RESPONSE TO A SIX(';LE SPARK I07 



dots being at intervals of 2 seconds. Record of response 

 was taken under moderate intensity of light acting from 

 above, and the latent period was found to be as short as 

 28 seconds (fig. 54). Under the action of stronger light 

 the latent period was further shortened. The latent period 

 for retardation of growth under diffuse stimulation by light 

 has also been found of the same order, namely, 35 seconds 

 [cf. fig. 34). 



Two different meanings are attached to the expression 

 latent period. It may connote the interval between the 

 application of stimulus and the initiation of response, being 

 in the cases just described of the order of about 30 seconds ; 

 or it may be taken to mean the shortest period of exposure 

 to stimulus for ensuring response. Taking the case of light, 

 it may well be asked what is the shortest exposure to light 

 for inducing a retardation of growth in extremely sensitive 

 plants ? For this investigation I employed the highly 

 sensitive method of the Balanced Crescograph. 



Growth-Variation Caused by Flash of Light 



FROM A Single Spark 



Experiment 62. — The duration of a spark-discharge 

 from a Leyden jar is almost instantaneous, being of the 

 order of TTrorni of ^ second. A single discharge was made 

 to take place between two small steel spheres, the light 

 given out by the spark being most effective in retarding 

 growth. The plant employed was a seedling of Wheat 

 mounted on the Balanced Crescograph ; after exact com- 

 pensation of its normal grow^th, the record was horizontal. 

 The spark gap was placed at a distance of 10 cm. from the 

 plant, which was also illuminated from behind by reflected 

 rays from a suitably inclined mirror. The flash of light 

 from a single spark is seen to have induced a sudden retarda- 

 tion of the rate of growth, which lasted for 1 1 minutes. 

 The record shows another interesting pecuharity, namely, 

 acceleration as an after-effect of moderate stimulation. 



