Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls all ligulate and perfect, yellow; invol campanulate or narrower; achenes columnar to fusiform, but scarcely beaked, with a whitish basal callosity, ca 10-ribbed; pappus of 5–many members, these commonly with paleaceous base and slender, bristle-like tip, varying (in spp. approaching Agoseris) to merely obscurely flattened capillary bristles; lactiferous herbs with a taproot, entire to pinnatifid lvs, and 1–many heads on long naked peduncles. 25, chiefly w. U.S.