20 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
in the chamber until next morning, when a fine glass rod was introduced between the 
marginal hairs for the purpose of excitation; the difference was now — 0-007 D. The 
leaf was subjected to a series of excitations, each of which was followed by a normal 
diphasic variation. A fourth leaf was led off in the same way, but it was not, like the 
preceding ones, restrained by the glass holder. It was placed in the warm chamber at 
1.30 p.m. and observed at intervals until 4.30 p.m. The difference varied from 
“ 0'032 D. to — 0"029 D. On returning next morning it had diminished to — 0*018 D. 
It was at once excited by gently touching the distal sensitive hair of the led off lobe, 
the leaf responding by a normal variation but not closing. At once the difference 
sunk to —0-054 D. After a second excitation of the same kind the leaf closed smartly, 
the subsequent difference being — 0-045 D. Another leaf (not of the same series) 
was led off by three contacts, one (/) at the midrib, the other two at the middle of 
the external surface of each lobe (to and m'), so that these surfaces could be severally 
compared with f Both m and m were negative to /, the difference being 
m —0-018 D., and m — 0'024 D. In another similar observation the corresponding 
differences were m —0-038 D., and m — 0‘033 D. After a few excitations these 
diminished to — O’Oll D. and —0*004 D. 
The above observations, with the exception of the two last, were made for the 
purpose of ascertaining the truth of Professor Munk’s statement, that the external 
surface of the lobe is negative to the under surface of the midrib. They tend to show 
that the difference in question is one of frequent (perhaps constant) occurrence in 
unexcited leaves, and that it diminishes in consequence of excitation. 
We now proceed to the study of the second, and much more important, of the two 
statements made in our previous paper relating to the electromotive properties of the 
resting leaf. The fact that the external surface of the lobe is positive to the internal 
surface opposite to it, we propose to designate by the term “cross difference,” which 
sufficiently expresses its own meaning. It will be remembered that the existence of 
any such difference is denied by Professor Munk, who holds that the opposite 
surfaces of the lobe are isoelectrical. 
In a preliminary series of observations made in the Laboratory of the Eoyal Gardens 
at the same time with those already referred to no exceptions were met with. No 
importance, however, could be attached to the bare fact of the negativity of the 
internal surface unless it could be shown to be physiological in its nature—that is, not 
a result of the merely chemical or physical differences of the two opposite surfaces of 
contact. The method which is employed for determining whether a phenomenon is 
physiological or not, must always be the same. It consists in observing whether its 
development is so associated with the functions of the living part or organism in which 
it has its seat as to indicate that it is dependent upon it or forms part of it. It was 
therefore the first step towards determining the physiological meaning of the cross 
difference, to ascertain whether it is modified by changes in the vital activity of the 
leaf, and particularly whether it is the same in unexcited and in excited leaves, and 
