LAWS OF RESPONSIVE GROWTH-CURVATURES 



527 



Here, too, after a transient movement away, there was an 

 energetic movement towards the stimulating heated wire 

 (fig. 220). 



2. Effect of unilateral stimulus applied at the respond- 

 ing growing region. — I shall now show that when stimulus 

 is applied near the growing region, it. induces effects which 

 are opposite to those resulting from stimulation of the tip. 



FlG. 220. Mechanical Response 

 of Root of Bindweed to very 

 strong Unilateral Stimulation 

 applied at the Tip 



This causes a preliminary negative, 

 followed by a positive, move- 

 ment, that is to say towards the 

 source of stimulus (cf. fig. 215). 



Fig. 221. Mechanical Re- 

 sponses of Peduncle of 

 Crocus, S, and Root of 

 Bindweed, R, to Unilate : 

 ral Thermal Stimulation 

 at the Growing Region 



The responses are positive 

 and towards the source of 

 stimulus (cf. fig. 216). 



(a) Moderate stimulus. — When moderate stimulation of 

 any kind is applied unilaterally in the growing region, the 

 consequent negative turgidity-variation of the side directly 

 excited makes it concave ; and a positive turgidity-variation 

 due to the indirect effect of stimulation occurs at the distal 

 side, making that side convex. Thus the induced concavity 

 of the proximal, and convexity of the distal, both conspire to 

 cause a movement of the organ towards the source of stimula- 

 tion. This is seen in the following records obtained with the 

 peduncle of Crocus and the root of Bindweed, the stimulus 

 used having been thermal (fig. 221). 



