RADIO-THERMAL STIMULATOR 175 



same both under variation of temperature (as in experiments 

 with unequally heated sawdust) and under radiation from a 

 heated plate of metal. With reference to this Jost maintains 

 that, ' so far as we know, thermotropism due to radiant 

 heat cannot be distinguished from thermotropism due to 

 conduction.' 



The effect of temperature, within optimum limits, is 

 physiological expansion and enhancement of the rate of 

 growth {cf. fig. 15). The effect of visible radiation is, on 

 the other hand, contraction and retardation of growth 

 (cf. fig. 35). Should radiant heat act like light, the various 

 tropic effects in the two cases would be similar ; the 

 temperature effect would in that case be opposite to 

 the radiation effect. In order to ascertain if thermal 

 radiation produces tropic curvature as does light, a crucial 

 experiment has to be devised in which the complicating 

 effect of rise of temperature on the responding organ is 

 eliminated. Before referring to that experiment, I will 

 describe the method of quantitative stimulation by thermal 

 radiation. 



Radio-Thermal Stimulator 



This consists of a V-shaped loop of wire, heated short of 

 incandescence by the passage of an electric current. The 

 intensity of incident radiation can thus be maintained 

 constant, and increased or decreased by approach or recession 

 of the radiating loop. A series of thermal shocks can also 

 be applied in rapid succession by means of a metronome, 

 which closes the electric circuit (fig. 100). 



I referred to the crucial test by which the complicating 

 effect of rise of temperature on the responding organ can 

 be eliminated. This experiment has already been described 

 in Chapter XII {cf. fig. 67), in which radio-thermal stimula- 

 tion was applied on the stem of Mimosa at a point opposite 

 to the indicating leaf. The effect of unilateral stimulation 

 of Mimosa by heat-rays was found to be exactly the same as 



