285 



Levitt (1939), in a study of cold hardiness in cabbage, 

 brings a quite new argument against the theory that re- 

 sistance to injur}" by cold is simply resistance to dehydra- 

 tion. He determined by calorimetric methods the propor- 

 tion of frozen and unfrozen water in hardened and in un- 

 hardened plants at their critical freezing temperatures, 

 that is, at temperatures at which 50-75 per cent of the 

 plants are killed (-5.6° for hardened, -2.1° for unhard- 

 ened). The "unhardened tissue retained 3.5 times as 

 much water in the liquid state per gram dry matter as did 

 hardened (6.30 and 1.75 gm. respectively)". Hardiness, 

 that is, resistance to injury by cold, ''is therefore not de- 

 termined simply by resistance to dehydration." 



It is quite evident that as yet one cannot draw any 

 general conclusion from the data here presented. 



VI. THEOEY OF DEATH BY VARIOUS PHYSIOLOGICAL, 

 PHYSICAL OR CHEMICAL ALTERATIONS 



Various jjliysiological changes which accompany freez- 

 ing have been assumed to cause injury and death. Though, 

 in the last analysis, the mechanism of their lethal action 

 might be physical or chemical, they will be treated here 

 as physiological changes, as they were presented by 

 their authors. The physical alterations, other than crys- 

 tallization of water, which have been observed to result 

 from freezing and thawing in aqueous colloids are : pre- 

 cipitation, agglomeration or dispersion of particles, coag- 

 ulation, gelation, changes in transparency (turbidity, 

 opalescence) and changes in consistency (cf. our previous 

 review of this subject). Any of these physical alterations, 

 as well as a number of chemical changes, might consti- 

 tute the mechanism of injury and death by freezing. In 

 the literature one finds such a general suggestion repeat- 

 ed again and again. But specific suggestions as to wiiich 

 protoplasmic constituents might be altered, and in what 

 manner, are few and still fewer the experimental attempts 

 to test these possibilities. We shall mention here the 

 more specific suggestions and those which are backed by 

 some experimental evidence. 



