THE GENUS BETULA.
The common Birch (Betula alba) is an exceedingly graceful tree. The male
Fig. 83.—Catkins of the Birch. 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
Fig. 84.—The Birch (Betula). 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.
The Common Alder (Alnus glutinosa), though so nearly allied to the Birch botanically, differs widely in its habits; as it always grows in low marshy situations, or near water, while the Birch prefers the summits of the loftiest hills. In the Alder, the male catkins are long and drooping, like those of the Birch; but they are generally produced in clusters of three or more together. The male flowers are furnished with three lobed bracts or scales, each containing three flowers, each flower having a calyx of four scales united at the base, and bearing four stamens. The female flowers are in close ovate catkins, produced in clusters of four or five together, instead of being cylindrical and solitary, as in the Birch; the scales of the catkins, though three-lobed, are only two-flowered, and the flowers have two long stigmas like those of the Birch. The ovary has two cells and two ovules, but it only produces one seed. The ripe fruit is a nut without wings, attached at the base to the scale of the cone-like catkin, the scales of the catkin becoming rigid, and opening, like those of the Scotch Pine, as the seed ripens. There are several species of Alder, some of which bear considerable resemblance to the American species of Birch; but they are easily distinguished by the female catkins of the Birch being always solitary, while those of the Alder are produced in clusters, and by the capsules of the Alder being without wings.