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5. Alnus Miller, Gard. Dict. Abr., ed. 4. 1: [51]. 1754.

桤木属 qi mu shu

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.

Three or more divergent subgroups (subgenera) of Alnus are often treated as separate genera (J. J. Furlow, J. Arnold Arbor. 71: 1-67. 1990).

Various Alnus species are grown to protect dikes, some are valued for timber, and the roots have nodules with nitrogen-fixing bacteria.

About 40 species: Bangladesh, Bhutan, China, India, Japan, Korea, Nepal, Sikkim; Europe, North and South America; ten species (five endemic) in China.

1 Female inflorescence 1.   (2)
+ Female inflorescences 2-numerous, in a raceme or panicle.   (4)
2 (1) Nutlet with papery wings, 1/4-1/3 as wide as nutlet; peduncle usually erect, 1-3 cm 1 A. ferdinandi-coburgii   (3)
+ Nutlet with membranous wings, ca. 1/2 as wide as nutlet; peduncle slender, pendulous, 3-8 cm.   (3)
3 (2) Branchlets, petioles, peduncles, and leaves sparsely white pubescent abaxially when young   2 Alnus cremastogyne
+ Branchlets, petioles, peduncles, and leaves densely yellow-brown tomentose abaxially, especially when young   3 Alnus lanata
4 (1) Female inflorescences in a panicle.   (5)
+ Female inflorescences in a raceme.   (6)
5 (4) Female inflorescences numerous; nutlet with membranous wings; leaves entire or remotely minutely serrate   4 Alnus nepalensis
+ Female inflorescences 5-13; nutlet with papery wings; leaves remotely serrate   5 Alnus henryi
6 (4) Buds sessile, with 3-6 scales; peduncle long   6 Alnus mandshurica
+ Buds stipitate, with 2 scales; peduncle very short.   (7)
7 (6) Leaves suborbicular, brownish pubescent abaxially, margin undulate-incised   7 Alnus hirsuta
+ Leaves oblong, obovate-oblong, ovate-elliptic, or ovate-lanceolate, glabrous or sparsely pubescent on both surfaces when young, margin minutely serrate.   (8)
8 (7) Peduncle of female inflorescence 3-5 mm   8 Alnus formosana
+ Peduncle of female inflorescence 10-20 mm.   (9)
9 (8) Leaves obovate, obovate-elliptic, or obovate-lanceolate, base cuneate   9 Alnus japonica
+ Leaves obovate-oblong, oblanceolate-oblong, or oblong, base subrounded, subcordate, or broadly cuneate   10 Alnus trabeculosa

Lower Taxa


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