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Genus Xanthium Family Asteraceae

Description: Annual monoecious herbs.

Leaves alternate, variously lobed and/or toothed. Male heads terminal or in upper leaf axils, falling after flowering; heads ± globose; involucral bracts numerous, 1–3-seriate; receptacle hemispherical with scales; florets numerous, corollas 5-toothed. Female heads axillary below male heads, ovoid, involucral bracts 2-seriate, the inner 2 fused and completely enclosing the florets; florets 2, corollas absent.

Heads forming a hard burr, the tips of the involucral bracts becoming hooked spines, 2-beaked at the summit.

Achenes 2, retained inside the burr, the burr shed as a unit; pappus absent.

Distribution and occurrence: World: c. 30 species, temperate & tropical regions. Australia: 6 species (naturalized), all States.

The four species X. cavanillesii, X. italicum, X. occidentale and X. orientale, are sometimes included in the one variable species, X. strumarium L. s. lat All species of Xanthium are declared noxious weeds in NSW..

Text by L. Murray
Taxon concept:

Taxa not yet included in identification key
Xanthium strumarium

 Key to the species 
1Stems with 3-pronged spines at base of leaves2
Stems without spines at base of leaves3
2Leaves dark green, upper surface slightly pubescent, lower surface white and felted, 3–8 cm long; widespread in most regionsXanthium spinosum
Leaves grey-green, both surfaces very finely pubescent, 2–4 cm long; SWP only
                       Back to 1
Xanthium ambrosioides
3Burrs with the 2 apical spines divergent, but straight4
Burrs with the 2 apical spines either incurved or incurved at tips
                       Back to 1
4Leaves with definite lobes, usually 3–5-lobed; burrs 16–18 mm longXanthium occidentale
Leaves shallowly and unevenly lobed; burrs 15–30 mm long
                       Back to 3
Xanthium cavanillesii
5Spines stout and incurved; burrs 15–20 mm long; grows in SWP & SFWPXanthium orientale
Spines incurved at tips only; burrs 20–30 mm long; grows in NC, CWS & NWP
                       Back to 3
Xanthium italicum

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