Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Invol bracts ± imbricate, scarcely herbaceous; pistillate fls numerous and slender, without rays, or in some spp. (all ours) with very short, narrow, and inconspicuous white or purplish rays barely or scarcely exceeding the pappus; disk-fls few, in ours not more than ca 20; style-appendages short, as in Erigeron; receptacle flat or nearly so, naked; achenes 1–2-nerved, or nerveless; pappus of capillary bristles, sometimes with a short outer series; herbs, often weedy, with alternate lvs and several to many, mostly rather small heads. 50+, mainly trop. and subtrop.