DIA-THERMOTROPISM I79 



Experiment 107. Response of pulvinated organ. — In 

 order to show that thermal radiation is an effective stimulus 



Fig. 104. Response of pulvinus of Mimosa to thermal radiation. 



for a motile organ, I reproduce the response of Mimosa to 

 the rays of heat (fig. 104). 



Dia-Thermotropism 



I shall henceforth use the shorter term thermotropism to 

 indicate tropic curvature under unilateral action of thermal 

 rays. In the case of photic stimulation, it has been 

 explained how the positive curvature is induced by retarda- 

 tion of growth at the proximal side and enhancement of 

 growth at the distal side, this latter being the effect of 

 indirect stimulation by transmitted positive impulse. 



But under long-continued action of stimulus the negative 

 or excitatory impulse reaches the distal side, inducing 

 diminution of turgor and retardation of the rate of growth. 

 This leads to neutralisation, the organ placing itself at 

 right angles to the direction of orienting stimulus. 



Experiment 108. — Neutralisation is seen in the record 

 given in fig. 105, where under continuous unilateral stimu- 

 lation the growing stem of Dregea exhibited its maximum 



