Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls perfect or unisexual, the plants then variously polygamous to monoecious to dioecious; cal deeply 5-parted; stamens 5; filaments connate at base; anthers bisporangiate and unilocular; ovary compressed; style very short; stigmas usually filiform; ovule 1; utricle compressed, membranous, indehiscent; herbs or shrubs with opposite, petiolate lvs and small or minute fls in spikes or aggregated into panicles. 70, irregular.