. Although in the region covered by this
book the trees include all the very large plants, size alone does not
make a tree.

A plant with a single trunk of woody structure that does not branch for
some distance above the ground, is called a _tree_. Woody plants that
branch directly above the soil, even though they grow to the height of
twenty feet or more, are called _shrubs_, or, in popular language,
_bushes_. Many plants which have a tendency to grow into the form of
shrubs may, by pruning, be forced to grow tree-like; some that are
shrubs in the northern States are trees further south.

All the trees that grow wild, or can be cultivated out of doors, in the
northern States belong to one class, the stems having a separable bark
on the outside, a minute stem of pith in the center, and, between these,
wood in annual layers. Such a stem is called _exogenous_
(outside-growing), because a new layer forms on the outside of the wood
each year.

Another kind of tree-stem is found abundantly in the tropics; one, the
Palmetto, grows from South Carolina to Florida. While in our region
there are no trees of this character, there are plants having this kind
of stem, the best illustration being the corn-stalk. In this case there
is no separable bark, and the woody substance is in threads within the
pithy material. In the corn-stalk the woody threads are not very
numerous, and the pith is very abundant; in most of the tropical trees
belonging to this group the threads of wood are so numerous as to make
the material very durable and fit for furniture. A stem of this kind is
called _endogenous_ (inside-growing). Fig. 1 represents a longitudinal
and a cross section of an exogenous stem, and Fig. 2 of an endogenous
one.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

Since all the stems with which we have to deal are exogens, a particular
description of that class will here be given. Fig. 1 shows the
appearance of a section of an Ash stem six years old. The central
portion, which is about as thick as wrapping-twine, is the _pith_; from
this outward toward the bark can be seen the six annual layers of the
_wood_; and then comes the _bark_, consisting of two portions. First
there is an inside layer of greenish material, the fresh-growing
portion, and lastly the outer or dead matter. This outer portion must
crack open, peel off, or in some way give a chance for the constant
growth of the trunk. The different kinds of trees are r