Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Much like Convolvulus in aspect, but the bracts usually large, inserted just beneath the cal, and ± concealing it; pollen spheroidal, pantoporate with 20–40 apertures; ovary ± unilocular, the partition incomplete; stigmas oblong, ± cylindric, blunt; fls usually solitary; our spp. rhizomatous perennials. 30, widespread.