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LECTURE XIII. 



We have seen that exposure to a temperature higher than 

 the optimum exercises a depressing influence upon the various 

 metabolic processes, and we shall now learn that a consi- 

 derable rise of temperature is fatal. We shall find, moreover, 

 that the conditions for the fatal action of an excessively high 

 temperature and the mode of that action are very much the 

 same as in the case of an excessively low temperature. The 

 higher the temperature, the longer the exposure to it, and 

 the larger the proportion of water which the plant or organ 

 contains, the more actively will the prejudicial effect be 

 produced. The following facts will illustrate these state- 

 ments. 



In the case of entire plants Sachs found that an exposure 

 to air at a temperature slightly above 51 C. for ten minutes 

 sufficed to injure them, and, in many instances, to kill them, 

 and that immersion for ten minutes in water at 51 C. proved 

 fatal in all cases. The plants with which he experimented 

 were Nicotiana rustica, Cucurbita Pepo, Zea Mais, Mimosa 

 pudicciy Tropceolum majus, Brassica Napus. Similar experi- 

 ments with water plants ( Vallisneria spiralis, Ceratophyllum 

 demersum, Chara sp., and Cladophora) shewed that immersion 

 in water at 50 C. for ten minutes was fatal ; Vallisneria and 

 Chara were killed when the water had a temperature of 

 45 C. 



The following determinations of fatal temperatures are due to 

 de Vries : 



I. Roots, and branches with leaves : 



