Sonoran milkweed vine

Rothrockia cordifolia A. Gray, Proc. Amer. Acad. Arts 20: 295. 1885; Matelea cordifolia (A. Gray) Woodson

Stems: bark tan, corky, shal­lowly furrowed. Leaves: 1 stip­ular colleter on each side of pet­iole; petiole 1–4.5 cm; blade ovate, 2.5–8 × 2–6 cm, charta­ceous, base deeply cordate, laminar colleters 2–4, mar­gins planar, apex acute to acuminate, ven­ation pinni­palmate, surfaces densely puberulent to hirsute, paler abaxially. Inflorescences umbelliform or racemiform, 4–6-flowered; peduncle 1–4 cm. Pedicels 5–10 mm. Flowers: calyx lobes lanceolate-ovate, apices acute, 3–5 mm, hirsutulous; corolla tube 3–4 mm, lobes ascending, sometimes twisted, linear-lanceolate to oblong, 6–12 mm, glabrous; corona segments basally connate and united to base of column, green, laminar, fleshy, ovate with minute teeth, much shorter than gynostegium; gynostegium sessile; style apex prolonged into bifid beak, papillose. Follicles 4.5–13.2 × 1–1.5 cm, apically attenuate. Seeds 100–150, reddish brown, 6 × 4 mm, granulate-tuberculate; coma white, 3–4 cm.

Flowering Jul–Sep; fruiting Oct–Mar. Arroyos, can­yons, hills, slopes, plains, rock outcrops, granite, sandy and gravelly soils, desert scrub; 200–300 m; Ariz.; Mexico (Baja California, Baja California Sur, Sonora).

Polystemma cordifolium is widely distributed in the drier parts of the Sonoran Desert and reaches its northern limit in Organ Pipe Cactus National Monument (Pima County), where it may no longer be extant (R. S. Felger et al. 2014). It should be considered to be of conservation concern in the flora area.