Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls 2-merous; hypanthium short, tubular, deciduous after anthesis; pet white (or pinkish in age), obcordate or deeply notched; stamens 2; ovary 1- or 2-locular, with a solitary ovule in each locule; stigma shallowly bilobed or capitate; fr small, indehiscent, obovoid or pyriform, usually slightly compressed, beset with ± hooked bristles; rhizomatous perennial herbs with opposite, petiolate lvs and small fls paniculately disposed in terminal, bracteate racemes; x=11. 7, N. Temp. and Arctic.