perhaps, is the meaning of the loss of the lateral movement.

‘The cotyledons donot sleep at night. ‘The first leaf consists of a single orlicular katlet, which twists at night so that the blade stands vertically. It is a remarkable fact that with M. Teurica, and in a somewhat less degree with M. macrorrhiza and petit- pierrcana, all the many small and young leaves produced during the early spring from shoots on some cut-down plants in the greenhouse, slept in a tutally different manner from the normal one; for the three leaflets, instead of twisting on their owu axes so as to present their lateral edges to the zenith, turned upwards and stood vertically with their apices pointing to the zenith. They thus assumed nearly the same position as in the allied genus Trifolium; and on the same principle that embryological characters reveal the lines of descent in the animal kingdom, so the movements of the small leaves in the above three species of Melilotus, perhaps indicate that this genus is descended from a form which was closely allied to and slept like a Trifolium. Moreover, there is one species, M. mvrs:anensis, the leaves of which, on full-grown plants between 2 and 3 feet in height, sleep like the foregoing small leaves and like those of a Trifolium. We were so much surprised at this latter case that, until the flowers and fruit were examined, we thought that the seeds of some Trifolium had been sown by mistake instead of those of a Melilotus. It appears therefore probable that d/. messanensis has either retained or recovered a primordial habit.

The circumnutation of a leaf of M. officinalis was traced, the stem being left free; and the apex of the terminal leaflet described three laterally extended ellipses, between 8 a.m. and 4p.m.; after the latter hour the nocturnal twi-ting movement commenced, It was afterwards ascertained that the above movement was compounded of the circumnutation of the stem on a small scale, of the main petiole which moved most, and of the sub-petiole of the terminal leaflet. The main petiole of a leaf having been secured to a stick, close to the base of the sub- petiole of the terminal leaflet, the latter described two small ellipses between 10.30 4am. and2pm. At 7.15 p.m, after this same leaflet (as well as another) had twisted themselves into their vertical nocturnal position, they began to rise slowly, and continued to do so until 10.35 p.u., after which hour they were no longer observed,

As M. messunensis sleeps in an anomalous manner, unlike that of any other species in the genus, the circumnutation of a terminal leaflet, with the stem secured, was traced during two days. On each morning the leaflet fell, until about noon, and then began to rise very slowly; but on the first day the rising movement was interrupted between 1 and 3 p.m. by the formation — of a laterally extended ellipse, and on the second day, at the sane time, by two smaller ellipses. The rising movement then

recommenced, and became rapid late in the evening, when the leaflet was beginning to go to sleep. The awaking or sinking movement had already commenced by 6.45 a.m on both mornings. Trifolium (Tribe 3).—The nyctitropic movements of 11 species were observed, and were found to be closely similar. If we select a leaf of 7. repens having an upright petiole, and with the three leaflets expanded horizontally, the two lateral leaflets will be seen in the evening to twist and approach each other, until their upper surfaces come into contact. At the same time they bend downwards in a plane at right angles to that of their former position, until their midribs form an angle of about 45° with the upper part of the petiole. ‘This peculiar change of position requires a considerable amount of torsion in the pul- vinus. The t.rminal leaflet merely rises up without any twist-

Trifolium repens: A, leaf during the day; B, leaf asleep at night. ing, and bends over until it rests on and forms a roof over the edges of the now vertical and united lateral leaflets. Thus the terminal] leaflet always passes through an angle of at least 90°, generally of 130° or 140°, and not rarely—as was often observed with 7. s bterraneum—of 180°. In this latter case the terminal leaflet stands at night horizontally (as in Fig. 141), with its lower surface fully exposed tothe zenith. Besides the difference in the angles, at which the terminal leaflets stand at night in the individuals of the same species, the degree to which the lateral leaflets approach each other often likewise differs.

We have seen that the cotyledons of some species and not of others rise up vertically at night. The first true leaf is generally unifoliate and orbicular; it always rises, and either stands verti- cally at night or more commonly bends a little over so as to expose the lower surface obliquely to the zenith, in the same manner as does the terminal leaflet of the mature leaf. But it does not twist itself like the corresponding first simple leaf of Melilotus.

With 7. Punnonicum the first true leaf was generally unifoliate, but sometimes trifoliate, or again partially lobed and in an intermediate condition. Circumnutation.—Sachs described in 1868* the spontaneous up and down movements of the leaflets of 7. ‘ncarnatum, when kept in darkness. Pfeffer made many observations on the similar movements in 7’ pratense.t He states that the terminal leaflet of this species, observed at different times, passed through angles of from 30° to 120° in the course of from 14 to4h. We observed the movements of 7. subterraneum, resupinatwm, and repens,

