THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 355 



the "A" effect under moderate stimulation may persist 

 longer. Thus owing to the difference in their time-relations 

 the A effect is capable of being unmasked at the onset 

 of stimulus or on its sudden cessation. For the detec- 

 tion of the relatively feeble expansive A effect, a special 

 recorder is required which combines lightness with high 

 power of magnification. The earlier expansive reaction and 

 acceleration of rate of growth, followed by normal retarda- 

 tion, are often found in the response of growing organs. 

 The corresponding effect of unilateral stimulation, even 

 when direct, is a transient expansion at the proximal side, 

 inducing a convexity of that side and movement away from 

 stimulus (negative curvature) ; this is followed by contrac- 

 tion and concavity with normal positive curvature. The 

 interval between the A and D effects is increased with 

 increasing sub-tonicity of the specimen. But it nearly 

 vanishes when the excitability of the specimen is high 

 and the iwo opposite reactions succeed each other too quick- 

 ly for the preliminary A reaction to become evident. It is 

 probable that in such a case the conflict between the two 

 opposite reactions prolongs the latent period. But in other 

 instances a preliminary expansive response is found to herald 

 the more pronounced contractile response. Example of this 

 is seen in figure 129 given in page 344. 



The A effect was detected in the records referred to 

 above by its earlier appearance. Its longer persistence 

 after moderate stimulation, is also to be found on the 

 cessation of moderate stimulation. This was seen in the 

 acceleration of growth which was the after-effect of 

 stimulation (Figs. 104, 115). The presence of two conflict- 

 ing physiological reactions is also made evident on sudden 

 cessation of long continued stimulation. This particular 

 phenomenon of " overshooting " will be more fully dealt 

 with in a subsequent chapter. 



