5. Taxodium Richard, Ann. Nat. Hist. 16: 298. 1810.
落羽杉属 luo yu shan shu
Trees deciduous or semievergreen, monoecious; main branches persistent; lateral branches alternate and deciduous in winter; winter buds globose, small. Leaves spirally arranged, alternate, dimorphic: those on main branches persistent, inclined or curved upward and ± appressed to branchlets, subulate; those on annual branchlets deciduous together with branchlet as a unit, 2-ranked, linear or usually subulate. Pollen cones borne in long, pendulous, catkinlike racemes or panicles, ovoid; microsporophylls 6-many, spirally arranged; pollen sacs 4-9. Seed cones solitary, attached laterally by minute peduncle to apex of previous year’s growth, globose or ellipsoid but faceted; bracts and cone scales falling at maturity, spirally arranged; bracts attached to a conical axis, not overlapping, peltate, rhombic, each almost completely connate with its cone scale but with free, protruding, triangular, acute apex; ovules 2 per bract axil; cone scales shield-shaped, woody at maturity, apex irregularly quadrangular. Seeds 2 per cone scale, irregularly triquetrous, large, strongly keeled, wingless, water dispersed. Cotyledons 4-9. Germination epigeal. 2n = 22.
Two species: Guatemala, Mexico, United States; both introduced in China.
The number of distinct species of Taxodium is open to dispute: T. mucronatum might be better treated as a variety of T. distichum. The genus is similar in vegetative characters to Metasequoia, differing in its alternate (not opposite) branching and leaf arrangement. It also resembles Glyptostrobus, differing chiefly in its seeds which are strongly 3-angled and wingless (instead of 1-winged).
Taxodium is now important in the afforestation of low lying river deltas with waterlogged soils, especially in SE China.
