52 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
4. If the same observation is made while the leaf is subjected to mechanical or 
electrical excitation, it is seen that at the moment that excitation takes place the 
under surface becomes suddenly negative to the upper. By the rheotome we learn 
that the change last mentioned does not begin until - 2 ^th of a second after its cause, 
that it culminates about the middle of the first second, the maximum difference 
amounting to as much as 0'08 Daniell, and then gradually subsides. 
5. It is further learnt by the same mode of observation that in the course of the 
second second or towards its close, the electrical relation of the surfaces to each other 
is reversed, the under surface becoming positive to the upper. The difference of 
potential in this direction having rapidly augmented until it amounts to about 0'02 
Daniell, very slowly subsides into the lasting effect of excitation described in 1 . 
6. When two symmetrical points on the under surfaces of opposite lobes are simi¬ 
larly compared during excitation, the excitatory effect differs in character according to 
the relation between the seat of excitation and the leading off contacts, the surface of 
the lobe which is directly excited becoming negative to the other, consequently by 
changing the positive seat of excitation the direction of the first effect may be 
reversed. * 
7. When a voltaic current is led across the lobe from the upper to the under surface 
by electrodes applied opposite to each other near the sensitive hairs, but not touching 
them, at the same time that the electrical state of the opposite lobe is observed as in 
4, a response occurs at the moment that the current is closed, provided that its 
strength is such that the available electromotive force amounts to about half that of 
a Daniell cell and that the temperature is not below 30° C. No response occurs at 
opening the current. If a stronger current (two Daniells) is used and the direction is 
downwards, the response at closing the current is followed by several others. This 
effect does not happen when the current is directed upwards. 
8. A voltaic current directed from the upper to the under surface, which is too weak 
to evoke an excitatory response, produces an increase of the positivity of the under 
surface, limited to the part of the lobe through which the current passes, which lasts 
several seconds after the current is broken. Its direction shows that it is not due to 
“ polarization.” 
9. Voltaic currents of less than xooth second duration, though of moderate strength, 
do not excite Dionsea. Weaker currents cease to act when their duration is less than 
T Joth second. But the relation between strength and duration has not been 
ascertained. 
10. Opening induction currents such as are yielded by du Bois-Reymond's smaller 
inductorium when the secondary coil is at a distance of 8 to 10 centims. from the 
* It cannot be concluded either from the result stated above or from the preceding ones that in Dionsea, 
as in muscle and nerve, the excited part becomes negative to other parts. For it is as consistent with the 
facts observed to say that the upper surface becomes positive as the immediate result of excitation, as to 
say that the under surface becomes negative, 
