Trees and shrubs, with astringent scaly bark, soft straight-grained wood, naked stipitate winter-buds formed in summer and nearly inclosed by the united stipules of the first leaf, becoming thick, resinous, and dark red. Leaves open and convex in the bud, falling without change of color; stipules of all but the first leaf ovate, acute, and scarious. Flowers vernal, or rarely opening in the autumn from aments of the year, in 1—3-flowered cymes in the axils of the peltate short-stalked scales of stalked aments formed in summer or autumn in the axils of the last leaves of the year or of those of minute leafy bracts; staminate aments elongated, pendulous, paniculate, naked and erect during the winter, each staminate flower subtended by 3—5 minute bractlets adnate to the scales of the ament, and composed of a 4-parted calyx, and 1—3 or usually 4 stamens inserted on the base of the calyx opposite its lobes, with short simple filaments; pistillate aments ovoid or oblong, erect, stalked, produced in summer in the axils of the leaves of a branch developed from the axils of an upper leaf of the year, and below the staminate inflorescence, inclosed at first in the stipules of the first leaf, emerging in the autumn and naked during the winter, or remaining covered until early spring; pistillate flowers in pairs, each flower subtended by 2—4 minute bractlets adnate to the fleshy scale of the ament becoming at maturity thick and woody, obovate, 3—5-lobed or truncate at the thickened apex, forming an ovoid or subglobose strobile persistent after the opening of its closely imbricated scales; calyx 0; ovary compressed; nut minute, bright chestnut-brown, ovoid to oblong, flat, bearing at the apex the remnants of the style, marked at the base by a pale scar, the outer coat of the shell produced into lateral wings often reduced to a narrow membranaceous border. Alnus inhabits swamps, river bottom-lands, and high mountains, and is widely and generally distributed through the northern hemisphere, often forming the most conspicuous feature of vegetation on mountain slopes, ranging at high altitudes southward in the New World through Central America to Colombia, Peru, and Bolivia, and to upper Assam and Japan in the Old World. Of the eighteen or twenty species now recognized nine are North American; of these, six attain the size and habit of trees. Of the exotic species, _Alnus vulgaris_ Hill., a common European, north African, and Asiatic timber-tree, was introduced many years ago into the northeastern states, where it has become locally naturalized. The wood of Alnus is very durable in water, and the astringent bark and strobiles are used in tanning leather and in medicine. _Alnus_ is the classical name of the Alder. CONSPECTUS OF THE NORTH AMERICAN ARBORESCENT SPECIES.