Taxon	Illustrator ⠉
	Ambrosia artemisiifolia Xanthium strumarium Parthenium integrifolium	Barbara Alongi Yevonn Wilson-Ramsey Linda Ann Vorobik

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 glanddotted 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.

Distribution

New World, nearly worldwide

Discussion

Species 2–3 (2 in the flora).

Selected References

None.

Lower Taxa

Xanthium spinosum, Xanthium strumarium

Key

1	Nodal spines 0; leaf blades suborbiculate to ± pentagonal or deltate	Xanthium strumarium
1	Nodal spines (1–)3-lobed, 15–30+ mm; leaf blades ± lanceolate to ovate or lance-linear	Xanthium spinosum