PLANT RESPONSE 77 



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 



