QUANTITY OP LIGHT AND PHOTOTROPIC CURVATURE 841 



(Fig. 124), If the tropic curvature increased proportion- 

 ately to the intensity, the two responses should have been 



Fi(j. 124. Fig. 125. 



Fig. 124. — Tropic effect of increasing intensity of light 1 : 2 ; on the response of 

 terminal leaflet of D^ifmodium gyrans. 



Fig. 125.— Tropic effect of increasing intensity of light 1 : 2 : 3 on growin"- 

 organ {Crimim). 



in the ratio of 1 : 2 ; the actual ratio was however 

 slightly greater, viz. 1 : 2-^. In this connection it will be 

 shown in the next chapteiv, that strict proportionality holds 

 good only in the median range, and that the susceptibility 

 for excitation undergoes an increase at the beginning of 

 the phototropic curve. 



Effect of increasing intensity of light on the tropic 

 curvature of growing organs. — As the tropic curvature is 

 primarily due to the retardation of growth induced by light 

 at the proximal side of the organ, it will be of interest to 

 recapitulate the results I obtained (p. 208) on the effects 

 of increasing intensity of light on growth itself. The 

 normal rate of growth of the specimen in the dark was 

 0*47 jx per second ; this was reduced to 0'29 /a under an 

 intensity of one unit, to 0'17 fx under two, and to O'JO fju 

 under three units, Growth became arrested when tte 



