15. Euphorbia obtusata Pursh (blunt-leaved spurge)

Map 1677, Pl. 380 f–h

Plants annual, with taproots. Stems 20–70 cm long, erect or ascending, unbranched below the inflorescence or occasionally few-branched, the branches not flattened toward the tip, usually green to yellowish green, sometimes faintly purplish-tinged, glabrous. Leaves alternate above the lowest node and below the inflorescence branches (those of the inflorescence branches usually opposite, those of the basal node opposite or whorled but absent at flowering), sessile. Stipules absent. Leaf blades 10–45 mm long, oblanceolate to oblong-oblanceolate (those of the leaves of the inflorescence branches broadly ovate to broadly ovate-triangular), unlobed, rounded to truncate or shallowly cordate at the base and often somewhat clasping the stem, rounded or broadly angled to a bluntly pointed tip, the margins finely toothed mostly above the midpoint (the teeth sometimes minute and visible only with magnification), the surfaces glabrous, yellowish green to green. Inflorescences terminal umbellate panicles with a whorl of leaves at the base and each of the usually 3 primary branches often branched 1–3 additional times, the cyathia solitary at the branch tips and at the branch points. Involucre 1.2–1.5 mm long, glabrous, the rim shallowly 4- or 5-lobed to nearly entire, the marginal glands 4 or 5, 0.3–0.6 mm long, oblong-oval to elliptic or slightly kidney-shaped, usually red or reddish-tinged (occasionally greenish yellow), lacking a petaloid appendage. Staminate flowers 5–10 per cyathium. Ovaries glabrous, but the surface densely warty, the styles 0.7–1.2 mm long, each divided 1/3–1/2 of the way from the tip into 2 slightly club-shaped lobes. Fruits 3.0–3.5 mm long, glabrous but the surface finely warty. Seeds 1.7–2.3 mm long, broadly elliptic-ovate to nearly circular in outline, slightly biconvex in cross-section, rounded at the base, the surface smooth, reddish brown to dark purplish brown but often appearing slightly glaucous, with a pale, irregularly winglike caruncle, this often breaking off as the seeds are dispersed. May–July.

Scattered mostly in the Ozark and Ozark Border Divisions (Iowa to Texas east to Pennsylvania and South Carolina). Banks of streams and rivers and bottomland forests; also rarely fallow fields, ditches, railroads, roadsides, and open, disturbed areas.

The taxonomic status of this species is not entirely understood. Some authors include it in a broadly circumscribed E. spathulata.

Steyermark (1963) included E. platyphyllos L. (under the variant spelling E. platyphylla) in the Missouri flora with some reservations. He noted that a collection by B. F. Bush from Barry County was originally misdetermined as this species, but it instead represented material of E. spathulata, which left only a collection made by Earl E. Sherff in the city of St. Louis, and Steyermark admitted this was possibly from a cultivated plant. In his introduction to the Missouri flora, Steyermark (1963) discussed the series of specimens collected by Sherff in St. Louis that lack any further label data, noting that most of these originated from cultivated plants, and thus he excluded from the flora most of the species documented from Missouri only by these collections. As E. platyphylla has not been collected in the state since that time, the species is here excluded from the flora.

Euphorbia platyphyllos (Pl. 383 c, d)is a European species that has been introduced sporadically into the eastern (mostly northeastern) and central United States. It is morphologically most similar to E. obtusata but differs in its basally fused styles, each shallowly 2-lobed toward the tip. It differs from E. spathulata in its smooth seeds (vs. the surface with a network of raised ridges). It further differs from both of these species in its more sharply pointed leaf blades that are somewhat hairy on the undersurface and in the cyathia with a usually hairy involucre.