1. Perideridia americana (Nutt. ex DC.) Rchb.

Pl. 210 a, b; Map 874

Plants perennial, glabrous, with clusters of mostly 2 or 3 tuberous-thickened roots. Stems 50–120 cm long, erect or ascending. Leaves alternate and occasionally also basal (1 or 2 basal leaves rarely present at flowering), mostly short-petiolate, the uppermost leaves often sessile or nearly so, the sheathing bases not or only slightly inflated. Leaf blades 3–15 cm long (basal leaves sometimes longer), narrowly oblong-ovate to ovate-triangular in outline, pinnately or ternately then pinnately (1–)2–4 times compound or dissected, the leaflets or segments 5–50 mm long, linear to narrowly oblong, mostly narrowed at the base, the margins entire, narrowed to a sharp point at the tip. Inflorescences terminal and axillary, compound umbels, short- to long-stalked. Involucre absent or more commonly of 1–6 bracts, these 1–5 mm long, narrowly elliptic to narrowly triangular, usually shed by fruiting. Rays 6–20, 2–9 cm long. Involucel of 8–14 bractlets, these mostly shorter than the flower stalks, narrowly triangular to narrowly ovate. Flowers 15–25 in each umbellet, the stalks 3–10 mm long, elongated to 16 mm at fruiting. Sepals minute triangular scales. Petals broadly elliptic-obovate, shallowly notched or narrowed or tapered abruptly to a short, slender tip, white. Ovaries glabrous. Fruits 3–5 mm long, elliptic-ovate in outline, rounded at the base, flattened laterally, glabrous, brown to greenish brown, the mericarps narrowed along the commissures, with 5 slender and nervelike ribs, these hardly raised from the surface. 2n=40. April–July.

Scattered in the Ozark and Unglaciated Plains Divisions, with a few populations in the eastern portion of the Ozark Border and Glaciated Plains (Ohio to Tennessee west to Missouri, Oklahoma, and Arkansas). Mesic to dry upland forests, tops and exposed ledges of bluffs, glades, and upland prairies, on calcareous substrates; also roadsides.