candicans_), which resembles the balsam Poplar, except in its leaves, which are very large and whitish on the under surface, and in the great rapidity of its growth, while that of the Balsam Poplar is rather slow. BETULACEÆ.—THE BIRCH TRIBE. The plants included in this tribe have single leaves, which are generally what is called feather-nerved; that is, the veins are marked strongly and deeply from the mid rib to the margin. The flowers are in cylindrical catkins, the male and female flowers being on the same plant. THE GENUS BETULA. [Illustration: FIG. 83.—CATKINS OF THE BIRCH.] [Illustration: FIG. 84.—THE BIRCH (_Betula_).] The common Birch (_Betula alba_) is an exceedingly graceful tree. The male catkins are produced singly, or two or three together. They are long, slender, loose, and gracefully drooping; (see _fig._ 83;) and each consists of a great number of flowers, pressed close together, and growing round a rachis or stem, as shown in the catkin _a_ in _fig._ 84, from which some of the flowers have been removed. The male flowers have each ten or twelve stamens enclosed in three or more scales or bracts, as shown in a reversed flower at _b_. The female flowers are produced in dense catkins, which are much shorter than the others, and always solitary; the flowers, which are arranged round a very slender axis, are furnished with lobed scales, and _c_ is a scale with three female flowers in its lobes, each having two long spreading stigmas (_d_). A ripe capsule is shown at _e_, with its membranaceous wings, and the cell _f_ open to show the seed. The ovary when young has two cells and two ovules (as shown at _g_); but the division between the cells wastes away as the seeds ripen, and one of the ovules proves abortive. There are several species of Birch natives of America, some of which have upright oval female catkins like those of the Alder, but they are always distinguished by being solitary. The bark of the Birch is remarkable for its tenacity, and for the great length of time that it will resist decay. In America they make canoes of the bark of _B. papyracea_; and in Lapland huts are thatched with that of _B. alba_. The Birch is remarkably hardy; and it grows nearer the limits of perpetual snow both on mountains and near the pole than any other tree. THE GENUS ALNUS.