t SLEEP OF PLANTS. 257 



of motion. We have only of late years began to 

 appreciate the fact that plant organs are capable of 

 spontaneous movement, and although the earlier 

 chapters in the history of such phenomena have been 

 written, these are, doubtless, only preliminary to 

 fuller and more elaborate details which the future will 

 reveal. 



The number of genera in which nocturnal move- 

 ments have been observed is not more than about 

 eighty-six. All the species in a genus will, as a rule, 

 exhibit the same kind of motion. Here and there 

 an erratic species may be found expressing its move- 

 ments in the language of another genus, but usually 

 they are tolerably uniform. The large natural order 

 of Leguminosa or pod-bearers, to which the pea and 

 bean belongs, contains the largest number of genera 

 possessed of distinct nocturnal movements. It is 

 in this order that the " sensitive plants " are located. 

 There are in fact more genera belonging to this order 

 in which " sleeping partners " have been observed 

 than in all the other families put together. 



It is difficult to establish a definite boundary 

 between plants which exhibit nocturnal movements 

 and those which exhibit similar movements, but in a 

 less pronounced degree, as for instance in those 

 which elevate or depress their leaves but slightly, or 

 those which only exhibit the same kind of rotation in 

 the evening and morning as at mid-day. This 



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