1923] Density of the Cell Sap of Plants 65 



association at an elevation of 10,000 feet or more. The samples of leaf 

 tissue were collected in air-tight test tubes, then frozen, and the sap 

 expressed in a specially designed press. The sap density determin- 

 ations were made by means of the standard method proposed by Dixon 

 and Atkins and further developed by Harris, Gortner and Lawrence, 

 w^hereby the osmotic pressure of the sap is calculated from the ob- 

 served- depression of the freezing point. The freezing point depres- 

 sions were determined by the well-known Beckmann thermometric 

 method. The Beckmann thermometer was read to thousandths of de- 

 grees and the results were then expressed in freezing point lowering 

 in degrees C. (A) and in atmospheres of osmotic pressure (P), after 

 corrections for undercooling were made by using the formula sug- 

 gested by Harris and Gortner as follows : 



A = A' — 0.0125 u A'^ in which A ^ the true depression of the 

 freezing point, A' the observed depression of the freezing point u^ 

 the amount of undercooling in degrees C, and a constant (0.0125) 

 experimentally determined, according to the tables calculated by 

 Harris and Gortner. 



Interesting and fundamental relationships are brought out when 

 some of the easily measured environmental factors — especially precipi- 

 tation, the march of the available soil moisture and the evaporating 

 power of the air — are correlated with sap density. Since the ability 

 of the plant to maintain a proper balance between absorption and 

 transpiration is of vital importance, even at the intermediate eleva- 

 tions of the mesophytic aspen-fir type and increasingly so toward 

 lower altitudes and xerophytic habitats, any practicable means which 

 can be found to determine the water relations within the plant and 

 between the plant and its environment will be of decided value. The 

 correlation between habitat and sap concentration which was found 

 in this study affords such a means. It may well be illustrated here by 

 one of the series of tests, which was made on and adjacent to the 

 Wasatch Plateau, near Ephraim, Utah, in July, 1921. The average 

 densities of the cell sap of all species on the several vegetative asso- 

 ciations studied were as follows, the associations arranged progres- 

 sively from low to high altitudes : 



