Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Monoecious; staminate fls with 5 distinct sep and 5 stamens; sep of the pistillate fls 4, the outer pair much smaller, or 3 or 2 by abortion; style elongate, pubescent, persistent; fr an oblique achene, much surpassing the 2 persistent sep; herbs, shrubs, or trees with small fls, often in large clusters. 25, widespread.