CHAP. VII. SLEEP OF LEAVES: 381 



(the rudimentary ones included) form together a single vertical 

 packet. The two pinnae at the same time that they approach 

 each other sink downwards, and thus instead of extending hori- 

 zontally in the same line with the main petiole, as during the 

 day, they depend at night at about 45, or even at a greater 

 angle, beneath the horizon. The movement of the main petiole 

 seems to be variable ; we have seen it in the evening 27 lower 

 than during the day ; but sometimes in nearly the same position. 

 Nevertheless, a sinking movement in the evening and a rising- 

 one during the night is probably the normal course, for this 

 was well-marked in the petiole of the first-formed true leaf. 



The circumnutation of the main petiole of a young leaf was 

 traced during 21 days, and was considerable in extent, but less 

 complex than that of M. pudica. The movement was much 

 more lateral than is usual with circumnutating leaves, and this 

 was the sole peculiarity which it presented. The apex of 

 one of the terminal leaflets was seen under the microscope to 

 travel ^ of an inch in 3 minutes. 



Mimosa marginata. The opposite leaflets rise up and approach 

 each other at night, but do not come into close contact, except in 

 the case of very young leaflets on vigorous shoots. Full-grown 

 leaflets circumnutate during the day slowly and on a small scale. 



Schrankia uncinata (Tribe 20). A leaf consists of two or three 

 pairs of pinnae, each bearing many small leaflets. These, when 

 the plant is asleep, are directed forwards and become imbricated. 

 The angle between the two terminal pinnae was diminished at 

 night, in one case by 15 ; and they sank almost vertically down- 

 wards. The hinder pairs of pinnae likewise sink downwards, 

 but do not converge, that is, move towards the apex of the leaf. 

 The main petiole does not become depressed, at least during the 

 evening. In this latter respect, as well as in the sinking of the 

 pinnae, there is a marked difference between the nyctitropic 

 movements of the present plant and of Mimosa pudica. It 

 should, however, be added that our specimen was not in a very 

 vigorous condition. The pinnae of Schrankia acuhata also sink 

 at night. 



Acacia Farnesiana (Tribe 22). The different appearance pre- 

 sented by a bush of this plant when asleep and awake is won- 

 derful. The same leaf in the two states is shown in the following 

 figure (Fig. 160). The leaflets move towards the apex of the 

 pinna and become imbricated, and the pinnae then look like bits 

 of dangling string. The following remarks and measurements 



