244 



The Living Plant 



yet one other marked response, and that most important, be- 

 cause, as is probable, it explains the original adaptation, — 

 viz. a marked sleep movement just like those which we have 

 noticed already under phototropism. The motor-mechanism 

 underlying the droop of the leaves of the Sensitive Plant is a 



Fig. 86. — Two Sensitive Plants, of which the one on the right was struck a sharp blow 



just before the photograph was taken. 



particularly efficient example of the hydraulic type already men- 

 tioned; and probably it is so highly perfected and delicately- 

 balanced that although developed originally in connection with 

 sleep movements, it can now be set ofT, so to speak, by various 

 other stimuli, such as touch, — ^precisely, for example, as a cannon 

 can be fired by a lighted match, an electric current, some chemi- 

 cals, or a sharp blow. The sensitiveness of the Sensitive Plant to 

 touch is upon this explanation accidental ; and there are probably 

 yet other examples of such accidental stimulation in other phases 



