356 PLANT PHYSIOLOGY 



of frost resistance. In the seedlings of cereals, for instance, the 

 most susceptible part is the plumule, and the hardiest are the 

 root tips and their central cylinders, while cortical parenchyma 

 of the root is comparatively little frost resistant. After moderate 

 freezing, the root tips, therefore, continue to grow for some time, 

 while the young leaves and the external portions of the roots are 

 killed. 



Frost resistance is likewise decreased as a result of alternate 

 freezing and thawing, which greatly weaken the protoplasm 

 because of the continuous change from a state of dehydration to 

 that of saturation. 



During autumn and winter, the frost resistance of winter plants 

 changes continually. When sown in early autumn, during fairly 

 warm weather, winter crops are not very frost resistant in the 

 beginning of their development, and freezing without a cool 

 period previous to it may be very injurious to them. But with 

 a gradual decrease in temperature, they become more hardened. 

 Clear sunny days alternating with moderately frosty nights are 

 most favorable in this respect. Usually up to the time of snow- 

 fall, the winter crops are in their most hardened state and are 

 capable of enduring the relatively severe frosts of the first winter 

 months. The snow cover, which smooths considerably the 

 fluctuations in temperature at this time of year, likewise exerts 

 a considerable protective influence. 



But a very prolonged period under snow, especially if the layer 

 is thick and prevents the passage of the sun's rays, leads to a 

 gradual decrease of frost resistance. As a result of continuous 

 respiration, the plants gradually lose their carbohydrate reserve 

 and cannot restore it because of insufficiency or even complete 

 absence of light for photosynthesis. This is accompanied, 

 besides, by other internal changes, which also decrease frost 

 resistance, such as vernalization, which will be discussed in detail 

 in Chap. XII. Toward spring, the plants are very weakened 

 and may succumb easily to the influence of a mild frost, which 

 they could endure without harm at the beginning or in the middle 

 of the winter. 



Not all plants can be hardened. Plants of southern latitudes, 

 e.g., cotton, rice, as well as cucurbits, after a prolonged exposure 

 at temperatures just above 0°C. do not increase their frost resist- 

 ance but on the contrary may be injured or killed. Rice and 



