846 
Journal of Agricultural Research 
Vol. XXVIII, No. 9 
will attain the best growth and development on a given aspect at a given eleva¬ 
tion. When the effects of climatic or other factors are once definitely known, 
failures in planting may be largely avoided by a judicious choice of sites 3 and 
of especially adapted species. 
Since reliable quantitative data on the physiological requirements of native 
forest vegetation are still very meager, investigations were undertaken to deter¬ 
mine site characteristics by studies of native plants as indicators. {72, 73 .) 4 
The physiological activities of a plant are, of course, a response to (and there¬ 
fore must be a measure of) the physical factors of the environment. By numer¬ 
ous close observations it ought, therefore, to be possible to ascertain the exact con¬ 
ditions required for optimum growth and development. If this could be done 
quantitatively, by the employment of native species as indicators, a way might 
be found to determine with precision the planting sites most suitable for species 
with known requirements. In the studies which were accordingly undertaken in 
the Intermountain Region, which is situated mainly in the Great Basin and the 
Snake River drainage, it was early found that the water relations of the plant and 
its environment are of paramount importance, especially where semiarid condi¬ 
tions prevail 5 {76, 77), It is very evident that no environmental factors are more 
significant than the forces with which the soil withholds and the air withdraws 
water from the plant. 
The results of a study of one phase of the water-relations problem are reported 
in this paper, namely, the relation which the density of the cell sap bears to 
environmental conditions in the Wasatch Mountains of Utah. 6 The importance 
of this relationship will be brought out more fully further on. Before taking up 
the details of the present investigation, however, the mechanism of absorption, 
diffusion, and osmotic pressure in plants will be considered briefly as the basis for 
a proper understanding and interpretation of what follows. 
RELATION BETWEEN OSMOTIC PHENOMENA AND CELL SAP 
CONCENTRATION 
Because of its restricting cell walls a plant is compelled to depend entirely upon 
fluid nutriment. The absorption of water and of dissolved substances by the 
root hairs and young rootlets is practically the only natural method by which the 
plant can obtain its supply of nutrients from the soil. The passage of water 
through the organism, however, is not a simple process. In the first place, not 
all water-soluble substances, as shown by Jost {69, p. 11-23), are able to enter the 
cell; secondly, to reach the interior of the cell, the fluid, with its solutes, must 
penetrate both the cell-wall and the protoplast—the layer of living protoplasm 
within the cell. The passage of water through the organism is therefore compli¬ 
cated by the presence of substances in solution, and this involves osmosis, or the 
8 “Site” is regarded by foresters as the summation or combined effect of the climatic and edaphic condi¬ 
tions of an area or environment when considered with reference to its forest-producing power; and is prac¬ 
tically synonymous with “habitat” as defined by the ecologist. The relation of the living tree to its inert 
environment embraces the most fundamental of silvical problems. In fact, a great many of the physiolog¬ 
ical problems of plant life involve to a greater or less extent the relation of the organism to its environment. 
The plant in relation to its environment must be regarded, therefore, as a physiologically active organism. 
The existence, limits, and movements of plant communities are controlled by the physical conditions of 
the environment. 
* Reference is made by number (italic) to “Literature cited,” pp. 900-907. 
8 This is further elaborated in a report by F. S. Baker and C. F. Korstian now in process of prepara¬ 
tion, covering climate, soil, and native vegetation of brushlands as indicators of planting sites for west¬ 
ern yellow pine in the intermountain region. 
6 Since the sap upon which the determinations were based was extracted by pressure from the tissues a 
part is doubtless intercellular rather than intracellular in origin. This is probably but a very small 
raction of the whole amount, and can introduce no appreciable error in the results. 
