OSMOTIC PRESSURE AS AN INDEX OF HABITAT 

 By Barrington Moore 



In studies of forest types, or forest associations, we must first know 

 the factors which cause each type, and then the maximum and minimum 

 quantities of these factors which permit the existence of a given type. 

 For example, we know that moisture is one of the factors. What range 

 of moisture conditions, other conditions being equal, will permit the 

 existence of the sugar pine-yellow pine type? What moisture the ex- 

 istence of pure yellow pine? 



Although the emphasis should be placed primarily on studies of the 

 manner in which each factor affects the forest, and measurements of 

 the range of each factor in each forest type, nevertheless it is some- 

 times helpful to find a single phenomenon, which, though its exact 

 workings may not be clearly understood, yet is subject to measurement 

 and serves as an index of the underlying factors. Such an index should 

 never bq confused with the causal factors which produce it. It is 

 simply a measure of these causes. The osmotic pressure of cell sap 

 appears to be such an index and to hold out considerable promise of 

 offering a convenient yardstick which will express with reasonable 

 accuracy the moisture conditions of different types. 



The importance of the cell sap is due to the fact that "all the activities 

 of the protoplasm, both somatic and germinal, must take place in the 

 environment of the cell solution."^ The osmotic pressure of the cell 

 solution must exert considerable influence on these activities. There- 

 fore the determination of osmotic pressure of the sap of a species grow- 

 ing under different conditions should throw considerable light on the 

 manner in which those conditions affect the species ; also the osmotic 

 pressure which characterizes different species suited to different habitats 

 should serve as a measure of the different habitats. 



There are three indirect ways of determining the osmotic pressure of 

 the sap of plants, namely, by the lowering of the freezing point, by 

 vapor pressure, and by the raising of the boiling point. Of these the 

 most used is the first. 



' Harris, J. A. ; Lawrance, J. V., and Gortner, R. A. : "The cryoscopic constants 

 of expressed vegetable saps as related to local environmental conditions in the 

 Arizona deserts." Physiological Researches, Vol. 2, No. i, pp. 1-49, 1916. 



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