DETERMINATION OF CRITICAL POINT OF DEATH 163 



We thus arrive at two conditions which must be regarded 

 as mutually somewhat antagonistic. In the first place, in 

 order to obtain the immediate point of death, it is essential 

 that the plant undergo an exposure which is not too pro- 

 longed. In order, on the other hand, to make due allowance 

 for the latent period and for attainment of the surrounding 

 temperature, the rate of rise must be gradual and definite. 

 In the case of tissues which are not too thick, the latter of 

 these conditions is amply fulfilled by a rate of i° C. per 

 minute and a half. We see, therefore, that in precise deter- 

 minations of the death-point, the rate of rise must always be 

 specified. 



With thick stems, however, owing to relative want of 

 thermal conductivity, the attainment of surrounding tem- 

 perature and occurrence of death throughout the whole of 

 the organ is a very protracted process. The experiments 

 which I shall describe were made with specimens which 

 were not too thick, death at the fatal point being ensured, 

 when the rate of rise of temperature was that prescribed, 

 namely, i° C. per every minute and a half. 



This definite rate of rise may be secured by using an 

 electric heating apparatus, such as is commonly employed 

 for boiling a tea-kettle. The current from the street-mains, 

 which is 220 volts, heats water too rapidly. But the desired 

 rate may be obtained by interposing an electrolytic rheostat 

 of copper sulphate, the current being brought to a suitable 

 value, by separation of the two electrodes through which the 

 current enters and leaves the electrolyte. 



In this case, when placing the specimen in the experi- 

 mental bath, it is advisable to secure it to a bent glass rod, 

 which rests outside. For if it is placed in the metallic vessel 

 itself, the record will be subject to a certain disturbance, 

 owing to the expansion of the supporting metal while heating. 

 The expansion of the glass rod is so small as to be negligible. 

 In this way, using for experiment a filament of the corona 

 of Passiflora, I obtained a record, showing a very abrupt 

 inversion, corresponding to the death-point, which was 



M 2 



