358 PLANT PHYSIOLOGY 



soil and are hardened for 5 to 10 days previous to the testing, for 

 in a nonhardened state the differences between the more and 

 the less frost-resistant varieties are quite insignificant. After 

 hardening, the boxes are placed for 1 or 2 days in the refrigerators 

 at a definite temperature ; and after gradual and careful thawing, 

 the degree of injury by freezing is determined, the greater injury 

 corresponding to a lesser frost resistance. 



The method of direct freezing requires complicated and 

 expensive refrigeration installations and hence is accessible only 

 to large institutions. The so-called ''indirect methods" of 

 determining frost resistance are therefore frequently applied. 

 They are based on the fact that in genetically closely allied plants, 

 e.g., in different varieties of wheat or rye, the degree of frost 

 resistance varies more or less proportionally to their chemical 

 composition, for instance, to their sugar content. The latter 

 may be readily determined either directly by chemical analyses 

 or more quickly by means of a refractometer, which is widely 

 applied for this purpose in the sugar industry. Another group 

 of indirect methods is based on the determination of the amount 

 of water bound by colloids. All these methods, like the 

 method of direct freezing, can be used only with such plants as 

 have been sufficiently hardened. 



Indirect methods are more rapid, more convenient, and less 

 expensive than the method of direct freezing, but they give less 

 reliable results; for they allow one to determine only one of the 

 items, the sum of which determines the degree of frost resistance. 

 For example, with one and the same sugar content, the frost 

 resistance of the varieties compared may be different because of 

 variation in the amount of bound water. Hence, indirect 

 methods can serve only for a very approximate preliminary 

 estimate of the hardening capacity of varieties. 



82. Other Harmful Effects during Wintering: Exhaustion, 

 Drowning, Heaving, Winter Desiccation. Physiological Basis 

 of Selection for Winter Resistance. — The direct influence of 

 freezing upon the cells is not the only danger threatening plants 

 during winter. They often perish from other unfavorable con- 

 ditions. Death of plants from exhaustion is usually observed in 

 mild winters with a deep snow cover, especially if the snow falls 

 on thawing soil. For a long time, this was explained by the 

 suggestion that the plants remaining at a relatively high tern- 



