136 



CHAP. XIIT. DIA-PHOTOTROPISM 



growth on a moving plate. The ascending part of the 

 curve (fig. 74) represents the normal rate ; direct applica- 

 tion of electric stimulus at horizontal arrow (intensity 

 2 units and duration 2 seconds) induced contraction indi- 

 cated by the down-curve. Slow recovery occurred till the 

 normal rate of growth was restored. After this normal 

 recovery, the plant exhibited cojitraction on fresh stimulation. 

 Experiment 81. Fatigue-relaxation of growth under 



continued stimulation. — After 

 the normal recovery the 

 identical specimen was con- 

 tinuously subjected to an 

 electric stimulus of 2 units. 

 The response now was a pre- 

 liminary down-curve of con- 

 traction, succeeded by an up- 

 curve of relaxation (fig. 75). 

 An outward resemblance will 

 be noticed in the two records 

 (figs. 74, 75). The inner differ- 

 ence lies in the fact that after 

 apparent recovery consequent 

 on fatigue-relaxation , the plant 

 does not exhibit contraction on 

 fresh stimulation, as it does 

 after normal recovery. 



In the experiments described 

 the stimulus employed was electric. I next investigated 

 the effect of intense photic stimulation on a growing 

 plant. 



Experiment 82. Fatigue-relaxation under strong light. — 

 I took a seedling of Oryza, and strong light was made to act, 

 by means of inclined mirrors, on all sides of the plant. 

 The result obtained at the first stage was normal contraction 

 and retardation of growth, followed later by . relaxation 

 under the prolonged stimulation. 



From the above facts it is clear that stimulation induces 



Fig. 74. Effect of electric shock of 

 short duration at arrow induces 

 contraction, followed by normal 

 recovery (Vigna Catjang). 



