during at least a part of the day they do not sleep at night; and a little difference in the degree of illumination determines whether or not they shall become vertical at night. We have observed no other so well-marked a case as this, of the influence of previous illumination on nyctitropic movements. The leaves present also another peculiarity in their habit of rising or awaking in the morning, being more strongly fixed or inherited than that of sinking or sleeping at night. The movements are caused by the bending of an upper part of the petiole, between ½ and 1 inch in length; but the part close to the blade, for about 1/4 of an inch in length, does not bend and always remains at right angles to the blade. The bending portion does not present any external or internal difference in structure from the rest of the petiole. We will now give the experiments on which the above conclusions are founded.
A large pot with several plants was brought on the morning of Sept. 3rd out of the greenhouse and placed before a north-east window, in the same position as before with respect to the light, as far as that was possible. On the front of the plants, 24 leaves were marked with thread, some of which had their blades horizontal, but the greater number were inclined at about 45°, beneath the horizon; at night all these, without exception, became vertical. Early on the following morning (4th) they reassumed their former positions, and at night again became vertical. On the 5th the shutters were opened at 6.15 A.M., and by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m. and had acquired their diurnal position, they were placed in a dark cupboard. They were looked at twice during the day and thrice in the evening, the last time at 10.30 P.M., and not one had become vertical.
At 8 A.M. on the following morning (6th) they still retained the same diurnal position, and were now replaced before the north-east window.
At night all the leaves which had faced the light had their petioles curved and their blades vertical; whereas none of the leaves on the back of the plants, although they had been moderately illuminated by the diffused light of the room, were vertical. They were now at night placed in the same dark cupboard; at 9 A.M. on the next morning (7th) all those which had been asleep had reassumed their diurnal position.
The pot was then placed for 3 h. in the sunshine, so as to stimulate the plants; at noon they were placed before the same north-east window, and at night the leaves slept in the usual manner and awoke on the following morning. At noon on this day (8th) the plants, after having been left before the north-east window for 5 h. 45 m. and thus illuminated (though not brightly, as the sky was cloudy during the whole time), were replaced in the dark cupboard, and at 3 P.M. the position of the leaves was very little, if at all, altered, so that they are not quickly affected by darkness; but by 10.15 P.M. all the leaves which had faced the north-east sky during the 5 h. 45 m. of illumination stood vertical, whereas those on the back of the plant retained their diurnal position. On the following morning (9th) the leaves awoke as on the two former occasions in the dark, and they were kept in the dark during the whole day; at night a very few of them became vertical, and this was the one instance in which we observed any inherited tendency or habit in this plant to sleep at the proper time.
That it was real sleep was shown by these same leaves reassuming their diurnal position on the following morning (10th) whilst still kept in the dark.
The pot was then (9.45 A.M. 10th) replaced, after having been kept for 36 h. in darkness, before the north-east window; and at night the blades of all the leaves (excepting a few on the back of the plants) became conspicuously vertical.
At 6.45 A.M. (11th) after the plants had been illuminated on the same side as before during only 25 m., the pot was turned round, so that the leaves which had faced the light now faced the interior of the room, and not one of these went to sleep at night; whilst some, but not many, of those which had formerly stood facing the back of the room and which had never before been well illuminated or gone to sleep, now assumed a vertical position at night. On the next day (12th) the plant was turned round into its original position, so that the same leaves faced the light as formerly, and these now went to sleep in the usual manner. We will only add that with some young seedlings kept in the greenhouse, the blades of the first pair of true leaves (the cotyledons being hypogean) stood during the day almost horizontally and at night almost vertically.
A few observations were subsequently made on the circumnutation of three leaves, whilst facing a north-east window; but the tracings are not given, as the leaves moved somewhat towards the light. It was, however, manifest that they rose and fell more than once during the daytime, the ascending and descending lines being in parts extremely zigzag. The nocturnal fall commenced about 7 P.M., and the leaves had risen considerably by 6.45 A.M. on the following morning.
