Grafts et al. — 134— Water in Plants 



Subsequently the sap was expressed and a third value obtained. From 

 Table 43 it is clear that the living tissue froze in every instance at a lower 

 temperature than did the dead tissue. The beet roots employed had pre- 

 viously been investigated with respect to plasmolytic and cryoscopic deter- 

 mination of OP. There was found to be considerable resistance to freez- 

 ing in the case of -j-PCD beets, with particular difficulty in inoculation. 

 The reverse was true of — PCD plants, where in some instances freezing 

 was spontaneous, i.e., required no inoculation. In line with other evidence, 

 the differences were believed due to dilution and concentration effects as 

 a result of freezing. 



Table 43. — Freesing points of living fisstie, dead tissue, and expressed sap. 



Atmospheres at 22.5° C: — 



PCD Living Dead Sap 



Positive 26.4 22.8^ 18.0 



Positive 23.6 15.3 13.3 



Zero 21.2 14.6 14.3 



Negative 12.9 11.7 12.9 



Negative 18.6 11.5 13.0 



• Freezing not complete. 



Recent reviews (Levitt, 1941 ; Scartii, 1944) on the subject of cold 

 hardiness of plants have emphasized the fact that resistance to freezing tem- 

 peratures by certain plants is due primarily to protoplasmic factors. While 

 there has been shown to be some correlation with other characteristics ac- 

 companying the hardened condition, e.g., small cells, lower moisture content, 

 higher sap concentration, they are considered to be of only secondary im- 

 portance. Considerable significance is attached to the physical state of the 

 protoplasm in hardened as compared to unhardened cells. Associated with 

 the former are (a) an increased permeability and (5) decreased consistency, 

 both of which are attributed to (c) a greater hydrophily of the protoplasm. 

 By means of this theory many of the observations on cold hardiness may be 

 explained. 



The experimental basis for this hypothesis is derived for the most part 

 from application of micrurgical and plasmolytic methods applicable to liv- 

 ing cells. Certain advantages are thus gained over the considerable number 

 of chemical and physical procedures that have been employed with dead 

 tissues and extracts. The following additional characteristics were found 

 to be associated with the hardened condition of living plant cells : increased 

 thickness of the cytoplasmic layer, high resistance to deplasmolysis injury, 

 ability of the protoplast to round up on plasmolysis, greater density of 

 protoplasmic strands, less rigidity of ectoplasm on dehydration, lower re- 

 fractive index of ectoplasm, less clumping of plastids and granules on plas- 

 molysis, greater non-solvent space (in some), less tendency of colloids to 

 coagulate. 



The literature of frost resistance is so extensive that only one phase of 

 the problem will be considered here — the matter of hydrophily of the proto- 

 plasm. Frost resistance implies ability to withstand freezing temperatures. 

 It means that the tissues are able to prevent formation of ice, or have an 

 ability to withstand it. The term hydrophily, used in connection with proto- 

 plasm, refers to the total water content or hydration ; it carries no implica- 

 tion as to the nature of the forces holding this water. The relationship 



