The most complex of all the movements performed by sleeping plants, is that when leaves or leaflets, after describing in the daytime several vertically directed ellipses, rotate greatly on their axes in the evening, by which twisting movement they occupy a wholly different position at night to what they do during the day. For instance, the terminal leaflets of Cassia not only move vertically downwards in the evening, but twist round, so that their lower surfaces face outwards. Such movements are wholly, or almost wholly, confined to leaflets provided with a pulvinus. But this torsion is not a new kind of movement introduced solely for the purpose of sleep; for it has been shown that some leaflets whilst describing their ordinary ellipses during the daytime rotate slightly, causing their blades to face first to one side and then to another. Although we can see how the slight periodical movements of leaves in a vertical plane could be easily converted into the greater yet simple nyctitropic movements, we do not at present know by what graduated steps the more complex movements, effected by the torsion of the pulvini, have been acquired. A probable explanation could be given in each case only after a close investigation of the movements in all the allied forms.
From the facts and considerations now advanced we may conclude that nyctitropism, or the sleep of leaves and cotyledons, is merely a modification of their ordinary circumnutating movement, regulated in its period and amplitude by the alternations of light and darkness. The object gained is the protection of the upper surfaces of the leaves from radiation at night, often combined with the mutual protection of the several parts by their close approximation. In such cases as those of the leaflets of Cassia—of the terminal leaflets of Melilotus—of all the leaflets of Arachis, Marsilea, etc.—we have ordinary circumnutation modified to the extreme extent known to us in any of the several great classes of modified circumnutation. On this view of the origin of nyctitropism we can understand how it is that a few plants, widely distributed throughout the Vascular series, have been able to acquire the habit of placing the blades of their leaves vertically at night, that is, of sleeping,—a fact otherwise inexplicable.
The leaves of some plants move during the day in a manner, which has improperly been called diurnal sleep; for when the sun shines brightly on them, they direct their edges towards it. To such cases we shall recur in the following chapter on Heliotropism. It has been shown that the leaflets of one form of Porlieria hygrometrica keep closed during the day, as long as the plant is scantily supplied with water, in the same manner as when asleep; and this apparently serves to check evaporation. There is only one other analogous case known to us, namely, that of certain Gramineæ, which fold inwards the sides of their narrow leaves, when these are exposed to the sun and to a dry atmosphere, as described by Duval-Jouve.[[26]] We have also observed the same phenomenon in Elymus arenareus.
[26] ‘Annal. des Sc. Nat. (Bot.),’ 1875, tom. i. pp. 326–329.
There is another movement, which since the time of Linnæus has generally been called sleep, namely, that of the petals of the many flowers which close at night. These movements have been ably investigated by Pfeffer, who has shown (as was first observed by Hofmeister) that they are caused or regulated more by temperature than by the alternations of light and darkness. Although they cannot fail to protect the organs of reproduction from radiation at night, this does not seem to be their chief function, but rather the protection of the organs from cold winds, and especially from rain, during the day. the latter seems probable, as Kerner[[27]] has shown that a widely different kind of movement, namely, the bending down of the upper part of the peduncle, serves in many cases the same end. The closure of the flowers will also exclude nocturnal insects which may be ill-adapted for their fertilisation, and the well-adapted kinds at periods when the temperature is not favourable for fertilisation. Whether these movements of the petals consist, as is probable, of modified circumnutation we do not know.
[27] ‘Die Schutzmittel des Pollens,’ 1873, pp. 30–39.
Embryology of Leaves.—A few facts have been incidentally given in this chapter on what may be called the embryology of leaves. With most plants the first leaf which is developed after the cotyledons, resembles closely the leaves produced by the mature plant, but this is not always the case. the first leaves produced by some species of Drosera, for instance by D. Capensis, differ widely in shape from those borne by the mature plant, and resemble closely the leaves of D. rotundifolia, as was shown to us by Prof. Williamson of Manchester. The first true leaf of the gorse, or Ulex, is not narrow and spinose like the older leaves. On the other hand, with many Leguminous plants, for instance, Cassia, Acacia lophantha, etc., the first leaf has essentially the same character as the older leaves, excepting that it bears fewer leaflets. In Trifolium the first leaf generally bears only a single leaflet instead of three, and this differs somewhat in shape from the corresponding leaflet on the older leaves. Now, with Trifolium Pannonicum the first true leaf on some seedlings was unifoliate, and on others completely trifoliate; and between these two extreme states there were all sorts of gradations, some seedlings bearing a single leaflet more or less deeply notched on one or both sides, and some bearing a single additional and perfect lateral leaflet. Here, then, we have the rare opportunity of seeing a structure proper to a more advanced age, in the act of gradually encroaching on and replacing an earlier or embryological condition.
The genus Melilotus is closely allied to Trifolium, and the first leaf bears only a single leaflet, which at night rotates on its axis so as to present one lateral edge to the zenith. Hence it sleeps like the terminal leaflet of a mature plant, as was observed in 15 species, and wholly unlike the corresponding leaflet of Trifolium, which simply bends upwards. It is therefore a curious fact that in one of these 15 species, viz., M. Taurica (and in a lesser degree in two others), leaves arising from young shoots, produced on plants which had been cut down and kept in pots during the winter in the green-house, slept like the leaves of a Trifolium, whilst the leaves on the fully-grown branches on these same plants afterwards slept normally like those of a Melilotus. If young shoots rising from the ground may be considered as new individuals, partaking to a certain extent of the nature of seedlings, then the peculiar manner in which their leaves slept may be considered as an embryological habit, probably the result of Melilotus being descended from some form which slept like a Trifolium. This view is partially supported by the leaves on old and young branches of another species, M. Messanensis (not included in the above 15 species), always sleeping like those of a Trifolium.
The first true leaf of Mimosa albida consists of a simple petiole, often bearing three pairs of leaflets, all of which are of nearly equal size and of the same shape: the second leaf differs widely from the first, and resembles that on a mature plant (see Fig. 159, p. 379), for it consists of two pinnae, each of which bears two pairs of leaflets, of which the inner basal one is very small. But at the base of each pinna there is a pair of minute points, evidently rudiments of leaflets, for they are of unequal sizes, like the two succeeding leaflets. These rudiments are in one sense embryological, for they exist only during the youth of the leaf, falling off and disappearing as soon as it is fully grown.
With Desmodium gyrans the two lateral leaflets are very much smaller than the corresponding leaflets in most of the species in this large genus; they vary also in position and size; one or both are sometimes absent; and they do not sleep like the fully-developed leaflets. They may therefore be considered as almost rudimentary; and in accordance with the general principles of embryology, they ought to be more constantly and fully developed on very young than on old plants. But this is not the case, for they were quite absent on some young seedlings, and did not appear until from 10 to 20 leaves had been formed. This fact leads to the suspicion that D. gyrans is descended through a unifoliate form (of which some exist) from a trifoliate species; and that the little lateral leaflets reappear through reversion. However this may be, the interesting fact of the pulvini or organs of movement of these little leaflets, not having been reduced nearly so much as their blades—taking the large terminal leaflet as the standard of comparison—gives us probably the proximate cause of their extraordinary power of gyration.