76 RESPONSE IN THE LIVING AND NON-LIVING 
variation. We read this effect by means of galvano- 
metric deflections. And if the resistance of the circuit 
remained constant, then an increase of galyanometer 
deflection would accurately indicate a heightened or 
depressed E.M. response, due to greater or less excita- 
bility of tissue caused by the reagent. But, by the 
introduction of the chemical reagent, the resistance of 
the tissue may undergo change, and owing to this 
cause, modification of response as read by the galvano- 
meter may be produced without any E.M. variation. 
The observed variation of response may thus be partly 
owing to some unknown change of resistance, as well as 
to that of the E.M. variation in response to stimulus. 
We may however discriminate as to how much of 
the observed change is due to variation of resistance by 
comparing the deflections produced in the galvanometer 
by the action of a definite small E.M.F. before and after 
the introduction of the reagent. If the deflections be 
the same in both cases, we know that the resistance has 
not varied. If there have been any change, the 
variation of deflection will show the amount, and we 
can make allowance accordingly. 
I have however adopted another method, by which 
all necessity of correction is obviated, and the galvano- 
metric deflections simply give E.M. variations, unaffected 
by any change in the resistance of the tissue. This is 
done by interposing a very large and constant resistance 
in the external circuit and thereby making other 
resistances negligible. An example will make this 
point clear. Taking a carrot as the vegetable tissue, 
I found its resistance plus the resistance of the non- 
