84 Journal of Agricultural Research voi. xv, No. a 



Undercooling was found by Mez (jj) to be a factor of great importance 

 in frost injury. Where ice formation occurs at once, the lower tissues 

 of trees are protected by the poorly conducting layers of frozen tissue on 

 the outside. Voigtlander (47) showed that, owing to the continual 

 movement of plants in the wind, but little undercooling of the cell sap 

 occurs in nature. 



A mechanical effect of ice formation upon the plasma membrane has 

 been ascribed by Maximow {30, 31, 32) as a cause contributing to the 

 frost injury of cells. He stated that the osmotic properties of the 

 plasma membrane are changed by freezing, beipg supported in this 

 opinion by Chandler (5). 



MoHsch (j6) and Miiller (57, 39) ascribed frost injury to the withdrawal 

 of water from the plasma membrane, and with this view Maximow partly 

 agrees. The former authors did not follow the effect of desiccation farther 

 than to state that the injury is due to the withdrawal of water during freez- 

 ing. The processes of freezing, desiccation, and plasmolysis were found to be 

 analagous in their effects upon the cell by Matruchot and MolHard {28, 29). 

 The analogy of these processes is further indicated by the work of Greely 



Gorke (jj) advanced the idea that frost injury is due to the precipita- 

 tion of proteins through salting out. He considered the concentration 

 of the salts of the cell sap on freezing to be sufficient to precipitate 

 irreversibly the proteins in solution in the cell sap and to cause similar 

 changes in the protoplasmic gels. He considered the effect of the in- 

 creased concentration of acid salts to be small, however, and insufficient 

 to account for the precipitation. He was supported in this statement 

 by later workers, including Schaffnit (44) and lyidforss {23). Voigtlander 

 (47) doubted that the salting out of the proteins accounts for frost 

 injury. Chandler (5) maintained that protein precipitation does not 

 occur to any significant degree. He found that plants increase in hardi- 

 ness when given nutrient salts in abundance, while, in accordance with 

 Gorke's (13) theory, these plants should be more easily injured, owing to 

 the increased salt content of their cell sap. It therefore appears that 

 some factor other than salting out is necessary to account for the injury. 



The accumulation of sugars and consequent increased depression of the 

 freezing point of the cell sap was observed by Miiller {38) in plants ex- 

 posed to low temperatures. The effect of this increased content of sugars 

 was followed by Lidforss {23), Schaffnit (44), Kovchoff (21) and Bar- 

 tetzko (z) . All of these authors agree in ascribing to sugars an important 

 role in the prevention of protein precipitation owing to its protective 

 effect for colloids. 



The lesser injury from freezing shown by hardened plants was ascribed 

 by Gorke (zj), Schaffnit (44), and Bartetzko (z) to changes in the proteins. 

 Schaffnit thought these changes consisted in a cleavage of the "high 



