360 PLANT PHYSIOLOGY 



siderable depth are broken off. In spring, when the ice layer 

 thaws and the soil that had been lifted by it settles down again, 

 the plants with the ruptured roots remain lying on the surface 

 and are easily killed by desiccation, if the spring is not moist 

 enough to allow them to form new roots. Their recovery may 

 be promoted by well-timed rolling of the field. 



Plants whose overwintering organs are situated largely above 

 the soil, chiefly shrubs and trees, are subject to an additional 

 danger in the winter, viz., excessive desiccation. Though the 

 evaporation of water is insignificant at temperatures below zero, 

 nevertheless the water balance of shrubs and trees in winter is 

 unfavorable for them owing to a complete cessation of water 

 absorption from the frozen soil. With strong permanent winds 

 or on clear sunny days, especially toward the end of the winter 

 when the sun's heat is considerable, the plants may lose too much 

 water and die as a result of drying out. Autumnal drought is a 

 major cause of the abscission of leaves of deciduous trees and the 

 formation of a thick layer of cork on branches of trees, which 

 are not found in grass plants with annual stems. Leaves of 

 conifers and other evergreen plants develop a thick cuticle, which 

 lends them the characteristic rigidity and protects them from 

 water losses. 



The snow cover prevents the desiccation of winter cereals, 

 and it is only in the absence of snow during a prolonged period 

 in autumn that wilting may be observed. But this may be 

 more profitable than harmful to the plant; for it increases con- 

 siderably the frost resistance, which is especially important in 

 the absence of a snow cover. But an excessively early and 

 severe drought in autumn checks normal tillering and rooting 

 and inhibits the accumulation of protective reserve carbohy- 

 drates, thus proving harmful for winter crops. 



The causes of winter injury and winterkilling of plants are 

 various and are not determined by the action of freezing alone. 

 Hence, the concept of winter resistance, or the capacity to 

 withstand diverse harmful effects that occur during winter, 

 does not coincide with the more narrow concept of resistance to 

 freezing. 



W-hen selecting winter-resistant plants, it is not sufficient to 

 determine frost resistance alone by means of one of the methods 

 discussed. It is necessary to subject the plants to the complex 



