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, 5–12, pink or purple, rarely white; invol cylindric, with 4–10 principal bracts and a few much reduced outer ones; achenes glabrous or scabrous, slender, columnar to fusiform; pappus of numerous capillary bristles; ± rush-like herbs with milky juice and alternate (or the lower opposite), narrow, mostly linear or subulate lvs; heads erect, terminating the branches, or sometimes in a racemiform infl. 10, N. Amer.