Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls perfect or unisexual; cal initially involute and inconspicuous, later enlarged and spreading, usually with several or many long, setaceous, plumose, pappus-like segments; cor-tube sometimes gibbous at base, the 5 lobes equal or subequal; stamens 3; ovary basically tricarpellate, but 2 of the carpels vestigial; stigma 3- lobed; fr a nerved achene; annual or (ours) perennial herbs with entire to bipinnatifid lvs, the fls in corymbiform to paniculiform or thyrsoid infls of basically determinate nature. 200, nearly cosmop.