Chap. VU SUMMARY ON SLEEP OF LEAVES. 397 



to leaves. We have seen that a leaf of Mimosa 

 yudica continued to move in the ordinary manner, 

 though somewhat more simply, until it withered and 

 died. It may be added that some leaflets of TrifoUum 

 pratense were pinned open during 10 days, and on the 

 first evening after being released they rose up and 

 slept in the usual manner. Besides the long con- 

 tinuance of the movements when effected by the aid 

 of a pulvinus (and this appears to be the final cause 

 of its development), a twisting movement at night, as 

 Pfeffer has remarked, is almost confined to leaves thus 

 provided. 



It is a very general rule that the first true leaf, 

 though it may diifer somewhat in shape from the 

 leaves on the mature plant, yet sleeps like them ; and 

 this occurs quite independently of the fact whether or 

 not the cotyledons themselves sleep, or whether they 

 sleep in the same manner. But with Phaseolus Eox- 

 iurghii the first unifoliate leaves rise at night almost 

 sufficiently to be said to sleep, whilst the leaflets of 

 the secondary trifoliate leaves sink vertically at night. 

 On young plants of Sida rhowhmfoUa, only a few 

 inches in height, the leaves did not sleep, though on 

 rather older plants they rose up vertically at night. 

 On the other hand, the leaves on very young plants of 

 Oytisus fragrans slept in a conspicuous manner, whilst 

 on old and vigorous bushes kept in the greenhouse, 

 the leaves did not exhibit any plain nyctitropic move- 

 ment. In the genus Lotus the basal stipule-like 

 leaflets rise up yertically at night, and are provided 

 with pulvini. 



As already remarked, when leaves or leaflets change 

 their position greatly at night and by complicated 

 movements, it can hardly be doubted that these must 

 be in some manner beneficial to the plant. If so, we 



