Trees and shrubs , deciduous. Winter buds stipitate or sessile, with either 2--3 valvate scales (stipules) or few to many imbricate scales (or occasionally naked); terminal bud absent. Leaves alternate, spirally arranged, 2--3-ranked, simple; stipules deciduous, distinct; petioles present. Leaf blade sometimes lobed, pinnately veined, margins toothed, serrate to nearly entire; surfaces glabrous to tomentose, abaxially often with resinous glands. Inflorescences unisexual; staminate catkins pendulous, elongate, cylindric, conspicuously bracteate, consisting of crowded, reduced, 1--3-flowered clusters; pistillate inflorescences either of erect to pendulous bracteate catkins, or of compact 2--3-flowered clusters subtended by leafy involucres; bracts often nearly foliaceous or woody in infructescences. Staminate flowers bracteate; stamens (1--)4--6; anthers 2-locular, dehiscing by longitudinal slits, pollen sacs often ± distinct; pistillode sometimes present. Pistillate flowers small, highly reduced; pistil 1, 2(--3)-carpellate; ovary inferior, usually 2-locular proximally, 1-locular distally; placentation axile; ovules 1--2 per locule, pendulous; styles 2, distinct or nearly so; stigmas dry; staminodes usually absent. Fruits nuts, nutlets, or 2-winged samaras, 1-seeded, without multibracteate cupule, often subtended or enclosed by foliaceous hull developed from 2--3 bracts; seed coat membranous; endosperm present, thin at maturity; embryo straight, as long as seed; cotyledons flat or greatly thickened, oily.
Trees or shrubs deciduous, monoecious. Stipules present, free, often deciduous, rarely persistent. Leaves alternate, simple, petiolate, usually doubly serrate, rarely simply serrate, lobulate, or entire; veins pinnate. Flowers unisexual. Male inflorescence precocious, elongate, pendulous, with numerous overlapping bracts; each bract usually subtending a small dichasium with 1-3 male flowers; stamens as many as and opposite sepals or, if sepals obsolete, then stamens of inflorescence to 20; filaments very short, connate or nearly so; anthers 2-loculed, thecae connate or separate, opening by longitudinal slits. Female inflorescence pendulous or erect, with numerous overlapping bracts; each bract subtending a small dichasium with 2 or 3 flowers; calyx with 1-6 scalelike lobes, or obsolete; petals absent; ovary inferior, 2-loculed; styles 2, free; ovules 2, or 1 by abortion, pendulous from near apex of each locule. Fruit a nut or nutlet, winged or not. Seed 1, with straight embryo and flat or thickened cotyledons, without endosperm.
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