While the duration of leaves in evergreens is not at all a new subject, very little systematic work seems to have been done toward determining durations for an extended list of evergreen species. This work was begun for the purpose of determining the leaf duration of the evergreen species of trees and shrubs in western Washington. It soon developed that the work would not be a mere cataloging of species with their accompanying leaf durations, since a very superficial examination of some of the coniferous evergreens growing under different conditions on the University campus, showed a wide but constant difference in the duration of their leaves. It was then decided to limit the species discussed to those growing under varying conditions that could be examined in the field, and to ascertain, if possible, the factors governing the duration of their leaves. The Pacific northwest is peculiarly an evergreen region. Sargent (I5) described the characteristic coniferous forests as the most luxuriant if not the most diversified on the continent. His report in the Tenth Census states that "Washington is covered with the heaviest continuous belt of forest growth in the United States. This magnificent coniferous forest extends over the slopes of the Cascade and Coast ranges, and occupies the entire drift plain surrounding the waters of Puget Sound." Evergreenness is not only characteristic of the forests, but is equally typical of the forest undergrowth, and of a large list of herbaceous species of the open fields. This is especially true of the Puget Sound region, in which the mild climate affords a practically continuous growing season. This may be one reason why many species elsewhere deciduous are here evergreen. There are, in the state of Washington, according to Frye and Rigg (2), 76 species of woody evergreens, 24 of which are gymnosperms, and 52 angiosperms. In western Washington there are 52 species, i6 of which are gymnosperms, and 36 angiosperms. Of these the writer has studied the following 9 gymnosperms and 22 angiosperms: