372 PLANT PHYSIOLOGY 



able to show that one of the important features of drought 

 resistance, at least in wheat varieties, is the diurnal changes of 

 the stomatal movements. In the dry climate of Southeastern 

 Russia, the resistant South-Russian wheat varieties keep their 

 stomata open all day long, while the less resistant Canadian 

 varieties close them early in the morning and thus starve and 

 produce a low yield. The capacity of enduring wilting is 

 probably connected with an increased content of osmotically 

 active substances and hydrophilic colloids in the cells of drought- 

 resistant plants. With sufficient water supply, the leaves of 

 drought-resistant wheats have a higher water content than those 

 of nonresistant varieties (Lit vino v). 



Although the capacity of enduring considerable dehydration 

 of their tissues without injury is one of the essential features of 

 drought-resistant plants, nevertheless the drought resistance of 

 all varieties and crops cannot be explained by this one feature. 

 Anatomical and morphological peculiarities, such as a well- 

 developed root system and the storage of water reserves in fleshy 

 stems and roots, may also be of great significance. Of no less 

 importance is the xeromorphic structure, which has been exam- 

 ined above. Hence in selecting drought-resistant varieties, it is 

 important to pay attention to the size of the plant cells, as has 

 been indicated by Kolkunov (1904). The possibility of the 

 plant withstanding drought by means of various physiological 

 and anatomical adjustments makes it insufficient to determine 

 the degree of drought resistance on the basis of a single feature, 

 even if it is a feature of major importance. It is more reliable 

 to apply the more compHcated methods of direct testing of the 

 resistance to different forms of drought, especially to soil drought 

 and to hot dry winds. To accelerate this testing, which up to 

 the present time was carried out under field conditions, different 

 forms of desiccators, dry wind chambers and other devices, have 

 been devised, which allow one to subject the plant to any com- 

 bination of soils and atmospheric factors (Shirley, Aamodt). 



In most arid regions, the soil dries completely toward the end 

 of the vegetative period in spite of the scant summer precipita- 

 tion. Early varieties in such regions produce higher yields than 

 late varieties, for they complete their life cycle and ripen their 

 grain before the complete consumption of all the available mois- 

 ture. Consequently, in the practice of agronomy earliness is 



