《中国植物志》|| 桦木科 BETULACEAE ||桦木族 Betuleae (Doll) Ascherson ||桦木属 Betula L.

6.桦木属——Betula L.
L., Sp. Pl. 982. 1753.
落叶乔木或灌木。树皮白色、灰色、黄白色、红褐色、褐色或黑褐色，光滑、横裂、纵裂、薄层状剥裂或块状剥裂。芽无柄，具数枚覆瓦状排列之芽鳞。单叶，互生，叶下面通常具腺点，边缘具重锯齿，很少为单锯齿，叶脉羽状，具叶柄；托叶分离，早落。花单性，雌雄同株；雄花序2-4枚簇生于上一年枝条的顶端或侧生；苞鳞覆瓦状排列，每苞鳞内具2枚小苞片及3朵雄花；花被膜质，基部连合；雄蕊通常2枚，花丝短，顶端叉分，花药具2个完全分离的药室，顶端有毛或无毛；花粉粒赤道面观为宽椭圆形，极面观具棱，大多数具3孔，呈三角形，很少为2、4、5-8孔，外壁两层均加厚，不形成孔室，外层在孔处无明显的带状加厚；雌花序单1或2-5枚生于短枝的顶端，圆柱状、矩圆状或近球形，直立或下垂；苞鳞覆瓦状排列，每苞鳞内有3朵雌花；雌花无花被，子房扁平，2室，每室有1个倒生胚珠，花柱2枚，分离。果苞革质，鳞片状，脱落，由3枚苞片愈合而成，具3裂片，内有3枚小坚果。小坚果小，扁平，具或宽或窄的膜质翅，顶端具2枚宿存的柱头。种子单生，具膜质种皮。
本属约100种，主要分布于北温带，少数种类分布至北极区内。我国产29种6变种。 全国均有分布。
本属模式种：B. alba L. p. (B. pendula Roth)
本属植物最早出现于白垩纪，第三纪开始繁茂，分布于北半球的温带与寒带。我国山 东山旺中新统有很好的叶化石保存，东北地区上白垩统曾发现化石花粉。

《云南植物志》|| 桦木科 Betulaceae nom. conserv. ||桦木科 Betulaceae nom. conserv. ||桦木属 Betula L.

2．桦木属 Betula L.
乔木或灌木；树皮光滑或成环状片层剥离，或成块状纵裂，灰色、褐色或黑色；芽无柄；芽鳞多数，呈覆瓦状排列。叶互生，叶面光滑，背面通常具腺点，边缘具重锯齿，稀单锯齿，托叶分离，早落。花单性，雌雄同株，雄花序2—4枚，簇生枝顶或侧生；苞鳞覆瓦状排裂，每苞鳞内具2枚小苞片和3朵雄花；雄花具花被，膜质，基部联合，雄蕊通常2枚，花丝短，顶端分叉，花药2室，药室分离，顶端通常具毛；雌花序单生或2—5枚组成总状，直立或下垂；苞鳞覆瓦状排列，每苞鳞内有3朵雌花，雌花无花被，子房2室，每室有1个倒生胚珠，花柱2枚，线形，分离。果苞鳞片状，革质，通常脱落，由1枚苞片和2枚小苞片发育而成，具3裂片，内有3枚小坚果；小坚果扁形，长约3毫米，具或宽或狭的膜质或纸质的翅，顶端具2枚宿存的花柱，种子单生，种皮膜质。
约100种，主要分布在北温带，有少数种类延伸至北极区内。我国约29种，7变种；云南11种，2变种。
本属为我省森林中主要阔叶树种之一，乔木树种为优良胶合板材及军工用材之一，有的树皮可提取单宁及香桦油。

《Flora of North America》|| Betulaceae || || Betula

2. Betula Linnaeus, Sp. Pl. 2: 982. 1753; Gen. Pl. ed. 5, 433, 1754.

Birch [Latin betula, birch]

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.

Species ca. 35 (18 species in the flora): throughout n temperate, boreal, and arctic zones of the Northern Hemisphere; North America, Asia.

Birches, like alders, are common trees and shrubs of northern temperate and boreal zones of the Northern Hemisphere. Like Alnus , the group is highly diversified, especially in the Old World. The species are well known for their free hybridization, and specimens are therefore frequently difficult to identify. Birches occupy habitats in cool, moist regions, including peatlands, stream banks, and lakeshores, cool, damp woods, and moist slopes in cool coves. The wood of species that grow to a large size (including especially B . alleghaniensis ) has many uses, including the manufacture of doors and windows, flooring, cabinetry, interior molding, wood paneling, furniture, and plywood.

Betula sect. Costatae (Regel) Koehne consists of large, mesophytic trees, often with dark, close or exfoliating bark, large thin leaves, infructescence scales with long narrow lobes, and fruits with narrow wings. North American representatives of this group include Betula alleghaniensis , B . lenta , and B . nigra . The mostly circumboreal Betula sect. Betula consists of small to medium trees with rather large thin leaves and fruits with wide wings (wider than the fruit body). A characteristic feature of trees in this group is their white bark that often peels apart in sheets. These include the familiar paper birch, B . papyrifera , and its European counterpart, B . pubescens , as well as the common eastern B . neoalaskana . A third line, Betula sect. Humiles W. D. Koch, consists of dwarf shrubby species of the cold circumpolar region. In North America this section is represented by B . glandulosa , B . pumila , and B . nana .

Birches are a difficult group taxonomically because of their high vegetative variability and frequent hybridization. Many morphologic and cytologic studies have attempted to deal with variation within the genus or its subgroups. Species of Betula form a polyploid series, with chromosome numbers of 2 n = 28, 56, 70, 84, and 112, plus additional numbers in some hybrids. This and other research in the genus has been reviewed by J. J. Furlow (1990).

SELECTED REFERENCES

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.

《Flora of China》|| Betulaceae || ||桦木属 Betula

6. Betula Linnaeus, Sp. Pl. 2: 982. 1753.

桦木属 hua mu shu

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.

Between 50 and 60 species: Afghanistan, China, Japan, Kazakhstan, Korea, Kyrgyzstan, Mongolia, Nepal, Russia, Sikkim; Europe, North and South America; 32 species (14 endemic) in China.