Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads disciform, the pistillate fls few, with tubular or obsolete cor; invol of a few equal or imbricate, ± herbaceous bracts in 1–3 series; receptacle small, chaffy, its bracts commonly linear or spatulate, often subtending the pistillate as well as the staminate fls, these outer pales sometimes larger and more like the bracts of the invol; disk fls 6–20 in our spp., staminate, with undivided style, connate filaments, scarcely united anthers, and abortive or no ovary; achenes obovate, thick but somewhat compressed parallel to the invol bracts; pappus none; herbs or shrubs with opposite (or the upper alternate) lvs and small heads of greenish-white fls sessile or short pedunculate in the axils of the upper lvs or bracts (infl bractless in one sp.). 15, N. Amer.