rked with dots (Birch), some roughened with corky
ridges (Sweet Gum), etc.

The taste and odor of the bark are characteristics worthy of notice: the
strong, fragrant odor of the Spice-bush; the fetid odor of the Papaw;
the aromatic taste of the Sweet Birch; the bitter taste of the Peach;
the mucilaginous Slippery Elm; the strong-scented, resinous, aromatic
Walnut, etc.

The branches of trees vary greatly in the thickness of their tips and in
their tendency to grow erect, horizontal, or drooping. Thus the delicate
spray of the Birches contrasted with the stout twigs of the Ailanthus,
or the drooping twigs of the Weeping Willow with the erect growth of the
Lombardy Poplar, give contrasts of the strongest character. In the same
way, the directions the main branches take in their growth from the
trunk form another distinctive feature. Thus the upward sloping branches
of the Elm form a striking contrast to the horizontal or downward
sloping branches of the Sour Gum, or, better still, to certain varieties
of Oaks.

When the main trunk of a tree extends upward through the head to the
tip, as in Fig. 3, it is said to be _excurrent_. When it is soon lost in
the division, as in Fig. 4, it is said to be _deliquescent_.

CHAPTER III.

_Leaves._

Leaves are the lungs of plants. The food taken in by the roots has to
pass through the stem to the leaves to be acted upon by the air, before
it becomes sap and is fit to be used for the growth of the plant. No
portion of a plant is more varied in parts, forms, surface, and duration
than the leaf.

No one can become familiar with leaves, and appreciate their beauty and
variety, who does not study them upon the plants themselves. This
chapter therefore will be devoted mainly to the words needed for leaf
description, together with their application.

THE LEAF.--In the axil of the whole leaf the bud forms for the growth of
a new branch. So by noting the position of the buds, all the parts
included in a single leaf can be determined. As a general thing the leaf
has but one blade, as in the Chestnut, Apple, Elm, etc.; yet the
Horse-chestnut has 7 blades, the Common Locust often has 21, and a
single leaf of the Honey-locust occasionally has as many as 300. Figs.
17-58 (Chapter VII.) are all illustrations of single leaves, except Fig.
43, where there are two leaves on a twig. A number of them show the bud
by which the fact is determined (Figs. 25, 26, 31, 33, 34, 36, 40,
etc.)