May 31, 1924 
Cell Sap Density and Environmental Conditions 
887 
tration than those of herbaceous species. 18 Variations in growth form are in 
turn dependent upon environmental conditions. Some species which are ordi¬ 
narily trees may develop as shrubs in certain environments as, for example, the 
dwarfing of tree growth at timber-line in parts of the Rocky Mountains. 
The existence of an osmotic gradient within the plant, which has been referred 
to above, is now readily demonstrable in the leaves of tall trees. 19 Determinations 
made in the present study showed higher concentrations in the top of the tree. 
The osmotic concentration of the cell sap of leaves increases from lower to higher 
levels in the same plant. A further study of this phase was made on the Wasatch 
National Forest, where the two-year-old leaves of 10 trees of western yellow pine 
growing in Whipple Gulch yielded the following data: 
Height of leaves selected 
Depression 
of freezing 
point in 
degrees C. 
Osmotic 
pressure in 
atmos¬ 
pheres. 
Base of trees. ........... 
1.44 
17.3 
Top of trees (average height above ground, 40 feet)..1.. 
1.54 
18.5 
These data afford additional proof of the soundness of the above-stated proposi¬ 
tion. It is a general principle that conditions are more favorable to the formation 
of soluble carbohydrates in the upper parts of the crowns of trees. It is therefore 
reasonable to assume that the greater osmotic pressures are regularly developed 
in the tops and that smaller pressures exist at the bases of the crowns, where 
these substances are not commonly stored. Variations in the sugar content of 
the leaves due to differences in illumination would also be expected to cause 
material fluctuations in the osmotic concentration of the sap. These sap-density 
figures consistently confirm these postulates. The fact that the higher osmotic 
pressures occur in the tree tops is in accord with Dixon’s (23-24, 32-33) theory 
that the transpiration stream is set in motion through the plant by secretory ac¬ 
tion occurring in the leaf cells or by evaporation and capillarity at their surfaces 
drawing water from the tracheae. The condition of saturation surrounding 
these cells determines which agency shall be effective. With normal transpira¬ 
tion unaffected by root pressure, the cohesion of the sap is sufficient to explain the 
transmission within tracheae of the tension downward, and the resultant ascent 
of the sap. With the evaporation of water through the stomata a somewhat 
higher concentration of the sap is produced in the cells immediately adjacent to 
the stomata. This upsets the osmotic equilibrium and a current of sap is set*in 
motion, extending all the wav from the soil to the -stomata! cells of the topmost 
leaf of the tree. Dixon ( 32 ) has shown that the tensile strength of the sap is 
much in excess of the tension necessary to raise the sap to the top of the tallest 
trees. Further, the osmotic pressures which he has measured have always been 
found adequate to resist the tension caused by the transpiration stream. In 
many cases, however, other factors enter, such as the presence of an unusually 
large amount of stored soluble carbohydrates or a high concentration of the soil 
solution, which develop pressures greatW in excess of those required by the trans¬ 
piration stream alone. 
18 The same thing has been observed by several other investigators (SO, 40, 50, 54, 56, 59, 60.) 
On the basis of determinations of the osmotic concentration of the leaf sap at different levels above the 
ground, made by the plasmolytic method, Ewart (SS) advanced the theory that the density of the leaf sap 
increases from lower to higher levels in the same plant. Further observations have been made by Dixon 
and Atkins (25,36), and Harris, Gortner and Lawrence (63), who used the refined freezing-point depression 
method. 
