May 3 i, 1924 Cell Sap Density and Environmental Conditions 
847 
movement of the solvent through the cell-wall membranes from the weaker to 
the stronger solution. 
The physicochemical laws of diffusion and osmosis, which underlie the water 
relations of plants, can only be roughly outlined in this paper. For more com¬ 
plete discussions of these laws, relating particularly to selective absorption, 
diffusion, and osmotic pressure, the reader is referred to the literature cited 
herewith, and to standard works on physical chemistry and plant physiology. 
Many investigations have been conducted to determine the influence upon the 
organism of the medium in which it is grown, but it is only recently that the 
osmotic pressure of the cell sap has been studied in this connection. Among 
such researches those of Drabble and Drabble (37), Fitting (40), Dixon and 
Atkins (26-31, 34~36), Harris, Gortner and Lawrence (46, 43-50, 52-61), Iljin, 
Nazarova and Ostrovskaja (68), and Livingston (86, 87) should be mentioned. 
The foundations of our present knowledge of osmotic phenomena were laid 
by Pfeifer (101) in 1877 and somewhat later were further amplified by Van’t 
Hoff (67). LeClerc du Sablon (79) and Livingston (83) have shown that the 
permeability of membranes is an essential factor in the physiology of plants and 
that all of the material exchanges, whether between the cells themselves or 
between the cells and the external medium (soil in the case of forest trees), are 
determined by the laws of osmosis. 
These laws set forth explicitly that a substance in solution tends to distribute 
itself uniformly throughout the entire volume of the solvent. In the plant cell 
no absorption will take place when the concentration of the solution outside is 
equal to that within the cell vacuole. If the outside solution is the more highly 
concentrated, water will actually be withdrawn from the vacuole, which will 
become smaller. An osmotic interchange through the cell walls thus accom¬ 
panies and complicates the simple movement of water, and though millions of 
cells may intervene there exists a direct osmotic relation between the topmost 
leaf of the tallest Sequoia tree and the soil solution at its root tips. 
Osmotic pressure can not be measured directly except with great difficulty, 
but is readily calculable from the amount by which a solute lowers the freezing 
point or raises the boiling point of the solvent or from the observed vapor ten¬ 
sion of a solution. These indirect methods depend upon the general principles 
that the depression of the freezing point, elevation of the boiling point, decrease 
of the vapor tension of solutions, and osmotic pressure are all related phenomena 
and may be obtained one from the other for any given solution. 
A review of the above-mentioned researches convinced the writer that an 
investigation of the osmotic properties of the intra-cellular fluids of forest vege¬ 
tation on various sites, selected with reference to wide differences in evaporation 
and the supply of available soil moisture, would throw considerable light upon 
the basic physiological problems of silviculture and forest distribution. Chief 
among these may be mentioned the relative ability of different species to extract 
water from the soil, and, to some extent, retard transpiration through increased 
sap concentration; the relation of sap density to frost resistance; and the r61e 
of the osmotic pressure of the cell sap in the adaptation of exotic species intro¬ 
duced on dry sites in semiarid regions. 
EXPERIMENTAL METHODS 7 
In conducting this investigation it was necessary to consider: (1) the selection 
of sites, (2) the collection of leaf material, and (3) the technique of determining 
the density of the cell sap. 
7 The writer desires to express his indebtedness to Doctors Harris and Gortner for further details of 
technique gained through a short sojourn at their field laboratory and through their subsequent visit to 
the writer’s field laboratory in 1920. 
