Asclepias stenophylla
Asclepias asperula
narrow-leaf milkweed, slimleaf milkweed
Antelope horns, spider milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
1–40, erect to decumbent, unbranched or branched at base, 15–60 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
alternate, subopposite, or opposite, sessile or petiolate, with 1 stipular colleter on each side of petiole;
petiole 0–1 mm, spreading to ascending, glabrate;
blade linear, conduplicate, 5–16 × 0.1–0.5 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, surfaces sparsely puberulent with curved trichomes, especially on midvein, to glabrate, margins ciliate, laminar colleters absent.
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.
extra-axillary, sessile or pedunculate, 9–28-flowered;
peduncle 0–1.3 cm, sometimes branched at apex, puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel.
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.
5–12 mm, puberulent with curved trichomes to pilosulous.
16–30 mm, puberulent with curved trichomes to pilose.
erect to pendent;
calyx lobes lanceolate, 1.5–2.5 mm, apex acute, puberulent with curved trichomes to pilosulous;
corolla pale green to greenish cream, lobes reflexed with spreading tips, elliptic, 3–5 mm, apex acute, minutely puberulent with curved trichomes at apex abaxially, glabrous adaxially;
gynostegial column 0–0.5 mm, fused anthers green, truncately obconic, 1.5–2 mm, wings narrowly crescent-shaped, wide open at base, apical appendages deltoid;
corona segments cream, often green-tinged, sessile, chute-shaped, margins incurved, appressed to anthers, 3–3.5 mm, equaling style apex, base saccate, auriculate, apex truncate, glabrous, internal appendage a short crest, the segment appearing 3-toothed, glabrous;
style apex shallowly depressed, green.
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.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
ovate, 5–8 × 4–6 mm, margin winged, remotely erose, faces minutely rugulose-papillose, minutely hirtellous;
coma 2.5–4 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
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.
Asclepias stenophylla
Asclepias asperula
Although Asclepias stenophylla is a distinctive species, it is difficult to distinguish from A. engelmanniana in the absence of flowers or fruits, where their ranges overlap in the Great Plains. The drooping leaves of A. engelmanniana can reliably distinguish that species from A. stenophylla. Asclepias stenophylla is also often mistaken for A. verticillata, but the nearly appendageless corona segments and alternate or opposite (versus whorled) leaves readily separate A. stenophylla from that species. Because of its slender habit, linear leaves, and small clusters of greenish cream flowers held close to the stem, it can be overlooked in its grassland habitats. Asclepias stenophylla is widespread and common in its core habitat of Ozark glades and dry sites in tallgrass in Missouri, and in mixed-grass prairies from South Dakota to Texas. It is quite rare at the margins of its range in Arkansas (Baxter County), Illinois (Adams, Calhoun, and Pike counties), Iowa (Guthrie, Plymouth, and Sioux counties), Louisiana (Winn Parish), Minnesota (Houston County), Montana (Carter County), and Wyoming (Crook and Weston counties). In Colorado, it exhibits an interesting disjunction between Yuma County in the east and the Front Range of the Rocky Mountains, where it is sporadic, but impacted by development and considered to be of conservation concern. A report from North Dakota has not been confirmed.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Subspecies 2 (2 in the flora).
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.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Corona segments reddish purple with white upper margin, 5–7 mm; follicle ribs inconspicuously muricate at apex or not at all. | subsp. asperula |
1. Corona segments white, dorsally reddish purple, 4.5–6 mm; follicle ribs sparsely to densely muricate for most of the length. | subsp. capricornu |
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