Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Hypanthium ovoid to cylindric or obconic; sep 5; pet 5–10; stamens 10–200, distinct or connate into fascicles; ovules 2–many on 3 parietal placentas; style 3-lobed; fr a capsule; annual or perennial herbs or shrubs, armed with stiff, barbed hairs, with alternate, serrate to pinnatifid lvs and 1–many often large, white to yellow or orange fls terminating the branches. (Nuttallia) 50, New World.