124 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE 



fortunate fact of the existence of the motile leaf of Mimosa 

 that makes possible the most convincing demonstration of 

 the reactions underlying the mechanics of tropic curvature. 



To recapitulate : Stimulation gives rise to dual impulses, 

 positive and negative ; of these the positive impulse is 

 not entirely dependent on the conducting power of the 

 tissue, but the propagation of the excitatory negative 

 impulse is greatly dependent on the conducting power. 

 No tissue is a perfect conductor, nor is any a perfect non- 

 conductor of excitation, the difference being a question of 

 degree. A semi-conducting tissue, on feeble stimulation, 

 will transmit only the positive impulse ; on strong or long- 

 continued stimulation it will transmit both positive and 

 negative impulses, the positive preceding the negative. 

 The transmitted positive gives rise to increase of turgor 

 and expansion ; the negative induces the opposite reaction 

 of diminution of turgor and contraction. 



In cases where transverse conductivity is feeble, the 

 impulse transmitted to the distal side (indirect effect of 

 stimulation) is positive, while the directly stimulated proxi- 

 mal side is negative. The tropic curvature is in such 

 cases positive, being brought about as the conjoint effect 

 of contraction of the proximal and expansion of the distal 

 side. It is only under strong and long-continued stimula- 

 tion that the excitatory negative impulse reaches the distal 

 side, and may thus neutrahse the positive curvature. Con- 

 sideration of this aspect of the subject is deferred to the 

 next chapter. 



Effects of Direct and Indirect Stimulation on 



A Growing Organ 



The characteristic effects of direct and indirect stimu- 

 lation have been definitely established by experiments 

 carried out with Mimosa. It has been shown that direct 

 stimulation induces contraction and diminution of turgor, 

 while indirect stimulation induces the opposite reaction of 

 expansion and increase of turgor. What are the effects 



