Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fr ovoid or oblong to subglobose, slightly flattened laterally, not readily separating into mericarps, densely covered with hooked bristles sometimes arranged in rows; ribs obsolete; oil-tubes in our spp. (except no. 5) solitary in the intervals, 2–3 on the commissure; umbel irregular, with unequal spreading primary branches, the umbellets dense or subcapitate, commonly with 3 sessile or short-pedicellate perfect fls, their ovary bristly, and several staminate fls with smooth ovary, these all or chiefly on much longer pedicels; sep relatively well developed, narrow, connate at base, persistent; pet greenish-white to greenish-yellow; stylopodium wanting; stems in our spp. arising from a cluster of fibrous or fleshy-fibrous roots; lvs palmately divided into 3–5(–7) broad segments, the basal long-petioled, the cauline progressively reduced and less petiolate; invol foliaceous. 40, nearly cosmop.