Trees, with sweet watery juice, without terminal buds, their slender terete branchlets marked by numerous pale lenticels and lengthening by one of the upper axillary buds formed in early summer, and alternate simple penniveined usually doubly serrate deciduous stalked leaves, obliquely plicately folded along the primary veins, their petioles in falling leaving small semioval slightly oblique scars showing three equidistant fibro-vascular bundle-scars; stipules inclosing the leaf in the bud, fugacious. Flowers vernal, appearing with or before the unfolding of the leaves, or rarely autumnal, monœcious, the staminate 1—3 together in the axils of the scales of an elongated pendulous lateral ament and composed of a 2—4-parted membranaceous calyx and 2—20 stamens inserted on a receptacle, with distinct filaments and 2-celled erect extrorse anthers opening longitudinally, or without a calyx, the pistillate in short lateral or capitate aments, with or without a calyx, a 2-celled ovary, narrowed into a short style divided into two elongated branches longer than the scales of the ament and stigmatic on the inner face or at the apex, and a single anatropous pendulous ovule in each cell of the ovary. Fruit a small mostly 1-celled 1-seeded nut, the outer layer of the shell light brown, thin and membranaceous, the inner thick, hard, and bony. Seed solitary by abortion, filling the cavity of the nut, suspended, without albumen, its coat membranaceous, light chestnut-brown; cotyledons thick and fleshy, much longer than the short superior radicle turned toward the minute hilum. Of the six genera, all confined to the northern hemisphere, five are found in North America; of these only Corylus is shrubby. CONSPECTUS OF THE NORTH AMERICAN ARBORESCENT GENERA.