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 
