Syst. Bot. 33: 181. 2008.
Annuals. Stems erect, simple or few-branched, (7–)9–21(–29) cm, glabrous or glabrate proximally, glabrate or glandular-puberulent distally. Leaves: dimorphic, submersed basal and clustered, floating paired at ends of branches to 70 mm, blade of submersed lanceolate to oblanceolate or oblong, 1–7 × 0.5–3 mm, margins entire, apex obtuse to acute, surfaces glabrous, blade of floating elliptic to ovate or nearly round, 3–10 × 2–9 mm, margins entire, apex obtuse to rounded or retuse, surfaces glabrous. Pedicels stout, 0.1–3 mm, length less than 0.1 times bract, glabrous; bracteoles 0. Flowers: sepals connate proximally, calyx lobes obovate to oblong, 0.7–1 mm; corolla 3–4 mm, tube and limb white tinged pink or purple, veins lavender or purple; style 0.5–1 mm. Capsules bilaterally symmetric, obcordiform, 2–3 × 3–4 mm. Seeds (0.3–)0.4–0.5 mm. 2n = 18.
Phenology: Flowering Mar–Apr.
Habitat: Shallow, ephemeral pools on exposed granite outcrops.
Elevation: 100–300 m.
Ala., Ga., S.C.
Phylogenetic studies confirmed that Gratiola amphiantha, long treated in monospecific Amphianthus, is embedded in Gratiola (D. Estes and R. L. Small 2008). Gratiola amphiantha is listed as a federally-threatened species by the United States Fish and Wildlife Service.