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 Gramineae, 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.* We have also observed the same phenomenon in Elymus arenareus.
* 'Annal. des Sc. Nat. (Bot.),' 1875, tom. i. pp. 326-329. [page 414]
There is another movement, which since the time of Linnaeus 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* 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.
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
* 'Die Schutzmittel des Pollens,' 1873, pp. 30-39. [page 415]
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.