Evergreen or deciduous trees or shrubs. Leaves usually alternate, rarely opposite or whorled, simple, entire, wavy, serrate, echinate or cleft, petiolate; stipules caducous, pinnately veined, glabrous or tomentose beneath. Plants are monoecious, flowers small, apetalous. Staminate flowers in spikes (catkins) or heads, with 4–7-lobed perianth; stamens 3–12 or more, anthers glabrous or hairy, 2-celled, longitudinally dehiscent; reduced ovary present or not. Pistillate flowers solitary or 3–4-clustered, usually in short spikes, sometimes at the base of staminate catkins, with 4–8-lobed perianth adnate to the ovary; ovary 3–7-celled, placentation axile, ovules 2 in each cell, but only 1 in the ripened ovary; styles 3–10, rarely 1–2, usually as many as cells; stigmas pointed or enlarged with flat or tuberculate portion. Nuts ovoid, oblong, depressed-globose or 3-angled, enclosed partly or wholly in a cupule (involucre), glabrous or hairy or winged; cupules regularly or irregularly dehiscent or not, covered with imbricate scales, scales free or united into 6–11 concentric rings, smooth, muricate, tomentose or long-spined, spines simple or branched, hard or soft, often tuberculate-scaled; seed usually solitary by abortion, without endosperm, cotyledons 2, fleshy.
Widely distributed in tropical, subtropical and temperate regions of the northern hemisphere, one genus in the southern hemisphere; about 10 genera and 600 species worldwide, eight genera with 60 taxa (including 41 native species, six introduced species., nine native varieties, one introduced variety and three native forms) in Taiwan.
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