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Volume IV (1988) - Flora of New Zealand Naturalised Pteridophytes, Gymnosperms, Dicotyledons
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Xanthium L.

XANTHIUM L.

Annual herbs. Lvs alternate, simple, entire to serrate or lobed. Capitula unisexual, sessile or shortly pedunculate, solitary, terminal and in panicles or racemes, or clustered in axils. ♂ capitula ± globose; involucral bracts in 1 row; receptacle conic; scales present; corolla tubular. ♀ capitula ovoid; involucral bracts in 2 rows, the outer small and free, the inner connate, prickly and ending in 1-2 beaks containing the 2 florets; corolla 0. Achenes all similar, ovoid, enclosed within the expanded prickly involucre; pappus 0.

Key

1
Lvs sessile or shortly petiolate, with stout axillary spines; lamina obtuse to cuneate at base, white or grey with dense tomentum on lower surface; beaks of expanded fruiting involucre 1-2, inconspicuous, 0.5-1 mm long
1. *spinosum
Lvs long-petiolate, spineless; lamina cordate or truncate at base, green with scattered hairs on lower surface; beaks of expanded fruiting involucre 2, conspicuous, 4-6 mm long
2. *strumarium

2 spp., N. temperate regions, C. and S. America. Naturalised spp. 2.

Recent European and American Floras follow the biosystematic study of Xanthium by Löve, D. and Dansereau, P., Canad. J. Bot. 37 : 173-208 (1959), which accepted only 2 spp. This approach is also adopted here. Both spp. are troublesome weeds as the prickly burs catch in wool causing discomfort to stock and decreasing fleece quality.

Fig. 25. Fruiting involucres of Xanthium. A X. spinosum; B X. strumarium. © All rights reserved. [Image: 4X4H]
Fig. 25. Fruiting involucres of Xanthium. A X. spinosum; B X. strumarium. © All rights reserved. [Image: 4X4H]
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