Plants annual. Taproots filiform. Stems 1–18+, erect to ascending, green or reddish, 15–30(–45) cm; internodes ± terete, 1–10 times as long as leaves, dull, retrorsely pubescent in 2 lines. Leaves usually connate basally, with narrow, scarious sheath 0.2–0.5 mm, petiolate (proximal leaves) or sessile; petiole 2–4 mm; blade 1-veined, vein prominent abaxially, usually narrowly lanceolate, elliptic, or oblanceolate, 10–17 × 2–4 mm, herbaceous, margins ± flat, herbaceous, dull, ciliate in proximal 1/2, apex acute to acuminate, not pustulate, glabrous; axillary leaf clusters absent. Inflorescences terminal, open, minutely bracteate, 3–45+-flowered cymes. Pedicels erect or ascending in fruit, 3–20 mm, retrorsely pubescent in 2 lines. Flowers: sepals green or often purple, 1-veined, strongly keeled proximally, ovate-lanceolate to narrowly lanceolate (herbaceous portion pale, narrowly lanceolate to linear), 3–4 mm, not enlarging in fruit, apex acuminate, not pustulate, glabrous; petals oblong to broadly elliptic, 2.8–4 mm, 1–11/3 times as long as sepals, apex rounded. Capsules loosely enclosed by calyx, ovoid, 3–3.5 mm, 4/5–1 times as long as sepals. Seeds 7–15, dark brown to black, suborbicular, slightly compressed, 0.6–0.7 mm, shiny, obscurely tuberculate (20×).
Phenology: Flowering late summer–early autumn.
Habitat: Igneous soil on cliffs and ledges
Elevation: 1000-2000 m
Tex., n Mexico (Coahuila).
Arenaria ludens may be more closely related to A. lanuginosa than to A. benthamii, the taxon with which it is often confused, if seed morphology is any indication.