40 MANUAL OF FRUIT DISEASES 
tree. It may be assumed, then, that irrespective of the time 
of the injury or of the organ affected, low temperature acts the 
same in bringing about such injuries. The explanation of the 
action of frost in the production of injury is based on the prin- 
ciple that there is a limit to low-temperature endurance by the 
plant. When this limit is reached or exceeded, the part is frozen 
to death or is seriously injured. This phenomenon is essen- 
tially a desiccation-process. The plant cell contains, on the 
average, about 75 per cent water which is necessary to the life 
of the protoplasm. The permanent removal of water from the 
cell, which occurs in freezing, is detrimental and the effect is 
expressed in serious injury or death. 
A consideration of the cause of frost-injury involves a dispo- 
sition of the many phases of the question of low temperature- 
action. To state that a given low temperature freezes certain 
tissues to death does not explain the manner of freezing. 
Among the more important phases in the process of injury the 
following may be noted: ice-formation in the tissues; slow 
versus rapid freezing; slow versus rapid thawing; bark ten- 
sion and shrinkage of bark from wood; effect of snow and ice 
on the bark; alternate high and low temperatures. 
It has been held in the past that ice-formation in the tissues 
takes place during the freezing-process. A warm, moist 
autumn, for example, offers conditions for excess water in the 
various tissues concerned. If winter should close in rather 
abruptly, the water in these parts would be frozen by the first 
period of low temperature. It was further believed that the 
formation of ice crystals in the tissues resulted in a tearing and 
rupturing of the cell walls. That ice is formed between the 
cells there is no doubt. Some, however, maintain that it is 
formed only when the freezing-process is very rapid. During 
such a process, it is thought, the water of the cell is withdrawn 
rapidly to the intercellular spaces; this rapid withdrawal of 
water from the cells results harmfully to them. At present the 
