Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Bractlets of the epicalyx 3, linear to obovate; stamen-column bearing anthers at the summit; pet truncate to (in our spp.) obcordate; carpels 10–20, beakless, one-seeded, indehiscent, the sides incompletely covering the seed; styles slender, stigmatic along the inner side; annual to perennial herbs with broad, toothed to lobed or parted lvs, the fls solitary or fascicled in the axils. 25, temp. Eurasia.