Asclepias stenophylla
Asclepias ruthiae
narrow-leaf milkweed, slimleaf milkweed
Ruth's milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
1–8, decumbent, unbranched or branched near base, 9–10 cm, densely puberulent with curved trichomes, not glaucous, rhizomatous.
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.
proximally opposite, distally alternate, petiolate, with 0 or 1 stipular colleter on either side of petiole;
petiole 2–5 mm, densely puberulent with curved trichomes;
blade ovate to lanceolate, 2–3 × 0.8–2 cm, chartaceous, base truncate to cuneate, margins entire, apex acute, venation eucamptodromous, surfaces persistently pilosulous, more densely so on veins, margins densely 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 and extra-axillary at upper nodes, sessile, 2–6-flowered, bracts few.
5–12 mm, puberulent with curved trichomes to pilosulous.
15–25 mm, densely pilosulous to tomentulose.
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;
calyx lobes elliptic, 2–2.5 mm, apex acute, pilose to tomentulose;
corolla red-violet, lobes reflexed with spreading tips, oval, 4–5 mm, apex acute, glabrous;
gynostegial column 0.2–0.5 mm;
fused anthers brown, cylindric, 1.5–2 mm, wings right-triangular, apical appendages ovate;
corona segments red-violet dorsally, white proximally, sessile, cupulate, 1.5–2 mm, slightly exceeded by style apex, apex truncate with a proximal tooth on each side, glabrous, internal appendage falcate, included in cavity, glabrous;
style apex shallowly depressed, pink to red-violet.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
broadly ovate, 8–9 × 6–7 mm, margin corky-winged, erose, faces rugulose, minutely hirtellous;
coma 1–1.5 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
erect to spreading on upcurved to spreading pedicels, ovoid, 3–3.5 × 0.7–1 cm, apex acuminate, smooth, pilosulous to tomentulose.
Asclepias stenophylla
Asclepias ruthiae
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.)
As discussed under Asclepias eastwoodiana, A. ruthiae is part of a complex of four species sometimes recognized as a single species. It is endemic to a small area in southeastern Utah (Emery, Sevier, and Wayne counties) on the periphery of the San Rafael Swell, the northeastern margin of Waterpocket Fold, and valleys north of the Henry Mountains. Reports from Arizona, New Mexico, and Monument Valley in Utah represent records of A. sanjuanensis. It commonly co-occurs with the similar A. macrosperma in this region and differs from the latter species by its more diminutive dimensions, red-violet corolla, and smaller, smooth fruit. Conservation status of this species requires re-assessment in light of the recent recognition of its more limited range.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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