260 CAUSES OF VARIATION 



rigor, which soon passes into death. The more rapid the rise 

 the lower the death point, and the more gradual the rise the 

 greater the resistance. Again, if the temperature does not rise 

 too high, the heat rigor may gradually pass off, and activity may 

 be resumed, even at temperatures which at first were followed 

 by entire suspension of activity, and even by rigor. This is the 

 first stage in the process of acclimatization. 



Sachs found that a sensitive plant kept at " 40 C. for one hour 

 suffered loss of sensibility during twenty minutes after removal.' 

 Raised slowly to 50, sensibility was only temporarily lost, but 

 52 proved fatal. Immersed in water, heat rigor occurred at a 

 temperature 5 to 10 lower." 1 Hofrneister found that hairs 

 from the stem and leaf of Ecballium agreste, showing lively 

 movement, when gradually raised from i6-i7 C. to 40 C. 

 "became motionless," but that " after one or two hours move- 

 ment returned and was very violent. Cooled and raised again to 

 45 C., the protoplasm was motionless at first, but after seven- 

 teen minutes movements recurred but were not rapid." 2 



The vital limit varies greatly with the species. Thus, roughly 

 speaking, for bacteria it is 45 C.; for cryptogams, generally 

 45-5O, with an occasional one at 60; flowering plants, 

 45-5; protozoa, 4O-45, with a few as high as 60; mol- 

 lusks, 30-4O ; worms, 45-5O (tardigrades, dried, 98); crus- 

 taceans, 26-43; insects, 27 -43.; ; fish, 2^-40 ; salamander, 

 44; frog, 4O-42; dog, rabbit, and man, 44-45; vertebrate 

 muscle, 4O-5O. 2 This series exhibits a wide range of resistance 

 to excessive heat, yet few organisms can endure much above 50. 



All experiments and observations indicate that death from 

 high temperatures is caused by coagulation of the albumen of 

 the protoplasm, a circumstance showing that albumen carries 

 into its vital relations its ordinary property of coagulation by 

 heat. That living matter contains many easily coagulable pro- 

 teids no longer admits of doubt, and that their coagulation causes 

 death is evidenced by the fact that once in this condition they 

 do not return to their normal state. 



1 C. B. Davenport, Experimental Morphology, Part I, p. 232. 



2 For full tables from which these abstracts are made, see C. B. Davenport, 

 Experimental Morphology, Part I, pp. 234-237. 



