Ammannia robusta Heer & Regel, robust ammannia. Annual, sometimes aquatic, diffuse–rooted, 1–stemmed at base, branches above, ascending to decumbent, in range 8—60 cm tall; shoots with conspicuously ranked cauline leaves and bracts in 4 vertical files, green, glabrous.
Stems 4–sided, to 4 mm diameter, 2 sides somewhat concave between the ridges descending from each leaf and 2 convex below each leaf, tough, internodes to 25 mm long, lower stem nearly cylindric and forming periderm at base.
Leaves opposite decussate, simple, sessile, without stipules; blades linear, < 30—100 × < 3.5—9 mm, the widest at base with rounded lobes but usually not clasping, entire, acuminate at tip, 1–veined with midrib conspicuously sunken on upper surface and raised on lower surface.
Inflorescence leafy cyme, axillary, cymes paired at nodes, each cyme (1)3—5–flowered with subsessile flowers, bracteate, glabrous; occasionally at a node on lower stem forming a pair of lateral branches from sessile cyme, having internodes to 7 mm long and alternate to subopposite bracts subtending only 1 flower; bract subtending cyme leaflike; bractlets subtending the oldest flower 2, paired, oblong, ca. 2 mm long, green with colorless margins; bractlets (bracteoles) subtending later flowers 2, awl–like and < bractlets of central flower.
Flower bisexual, radial, ca. 4 mm across; in population of only 1 type (homostylous); hypanthium (floral tube) at anthesis pleated, bell–shaped with 8 ribs, ca. 2.5 × 2 mm, expanding and spheric in fruit, appendaged, appendages (epicalyx lobes) spreading, prow–shaped, 1 mm long, green; sepals 4, alternate with hypanthium appendages, broadly triangular with short reddish tip, not spreading; petals 4, opposite hypanthium appendages, obovate, 2.5—2.9 × 1.6—1.9 mm, pale pink to light pink, abscising soon after anthesis (fugacious); stamens 4, opposite petals, free but inserted on hypanthium in green furrows ca. 1 mm from base, exserted 1.4—2 mm above hypanthium; filaments 3.4—3.9 mm long, white; anthers dorsifixed, dithecal, ± 0.5 mm, cream–colored, longitudinally dehiscent; pollen pale yellow; pistil 1; ovary superior, 4–lobed, ca. 1 mm long, yellow–green, 1–chambered with numerous ovules attached to center; style exserted from hypanthium, cylindric, 2.5—3 mm long, light green aging whitish; stigma terminal, capitate and hemispheric, ± 0.35 mm diameter, green.
Fruit capsule, dehiscing irregularly, many–seeds (200+ seeds), spheric,mm, hidden by papery hypanthium and top at same level as hypanthium margin, at least exposed top crimson drying brown.
Seed stout wedge–shaped or inversely pyramid–shaped, 0.35—0.4 mm long, with blunt angles, glossy golden brown on fresh seeds.
A. C. Gibson