Radio-thermotropism 411 



Zea Mays was positively, but that of Lepidium negatively, 

 thermotropic . . . Steyer, however, found that both plants 

 were positively thermotropic. Wortmann has also investi- 

 gated the radicles of seedlinsrs by growing them in boxes 

 of saw-dust, one side being kept hot, the other cold." 



It will be noted that in the investigations described 

 above, thermotropic reaction has been assumed to be the 

 same under variation of temperature (as in experiments 

 with unequally heated saw-dust), and under radiation from 

 heated plate of metal. With reference to this Jost main- 

 tains 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 a 

 physiological expansion and enhancement of the rate of 

 growth. The effect of visible radiation is, on the other hand, 

 a contraction and retardation of growth. 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 find whether 

 the thermal radiation produces tropic curvature similar to 

 that of light, we have to devise a crucial experiment in 

 which the complicating factor of rise of temperature on the 

 responding organ is eliminated. 



Experiment 156. — I have described the effect of light 

 applied unilaterally to the stem of Mimosa, at a point dia- 

 metrically opposite to the indicating leaf (Expt. 104). It was 

 shown that the effect of indirect stimulus induced at first 

 an erectile movement of the leaf, and that this was followed 

 by a fall of the leaf on account of transverse transmission 

 of excitation. In the present experiment I applied thermal 

 radiation instead of light. The source of radiation was a 

 spiral of platinum wire heated short of incandescence by 



* JoBt— /6tJ— p. 480. 



