HEAT INJURY AND HEAT RESISTANCE 595 



point of most living active plant cells Is in the approximate range of 50-60° C. 

 The exact temperature at which death of the protoplasm will occur depends 

 upon the length of the period during which the cells are warming up to the 

 lethal temperature. For example, according to Lepeschkin (1912), if the leaf 

 epidermal cells of Rhoeo discolor were heated at such a rate that death 

 occurred in four minutes the lethal temperature was 72.1° C. If the rate 

 of warming was so slow that death did not take place until 150 minutes had 

 elapsed the thermal death point was only 52.0° C. 



Air temperatures in temperate regions seldom exceed 40° C, and although 

 the temperature of an insolated plant organ often exceeds that of the atmos- 

 phere, lethal temperatures are seldom attained for reasons discussed in Chap. 

 XII. The surface temperatures of some soils, however, may attain values of 

 70° C. or even higher when exposed to intense insolation. Attempts to 

 reforest denuded areas in certain regions have sometimes failed because of 

 such high soil surface temperatures. The living cells of the stems of the young 

 trees which had been set out were killed at the soil line by contact with soil 

 at a temperature above their thermal death point, thus causing death of the 

 entire transplant. On the other hand, many woody species are habitants of 

 semi-desert regions in which high soil temperatures often prevail. The stems 

 of such species are obviously more heat resistant than those which are injured 

 or killed by contact with hot soils. 



Similarly lichens, certain species of mosses, and other species which grow on 

 rock cliffs exposed to the full glare of the sunlight often possess considerable 

 heat resistance because of the heating effect of the sun's rays upon their rock 

 substratum. 



Another example of direct heat injury to plants is often evident after a 

 "ground" forest fire sweeps through a woods. Such fires burn only fallen 

 leaves and branches on the forest floor and are often without any apparent 

 immediate effects upon living trees and saplings. Subsequently the tops of 

 many of the trees on an area burned over by such a fire often die as a result 

 of the killing of an encircling zone of living cells at the base of the trunk by 

 the high temperatures to which they have been exposed. 



II. Theories of Heat Injury. — The most generally advocated theory of 

 the cause of direct heat injury to plant cells is that it is due to a coagulation 

 of the protoplasm. Many proteins are heat coagulable and since protoplasm 

 is largely composed of proteins it seems highly probable that heat injury to 

 cells is due largely if not entirely to the coagulation of protein components 

 of the protoplasm (Lepeschkin, 1935). 



Several other theories of the mechanism of heat injury to plant cells have 

 been advanced. It has been suggested, for example, that heat injury is due 



