DEATH-SPASM IN PLANTS loi 



constitute the death-point. For precision in such a deter- 

 mination 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 mani- 

 fested. 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 anta- 

 gonistic. 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 tempera- 

 ture 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 i"^ C. per minute. For the precise determina- 

 tion 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 



