years, has developed a wide-spreading system of trunk and branches,
bearing on the ultimate twigs or branchlets innumerable leaves, while
beneath the soil a widely-branching root-system covers an area of
corresponding extent. Between these two extremes is every conceivable
gradation, embracing aquatic and terrestrial herbs, creeping, erect or
climbing in habit, shrubs and trees, and representing a much greater
variety than is to be found in the other subdivision of seed-plants,
the Gymnosperms.

_Internal structure._

In internal structure also the variety of tissue-formation far exceeds
that found in Gymnosperms (see PLANTS: _Anatomy_). The vascular
bundles of the stem belong to the collateral type, that is to say,
the elements of the wood or xylem and the bast or phloem stand side
by side on the same radius. In the larger of the two great groups into
which the Angiosperms are divided, the Dicotyledons, the bundles in
the very young stem are arranged in an open ring, separating a central
pith from an outer cortex. In each bundle, separating the xylem and
phloem, is a layer of meristem or active formative tissue, known as
cambium; by the formation of a layer of cambium between the bundles
(interfascicular cambium) a complete ring is formed, and a regular
periodical increase in thickness results from it by the development
of xylem on the inside and phloem on the outside. The soft phloem soon
becomes crushed, but the hard wood persists, and forms the great bulk
of the stem and branches of the woody perennial. Owing to differences
in the character of the elements produced at the beginning and end
of the season, the wood is marked out in transverse section into
concentric rings, one for each season of growth--the so-called annual
rings. In the smaller group, the Monocotyledons, the bundles are more
numerous in the young stem and scattered through the ground tissue.
Moreover they contain no cambium and the stem once formed increases in
diameter only in exceptional cases.

_Vegetative organs._

As in Gymnosperms, branching is monopodial; dichotomy or the forking
of the growing point into two equivalent branches which replace the
main stem, is absent both in the case of the stem and the root. The
leaves show a remarkable variety in form (see LEAF), but are generally
small in comparison with the size of the plant; exceptions occur in
some Monocotyledons, _e.g._ in the Aroid family, where in some genera
the plant produ