Trifolium subterranenm.—A petiole was secured close to the base of the three leaflets, and the movement of the terminal leafict was traced during 263 h., as shown in the figure on the next page Between 6.45 a.m.and 6 p.m. the apex moved 8 times up and 3 times down, completing 8 ellipses in 11h. 15 m. The ascending and descending lines stand nearer to one another than is usual with most plants,. yet there was some lateral motion. At 6 p.m. the great nocturnal rise commenced, and on the next morning the sinking of the leaflet was continned until 8.30 a.m., after which hour it cireumnutated in the manner just described. In the figure the great nocturnal rise and the morning fall are greatly abbreviated, from the want of space, and are merely represented by a short curved line. ‘The leaflet stood horizontally when at a point a little beneath the middle of the diagram; so that during the daytime it oscillated almost equally above and beneath a horizontal position. At 8.30 a.m. it stood 48° beneath the horizon, and by 11.50 a.m. it had risen 50° above the horizon; so that it passed through 98° in 8h. By the aid of the tracing we ascertained that the distance travelled in the 3 h. by the apex of this leaflet was 1:03 inch. If we look at the figure, and prolong upwards in our mind’s eye the short curved broken line, which repre- sents the nocturnal course, we sec that the latter movement is merely an exaggeration or pro'ongation of one of the diurnal ellipses. The same leaflet had been observed on the previous day, and the course then pursued was almost identically the same as that here described.

Trifolium res.pinatum.—A plant left entirely fre. was placed dow, in such a position that a terminal leaflet projected at right angles to the source of the light, the sky being uniformly clouded all day. The movements of this leaflet were traced during two days, and on both were closely similar, Those executed on the second day are shown in Fig. 143. The obliquity of the several lines is due partly to the manner in which the leaflet was viewed, and partly to its having moved a little to- wards the light. From 7.5) am. to 840 a.m. the leaflet fell, that is, the awakening movement was continued. It then rose and moved a little late- rally towards the light. At 12.30 it retrograded, and at 2.30 resumed its original course, having thus completed a small ellipse during the middle of the day. In the even- ing it rose rapidly, and by 8 a.m. on the following morning had returned to exactly the same spot as on the previous morning. The line representing the nocturnal course ought to be extended much higher up, and is here abbreviated into a short,

curved, broken line. The terminal leaflet, therefore, of this species described during the daytime only a single additional ellipse, instead of two ad- But we should remember that it was shown in the fourth chapter that the stem circumnutates, as no doubt does the main petiole and the sub-petioles; sa that the movement repre- sented in fig. 143 is a com- pounded one We tried to observe the movements of a leaf kept during the day in darkness, but it

Trifolium resupinatum: circumnutation began to po Aes after and nyctitropic movements of the ter- 2h. 15 m, an this was minal leaflet during 24 hours. well pronounced after 4 h. Trifolium repens.—A stem was secured close to the base of a moderately old leaf, and the movement of the terminal leaflet was observed during two days. This case is interesting solely from the simplicity of the movements, in contrast with those of the two preceding species. On the first day the leafict fell between 8 a.m. and 8 p.m., and on the second between 7 a.m. and 1 p.m. On both days the descending course was somewhat zigzag, and this evidently represents the circumnutating move- ment of the two previous species during the middle of the day. After 1 p.m., Oct. lst (Fig. 144), the leaflet began to rise, but the movement was slow on both days, both before and after this hour, until4 pm. The rapid evening and nocturnal rise then commenced. Thus in this species the course during 24h. consists of a single great ellipse; in 7. resupinatum of two ellipses, one of which includes the nocturnal movement and is much elongated; and in T. subterranenm of three ellipses, of which the nocturnal one is likewise of great length.

Securigera covonilla (Tribe 4).—The leaflets, which stand opposite one another and are numerous, 1ise up at night, come into close contact, ard bend backwards at a moderate angle towards the base of the petiole. main petiole rises a little at night, and the three leaflets rise till they become vertical, and at the same time approach each other. This was conspicuous with di, Jacobeus, in which the leaflets are almost linear. In most of the species the Jeaflets rise so much as to press against the stem, and not rarely they become inclined a little inwards with their lower surfaces exposed obliquely to the zenith. This was clearly the case with L. major, as its petioles are unusually long, and the leaflets are thus enabled to bend further inwards. The young leaves on the summits of the stems close up at night so much, as often to resemble large buds. The stipule-like leaflets, which are often of large size, rise up like the other leaflets, and press against the stem (Fig. 145). All the leaflets of LZ. Gebelii, and pro- bably of the other species, are provided at their bases with distinct pulvini, of a yellowish colour, and formed of very small cells. The circumnutation of a terminal leaflet of L. perigrinus (with the stem secured) was traced during two days, but the movement was so simple that it is not worth while to give the diagram. The leaflet fell slowly from the early morning till about 1 p.m. It then rose gradually at first, but rapidly late in the evening.

