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USES OF FORESTS. 213
this acid helps to decompose the par ticles of stone.
In tills way the rootlets of plants serve in part to
make from the solid rocks the soil that gives th em
support.
3. Not only do trees help to make the soil upon
whi ch th ey live, but th ey also pr eserv e it fr om
destruction. In a time of heavy min, th e soil is
rapidly born e away to the riv ers, and th ence to the
sea, in the form of mud. In countries wh ere th e
soil has long been till ed, it constantly diminishes
in depth i and unless great care is taken, in a few
centuries it all passes away into th e strea ms, except
where the surface is very level. Thus in Italy, and
in many of th e countries th at have long been cultivated,
th e soil on th e steeper slopes, whi ch once were
fertile, has so far disappeared that many extensive
districts ar e now barren wastes.
4. Forests serve not only to prevent th e wasting
of the soil und er th e pelting influence of th e rain,
but they also gr eatly restrain th e action of even the
largest rivers. Willows, poplars, and oth er waterloving
plants thrive along th e banks of a stream, send
th eir roots downward beneath th e surface, and so
make a strong net-work whi ch resists th e cutting
action of the river, and keeps it within narrow bounds.
5. Forests also help to prevent floods. If the rain
falls on an unforested country, the wat er flows quickly
over the bare surface to th e brooks, and th ence to th e
larger ri vers, on its way to th e sea. When, however,
th e rain falls on forests, th e water enters a thick,
spcngy lay er, composed of partly-decay ed leaves, togeth
er with fallen trunks and branches of trees.
Through thi s sponge the water moves but slowly on