Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls greatly reduced, the staminate consisting of a single stamen, the pistillate of a single pistil; several staminate fls surrounding one pistillate fl inserted at the base of a cupulate involucre, the whole constituting an infl termed a cyathium but simulating a single fl with a stipitate ovary; involucre 4–5-lobed, usually bearing glands in the sinuses; these in some spp. with white or colored petaloid margin; ovary 3-locular and 3- ovulate; styles 3, bifid; capsule 3-lobed, 3-seeded; herbs, shrubs, or trees of diverse form, with milky, often highly acrid juice. The several subgenera and even some of the sections have often been taken as distinct genera, but the boundaries are difficult to draw. 1500, cosmop.