5. Asclepias perennis Walter (white milkweed, smoothseed milkweed)

Pl. 221 h, i; Map 919

Plants with white latex and a fibrous or sometimes slightly woody rootstock. Stems 20–50(–80) cm long, unbranched or more commonly branched, erect or ascending, glabrous or sparsely pubescent with minute hairs in longitudinal lines, with 6 to numerous nodes. Leaves opposite, short-petiolate. Leaf blades 4–15 cm long, 0.5–3.5(–6) cm wide, narrowly lanceolate or narrowly elliptic to elliptic, the base gradually narrowed or tapered, the tip gradually narrowed or tapered to a sharp point, the margins flat, glabrous. Inflorescences 1–8, terminal and in the upper leaf axils, short- to long-stalked, with 8–25 flowers. Calyces reflexed, sparsely and minutely hairy on the outer surface, the lobes 1.0–1.5 mm long, oblong-elliptic. Corollas reflexed, glabrous, white, sometimes tinged with pale pink, the lobes 2.5–4.0 mm long, lanceolate to narrowly elliptic. Gynostegium appearing stalked (the column visible below the bases of the hoods), white, the corona slightly shorter than to about as long as the tip of the anther/stigma head. Corona hoods 2.0–2.8 mm long, ascending, attached near their bases, oblong-ovate in outline, the tips broadly rounded, the margins not toothed, the bases not pouched. Horns attached toward the hood bases, extended conspicuously beyond the tips of the hoods and incurved over the anther/stigma head, linear, not flattened, tapered to a sharp point at the tip. Fruits 4–7 cm long, pendant from usually deflexed stalks, ovate to broadly elliptic-ovate in outline, the surface smooth, glabrous. Seeds with the body 12–17 mm long, the margins broadly winged, the terminal tuft of hairs absent. May–September.

Scattered in the Mississippi Lowlands Division and adjacent Ozarks (southeastern U.S. west to Texas, mostly in the Atlantic and Gulf Coastal Plains, north up the Mississippi and Ohio River valleys). Swamps, sloughs, bottomland forests, margins of ponds and lakes, and occasionally banks of streams, often an emergent aquatic; also ditches and moist roadsides.

Edwards et al. (1994) studied the dispersal ecology of A. perennis and concluded that the drooping rather than erect follicles and large, winged seeds lacking terminal tufts of hair are adaptations to seed dispersal by water, in contrast to wind dispersal of the plumed seeds of most other milkweeds.