Leguminosae.—This Family includes many more genera with sleeping species than all the other families put together. The number of the tribes to which each genus belongs, according to Bentham and Hooker’s arrangement, has been added.
Crotolaria (sp.?) (Tribe 2).—This plant is monophyllous, and we are informed by Mr. T. Thiselton Dyer that the leaves rise up vertically at night and press against the stem.
Lupinus (Tribe 2).—The palmate or digitate leaves of the species in this large genus sleep in three different manners. One of the simplest, is that all the leaflets become steeply inclined downwards at night, having been during the day extended horizontally. This is shown in the accompanying figures (Fig. 137), of a leaf of L. pilosus, as seen during the day from vertically above, and of another leaf asleep with the leaflets inclined downwards. As in this position they are crowded together, and as they do not become folded like those in the genus Oxalis, they cannot occupy a vertically dependent position; but they are often inclined at an angle of 50° beneath the horizon. In this species, whilst the leaflets are sinking, the petioles rise up, in two instances when the angles were measured to the extent of 23°. The leaflets of L. sub-carnosus and arboreus, which were horizontal during the day, sank down at night in nearly the same manner; the former to an angle of 38° and the latter of 36°, beneath the horizon; but their petioles did not move in any plainly perceptible degree. It is, however, quite possible, as we shall presently see, that if a large number of plants of the three foregoing and of the following species were to be observed at all seasons, some of the leaves would be found to sleep in a different manner.
Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in daytime; B, leaf asleep, seen laterally at night.
In the two following species the leaflets, instead of moving downwards, rise at night. With L. Hartwegii some stood at noon at a mean angle of 36° above the horizon, and at night at 51°, thus forming together a hollow cone with moderately steep sides. The petiole of one leaf rose 14° and of a second 11° at night. With L. luteus a leaflet rose from 47° at noon to 65° above the horizon at night, and another on a distinct leaf rose from 45° to 69°. The petioles, however, sink at night to a small extent, viz., in three instances by 2°, 6°, and 9° 30 seconds. Owing to this movement of the petioles, the outer and longer leaflets have to bend up a little more than the shorter and inner ones, in order that all should stand symmetrically at night. We shall presently see that some leaves on the same individual plants of L.
luteus sleep in a very different manner.
We now come to a remarkable position of the leaves when asleep, which is common to several species of Lupines. On the same leaf the shorter leaflets, which generally face the centre of the plant, sink at night, whilst the longer ones on the opposite side rise; the intermediate and lateral ones merely twisting on their own axes. But there is some variability with respect to which leaflets rise or fall. As might have been expected from such diverse and complicated movements, the base of each leaflet is developed (at least in the case of L. luteus) into a pulvinus. The result is that all the leaflets on the same leaf stand at night more or less highly inclined, or even quite vertically, forming in this latter case a vertical star. This occurs with the leaves of a species purchased under the name of L. pubescens; and in the accompanying figures we see at A (Fig. 138) the leaves in their diurnal position; and at B the same plant at night with the two upper leaves having their leaflets almost vertical. At C another leaf, viewed laterally, is shown with the leaflets quite vertical. It is chiefly or exclusively the youngest leaves which form at night vertical stars. But there is much variability in the position of the leaves at night on the same plant; some remaining with their leaflets almost horizontal, others forming more or less highly inclined or vertical stars, and some with all their leaflets sloping downwards, as in our first class of cases.
It is also a remarkable fact, that although all the plants produced from the same lot of seeds were identical in appearance, yet some individuals at night had the leaflets of all their leaves arranged so as to form more or less highly inclined stars; others had them all sloping downwards and never forming a star; and others, again, retained them either in a horizontal position or raised them a little.
Fig. 138. Lupinus pubescens: A, leaf viewed laterally during the day; B, same leaf at night; C, another leaf with the leaflet forming a vertical star at night. Figures reduced.
