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Staminate catkins elongate, the scales ovate to rotund, each subtending 3 fls; cal minute, 4-parted or usually 2-parted, one lobe much exceeding the other; stamens 2, each short filament divided near the summit and the anther-sacs separate; pistillate catkins ovoid to cylindric, the scales closely imbricate, generally 3-lobed, relatively thin, at maturity herbaceous or coriaceous, deciduous with or soon after the frs, the catkin disintegrating; fr ordinarily a small samara, the elliptic body with 2 membranous lateral wings and terminated by the short, persistent styles, or sometimes the wings virtually obsolete; trees or shrubs, the bark often separable into thin layers; the catkins appear in the fall and come into anthesis early in the spring (midsummer in B. michauxii). 50, N. Hemisphere. The taxonomy of Betula is complicated by extensive hybridization and introgression, yet the species maintain their identity over large areas of sympatry. Many of the hybrids have been given names. B. ×borealis Spach is B. papyrifera × pumila; B. ×caerulea Blanch. and B.×caerulea-grandis Blanch. are B. papyrifera var. cordifolia × populifolia; B. ×minor (Tuck.) Fernald is B. glandulosa var. glandulosa × a dwarf, alpine form of B. papyrifera, perhaps locally alloploid; B. ×purpusii C. K. Schneid. is B. alleghaniensis × pumila; B. ×murrayana B. V. Barnes & Dancik is an octoploid back-cross of B. ×purpusii with B. alleghaniensis; and B. ×sandbergii Britton is B. glandulosa var. glandulifera × papyrifera.
Trees or shrubs deciduous. Buds sessile, with several overlapping scales. Leaves often glandular punctate abaxially, usually serrate or dentate, rarely lobulate. Male inflorescence pendulous, elongate, cylindric, with numerous overlapping bracts, each bract subtending 2 bracteoles and 3 flowers; calyx 4-lobed; stamens 2; anthers 2-loculed; thecae connate, apex pubescent or glabrous. Female inflorescence 1, or 2-several in a raceme, spicate, ellipsoid, or cylindric; bracts numerous, overlapping, leathery, deciduous, apex deeply 3-lobed, each bract subtending 3 flowers. Nutlet compressed, usually with membranous wings.
Trees or shrubs. Buds sessile, with several imbricate scales. Lvs toothed, few-to many-veined. ♂ catkins formed in autumn, opening in spring often before the lvs; fls with a bract and 2 bracteoles; fls 3 in the axil of each bract; perianth minute, 4-lobed; stamens 2, divided below the anthers. ♀ catkins shorter; fls in groups of 3 each with a bract and 2 bracteoles. Fruiting catkins cylindric to oblong or narrow-ovoid; scales 3-lobed, ± membranous, falling at fr. maturity. Fr. small; wing broad or narrow.
