QUANTITY OP LIGHT AND PHOTOTROPIC CURVATURE 345 



We may now recapitulate the tropic effects of light of 

 increaaing intensity, directive angle, and duratioa of ex- 

 posure. It has been shown that the tropic effect is en- 

 hanced under increasing intensity of light ; it is also in- 

 creased with the angle increasing from grazing to perpendi- 

 cular incidence. And finally, the tropic effect is enhanced 

 with the duration of exposure. Taking into consideration 

 the effects of these different factors we arrive at the con- 

 clusion that pJiolotropic effect increases with the quantity 

 of incident light. It will be shown in the next chapter 

 tliat strict proportioaality of cause and effect holds good 

 in the mecjian range of stimulation, and the slight de- 

 viation from this, above and below the median range, is 

 due to the fact that susceptibility for excitation is low 

 at these two regions. 



SUMMARY. 



Increasing intensity of light induces increasing tropic 

 curvature. 







Tropic curvature increases with the directive angle, the 

 etfect being approximately proportional to sin 0, where 6 

 is the angle made by the rays with the responding sur- 

 face. 



Tropic curvature also increases with the duration of 

 exposure. 



The intensity of induced tropic effect is determined by 

 the quantity of incident light. 



27 a 



