THE NATURAL CURRENT AND ITS VARIATIONS 125 



direction, not only of the current of rest, but also of the 

 current of response. We shall be able to obtain a clear 

 understanding of these effects, on subjecting the underlying 

 phenomena to close analysis. As a concrete example, we 

 may take for our investigation the various effects to be 

 observed in the pulvinus of Mimosa. 



In this anisotropic organ, the directions of the resultant 

 current of rest and the current of response are determined, 

 as we have seen, by the differential excitability and differential 

 excitation of the two sides of the organ. We shall fix our 

 attention, however, for the sake of simplicity, on the changes 

 which occur in the more effective lower half. I shall here 

 succinctly describe the various post- primary phases in the 

 changing effect, culminating in the onset of fatigue from over- 

 stimulation. In the primary condition, as we have seen, the 

 lower half of the pulvinus is positive to the upper, the 

 direction of the resting-current being down I. On stimula- 

 tion, the lower half becomes negative and the response 

 current is up t. Response thus takes place by a negative 

 variation of the resting-current. 



We may now suppose the pulvinus to be in a state of 

 slight excitation, its molecular condition at or about the B 

 stage. This existing state of moderate excitation will annul 

 the previous positivity, and the current of rest will now be 

 zero. At this stage, however, as we have seen, the excit- 

 ability of a tissue is enhanced. Hence, on stimulation, the 

 lower half of the pulvinus will exhibit responsive negativity, 

 and the direction of the response current will be up t. We 

 are, however, unable to describe this variation in terms of the 

 current of rest, since that, as we have already seen, is zero. 



A condition of still stronger excitation, bringing the 

 tissue to the C condition, will induce galvanometric negativity 

 of the lower half. The so-called current of rest is thus 

 upwards t. But as the excitability of the lower half is still 

 relatively greater than that of the upper, it follows that 

 external stimulus will bring about a responsive current whose 

 direction is upwards t. This normal excitatory response, 



