11. Asclepias sullivantii Engelm. ex A. Gray (smooth milkweed, prairie milkweed)

Pl. 220 i; Map 925

Plants with white latex and deep-set rhizomes. Stems 40–110 cm long, unbranched or less commonly with 1 or 2 branches toward the tip, erect or ascending, glabrous, often somewhat glaucous, with 7–15 nodes. Leaves opposite, sessile or nearly so. Leaf blades 4–16 cm long, 1.5–9.0 cm wide, narrowly oblong or lanceolate to oblong, oblong-elliptic, or broadly ovate, the base rounded to truncate or shallowly cordate, the tip narrowed to a blunt or sharp point or rounded but usually with a short, sharp point, less commonly nearly truncate or very shallowly notched, the margins flat, the upper surface glabrous, the undersurface glabrous or rarely sparsely and minutely hairy along the midvein or toward the base, often somewhat glaucous. Inflorescences 1–6, terminal and in the upper leaf axils, short- or long-stalked, with 15–40 flowers. Calyces reflexed, glabrous, the lobes 4–6 mm long, lanceolate to ovate. Corollas reflexed, glabrous, light pink to purplish-pink, sometimes tinged with green, the lobes 9–12 mm long, lanceolate to oblong-lanceolate. Gynostegium appearing sessile or very short-stalked (the column sometimes barely visible below the bases of the hoods), pale pink to pink, the corona conspicuously longer than the tip of the anther/stigma head. Corona hoods 5.0–6.5 mm long, loosely ascending to ascending, attached near their bases, oblong to oblong-ovate in outline, the tip rounded to broadly rounded, the margins not toothed, with a pair of small, pouched areas near the base. Horns attached toward the hood bases, relatively short, bent or curved abruptly inward over the anther/stigma head and not extended past the tips of the hoods, sickle-shaped, relatively stout, slightly flattened, tapered to a sharp point at the tip. Fruits 8–10 cm long, erect or ascending from deflexed stalks, narrowly ovate to ovate and slightly arched in outline, the surface with small, warty, spinelike tubercles above the middle, glabrous or minutely hairy. Seeds with the body 7–8 mm long, the margins narrowly or relatively broadly winged, the terminal tuft of hairs white. 2n=22. June–July.

Scattered in northern and western Missouri, mostly in the Glaciated Plains Division (Ohio to North Dakota and Oklahoma; Canada). Bottomland and upland prairies, margins of lakes and marshes, banks of rivers and streams, and less commonly openings of bottomland and mesic upland forests; also pastures, ditches, roadsides, and railroads.