APPLE DISEASES 41 



general opinion is that while the removal of water from the 

 cells does act detrimentally, ice-formation itself is not di- 

 rectly responsible for the injury; it is only incidental and 

 secondary. 



The rate of the freezing has been a point of attention by many 

 students of winter-injury. Prominent writers are agreed, 

 however, that it is the sudden or rapid temperature-fall that 

 brings about the injury. But this is not the final cause of the 

 injury or death of tissues. During the downward jump of the 

 temperature, water is rapidly removed from the cell, and death 

 follows. On a cold sunny day tissues exposed to the sun are 

 warmed, while tissues not exposed remain at or near the tem- 

 perature of the atmosphere. Toward evening with the with- 

 drawal of the sun's rays the temperature of the sun-exposed 

 area falls so rapidly the first part of the period as to produce 

 death of the tissues on the southwest side of the trunk, while 

 that on the northeast side, protected by the slower temperature- 

 fall, remains uninjured. Thus, in brief, the uniform presence 

 of frost-injury on the southwest side of trees is explained. 

 Localization of the injury at certain points, as, for example, the 

 crown or the crotch, is due to the fact that the tissues at such 

 places become mature more slowly than at other points. Per- 

 haps, too, bark-tensions set up by rapid growth at these certain 

 points also assist in localizing the injuries. And finally, rapid 

 evaporation due to high winds may be a contributory factor. 



Sudden thawing of frozen tissue is thought by some to be 

 harmful in effect. In fact, one of the earlier theories advanced 

 to explain the cause of sun-scald is that rapid thawing, subse- 

 quent to extreme temperatures, brought about by the action 

 of the sun, produces the injury. This theory still has adherents. 

 Those who hold to this explanation believe that during a rapid 

 thawing the plant cell is unable to recover the water which 

 has been removed during the freezing-process, and as a result 

 the cell dries out and dies. But there are authorities who take 



