V Movements of Plants 85 



in a larger number the cotyledons sleep, but not the leaves ; 

 in many both sleep, but in widely different positions. Yet 

 if sleep occur the general result is always the same ; the 

 blade is placed in such a position at night that its upper 

 surface is exposed as little as possible to full radiation. 



The movements are without doubt associated with the 

 daily alternation of light and darkness ; but it is the 

 difference in the illumination rather than the darkness 

 which excites the change, for Darwin showed that in 

 several species, if the leaves have not been brightly 

 illuminated during the day, they do not sleep at night. 

 Moreover, in the North, where the sun does not set, 

 Mimosa still goes regularly to sleep ; and by artificial 

 light and darkness the daily movements may be reversed. 

 " The presence of light or its absence cannot be supposed 

 to be the direct cause of the movements, for these are 

 wonderfully diversified even with the leaflets of the same 

 leaf, although all have of course been similarly exposed. 

 The movements depend on innate causes, and are of an 

 adaptive nature. The alternations of light and darkness 

 merely give notice to the leaves that the period has arrived 

 for them to move in a certain manner. We may infer 

 from the fact of several plants (Tropseolum, Lupine, etc.) 

 not sleeping unless they have been well illuminated during 

 the day, that it is not the actual decrease of the light in 

 the evening, but the contrast between the amount at this 

 hour and during the early part of the day, which excites 

 the leaves to modify their ordinary mode of circumnuta- 

 tion. As the leaves of most plants assume their proper 

 diurnal position in the morning, although light be excluded, 

 and as the leaves of some plants continue to move in the 

 normal manner in darkness during at least a whole day, 

 we may conclude that the periodicity of their movements 

 is to a certain extent inherited. The strength of such 



