Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Sep ascending, cucullate at the tip; pet yellow, narrowly obovate, gradually narrowed to the claw; short stamens subtended by a flat gland, long stamens by a short-conic gland; anthers oblong; ovary cylindric, the style about as thick, the stigma truncate; fr short, terete, coriaceous, tipped by the elongate, persistent style, transversely divided into 2 members, the lower cylindric to obovoid, dehiscent by 2 valves, bearing 1–3 suspended seeds, the upper larger, subglobose, indehiscent, obtusely 5-ribbed and usually verrucose, containing one erect seed; hairs simple. 2, w. Eurasia, Mediterranean.