Alternanthera sessilis

Name and classification

Alternanthera sessilis (L.) R. Br. ex DC
Family Amaranthaceae

Common names

sessile joyweed

Disseminule

fruit

Description

Fruit a utricle with attached perianth, 1.8–3 mm long, 1.3–2 mm wide, 0.3–0.7 mm thick. Perianth shorter than or same length as utricle; tepals about 2 mm long, lanceolate, scarious, white, lustrous, finely striate. Utricle strongly compressed, broadly obcordate. Scar knobby. Surface somewhat glistening, cream to light beige-yellow, minutely pebbled.

Seeds lenticular, ovate in outline, ca. 0.9–1.5 mm long, 0.8–1 mm wide, 0.3–0.6 mm thick, with marginal hilar notch at broad end. Hilum small, inconspicuous. Testa glossy, smooth, dull orange or dark brown to black, with faintly visible reticulation. Embryo peripheral, visible on surface under testa, encircling perisperm.

Identification considerations

Alternanthera sessilis is unique in the genus in that its tepals are generally shorter than the utricle. The utricles of other Alternanthera species sit well below the tops of the tepals. Compare with Alternanthera philoxeroides (Mart) Griseb (non-FNW).

Distribution

widespread throughout the tropics and subtropics: tropical Africa, southern Asia to Japan, Southeast Asia, Australia, and the Pacific Islands; in the western hemisphere: the West Indies, Central America, tropical South America including Argentina, and the United States

Habitat

pantropical; damp shady areas, swamps, pond margins, shallow ditches, roadsides, low-lying waste places, damp pastures, cultivated areas

General information

Alternanthera sessilis is an annual or perennial herb, up to 1 m tall. It prefers wet conditions, but occurs in both wetlands and uplands and can grow on a variety of soil types. It is a weed of rice throughout tropical lands, and of other cereal crops, sugarcane, and bananas. Young shoots and leaves are eaten as a vegetable in Southeast Asia. The plant spreads by seeds, which are wind and water-dispersed, and by rooting at stem nodes.

fruits

fruits

seeds

seeds

A, utricle with perianth; B, utricle; C, seed; D, longitudinal section of fruit showing embryo; E, transverse section of fruit showing embryo; drawing by Lynda E. Chandler

A, utricle with perianth; B, utricle; C, seed; D, longitudinal section of fruit showing embryo; E, transverse section of fruit showing embryo; drawing by Lynda E. Chandler