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31a. Betuloideae Gray

Trunks and branches terete. Bark thin, close or exfoliating in thin sheets, becoming thicker and frequently furrowed or broken in age; lenticels often present, prominent, sometimes becoming greatly expanded horizontally. Bark and wood strongly tanniferous. Young twigs and buds often covered with small to large, resinous glands; pith triangular in cross section. Leaves 3-ranked, occasionally nearly 2-ranked. Staminate flowers: perianth of 4(--6) sepals, well defined, minute, membranaceous. Pistillate flowers 2--3 per scale, scales arranged in conelike catkins; perianth not obvious; ovules with 1 integument. Infructescences 1--4 cm, conelike, composed of many scales; scales either persistent or deciduous with fruits, crowded, small, woody or leathery. Fruits tiny samaras, lateral wings 2, membranous, sometimes reduced to ridges; pericarp thin, leathery.

Genera 2, species 60 (2 genera, 26 species in the flora): primarily boreal and cool temperate zones of Northern Hemisphere


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