RESISTANCE OF PLANTS 357 



cotton are especially sensitive. A temperature of to +5°C. 

 for 24 hr. may be lethal to them. But death of such plants from 

 low temperatures differs in character from death following 

 freezing. It occurs not all of a sudden but gradually and fre- 

 quently is observed only after the plants are returned to a warmer 

 temperature. The actual causes of death in such cases are not 

 known. Death is usually preceded by the appearance of brown 

 spots, by the destruction of chlorophyll, by a loss of turgor, and 

 by abscission of the leaves. This general behavior leads to the 

 supposition that there is a disturbance in the regular changes of 

 the biochemical processes in the plants and that an accumulation 

 of poisonous products of metabolism takes place, which further 

 causes the death of the whole organism. 



Various winter plants possess a widely differing frost resistance. 

 Thus varieties of wheat originating in Eastern Russia are very 

 frost resistant, while varieties from Southwestern Russia and 

 Western Europe are very susceptible to frost. Still more sus- 

 ceptible are varieties originating from the Transcaucasus and 

 Central Asia, where they are grown in a mild and almost frostless 

 winter. 



But the origin of the variety alone does not give a full knowl- 

 edge of its resistance. For a systematic selection of winter 

 varieties, it is quite indispensable to have more convenient and 

 reliable methods of determining their frost resistance. Not so 

 very long ago, the only method for this was the testing of varieties 

 under field conditions during several winters, keeping an account 

 of the number of plants that had overwintered in each plot. But 

 the field method requires many years of testing; for in mild 

 winters all the varieties overwinter w^ell, and one has to wait for 

 a more severe winter. Besides, the results obtained are not 

 reliable; for death during the winter m,ay be the result of injuries 

 not only from low temperatures but likewise from fungi or insects. 



That is why much attention has recently been given to the 

 elaboration of rapid laboratory methods for the determination 

 of frost resistance. In the first place, there may be mentioned 

 the so-called ''direct freezing method," or ''artificial winter test" 

 introduced by Harvey and widely used for testing the hardening 

 capacity of plants in special refrigerators, which are cooled to 

 the required temperature by means of refrigerating machines. 

 The varieties intended for testing are sown in boxes filled with 



