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Key to Xanthium

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1 Leaf axil with a 1-3 cm long, yellow, 3-forked spine; leaf blades lanceolate to ovate, mostly 2-5× as long as wide, cuneate at the base; burs bearing 0-1 apical beak (if 2, then unequal); [section Acanthoxanthium].
  2 Decumbent annual 20-30 cm tall; leaves pinnatifid, about 2× as long as wide; burs woolly-hairy
  2 Erect to decumbent annual to 50-100 cm tall; leaves not pinnatifid, mostly 3-4× as long as wide; burs only lightly hairy
1 Leaf axil lacking a spine; leaf blades orbicular or broadly ovate, mostly 0.8-1.5× as long as wide, cordate, truncate, rounded, or cuneate at the base; burs bearing 2 equal apical beaks; [section Xanthium].
    3 Burs glabrous and small, rarely exceeding 1 cm long, with few, thin prickles and straight beaks; leaf blades usually cordate
    3 Burs glabrous or hirsute, 1 to 3 cm long, densely covered by prickles (if not, then burs longer than 1 cm and with strong prickles and beaks), with strait or curved beaks; leaf blades truncate, rounded, or cuneate at the base.
      4 Bur densely covered (ca. 400 per bur) with thin prickles, 7-9 mm long; bur body 3-4 cm long
      4 Bur less densely covered with thicker prickles, 2-7 (-10) mm long); bur body 1-3 cm long.
        5 Burs generally glabrous or puberulent, usually light tan at maturity; burs 1.0-2.0 (-2.5) cm long (including the beaks); prickles mostly straight, hooked prickles restricted to the upper portion of the bur; beaks straight
        5 Burs generally hirsute, usually brown at maturity; burs up to 3 cm long (including the beaks); prickles uncinate; beaks straight or curved
          6 Bur densely long-hirsute; upper stem whitened with hispid hairs; [of dunes and coastal sands]
          6 Beaks glandular-puberulent; upper stem sparsely strigose; [more widespread in distribution and habitat]