SENSITIVE PLANTS. 231 



susceptibility, by and by ceased to respond to the 

 stimulus, and its leaves became motionless as a result 

 of the continued vibration. Dr. Masters experienced 

 similar results in a plant carried by railwa}-. It is 

 stated that the Mimosa, which is an annual, is more 

 feeble towards the close of the year than when in full 

 vigour. In its native country, where the plant grows 

 wild, it is said that the touch of children affects the 

 plant's movements more than that of adults will do ; 

 and Dr. Sigerson has stated that it is more active 

 in its movements when excited by a person in a 

 tonic condition than when he is weary or exhausted. 

 These are curious facts, although they contribute 

 but little to the solution of the problem. " Why a 

 touch," writes Mr. Darwin, " slight pressure, or any 

 other irritant, such as electricity, heat, or the absorp- 

 tion of animal matter should modify the turgescence 

 of the affected cells in such a manner as to cause 

 movement, we do not know. But a touch acts in 

 this manner so often, and on such widely distinct 

 plants, that the tendency seems to be a very general 

 one ; and, if beneficial, it might be increased to any 

 extent." ^ We may, therefore, be excused from any 

 attempt to explain that which such an experienced 

 authority confesses that " wc do not know." The 



1 " Movements of Plants," p. 571, 



