10

"From her unpolluted dust,
Shall violets spring,"

can readily be admitted, without poetical license.

The mode in which the various changes incident to vegetation
are effected deserves a passing notice. In the process of germi-
nation, the sprouting of the seed, oxygen is absorbed, heat is
developed, and probably acetic acid is formed, whose use is to
extract from the soil bases useful for the further progress of the
plants. The substance of the seed (starch and albumen) becomes
soluble, and undergoes certain changes by which the woody fibre
required for the stem and leaves is produced. The plant now
must absorb from the air and soil, matter necessary for its growth.

By means of its roots, the necessary mineral matter is taken
up, and also ammonia and some portions of its carbonic acid.
By far the greater portion of the latter is, however, absorbed
by numerous pores on the bottom of the leaf, and under the in-
fluence of light, carbon necessary for its woody fibre, gum, &c,
is separated and retained; whilst oxygen, fit for the respira-
tion of animals, is thrown out by pores on the top of the leaf.
During daylight, plants are continually absorbing carbonic acid
and giving out oxygen; in the night the contrary, to some
extent, prevails; they then throw out carbonic acid. This ex-
plains partially the fact of the greater and more rapid growth of
vegetable life during the summer season, when the days—the
period of light in which the plants assimilate carbon and build up
their structure—are much longer than in other seasons of the
year.

Oxygen.—Another of the elementary constituents of organic
matter is oxygen, and one of the most generally diffused in nature;
though, contrary to what some have said, it performs no more im-
portant part than any of the other elements.

All are necessary; each performs the special part assigned to it;,
they are all links in the great chain of matter. It forms a large part
of the atmosphere which surrounds the earth, about eight-ninths of
all the water in existence, besides forming a very large proportion of
rocks, minerals and soils. In water, it is in chemical union with
hydrogen, and in the air it is only mechanically mixed, not chemi-
cally combined with nitrogen, both elements of organic matter here-
after to be described; but in soils and rocks it is combined with
various bodies, affecting their properties in a very great degree. In
its pure state it is invisible, colorless, tasteless, and without smell.
It has a very strong affinity for almost all bodies. The act of com-
bining with oxygen is called oxydation; and the compounds thus
formed are called either Acids or Oxyds. The former of these
possesses the general properties of acids; the latter of bases,