RESISTANCE OF PLANTS 371 



Similar changes are produced by increased light intensity. 

 Generally, sun leaves differ from shade leaves by the same char- 

 acteristics as do the upper leaves from the lower ones. Espe- 

 cially marked are the changes in the development of palisade 

 parenchyma, as has been seen in Art. 45. Among the physi- 

 ological characteristics of sun leaves may be mentioned an 

 increased assimilation, at least in strong hght, and an intensive 

 transpiration. Consequently, sun leaves may be considered 

 xeromorphic leaves. 



The study of the anatomical and physiological peculiarities 

 of xeromorphic structures is of great importance in the determi- 

 nation of differences in drought resistance among varieties of 

 crop plants. 



Formerly, it was assumed that the capacity of a plant to expend 

 the least possible amount of water is the basic feature of drought- 

 resistance. Hence, it was concluded that low transpiration 

 intensity and a low transpiration coefficient were the most reliable 

 physiological pecuharities of drought-resistant plants. However, 

 the basic contradiction in the life of land plants was omitted, 

 which has been indicated in Art. 64 and according to which a 

 decrease in water expenditure inevitably leads to a change for 

 the worse in the conditions of both aerial and mineral nutrition 

 of the plants. It has already been seen that in the xerophytes 

 of the deserts this contradiction is solved in different ways. 

 In drought-resistant crop plants, Hkewise, it is solved, but differ- 

 ently. There is not a uniform type of drought resistance, just 

 as there cannot be universal features of drought resistance. 



As crop plants, unhke the desert xerophytes, are not subject 

 to drought throughout their vegetative period but only during 

 more or less short intervals of time, one of the most important 

 features of drought-resistant plants is their capacity of enduring 

 temporary dehydration of their tissues without decreasing, or 

 with the least decrease of, their yield (Maximov, H. Walter). 

 Thus for instance, according to Tumanov, permanent wilting 

 caused by drying of the soil does not affect the yield of millet, 

 a plant which is very drought resistant, while it sharply decreases 

 the yield of oats. The capacity of enduring without injury the 

 loss of a considerable part of their water allows drought-resistant 

 plants to keep their stomata open and to continue assimilation 

 even during periods of intense drought. Wassiliev has been 



