22 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
observations were as follows :—Before excitation the cross difference was + 0*024 D.; 
immediately after excitation, by exciting mechanically the sensitive hairs of the 
opposite lobe, it was +0-031 D., but soon diminished to +0-026 D. A second excita¬ 
tion had a similar effect—it rose to + 0*029 D. After an hour’s rest in the chamber the 
experiment was repeated. Before excitation the cross difference was + 0*025 D.; on 
compensating as quickly as possible it was found to have risen (about half a minute 
after excitation) to + 0'035 D. ; in five minutes it had fallen to +0"028 D. A second 
excitation similarly observed raised the cross difference to +0*031 D., after which it 
fell to +0*028 D. Four similar observations gave corresponding results, but the effect 
produced (increase of the cross difference), which in the first experiment of the series 
amounted to one-hundredth of a Daniell, did not in the later ones exceed one-five 
hundredth. The indication that these phenomena afford that the electrical difference 
which exists in the unexcited leaf of Diontea between the opposite surfaces is in 
physiological relation with the excitatory properties of the leaf wih receive confirma¬ 
tion when, in the next part of this paper, we have before us the immediate effects of 
excitation. 
In the short communication I made to the Royal Society in 1873, in which I 
announced the discovery of the excitatory electrical change in the leaf of Dionsea, I 
stated the existence of a “ leaf current ” directed from the proximal to the distal end 
of the midrib in the uninjured leaf. I had observed that, in general, the attached end 
of the midrib was negative to the further end, and expressed this fact in the language 
commonly used in physiological writings. I had also found that the under surface of 
the leaf stalk in the neighbourhood of the joint by which the leaf is attached to it, 
was negative to the rest of the leaf stalk, and designated this negativity by the term 
“ stalk current.” I had further observed that if the current from a battery (a small 
Daniell’s cell) was directed through the 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 was increased if the current led through the 
leaf stalk were in the same direction with the leaf current, diminished if it were in the 
opposite direction. I had finally observed that in leaves severed from the plant, the 
“leaf current” was curiously dependent on the length of the petiole/' 5 ' so that the 
shorter the petiole the greater was the negativity of the proximal end as compared, 
with the distal. Of these four observations relating to the electromotive properties of 
the unexcited leaf, all may be readily demonstrated in most leaves. The more com¬ 
plete knowledge of the subject which I now possess leads me to regard the first two 
as of little importance. The third possesses a greater interest, for it affords evidence 
that in the plant a voltaic current conducted through a living part not only influences 
the condition of the part through which it flows, but extends that influence to extra- 
* The experiment was related as follows : —“ In a leaf with a petiole an inch long I observed a deflec¬ 
tion of 40 scale. I then cut off half, then half the remainder, and so on. After these successive 
amputations, the deflections were respectively 50, G5, 90, 120.” (Proceedings, vol. 21, p. 495.) 
