Polycarpon tetraphyllum subsp. tetraphyllum
Stems prostrate to erect, 3–15 cm. Leaves in whorls of 4 or opposite; stipules 1.8–2.8 mm; blade elliptic, ovate, or obovate, 2–12 mm. Inflorescence bracts absent. Flowers: sepals with keel and apex sometimes purplish, keeled, 1.5–2.5 mm; petals linear to oblong, 0.5–0.8 mm, ca. 1/2 times as long as sepals; stamens 3(–5). Capsules clasped by calyx, broadly ovoid, valve margins rolled inward after dehiscence. Seeds ovoid to lenticular, 0.4–0.8 mm, finely papillate. 2n = 32, 48, 64 (all Europe).
Phenology: Flowering spring–fall.
Habitat: Waste places, cultivated land
Elevation: 0-1400 m
Introduced; B.C., Ala., Calif., Fla., Ga., S.C., Tex., s Europe (Mediterranean region), considered cosmopolitan via introduction, at least in part from ships' ballast.
Historic records are known from Massachusetts [wool waste, North Worcester (Woodward in 1917, GH)] and Pennsylvania [ballast at Philadelphia (Burk in 1880, PH), South Bethlehem, 1890s (T. C. Porter 1892)].