1. Mecardonia acuminata (Walter) Small

Bacopa acuminata (Walter) B.L. Rob.

Pl. 482 h, i; Map 2210

Plants perennial herbs, with fibrous roots, terrestrial. Stems 20–55 cm long, erect or ascending, usually not rooting at the lower nodes, 4-angled, glabrous. Leaves opposite, sessile. Leaf blades 1–5 cm long, 2–11(–15) mm wide, somewhat thickened and leathery, linear to more commonly oblanceolate, spatulate, elliptic, or narrowly rhombic, unlobed, rounded or bluntly to sharply pointed at the tip, angled or tapered at the base, not clasping the stem, the margins sharply toothed, mostly above the midpoint, the venation pinnate but often with the lowermost pair of secondary veins branching well above the base and relatively prominent, the surfaces glabrous, not shiny, the undersurface inconspicuously gland-dotted. Inflorescences axillary, the flowers 1 per leaf axil, the flower stalks 7–35 mm long; bractlets 2, at the base of the flower stalk, 6–10 mm long, linear to narrowly oblanceolate. Flowers perfect. Calyces 6–10 mm long at flowering, not or only slightly enlarged at fruiting, deeply 5-lobed nearly to the base, glabrous but minutely gland-dotted, the lobes subequal (the outer 3 lobes slightly broader than the inner 2), linear to narrowly lanceolate. Corollas 6–10 mm long, somewhat bilabiate, 5-lobed, weakly zygomorphic, narrowly bell-shaped, the tube longer than the lobes, white or with pinkish or purplish lines or streaks and a yellow throat, spurless, the throat open, hairy on the inner surface of the upper side, the upper lip shallowly 2-lobed, the lower lip with 3 broader, deeper lobes. Fertile stamens 4, the filaments of 2 lengths, not exserted, the anthers attached near their midpoints, the anther sacs parallel; staminodes absent (perhaps represented by the bearded throat). Style 1, not exserted, forked near the tip, the short branches flattened and broad (appearing similar to a pair of lips). Fruits capsules, 5–8 mm long, narrowly ovoid to narrowly oblong-ovoid, glabrous, the 2 locules equal in size, dehiscent longitudinally along the 2 sutures. Seeds numerous, 0.5–0.6 mm long, more or less cylindric to somewhat conic, not flattened, but usually appearing several-angled, the surface dark brown, with a network of prominent ridges, these arranged into vertical ranks. 2n=42. July–September.

Uncommon in the southwestern and southeastern portions of the Ozark Division and the Mississippi Lowlands (southeastern U.S. west to Kansas and Texas). Swamps, bottomland forests, margins of ponds, lakes, and sinkhole ponds, sloughs, fens, wet swales in sand prairies and glades, and occasionally banks of spring branches; also gardens and wet portions of old fields; terrestrial or emergent aquatics in shallow water.

Steyermark (1963) noted that specimens of this species tend to blacken as they dry.