6l6 PLANT RESPONSE 



mica shield is again dropped the action of radiation on the 

 organ is cut off. 



Experimenting in this manner, and obtaining a horizontal 

 record with the shield down, we find, when the shield is 

 lifted, that there is at once an upsetting of the balance, which 

 indicates a retardation of growth. When the shield is once 

 more allowed to drop, the deflected record becomes again 

 horizontal, indicating the restoration of the original rate of 

 growth. This experiment conclusively shows that radiation 

 by itself acts as an external stimulus, retarding the rate of 

 growth. It will be remembered that this is quite distinct 

 from the effect of temperature, which, up to the optimum, 

 always induces the opposite result, namely, an enhancement 

 of the rate of growth. The same distinction is also to be 

 borne in mind in dealing with the excitatory effect of light, 

 for incident light also has the twofold effect of causing 

 external stimulation and at the same time raising the tem- 

 perature of the tissue. 



Response to successive uniform stimuli of thermal 

 radiation. — Having thus demonstrated the fundamental 

 action of thermal radiation, we shall now study the effect of 



the unilateral appli- 

 cation of this form 

 of stimulus, which 

 will be found similar 

 to that caused by 

 visible light. The 

 responsive effect of 



successive uniform 

 Fig. 248. Responses to Successive Uniform .. , ,. , 



Stimuli of Thermal Radiation in Pistil of Musa Stimulations may De 



studied by placing a 

 V-shaped platinum wire with its point opposite to the 

 region of growth, and heating it periodically by definite 

 currents of short duration ; the flashes of thermal radiation 

 thus produced bring about the usual responsive concavity. 

 These responses and recoveries are recorded in the usual 

 manner. I give in fig. 248 a series of such responses 



