RESISTANCE OF PLANTS 373 



very often confused with droaight resistance, and varieties that 

 ripen in time to evade drought are generally accepted as drought 

 resistant. The method commonly adopted for the determination 

 of drought resistance in field variety testing contributes to this 

 confusion of concepts. The yield of varieties is compared in dry 

 years, and those that are the most productive are accepted as 

 most drought resistant. It is the fearly varieties that produce a 

 higher yield when there is a late drought. 



Plants show a different resistance to drought at various growth 

 stages. In cereals, there is the greatest depression when they 

 are subjected to drought at the time of the formation of the 

 floral organs. Withdrawal of water by the leaves in this period 

 stunts the developing embryo and leads to alternate setting 

 or to poor filling of the grain. This sensitive period has been 

 designated the ''critical period." It was first observed by 

 agrometeorologists (Browmov, Hauer, Azzi) by means of com- 

 paring yield with the time of the summer precipitation. Later 

 on, the existence of critical periods was proved by means of 

 physiological experiments. 



The complexity of the conditions and the existence of different 

 types of drought resistance make the selection of plants difficult. 

 Plants cannot be chosen according to one peculiarity, as for 

 instance the intensity of transpiration or the coefficient of 

 transpiration, as it has been customary to do. First of all, there 

 must be established the factors of drought that are found in the 

 various geographical regions, as has been indicated in the case 

 of winter resistance. For instance, one should know the preva- 

 lence of soil and atmospheric drought; the time of its manifesta- 

 tion, whether in spring or summer; and finally, the reaction of 

 the different varieties to the different types of drought. As yet 

 there are no satisfactory methods of directly determining drought 

 resistance that are as reliable as the methods of determining 

 frost resistance by means of direct freezing. This hinders selec- 

 tion work and compels recourse to field methods, which require 

 a long time and do not always give clear results. At present, 

 the wilting method suggested by Tumanov is most widely used. 

 Plants are grown in pots and at definite stages of their develop- 

 ment are subjected to permanent wilting, by stopping watering^ 

 for some time. The decrease in yield of the wilted plants as 

 compared with the control plants serves as an index of their 



