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Trees, rarely shrubs , deciduous, with gray or brownish bark. Terminal buds larger than lateral buds. Leaves alternate [or opposite], aromatic, usually odd-, rarely even-, pinnately compound; stipules absent; petiole present. Leaflets 3-23, margins serrate or entire. Inflorescences: staminate catkins solitary or fasciculate, pendulous, elongate, on reduced shoots arising on branches of previous year or at base of current year's growth; pistillate catkins solitary or few-flowered spikes [or many-flowered racemes]. Flowers unisexual, staminate and pistillate on same plants; bract 1, bracteoles (0-)2. Staminate flowers: calyx 2-6-lobed or absent; corolla absent; stamens 3-50; filaments very short or absent; anthers usually pubescent. Pistillate flowers: calyx 4-lobed or absent; corolla absent; ovary 1, inferior, usually 2-carpellate, 1-locular distally; ovule 1; stigmas 2, fleshy or plumose. Fruits large nuts [or samaras], nuts enclosed in dehiscent or indehiscent, fibrous-fleshy or hard involucres (husks), thus ± drupelike. Seeds 1; endosperm absent; cotyledons fleshy and oily, variously lobed.
Trees or shrubs, usually deciduous, monoecious, rarely dioecious; branchlets solid or with partitioned pith. Leaves alternate, rarely opposite, odd-or even-pinnate; leaflets with glandular scales, often aromatic; stipules absent, rarely present (not in Australia). Flowers unisexual, arranged in catkins, produced on the previous or current year’s growth. Catkins solitary or clustered, pendulous or erect, elongate. Male flowers subtended by an entire or 3-lobed bract, bracteoles 2 or absent; perianth 2–6-lobed or absent; stamens 3–50, filaments short. Female flowers subtended by an entire or 3-lobed bract, bracteoles 2 or 3 or absent; perianth 4-lobed or absent; ovary inferior, unilocular at apex, carpels usually 2; style 1, stigmas 2. Fruit a large nut, sometimes fleshy and drupe-like, dehiscent or indehiscent, or rarely a samara. Seed 1, lacking endosperm.
