CHARACTER OF TRANSMITTED IMPULSE 159 



mechanical disturbance is transmitted to a distance, but 

 the determination of propagation of physiological change, 

 under normal modes of stimulation. By employing stimulus 

 of graduated intensity, it should be easy to determine the 

 character of a given impulse by observing the effects of 

 various physiological depressors in modifying the power 

 of conduction. 



In order to bring the question — whether in a plant 

 there is true transmission of excitation or mere passage of a 

 mechanical disturbance — to a satisfactory issue, it is clear 

 that we ought to proceed in the following way : First, we 

 have to inquire whether it is not possible to find modes of 

 excitation for the plant which would be purely physiological, 

 and in which there can be no element of physical disturbance. 

 Transmitted effect in such a case could only be due to propa- 

 gation of excitatory protoplasmic change. We will next sub- 

 ject the question to the final test of the physiological block 

 which would arrest an excitatory impulse, but could have 

 no effect on the passage of a hydro-mechanical disturbance. 



I shall now describe four different lines of investigations 

 each of which furnishes an independent proof of the excita- 

 tory character of the transmitted impulse : — 



(i) On methods of excitation by the discriminative polar 

 action of electric currents. 



(2) On the block of transmission of excitation by local 

 application of cold. 



(3) On the electrotonic arrest of excitatory impulse. 



(4) On the action of poison in inducing block of conduc- 

 tion. 



Characteristic Polar Excitation by Constant Current 



If in a muscle-and-nerve preparation of frog two electrodes 

 are applied on the conducting-nerve, at a certain distance 

 from the responding muscle, it is found that on sending a 

 current through the included portion of the nerve excitation 

 is induced, which on reaching the responding muscle brings 

 about contraction. There is no excitatory action in the case 



