Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls regular, but complex in structure; sep 5, short- clawed; pet 5, the blade prolonged backward from the base below the fl into an elongate, hollow spur; stamens numerous, often ± connivent, the innermost staminodial; follicles mostly 5, erect; perennial herbs from a stout, caudex-like rhizome, with petiolate, 2–3 times ternately compound, alternate lvs and few but showy fls. 70, N. Hemisphere.