a more even distribution of nutritious matters among
those parts of soil traversed by roots_, because it increases the ease
with which water travels around, descending by its own weight, moving
sideways by a desire to find its level, or carried upward by attraction
to supply the evaporation at the surface. By this continued motion of
the water, soluble matter of one part of the soil may be carried to some
other part; and another constituent from this latter position may be
carried back to the former. Thus the food of vegetables is continually
circulating around among their roots, ready for absorption at any point
where it is needed, while the more open character of the soil enables
roots to occupy larger portions, making a more even drain on the whole,
and preventing the undue impoverishment of any part.

7. Under-drains _improve the mechanical texture of the soil_; because,
by the decomposition of its parts, as previously described (4 and 5), it
is rendered of a character to be more easily worked; while smooth round
particles, which have a tendency to pack, are roughened by the oxidation
of their surfaces, and move less easily among each other.

8. Drains _cause the excrementitious matter of plants to be carried out
of the reach of their roots_. Nearly all plants return to the soil those
parts of their food, which are not adapted to their necessities, and
usually in a form that is poisonous to plants of the same kind. In an
open soil, this matter may be carried by rains to a point where roots
cannot reach it, and where it may undergo such changes as will fit it to
be again taken up.

[Why do they prevent grasses from running out?]

9. By under-draining, _grasses are prevented from running out_, partly
by preventing the accumulation of the poisonous excrementitious matter,
and partly because these grasses usually consist of _tillering_ plants.

These plants continually reproduce themselves in sprouts from the upper
parts of their roots. These sprouts become independent plants, and
continue to tiller (thus keeping the land supplied with a full growth),
until the roots of the _stools_ (or clumps of tillers), come in contact
with an uncongenial part of the soil, when the tillering ceases; the
stools become extinct on the death of their plants, and the grasses run
out.

The open and healthy condition of soil produced by draining prevents the
tillering from being stopped, and thus keeps up a full growth of grass
until