1. Vernonia arkansana DC.

V. crinita Raf.

Pl. 300 g–j; Map 1261

Stems 70–160 cm long, glabrous or nearly so, sometimes minutely hairy toward the tip, often appearing somewhat glaucous. Leaf blades 8–20 cm long, linear to narrowly elliptic-lanceolate, tapered at both ends, the margins sharply toothed or less commonly entire or nearly so, usually appearing somewhat turned under, both surfaces glabrous to sparsely hairy and appearing dotted with minute, impressed resin glands (these sometimes difficult to observe in fresh material, but darkening and becoming more noticeable in dried leaves). Heads with 50–120 florets. Involucre 9–16 mm long, hemispherical or somewhat bell-shaped, the bracts 6–14 mm long, all but the outermost ones linear to narrowly lanceolate, long-tapered and somewhat curled to a threadlike, sharply pointed tip, cobwebby-hairy along the margins and sometimes also minutely hairy on the outer surface, green, the midvein indistinct. Pappus brownish purple, the inner bristles 6–7 mm long, the outer scales 0.7–1.0 mm long. Corollas 10–12 mm long. Fruits 4–5 mm long. 2n=34. July–October.

Scattered in the Ozark and Ozark Border Divisions, and uncommon in the Big Rivers (Kansas and Oklahoma east to Missouri and Arkansas; introduced north- and eastward to Wisconsin and New York). Banks of streams, margins of sloughs, fens, openings of bottomland forests, mesic upland forests, bottomland prairies, and rarely glades; also pastures and roadsides.

Vernonia arkansana is a characteristic species of stream banks and gravel bars in the Ozarks, but it can occur in a variety of other habitats. Hybrids with other Missouri species of Vernonia are relatively easily noted, because V. arkansana is relatively distinctive in its larger heads and elongate involucral bracts. Cora Steyermark (1939) grew seeds harvested from natural Ozarkian populations of V. arkansana (as V. crinita) in a garden, and she concluded that hybridization involving this species was relatively common, as the progeny often exhibited variable and intermediate morphologies. Hybrids with V. baldwinii are especially common (Jones et al., 1970) and often occur where a road near a stream brings drier roadside and moister streamside habitats into close proximity. Harms (1969) studied ironweeds in southeasternmost Kansas and documented a population showing introgression between V. arkansana, V. baldwinii, and V. missurica.