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.]

The leaves are the stomach and lungs of the tree.  Their broad blades are a device to catch the sunlight which is needed in the process of digesting the food of the tree.  The leaves are arranged on the twigs in such a way as to catch the most sunlight.  The leaves take up the carbonic acid gas from the air, decompose it under the influence of light and combine it with the minerals and water brought up by the roots from the soil.  The resulting chemical combinations are the sugars and starches used by the cambium layer in building up the body of the tree.  A green pigment, chlorophyll, in the leaf is the medium by which, with the aid of sunlight, the sugars are manufactured.

[Illustration:  FIG. 88.—­Roots of a Hemlock Tree in their Search for Water.]

The chlorophyll gives the leaf its green color, and this explains why a tree pales when it is in a dying condition or when its life processes are interfered with.  The other colors of the leaf—­the reds, browns and yellows of the fall or spring—­are due to other pigments.  These are angular crystals of different hues, which at certain times of the year become more conspicuous than at others, a phenomenon which explains the variation in the colors of the leaves during the different seasons.

It is evident that a tree is greatly dependent upon its leaves for the manufacture of food and one can, therefore, readily see why it is important to prevent destruction of the leaves by insects or through over-trimming.