18. Euphorbia serpyllifolia Pers. (thyme-leaved spurge)

Chamaesyce serpyllifolia (Pers.) Small

Map 1680, Pl. 381 c, d

Plants annual, with taproots. Stems 5–40 cm long, prostrate to loosely ascending, several- to many-branched, the branches often overlapping (plants sometimes mat-forming), often appearing somewhat flattened or narrowly winged toward the tip, usually yellowish green, glabrous, sometimes slightly glaucous. Leaves opposite, sessile or very short-petiolate. Stipules small scales 0.5–1.5 mm long, these not fused, entire or irregularly fringed or lobed. Leaf blades 3–15 mm long, oblong to oblong-obovate, asymmetrical at the base with the side toward the stem tip usually angled and the other side rounded to shallowly cordate and expanded into a small, rounded auricle, more or less rounded at the tip, the margins minutely few- to several-toothed toward the tip (best observed with magnification), the surfaces glabrous, sometimes with an irregular reddish spot, the undersurface usually pale grayish green. Inflorescences axillary, of solitary cyathia or appearing as small clusters on short axillary branches. Involucre 0.8–1.2 mm long, glabrous, the rim shallowly 4-lobed or 4-toothed, the marginal glands 4, 0.1–0.3 mm long and more or less equal in size, the body narrowly oblong, greenish yellow, with a relatively inconspicuous petaloid appendage 0.1–0.3 mm long, this white or pinkish-tinged. Staminate flowers 5–18 per cyathium. Ovaries glabrous, the styles 0.2–0.5 mm long, each shallowly notched or divided less than 1/3 of the way from the tip into 2 somewhat club-shaped lobes. Fruits 1.5–2.0 mm long, glabrous. Seeds 1.0–1.6 mm long, ovate to oblong-ovate in outline, angular in cross-section, flat to slightly convex at the base, the surface smooth or with 1–4 low, indistinct cross-wrinkles (these not extending the full width of the face), rarely appearing faintly roughened or pitted, white to pinkish white, light tan, or pale purplish-tinged, lacking a caruncle. July–October.

Uncommon, known thus far from a presumably native historical collection from Jackson County and an introduced occurrence in Butler County (western U.S. east to Michigan and Texas; Canada, Mexico; introduced eastward to New Hampshire and Florida). Open, disturbed areas.

Steyermark (1963) was unsure whether the historical occurrence in Jackson County was native or not. The Butler County plants were growing in disturbed ground beneath a bird feeder.