104 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
studies of the method of determining the freezing point of cell saps, 
and also applied this knowledge to the study of plants under various 
habitat conditions. The latter were probably the first to point out 
that the osmotic pressure of the cell sap varied rather directly with 
the dryness of the habitat. They also showed that trees and shrubs 
possess higher pressures than the lower and shorter lived forms of 
vegetation, which furnishes the basis for considering height as a 
factor affecting the osmotic pressure in the leaves. 
McCool and Millar (131). 1917, experimented with impressed plant 
tissues and obtained practically the same results as when the ex- 
tracted saps were used. This was a distinct step forward in simplify- 
ing the process, and therefore no attempt is made to describe the 
method of sap extraction. McCool and Millar found it only neces- 
sary to macerate slightly the material with a stiff wire, in the freezing 
tube. These investigators also brought out much new information 
on the changes in osmotic pressure in the leaves with atmospheric 
changes, and the close correlation between root pressures and condi- 
tions of the soil moisture, the former being little influenced by atmos- 
pheric conditions. 
Bates (105), in 1917, seeking an explanation of the great difference 
in the transpiring capacity of different species of tree seedlings, and 
not being equipped with freezing-point apparatus, obtained the sap 
density of the aerial portions of whole seedlings by grinding them in 
a food grinder, extracting the water-soluble substances, filtering the 
liquid, and then drying the water-soluble solids and the washed pulp 
separately. The weight of these two. when deducted from the origi- 
nal weight of the plant, gives the weight of the original solvents, and 
the " sap density " is expressed by the ratio between solutes and 
solvent. These first results were found to have a close relation to 
the transpiration rates that had been observed, and it was therefore 
concluded that sap density might very largely serve as an automatic 
restriction on transpiration. 
Although realizing that an expression of osmotic pressures would 
2five a more reliable basis for comparing the species, this was not 
undertaken for some time, since it was desired to establish first the 
importance of the sap density as a measure of the condition of the 
plant and its response to various atmospheric conditions. This work 
has been pursued to some extent. 13 It is only desired here to state 
that, within the limits- of experimental error, the osmotic pressures 
shown by a number of the conifers appear to be the same when the 
sap densities by the above method are the same. Considering all of 
" " Forest Types of the Central Rocky Mountains," by C. G. Bates. Unpublished 
renort. 
