210 HOW CROPS FEED. ~- 
saturation, furnish the tobacco plant with 44.1°|, of its 
weight of water. 
A coarse sand that could hold 20.8°|, of water was 
found to yield all but 1.5°|, to a tobacco plant. 
From these trials we gather with at least approximate 
accuracy the power of the plant to extract water from 
these several soils, and by difference, the quantity of wa- 
ter in them that was unavailable to the tobacco plant. 
How do the Roots take Hygroscopic Water from the 
Soil ?—The entire plant, when living, is itself extremely 
hygroscopic. Even the dead plant retains a certain pro- 
portion of water with great obstinacy. Thus wheat, 
maize, starch, straw, and most air-dry vegetable substances, 
contain 12 to 15°|, of water; and when these matters are 
exposed to damp air, they can take up much more, Ac- 
cording to Trommer (Bodenkunde, p. 270), 100 parts of 
the following matters, when dry, absorb from moist air in 
12 24 48 72 
a a 
hours. 
Fine cut barley straw, 15 24 o4 45 parts of water. 
ac “cc rye ce 12 20 97 99 “ ce “ce 
‘¢  ~white unsized paper, 8 12 17 1p Se ae 
As already explained, a body is hygroscopic because 
there is attraction between its particles and the particles 
of water. The form of attraction exerted thus among 
different kinds of matter is termed adhesive attraction, or 
simply adhesion. 
Adhesion acts only through a small distance, but its in- 
tensity varies greatly within this distance. If we attempt 
to remove hygroscopic water from starch or any similar 
body by drying at 212°, we shall find that the greater 
part of the moisture is easily expelled in a short time, 
but we shall also notice that it requires a relatively much 
longer time to expel the last portions. A general law of 
attraction is that its force diminishes as the distance be- 
tween the attracting bodies increases, This has been ex- 
