May 3i, 1924 Cell Sap Density and Environmental Conditions 
893 
time maintain an economically efficient transpiration rate under temperature 
conditions favorable to photosynthesis. This dual relation between absorption 
and transpiration, then, is more often the determining factor in the composition 
of the stand than absorption alone. It is to this that the several forest types 
are due. 
Evidently, therefore, (a) the water relations of the plant have a vital bearing 
on the succession of plant associations and ( b) the causes of these phenomena 
are associated with the osmotic properties of the plant juices and the soil solu¬ 
tion. The densities of the sap of all species tested in the main Wasatch Moun¬ 
tains, which are averaged by types, growth forms and seasons in Table X, illus¬ 
trate this point. The average densities of the sap of all species tested in each 
type in the Ephraim Canyon series in July, 1921, are given in Table XVII. 
Table XVII .—Average sap densities by types for the Ephraim Canyon series 
1 
Type 
Depression 
of freezing 
point 
Osmotic 
pressure 
Degrees C. 
Atmospheres 
Greasewood-shadscale......... 
4.29 
51.4 
Sagebrush......... 
2.35 
28.2 
Pinon-juniper......... 
1.78 
21.4 
Oak brush: 
South aspect.............. 
1.66 
20.0 
North aspect........ 
1.36 
16.4 
Average.......... 
1.47 
17.7 
Aspen-fir....... 
1.25 
16.0 
Spruce-fir: 
South aspect.......... 
1.25 
15.0 
North aspect..........I 
| 1.05 
12.6 
Average______ 
1.16 
14.0 
These averages, together with the principal climatic factors, are plotted in 
figure 5, which further emphasizes the correlation between site and sap density. 
The sap of sagebrush, the extreme range of which covers several types, shows 
the same characteristic decrease in density with an increase in altitude. (Fig. 5.) 
A comparison of the opposite north and south aspects at the same elevations 
in the oak brush and spruce-fir types also shows a decrease in sap density cor¬ 
responding with an improvement in site conditions. The plants have the 
lower densities on the northern aspects, which in these two cases are the more 
favorable for plant growth. Similar comparisons can also be drawn from the 
majority of the species listed. The common progressive succession is from the 
high osmotic pressures in the drought-resistant species of the pinon-juniper 
and chaparral zones of the lower mountains, through the western yellow pine, 
aspen-fir, and logdepole pine types composed of species having successively lower 
sap concentrations, to the climax spruce-fir type of uniformly low osmotic 
pressures. The establishment of a forest cover is conducive to greater stability 
and the development of progressively lower osmotic concentrations in the pe¬ 
rennial species. It is somewhat different in the case of annual herbs which are 
capable of completing their life cycle in shorter periods, often during a rainy 
season when conditions are optimum for vegetative growth. 
A correlation of plant succession with the underlying casual factors shows 
that the succession is from drought-resistant species toward species of less 
drought resistance as the atmospheric and soil moisture increases and produces 
greater stability and lower sap densities. It is evident that the existence of a 
high osmotic concentration of the soil solution tending toward physiological 
dryness of the soil would prove most detrimental to species of low sap con¬ 
centration by cutting off the moisture supply. Shallow-rooted species having 
