Emory’s milkweed

Podostemma emoryi Greene, Pittonia 3: 237. 1897

Herbs. Stems 1–4, erect to spreading, unbranched or rarely branched near base, 6–30 cm, puberulent with curved trichomes to hirtellous, not glaucous, rhizomes absent. Leaves persistent or gradually caducous from base, opposite, sessile or petiolate, with 1 stipular colleter on each side of petiole; petiole 0–17 mm, puberulent with curved trichomes to hirtellous; blade elliptic to lanceolate or lance-ovate, 3–7.5 × 0.4–2.5 cm, chartaceous, base cuneate, margins often crisped, apex acute, venation eucamptodromous, surfaces hirtellous, usually conduplicate, laminar colleters absent. Inflorescences extra-axillary, sessile or pedunculate, 4–8-flowered; peduncle 0–2 cm, hirtellous, with 1 caducous bract at the base of each pedicel. Pedicels 7–10 mm, hirtellous. Flowers erect; calyx lobes linear-lanceolate to narrowly elliptic, 3–4 mm, apex acute, hirtellous; corolla green, sometimes tinged red or brown, faintly striate, lobes reflexed, elliptic, 5–7 mm, apex acute, hirtellous throughout or glabrate at tips abaxially, glabrous adaxially; gynostegial column 0.5–1 mm; fused anthers green, obconic, 1–1.5 mm, wings trapezoidal, closed, apical appendages ovate; corona segments proximally green, distally white or cream, sessile, tubular, 3.5–5.5 mm, greatly exceeding style apex, apex flared, deeply emarginate, minutely papillose, internal appendage lingulate, sharply incurved, at the same level as and closing the segment apex, minutely papillose. Follicles erect on upcurved pedicels, lance-ovoid, 5–9 × 1.2–2 cm, apex attenuate to acuminate, smooth, puberulent with curved trichomes to hirtellous, sometimes faintly striate. Seeds oval, 7 × 5–6 mm, margin winged, faces smooth; coma 2.5–3.5 cm.

Flowering Mar–Aug(–Oct); fruiting Jul–Nov. Plains, hills, slopes, limestone, caliche, sandy, clay, rocky, calcareous, and gravelly soils, prairies, mesquite grasslands, thorn scrub; 0–800 m; Tex.; Mexico (Coahuila, Nuevo León, San Luis Potosí, Tamaulipas).

Asclepias emoryi is distributed entirely within the range of its close relative, A. oenotheroides. Distin­guishing them is discussed under the latter species. A few putative hybrid specimens have been collected. These can be distinguished from A. emoryi by slightly longer corona segments (usually shorter than in A. oenotheroides) with sinuate apices, slightly longer corolla lobes, and slightly broader leaves. Although not accorded conservation concern, A. emoryi is very rarely encountered (across its entire range) and merits study for evaluation of needed protections. Reports of A. emoryi from New Mexico are based upon misidentifications. It is restricted in the flora area almost entirely to southern Texas, but there are a few scattered occurrences to the northwest.