SLEEP OF PLANTS. 243 



fied by the evidence. Under these circumstances we 

 see no difficulty in accepting it as an interpretation. 



We return now to the sleep of leaves. This is a 

 phenomenon not unknown to those who have been in 

 the habit of observing plants, as even children have 

 noticed it. The different appearance which certain 

 leaves present in the evening to that of mid-day could 

 not escape recognition. Such, for instance, as the 

 drooping leaflets of the common acacia-tree, the 

 Robinia of gardeners, and the rcflexcd leaflets of the 

 wood-sorrel, and white clover. These are the most 

 common and readily observed. Investigation has 

 shown that movements of leaves, upwards or down- 

 wards, in the evening, are far more common than ha.'^ 

 been supposed ; to some of the details of which we 

 shall have to refer. IMovemcnts of thij kind must 

 not be confounded with the closing of flowers on 

 dull days, or at eventide, nor with their turning 

 towards, or in opposition to the sun. For the present 

 we concern ourselves entirely with the periodic leaf 

 movements which are observed night and morning, 

 and which bear an intimate relationship to the 

 elevation and depression of the cotyledons, to which 

 we have already directed attention. The movements 

 called by Darwin the " nyctitropism of leaves." 



The pretty little wood-sorrel {Oxalis acetosella) is 

 almost universally known and admired. The leaves 

 are of a beautiful pale green, not unlike in form the 



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