Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Sep 5, ovate to subulate; cor mostly small, funnelform to rotate, entire to 5-lobed; stamens at the top of the short cor-tube; ovary with 2 locules and 4 ovules; styles 2, distinct or barely united at base, each 2-cleft, the fl with 4 linear stigmas; capsule typically 4- valved and 4-seeded, or with fewer seeds by abortion; plants never twining; ours small, hairy perennials with several stems from a thick root, and with small axillary fls usually solitary or occasionally few together at the end of the peduncle. 100, warm reg., mainly Amer.