CHAP. VII. SUMMARY ON SLEEP OF LEAVES. 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 were traced. It is troublesome 

 to observe the movements of leaves in the middle of 

 the night, but this was done in a few cases; and 

 tracings were made during the early part of the night 

 of the movements, in the case of Oxalis, Amphicarpaea, 

 two species of Erythrina, a Cassia, Passiflora, Euphorbia 

 and Marsilea ; and the leaves after they had gone to 

 sleep, were found to be in constant movement. When, 

 however, opposite leaflets come into close contact with 

 one another or with the stem at night, they are, as we 

 believe, mechanically prevented from moving, but this 

 point was not sufficiently investigated. 



When the movements of sleeping leaves are traced 

 during twenty-four hours, the ascending and descend- 

 ing lines do not coincide, except occasionally and by 

 accident for a short space ; so that with many plants a 



