petioles rose 30o, others only 6o, and others sank 4o at night.  The leaflets of Cassia Barclayana moved so little at night that they could not be said to sleep, yet the petioles of some young leaves rose as much as 34o.  These several facts apparently indicate that the movements [page 402] of the petioles are not performed for any special purpose; though a conclusion of this kind is generally rash.  When the leaflets sink vertically down at night and the petioles rise, as often occurs, it is certain that the upward movement of the latter does not aid the leaflets in placing themselves in their proper position at night, for they have to move through a greater angular space than would otherwise have been necessary.

Notwithstanding what has just been said, it may be strongly suspected that in some cases the rising of the petioles, when considerable, does beneficially serve the plant by greatly reducing the surface exposed to radiation at night.  If the reader will compare the two drawings (Fig. 155, p. 371) of Cassia pubescens, copied from photographs, he will see that the diameter of the plant at night is about one-third of what it is by day, and therefore the surface exposed to radiation is nearly nine times less.  A similar conclusion may be deduced from the drawings (Fig. 149, p. 358) of a branch awake and asleep of Desmodium gyrans.  So it was in a very striking manner with young plants of Bauhinia, and with Oxalis Ortegesii.

We are led to an analogous conclusion with respect to the movements of the secondary petioles of certain pinnate leaves.  The pinnae of Mimosa pudica converge at night; and thus the imbricated and closed leaflets on each separate pinna are all brought close together into a single bundle, and mutually protect one another, with a somewhat smaller surface exposed to radiation.  With Albizzia lophantha the pinnae close together in the same manner.  Although the pinnae of Acacia Farnesiana do not converge much, they sink downwards.  Those of Neptunia oleracea likewise [page 403] move downwards, as well as backwards, towards the base of the leaf, whilst the main petiole rises.  With Schrankia, again, the pinnae are depressed at night.  Now in these three latter cases, though the pinnae do not mutually protect one another at night, yet after having sunk down they expose, as does a dependent sleeping leaf, much less surface to the zenith and to radiation than if they had remained horizontal.

Any one who had never observed continuously a sleeping plant, would naturally suppose that the leaves moved only in the evening when going to sleep, and in the morning when awaking; but he would be quite mistaken, for we have found no exception to the rule that leaves which sleep continue to move during the whole twenty-four hours; they move, however, more quickly when going to sleep and when awaking than at other times.  That they are not stationary during the day is shown by all the diagrams given, and by the many more which