Lanceleaf primrose-willow

Isnardia lanceolata (Elliott) de Candolle

Herbs slender, with well-developed aerenchyma when base submerged, often forming stolons 10–40 cm, 2–3 mm thick, stolons with widely spaced leaves. Stems erect, sub­terete or slightly ridged, well branched distally, 45–100 cm, glabrous, with raised lines decurrent from leaf axils. Leaves alternate; stipules ovate to very broadly ovate, 0.2–0.5 ×0.1–0.3 mm, succulent; stolons: petiole winged, 0.2–1 cm, blade orbiculate or elliptic to broadly elliptic, 0.5–2.7 × 0.6–1.2 cm, base attenuate, apex rounded to acute; main stem: petiole winged, 0.1–0.5 cm, blade elliptic, oblanceolate, or narrowly oblanceolate to sublinear, 2–7.5 × 0.2–0.8(–1.4) cm, base narrowly cuneate, sometimes attenuate, margins entire with minute hydathodal glands, apex acute to narrowly acute, leaves on side branches much reduced; bracts sublinear, moderately reduced. Inflorescences open spikes, flowers solitary in distal leaf axils; bracteoles attached in opposite pairs at or slightly distal to base of ovary, ovate-elliptic to very narrowly elliptic, 1.5–4.3 × 0.4–1.4 mm, often with a swollen base, margins minutely papillose, apex subacute. Flowers: sepals ascending, pale green, broadly ovate-deltate, 1.5–2.5 × 1.8–3.3 mm, margins minutely papillose, apex usually acute, rarely acuminate, surfaces gla­brous; petals 0; filaments nearly translucent, 1–1.4 mm, base dilated, anthers 0.4–0.6(–0.8) × 0.5–0.6 mm; pollen shed in tetrads; ovary broadly obovoid or cup-shaped, 2.5–3.5 × 2.2–3.2 mm; nectary disc elevated 0.4–0.6 mm on ovary apex, yellowish green, 1.8–2.6 mm diam., 4-lobed, glabrous; style yellowish green, 0.5–0.7 mm, glabrous, stigma yellowish green, broadly capitate to subglobose, 0.3–0.5 × 0.7–1 mm, not exserted beyond anthers. Capsules obpyramidal, sharply 4-angled and 4-winged, wings 0.3–0.7 mm wide, 3.5–5 × 2.5–4.5 mm, hard-walled, dehiscent by apical ring, pedicel 0.1–0.5 mm. Seeds light brown, narrowly oblong with constricted ends, 0.6–0.8 ×0.2–0.3 mm, surface cells nearly isodiametric. 2n = 32.

Flowering Jun–Oct. Ditches, low meadows, cypress swamps, moist pinelands, edges of pocosins, sandy peaty soil; 0–100 m; Fla., Ga., N.C., S.C.

Ludwigia lanceolata is fairly uncommon, with scattered populations occurring along the Atlantic coast of southern North Carolina, South Carolina, eastern and southern Georgia, and peninsular Florida. It reaches its western limit in the central panhandle of Florida. C. I. Peng (1988, 1989) found this tetraploid species to be interfertile with other tetraploid species in the section, but few natural hybrids are found, perhaps due to persistent autogamy and habitat specialization.