RESISTANCE OF PLANTS 361 



influences of all the unfavorable factors. This may be best 

 achieved under the conditions of field experiments. 



In various climatic regions, different factors may be of major 

 importance. It is ciuite natural, therefore, that varieties that 

 are winter resistant in some regions may be considerably less 

 resistant in others. For instance, the extremely frost-resistant 

 varieties of the Volga region, where the killing of winter cereals 

 is conditioned almost exclusively by winter temperatures with 

 insufficient snow cover, when cultivated in West-central or 

 Northwestern Russia yield the first place to the local winter- 

 resistant varieties, which better withstand rotting, a phenomenon 

 common in these regions. In systematic selection for frost 

 resistance, it first of all must be ascertained which of the harmful 

 winter factors is the most important in the given region. Great 

 attention should be directed to the study of resistance in relation 

 to these leading factors. The influence of these numerous factors 

 being very diverse and even to a certain degree contradictory, 

 we cannot expect to select such a variety as would have winter 

 resistance in all regions. 



83. Effect of High Temperatures on Plants. Heat Resistance 

 and Its Physicochemical Basis.— In examining the effect of 

 temperature on the various physiological functions, it has 

 repeatedly been mentioned that beyond the optimum tempera- 

 ture, when a process reaches its highest rate, there follows a 

 decrease up to the maximum temperature, when it ceases entirely. 

 This sharp decrease in the rate of the separate physiological 

 functions at temperatures above the optimum is explained by 

 the fact that beginning from certain temperatures pathological 

 processes begin to prevail in the plant, which suppress the normal 

 physiological functions and finally cause the death of cells. 



The action of superoptimal temperatures as it is usually 

 specified is often too simple. It is assumed that the Avhole 

 phenomenon can be explained by the coagulation of the protein 

 substances of the protoplasm at high temperatures. Actually, 

 the phenomenon is much more complicated; for death usually 

 begins at temperatures slightly above 40°, which are far below 

 the coagulating point of proteins. Probably, as in the case of 

 thermophilous plants, at temperatures close to zero, death is 

 preceded by a disturbance in the coordination of the biochemical 

 processes taking place in the cell and by an accumulation of 