We have as yet referred only to the different positions of the 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 30°. The petiole of another leaf, the leaflets of which occupied a 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 subsequently observed; both of which stood during the day at exactly the same angle, viz., 50° above the horizon, and one of these rose 7°–8°, and the other fell 3°–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 their leaflets sloping downwards at 46° beneath the horizon, but their petioles had hardly moved. Again, L. luteus 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 downwards 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 Lupinus; several, namely, L. polyphyllus, nanus, Menziesii, speciosus, and albifrons, though observed out of doors and in the greenhouse, 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 Tropaeolum 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 brought 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 arboreus. It will suffice to say that the leaflets of the latter exhibited a double oscillation in the course of 24 h.; for they fell from the early morning until 10.15 A.M., then rose and zigzagged greatly till 4 P.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.
Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3).—Only a few observations were made on these three genera. The petioles on a young plant, about a foot in height, of Cytisus fragrans rose at night, on one occasion 23° and on another 33°. The three leaflets also bend 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 young 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.
Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep at night.
Trigonella Cretica resembles a Melilotus in its sleep, which will be immediately described. According to M. Royer,[9] the leaves of Medicago maculata rise up at night, and “se renversent un peu de manière à presenter obliquement au ciel leur face inférieure.” 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 edge presented to the zenith (Fig. 140). We shall best understand the other and more complicated movements, 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 so 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 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 leaflets of which twisted so that their upper surfaces faced west, and 14 white threads to leaflets which twisted to the east. These were observed occasionally during 14 days, and they all continued, with a single exception, to twist and bend in the same direction; for one 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.
Fig. 140. Melilotus 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 lateral one, as is usual.
We believe that the leaflets, 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 movement 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 circumnutating during the whole 24 h., as we shall presently see.
The leaves of the following 15 species, M. officinalis, suaveolens, parviflora, alba, infesta, dentata, gracilis, sulcata, elegans, coerulea, petitpierreana, macrorrhiza, Italica, secundiflora, 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 M. petitpierreana and secundiflora the terminal leaflet was rarely seen to bend to one side. In young plants of M. Italica 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 leaves had bent to one side, though they stood vertically; nor had the two lateral leaflets, though standing vertically, moved towards the terminal one. At 10.30 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 position of the leaflets even at this late hour was far from the ordinary one. Again, with M. Taurica 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 do not sleep at night. the first leaf consists of a single orbicular leaflet, which twists at night so that the blade stands vertically. It is a remarkable fact that with M. Taurica, and in a somewhat less degree with M. macrorrhiza and petitpierreana, 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 totally different manner from the normal one; for the three leaflets, instead of twisting on their own 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. messanensis, 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 M. 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 4 P.M.; after the latter hour the nocturnal twisting 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 A.M., and 2 P.M. 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.M., after which hour they were no longer observed.
As M. messanensis 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 same 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 T. 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 pulvinus. The terminal leaflet merely rises up without any twisting 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 T. subterraneum—of 180°. In this latter case the terminal leaflet stands at night horizontally (as in Fig. 141), with its lower surface fully exposed to the 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.
Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at night.
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 vertically 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 T. Pannonicum the first true leaf was generally unifoliate, but sometimes trifoliate, or again partially lobed and in an intermediate condition.
Circumnutation.—Sachs described in 1863[10] the spontaneous up and down movements of the leaflets of T. incarnatum, when kept in darkness.
Pfeffer made many observations on the similar movements in T.
pratense.[11] 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 1½ to 4 h. We observed the movements of T.
subterraneum, resupinatum, and repens.
Trifolium subterraneum.—A petiole was secured close to the base of the three leaflets, and the movement of the terminal leaflet was traced during 26½ h., as shown in the figure on the next page.
Between 6.45 A.M. and 6 P.M. the apex moved 3 times up and 3 times down, completing 3 ellipses in 11 h. 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 continued until 8.30 A.M., after which hour it circumnutated 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 11.30 A.M. it had risen 50° above the horizon; so that it passed through 98° in 3 h. 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 represents the nocturnal course, we see that the latter movement is merely an exaggeration or prolongation 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.
