constitute the death-point. For precision in such a determination it is necessary to discover a temperature which is of itself efficient to initiate an abrupt death-change. On the other hand, there must be a certain latent period after the expiration of which the change would be outwardly manifested. An interval will elapse, moreover, during which the tissue is attaining the temperature of the bath. If the rate of rise of temperature be too rapid, then, owing to the lag caused by the two factors, by the time the death-spasm commences the recorded temperature may have gone beyond the actual death-point.
There are thus two points which are somewhat antagonistic. In the first place, in order to obtain the immediate point of death it is necessary that the plant should undergo an exposure which is not too prolonged. Nevertheless, to make due allowance for the latent period and for attainment of the surrounding temperature, the rate of rise of temperature must be gradual. In the case of tissues which are not too thick, the latter condition is sufficiently fulfilled by a rate of rise of 1° C. per minute. For the precise determination of the death-point the rate of rise of temperature must be specified. It must also be borne in mind that after the initiation of the death-change a certain time must elapse before the whole mass of tissue in the interior is killed. With a thick mass of tissue, owing to its inefficient thermal conductivity, the attainment of the surrounding temperature and occurrence of death throughout the tissue will be a protracted process.
The definite rate of rise of temperature may be simply secured by moving the heating flame nearer to, or further from, the bath. With thin organs, such as the pulvinus of Mimosa, I find that a spasmodic contraction takes place at or very near 60° C., when the rate of rise of temperature is approximately 1° C. per minute. This is seen in fig. 58; the record was commenced at 25° C., and the successive dots in the record are at intervals of 1° C. The down-curve indicates the expansive erection of leaf. As soon as the
