iSz CHAP. XVI. RADIO-THERMOTROPISM 



curvature continued for 40 seconds. Neutralisation then 

 occurred, after which the tropic curvature became reversed 

 into a very pronounced negative (fig. 106). 



Summary 



The effects of rise of temperature and of radiation are 

 antagonistic to each other. 



The response to unilateral stimulation by thermal 

 radiation is a positive curvature induced in both shoot 

 and root by the retardation of growth at the proximal, and 

 acceleration of growth at the distal, side of the organ. 



There is complete recovery on the cessation of stimu- 

 lation of moderate intensity and short duration. Repeated 

 responses may therefore be obtained similar to the repeated 

 responses of pulvinated organs. 



In certain cases the power of conduction in a transverse 

 direction is wanting ; excitation remains locaHsed at the 

 proximal side, and the responsive curvature remains positive. 

 In other cases there is a slow conduction of excitation to 

 the distal side. The result of this under different circum- 

 stances is dia-radio-thermotropic neutralisation, or even a 

 negative curvature. 



The heat-rays in sunhght play as important a part in 

 inducing phototropic curvature as the more refrangible 

 rays of the spectrum. 



The root, under unilateral action of thermal radiation, 

 exhibits positive, neutral, and negative curvature, as it does 

 under unilateral photic stimulation. 



