280 MODIFIED CIRCUMNUTATION. CHAP. VI. 



CHAPTER VI. 



MODIFIED CIRCUMNUTATION : SLEEP OR NYCTITROPIC MOVEMENTS, 

 THEIR USE: SLEEP OF COTYLEDONS. 



Preliminary sketch of the sleep or nyctitropic movements of leaves- 

 Presence of pulvini The lessening of radiation the final cause of 

 nyctitropic movements Manner of trying experiments on leaves of 

 Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, and on the 

 cotyledons of Mimosa Concluding remarks on radiation from leaves 

 Small differences in the conditions make a great difference in the 

 result Description of the nyctitropic position and movements of 

 the cotyledons of various plants List of species Concluding 

 remarks Independence of the nyctitropic movements of the leaves 

 and cotyledons of the same species Reasons for believing that the 

 movements have been acquired for a special purpose. 



THE so-called sleep of leaves is so conspicuous a 

 phenomenon that it was observed as early as the 

 time of Pliny ;* and since Linnaeus published 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 being effected, as 

 far as we can judge, for a different purpose. Hardly 

 any one supposes that there is any real analogy 



* Pfeffer has given a clear and riodigchen Bewegungen der Blat 

 interesting sketch of the history torgaue,' 1875, p. 163. 

 of this subject in his * Die Pe- 



