48 MANUAL OF TREE DISEASES 



general effect on the health and development of the branches, 

 the injury is not directly noticeable, unless the tree is seriously 

 damaged by such leaf troubles year after year. It was formerly 

 thought that the characteristic cankers in the bark of trees 

 associated with extremes of temperature were of two sorts, 

 called respectively, sun-scald, when the drying-out effect of 

 the sun's rays was the cause, and winter-injury or freezing-to- 

 death, when areas of bark were killed by extremely low tem- 

 peratures in winter. It is now held that a very large pro- 

 portion of such cankers are due to freezing-to-death and that 

 sun-scald cankers caused by extremes of heat in summer are 

 rare. 



Cankers or dead areas of bark due to freezing often occur in 

 crotches, on the south and southwest sides of the trunk, and 

 around the base of the tree. Crotch-cankers are common and 

 are thought to be due to the tissues at these places being 

 more parenchymatous and much more slowly matured than 

 the adjacent bark-tissues. Thus the bark at crotches is more 

 susceptible to freezing-to-death, and injury occurs at a higher 

 temperature than would cause injury to properly matured tissue. 

 The cankered dead areas, often with the bark fallen away, on 

 the south and southwest sides of the trunk aire common in 

 certain kinds of trees, notably the Norway maple. The injury 

 occurs in late winter when the sun's rays in the afternoon raise 

 the temperature of the bark above the freezing point, to be 

 followed at night by a temperature below freezing. The dif- 

 ference in temperature between the north and south sides of 

 a tree often amounts to as much as ten degrees. Several 

 explanations can be offered to account for the injury : (1) with 

 the daily rise in temperature new growth is started and the 

 tissues formed are more susceptible to low temperatures at 

 night; (2) repeated thawing and freezing of the tissues de- 

 crease their resistance to freezing; (3) the rapid fall, from a 

 high to a low temperature causes the death of the tissue at a 



