Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Dioecious; invol bracts imbricate in several series, scarious at least at the tip, white or often colored; receptacle flat or convex, naked; staminate heads discoid, with tubular corollas, usually undivided styles, and scanty pappus, the bristles commonly barbellate or clavellate; anthers tailed; pistillate heads disciform, with filiform-tubular cors, bifid styles, and copious pappus of capillary, naked bristles weakly united at base; achenes nerveless, terete or nearly so, glabrous or papillate; woolly perennial herbs, ours fibrous-rooted, stoloniferous, and mat-forming or colonial; lvs alternate, simple, entire, in our spp. the largest ones basal and at the ends of the stolons; heads rather small, (1–) several in terminal infls; x=14. 30+, mainly N. Amer., especially the w. cordillera. Our spp. 3–6 form a polyploid-apomictic complex. Absence of staminate plants from a local population indicates apomixis.