Fig. 142. Trifolium subterraneum: circumnutation and nyctitropic movement of terminal leaflet (.68 inch in length), traced from 6.45 A.M. July 4th to 9.15 A.M. 5th. Apex of leaf 3 7/8 inches from the vertical glass, and movement, as here shown, magnified 5 1/4 times, reduced to one-half of original scale. Plant illuminated from above; temp. 16°–17° C.
Trifolium resupinatum.—A plant left entirely free was placed before a north-east window, 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 towards the light. From 7.50 A.M. to 8.40 A.M.
the leaflet fell, that is, the awakening movement was continued. It then rose and moved a little laterally 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 evening 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 additional ones, as in the case of T. subterraneum. 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; so that the movement represented in Fig. 143 is a compounded one. We tried to observe the movements of a leaf kept during the day in darkness, but it began to go to sleep after 2 h. 15 m., and this was Fig 143. Trifolium resupinatum: circumnutation and nyctitropic movements of the terminal leaflet during 24 hours.
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 leaflet fell between 8 A.M. and 3 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 movement of the two previous species during the middle of the day.
After 1 P.M., Oct. 1st (Fig. 144), the leaflet began to rise, but the movement was slow on both days, both before and after this hour, until 4 P.M. The rapid evening and nocturnal rise then commenced. Thus in this species the course during 24 h. consists of a single great ellipse; in T. resupinatum of two ellipses, one of which includes the nocturnal movement and is much elongated; and in T. subterraneum of three ellipses, of which the nocturnal one is likewise of great length.
Securigera coronilla (Tribe 4).—The leaflets, which stand opposite one another and are numerous, rise up at night, come into close contact, and bend backwards at a moderate angle towards the base of the petiole.
Fig. 144. Trifolium repens: circumnutation and nyctitropic movements of a nearly full-grown terminal leaflet, traced on a vertical glass from 7 A.M. Sept. 30th to 8 A.M. Oct. 1st. Nocturnal course, represented by curved broken line, much abbreviated.
Lotus (Tribe 4).—The nyctitropic movements of 10 species in this genus were observed, and found to be the same. The 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 L.
Jacoboeus, in which the leaflets are almost linear. In most of the species the leaflets 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 L. Gebelii, and probably 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. peregrinus (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. It occasionally stood still for about 20 m. during the day, and sometimes zigzagged a little. The movement of one of the basal, stipule-like leaflets was likewise traced in the same manner and at the same time, and its course was closely similar to that of the terminal leaflet.
Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B, with leaves asleep at night. SS, stipule-like leaflets.
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. Psoralea acaulis raises its three leaflets at night; whilst Amorpha fruticosa,[12] Dalea alopecuroides, and Indigofera tinctoria depress them. Ducharte[13] states that Tephrosia caribaea is the sole example of “folioles couchées le long du pétiole et vers la base;” but a similar movement occurs, as we have already seen, and shall again see in other cases.
Wistaria Sinensis, according to Royer,[14] “abaisse les folioles qui par une disposition bizarre sont inclinées dans la même 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 greenhouse merely sank vertically downwards at night. The leaflets are raised in Sphaerophysa salsola, Colutea arborea, and Astragalus uliginosus, but are depressed, according to Linnæus, in Glycyrrhiza. The leaflets of Robinia pseudo-acacia likewise sink vertically down at night, but the petioles rise a little, viz., in one case 3°, and in another 4°. The circumnutating movements 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 A.M. to 5 P.M., and then more rapidly; by 7 A.M. on the following morning it had risen to its diurnal position. There was only one peculiarity in the movement, 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.
[13] Ibid., p. 347.
[14] ‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868.
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 bend 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, sometimes 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 Leguminosae, for instance, in Mimosa pudica; but, from standing further apart, they do not overlap one 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 3° above the horizon at noon, to 9° at 10 P.M.; from 11° to 33°; and from 5° to 33°—the amount of angular movement in this latter case amounting to 28°. In several other species of Coronilla the leaflets showed only feeble movements of a similar kind.
Fig. 146. Coronilla rosea: leaf asleep.
Hedysarum coronarium (Tribe 6).—The small lateral leaflets on plants