Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads discoid, the fls sometimes all tubular and perfect, or more often the marginal ones neutral, with enlarged, irregular, falsely radiate cor; invol bracts imbricate in several series, dry, sometimes spine-tipped, more often some of them with enlarged, scarious or hyaline, erose or lacerate or pectinate terminal appendage, seldom all merely rounded above and entire; receptacle nearly flat, densely bristly; cors anthocyanic to yellow or white, with slender tube, short throat, and long narrow lobes; anthers with a narrow, firm, apical appendage, evidently (though often shortly) tailed at base; styles with a thickened, often hairy ring and an abrupt change of texture at the base of the papillate branches; achenes obliquely or laterally attached to the receptacle, seldom evidently nerved; pappus of several series of graduated bristles or narrow scales, often much reduced, or wanting; herbs with alternate or all basal, entire to pinnatifid lvs. 500+, mostly Old World.