Infl composed of dense heads, each head subtended by bracts, each fl by a bractlet; fr globose to obovoid, not or scarcely compressed, variously covered with scales or tubercles, the ribs obsolete; stylopodium and carpophore wanting; sep usually conspicuous; pet white to purple; biennial or perennial, often spiny herbs. 200, widespread.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.