PHOTOTROPISM 325 



retardation of growth ? For this investigation I employed 

 the very sensitive method of the Balanced Crescograph. 



GROWTH-VARIATION BY FLASH OF LIGHT FROM A 

 SINGLE SPARK. 



Experime)it 123. — I stated that the more intense is the 

 light, the shorter is the latent period. The duration of a 

 single spark discharge from a Leyden jar is almost 

 instantaneous, the duration of discharge being of the order of 

 ^p^^QQO th of a second. The single discharge was made to 

 take place between two small steel spheres, the light 

 given out by the spark being rich in effective ultra-violet 

 rays. The plant used for the experiment was a seedling 

 of wheat. It was mounted on the Balanced Crescograph, 

 and its normal growth was exactly compensated as seen. 



Fig. 117.— Effect of a single elect ric spark on variation of growth. Record taken 

 by Balanced Crescograph. Up-curve shows induced retardation of growth; the after- 

 effect is an acceleration (down-curve) followed by restoration to normal. 



in the first part of the record. The spark gap was 

 placed at a distance of 10 cm. from the plant ; there 



