THE NATURAL CURRENT AND ITS VARIATIONS 1 17 



We have thus seen that in order to maintain a high state 

 of excitability, absorption of energy is necessary. On 

 excitation, emission of energy occurs. In this latter case, of 

 emission, we observe a concomitant galvanometric negativity 

 of the more excited lower side. Since to have maintained its 

 excitability the opposite process of absorption would have 

 been necessary, it follows that the more excitable lower side 

 must under normal conditions be galvanometrically positive. 

 This is found to be the case. For when the leaf is in an 

 excitable condition, there is an electro-motive difference 

 between the upper and lower halves of the pulvinus, in con- 

 sequence of which a current flows across the tissue, from the 

 less excitable upper to the more excitable lower half, which 

 is thus galvanometrically positive, in relation to the upper. 

 We have here, then, an additional instance of the opposite 

 effects of internal energy and external stimulus. Internal 

 energy, maintaining a greater excitability of the lower half 

 of the pulvinus, induces in it a relative galvanometric positivity. 

 External stimulus, on the other hand, gives rise to precisely 

 the opposite effect namely, the relative galvanometric 

 negativity of the lower half. Under typical conditions, then, 

 we may expect the more excitable point to be galvano- 

 metrically positive ; and the more excited to be galvano- 

 metrically negative. 



Turning next to non-motile tissues, we find the same 

 conclusions to hold good. We saw that in the case of the 

 sheathing petiole of Musa, the concave was more excitable 

 than the convex side. The concave is thus normally positive 

 to the convex side, and the natural current flows across the 

 tissue from the convex to the concave. While the natural 

 current flows from the more excitable to the less excitable, 

 external stimulus gives rise to a responsive current in the 

 opposite direction, from the more excited to the less excited, 

 constituting a negative variation of the current of rest. 



Let us next consider what would be the effect of an 

 increase of internal energy on the natural current. Since the 

 action of internal energy is opposite to that of external 

 stimulus, we should expect it to induce a positive variation of 



