208 PLANT RESPONSE 



reversal of normal polar effects may be brought about. We 

 shall next study other circumstances which may also be 

 efficient to induce this reversal. This subject assumes the 

 greater importance from the difference of opinion which exists 

 among investigators in animal physiology as to the possi- 

 bility of such reversal. The question has not yet, as far as 

 I am aware, been definitely settled. Thus, ' Aeby thought 

 he had proved that under certain conditions, more par- 

 ticularly with progressive fatigue of the preparation, the 

 normal reaction — in which the excitatory action of the 

 kathode far exceeds that of the anode — -was exactly reversed. 

 Aeby's experiments, however, are by no means unimpeach- 

 able, as both Engelmann and Hering pointed out later. 

 Engelmann, also, came to the conclusion later, that such a 

 complete reversal of phenomena {i.e. of the law of polar ex- 

 citation) might take place. But until it has been determined 

 by unexceptional experiments, there must be great scepticism 

 in regard to such statements.' x 



We now turn our attention to that of the changed 

 condition of the tissue by which the normal polar response 

 may become reversed, and in this regard the experiments 

 which I shall describe are very instructive, as these changes 

 are there seen to occur progressively. I took a specimen of 

 Mimosa and carried out on it five consecutive experiments. 

 The two electrodes were attached to the pulvini of different 

 leaves on the same stem, and the E.M.F. used was fifty volts ; 

 an interval of about seven minutes was allowed in each case 

 for recovery. For easy inspection, the results are given in 

 somewhat tabular form. 



(i) At make— Leaves fell both at kathode and anode. 

 The kathodic fall was earlier and more energetic. 



At break — No decisive effect observed at either 

 electrode. 



(2) At make — The kathodic leaf fell, and the anodic fall 

 was slight. 



At break — No action at kathode, but energetic fall 



1 Biedermann, Electro- Physiology ', English translation, 1898, vol. i. p. 271. 



