Prostrate mat-forming perennial herb with a stout vertical rootstock, also rooting at the lower nodes, much-branched from the base outwards, mats up to c. 1 m across. Stem and branches terete, striate, ± densely villous with long, white hairs but frequently glabrescent. Leaves broadly rhomboid-ovate to broadly oval or obovate, 1.5-4.5 x 0.3-2.7 cm, rounded to subacute at the apex with a mucro which in the young leaves is often fine and bristle-like, narrowed below to a petiole up to 1 cm long, glabrous or thinly appressed-pilose on both surfaces, especially on the lower surface of the primary venation. Inflorescences sessile, axillary, solitary or more commonly 2-3 together, globose to shortly cylindrical, 0.5-1.5 cm long and 0.5-1 cm wide; bracts membranous, white or stramineous, 4-5 mm, lanceolate-ovate, glabrous or dorsally pilose, distinctly aristate with the excurrent midrib, strongly denticulate around the upper margin; bracteoles similar but smaller, 3-4 mm, falling with the fruit. Tepals extremely dissimilar; the 2 outer (abaxial) deltoid-lanceolate, 5 mm, very rigid, 5-nerved below (the intermediate pair of nerves much shorter and finer), outer 2 nerves meeting above to join the excurrent, pungently mucronate midrib; inner (adaxial) tepal oblong, flat, 3 mm, blunt and strongly dentate at the apex, 3-nerved below but the nerves meeting well below the apex and the apical mucro short and fine; lateral tepals c. 3 mm, sinuate in side view with the two sides of the lamina connivent and denticulate above, sharply mucronate; abaxial and adaxial tepals with small tufts of glochidiate and barbed whitish bristles about the basal angles, the lateral tepals each with a large tuft about the centre of the midrib. Stamens 5, all with anthers, at anthesis slightly exceeding the ovary and style, the alternating pseudostaminodes broad, subquadrate, shorter than the filaments, entire to dentate. Ovary compressed, squat, narrowed below, style very short, wider than long. Fruit orbicular, rounded or retuse above, 2 mm. Seed discoid, c. 1.25 m, brown, shining faintly reticulate.

Type: Colombia (Maypur waterfall, R. Orinoco), Humboldt and Bonpland (P, isotype, photo!).

Distribution: A native of tropical America, now widespread as a weed of waste ground etc. in the tropics and subtropics of both Old and New Worlds. Seems to thrive in bare, heavily trodden places.

For the nomenclautre of Alternanthera sessilis and Alternanthera pungens, see Melville in Kew Bull. 1958: 171-174. 1958.