CHAPTER XV 



WINTER KILLING AND HARDINESS 



The limits to fruit growing set by low winter temperatures have been 

 indicated. This limitation has been shown to be influenced more or 

 less by other factors, precipitation in some cases, summer temperatures 

 in others. Low winter temperatures are important, however, in other 

 respects than merely marking boundaries separating a section where 

 a given fruit is grown from another section where it is not. Damage 

 by freezing is not confined to any one region; it is as definitely an injurious 

 factor in CaUfornia and Florida for tender species as it is in Montana 

 or Wisconsin for the more hardy fruits. It is not confined to the border- 

 lands of a fruit zone but in one way or another makes itself felt well 

 within the regions adapted to fruit growing. It is not a simple matter 

 of uniform, predictable reaction to a given temperature but is modified, 

 intensified or palliated by varying factors and is itself probably a group 

 of fatal or damaging reactions assembled for convenience or for want 

 of discriminating classification under the single name of winter killing. 



DEATH FROM FREEZING 



Several explanations of the actual process of killing of tissue by low 

 temperatures have been made; it seems possible that there may be more 

 than one way by which the killing is brought about. Parenthetically, 

 it should be stated that the original theory and the one still most fre- 

 quently advanced by practical men, i.e., that death by cold is due to 

 expansion accompanying freezing and a consequent rupture of the cell 

 walls, is not tenable as can be proved mathematically or by microscopic 

 examination. The bursting of trunks and limbs, cited to justify this 

 contention, is considered later. The view held most generally by inves- 

 tigators ascribes death to withdrawal of water from the cell, a process 

 comparable to death by plasmolysis. 



Tissue Freezing is Accompanied by Cell Dehydration. — Numerous 

 investigators have shown that ice is very rarely formed within the cell 

 unless the cooling is very rapid, more rapid, in fact, than would occur in 

 nature. Before freezing begins, since the cell sap contains substances 

 in solution and because of capillary supercooling, most tissues must be at 

 a temperature several degrees below the freezing point. The first evident 

 step in the process of freezing is a contraction of the protoplasm and the 

 appearance of water in the intercellular spaces where it has been forced 

 or drawn from the cell. Ice formation begins at various points in the 



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