112 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT 



minutes. The amplitudes of the responses are in the ratio 

 of I : 1-25 : 5 (fig. 59)- 



Fig. 59. Effect of increasing duration of exposure of 

 1:2:3 minutes on phototropic curvature of growing 

 organ (Crinum). 



Effect of the Angle of the Incident Light 



The quantity of Hght which falls on a unit area of the 

 responding organ varies as sin 0, where is the directive 

 angle — i.e. the angle made by the rays with the surface. 



Some allowance has to be 





made for the loss of light 

 O \ I 1 reflected from the surface, 



this being greater at 45° 

 than at 90°. 



Experiment 66. Re- 



FiG. 60. The Collimator for application , . * 7 • j j 



of light at various angles. ■ SpOUSC of a pulvmated 



organ. — For application 

 of light at various angles, an incandescent electric lamp 

 was mounted at one end of a brass tube, a colhmating 

 lens being placed at the other end (fig. 60). The parallel 

 beam of light from the Collimator could be thrown at 

 various angles by rotating the coUimator-tube round an 

 axis at right angles to the tube. Light was directed for 

 a minute, in the two successive experiments with the 



