202 PLANT RESPONSE 



excitatory power of the kathode at make is seen to undergo 

 a gradual diminution, beyond this optimum, it follows that at 

 some certain high E.M.F. the excitation produced by it would 

 be zero. In other words, the kathode would cease to excite. 



Possibility of two distinct stages of reversal, A and B. 

 Further, since the effects at anode and kathode are, generally 

 speaking, contrary in character, we might expect a corre- 

 sponding change, but of opposite nature, to make its appear- 

 ance progressively at the anode. In other words, it might 

 happen that at a certain stage in the raising of the E.M.F. 

 the exciting value of the kathode would be considerably 

 diminished, and that the anode would begin to show 

 excitatory effect. This might be designated as the A stage. 



On still further raising the E.M.F. the same contrary- 

 directioned change might be expected to continue progres- 

 sively at the anode and kathode, and to reach a stage at 

 and beyond which it would be the anode which excited at 

 make, while the kathode produced either no excitation or 

 actual depression. This might be designated as the B stage. 

 We should then have a complete reversal of the normal 

 polar effects. 



If we exhibit these inferences, as to the relative excitatory 

 powers of anode and kathode with increasing E.M.F., by 

 means of curve.s, whose abscissae represent the E.M.F. while 

 their ordinates give the corresponding excitator)^ values, 

 the kathode curve would first rise to a maximum, and 

 then fall continuously, till, reaching the zero line, it might 

 even proceed still further in the negative direction, thus 

 representing depression. The anodic curve, on the contrary, 

 would at first descend in the negative direction, thus indicat- 

 ing increasing depression of excitability, until it reached a 

 negative maximum, after which there would be a reversal, 

 and it would begin to ascend and reach the zero-line. Here 

 the anode would cease to depress. After this it would 

 proceed upwards in the positive direction, indicating a con- 

 tinuously increasing power of excitation. The anodic and 

 kathodic curves in the course of this ascent would cross at a 



