Trees and shrubs , deciduous. Winter buds stipitate or sessile, with either 2--3 valvate scales (stipules) or few to many imbricate scales (or occasionally naked); terminal bud absent. Leaves alternate, spirally arranged, 2--3-ranked, simple; stipules deciduous, distinct; petioles present. Leaf blade sometimes lobed, pinnately veined, margins toothed, serrate to nearly entire; surfaces glabrous to tomentose, abaxially often with resinous glands. Inflorescences unisexual; staminate catkins pendulous, elongate, cylindric, conspicuously bracteate, consisting of crowded, reduced, 1--3-flowered clusters; pistillate inflorescences either of erect to pendulous bracteate catkins, or of compact 2--3-flowered clusters subtended by leafy involucres; bracts often nearly foliaceous or woody in infructescences. Staminate flowers bracteate; stamens (1--)4--6; anthers 2-locular, dehiscing by longitudinal slits, pollen sacs often ± distinct; pistillode sometimes present. Pistillate flowers small, highly reduced; pistil 1, 2(--3)-carpellate; ovary inferior, usually 2-locular proximally, 1-locular distally; placentation axile; ovules 1--2 per locule, pendulous; styles 2, distinct or nearly so; stigmas dry; staminodes usually absent. Fruits nuts, nutlets, or 2-winged samaras, 1-seeded, without multibracteate cupule, often subtended or enclosed by foliaceous hull developed from 2--3 bracts; seed coat membranous; endosperm present, thin at maturity; embryo straight, as long as seed; cotyledons flat or greatly thickened, oily.
Trees or shrubs deciduous, monoecious. Stipules present, free, often deciduous, rarely persistent. Leaves alternate, simple, petiolate, usually doubly serrate, rarely simply serrate, lobulate, or entire; veins pinnate. Flowers unisexual. Male inflorescence precocious, elongate, pendulous, with numerous overlapping bracts; each bract usually subtending a small dichasium with 1-3 male flowers; stamens as many as and opposite sepals or, if sepals obsolete, then stamens of inflorescence to 20; filaments very short, connate or nearly so; anthers 2-loculed, thecae connate or separate, opening by longitudinal slits. Female inflorescence pendulous or erect, with numerous overlapping bracts; each bract subtending a small dichasium with 2 or 3 flowers; calyx with 1-6 scalelike lobes, or obsolete; petals absent; ovary inferior, 2-loculed; styles 2, free; ovules 2, or 1 by abortion, pendulous from near apex of each locule. Fruit a nut or nutlet, winged or not. Seed 1, with straight embryo and flat or thickened cotyledons, without endosperm.
Fls tiny, inconspicuous, unisexual (the plants monoecious), in unisexual catkins of rather different appearance, the male pendulous and more or less elongate, with a flexuous axis, the female pendulous or erect, shorter, firm, often woody; each bract of the catkin subtending 1–3 fls; cal of 1–6 minute, scale-like sep, or obsolete; pet none; stamens commonly as many as and opposite the sep, or (when the sep are obsolete) sometimes to as many as 18; filaments very short, distinct or basally connate; anthers often deeply divided from the summit so that the pollen sacs are ± distinct; ovary inferior or nude, bicarpellate, with 2 styles, bilocular below, unilocular above, with 1–2 pendulous, anatropous ovules in each locule; fr a nut or more commonly a 2-winged samara, without a basal cupule, but sometimes with a foliaceous hull derived from 2 or 3 involucral bracts; seed solitary, with a large, dicotyledonous embryo and little or no endosperm; anemophilous trees or shrubs with alternate, simple, pinnately straight-veined, usually toothed lvs and deciduous stipules. 6/120.
Abbe, E. C. 1935. Studies in the phylogeny of the Betulaceae. I. Floral and inflorescence anatomy and morphology. Bot. Gaz. 97: 1--67. Abbe, E. C. 1938. Studies in the phylogeny of the Betulaceae. II. Extremes in the range of variation of floral and inflorescence morphology. Bot. Gaz. 99: 431--469. Bousquet, J., S. H. Strauss, and Li P. 1992. Complete congruence between morphological and rbcL-based molecular phylogenies in birches and related species (Betulaceae). Molec. Biol. Evol. 9: 1076--1088. Brunner, F. and D. E. Fairbrothers. 1979. Serological investigation of the Corylaceae. Bull. Torrey Bot. Club 106: 97--103. Chase, M. W. et al., eds. 1993. Phylogenetics of seed plants: An analysis of nucleotide sequences from the plastid gene rbcL. Ann. Missouri Bot. Gard. 80: 528--580. Endress, P. K. 1977. Evolutionary trends in the Hamamelidales-Fagales group. Plant Syst. Evol. Suppl. 1: 321--347. Furlow, J. J. 1983. The phylogenetic relationships of the genera and infrageneric taxa of the Betulaceae. [Abstract.] Amer. J. Bot. 70(suppl.): 114. Furlow, J. J. 1990. The genera of Betulaceae in the southeastern United States. J. Arnold Arbor. 71: 1--67. Hall, J. W. 1952. The comparative anatomy and phylogeny of the Betulaceae. Bot. Gaz. 113: 235--270. Hardin, J. W. and J. M. Bell. 1986. Atlas of foliar surface features in woody plants, IX. Betulaceae of eastern United States. Brittonia 38: 133--144. Hjelmquist, H. 1948. Studies in the floral morphology and phylogeny of the Amentiferae. Bot. Not., Suppl. 2(1): 1--171. Jäger, E. J. 1980. Progressionen im Synfloreszenzbau und in der Verbreitung bei den Betulaceae. Flora 170: 91--113. Petersen, F. P. and D. E. Fairbrothers. 1985. A serotaxonomic appraisal of the "Amentiferae." Bull. Torrey Bot. Club 112: 43--52. Regel, E. 1861. Monographische Bearbeitung der Betulaceen. Nouv. Mém. Soc. Imp. Naturalistes Moscou 13(2): 59--187. Winkler, H. 1904. Betulaceae. In: H. G. A. Engler, ed. 1900--1953. Das Pflanzenreich.... 107 vols. Berlin. Vol. 19[IV,61], pp. 1--149.