y sight; but they
will well repay examination. For the vegetation that is carried on under
ground is hardly less varied or important than that above ground. All
their forms may be referred to four principal kinds: namely, the
_Rhizoma_ (_Rhizome_) or _Rootstock_, the _Tuber_, the _Corm_ or solid
bulb, and the true _Bulb_.

[Illustration: Fig. 97. Rootstocks, or creeping subterranean branches,
of the Peppermint.]

104. =The Rootstock, or Rhizoma=, in its simplest form, is merely a
creeping stem or branch growing beneath the surface of the soil, or
partly covered by it. Of this kind are the so-called _creeping_,
_running_, or _scaly roots_, such as those by which the Mint (Fig. 97),
the Couch-grass, or Quick-grass, and many other plants, spread so
rapidly and widely,--"by the root," as it is said. That these are really
_stems_, and not roots, is evident from the way in which they grow;
from their consisting of a succession of joints; and from the leaves
which they bear on each _node_, in the form of small scales, just like
the lowest ones on the upright stem next the ground. They also produce
buds in the axils of these scales, showing the scales to be leaves;
whereas real roots bear neither leaves nor axillary buds. Placed as they
are in the damp and dark soil, such stems naturally produce roots, just
as the creeping stem does where it lies on the surface of the ground.

105. It is easy to see why plants with these running rootstocks take
such rapid and wide possession of the soil, and why they are so hard to
get rid of. They are always perennials; the subterranean shoots live
over the first winter, if not longer, and are provided with vigorous
buds at every joint. Some of these buds grow in spring into upright
stems, bearing foliage, to elaborate nourishment, and at length produce
blossoms for reproduction by seed; while many others, fed by nourishment
supplied from above, form a new generation of subterranean shoots; and
this is repeated over and over in the course of the season or in
succeeding years. Meanwhile, as the subterranean shoots increase in
number, the older ones, connecting the successive growths, die off year
by year, liberating the already rooted side-branches as so many separate
plants; and so on indefinitely. Cutting these running rootstocks into
pieces, therefore, by the hoe or the plough, far from destroying the
plant, only accelerates the propagation; it converts one many-branched
plant into a grea