888 
Journal of Agricultural Research 
Vol. XXVIII, Xo. 9 
One of the strongest arguments in support of the theory that water intake by 
the plant depends on the osmotic concentration of its tissue fluids is to be found 
in a comparison of the saps of parasite and host. The osmotic concentration 
determinations on parasitic Loranthaceae in relation to those of their hosts, dis¬ 
cussed on pages 897-898, show that osmotic pressure is one of the most impor¬ 
tant forces by which the solutes obtained by the parasite are withdrawn from 
the tissues of the host. 
The facts just presented are regarded as fully justifying the assertion made 
on an earlier page that an intimate relation exists between the concentration of 
the cell sap and the water relations of the plant. The correlation of data on 
water relations is facilitated by comparative studies of sap density. Sap density 
may be used as a convenient measure for expressing with an accuracy sufficient 
for practical purposes the moisture conditions of different sites and different 
forest types. Such an index must not be confused with the underlying causal 
factors which produce it. The osmotic concentrations characteristic of species 
adapted to different sites serve as criteria of these sites, and, in view of the fact 
that all of the activities of the protoplasm must take place in the environment 
of the cell sap, the relative densities of the sap of a species growing under diverse 
conditions evidently indicate the manner in which those conditions affect the 
species. In short, the concentration of the cell sap is an expression of the bio¬ 
logical requirements of a species since it indicates the ability of a plant to with¬ 
draw water from the soil. 
SEASONAL VARIATION IN SAP CONCENTRATION 
Table VII shows that the sap concentration for a given species varies not 
only from site to site but on the same site from season to season. The Septem¬ 
ber concentrations were in general considerably lower than those found in July, 
a fact which is directly related to the influence of the environment upon the 
plant. Dixon and Atkins ( 35) have pointed out that since the major part of 
the osmotic pressure in the cell sap of plants is due to dissolved carbohydrates, 
it is evident that the variations in the sap density are largely due to fluctuations 
in the synthesized solute content of the cells. These differences, therefore, are 
dependent not only upon the moisture content of the soil but also upon atmos¬ 
pheric conditions which may materially influence current sap concentrations 
through their effects upon the photosynthetic activities of the plant and the 
production of carbohydrates. In the course of a year the plant passes through 
a definite cycle of changes directly dependent upon these varying conditions. 
A number of factors combine to start growth in the early spring. The soil is 
abundantly supplied with water; the snow is melting under the influence of rising 
temperatures and spring rains; the intensity and duration of the sunshine are 
increasing; and yet transpiration is still moderate. All of these conditions favor 
the maintenance of a low sap density. With the advance of the season and the 
advent of midsummer the supply of available soil moisture is usually much 
depleted, as has been shown, and the rate of water loss from the plant through 
transpiration has been materially increased by the occurrence of the highest 
temperatures of the season and the greatest intensities of sunlight. Summer is, 
furthermore, the season of minimum rainfall. All these factors unite to produce 
the maximum sap densities of the growing season. It is during this period that 
death from drought most frequently occurs. To survive, a species must be able 
to adapt itself to the changing conditions and to resist the adverse stresses. It 
can do this by a reduction of transpiration or by such an increase of sap density 
that absorption continues even in spite of an increasingly concentrated soil 
