This volume is the first of four which are to deal with all the native forest trees of North America north of the Mexican boundary. It contains an account of the tree species known to inhabit the Pacific region, 150 in all. Part II will be devoted to the Rocky Mountain trees, Part III to the trees of the southern States, and Part IV to the trees of the northern States.<br><br>The region covered by Part I includes Alaska, British Columbia, Washington, Oregon, and California (see maps, frontispiece). Many trees described occur wholly within this region, but none are represented throughout it. A few are found on its southern border and range into Mexico, while three or four trees stretch from within the Pacific region to the Atlantic.<br><br>The definition of a tree followed by the author includes woody plants having one well-defined stem and a more or less definitely formed crown (but not excluding unbranched cactuses, yuccas, and palms), and attaining somewhere in their natural or planted range a height of at least 8 feet and a diameter of not less than 2 inches. It has been difficult to apply this definition in all cases, for there is no sharp line between some shrub-like trees and some tree-like shrubs. However, though wholly arbitrary, it has been serviceable. A considerable number of species included are, over much of their range, little more than chaparral shrubs, becoming tree-like only in exceptionally favorable places. Recent discoveries in this region have made it necessary to class as trees several species previously regarded as shrubs. Some species are shrubs within this territory, but are trees outside of it. There still remain for further careful consideration several species of Arctostaphylos, Ceanothus, and Styrax, which may prove to be trees.<br><br>Since this work was prepared solely for the layman, the use of technical terms has been avoided.