178 CHAP. XVI. RADIO-THERMOTROPISM 



Experiment 105. — The effective intensity of stimulation 

 was next made stronger by reducing the intervening distance 

 to 4 cm. The duration of stimulation was the same as 

 before, namely, 40 seconds. The response was, as in the 

 case of photic stimulation, diphasic negative curvature, 

 followed by positive (fig. 102). 



Positive Radio-Thermotropism 



Experiment 106. — The growing region of a stem was next 

 stimulated unilaterally for a short time by thermal radiation. 



Fig. 103. Positive response to stimulation by short 

 exposures to thermal radiation. Successive dots 

 at intervals of 5 seconds (Dregea volubilis). 



Fig. 103 gives a record of response of the stem of Dregea ; 

 the induced curvature is positive or towards the source 

 of heat. On the cessation of stimulation, there is a 

 recovery which is practically complete, and which takes 

 place at a slower rate than the excitatory positive curva- 

 ture. Repetition of stimulation gives rise to responses 

 similar to the first. Successive stimulations of moderate 

 intensity thus give rise to repeated responses of positive 

 growth-curvature. 



