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 , to 30 m; trunks often several, branching excurrent, becoming deliquescent. Bark of trunks and branches dark brown to chalky white, smooth, often exfoliating; lenticels dark, prominent, sometimes horizontally expanded. Wood nearly white to reddish brown, light and soft to moderately heavy and hard, texture fine. Branches, branchlets, and twigs nearly 2-ranked; young twigs differentiated into long and short shoots, sometimes with taste and odor of wintergreen. Winter buds sessile, slender, terete, apex acute; scales several, imbricate, smooth. Leaves mostly on short shoots, nearly 2-ranked. Leaf blade ovate to deltate, elliptic, or nearly orbiculate, 0.5--10(--14) × 0.5--8 cm, thin, margins doubly serrate or serrate (or crenate to shallowly round-lobed in dwarf northern species); surfaces glabrous to tomentose, sometimes abaxially resinous-glandular. Inflorescences: staminate catkins mostly terminal on branchlets, solitary or in small racemose clusters, formed previous growing season and often exposed during winter, expanding with leaves; pistillate catkins proximal to staminate catkins, mostly solitary, erect, ovoid to cylindric, firm; scales and flowers crowded, enclosed within buds during winter, expanding with leaves. Staminate flowers in catkins 3 per scale; stamens (1--)2--3(--4), filaments divided below anthers, nearly to base. Pistillate flowers (1--)3 per scale. Infructescences erect or pendulous; scales usually deciduous with release of fruits (although persisting into winter in a few species), (1--)3-lobed, thickened or leathery but not woody. Fruits samaras, lateral wings 2, moderately wide to broad, membranaceous. x = 14.
"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."
Alam, M. T. and W. F. Grant. 1972. Interspecific hybridization in birch (Betula). Naturaliste Canad. 99: 33--40. Brittain, W. H. and W. F. Grant. 1965. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. I. B. papyrifera in eastern Canada. Canad. Field-Naturalist 79: 189--197. Brittain, W. H. and W. F. Grant. 1965b. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. II. B. papyrifera var. cordifolia. Canad. Field-Naturalist 79: 253--257. Brittain, W. H. and W. F. Grant. 1966. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. III. B. papyrifera from British Columbia. Canad. Field-Naturalist 80: 147--157. Brittain, W. H. and W. F. Grant. 1967. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. IV. B. caerulea-grandis and hybrids. Canad. Field-Naturalist 81: 116--127. Brittain, W. H. and W. F. Grant. 1967b. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. V. B. papyrifera and B. cordifolia from eastern Canada. Canad. Field-Naturalist 81: 251--262. Brittain, W. H. and W. F. Grant. 1968. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. VI. B. papyrifera from the Rocky Mountains. Canad. Field-Naturalist 82: 44--48. Brittain, W. H. and W. F. Grant. 1969. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. VII. B. papyrifera and B. resinifera from northwestern Canada. Canad. Field-Naturalist 83: 185--202. Brittain, W. H. and W. F. Grant. 1969b. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. VIII. Betula from Grand Manon Island, New Brunswick. Canad. Field-Naturalist 83: 361--383. Dugle, J. R. 1966. A taxonomic study of western Canadian species in the genus Betula. Canad. J. Bot. 44: 929--1007. Fernald, M. L. 1902. The relationships of some American and Old World birches. Amer. J. Sci. 169: 167--194. Fernald, M. L. 1945. Some North American Corylaceae (Betulaceae). 1. Notes on Betula in eastern North America. Rhodora 47: 303--329. Fredskild, B. 1991. The genus Betula in Greenland---Holocene history, present distribution, and synecology. Nordic J. Bot. 11: 393--412. Grant, W. F. and B. K. Thompson. 1975. Observations on Canadian birches, Betula cordifolia, B. populifolia, B. papyrifera and B. ×caerulea. Canad. J. Bot. 53: 1478--1490. Johnsson, H. 1945. Interspecific hybridization within the genus Betula. Hereditas (Lund) 31: 163--176. Lepage, E. 1976. Les bouleaux arbustifs du Canada et de Alaska. Naturaliste Canad. 103: 215--233. Sulkinoja, M. 1990. Hybridization, introgression and taxonomy of the mountain birch in SW Greenland compared with related results from Iceland and Finnish Lapland. Meddel. Grfnland, Biosci. 33: 21--29.
|Birch [Latin betula, birch]|