9O RELATION OF PLANTS TO TEMPERATURE 



which it may revive when brought into a lower one. Above the 

 temperature producing heat-rigor, is a point at which the condi- 

 tion of inactivity induced in heat rigor becomes one of death-rigor 

 from which the organism may not recover. Death rigor sets in 

 at 46 C. with some of the higher plants, though higher than this 

 in others. The bodies of fleshy cacti have been found by the 

 author to reach a temperature of 46 C. in the desert plains of 

 Arizona. Seeds and propagative organs have special adaptations 

 for the endurance of much higher temperatures however, and 

 spores of certain bacteria (anthrax spores) are reputed to live and 

 develop after being subjected to 140 C. 



The minimum at which activity may proceed varies with the 

 same conditions that influence the determination of the maximum. 

 It is, however, rarely below o C. and generally lies a few degrees 

 above that point. 



The discussion of the relations of temperatures below the 

 minimum brings out some of the most interesting facts in the 

 entire subject. The degree of cold necessary to produce death- 

 rigor varies with the experience of the organism, and its species, 

 the rate of cooling of the organism, duration and exposure, and 

 rapidity of rise in temperature at the close of the tests. The 

 amount of water in the protoplasts, and the stage of develop- 

 ment are very important factors in such endurance. 252 

 C. is the lowest temperature to which vegetal protoplasm has 

 been exposed, and this condition was obtained by immersing 

 seeds in liquid hydrogen, in which they remained for an hour. 

 Brassica alba, Pisum sativum, Mimulus Moschatus, Cucurbita Pepo, 

 Triticum sativum, and Hordeum vulgare are represented in this 

 result. * 



It is to be seen that protoplasm has taken such adaptive forms 

 in different plants that it may withstand a total range of 392 C. 

 (705.6 F.), which is far beyond the capacity of but few single 

 organisms, however. 



1 Thiselton-Dyer, W. T. On the influence of the temperature of liquid hydrogen 

 on the germinative power of seeds. Proc. Roy. Soc. 65 : 361. 1899. 



