114 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT 



induced an amplitude of response of 5 mm. ; the second 

 stimulation at 45°, i.e. the third response of the series, 

 induced a slightly larger response 7 mm. in amplitude. 

 Similarly the two responses at 90° gave amplitudes of 

 9 and II mm. respectively (fig. 62). Taking the mean 

 value of each pair, the ratio of tropic effects for 90° and 45° 

 is = 10/6 = 1-7, nearly — a value which is slightly greater 

 than the ratio of the sines of the two angles. 



The tropic effect of light as affected by increasing 

 intensity, duration, and change of directive angle, may now 

 be recapitulated : (i) the tropic effect is enhanced under 

 increasing intensity of light ; (2) it is increased with the 

 duration of exposure ; and finally (3) it is increased with 

 the directive angle from grazing to perpendicular incidence. 

 Taking into consideration the effects of these different 

 factors the conclusion is that the phototropic effect increases 

 with the quantity of incident light. It will be shown in 

 a subsequent chapter that strict proportionality of cause 

 and effect holds good only in the median range of stimula- 

 tion, and the slight deviation from this is due to the fact 

 that the susceptibility for excitation is feeble below that 

 range. 



Summary 



Phototropic response is similar in pulvinated and in 

 growing organs. 



One important factor in positive curvature is the con- 

 traction of the directly stimulated proximal side of the 

 organ. The modifying effect of another factor will be 

 described in the next chapter. 



The induced curvature is followed by complete recovery 

 after brief stimulation by light. The recovery is hastened 

 by an acceleration of the rate of growth above the normal 

 of the previously stimulated side. The recovery is pro- 

 longed after strong and long-continued stimulation. 



The latent period for phototropic reaction is very much 

 shorter than has been previously supposed. In certain 



