15. Asclepias verticillata L. (whorled milkweed, horsetail milkweed)

Pl. 222 b; Map 929

Plants with white latex and a fibrous rootstock. Stems 20–60(–90) cm long, occasionally few-branched toward the tip and with axillary branchlets with pairs or clusters of leaves absent or rare, mostly erect or ascending, sparsely short-hairy in longitudinal lines, with numerous nodes. Leaves all or nearly all in whorls of 3–6, sessile. Leaf blades 2–8 cm long, 0.5–2.5 mm wide, linear, the base narrowed or tapered, the tip narrowed or tapered to a sharp point, the margins curled under, glabrous or sparsely and minutely hairy. Inflorescences (1)2–10 in the leaf axils, mostly short-stalked, with 6–20 flowers. Calyces reflexed, glabrous or sparsely short-hairy on the outer surface, the lobes 1.5–2.5 mm long, narrowly lanceolate to ovate. Corollas reflexed, glabrous, white to pale green (usually with age), sometimes slightly purple-tinged at the tip, the lobes 3.5–5.0 mm long, elliptic. Gynostegium appearing stalked (the column visible below the bases of the hoods), pale green to white, the corona slightly shorter than to about as long as the tip of the anther/stigma head. Corona hoods 1.5–2.0 mm long, erect, attached toward their bases, broadly oblong in outline, the tips broadly rounded, the margins not toothed, the bases not pouched. Horns attached near 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 7–10 cm long, erect or ascending from erect to deflexed stalks, narrowly elliptic-lanceolate in outline, the surface smooth, glabrous or minutely hairy. Seeds with the body 5–6 mm long, the margins narrowly to somewhat more broadly winged, the terminal tuft of hairs white. 2n=22. May–September.

Scattered nearly throughout the state, but nearly absent from the Mississippi Lowlands Division (eastern U.S. and adjacent Canada west to Montana and Arizona). Upland prairies, savannas, glades, exposed ledges and tops of bluffs, and less commonly dry upland forests; also pastures, roadsides, and railroads.