The World Flora Online

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.

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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.

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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.

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SELECTED REFERENCES

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.

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