QUANTITY OF LIGHT IO9 



1. The contraction of the cells must have a limit ; 



2. The bending organ offers increasing resistance to 



curvature ; and 



3. The induced curvature tends to place the organ 



parallel to the direction of light, when the tropic 

 effect is reduced to a minimum. 



Relation between Quantity of Light and 

 Induced Curvature 



I now describe the effects of increasing intensity and 

 duration of exposure to hght on a pulvinated and on a 

 growing organ. The pulvinus of the terminal leaflet of 

 Desmodiiim gyrans was employed for the former, and a 

 growing bud of Crinum for the latter. 



Effect of Increasing Intensity of Light 



Experiment 63. Effect on pulvinus of Desmodium. — A 

 petiole bearing the terminal leaflet was suitably mounted 

 in a U-tube filled with water. The 

 wound-effect of section passed off 

 in the course of an hour. Light of 

 increasing intensity was successive- 

 ly applied from above, producing 

 increasing contraction of the upper 

 half of the pulvinus and increased 

 up-movement of the leaflet, re- 

 sulting in enhancement of the 

 amplitude of response. The first 

 record was obtained under light 

 of a given intensity, and the second 

 under an intensity twice as great 

 (fig. 56). If tropic curvature 

 increased in proportion to light- 

 intensity, then the two responses 

 would have been in the ratio of 



1:2. The actual ratio was however slightly greater, 

 namely, 1:2-6. It will be shown, in a succeeding chapter, 



Fig. 5G. Tropic effect of in- 

 creasing intensity of light, 

 I : 2, on the positive up- 

 response of terminal leaf- 

 let of Desmodium. 



