Trees or shrubs deciduous. Buds stalked with 2(or 3) scales or sessile with several overlapping scales. Leaves usually serrate or dentate, rarely incised or entire. Male inflorescence elongate, pendulous, cylindric, with numerous overlapping bracts, each bract subtending (3 or)4(or 5) bracteoles and 3 flowers; calyx 4-lobed; stamens (1-)4; anthers 2-loculed, thecae connate, apex glabrous. Female inflorescence 1, or 2-numerous in a raceme or panicle, ovoid or ellipsoid, conelike; bracts numerous, overlapping, woody, persistent, apex 5-lobulate, each bract subtending 2 flowers. Nutlets 2 in each bract axil, compressed, with membranous or papery wings. Fl. mainly spring, Alnus formosana and A. nepalensis autumn.
Trees or shrubs , to 35 m; trunks usually several, branching excurrent to deliquescent. Bark of trunks and branches light gray to dark brown, thin, smooth, close; lenticels often present, pale, prominent, sometimes horizontally expanded. Wood nearly white, turning reddish upon exposure to air, moderately light and soft, texture fine. Branches, branchlets, and twigs nearly 2-ranked to diffuse; young twigs uniform or ( Alnus subg. Alnobetula ) differentiated into long and short shoots. Winter buds stipitate (nearly sessile in Alnus subg. Alnobetula ), narrowly to broadly ovoid or ellipsoid, terete, apex acute to rounded; scales 2--3, valvate, or ( Alnus subg. Alnobetula ) several, imbricate, smooth, or ( Alnus subg. Clethropsis ) sometimes none. Leaves borne on long or short shoots, 3-ranked to nearly 2-ranked. Leaf blade ovate to elliptic or obovate, thin to leathery, base variable, cuneate to rounded, margins doubly serrate, serrate, serrulate, or nearly entire, apex variable, acute to obtuse or acuminate to rounded; surfaces glabrous to tomentose, abaxially sometimes resinous-glandular. Inflorescences: staminate catkins lateral, in racemose clusters or ( Alnus subg. Clethropsis ) solitary, formed ( Alnus subg. Alnus and Clethropsis ) during previous growing season and exposed or enclosed in buds during winter, or ( Alnus subg. Clethropsis ) formed and expanding during same growing season, expanding before or with leaves; pistillate catkins proximal to staminate catkins, solitary or in relatively small racemose clusters, erect to nearly pendulous, ovoid to ellipsoid, firm; scales and flowers crowded, developing and maturing at same time as staminate catkins. Staminate flowers in catkins, 3 per scale; stamens (3--)4(--6); anthers and filaments undivided. Pistillate flowers usually 2 per scale. Infructescences erect or pendulous; scales persistent long after release of fruits, with 5 lobes, greatly thickened, woody. Fruits tiny samaras, lateral wings 2, leathery or membranaceous, reduced or essentially absent in some species. x = 7.
Terminal buds present; stipules caducous. Staminate catkins borne at the end of last year's leaf shoot; cymules 3-flowered (often reduced at distal portion of catkin), subtended by a primary bract, 2 secondary bracts and each lateral flower by a single abaxial bract (rarely an adaxial bract develops); tepals 0-6; stamens 4(-6), adnate to the tepals. Pistillate catkins borne on few-leaved dwarf-shoots or borne in a leaf axil subtending the staminate catkin, often several lateral catkins develop to form a raceme, each cymule composed of 2 lateral flowers, the bracts as in staminate cymules; perianth reduced to !small glands which are sometimes adnate to the ovary; the catkin at maturity resembling a small cone; ovary 2- or 3-carpellate. Fruit a nut more or less winged. The presence of polymerous flowers in the same cymule is not unusual. Hermaphroditic flowers are sometimes found toward the base of the catkin (either staminate or pistillate).
"Staminate catkins pendulous, usually clustered, each bract subtending 3 fls with minute, 4-parted cal and 4 stamens with short, undivided filaments; pistillate catkins becoming woody, long-persistent, short, ovoid to ellipsoid or short-cylindric, each bract subtending 2 fls without cal; bracts of the pistillate catkins adnate to the bractlets, the compound unit cuneate, rounded or truncate and lobed at the summit; fr a small achene or samara, crowned with the 2 short, persistent styles and surrounded by a thin margin or membranous wing; trees or shrubs with 3-angled pith and broad, ovate to obovate, deciduous lvs. 30, mostly N. Hemisphere."
Furlow, J. J. 1979. The systematics of the American species of Alnus (Betulaceae). Rhodora 81: 1--121, 151--248. Hylander, N. 1957. On cut-leaved and small-leaved forms of Alnus glutinosa and A. incana. Svensk Bot. Tidskr. 51: 437--453. Murai, S. 1964. Phytotaxonomical and geobotanical studies on gen. Alnus in Japan (III). Taxonomy of whole world species and distribution of each sect. Bull. Gov. Forest Exp. Sta. 171: 1--107. Trappe, J. M., J. F. Franklin, R. F. Tarrant, and G. M. Hansen, eds. 1968. Biology of Alder.... Portland.
|Alder, aulne, aune [Latin alnus, alder]|