CHARACTER OF TRANSMITTED IMPULSE 169 



round the petiole and soaking it with strong copper-sulphate 

 solution. Warm solution was found to be more quickly 

 absorbed than cold. Out of six sensitive leaves, two were 

 subjected to the local action of poison. The selective and 

 local arrest of the excitatory wave in the poisoned leaves 

 would then afford a conclusive demonstration of the physio- 

 logical character of the transmitted impulse. It should be 

 remembered that the absorption of poison is likely to be 

 a slow process. Hence the physiological block of conduc- 

 tivity induced by poison will become increasingly effective 

 with the duration of application. We may therefore expect 

 the following sequence of events after the application of 

 poison on a narrow zone of the petiole : — 



(i) There will be no noticeable variation of conductivity 

 at the beginning. 



(ii) After the lapse of a certain length of time the quantity 

 of poisonous solution absorbed will be sufficient to induce a 

 certain depression of conductivity. The minimal excitation 

 which was formerly transmitted will now undergo an arrest. 

 But the blocking action will not be sufficiently great to arrest 

 maximal excitation. 



(iii) After a still longer interval, the depression of 

 conductivity induced by poison will be very great. Even a 

 maximal excitation will now be practically arrested. 



(iv) This arrest will be due to the abolition of conduc- 

 tivity in the localised poisoned zone. The conductivity of 

 the petiole beyond the poisoned area, and the motile 

 excitability of the leaflets will remain unaffected. 



I shall now describe experiments on the effect of poison 

 on conductivity. The experiment was repeated with 

 twelve different specimens of Biophytum, all of which gave 

 similar results. The following experiment may be taken as 

 representative of the rest. Among the whorl of leaves in 

 the particular specimen of Biophytum there were six which 

 were fairly sensitive. The additive effect of four successive 

 thermal shocks applied on the stem was found to be sufficient 

 to cause excitatory fall of the leaflets in all the six leaves. 



