XVI. On the electromotive properties of the leaf of Dionæa in the excited and unexcited states.— Second paper

Abstract

In the paper which I communicated to the Royal Society in 1881, I presented the results of experiments made at various periods during the preceding five years, and the conclusions which I was then able to found upon them. These conclusions, which are set forth in full in the closing pages of my paper (‘Phil. Trans.,' 1882, Part 1, p. 1) may be shortly stated as follows:—If by proper means the electrical relation between opposite points of the upper and under surfaces of the leaf of Dionœa is investigated while the leaf is "excited" either by touching one of its sensitive filaments or by passing an induction current through the opposite lobe, it is found that the excitation is followed after a few hundredths of a second by a sudden electrical disturbance of less than half a second duration, in which the under surface becomes negative to the upper; and that this is succeeded towards the end of the first second after the excitation by a change of less intensity, but of longer duration and opposite sign . From the character and relative durations (time relations) of these changes, which together, constitute what may be conveniently termed the "excitatory disturbance” or “response,” it was concluded that the first was of the same nature with the "excitatory variation” or “action current” of animal physiology, and must be regarded as the expression of a molecular change similar to that which occurs in nerve, muscle, or the electrical organ under analogous conditions. As regards the second, which was designated the "after effect,” it was observed first that it was present only when the leaf was "fresh,” that is, had not been excited immediately before; and, secondly, that it was associated with the previous and subsequent electrical state of the two "led off” surfaces in such a way that each excitation appeared to leave behind it what might be described as a lasting remainder of the after effect, that is, a permanent and homonymous difference of potential: in other words, that each excitation tended to make the under surface relatively more positive if it were before positive, less negative if it were before negative; and that, if the leaf were subjected to repeated excitations, it generally happened that the lasting electrical relation between opposite points of its upper and under surfaces underwent a gradual change, that is, the under surface became more positive the longer the observation was continued. It was further observed that in any leaf a similar modification could be brought about much more rapidly by leading through the explored part a very weak voltaic current in the direction of the after effect, that is, from the upper to the under surface.