e dividing line between
trees and shrubs. In accordance with the usual definition, a tree must
have a single trunk, unbranched at or near the base, and must be at
least fifteen feet in height.

Trees that are native in New England, or native in other sections of the
United States and thoroughly established in New England, are described
and, for the most part, figured. Foreign trees, though locally
established, are not figured. Trees may be occasionally spontaneous
over a large area without really forming a constituent part of the
flora. Even the apple and pear, when originating spontaneously and
growing without cultivation, quickly become degenerate and show little
tendency to possess themselves of the soil at the expense of the native
growths. Gleditsia, for example, while clearly locally established, has
with some hesitation been accorded pictorial representation.

The geographical distribution is treated under three heads: Canada and
Alaska; New England; south of New England and westward. With regard to
the distribution outside of New England, the standard authorities have
been followed. An effort extending through several years has been made
to give the distribution as definitely as possible in each of the New
England states, and while previous publications have been freely
consulted, the present work rests mainly upon the observations of living
botanists.

All descriptions are based upon the habit of trees as they appear in New
England, unless special mention is made to the contrary. The
descriptions are designed to apply to trees as they grow in open land,
with full space for the development of their characteristics under
favorable conditions. In forest trees there is much greater uniformity;
the trunks are more slender, taller, often unbranched to a considerable
height, and the heads are much smaller.

When the trunk tapers uniformly from the ground upward, the given
diameter is taken at the base; when the trunk is reinforced at the base,
the measurements are made above the swell of the roots; when reinforced
at the ground and also at the branching point, as often in the American
elm, the measurements are made at the smallest place between the swell
of the roots and of the branches.

A regular order has been followed in the description for the purpose of
ready comparison. No explanation of the headings used seems necessary,
except to state that the _habitat_ is used in the more customary present
acceptation