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]