436 CHAP. XXXVII. GENERAL REVIEW 



often complex, owing either to the organisation of the organ, 

 as, for instance, when it is anisotropic, or to the fact that 

 they are the resultants of simultaneous action of two or 

 more stimulating agents. But it has been shown that such 

 complex movements are susceptible of analysis, and can 

 then be adequately accounted for on general principles. 



The fundamental principle is that growth is retarded by 

 strong and accelerated by weak stimulation of whatever 

 kind. Closely connected with it is the further principle 

 that direct stimulation retards and indirect stimulation 

 accelerates the rate of growth : this is the essential feature 

 of the mechanism of tropisms. There is no longer any 

 ground for assuming distinct irritabilities, such as the 

 phototropic and the geotropic, or negative and positive 

 phototropism and geotropism : these terms may remain as 

 merely descriptive of the visible response. There is but 

 one irritability of the growing organ which responds to all 

 stimuli that may act upon it, and in essentially the same 

 manner. 