nutation and nyctitropic movements of a nearly full-grown terminal leaflet, traced on a ver- tical glass from 7 A.M. Sept. 30th to 8 a.m. Oct. 1st. Nocturnal course, represented by curved broken line, much ab- breviated. It occasionally stood still for about 20 m. during the day, and basal, stipule-like leaflets was likewise traced in the same manner and at the same time, and its course was closely similar In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12 genera have been observed by ourselves and

others, but only in Robinia with any care. /'suralea acaulis raises its three leaflets at night; whilst Amorpha fruticosu,* Dalea alopccuroid.s, and Indigofera tinctoria depress them. Duchartre ¢ states that Tephrosia caribea is the sole example xf “ folioles couchées le long du petiole et vers la base;” but a Lotus C.eticus : A, stem with leaves awake during the day ; B, with leaves asleep at night. SS, stipule-like leaflets. similar movement occurs, as we have already seen, and shall again see in other cases. Histaria Sinensis, according to Royer.{ “abaisse les folioles qui par une disposition bizarre sont inclinées dans la méme feuille, les supérieures vers le

sommet, les inférieures vers la base du petiole commun;” but the leaflets on a young plant observed by us in the green- house merely sank vertically downwards at night. The leaflets are raised in Spherophysa salsola, Colutea arborea, and <As!ra- galus uliyinosus, but are depressed, according to Linneeus, in Glycyrrhiza. The leaflets of Robinia ps:udu-acacia likewise sink vertically down at night, but the petioles rise a little, viz., in one case 3°, and in another 4°. The circumnutating move- ments of a terminal leaflet on a rather old leaf were traced during two days, and were simple. The leaflet fell slowly, in a slightly zigzag line, from 8 am. to 5 p.m., and then more rapidly; by 7 4.u. on the following morning it had risen to its diurnal position. There was only one peculiarity in the move- ment, namely, that on both days there was a distinct though small oscillation up and down between 8.30 and 10 a.m., and this would probably have been more strongly pronounced if the leaf had been younger.

Coronilla rosea (Tribe 6).—The leaves bear 9 or 10 pairs of opposite leaflets, which during the day stand horizontally, with their midribs at right angles to the petiole. At night they rise up, so that the opposite leaflets come nearly into contact, and those on the younger leaves into close contact. At*the same time they hend back towards the base of the petiole, until their midribs form with it angles of from 40° to 50° in a vertical plane, as here figured (Fig. 146). The leaflets, however, some- times bend so much back that their midribs become parallel to and lie on the petiole. They thus occupy a reversed position to what they do in several Leguminose, for instance, in Mimosa

pudica; but, from standing further apart, they do not overlap oue another nearly so much as in this latter plant. ‘The main petiole is curved slightly downwards during the day, but straightens itself at night. In three cases it rose from 8° above the horizon at noon, to 9° at 10 p.m.; from 11° to 83°; and from 5° to 33°-—the amount of angular movement in this latter case amounting to 28°. In several other species of Coronilla the leaflcts showed only feeble movements of a similar kind,

Ledysarum coronarium (Tribe 6).—The small lateral leaflets on plants growing out of doors rose up vertically at night, but the large terminal one became only moderately inclined. The petioles apparently did not rise at all. Smithia P/undii (Tribe 6).—The leaflets rise up vertically, and the main petiole also rises considerably. Arachis hypoyea (Tribe 6).—The shape of a leaf, with its two pairs of leaflets, is shown at A (Fig. 147); and a leaf asleep,

Arachis hypoqgea: A, leaf during the day, seen from vertically above; B, leaf asleep, seen laterally ; copied from a photogriph, Figures much reduced. traced from a photograph (made by the aid of aluminium light), is given at B. The two terminal leaflets twist round at night until their blades stand vertically, and approach each other until they meet, at the same time moving a little upwards and backwards. The two lateral leaflets meet each other in the same manner, but move to a greater extent forwards, that is, in a contrary direction to the two terminal leaflets, which they partially embrace. Thus all four leaflets form together a single packet, with their edges directed to the zenith, and with their lower surfaces turned outwards. On a plant which was not growing vigorously the closed leaflets seemed too heavy for the

petioles to support them in a vertical position, so that each night the main petiole became twisted, and all the packets were extended horizontally, with the lower surfaces of the leaflets on one side directed to the zenith in a most anomalous manner. This fact is mentioned solely as a caution, as it surprised ug greatly, until we discovered that it was an anomaly. The petioles are inclined upwards during the day, but sink at night, so as to stand at about right angles with the stem. The amount of sinking was measured only on one occasion, and found to be 39°. A petiole was secured to a stick at the base of the two terminal leaflets, and the circumnutating movement of one of these leaflets was traced from 6.40 a.m. to 10.40 p.m., the plant being illuminated fromabove. The temperature was 17°—174° C., and therefore rather toolow. During the 16 h. the leaflet moved thrice up and thrice down, and as the ascending and desceud- ing lines did not coincide, three ellipses were formed. Desmodium gyrans (Tribe 6).—A large and full-grown leaf of this plant, so famous for the spontaneous movements of the two little lateral leaflets, Fig. 148. is here represented (Fig. 148). The large terminal leaflet sleeps by sinking vertically down, whilst the petiole rises up. The coty- \ ledons do not sleep, but the first-formed leaf / sleeps equally well as the older ones. The appearance presented by a sleeping branch XI and one in the day-time, copied from two \ , photographs, are shown at A and B (Fig. 149), and we see how at night the leaves are “ C/) crowded together, as if for mutual pro- tection, by the rising of the petioles. The petioles of the younger leaves near the sum- 5 mits of the shoots rise up at night, so as to Sy stand vertical and parallel to the stem ; whilst those on the sides were found in four cases to have risen respectively 463°, 36°,