Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal divided to below the middle or near the base; cor campanulate to tubular, white to purple; filaments exserted, each flanked at base by a pair of linear appendages of the cor; style shortly bifid; ovary unilocular, with 2 dilated placentas; ovules 4; seeds 1–3; capsule globose, 2-valved; herbs, mostly perennial and with fleshy-fibrous roots from a very short to well developed rhizome, the lvs variously cleft; fls in compact (often capitate) mostly subdichotomously branched cymes that lack a well developed main axis. 8, N. Amer.