LEAFLET OF AVERRHOA 167 



as evidenced by partial recovery ; the contractile response 

 of the upper half of the pulvinus next induced an erectile 

 movement from atob. The excitatory impulse then reached 

 the uppermost layer of the lower half of pulvinus and in- 

 duced reversal of response, which was slow at the beginning 

 from b to c. But when the impulse reached the most 

 excitable layer c, there was an abrupt movement of fall 

 indicated by the scratch-line in the record. 



I had been greatly puzzled by the triphasic electromotive 

 response which sometimes appeared in the records. The 

 presence of a highly excitable layer in the tissue would 

 appear to offer an explanation of the phenomenon. 



Experiment 98. Effect of radio-thermal stimulation. — 

 The fact that triphasic response is universal is borne out by 

 the record taken under radio-thermal stimulation T, applied 

 on the upper half of the organ (fig. 93). The results are 

 essentially similar to those in the previous experiment. 

 The only difference is that on account of the stronger 

 intensity of the stimulus the reversal to negative occurred 

 somewhat earlier. 



Experiment 99. Effect of application of strong light on 

 lower half of the pulvinus. — The preliminary twitch was still 

 erectile though the stimulus was applied below. Stimulation 

 of the outermost layer initiated the positive impulse which, 

 reaching the layer c, induced the erectile twitch. The 

 excitatory contraction of the layers from d io c caused 

 a slow fall of the leaf, which was next transformed into a 

 very rapid fall by the contraction of the highly excitable 

 and contractile layer c (fig. 94). 



Thus, by the application of adequately sensitive physio- 

 logical tests, the gradations of excitability in the interior of 

 a tissue can be revealed. 



Response of the Leaflet of Averrhoa 



Diffuse stimulation induces a downward folding of the 

 leaflet, proving that the excitability of the pulvinule is 

 greater on the lower side. 



