PROPERTIES OE THE LEAP OF DiOJSTiEA. 
45 
local change in the path of the current, which is probably of the same nature as that 
which, when stronger currents are used, excites a propagable explosion. 
By the experiments already related it has been shown that when a leaf is traversed 
by a voltaic current in a direction vertical to its surface for a short period (one-tenth 
of a second to a second) from the upper to the under surface, excitation follows as 
soon as the difference between the two electrodes applied to the two surfaces of a leaf 
at opposite points, amounts to something like half the electromotive force of a Daniell 
cell. But they afford no information either as to the time or place at which the 
excitation occurs. It remains to be determined whether it occurs at the closure or 
opening of the circuit, and whether at the anode or cathode. The first of these 
questions was easily decided in experiments in which the leaf was led off as in the 
fundamental experiment and excited by currents for periods of sufficient length to 
render it possible to distinguish easily between the effects of opening and closing. 
The method consisted in making a series of observations in which the current was alter¬ 
nately directed upwards and downwards, and lasted from five to seven seconds, the 
opposite lobe being led off to the electrometer. A succession of experiments were made 
on different leaves in which various strengths were used, from one Daniell, without 
rheochord, to two Groves, always with the same result, namely, that the leaf responded 
at make, and at make only, whatever was the strength of the current or its direction. 
When strong currents were used, as will be noticed further on, the excitation at the 
closure of the circuit was succeeded by another from three to six seconds later, and 
might be easily mistaken for a break effect, if the moment at which it occurred 
happened to follow that at which the current was opened. 
To the question whether excitation proceeds from the anode or cathode, we have 
failed entirely in our efforts to obtain an answer. The modes of experiment used were 
as follows :—The leaf having been led off to the electrometer as in the fundamental 
experiment, non-polarizable electrodes for excitation were applied on the surface of 
the opposite lobe and on the petiole. In some cases the lobe electrode was on the 
under surface, in others on the upper. A reverser was introduced into the exciting 
circuit, so that the current of two Groves could be directed either towards the leaf 
or towards the leaf stalk. In either case the excitatory effect showed itself at make, 
and make only. But when the electrode was on the upper surface it was anodic, and 
when it was on the lower surface, cathodic. At first sight it seemed as if this 
indicated a difference of action between the poles, but on consideration it appeared 
to be another form of the result already recorded, viz. : that in the lamina the down¬ 
wards current is more effectual than the upwards. A current from the leaf stalk 
which passes out at the under surface acts in the same way as a current to the leaf 
stalk which enters at the upper surface, because in the excitable part the direction of 
both is descending. The next attempt to settle the question was by a rheochord 
experiment, in which the leaf was excited by electrodes applied symmetrically to corre¬ 
sponding surfaces on the right and left lobes respectively, the one lobe (right or left as 
