DIRECT AND INDIRECT STIMULATION 425 



on growth is modified in a similar manner ; the organ in a 

 subtonic condition responds to stimulation by enhancement 

 of its feeble rate of growth ; in extreme cases growth at 

 standstill becomes revived under stimulation. Continuous 

 stimulation transforms the response from abnormal ac- 

 celeration into normal retardation. This is equally true 

 of photic and of electric stimulation (p. 86). 



Opposite Effects of Direct and Indirect 



Stimulation 



Stimulation gives rise to dual impulses : the positive, 

 which is of a hydraulic nature, not so dependent on the 

 conductivity of the tissue as is the excitatory negative, 

 is transmitted quickly ; the negative, which is the propa- 

 gation of protoplasmic excitation, is conducted slowly. 

 The positive impulse gives rise to expansion, positive 

 electric response, and acceleration of the rate of growth. 

 The excitatory negative impulse causes contraction, negative 

 electric response, and retardation of the rate of growth. 

 The negative reaction is more intense than the positive, 

 so that when the intervening distance between the point 

 of application of stimulus and the responding organ is 

 sufficiently small, the positive response is masked by the 

 predominant negative. When the intervening distance is 

 considerable, the negative impulse lags behind the positive, 

 and the response is diphasic, positive followed by negative. 

 When the distance to be traversed is still greater, the 

 excitatory negative impulse becomes weakened to the 

 point of extinction ; consequently, the hydraulic impulse 

 alone is effective and the response is positive, as show^n by 

 the movements of various motile leaves and by an enhanced 

 rate of grow'th in growing organs (pp. 124, 131). 



Tropic Movement under All Modes of Unilateral 



Stimulation 



The difficulty in arriving at an explanation of the diverse 

 effects induced by unilateral stimulation is attributable to 



