32 
PROFESSOR ff. SANDERSON OK THE ELECTROMOTIVE 
On repeating the excitations at half minute intervals, the after effect was again 
entirely abolished, reappearing again after a period of rest. It was further found, in 
another experiment, that if, after a succession of excitations at half minute intervals, 
a leaf was excited twice at longer ones, e.y., two minutes and a half, the after effect 
had, by the time the second was made, already reappeared, or if previously diminished 
had much increased. 
In Part III. it was shown that the influence of an excitation in increasing the pre¬ 
vious difference (that is, the positivity of the external surface) or in other words, in 
rendering the external surface permanently more positive than it was before excitation, 
depended on the interval of time which had elapsed since any previous excitation. 
In a leaf which has been repeatedly excited at frequent intervals, excitation leaves 
behind no change in the electrical relation of the opposed surfaces, because there is no 
after effect. In other words, the persistent change of the potential, consequent on 
excitation, described in Part III., is a small remainder of the after effect, as it seldom 
amounts to more than 0‘002 D, whereas the after effect itself often attains at its maxi¬ 
mum an electromotive force of 0'02 D. It may be roughly stated that the latter is apt 
to leave behind a remainder of something like one-tenth of its amount. All that can 
be insisted on as resulting from observation is, that the subsidence of the after effect 
is extremely slow and incomplete, and that whenever it exists it leaves behind it 
a remainder—-the excitatory increase of positivity of the external surface, already 
described. 
The excitatory process as observed when the leaf is led off by symmetrical (identical) 
surfaces of contact of opposite lobes. —The excitatory variation which is observed in a 
leaf led off symmetrically, that is, by corresponding surfaces on opposite lobes, is the 
combined result of equal and opposite electromotive changes which have their seat in 
the tissues between the contacts. Consequently, if in an unsevered leaf so led off we 
suppose an excitation to take place at the same moment from each electrode, no varia¬ 
tion would be observed, for the electrical changes due to the one excitation would 
exactly balance those due to the other. The appearance of a variation would mdicate 
inequality of action between the two lobes. If the excitatory response of the right 
lobe were either stronger than that of the left, or were accomplished in a shorter time, 
the variation would express the difference. In the former case the right would have 
the advantage throughout; in the latter, the right would have the advantage at first, 
afterwards the left. 
The case in which two lobes are led off and excited simultaneously and symmetrically 
has not been realised experimentally. The consideration of it, however, serves to 
facilitate the understanding of the case which actually presents itself in which the leaf 
is led off by symmetrically placed contacts on opposite sides, but excited on one side 
only. In this case experiment shows that there is always a galvanometric effect, but 
that the character of the variation observed differs in different leaves very considerably 
—so much so indeed that at first sight the differences appear irreconcilable. Before 
