Scales of the pistillate ament deciduous; nut wingless, more or less inclosed in an involucre formed by the enlargement of the bract and bractlets of the flower; staminate flowers solitary in the axils of the scales of the ament; calyx 0; pistillate flowers with a calyx. Staminate aments covered during the winter: involucre of the fruit flat, 3-cleft, foliaceous. 1. Carpinus. Staminate aments naked during the winter: involucre of the fruit bladder-like, closed. 2. Ostrya. Scales of the pistillate ament persistent and forming a woody strobile; nut without an involucre, more or less broadly winged; staminate flowers 3—6 together in the axils of the scales of the ament; calyx present; pistillate flowers without a calyx. Pistillate aments solitary, their scales 3-lobed, becoming thin, brown, and woody, deciduous; stamens 2; filaments 2-branched, each division bearing a half-anther; winter-buds covered by imbricated scales. 3. Betula. Pistillate aments racemose, their scales erose or 5-toothed, becoming thick, woody, and dark-colored, persistent; stamens 1—3 or 4; filaments simple; wings of the nut often reduced to a narrow border; winter-buds without scales. 4. Alnus. 1. CARPINUS L. Hornbeam.