PLANT RESPONSE 7 
polarisable electrode equal to 20,000 ohms. The 
introduction of a chemical reagent reduced it to 
19,000 ohms. The resistance of the galvanometer is 
equal to 1,000 ohms. The high external resistance was 
1,000,000 ohms. The variation of resistance produced 
in the circuit would therefore be 1,000 in (1,000,000 + 
19,000+1,000) or one part in 1,020. Therefore the 
variation of galvanometric deflection due to change of 
resistance would be less than one part in a thousand 
(cf. fig. 49). 
The advantage of the block method.—In these in- 
vestigations I have used the block method, instead of 
that of negative variation, and I may here draw 
attention to the advantages which it offers. In the 
method of negative variation, one contact being 
injured, the chemical reagents act on injured and 
uninjured unequally, and it is conceivable that by this 
unequal action the resting difference of potential may 
be altered. But the intensity of response in the method 
of injury depends on this resting difference. It is thus 
hypothetically possible that on the method of negative 
variation there might be changes in the responses 
caused by variation of the resting difference, and not 
necessarily due to the stimulating or depressing effect 
of the reagent on the tissue. 
But by the block method the two contacts are 
made with uninjured surfaces, and the effect of reagents 
on both is similar. Thus no advantage is given to one 
contact over the other. The changes now detected in 
response are therefore due to no adventitious circum- 
stance, but to the reagent itself. If further verification 
