228 COMPARATIVE ELECTRO-PHYSIOLOGY 



quite untenable. He failed to see that the effect was, on the 

 contrary, due to the increasing excitatory action of the 

 sections themselves. Similar results may be obtained, even 

 without the bodily removal of the supposed antagonistic 

 element, if, instead, we apply an increasing intensity of 

 stimulus, as say, by contact of a hot wire at points nearer 

 and nearer to that of junction. In the case of the trans- 

 verse section, the cut acts as a stimulus, and the respon- 

 sive current flows from the left to the right. Algebraical 

 summation of this with the existing leaf-current, which is 

 also from left to right, causes an increase, or positive variation 

 of it, in a manner exactly the converse of the negative varia- 

 tion induced in the leaf, when the stimulus was applied on 

 the lamina. As the section is made nearer and nearer to the 

 point of junction, the degree of stimulation, and the con- 

 sequent positive variation of the resting-current, must become 

 greater and greater. 



And lastly, in the case of the longitudinal leaf-current, 

 Burdon Sanderson found that if a current from a battery 

 were directed through a leaf-stalk, at the same time that the 

 two ends of the midrib were led off to the galvanometer, the 

 difference previously existing between the ends of the midrib 

 would be increased, if the current led through the leaf-stalk 

 were in the same direction with the leaf- current, and 

 diminished, if it were in the opposite direction. A similar 

 effect, as seen in the conducting tissues of ordinary plants, 

 will be studied in detail, when we take up the question of 

 the extra-polar effects induced by electrotonic currents 

 (Chap. XXXIX.). 



We have already seen that, by means of induced varia- 

 tion of the longitudinal stalk-current, under the stimulation 

 caused by section of the petiole, it is easy to obtain an un- 

 mistakable indication of the nature of the true excitatory 

 electrical change. Burdon Sanderson, however, laboured 

 under the disadvantage, as already said, of having failed to 

 recognise that a section acts as a stimulus. His investi- 

 gation, therefore, on the character of the excitatory variation, 



