Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Staminate catkins protruding from the buds in autumn, elongating in spring, densely fld, pendulous; bract adherent to the "perianth," except at its summit; "perianth " spreading, 3–6-lobed, with 8–40 stamens on its upper side; anthers glabrous; pistillate fls in short spikes terminating the branches, closely subtended by a 3-lobed, cup-shaped involucre that ripens with the fr to form a husk; perianth minute, 4-parted; husk indehiscent, clammy- glandular; nut indehiscent but ± distinctly 2-valved; trees with partitioned pith and odd-pinnate lvs, the median-lateral lfls the largest, inequilateral or falcate; lfls with conduplicate vernation. 20, widespread.