QUANTITY OF LIGHT AND PHOTOTROPIC CURVATURE 339 



of exposure. I shall give a detailed accouut of the investi- 

 gation relating to the individual eflects of each of these 

 factors on the tropic reactions not merely in pulvinated but 

 also in growing organs. 



EFFECT OF INCREASING INTENSITY OF LIGHT ON TROPIC 

 CURVATURE. 



The intensity of light was increased in successive experi- 

 ments, in arithmetical progression 1 : 2 : 3 by suitably dimi- 

 nishing the distance between the plant and the source of 

 light, and the resulting tropic curvatures recorded. 



Effect of increasing intensity of light on the pulvinus of 

 Desmodium gyrans : Experiment 129. — The source of light 

 was a 50 candle-power incandescent lamp, and the dura- 

 tion of exposure was 1 minute. The specimen employed was 

 a terminal leaflet of Desmodium gyrans (Fig. 122) the 



pulvinus of which is very 

 sensitive to light. It is more 

 convenient to manipulate a 

 cut specimen of the leaf, 

 instead of the whole plant. 

 The petiole is placed in 

 water contained in a U-tube ; 

 the depressing effect of wound 

 passes off in the course of 



Fig. 122.-Leaf of Desmodium g;,rans, ^u hour or SO. Light of 

 with the terminal lar'^e, and two lateral . 



small leafletB. These latter exhibit Increasing intensity is applied 

 automatic pulsations. from above ; this induces 



a contraction of the upper half of the pulvinus, and 



