Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Pet yellow, varying to white, gradually narrowed to the base; short filaments flanked by a single gland on each side; anthers short and blunt; ovary short, compressed; ovules 1–8 per locule; style slender but short; fr elliptic to orbicular, flattened parallel to the septum, each locule with a few seeds; herbs with entire or serrate lvs; ± pubescent with stellate hairs. 170, mainly Eurasia.