Xanthium

Annuals (coarse), 10-200+ cm. Stems erect, branched. Leaves cauline; mostly alternate (proximal 2-6 sometimes opposite); petiolate; blades lanceolate, linear, ovate, rounded-deltate, or suborbiculate, often ± palmately or pinnately lobed, ultimate margins entire or ± toothed, faces hirtellous or ± strigose, usually gland-dotted as well. Heads discoid, either pistillate (proximal) or functionally staminate (distal), in racemiform to spiciform arrays or borne singly (in axils). Pistillate heads: involucres ± ellipsoid, 2-5+ mm diam. at anthesis (6-20+ mm diam. at maturity); phyllaries 30-75+ in 6-12+ series, outer 5-8 distinct, the rest (sometimes interpreted as paleae) proximally connate, their distinct tips mostly ± hooked (the distal 1-3 usually longer, stouter, and not hooked), the whole becoming a hard, prickly perigynium (a bur); florets 2, corollas 0. Staminate heads: involucres ± saucer-shaped, 3-5 mm diam.; phyllaries 6-16+ in 1-2+ series, distinct to bases; receptacles conic to columnar; paleae spatulate to cuneiform or linear, membranous, distally ± villous or hirtellous; florets 20-50+, corollas whitish, ± funnelform, lobes 5, erect or reflexed (filaments connate, anthers distinct or weakly coherent). Cypselae (black) ± fusiform, enclosed in obovoid to ellipsoid, hard, prickly, 2-chambered burs; pappi 0. x = 18.

Heads small, unisexual; staminate heads uppermost, many-fld, with poorly developed or no invol, the receptacle short, cylindric, chaffy, the filaments monadelphous, the pistil composed mainly of the undivided style; invol of the pistillate heads completely enclosing the 2 fls, forming a conspicuous 2-chambered bur with hooked prickles; pistillate fls lacking a cor, and with the styles exsert from the invol; achenes thick, solitary in the chambers of the bur; pappus none; coarse, taprooted, annual weeds with alternate lvs and solitary or clustered axillary and terminal heads. 3, originally New World.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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