16. Euphorbia prostrata Aiton (groundfig spurge, milk spurge)

Chamaesyce prostrata (Aiton) Small

Map 1678, Pl. 382 d–f

Plants annual, with taproots. Stems 5–40 cm long, usually prostrate, occasionally with ascending tips, several- to many-branched, the branches often overlapping (plants mat-forming), not flattened toward the tip, usually reddish brown, moderately to densely and more or less evenly pubescent with short, incurved hairs toward the branch tips, often nearly glabrous toward the stem base. Leaves opposite, sessile or very short-petiolate. Stipules small scales 0.3–1.0 mm long, these not fused or, on 1 side of the stem, those from the adjacent leaf in each pair fused toward the base into a single small structure positioned between the leaf bases, this irregularly and deeply fringed or lobed. Leaf blades 3–10(–15) mm long, oblong to oblong-ovate, occasionally some of the leaves narrowly oblong, asymmetrical at the base with the side toward the stem tip usually angled or rounded and the other side more or less truncate and expanded into a small, rounded auricle, broadly rounded to occasionally broadly and bluntly pointed at the tip, the margins minutely several-toothed especially toward the tip (best observed with magnification) and usually reddish-tinged, the upper surface glabrous or nearly so and lacking a reddish spot, the undersurface sparsely to moderately pubescent with somewhat tangled, slender hairs and usually pale or light green. Inflorescences axillary, of solitary cyathia or appearing as small clusters on short axillary branches. Involucre 0.4–0.8 mm long, sparsely to moderately hairy on the outer surface, the rim shallowly 4-lobed, the marginal glands 4, 0.1–0.3 mm long and usually more or less equal in size, the body oblong to nearly circular, reddish purple to dark purple, with a relatively inconspicuous petaloid appendage 0.1–0.3 mm long, this white to strongly pinkish- or reddish-tinged. Staminate flowers 2–5 per cyathium. Ovaries hairy, the styles about 0.1 mm long, each divided nearly to the base into 2 slender lobes. Fruits 1.0–1.5 mm long, moderately to densely pubescent with more or less spreading hairs toward the angles, less densely hairy to nearly glabrous between the angles. Seeds 0.8–1.2 mm long, more or less oblong-ovate in outline, angular in cross-section, flat to slightly convex at the base, the surface with 4–7 relatively sharp, slender cross-ridges, light to dark brown, usually with a thin, white to pinkish white coating, this sometimes wearing away irregularly, lacking a caruncle. 2n=18, 20. June–October.

Scattered, mostly in the floodplains of the Missouri and Mississippi Rivers (southern U.S. north to Wyoming, Illinois, and Massachusetts, introduced in most of the northern portion of this range; also native to Mexico, Central America, South America, Caribbean Islands; also introduced in Hawaii and other Pacific Islands, Europe, Africa, Asia). Banks of streams and rivers and sloughs; also levees, cracks and edges of pavement, railroads, roadsides, and open, disturbed areas.

Steyermark (1963) knew this species from a single historical specimen from Jackson County. Since that time, a number of collections have come to light, both previously misdetermined older specimens and newer ones. The native distribution of this widespread weedy species is not well understood.