Mecardonia procumbens (Mill.) Small,prostrate water–hyssop, prostrate mecardonia, stalked water–hyssop. Annual, taprooted, 1—several–stemmed at base, decumbent with spreading to ascending shoots, axillary shoots soon having 1—2 flowers arising per node; shoots with only cauline leaves, leaf clasping young axillary shoot, glabrous.
Stems± 4–sided, < 2 mm diameter, leaf bases decurrent with a pair of low, colorless ridges along each internode, internodes to 30 mm long, each with a pair of long, colorless ridges.
Leaves opposite decussate with pair not fused across nodes, simple, subsessile, without stipules; petiole to 2 mm long; blade ovate to elliptic, (3—)8—16 × 4—8+ mm, tapered at base, low–serrate below midblade, acute at tip, pinnately veined with midrib slightly raised on lower surface.
Inflorescence leafy raceme, terminal, with paired stalked flowers at each node; bractlets subtending pedicel 2 in axil, leaflike, narrowly oblanceolate, 4—5 × 1 mm, entire to remotely toothed; pedicel straight and ascending, 4–sided, at anthesis ca. 20 × 0.5—0.6 mm, >> leaf , 24—29 × 0.4 mm and blackish purple in fruit, aging slender and wiry, persistent.
Flower bisexual, bilateral, in range ca. 3 mm across, ca. 8 mm long; sepals 5 in 2 series, unequal, green or tinged purplish red, outer 3 sepals whorled and fused at base, elliptic–lanceolate, 6.5—7.2 × 2—2.8 mm (1 slightly larger and 5–veined)), inner 2 sepals on lower side of flower and linear, < outer sepals and 0.5 mm wide, ± opposite and keeled and 1–veined, persistent and ascending in fruit; corolla 2–lipped, ca. 7.5 mm long, light yellow with purple–red nectar lines; tube ± cylindric slightly compressed top–to–bottom, without nectar lines; throat yellow, throat and lip lines above midpoint of flower; upper lip 2–lobed, mostly lacking nectar lines, lobes obovate rounded at tip, ca. 2 × 2.5 mm; lower lip weakly 3–lobed, curled over rest of corolla, ca. 1.8 × 4.5 mm, lobes 1.5 mm wide, having a bear of yellow hairs to 1 mm long from upper throat to lower lip; stamens 4, of 2 lengths, attached 1 mm from base of corolla tube with shorter 2 on lower side; filaments curved below anther to display anther inward, 3 mm and 4 mm long, yellowish; anthers basifixed, dithecal, ca. 0.65 mm long, white, longitudinally dehiscent; pollen white; nectary disc below ovary, cuplike, 1 mm diameter, green; pistil 1, ca. 4 mm long; ovary superior, oblong–lanceoloid, ca. 2.5 × 0.9 mm, pale green, glabrous with slightly raised sutures, 2–chambered, each chamber with many ovules; style flaring to and slightly bent approaching stigma, white; stigma at same level as anthers, capitate, green.
Fruit capsule, loculicidal, 2–valved, many–seeded (appearing to have full fruit set), ascending to spreading 30—45°, ± 6 mm long, light brown with dark brown septum pitted by where seeds were embedded.
Seed oblong, 0.25—0.3 × 0.1 mm, mostly colorless but with a fine red network of low ridges defining cells in longitudinal files.
A. C. Gibson