17 



must all be crystallized out at -6° C. can not be true since sugars 

 would remain in solution at lower temperatures than that. Further, 

 when we have evaporated the cortex sap of peach twigs in winter 

 condition to one-sixth or one-eighth of its volume without permitting 

 the temperature to go above 50° C. ice would not form when the 

 temperature was lowered to -22° C. though niany of the buds would 

 be killed at that temperature. Mez seems also to ignore the force 

 of imbibition which would tend to hold the water in the protoplasm 

 even after the sap solute may be crystallized out. The fact, however, 

 that when great supercooling takes place, plants are more liable to be 

 killed, is of interest and is, of course, associated with the fact which 

 will be discussed later that rapid cooling is more injurious to plant 

 tissue than is slow cooling. 



EFFECT OF SAP DENSITY ON KILLING TEMPERATURE. 



If the theory of Miiller-Thurgau and Molisch be true (even as it 

 is modified by Maximow) it would seem that some plants might 

 be hardy because the plasma membrane has the property of with- 

 standing great loss of water. Some might be relatively hardy be- 

 cause of a property by which sufficient water to protect the plasma 

 membrane at low temperatures is prevented from freezing, and some 

 might be relatively hardy on account of the presence of both condi- 

 tions. It is generally considered that after the effect of the sap solute 

 in holding water unfrozen is exhausted, there is still left the force of 

 imbibition. The relative importance of these two forces, however, 

 is not determined. Disregarding the force of imbibition (which, 

 however, may be the more important), it would appear to be true that 

 if the sap density (by sap density is meant not specific gravity but 

 molar concentration of the sap; that is, the number of gram molecules 

 of the sap solute in one thousand grams of water) were doubled, then 

 at any given temperature below the freezing point, but above the 

 eutectic point of the solute, twice as much water would be held un- 

 frozen to protect the protoplasm. 



With this idea in mind, experiments were started in September, 

 1908, to determine whether or not an increase in the sap density 

 would lower the killing temperature. 



Seedlings of corn, cowpeas, garden peas, tomatoes, squash, 

 cabbage and lettuce were grown in sand and watered with varying 

 strengths of potassium chloride and ammonium chloride at first — 

 later magnesium chloride, sodium chloride and sodium nitrate were 

 also used — while check plants were grown under similar conditions 



