COLD INJURY AxND COLD RESISTANCE 593 



the crj'stalHzation of water takes place within the cells, either in the vacuole, 

 or the cytoplasm, or both. The factors which determine whether freezing will 

 occur within the cells or between them are not well understood. Freezing in 

 the intercellular spaces is apparently much less often injurious to cells than 

 freezing within the cells. 



III. Causes of Freezing Injury in Plants. — A number of suggestions have 

 been made regarding the possible causes of freezing injury to plant cells, the 

 principal ones being the following: 



( 1 ) Ice formation in the intercellular spaces results in withdrawal of 

 water from the cells. The consequent dehydration results in disorganization 

 and death of the protoplasm (Molisch, 1897). 



(2) The ice formed in the intercellular spaces results in mechanical com- 

 pression of the cells which in turn causes deformation and death of the proto- 

 plasm (Alaximov, 1914). 



(3) Withdrawal of water from the cells due to the formation of ice 

 crystals in the intercellular spaces results in an increase in the concentration of 

 electrolytes in the cell sap which may have a "salting out" or other destructive 

 effect on the protoplasmic proteins (Harvey, 1918, and others). 



(4) Ice crystals may form within the cell, resulting in compression or 

 laceration of the protoplasm or other destructive effects (Stiles, 1930, and 

 others). 



(5) Death occurs, not at the time of ice formation, but as a result of 

 the subsequent thawing of the tissue. This idea, originally sponsored by Sachs 

 in i860, largely dropped into disrepute but has been revived by the findings of 

 Iljin (1933) and others that some plant tissues which can withstand freezing 

 are killed by rapid thawing. Death is apparently due to various types of 

 mechanical disturbances attendant upon the entry of water into the cells upon 

 thawing. 



Most modern workers seem to favor the concept that injury to protoplasm 

 during freezing is fundamentally due to mechanical effects of ice formation 

 either within or between the cells rather than to dehydration per se or to 

 chemical effects. The ultimate effect of such mechanical disturbances is pre- 

 sumably the disruption of the delicate organization of the protoplasm. 



IV. Hardening. — Exposure of many species to low temperatures just above 

 the freezing point results in a marked increase in their cold resistance. This 

 phenomenon is called hardening. Crop plants such as cabbage which are to 

 be planted early in the spring are often hardened artificially before being set 

 out in the field. This is usually done by transferring the cabbage seedlings 

 from the greenhouse to a cold-frame for a few days before they are trans- 

 planted into the field. 



