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 w r ater 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 such a process, it is thought, the water of the cell is withdrawn 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 



