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, equal or nearly so, appressed; cor funnelform; stamens included, inserted near the base of the cor; pollen prolate to subspheroidal, with 3(4) elongate colpi; ovary 4- ovulate, bilocular or becoming 4-locellar by the development of secondary partitions between the ovules; style 1, slender, elongate; stigmas 2, linear, somewhat flattened, gradually tapering; erect, twining, or trailing perennial herbs with oblong to cordate-ovate or hastate lvs and pink and white fls borne singly or often several together on axillary peduncles; bracts 2, mostly small and well removed from the fl, not concealing the cal. 200+, cosmop., mainly Old World.