280 MODIFIED CIRCUMNUTATIOX. OHAP. VI. 



CHAPTER VI. 



MODIFIED CIBCTTMNUTATION : SLEEP OR NYCTITROPIC MOVEMENTS, 

 THEIR USE: SLEEP OP COTYLEDONS. 



Preliminary sketch of the sleep or nyctitropic movements of leaves 

 Piesence of pulvini The lessening of radiation the final cause of 

 nycrtitropie movements Manner of trying experiments on leaves of 

 Oxalis. Arachis, Cassia, Melilotus, Lotus and Murnilea, and on the 

 cot\ ledons of Miuuua 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 cotyl.dons of various plants List of species Concluding 

 reiiiaiks Independence of the mctifropic movements of the leaves 

 and cotyledons of the s;ime 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, ' Sornnus Plantarurn,' 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 riodis-clien Bewegungen der Blat 

 interesting sketch of the history toryane,' 1S75, p. 1G3 

 of this subject in his 'Die Pe- 



