Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Scales distichous, the lowermost one empty and ± modified; fls perfect, each in the axil of a scale; perianth wanting; stamens 3, less often 1 or 2; style 2–3- cleft, the beakless (or nearly beakless) achene accordingly lenticular or ± trigonous; spikelets few–many in dense or loose spikes or heads, each subtended by 2 small bracts; spikes or heads commonly in a simple or compound terminal umbel that is subtended by sheathless, leafy invol bracts; each ray of the umbel surrounded at base by a tubular prophyll; herbs with solid, ± triangular stems, the lvs with closed sheath and usually an elongate, grasslike blade. 600+, cosmop. Our spp. belong to 5 subgenera, as in the following key. Spp. 14–22 and 23–28 appear to be intergradient, presumably reflecting extensive hybridization.