RESPONSE BY VARIATION OF ELECTRICAL RESISTIVITY 543 



and whether, if so, such variation is or is not of two opposite 

 signs, according to the tonic condition of the tissue con- 

 cerned. In subjecting this question to experimental investi- 

 gation, it is well to employ a non-electrical form of stimula- 

 tion, in order to avoid any possible disturbance of the 

 galvanometer record from polarisation or current escape. 



The first point to be decided is the character, positive or 

 negative, of that resistivity variation by which the true 

 excitatory change finds expression. We have already seen 

 that when a tissue is subjected to a gradually rising tem- 

 perature it exhibits response, which is expressed mechani- 

 cally as increasing expansion, and electrically as increasing 

 positivity. When the temperature, however, has reached 

 the definite critical point of death, we have seen that 

 there is a sudden excitatory effect induced, attended by 

 a reversal of the sign of response. This is expressed 

 mechanically by a sudden contraction, and electrically by a 

 change to galvanometric negativity. I have already ex- 

 plained in Chapter XVI. that, in mechanical and electrical 

 morographic curves, the abrupt point of inversion represents 

 the death-point. I have also shown that this death-response 

 is a true physiological response ; that the temperature at 

 which it takes place is definite in all phanerogamous plants, 

 being at, or very near, 60 C. in normal specimens ; and that 

 it displays depression, by transposition to a lower tempera- 

 ture, when the tissue is physiologically depressed by such 

 influences as fatigue. 1 



From these facts we might expect, if a tissue showed 

 response by variation of resistivity, that up to 60 C., or so, 

 there would be a continuous one-directioned change of 

 resistance, succeeded on reaching 60 C. by an abrupt 

 reversal to the opposite-directioned change. In that case, it 

 would be the second of the two, which would be indicative 

 of true excitation. To carry out this experiment I took a 

 radial and physiologically isotropic pistil of. Hibiscus, and 

 mounted it on two non-polarisable electrodes. The specimen 



1 Bose, Plant Response, p. 177. 



