Xanthium L.

Tutin, T. G., Heywood, V. H., Burges, N. A., Moore, D. M., Valentine, D. H., Walters, S. M. & Webb, D. A., 1976, Flora Europaea. Volume 4. Plantaginaceae to Compositae (and Rubiaceae), Cambridge University Press : 143

50. Xanthium L. 1

Annual herbs. Leaves alternate, entire or variously lobed. Capitula solitary or in axillary clusters, unisexual, the male above the female. Male capitula subglobose; involucral bracts in 1 row; receptacle cylindrical, with scales; florets numerous; stamens 5; anthers free and hooked at apex; filaments connate; style and ovary rudimentary. Female capitula ovoid; involucral bracts in 2 rows, the outer small, free, the inner connate, coriaceous, prickly, ending in 2 (rarely 1) beaks and forming a 2-locular structure containing 2 florets; corolla absent; styles exserted through a hole on the inside of the beak near its base. Achenes ovoid; pappus absent.

Literature: F. J. Widder, Feddes Repert. (Beih.) 20: 1-223 (1923); 21: 273-305 (1925); Phyton (Austria) 11: 69-82 (1964); 12: 182-190 (1967). A. Cronquist, Rhodora 47: 402-403 (1945). D. Love & P. Dansereau, Canad. Jour. Bot. 37: 173-208 (1959).

Leaves long-petiolate, green beneath, without spines at base of petiole ' 1. strumarium

Leaves sessile or shortly petiolate, white- or grey-tomentose beneath, with 1-2, 3-fid, yellow spines at base (rarely replaced by small leaves) 2. spinosum