May 3i, 1924 Cell Sap Density and Environmental Conditions 
885 
CORRELATION AND DISCUSSION OF RESULTS 
INTERDEPENDENCE OF SAP DENSITY AND WATER RELATIONS 
Of the fundamental relationships indicated by the sap concentrations of the 
individual species in Tables VII to IX and the summaries in Table X the most 
significant are connected with the water relations within the plant and between 
the plant and its environment. Some are capable of direct measurement and 
may be readily correlated, while others can not as yet be measured with precision 
and are accordingly difficult to interpret. 
A study of the easily measured environmental factors—especially precipita¬ 
tion, the changes in the available moisture and the evaporation on those sites 
which were under investigation in the aspen-fir type in Big Cottonwood Canyon— 
indicates that the ability of the plant to maintain a proper balance between 
absorption and transpiration, even at the intermediate elevations of the aspen-fir 
type, is of vital importance; increasingly so with lower elevations. The Ephraim 
Canyon series of tests, the results of which are shown in Tables XI, XII, and 
XIII confirm these relations. 
The Wasatch Mountain series of determinations are segregated by growth 
forms and species in Tables VII, VIII, and IX, while those for the Ephraim 
Canyon series are given in Tables XI, XII, and XIII. 14 In the case of duplicate 
tests made at the same time merely the averages of the determinations are given. 
The type, situation, altitude, date of collection, depression of the freezing point 
in degrees C. (A), and the osmotic pressure in atmospheres (P), are given for 
each determination. Inasmuch as the thermometric readings were taken to 
thousandths of a degree the depressions were also computed with the same 
accuracy, but the osmotic pressures were interpolated from the Harris and 
Gortner tables (46, 4?)] the freezing point depressions were rounded off to the 
nearest hundredth and the osmotic pressures to the nearest tenth, greater pre¬ 
cision being inconsistent with the experimental error, incidental to the collection 
of the samples. The osmotic concentrations for all Wasatch Mountain species 
upon which determinations were made are summarized in Table X by type, 
growth form and season of the year. 
From the investigations of Livingston and his associates (84, 85, 88, 89) it is 
evident that when the soil is unable to supply moisture as rapidly as the root 
system of the plant can absorb it, soil moisture then becomes a limiting factor 
in plant growth. From the standpoint of moisture supply, a soil is satisfactory 
for the growth of a plant when it can supply water to the absorbing surfaces of 
the root system fast enough to meet the vital activities of the plant. Osmosis, 
which plays such an important role in the ascent of sap, plays an equally im¬ 
portant part in the absorption of soil moisture and solutes. A very important 
relation is therefore set up between the density of the soil solution and the den¬ 
sity of the sap in the roots. The osmotic concentration of the cell sap of the 
roots is closely correlated with the concentration of the soil solution which in 
turn is dependent upon the amount of moisture in the soil. Shull (116) has, in 
fact, found that roots have an osmotic concentration somewhat above that of 
the soil solution of a soil in which the water has been reduced to the wilting 
coefficient. 
This suggested that for the present investigation a comparative determina¬ 
tion of the cell sap density of leaves and roots would afford a valuable index of 
soil moisture content, since the frequently important variables of soil fertility 
u The species about which there was some doubt as to their specific identity were referred to the Na¬ 
tional Herbarium for confirmation by specialists. 
96039—24t-4 
