vegetation of the earth's surface at the present epoch. There is no
land-area from the poles to the equator, where plant-life is possible,
upon which Angiosperms are not found. They occur also abundantly in
the shallows of rivers and fresh-water lakes, and in less number in
salt lakes and in the sea; such aquatic Angiosperms are not, however,
primitive forms, but are derived from immediate land-ancestors.
Associated with this diversity of habitat is great variety in general
form and manner of growth. The familiar duckweed which covers the
surface of a pond consists of a tiny green "thalloid" shoot, one, that
is, which shows no distinction of parts--stem and leaf, and a
simple root growing vertically downwards into the water. The great
forest-tree has a shoot, which in the course perhaps of hundreds of
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