890 
Journal of Agricultural Research 
Vol. XXVIII, No. 9 
of the preceding July and September. Inasmuch as these results were so 
completely at variance with the other two series a special study was made to 
determine the underlying causes. The results of this investigation are incor¬ 
porated in the subsequent discussion of “ Seasonal Changes in Food Reserves.” 21 
It has been shown that young, newly developed leaves have uniformly lower 
sap densities in July than one-year-old leaves. In all cases the first and second 
year leaves were taken from the same twigs. In September the relative densities 
w'ere found to be variable; about one-half of the species showed higher densities 
in the leaves of the current year and half in those of the second year. By Decem¬ 
ber and January, however, all of the species tested showed uniformly higher den¬ 
sities in their second-year leaves, as shown in Table XV. 
These data are in general agreement with the preceding discussion and also 
with that which is presented in the next section. This difference in density of the 
sap of the first and second year leaves is possibly due to the greater amount of 
dissolved carbohydrates in the second-year leaves with resultant higher sap 
densities. 
Another interesting relation was noted in September between yellow and red 
leaves and normal green ones on the same branches of aspen, large-tooth maple, 
and chokecherrv. The results obtained in Big Cottonwood Canyon are given in 
Table XVI. 
The leaves on these deciduous broad-leaved species were just beginning to take 
on their gorgeous autumnal colorations. It is probable that in the case of the 
colored leaves the abscission layer was forming at the base of the petiole and 
that the greater part of the carbohydrates had already passed from the leaves 
to the storage organs. This offers a possible explanation for the lower densities 
in the sap of the colored leaves taken from the same branches as the green leaves. 
Table XVI .—Sap density as influenced by autumnal color change 
Species 
Depression 
of freezing 
point 
Osmotic 
pressure 
Pop-ulus tremuloides: 
Yellow leaves... 
Degrees C. 
1.39 
Atmos¬ 
pheres 
16.7 
Green leaves...... 
1.83 
22.0 
Acer grandidentatum: 
Red leaves.......... 
1.44 
17.3 
Green leaves.......... 
1.52 
18.3 
Prunus melanocarpa: 
Red and yellow leaves________ 
1.28 
15.4 
Green leaves._.-..._... 
1.75 
21.0 
It is obvious that the concentration of the sap of any species is subject to 
wide variations. This is, nevertheless, of great significance in showing that the 
striking variations in the cell sap concentration of a given species encountered 
in different seasons and on different sites serve to emphasize the necessity of 
having a large number of determinations available for the same species on different 
sites and at different seasons of the year before attempting a correlation of the 
osmotic concentration of the cell sap with plant activities, dynamically exemplified 
in growth, distribution, and succession. 
It is of interest to note in passing that Lewis and Tattle (SO) found that the maximum sap concentra¬ 
tions are reached in the leaves of Picea canadensis, Pyrola rotundifolia , and Linnaea borealis in the winter, 
and that the variations are due chiefly to the noneleetrolytes of the sap, the sugar content variation of 
which followed closely the fluctuations in the sap concentration. 
