LAWS OF RESPONSIVE GROWTH-CURVATURES 



529 



the plant-tissue, for they respond to direct external stimula- 

 tion by contraction. But their power of transmitting stimulus 

 is relatively feeble. For this reason, under ordinary circum- 

 stances, they transmit only the indirect effect of stimulus, 

 and it is only when the unilateral stimulus is very strong 

 that the direct excitatory effect is transmitted, inducing the 

 opposite to the usual result, in the 

 responsive concavity of the same side 

 of the growing region. 



The fact that the sensitiveness of 

 the tip is not fundamentally different 

 from that of the growing region may 

 be demonstrated by applying stimulus 

 to a given point D in the growing 

 region, and observing the responsive 

 effect induced at R, diametrically 

 opposite. The power of the tissue to 

 conduct stimulus transversely being 

 feeble, the result is in this case 

 the same as in that of the ordinary 

 longitudinal transmission from the 

 tip ; that is to say, it is the indirect 

 effect that reaches the diametrically 

 opposite point, R, and induces con- 

 vexity there, this effect being aided, 

 as it happens, by the concavity of 

 the proximal side, due to the direct 

 effect of stimulation. Here again, as before, a stronger or 

 long-continued stimulus may later transmit the direct effect, 

 and neutralise or reverse this first responsive curvature. A 

 third case arises when unilateral stimulus is applied at L, 

 lower down on the stem, at some distance from the respond- 

 ing region, and if this be sufficiently feeble, it will be the 

 indirect effect which will reach the same side of the respond- 

 ing region, and produce convexity there. Stronger or long- 

 continued stimulation will in this case, as before, neutralise or 

 reverse the first effect. 



M M 



L— 



Fig. 222. Diagram showing 

 the various Responsive 

 Effects induced at the 

 Growing Region, R 



When moderate stimulus is 

 applied unilaterally at 

 the tip, '1', or distant 

 point, I,, it is the indirect 

 effect that reaches r, and 

 produces convexity. The 

 same convexity of r is 

 induced by stimulation 

 of the transverse point, D; 

 but here the induced 

 curvature is aided by the 

 concavity of the directly 

 excited D. 



