620 PLANT RESPONSE 



these high frequency currents is often found to retard growth. 

 It is probable that this is due to certain material emanations 

 that proceed from the wire. A coil of iron wire was made to 

 surround the growing organ, and during the excitation of 

 this coil the normal growth-rate was found to be retarded ; 

 but there was no such retardation when a glass cylinder was 

 interposed between the specimen and the coil. During the 

 short period of the experiment there was but little rise of 

 temperature within the coil, and the retardation of growth 

 could not have been due to any thermal radiation. 



Summary 



Variation of temperature acts as a stimulus. Below the 

 optimum the direct effect of variation of temperature is a 

 retardation of growth, and the negative after-effect, owing to 

 absorption of stimulus, is an acceleration of growth. Hence 

 repeated variations of temperature below the optimum have 

 little resultant effect on growth. 



Above the optimum, stimulus is not held latent, and there 

 is no negative after-effect of accelerated growth. Hence, 

 repeated variations of temperature above the optimum will 

 retard growth. 



While rise of temperature up to the optimum accelerates 

 growth, thermal radiation, as such, acts as a stimulus, and 

 retards growth. 



The unilateral action of thermal radiation is similar to 

 that of luminous radiation — that is to say, thermal radiation, 

 when of moderate intensity, gives rise to positive, and when 

 strong, to neutral, or negative movement. 



Electric waves induce retardation of growth. The uni- 

 lateral application of electric radiation is also found to induce 

 responsive movements. 



High frequency Tesla currents retard growth. 



