Asclepias subulata
Decaisne in A. P de Candolle and A. L. P. P. de Candolle, Prodr. 8: 571. 1844.
Shrubs. Stems 3–100 (usually few–numerous), erect to ascending, branched, especially in lower half, 50–175 cm, sparsely pilosulous to glabrate, thickly glaucous, rhizomes absent. Leaves ephemeral, often present on flowering stems, opposite (rarely whorled), sessile, with 0 or 1 stipular colleter on each side of leaf base; blade filiform, 2–6 × 0.1 cm, succulent, base cuneate, margins entire, apex acute, mucronate, venation obscure, surfaces pilosulous, laminar colleters absent. Inflorescences extra-axillary, pedunculate, 3–21-flowered; peduncle 0.4–1.7 cm, sparsely pilose, glaucous, with 1 caducous bract at the base of each pedicel. Pedicels 11–21 mm, pilose. Flowers erect to spreading; calyx lobes lanceolate, 4–4.5 mm, apex acute, pilose; corolla green, sometimes tinged cream, yellow, or red, faintly striate, lobes reflexed, lanceolate, 7–12 mm, apex acute, glabrous; gynostegial column 1–1.5 mm; fused anthers brown, cylindric, 2–3 mm, wings right-triangular, distended at base, open at tip, apical appendages ovate; corona segments cream, often tinged pink, yellow, or green, shiny, sessile, tubular, 7–9 mm, greatly exceeding style apex, base saccate, apex truncate, spreading, with proximal flaps, glabrous, internal appendage crested, apically falcate and sharply inflexed towards style apex, barely exserted, minutely papillose; style apex shallowly depressed, cream to green. Follicles pendulous on spreading pedicels, fusiform, 6.5–13.5 × 1–1.8 cm, apex acuminate, smooth, sparsely pilosulous or puberulent with curved trichomes to glabrate, glaucous. Seeds flat to somewhat naviculate, ovate, 6–8 × 4–5 mm, margin narrowly winged, faces papillose-rugulose, concave face with a low keel; coma 1.5–3 cm. 2n = 22.
Phenology: Flowering and fruiting year-round.
Habitat: Arroyos, dunes, hills, slopes, flats, depressions, bajadas, alluvial fans, basalt, granite, rhyolite, caliche, sandy, rocky, and clay soils, desert scrub.
Elevation: 0–1100 m.
Distribution
Ariz., Calif., Nev., Mexico (Baja California, Baja California Sur, Sinaloa, Sonora).
Discussion
Morphological and ecological distinctions between Asclepias subulata and its close relative, A. albicans, are discussed under the latter species. The range of A. subulata is more extensive than A. albicans and is almost exactly confluent with that of the Sonoran Desert, although it extends into the southeastern part of the Mohave Desert. In Nevada, it is restricted to Clark and Lincoln counties. The elongate, tubular, cream corona segments are remarkably similar to those of A. nyctaginifolia. These species were formerly considered close relatives (R. E. Woodson Jr. 1954), but they are highly dissimilar morphologically, other than the corona segments. They appear to be only distantly related (M. Fishbein et al. 2011, 2018), and the corona similarities represent a remarkable convergence. Both species are commonly visited by long-tongued tarantula hawk wasps (Pompilidae, Pepsinae), but it is not known whether they are important pollinators for these milkweeds.
Selected References
None.