CHAPTER VIII 



EFFECT OF LIGHT OX GROWTH 



I SHALL first deal with the effect of hght on vigorously 

 growing organs, leaving the consideration of the abnormal 

 reaction to light, the underlying cause of which was at 

 first difficult to trace, for the next chapter. The effect of 

 light is studied by taking the normal record of growth in 

 darkness or under uniformly diffused feeble light, and 

 then records under increasing intensities of illumination. 

 The subjects discussed in the present chapter are : 



1. Normal effect of hght on growth. 



2. Determination of the latent period of response. 



3. Effect of increasing intensity of light. 



4. Effect of continuous stimulation. 



5. Immediate and after-effect of light. 



6. Effects of different rays of the spectrum. 



Method of Experiment 



The plant was placed in a glass chamber kept moist. 

 Strong light was obtained from a small arc-lamp with a 

 self-regulating device for ensuring steadiness of illumination. 

 An incandescent electric bulb was also employed as a less 

 strong source of light. Two inclined mirrors were placed 

 close behind the plant so that light acted on it from all 

 sides. 



Normal Effect of Light 



Experiment 37. — The record of normal growth N, of 

 Kysoor, was at first taken in darkness on a stationary plate. 



