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 

 give 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 



13 " Forest Types of the Central Rocky Mountains," by C. G. Bates. Unpublished 

 report. 



