SLEEP-MOVEMENTS. 



411 



continued long after the movements in }'oung leaves destitute 

 of such an apparatus have ceased. 



1065. The sleep-movements of cotyledons are extremely diverse, 

 -but in general consist in an elevation of the tips, bringing the 

 upper faces into proximity, and sometimes into contact. It ma}- 

 happen also that one or more of the early leaves developed 

 from the plumule approaches the elevated cotyledons. Dar- 

 win has noted that in some cases the cotyledons of plants, 

 with ordinary leaves which exhibit sleep-movements, ma}' not 

 change their position at night, except as they do in simple 

 circumnutation. 



A 



185 



1066. The utility of the sleep- movements of leaves and cotyle- 

 dons is believed to consist in protection from too great radiation 

 during the night. Darwin has shown by simple and conclusive 

 experiments that in the case of some plants this change of the 

 position of leaves at the approach of a chilly night is a matter 

 of life and death. 



When leaves which naturally assume nyctitropic positions are 

 pinned or otherwise kept from changing their position, and the 

 plant is exposed to a temperature a little below freezing, under 

 a clear sky, into which the radiation of heat must go on rapidly 

 from the upper surface of the leaves, serious injuries result, the 

 leaves becoming browned and even killed ; whereas, leaves on 



FlG. 185. Desmodium gyrans. A, position during the day ; B, position at night. 



