202 ; HOW CROPS FEED. 
small, and its adhesion to the particles of soil more firm 
for that reason. Again, no precise boundary can always 
be drawn between capillary and hydrostatic water, espe- 
cially in soil having fine pores. The terms are neverthe- 
less useful in conveying an idea of the degrees of wet- 
ness or moisture in the soil. 
Roots Absorb Capillary or Hygroscopic Water.—It is 
from capillary or hygroscopic water that the roots of most 
agricultural plants chiefly draw a supply of this liquid, 
though not infrequently they send roots into wells and 
drains. The physical characters of soils that have been 
already considered suffice to explain how the earth acquires 
this water; it here remains to notice how the plant is re- 
Jated to it. 
As we have seen (pp. 35-38), the aerial organs appear 
incapable of taking up either vapor-or liquid water from 
the air to much extent, and even roots continually exhale 
vapor without absorbing any, or at least without being able 
to make up the loss which they continually suffer. 
Transpiration of Water through Plants,—It is a most 
familiar fact that water constantly exhales from the surface 
of the plant. The amount of this exhalation is often very 
great. Hales, the earliest observer of this phenomenon, 
found that a sunflower whose foliage had 39 square feet 
of surface, gave off in 24 hours 8 lbs. of water. A cab- 
bage, whose surface of leaves equaled 19 square feet, ex- 
haled in the same time very nearly as much, Schleiden 
found the loss of water from a square foot of grass-sod to 
be more than 14 Ibs. in 24 hours. Schiibler states that in 
the same time 1 square foot of pasture-grass exhaled 
nearly 5} lbs. of water. In one of Knop’s more recent 
experiments, (Vs. St, VI, 239), a dwarf bean exhaled 
during 23 days, in September and October, 13 times its 
weight of water. In another trial a maize-plant transpir- 
ed 36 times its weight of water, from May 22d to Sept. 
4th. According to Knop, a grass-plant will exhale its own 
