PffOPEffTlES OP THE LEAP OE DlOMA. 
55 
I need scarcely say that the first phase of the variation—the effect which 
immediately follows excitation and has an opposite sign to the after effect and a 
much higher electromotive force—does not admit of a similar explanation; for it 
cannot be imagined that a change which spreads over the whole lamina in less than 
one-twentieth of a second can be dependent on migration of water. The excitatory 
disturbance which immediately follows excitation is an explosive molecular change, 
which by the mode of its origin, the suddenness of its incidence, and the rapidity of 
its propagation is distinguished from every other phenomenon except the one with 
which I have identified it, namely, the corresponding process in the excitable tissues 
of animals. 
Of the nature of this preliminary disturbance (to which alone the term excitatory 
variation ought to be applied, it alone being the analogue of the “ action current” of 
animal physiology) we know nothing. Just as in the case of muscle and nerve, the 
proof that it is attended by any chemical alteration or by the separation of any 
product of disintegration is wanting, so it is here. In the one case as in the other, 
we must regard the electrical change as a visible sign of an unknown molecular 
process. That it is not the primary change which occurs in protoplasm when it enters 
into the state of excitation we have evidence in two facts, first, that even when the 
exciting agent is an induction current which passes through one of the opposite 
surfaces by which the leaf is led off to the galvanometer, so that no time whatever is 
lost in transmission, there is still an interval between excitation and response of about 
(P03", during which interval molecular changes are obviously in progress; and, 
secondly, that excitation just insufficient to evoke a response gives effect to a second 
which follows it, provided that the interval between the first and the second does not 
exceed one-third of a second—a result which can only be explained on the supposi¬ 
tion that the first excitation actually effectuates a molecular change, notwithstanding 
that the visible sign is wanting. 
The direction of the excitatory effect in the fundamental experiment is such as to 
indicate that in excitation, excited cells become positive to unexcited, whereas in 
animal tissues excited parts always become negative to unexcited. The apparent 
discrepancy will probably find its explanation in the difference of the structural 
relations of the electromotive surfaces. 
