46 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
the case might be) being led off to the galvanometer as in the fundamental experiment. 
The time of closure of the exciting circuit (two Leclanche’s) was one-twentieth of 
a second. The galvanometer circuit was also closed for the same period, an interval 
of 0*03" intervening between the opening of the one circuit and the closure of the other. 
In the first leaf experimented on, the results were remarkable. In a series of twelve 
observations in which the current was directed across the leaf, alternately from and to 
the led off lobe, the needle remained motionless when, as in the first case, the anode 
was next the leading off surface, but swung to the negative side from 12 to 14 scale 
when the cathode was in the same position. As repetition confirmed the result it was 
clear that, as regards the particular leaf under observation, the proximity of the cathode 
was favourable to the early occurrence of the excitatory effect. Doubt was, however, 
suggested by the consideration that here, when the cathode was on the left lobe, the 
direction of the current near the leading off electrodes was downwards, that is, 
favourable to the excitatory effect, whereas in the opposite lobe it was unfavourable, 
and this surmise we confirmed by the observation that in another leaf, in which an 
exactly similar experiment was made, with the exciting electrodes applied to the 
internal surface, the response was strongest when the anode was on the led off lobe. 
All that was learned from these attempts was, that by the method employed, which 
seemed to be the only one likely to yield an answer to the question, no answer could 
be obtained. The want of material made it impossible to proceed further, but even 
had it been forthcoming I should have been at a loss how to use it. 
Influence of the Duration of the Current on the Effect. 
1. Short periods. —In order to determine the shortest period of closure of a voltaic 
current directed transversely from the upper to the under surface of a leaf, experiments 
were made in which the time of closure was gradually shortened by means of the 
pendulum rheotome. It was found that there was no diminution of effect until the 
duration of the closure was less than one-hundredth of a second. Beyond this, the 
response became uncertain when two Daniell’s with a rheochord of 20 ohms were 
used, and disappeared when the closing time was reduced to 0*007. Want of material 
made it impossible to investigate the important question of the influence of strength 
of current on the minimum. It was, however, ascertained that it was necessary to 
lengthen it when weaker currents were used. 
2. Currents of long duration. —When a voltaic current of five seconds’ duration and of 
moderate strength is led through the leaf from the upper surface between the sensitive 
hairs to the under surface, there is a response at make, and no other. When stronger 
currents are used (from one Daniell to two Groves) the first variation is followed by 
a second. If the current is continued for a long period (30 seconds) a succession of 
excitatory variations takes place at irregular intervals, as shown in the following 
