Alternanthera sessilis

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Alternanthera sessilis

Description

Annual or perennial herb, terrestrial or sometimes shallow-water aquatic, creeping, trailing, scandent or erect, to 60 cm or more, usually smaller; stems ribbed, pubescent in 2 lines in grooves between ribs, or sometimes glabrous, rooting at lower nodes. Leaves of a pair sometimes unequal; petiole short, to 5 mm long; blade ovate, oblanceolate or elliptical, acute to obtuse and mucronulate at apex, cuneate and tapering to base, 1-6 x 0.5-1.5 cm, shallowly serrulate, glabrous on both sides or sparsely puberulent on midvein below. Inflorescence of axillary, congested, sessile, globose heads; heads 2-4 x 3-6 mm; bracts and bracteoles ovate, 0.3-0.8 mm long, acuminate, mucronate, hyaline, prominently 1-veined. Tepals white, hyaline, lance-ovate, narrowly ovate or elliptic-ovate, 1-2 mm long, acute, apiculate, slightly concave, glabrous, 1-veined; stamens 3, shorter than tepals, pseudostaminodia entire, or dentate at apex, shorter than to as long as filaments; style 0.1-0.2 mm long, stigma capitate. Utricle yellowish, bifacially compressed, obcordate, 1.5-2.0 x 1.5-2.2 mm, emarginate, much protruding from and exceeding tepals at maturity, with peg-like persistent style; seed brown or reddish-brown, cochleate-orbicular or lenticular, 1.0-1.2 mm wide, with a sharp margin, shining.

Distribution

Guianas present, pantropical and subtropical present
Pantropical and subtropical; 85 collections studied, all from the Guianas (GU: 20; SU: 37; FG: 29).

Notes

The very small, glabrous tepals, 3 stamens and obcordate, protruding fruit are characteristic. As exhibited by axes from which fruits have fallen, the globose clusters of flowers are, technically, very densely flowered spikes. Kellogg (1988) noted a phenomenon which tends to be confirmed by our examination of Guianan specimens: "The habit of this species varies considerably depending on the environment. There are two widespread forms of Alternanthera sessilis: a graceful, large-leaved (usually) form with large internodal lengths and relatively small inflorescences — in humid to aquatic environs; and a tighter, scrubbier form with smaller leaves, shorter internodes and sometimes very large compactions of axillary inflorescences — in mesic to drier regions.".