Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fr-body oblong or ovoid, flattened dorsally, the primary ribs low, with a row of short inconspicuous bristles, the 4 secondary ribs prominently winged, the wing divided into a row of flattened-subulate, hooked or straight spines; oil-tubes 1 under each secondary wing, 2 on the commissure; carpophore entire or bifid at the top; umbels compound, terminal and from the upper axils, long-pedunculate; bracts commonly large, pinnately dissected; bractlets linear or rarely pinnate; fls mostly white, the marginal ones often enlarged and irregular; stylopodium conic; annual or biennial herbs with pinnately decompound lvs. 60, widespread.