THE FREE WATER OF THE SOIL. 213 
teo little, in fact, to be estimated by the methods hitherto 
employed. Sachs explains this as follows: Assuming that 
the roots have at a given temperature as strong an attrac- 
tion for water in the state of vapor as for liquid water, the 
amount of each taken up in a given time under the same 
circumstances would be in proportion to the weight of 
each contained in a given space. <A cubic inch of water 
yields at 212° nearly a cubic foot (accurately, 1,696 times 
its volume, the barometer standing at 29.92 inches) of 
vapor. We may then assume that the absorption of liq- 
uid or hygroscopic water proceeds at least one thousand 
times more rapidly than that of vapor, a difference in 
rate that enables us to comprehend why a plant may gain 
water by its roots from the soil, when it would lose water 
by its roots were they simply stationed in air saturated 
with vapor. 
Again, the soil need not be more hygroscopic than roots, 
to supply the latter with water. It is important only that 
it present a sufficient surface. As is well known, a plant 
requires a great volume of earth to nourish it properly, 
and the root-surface is trifling, compared to the surface 
of the particles which compose the soil. 
Boussingault found by actual measurement that, accord- 
ing to the rules of garden culture as practiced near Stras- 
burg, a dwarf bean had at its disposition 57 pounds of 
soil; a potato plant, 190 pounds; a tobacco plant, 470 
pounds; and a hop plant, 2,900 pounds. These weights 
correspond to about 1, 3,'7, and 50 cubic feet respectively. 
The Quantity of Water in Vegetation is influenced by 
that of the Soil_—De Saussure observed that plants grow- 
ing ina dry lime soil contained less water than those from 
aloam. It is well known that the grass of a wet summer 
is taller and more succulent, and the green crop is heavier 
than that from the same field ina dry summer. It does 
not, however, make much more hay, its greater weight 
consisting to a large degree of water, which is lost in dry- 
