55. Asclepias asperula (Decaisne) Woodson, Ann. Missouri Bot. Gard. 41: 193. 1954.
Antelope horns, spider milkweed, spider antelope horns
Acerates asperula Decaisne in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 8: 522. 1844; Asclepiodora asperula (Decaisne) E. Fournier
Herbs. Stems 1–40, erect to decumbent, unbranched or branched at base, 15–60 cm, puberulent with curved trichomes, not glaucous, rhizomes absent. Leaves alternate to subopposite, petiolate, with 1–3 stipular colleters on each side of petiole; petiole 2–4 mm, puberulent with curved trichomes to pilosulous; blade lanceolate to linear, 5–17 × 0.4–3.7 cm, chartaceous, base cuneate, margins entire, apex attenuate to acute, mucronate, venation eucamptodromous to brochidodromous, surfaces puberulent with curved trichomes to glabrate, more densely so on veins, margins ciliate, laminar colleters absent. Inflorescences terminal, sessile or pedunculate, 10–60-flowered; peduncle 0–22.5 cm, puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel. Pedicels 16–30 mm, puberulent with curved trichomes to pilose. Flowers erect to spreading; calyx lobes ovate to linear-lanceolate, 3–5 mm, apex acute, pilosulous to puberulent with curved trichomes; corolla pale green, sometimes tinged red abaxially, campanulate, lobes ascending and exceeding corolla segments, ovate to oval, 7–10 mm, apex acute, puberulent with curved trichomes at apex abaxially, glabrous adaxially; gynostegium sessile; fused anthers brown and green, turbinate, 2–2.5 mm, wings trapezoidal, widest above middle, closed, apical appendages ovate, erose; corona segments reddish purple and white, sessile, clavate-tubular, 4.5–7 mm, slightly exceeded by to equaling style apex, deflexed at base, margins connivent, apex incurved, rounded, upper margin and cavity papillose, internal appendage a low internal crest, papillose; style apex depressed, green. Follicles erect on upcurved pedicels, lance-ovoid, 6–11.5 × 1–2.5 cm, apex short- to long-acuminate, weakly to strongly arcuate, shallowly rugose-ribbed, ribs sometimes muricate, striate, pilosulous. Seeds ovate, 5–8 × 4–6 mm, margin winged, remotely erose, faces minutely rugulose-papillose, minutely hirtellous; coma 2.5–4 cm.
Subspecies 2 (2 in the flora): w United States, Mexico.
With terminal umbels of large, green and purple flowers, Asclepias asperula is highly distinctive and in flower can be confused only with its sister species, A. viridis. Distinguishing characteristics and the existence of interspecific hybrids are discussed under A. viridis. Large bees, notably Bombus and Xylocopa, are commonly observed visiting the flowers of A. asperula.
The subspecies of Asclepias asperula are strongly differentiated away from their region of contact, which extends from south of the Texas Panhandle to the extreme tip of the Oklahoma Panhandle. In the region of contact, their distinguishing traits intermix. The common occurrence of intermediates and apparent introgressants argues against elevation of the subspecies to the specific rank. There is a surprising gap in the distribution of the species as a whole on the Llano Estacado in eastern New Mexico and the Texas Panhandle that may contribute to the differentiation of the subspecies.
