complex until we reach the tree, which is the largest and highest
form of all plants. The tree is a living organism composed of cells
like any other living organism. It has many parts, every one of
which has a definite purpose. The three principal parts are: the
stem, the crown, and the root.

The stem: If we examine the cross-section of a tree, Fig. 86, we will
notice that it is made up of numerous rings arranged in sections of
different color and structure. The central part is known as the
_pith_. Around the pith comes a dark, close-grained series of rings
known as the _heartwood_, and outside the heartwood comes a lighter
layer, the _sapwood_. The _cambium layer_ surrounds the sapwood and
the _bark_ covers all. The cambium layer is the most important
tissue of the tree and, together with part of the sapwood,
transports the water and food of the tree. It is for this reason
that a tree may be hollow, without heart and sapwood, and still
produce foliage and fruit.

[Illustration: FIG. 86.--The Cross-Section of a Tree.]

The crown: The crown varies in form in different species and is
developed by the growth of new shoots from buds. The bud grows out
to a certain length and forms the branch. Afterwards it thickens
only and does not increase in length. New branches will then form
from other buds on the same branch. This explains in part the
characteristic branching of trees, Fig. 87.

[Illustration: FIG. 87.--Characteristic Form and Branching of Trees.
The trees in the photograph are pin oaks.]