Dune or flat-seeded spurge, flatseed sandmat Dune or flat-seeded spurge, flatseed sandmat

Chamaesyce platysperma (Engelmann) Shinners

Herbs, annual or short-lived perennial, with slender taproot. Stems prostrate, spreading and often mat-forming, 10–100 cm, glabrous. Leaves opposite; stipules usually distinct, occasionally connate basally, rarely to middle, subulate, narrowly triangular, or divided into 2–4 subulate segments, 0.5–1.1 mm, glabrous; petiole 1–3.6 mm, glabrous; blade oblong to obovate, 5–12 × 3–5 mm, base subsymmetric, cuneate to attenuate, margins entire, apex usually acute to mucronulate, rarely obtuse, surfaces glabrous; only midvein conspicuous. Cyathia solitary at distal nodes; peduncle 1.6–4.1 mm. Involucre campanulate to obconic, 1.5–2 × 1.3–2.5 mm, glabrous; glands 4, yellowish, subcircular to oblong, 0.5–0.6 × 0.5–0.6 mm; appendages white, ovate to oblong or almost triangular, 0.3–0.6 × 0.3–0.8 mm, distal margin entire or shallowly 2–3-lobed. Staminate flowers 45–50. Pistillate flowers: ovary glabrous, styles 0.4–0.6 mm, 2-fid 1/2 length. Capsules ovoid to ellipsoid, 2.7–3.2 × 2.2–2.9 mm, glabrous; columella 2.6–2.8 mm. Seeds whitish, pinkish, or light brown, ellipsoid-oblong, weakly dorsiventrally compressed and semielliptic in cross section, 2.2–2.5 × 1.3–1.6 mm, with sharp linelike longitudinal ridge on adaxial side, smooth and rounded on back.

Flowering and fruiting year-round in response to sufficient moisture. Sand dunes in Sonoran Desert scrub; of conservation concern; 60–200 m; Ariz., Calif.; Mexico (Baja California, Sonora).