PLANTS UNDER MECHANICAL STIMULUS. 277 
points, and there being thus no relative difference between the 
two, the galvanometer will still indicate no current. This null 
effect is due to the balancing action of B as against A. 
If the electrical effect at A is represented by Ey that at B 
by E,, then the resultant electromotive force round the 
circuit E,= E,-E, ; B 
to obtain any resultant effect in the galvanometer, we may 
employ two different means:—(1) We may so arrange matters 
that the disturbing stimulus reaches one point, say A, and not B. 
This may be accomplished by interposing a block between A and 
B. (2) Or even when the disturbance reaches both A and B, 
the balance might be destroyed by rendering A and B unequally 
responsive. This may be accomplished by physico-chemical 
means. For example, one point, say B, may be rendered more or 
less permanently irresponsive by injuring it by a cut, a burn, 
or the action of strong chemical reagents. In that case, stimulus 
will cause greater electrical disturbance at the more responsive 
point A, and this will be shown by the galvanometer as the 
resultant current of response. I may mention here in passing, 
that, in addition to the above method of relative depression, it 
is possible to obtain a resultant response by a relative exaltation 
of the sensitiveness of B. 
when E,=E , E,=0. If we wish 
Method of Injury, or Negative Variation. 
In obtaining electric response in animal tissues, one of: the 
two contacts, say B, is injured. This gives rise to a “current 
of injury ” which usually flows in the tissue from the injured 
B to uninjured A (fig. 1). On stimulating the tissue, there is 
Fig. 1, 
Method of negative variation. 
produced, in muscle and in nerve, a diminution of current 
