Alternanthera

Forsskål

Fl. Aegypt.-Arab., 28. 1775

Etymology: Latin alternans, alternating, and anthera, anther, referring to the alternation of pseudostaminodes and stamens
Treatment appears in FNA Volume 4. Treatment on page 447. Mentioned on page 406, 435.
Herbs or subshrubs, annual or perennial. Stems prostrate, decumbent, ascending, erect, or floating, indumentum of simple trichomes. Leaves opposite, sessile or petiolate; blade lanceolate to ovate, ovate-rhombic, or obovate-rhombic, margins entire. Inflorescences axillary or terminal, sessile or pedunculate, several-flowered cylindric spikes or globose heads, without immediately subtending leaves; bracts and bracteoles scarious. Flowers bisexual; tepals 5, distinct; stamens 3–5; filaments connate basally into tube or short cup; pseudostaminodes 5, alternating with stamens; ovule 1; style 1, ca. 0.2 mm; stigma capitate or rarely 2-lobed. Utricles compressed, ovoid or obovoid, indehiscence. Seeds 1, reddish brown, lenticular or ovoid-oblong.

Distribution

Primarily American tropics and subtropics, some in Asia, Africa, and Australia.

Discussion

Species ca. 80 (9 in the flora).

Key

1 Inflorescences pedunculate, terminal and axillary > 2
1 Inflorescences sessile, axillary > 4
2 Bracts not keeled; tepals glabrous Alternanthera philoxeroides
2 Bracts keeled; tepals pilose > 3
3 Bracts shorter than to equaling tepals Alternanthera brasiliana
3 Bracts less than 1/2 as long as tepals Alternanthera flavescens
4 Pseudostaminode margins fimbriate; anthers 5, oblong > 5
4 Pseudostaminode margins laciniate, dentate or, entire; anthers 3-5, globose > 6
5 Tepals rigid, margins coriaceous, glabrous; leaves succulent Alternanthera maritima
5 Tepals not rigid, margins chartaceous, pilose or hispid; leaves not succulent Alternanthera ficoidea
6 Tepals monomorphic, hairs not barbed > 7
6 Tepals dimorphic, hairs barbed > 8
7 Mature fruit exerted between tepals Alternanthera sessilis
7 Mature fruit included within tepals Alternanthera paronychioides
8 Tepals 5-7 mm, sparsely villous; leaf blades usually as broad as long; pseudostaminode margins dentate Alternanthera pungens
8 Tepals 3-5 mm, densely villous; leaf blades longer than broad; pseudostaminode margins usually entire Alternanthera caracasana