The Power of Movement in Plants by Charles Darwin (book club books TXT) 📖

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 well pronounced after 4

h. 30 m.

 

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.

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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.

 

In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12

genera have been observed by ourselves and [page 354]

others, but only in Robinia with any care. Psoralea acaulis raises its three leaflets at night; whilst Amorpha fruticosa,* Dalea alopecuroides, and Indigofera tinctoria depress them. Ducharte** states that Tephrosia caribaea is the sole example of “folioles couch�es le long du p�tiole et vers la base;” but a

 

Fig. 145. Lotus Creticus: 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. Wistaria Sinensis, according to Royer,*** “abaisse les folioles qui par une disposition bizarre sont inclin�es dans la m�me feuille, les sup�rieures vers le

 

* Ducharte, ‘El�ments de Botanique’, 1867, p. 349.

 

** Ibid., p. 347.

 

*** ‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868.

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sommet, les inf�rieures 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 Linnaeus, in Glycyrrhiza. The leaflets of Robinia pseudo-acacia likewise sink vertically down at night, but the petioles rise a little, viz., in one case 3o, and in another 4o. 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.

 

Coronilla rosea (Tribe 6).—the leaves bear 9 or 10 pairs of opposite leaflets, which during the day stand horizontally, with Fig. 146. Coronilla rosea: leaf asleep.

 

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 40o to 50o 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 [page 356]

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 3o above the horizon at noon, to 9o at 10 P.M.; from 11o to 33o; and from 5o to 33o—the amount of angular movement in this latter case amounting to 28o. In several other species of Coronilla the leaflets showed only feeble movements of a similar kind.

 

Hedysarum 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 Pfundii (Tribe 6).—The leaflets rise up vertically, and the main petiole also rises considerably.

 

Arachis hypogoea (Tribe 6).—The shape of a leaf, with its two pairs of leaflets, is shown at A (Fig. 147); and a leaf asleep, Fig. 147. Arachis hypogoea: A, leaf during the day, seen from vertically above; B, leaf asleep, seen laterally, copied from a photograph. 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 this 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

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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 us 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 39o. 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 from above. The temperature was 17o - 17 1/2o C., and therefore rather too low. During the 16 h. the leaflet moved thrice up and thrice down, and as the ascending and descending lines did not coincide, three ellipses were formed.

 

Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-half natural size. The minute stipules unusually large.

 

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, is here represented (Fig. 148). The large terminal leaflet sleeps by sinking vertically down, whilst the petiole rises up. The cotyledons do not sleep, but the first-formed leaf sleeps equally well as the older ones. The appearance presented by a sleeping branch and one in the daytime, copied from two photographs, are shown at A and B (Fig. 149), and we see how at night the leaves are crowded together, as if for mutual protection, by the rising of the petioles. The petioles of the younger leaves near the summits of the shoots rise up at night, so as to stand vertical and parallel to the stem; whilst those on the sides were found in four cases to have risen respectively 46 1/2o, 36o, 20o, and 19.5o above the inclined positions which they had occupied during the day. For instance, in the first of these four cases the petiole stood in the day at 23o, and at night at 69 1/2o above the horizon. In the evening the rising of the petioles is almost completed before the leaflets sink perpendicularly downwards.

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Circumnutation.—The circumnutating movements of four young shoots were observed during 5 h. 15 m.; and in this time each completed an oval figure of small size. The main petiole also circumnutates rapidly, for in the course of 31 m. (temp. 91o F.) it changed its course by as much as a rectangle six times, describing a figure which apparently represented two ellipses.

 

Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves asleep. Figures reduced.

 

The movement of the terminal leaflet by means of its sub-petiole or pulvinus is quite as rapid, or even more so, than that of the main petiole, and has much greater amplitude. Pfeffer has seen* these leaflets move through an angle of 8o in the course of from 10 to 30 seconds.

 

A fine, nearly full-grown leaf on a young plant, 8 inches in height, with the stem secured to a stick at the base of the leaf,