24 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
polar effect (which so strikingly resembles the electrotonic variation of nerve) and the 
strength of the current which produces it, more accurately. For this purpose currents 
of various strengths, derived from a rheochord, were conducted through the leaf stalk, 
the leaf being led off as in the experiment last described. The effects observed were 
as follows:—With 25 centims. of rheochord wire,'" corresponding to a difference of 
potential of about a quarter of a Daniell, the difference between the two leaf 
electrodes was diminished when the derived current was directed from the leaf, from 
— 0'007 to —0’0058, and increased to about —0‘008 when it was reversed. With 
twice the length of wire (about half a Daniell) it was diminished from — (POOS to 
— 0'004 when the derived current was directed from the leaf. Finally, with three 
times the length of wire the difference, which had now fallen to — 0‘005, was 
abolished; so that when the galvanometer was uncompensated, the needle was 
brought to zero by the derived current through the petiole. 
It was ascertained by careful experiments that the electromotive change in the 
leaf ceased with the current which induced it. When the experiment was so arranged 
that the galvanometer circuit having been open during the passage of the “polarizing” 
current was closed at the moment that it ceased, the needle remained motionless. 
In the above experiment it is seen that the induced difference of potential between 
the leading off contacts was very small indeed as compared with that of the contacts 
by which the derived current entered and left the petiole. But it is to be borne in 
mind that this relation does not express the true relation between the electromotive 
forces, for this reason : We have already assumed that all electrical changes dependent 
on physiological action, have their seat in the parenchyma, and this assumption is as 
applicable to the leaf-stalk as to the leaf itself. Consequently the “polarizing ” effect 
of a voltaic current led from one point to another of the surface of the leaf-stalk 
depends, not on the electrical difference thereby determined between the surfaces of 
contact, but on the differences between the end cells of the interpolar stratum of 
parenchyma. Now it is known experimentally, first, that the resistance encountered 
by a current led as above described through a leaf-stalk is enormous, and secondly, 
that this resistance has its seat at the surface, whereas the longitudinal resistance of 
the middle stratum is relatively small. The result is that the physiological or 
“polarizing” effect of the current on the parenchyma which is the immediate effect of 
the electrical difference determined by the current between the most and the least 
positive (or negative, as the case may be) cells of the parenchyma, is a very inconsider¬ 
able fraction indeed of what it would be if the polarizing electrodes could be apjdied 
directly to the parenchyma itself. These considerations lead me to believe that 
although in the experiment related on the previous page the currents used were 
relatively considerable, there is no reason to suppose that the electrical changes in the 
parenchyma exceeded those (to be described in the next part of this paper) which, in 
* With, reference to this mode of using the rheochord, see p. 42 (Excitatory influence of the voltaic 
current). 
