no CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT 



that Strict proportionality holds good only in the median 

 range of stimulation, and that in a complete phototropic 

 curve the susceptibility to excitation undergoes an increase 

 at the beginning of the curve (p. 150). 



The effect of light of increasing intensity on tropic 

 curvature of growing organs will now be considered. Since 



4 



15 



^ 



> 



15 



:4_\ .. 



•3 \ 



'1 _v 



I I I I K 



12 S 4 



Fig. 57. Curve showing the relation between intensity of 

 light and retardation of rate of growth (Crinum) . 



phototropic curvature is mainly due to one-sided retardation 

 of growth, I will recapitulate the results previously obtained 

 on the effect of light of increasing intensity on growth 

 itself. The normal rate of growth of Crinum in the dark 

 was 0*47 [JL per second ; this was reduced to 0-29 \x under 

 the intensity of light of i unit, to 0-17 (j, under 2, and 

 to 0-09 [JL under 3 units; growth became arrested when 

 the intensity was raised to 4 units (p. 74). The curve of 

 variation of growth under increasing intensity of light is 



