362 PLANT PHYSIOLOGY 



poisonous substances of the type of toxins, which poison the 

 protoplasm. At higher temperatures, about 50°C. and above, 

 this toxic effect is aggravated by the coagulation of the proto- 

 plasm, and this leads to a very rapid death of the cells. 



Various plants differ in their degree of resistance to high 

 temperatures. Blue-green algae and bacteria living in hot 

 mineral springs at temperatures about 70°C., at which all other 

 organisms quickly die, are the most heat resistant. Of the higher 

 plants, succulents of the type of cacti and different representa- 

 tives of the family of Crassulaceae endure without injury heating 

 by the sun's rays up to a temperature of 50 to 60°C. Plants of a 

 xerophytic type are more resistant than mesophytes. The most 

 heat resistant of the crop plants are the thermophilous varieties 

 from the southern latitudes, such as sorghum, rice, cotton, and 

 the castor-oil plant. 



In estimating the degree of heat resistance of plants in natural 

 environments, it must be kept in mind that the temperature of 

 plant tissues may not coincide with that of the surrounding 

 air. In spite of the fact that the leaves absorb radiant energy, 

 their temperature in the hot noon hours may be considerably 

 below the air temperature, for most of this energy is consumed in 

 the evaporation of water. Unimpeded transpiration protects 

 the leaves from excessive heating. Hence, dry heat is more 

 easily endured not only by man but also by plants. Plants 

 transpiring intensely are less subject to overheating by the sun's 

 rays. This explains the high transpiration intensity of many 

 desert plants. Wilting plants with closed stomata suffer more 

 readily from overheating than those that are sufficiently provided 

 with water. 



Nowhere on the earth's surface, with the exception of the 

 volcanoes, are there found places where plants could not develop 

 on account of high temperatures alone. The barrenness of 

 deserts is explained not by excessive heat but by a deficiency of 

 water. Sometimes, with hot dry winds the temperature may 

 rise beyond the limits that can be endured by plants, and in such 

 cases, the crop plants may be severely injured in a short time. 

 The hot winds being dry as well, it is sometimes very difficult 

 to distinguish the direct effect of a high temperature upon the 

 protoplasm from its indirect influence as a desiccating agent. 

 According to Zalensky's observations, in hot dry air the stomata 



