Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads radiate, the rays few, short and rather inconspicuous, white or yellowish, pistillate and fertile, the expanded part rarely wanting and the heads thus disciform; invol of 2–4 series of broad, ± imbricate, dry and scarcely herbaceous bracts, the inner subtending the rays; receptacle small, convex or conic, chaffy; disk-fls staminate, with undivided style; ray-achenes flattened parallel to the invol bracts, their subtending bracts joined at base to the 2 or 3 adjacent receptacular bracts, thus partly enclosing the achene, the whole commonly falling as a unit; pappus of 2 or 3 short or elongate awns or scales, or nearly obsolete; bitter, aromatic herbs (ours) or shrubs with alternate, entire to pinnatifid lvs and rather small hemispheric heads. 16, New World.