EXCITABILITY UNDER CHEMICAL REAGENTS 139 



tissue subjected to stimulation. One of these is the positive 

 effect, and the other, the true excitatory change of galvano- 

 metric negativity. As the latter is, under normal conditions, 

 predominant, the simultaneous effect of both is a resultant 

 negativity. The positive effect may, however, be unmasked, 

 as we have seen, by abolishing the true excitatory effect 

 of negativity (p. 66). This positivity may also be un- 

 lasked, if, by the action of a chemical reagent, the time- 



FIG. 98. Photographic Record of Effect of 5 per cent. KHO 



Note the complete reversal of response to positive at the beginning, 

 and its subsequent abolition. 



relations of the two responses are changed, so that instead of 

 occurring simultaneously, the one is made to lag behind the 

 other. This case will be seen very strikingly illustrated in 

 fig- 97> which exhibits the effect of a i per cent solution of 

 KHO, on response to transmitted stimulation in the petiole 

 of fern. In the normal responses here given, we observe 

 the resultant response of galvanometric negativity. The 

 application of KHO is first seen to reduce the excit- 

 ability, as indicated by the reduced height of the responses. 

 Later, we observe that the true excitatory effect is delayed. 



