SUMMARY 79 



responsive retardation occurred in the course of 14 seconds 

 and the depression of the rate was two-fifths of the normal. 



Experiment 45. Effect of infra-red rays. — In passing 

 from the highl}^ refrangible blue to the less refrangible red 

 rays, the responsive retardation of growth undergoes a 

 diminution or even abolition. Proceeding further into the 

 infra-red region of thermal rays, these are found to become 

 highly effective in inducing very marked retardation in the 

 rate of growth. 



The curve drawn with the wave-length of light as 

 abscissa, and the effectiveness of the ray as ordinate, shows 

 a fall towards zero from the blue to the red ; the curve 

 however shoots up on proceeding further into the region of 

 the infra-red towards the invisible thermal rays. 



Summary 



The normal effect of light is retardation of the rate of 

 growth, which is, in fact, incipient contraction. 



The latent period may in some cases be as short as 

 5 seconds, in others it varies from 15 seconds to several 

 minutes. The latent period is shortened under stronger 

 intensity of light. Increasing intensity of light induces in- 

 creasing retardation culminating in an arrest of growth. 



The response to continuous stimulation by light is 

 essentially similar to that to continuous electric stimulation. 



The after-effect of brief and moderate stimulation by 

 light is a short-lived acceleration of growth above the 

 normal rate. 



The effectiveness of different rays of the visible spectrum 

 in retarding growth undergoes a decline from the blue to 

 the red rays. The thermal rays in the infra-red region are, 

 however, very effective in retarding growth. 



