i6o RESEARCHES ON IRRITABILITY OF PLANTS 



when the current is estabHshed very gradually. The 

 electrical current, as such, has generally speaking Httle or 

 no excitatory action. It is only at the moment of its sudden 

 initiation, or sudden cessation, that the excitatory effect is 

 most conspicuously induced. It is found, moreover, that 

 an excitatory effect is induced by the * make ' of the current 

 at the kathode, or the point where the current leaves the 

 nerve ; at the * break ' of the current, on the other hand, 

 excitation is induced at the anode or the point of entry. 



Precisely parallel effects I find to take place when an 

 electrical current is sent through a portion of the conducting 

 petiole. A detailed description of the polar effects of currents 

 will be given in a subsequent chapter. I shall here only 

 give what is essential to my present purpose. 



Two non-polarisable electrodes are applied, the proximal 

 on the conducting petiole at a distance of lo mm. from the 

 responding pulvinus, and the distal on the parenchyma of 

 one of the leaflets, such tissue being non-conducting. If the 

 current now be gradually applied by continuously increasing 

 the E.M.F. from zero to 3 volts by means of a suitable potential 

 slide, we shall find that there is no excitatory effect. But 

 if the E.M.F. of 3 volts be applied suddenly, the direction of 

 the current being such that the proximal electrode is kathode, 

 we shall find that the kathode now becomes the seat of 

 excitation and the leaf undergoes a responsive fall after the 

 short and definite period required for the transmission of 

 excitation through the intervening distance. 



If the experiment be then repeated with the proximal 

 electrode as anode, and the distal indifferent parenchyma as 

 the kathode, we shall observe no excitatory effect. This is 

 because the effective proximal electrode, which is anode, does 

 not excite at ' make.' The excitatory effect will however be 

 found to take place at the anode, but only at the * break ' of 

 current. 



Reverting to the hydro-mechanical theory, we are con- 

 fronted with great difficulties in accounting for the excita- 

 tory effects in the petiole initiated locally at the electrodes. 



