This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trees or shrubs. Leaves alternate, petioled, simple, dentate, serrate, lobed, cleft or entire, pinnately veined, the stipules, if any, deciduous. Flowers small, monoecious, the staminate in pendulous erect or spreading aments, or capitate, the pistillate solitary or several together, subtended by an involucre of partly or wholly united bracts, which becomes a bur or cup. Petals none. Staminate flowers with a 4-7-lobed perianth and 4-20 stamens; filaments slender, distinct, simple; anther-sacs adnate, longitudinally dehiscent. Pistillate flowers with a 4-8-lobed urn-shaped or oblong perianth, adnate to the 3-7-celled ovary; ovules 1-2 in each cavity, only 1 in each ovary ripening, pendulous, anatropous; styles as many as the cavities of the ovary, linear, terminally or longitudinally stigmatic. Fruit a i-seeded nut, with a coriaceous or somewhat bony exocarp. Testa thin. Endosperm none; cotyledons large, fleshy, often rugose; radicle short.
About 5 genera and 375 species, of very wide geographic distribution.
Staminate flowers capitate; nut sharply triangular.
Staminate flowers in slender aments; nut rounded or plano-convex.
Pistillate flowers 2-5 in each involucre; involucre becoming globose and very prickly in fruit,
enclosing the nuts.
Pistillate flower 1 in each involucre; involucre of numerous scales forming a cup in fruit and
subtending the acorn.