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 railway. It is 

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

 feeble towards the close of the year than when in full 

 vig-our. 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." x We may, therefore, be excused from any 

 attempt to explain that which such an experienced 

 authority confesses that " we do not know." The 



1 " Movements of Plants," p. 571. 



