236 PLANT RESPONSE 



for the purpose of stimulation. It is then found that the 

 hitherto ineffective stimulus of a'k' is thus rendered effective. 

 An-electrotonic depression of excitability. — The de- 

 pressing action of the anode has been already demonstrated 

 in the case of Biophytum (p. 234). The following experiment 

 exhibits the same effect in a different manner in the case of 

 Mimosa. In this instance, I used an E.M.F. of four volts in 

 each of the two circuits AK and a'k'. When each circuit 

 was made separately, the leaf responded by depression. 

 At make, then, of one of the circuits the leaf responds, but as 

 the stimulus is only effective at make, the leaf recovers 

 during the continuation of the current. After this, on the 

 second circuit being completed, the excitement at make 

 again caused response. 



The experiment was now modified in the following way. 

 The AK circuit was reversed, the pulvinus becoming anode. 

 The excitation of the distant kathode, however, was still 

 strong enough to cause response of the leaf. The current 

 was kept on till the leaf recovered. On now making the 

 a'k' circuit, the leaf did not respond. Thus the stimulus of 

 a'k' at make, which was formerly effective, now became in- 

 effective, by the depressing action of A. 



Developing action of kathode. — -Another experiment, 

 showing the latent excitatory action of the kathode, is very 



striking. This experiment, 

 however, is somewhat diffi- 

 cult, as it requires a very 

 delicate adjustment of the 

 stimulus. The specimen 

 Fig. 103. ' Developing ' Action of used was a leaf of Biophytum. 



Kathode 



A subminimal stimulus applied at . 



ineffective to produce excitation of duce any direct excitation 



A current insufficient to pro- 

 duce any direct excitation 



nearer leaHets, becomes effective in was k t fl ow j ng through 

 the neighbourhood of kathode. l o & 



the circuit AK (fig. 101). The 



point of special difficulty was to apply a stimulus of exactly 



subminimal intensity at x, so as not to excite the adjacent 



leaflet. I have sometimes succeeded in obtaining this condition. 



