226 COMPARATIVE ELECTRO-PHYSIOLOGY 



variation of the former, and positive variation of the latter 

 (fig. 147 ()). The same stimulus thus induces effects which 

 are apparently opposite. Or an interesting variation of the 

 phenomenon may be obtained, on repeating the experiment 

 with the leaf of Citrus. Here, on stimulating the lamina, 

 we observe a positive variation of the leaf-current, and a 

 negative variation of the stalk-current (fig. 146 (<:)). This is 

 because the currents of reference or resting-currents are the 

 opposite of those in Dioncea and Ficus religiosa. 



Another series of variations exactly the reverse of these, 

 and therefore at first sight anomalous, is caused by simply 

 changing the point of application of stimulus, from the right 

 end on the lamina, to the left end on the stalk. The direc- 

 tion of the excitatory current is thus reversed, being now 

 from left to right (fig. 147 (a) ). By algebraical summation, 

 there now occurs a negative variation of the stalk-current, 

 and a positive variation of the leaf-current, in Dioncea and 

 Ficus religiosa, while the very opposite takes place in Citrus. 

 I shall here draw attention once more to those errors to 

 which an investigator becomes liable when he infers that 

 positive and negative variations must necessarily be the 

 expression of assimilatory and dissimilatory processes. For 

 we have just seen that the same responsive current, by alge- 

 braical summation with two opposite-directioned resting- 

 currents, may appear to be both positive and negative, at 

 one and the same time. Again, with a single resting-current, 

 it is possible to obtain either a positive or a negative varia- 

 tion, according as the same stimulus is applied to the right 

 or the left. It is now abundantly clear that the one uni- 

 versal effect of stimulus is to give rise to a responsive current 

 which flows from the more to the less excited portions of the 

 tissue. If there be already an existing current, the responsive 

 current is added to this algebraically, and induces, according 

 to circumstances, either a positive or a negative variation. 

 Much confusion, and many erroneous inferences would be 

 avoided, if instead of looking at these variable indications 

 attention were centred on the one constant criterion, namely 



