78 CHAP. VIII. EFFECT OF LIGHT ON GROWTH 



above the critical minimum which will be shown later to 

 demarcate the abnormal from the normal response ; (2) the 

 tonic condition of the organ was now improved by previous 

 stimulation. 



The important result regarding the after-effect of moderate 

 intensity of light is that, for a brief period, the rate of growth 

 is enhanced above the normal. This will be shown to be the 

 characteristic after-effect of all modes of stimulation, being 

 a factor of much importance in accelerating the recovery 

 of the organ from the effect of brief stimulation. 



Effects of Different Rays of the Spectrum 



Previous observers have found that it is the more 

 refrangible rays which exercise the greatest influence upon 

 growth and tropic curvature. The relative effects of lights 

 of different colours are, however, more precisely indicated 

 by the curve of response to the action of different rays. 

 For this purpose I first employed monochromatic light 

 from different parts of the spectrum, produced by a prism 

 of high dispersion. In practice the usual colour-filters 

 were found very convenient, as they allow the application 

 of more intense light. Certain complications arise from 

 the slight rise of temperature due to the absorption of 

 radiant energy by the organ. Moderate rise of temperature 

 has been shown to enhance the rate of growth (p. 36), while 

 radiation, in general, causes a retardation. In spite of this, 

 it is easy to demonstrate the predominant effect of certain 

 rays in retarding growth. 



Experiment 43. Effect of red and of yellow light. — The 

 highly sensitive Balanced Method of record fully supported 

 the results previously obtained, that these rays are practi- 

 cally ineffective in inducing any retardation in the rate of 

 growth. 



Experiment 44. Effect of blue light. — On application of 

 light even for such a short period as 34 seconds, the effective- 

 ness of the blue rays became fully demonstrated. For the 



