RESPONSE OF THE ROOT 



l8l 



fatigue-relaxation of the proximal side. As the thermal 

 radiation is relatively more effective than light, the reversal, 

 generally speaking, occurs much earlier. I have obtained 

 numerous records in confirmation ; the second part of the 

 record (fig. 105) is, however, sufficient to illustrate this. 

 After neutralisation the curve is seen to undergo a reversal 

 indicating negative 

 thermotropic curva- 

 ture. 



It is interesting 

 to note in this con- 

 nection that in the 

 phototropic curva- 

 ture induced by sun- 

 light the heat-rays 

 play as important a 

 part as the more re- 

 frangible rays of the 

 spectrum. 



The Response of 

 THE Root 



It has been shown 

 that under unilateral 

 action of light a root 

 exhibits first a posi- 

 tive, then a neutral, 

 and finally a negative 

 movement. This last 



phase is exhibited under very prolonged exposure (p. 145). 

 The advantage of thermal radiation is, as already explained, 

 the relatively greater effectiveness of the stimulus, on 

 account of which the three stages can be observed within a 

 shorter period. 



Experiment no. — I reproduce a record given by the 

 root of Ipomoea reptans under unilateral thermal radia- 

 tion. The latent period was 10 seconds, and the positive 



Fig. 106. Response of root to unilateral 

 and continuous thermal radiation, T. 

 Positive, neutral, and negative curvature. 

 (Ipomoea reptans.) 



