THE MATERIAL INCOME OF PLANTS 313 
mixed, especially in the upper part, with more or less organic matter, 
the offal of antecedent animal and plant life. The soil particles 
are of various sizes and kinds, and the soil is often named accordingly. 
Thus there are gravelly, sandy, clayey, and humus soils according to 
the amount of gravel, sand, clay, or humus present. An indefinite 
variety of mixtures also occurs, as in loam, with appropriate descriptive 
names. The texture of the soil depends chiefly upon the size of the 
individual particles; but when very fine, and especially when repeatedly 
wetted and dried, these often become aggregated into compound grains, 
as is obvious in clay. The sort of rock from which the soil was made, 
the size of the particles, their state of aggregation, and the proportion 
and character of organic matter, determine the relation of water to the 
soil, and so the freedom and extent of its movement. 
Soil water. Of the water which falls upon the surface as rain all may 
percolate into the soil, or part may run off. The character of the soil 
and of the vegetation on the surface, the slope, the rate of precipitation, 
and the existent water content, determine the fate of the falling water. 
A loose dry soil of level surface, a soil cover of leaves or grass, and a 
gentle rainfall, tend to reduce the run-off to a minimum. The water 
which percolates into the soil enters the spaces between the soil particles, 
which it fills more or less, driving out the air and adhering in the form 
of films to the component particles, when it does not fill the spaces com- 
pletely. The thicker the films, the less firmly the molecules more distant 
from the surface of the soil particles are held; so that gravity suffices 
to carry down to lower and lower levels a certain amount of the perco- 
lating water. This may drain away as subterranean streams or may 
remain, saturating the soil at a certain level and forming thus the " water 
table," approximately parallel to the surface and at a variable distance 
from it. 
Capacity of soils for water. When all the water that will sink to 
the water table in a well-drained soil has drained out of the upper regions, 
an amount varying according to the physical characters of the soil re- 
mains, adhering to the grains. The smaller spaces are still filled; the 
larger contain bubbles of air which have come in from above as the water 
sank. If the soil particles be very small and close together, a greater 
quantity of water will be held than in a loose, coarser soil. 
This seems anomalous, but as the amount of water adhering to the surfaces 
will be almost proportional to the surfaces themselves, it may easily be comprehended 
by calculating the area of 1000 spheres each i mm. in diameter, which could 
