114 PRINCIPLES OF PLANT CULTURE 



part to suffer injury. Many bulbs, tubers and roots which 

 survive the severest winters within the soil, where they 

 thaw slowly, are destroyed by moderate freezing if quickly 

 thawed. Frost-bitten plants are seldom injured when 

 sheltered from the morning sun by a dense fog, which 

 causes them to thaw slowly. Apples, covered in the or- 

 chard in autumn by leaves, sometimes pass a severe winter 

 with little harm. 



When the water that is withdrawn from the tissues in 

 the freezing process is gradually set free by slow thawing, 

 it may be absorbed by them again and little or no harm 

 results. 



The length of time the tissues remain frozen. — A com- 

 paratively slight degree of frost, if prolonged, may act 

 more injuriously than a severer degree of shorter duration. 

 Prolonged freezing is especially injurious when the frozen 

 parts are subjected to drying wind, which evaporates 

 their water, while the frozen condition prevents move- 

 ments of their fluids. 



The frequency with which freezing and thawing are re- 

 peated. — Frequent slight freezing and thawing are far 

 more injurious than a prolonged frozen condition, even 

 though the latter occurs at a much lower temperature. 

 Winter wheat and rye, and strawberry beds are often 

 more damaged in mild winters, in which freezing and 

 thawing weather alternate, than in more severe ones, 

 when the temperature is mostly below freezing. The 

 chief damage is usually done to these crops in late autumn 

 and early spring. 



The previous treatment of the plant. — • Plants grown by 

 artificial heat may be far less able to endure cold than 

 others of the same varieties grown in the open air, possibly 

 owing to the more succulent condition of the former. 



