INTENSITY Oi- LIGHT 73 



delicate means of observation ; for my High Magnification 

 Recorder indicated, in some cases, response within a period 

 as short as 5 seconds of exposure to Hght. In other cases 

 the latent period was found to vary from 15 seconds to 

 several minutes. 



Experiment 38. Determination of the latent period. — 

 The record of the effect of an arc-light on a seedling of 



Fig. 34. Determination of the latent period. 



Light apphed at thick dot. Successive dots at intervals of 



5 seconds (Cucurbita). 



Cucurbita was taken on a moving plate (fig. 34). The first 

 part of the record gives the normal rate of growth ; light 

 was applied at the thick dot, and the flexure of the curve 

 after the seventh dot indicates the responsive retardation. 

 As the successive dots are at intervals of 5 seconds, the 

 latent period in this case was 35 seconds. 



Effect of Increasing Intensity of Light 



Experiment 39. — The specimen employed was Crinum ; 

 it was subjected to light emitted by a half- watt incandescent 

 electric lamp of 200 candle-power. The intensity of light 

 when the lamp was placed at a distance of 100 cm. from 

 the plant was taken as the unit. Much feebler light would 



