236 COMPARATIVE ELECTRO-PHYSIOLOGY 



less vigorous leaves respectively of Dioncza, when stimulation 

 was applied on the distal lobe, according to the fundamental 

 experiment of Burdon Sanderson. 



We must next refer to the reason why a leaf that origin- 

 ally gives diphasic response positive followed by negative 

 undergoes such ' modification,' in consequence of previous 

 excitation, as thereafter to give only positive response. We 

 have seen that the negative element of the diphasic response 

 is due to the arrival at the responding point of the true 

 excitatory wave originated at the distant point of stimula- 

 tion. Now it has been shown (p. 65), that if by any 

 means the conductivity of an intervening region should 

 become diminished, we may expect that the hydro-positive 

 effect will continue to be transmitted, although the passage 

 of true excitation is partly or wholly blocked. By means of 

 this selective block, I was able to unmask the hydro-positive 

 component present in resultant response (cf. fig. 49). 



I have shown elsewhere ] that the conducting power of a 

 tissue will be impaired by the fatigue consequent on previous 

 stimulation. Thus, in the petiole of Biophytum, I found that 

 while the plant, when fresh, had a conductivity measured 

 by the velocity of transmission of excitation, at a rate of 

 r88 mm. per second, the same plant, when partially fatigued 

 by four successive stimulations, had its conductivity dimi- 

 nished, the velocity of transmisson being now only 1^54 mm. 

 per second. The diminution in this case, then, was about 

 1 8 per cent. I shall moreover . show in a later chapter 

 that in consequence of growing fatigue the passage of true 

 excitation may at a certain stage be arrested, the hydro- 

 positive effect alone being then transmitted. It is thus easy 

 to explain how it was that in Burdon Sanderson's experi- 

 ment, of stimulus applied on the distal lobe, the wave of 

 true excitation became blocked, and the 'modified' leaf gave 

 positive response alone. These considerations will be found 

 as I think, to offer a satisfactory explanation of the conflicting 

 results arrived at by Burdon Sanderson. 

 1 Plant Response, p. 244. 



