1916] SHULL—SOILS 25 
to particles become very great. This force would reach its maxi- 
mum presumably when the particles were surrounded by a film of 
water just one molecule thick. The curve for the sand is very simi- 
lar, except that the period of slow increase of the soil forces is very 
short, and that the whole curve lies much nearer 
to the absolutely dry condition. These rela- ns 
tions, of course, are conditioned mainly by the 
size of the particles. | 900 
Let us see now how the soil forces at the 
wilting coefficient compare in value with the L 00 
average osmotic pressure of the root hairs. A 
few years ago Hitt (18) showed that the root 
hairs of Salicornia can in a measure accommo- | 700 
date themselves to changes in the osmotic con- 
centration of the surroundings, through increase F600 
or decrease of the cell sap concentration parallel 
to. the changes in the environment. That the + 500 
cell sap of leaves and other exposed parts in- 
creases in concentration with xerophytic habitat r 400 
has been shown by DraBBLE and LAKE (13), 
FittTiInG (14), and others. The general conclu- - 300 
sion reached by DRABBLE and DRABBLE (12), 
that the osmotic strength of cell sap varies - 2.00 
with the physiological scarcity of water in any 
area, seems most reasonable, and it doubtless - 100 
. —<——- ~ = 0 
807% 60 40 20 ) 
Fic. 5.—Curves showing increase in the surface forces of ‘ike as drying proceeds; 
to the left, ie subsoil of the Oswego silt loam; to the right, for no. 2/o sand. 
holds true of root cells under xerophytic soil conditions. As 
the soil becomes deficient in water supply, and the surface forces 
are rapidly increasing in magnitude, we might then expect this 
increased force of the soil to be paralleled by increase in the osmotic 
forces of the root hairs, owing to progressive concentration of the 
cell sap. Such increase might amount to many atmospheres. 
