1012 JOURNAL OF FORESTRY 



was considered advantageous to subject the material to a preliminary 

 freezing before extracting the sap. McCool and Millar have found 

 that all this troublesome pressing out of liquid and preliminary freezing 

 is unnecessary, and that practically the same results can be obtained by 

 placing the material directly in the freezing tube and slightly macerating 

 it with a stiff wire flattened and sharpened at one end. 



A series of tests using a number of plant tissues showed that the 

 values secured by their method of direct determination and by deter- 

 minations on extracted liquid were practically the same for the leaves 

 (Table IV, p. ii8). With roots the two methods showed differences 

 in some cases, the freezing-point depression being greater with the 

 direct method than with the extracted liquid. This they are as yet 

 unable to explain unless it is due to adsorption by the solid mass in the 

 direct method. 



In studying the relation between the osmotic pressure in plant tissues 

 and environmental factors they found, first, that the osmotic pressure 

 changes throughout the day, increasing as the day advances, then drop- 

 ping off toward evening. It is at a minimum in the early morning. 

 Apparently there is a relation between osmotic pressure on the one 

 hand and photosynthesis and metabolism on the other. 



Next they investigated the relation between the osmotic pressure of 

 the cell sap and the concentration of the soil solution in which the plant 

 is growing. Their data (Table XIII, p. 130) "show very strikingly 

 that the concentration of the soil solution is rather closely correlated 

 with the freezing-point lowerings of the roots of plants in contact with 

 it, but the tops of the plants are less sensitive to changes in the soil 

 solution." 



In studying the relation of freezing-point lowerings to soil moisture 

 they found that "there is no relation between the water content of the 

 soil and the freezing-point lowerings of the leaves . . . until the 

 critical water content of the soil is reached." The roots, on the other 

 hand, show a close correlation between osmotic pressure of cell sap 

 and soil moisture. Perhaps most significant of all is the indication 

 from their data that the osmotic pressure of the soil solution at the 

 wilting point of the soil is equal to or greater than that of the roots of 

 the plants. This would indeed afford a simple explanation of the 

 wilting coefficient, but is contrary to the work of Shull, who found that 

 roots possessed an osmotic pressure somewhat higher than that of a 

 soil in which the water had been reduced to the wilting coefficient.^ 



8 Shull, Chas. H. : "Measurement of the surface forces in soils." Botanical 

 Gazette 52: 1-31, 1916. Reviewed by Barrington Moore in Journal of Forestry, 

 Vol. IS, No. I, 1917. 



