42 MANUAL OF FRUIT DISEASES 
the opposite view of the question and hold that rapid thawing 
is of no concern. 
The question of crotch-injury is often explained on the basis 
that snow and ice lodge therein, soften the bark on melting, 
and injury results. Here, it is maintained, the sun’s rays are 
concentrated through the water acting as a lens and the intense 
heat injures the protoplasm. 
It has been suggested that possibly tissues become more 
tender as a result of frequent alternation of freezing and thaw- 
ing. While sudden changes in temperature are destructive 
in effect, yet it is generally agreed that the sudden fall is of 
\ vital importance but that the injury is independent of the rate 
of thawing. 
In general, then, whatever the manner of killing, it may be 
said that when, during the freezing-process, the water with- 
drawal from the cell passes a certain limit, the cell is killed. 
Some lay emphasis on the effect of the rate of freezing and thaw- 
ing; others lay emphasis on the réle of winds, snow and ice in 
assisting the action of low temperatures to bring about the 
injurious effects. 
Frost-cankers, irrespective of their position on the aérial 
parts of the tree, are commonly inhabited by various fungi. 
The most common of these perhaps in the northeastern United 
States is the New York apple-canker fungus, Physalospora 
Cydonia. This fungus may live saprophytically, or it may 
assume the réle of a facultative parasite. In the latter case it 
enlarges the wound and does considerable damage. 
\ The topography of the land is influential in its relation to 
winter-injury. Trees on low, and consequently less-drained, 
land suffer more than others. The amount of water in the 
tissues at the time the low temperatures occur is a factor. The 
more water the cells contain, beyond certain limits, the more 
liable is the tissue to injury by freezing. On the other hand it 
is held that trees which have endured a drought during the 
