72 



CHAP. VIII. EFFECT OF LIGHT OX GROWTH 



The effect of light is shown in the record S, where shortening 

 of the spacings between successive dots indicates retardation 

 of growth (fig. 33). For the same extent of growth-elonga- 

 tion there are 12 spacings in darkness and 24 spacings in 



Fig. 33. Effect of light in retardation of growth. 



N, normal ; s, retarded rate in response to light ; n', recovery- 

 half an hour after cessation of light (Kysoor). 



light. The rate of growth under the action of light was 

 thus reduced to half. Record N' was taken once more 

 in darkness after an interval of half an hour, showing 

 restoration of the normal rate. 



The Latent Period 



A prevalent impression exists that a considerable period, 

 from several minutes to an hour, intervenes between the 

 incidence of light and the responsive variation of growth. 

 This over-estimate must be due to the absence of sufficiently 



