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 to indicate the place where a plant naturally grows, as in swamps or upon dry hillsides. Under the head of "Horticultural Value," the requisite information is given for an intelligent choice of trees for ornamental purposes. The order and names of families follow, in the main, Engler and Prantl. In accordance with the general tendency of New England botanists to conform to the best usage until an authoritative agreement has been reached with regard to nomenclature by an international congress, the Berlin rule has been followed for genera, and priority under the genus for species. Other names in use at the present day are given as synonyms and included in the index. Only those common names are given which are actually used in some part of New England, whether or not the same name is applied to different trees. It seems best to record what is, and not what ought to be. Common names that are the creation of botanists have been disregarded altogether. Any attempt to displace a name in wide use, even by one that is more appropriate, is futile, if not mischievous.