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 role 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 aerial 

 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 

 Cydonioe. This fungus may live saprophytically, or it may 

 assume the role 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 



