24 DARWINISM STATED BY DARWIN HIMSELF. 



THE SLEEP OF THE PLANTS. 



The Power The so-called sleep of leaves is so conspicu- 



in Plant™ 00 ous a Phenomenon that it was observed as early- 

 page 280. as the time of Pliny ; and since Linnseus pub- 

 lished his famous essay, " Somnus Plantarum," it has 

 been the subject of several memoirs. Many flowers close 

 at night, and these are likewise said to sleep ; but we are 

 not here concerned with their movements, for although 

 effected by the same mechanism as in the case of young 

 leaves, namely, unequal growth on the opposite sides (as 

 first proved by Pfeffer), yet they differ essentially in being 

 excited chiefly by changes of temperature instead of light ; 

 and in beiDg effected, as far as we can judge, for a differ- 

 ent purpose. Hardly any one supposes that there is any 

 real analogy between the sleep of animals and that of 

 plants, whether of leaves or flowers. It seems, therefore, 

 advisable to give a distinct name to the so-called sleep- 

 movements of plants. These have also generally been con- 

 founded, under the term "periodic," with the slight daily 

 rise and fall of leaves, as described in the fourth chapter ; 

 and this makes it all the more desirable to give some dis- 

 tinct name to sleep-movements. Nyctitropism and nycti- 

 tropic, i. e., night-turning, may be applied both to leaves 

 and flowers, and will be occasionally used by us ; but it 

 would be best to confine the term to leaves. 



p 2gl Leaves, when they go to sleep, move either 



upward or downward, or, in the case of the 

 leaflets of compound leaves, forward, that is, toward the 

 apex of the leaf, or backward, that is, toward its base ; or, 

 again, they may rotate on their own axis without mov- 

 ing either upward or downward. But in almost every 

 case the plane of the blade is so placed as to stand nearly 



