Herbs, perennial, occasionally annual, rosulate. Stems simple, erect, sometimes caudiciform, short to branching, elongate. Leaves alternate, 2-ranked, equitant; blade mostly linear to filiform, flattened to nearly terete, margins smooth to variously papillate or scabrous. Inflorescences: scapes variously elongate; spike bracts imbricate, the proximal sterile, decussate, distal ones fertile, in spiral [decussate], bearing medially, usually subapically, a distinct and mostly differently textured and colored 'dorsal area' or this indistinct or absent. Flowers solitary in bract axils; sepals 3, distinct, unequal, lateral (outer) sepals distinct [connate], chaffy, boat-shaped, chaffy, inner one membranous, capping corolla; keel papery, or scarious, variously winged, variously margined; petals 3, distinct [to connate], equal, strongly clawed; blade ephemeral, spreading, yellow to white; staminodes 3, distinct, strongly clawed, apically yoked, bearded with moniliform hairs (rarely reduced, beardless); stamens 3, epipetalous; anthers 2--4-locular; ovaries y thin-walled, placentation marginal to parietal [basal, free-central, axile]; styles elongate, tubular, 3-branched; stigmas 3, U-shaped or funnelform. Capsules 3-valved, usually thin, loculicidal. Seeds [1--]15--90 or more, mealy or translucent, ovoid to cylindric, 0.3--1(--4) mm, variously lined and cross-lined; endosperm starchy and proteinaceous; embryo basal-lateral, minute. x = 9. Most species of Xyris occur in the Guiana Highlands, Amazonia, and Brazilia in South America, with other smaller centers of endemism in Africa and Australasia. The key below is constructed to work with healthy plants that are in flower and/or fruit. Identifications based on sterile material are doubtful at best, then attempted only after long experience with such plants.
In the descriptions, scapes are described as linear if, 1 cm below the spike, they are 1 mm or more wide, or as filiform if, at that level, they are less than 1 mm wide. Petal color is assumed to be yellow, although X. caroliniana and X. platylepis have white-petaled forms. Leaf widths or thicknesses are measured at widest or thickest part of blade. Spike measurements are taken from mature spikes, those having at least some ripe fruit.
Fls trimerous; anterior sep well developed, deciduous; pet distinct, with slender claw (hidden by the bract) and broad, yellow (white), toothed blade; staminodes present, bifid; placentas in most spp. (incl. ours) parietal, ±intruded distally; style- branches 3, alternate with the staminodes; seeds longitudinally lined. 250+, mainly warm reg.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.