Cynanchum arizonicum (A. Gray) Shinners; Metastelma watsonianum Standley

Stems woody and corky at base, densely puberulent in single line. Leaves: 1 stipular colleter on each side of petiole; petiole 1–2 mm, puberulent; blade linear to narrowly lanceolate, 7–20 × 1.5–4 mm, chartaceous, single-veined, base rounded, with 1–2 laminar colleters, margins revolute, exaggerated on drying, apex acute, mucronulate, surfaces glabrous abaxially, minutely puberulent adaxially, primarily on midvein and near margins. Inflorescences umbelliform, sessile, 3–4-flowered. Pedicels 1–1.5 mm, puberulent. Flowers: calyx lobes linear-lanceolate, apices attenuate, 2–2.5 mm, puberulent, with 1 colleter per sinus; corolla cream, campanulate, tube 0.5–1 mm, lobes erect with spreading and recurved tips, opening not obscured, linear-lanceolate, 2.5–3.5 mm, abaxially glabrous, adaxially densely long-villous marginally and apically with erect, opaque, flattened hairs, densely hispid centrally with downward-pointing acicular trans­lucent hairs; corona segments united to base of column, laminar, subulate, 1.5–2 mm, slightly longer than gyno­stegium; gynostegial column 0.5–0.7 mm; style apex convex. Follicles dark brown, 4–6 × 0.3–0.5 cm, apic­ally acuminate, glabrous. Seeds 10–16, brown, flat to somewhat navicular, 5–6 × 2–3 mm, broadly winged, minutely tuberculate; coma white, 2–2.5 cm.

Flowering year-round, especially following winter and summer rains; fruiting year-round. Rocky substrates, val­leys, canyons, alluvial plains, arroyos, hills, desert scrub (desert grasslands); 400–1400 m; Ariz.; Mexico (Sonora).

Metastelma arizonicum is a characteristic species of the upland margins of the Sonoran Desert. In Arizona, it is known from Cochise, Maricopa, Pima, and Pinal counties. Populations are widely spaced, and the plants are easily overlooked.