Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cor funnelform or campanulate, with or without well developed fornices; stamens included; nutlets in most spp. evidently roughened, keeled ventrally and often also dorsally, the scar typically placed at the lower end of the ventral keel, often elevated and caruncle-like; gynobase mostly short-pyramidal; cotyledons deeply bifid, thus apparently 4; taprooted, rough-hairy annuals with rather small, often narrow lvs, the orange or yellow fls in elongating, mostly naked, helicoid false spikes or racemes. 15, w. N. Amer. and w. and s. S. Amer.