Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls 4-merous, perfect or unisexual; hypanthium urceolate, contracted at the mouth, ± 4-angled, not prickly; sep petaloid; pet none; stamens (2)4 or many; pistils 1 or 2; ovaries included in the hypanthium; ovule 1, suspended; style terminal; fr an achene, enclosed by the indurate hypanthium; perennial herbs from a thick rhizome, or annuals, the lvs pinnately compound with serrate to pinnatifid lfls, the fls small, borne in dense spikes or heads. 25, N. Amer., Eurasia.