2. Aristolochia tomentosa Sims (woolly pipe-vine, Dutchman’s pipe)

Pl. 219 a, b; Map 913

Plants lianas. Aerial stems to 25 m long, twining and climbing on other vegetation, the younger ones ridged and hairy, the older ones often appearing irregularly flattened, the bark ridged, gray to more commonly brown or blackish brown. Petioles 1–5 cm long. Leaf blades 4–20 cm long, ovate to nearly circular, rounded to pointed at the tip, cordate or less commonly truncate at the base, the main veins palmate, the undersurface hairy. Flowers solitary or less commonly paired, appearing axillary near the tips of young branches or at the nodes opposite the leaves. Calyx 2.5–6.0 cm long, densely hairy on the outer surface, the tube strongly hooked or S-shaped, expanded at both ends, pale yellowish green, purple to maroon on the inner surface at the mouth, the lobes spreading, triangular to nearly oblong, bright greenish yellow and glabrous on the inner (upper) surface. Fruits 5–8 cm long, barrel-shaped or somewhat obovoid, strongly 6-ribbed. Seeds 8–10 mm long, strongly flattened, triangular in outline, brown, smooth. 2n=28. May–June.

Relatively common, mostly south of the Missouri River (southeastern U.S. west to Kansas and Texas). Bottomland and mesic upland forests, usually associated with banks of streams and rivers, less commonly on open gravel bars, rarely margins of sand prairies; also open, disturbed, floodplain areas.

Aristolochia tomentosa is a characteristic woody climber along streams and rivers in the Ozarks, where hikers and canoeists often overlook the plants because the leaves and flowers can be located high in the adjacent forest canopy.