Asclepias tuberosa |
Asclepias albicans |
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asclépiade tubéreuse, butterfly milkweed |
candelilla, wax milkweed, white-stem milkweed |
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Habit | Herbs. | Shrubs. | ||||||||
Stems | 1–numerous, erect to ascending, branched in inflorescence, 15–90 cm, densely hirsute, not glaucous, rhizomes absent. |
5–40 (usually 1–few), erect to ascending, branched, especially in lower half, 140–400 cm, sparsely pilose to glabrate, thickly glaucous, rhizomes absent. |
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Leaves | alternate, petiolate, with 1 or 2 stipular colleters on each side of petiole; petiole 1–4 mm, densely hirsute; blade elliptic, oblong, or oblanceolate to lanceolate or linear, 2–12 × 0.3–3 cm, chartaceous, base cuneate or obtuse to rounded, truncate, hastate, or cordate, margins entire, apex acute to attenuate or obtuse to rounded, venation brochidodromous to eucamptodromous, surfaces hirsute, more densely so on midvein abaxially, margins ciliate, 0–4 laminar colleters. |
ephemeral, rarely present on flowering stems, opposite, sessile, with 0 or 1 stipular colleter on each side of leaf base; blade filiform, 1.5–2.5 × 0.1 cm, succulent, base cuneate, margins entire, apex acute, mucronate, venation obscure, surfaces pilosulous, laminar colleters absent. |
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Inflorescences | corymbs of extra-axillary umbels on branches, sessile or pedunculate, 5–27-flowered; peduncle 0–4 cm, sometimes branched, hirsute, with 1 caducous bract at the base of each pedicel. |
terminal, branched, also extra-axillary at leafless upper nodes, pedunculate, 8–50-flowered; peduncle 0.2–4.5 cm, pilose, with 1 caducous bract at the base of each pedicel. |
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Pedicels | 9–24 mm, puberulent with curved trichomes to pilosulous. |
8–16 mm, pilose. |
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Flowers | erect; calyx lobes narrowly lanceolate, 2–3 mm, apex acute, hirsute to puberulent with curved trichomes; corolla reddish orange (nearly red) to orange or yellow, lobes reflexed with spreading tips, narrowly elliptic, (5–)6–8 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 1.2–1.5 mm, fused anthers yellow to yellowish green, cylindric, 2–3 mm, wings right-triangular, closed, apical appendages ovate; corona segments reddish orange (nearly red) to orange or yellow, substipitate, conduplicate, dorsally flattened, sulcate, 5.5–7 mm, greatly exceeding style apex, apex acute, glabrous, internal appendage subulate, exserted, arching above style apex, glabrous; style apex shallowly depressed, yellow to yellowish green. |
erect to pendent; calyx lobes lanceolate to linear-lanceolate, 2.5–3 mm, apex acute, densely pilose; corolla ochroleucous to greenish cream, sometimes tinged red, faintly striate, lobes reflexed or sometimes spreading, oval, 4.5–6 mm, apex acute, glabrous; gynostegial column 1–1.8 mm; fused anthers brown, cylindric, 1.8–2 mm, wings right-triangular, closed, apical appendages ovate, erose; corona segments cream, often tinged pink, yellow, or green, shiny, sessile, conduplicate, 2–3 mm, exceeded by style apex, base saccate, apex truncate, oblique, glabrous, internal appendage falcate, exserted, sharply inflexed towards style apex, densely papillate; style apex shallowly depressed, ochroleucous to green. |
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Seeds | ovate, 8–9 × 4–5 mm, margin winged, faces minutely rugulose; coma 3–5 cm. |
naviculate, lanceolate, 5–7 × 2.5–3 mm, margin narrowly winged, faces papillose and rugulose, concave face with a low keel; coma 1.5–2 cm. |
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Follicles | erect on upcurved pedicels, fusiform, 7–14 × 1.2–2 cm, apex long-acuminate or attenuate, smooth, hirsute. |
spreading to pendulous on spreading to pendulous pedicels, fusiform to lance-ovoid, 5.5–12 × 0.7–1.8 cm, apex acuminate, smooth, pilosulous. |
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2n | = 22. |
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Asclepias tuberosa |
Asclepias albicans |
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Phenology | Flowering and fruiting year-round. | |||||||||
Habitat | Mountain slopes, ridge tops, bajadas, flats, arroyos, granite, basalt, tuff, coarse rocky soils, cracks in boulders, sand, desert scrub. | |||||||||
Elevation | 50–800 m. (200–2600 ft.) | |||||||||
Distribution |
North America; n Mexico
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AZ; CA; Mexico (Baja California, Baja California Sur, Sonora)
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Discussion | Subspecies 3 (3 in the flora). Asclepias tuberosa is one of the most familiar and beloved North American milkweeds and is a favored element of pollinator gardens because of the cheery orange flowers that attract abundant insect visitors. The clear latex is unusual in the genus and is often commented upon by collectors. The subspecies of A. tuberosa are highly intergrading. It is often difficult to satisfactorily place a given specimen in a particular subspecies; however, the great majority are readily assigned. It appears that the conspicuous variation in leaf morphology across the subspecies corresponds to genetically structured population variation (R. E. Woodson Jr. 1947). However, it is unknown to what extent cultivation and other human activities have blurred the distinctions among the geographic variants. Future recognition of the subspecies should be supported by genetic study with modern techniques. Although yellow-flowered plants predominate in the western plains, color variation is often pronounced in single populations, and yellow flowers may be encountered anywhere in the range. Hybridization with A. syriaca is documented, but is exceedingly rare. Presumed hybrids can be recognized by intermediate floral and vegetative traits. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Asclepias albicans reaches the greatest height of any Asclepias species in the flora area and among congeners can be confused only with A. subulata, the only other shrubby, leafless milkweed in the region. In the absence of flowers, it may be difficult to distinguish these species. In addition to the key characters, A. albicans often differs by thicker, waxier stems. The species are also ecologically divergent: A. albicans is most commonly found on rocky slopes and ridges (usually basalt or granite), and A. subulata is usually found on flats and in arroyos, often in sandy soils. However, A. albicans can occur on bajadas and in arroyos, often distant from the mountains and ridges harboring source populations. These species have extensively overlapping ranges but rarely hybridize at widely scattered locations in Arizona, California, and Baja California Sur. Hybrids are identified by intermediate floral morphology, especially corona size and shape. The range of A. albicans is within the limits of the Sonoran Desert. In Arizona, it is found in La Paz, Maricopa, Mohave, Pima, Pinal, and Yuma counties; in California only in Imperial, Riverside, San Bernardino, and San Diego counties. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
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Key |
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Source | FNA vol. 14. | FNA vol. 14. | ||||||||
Parent taxa | Apocynaceae > Asclepias | Apocynaceae > Asclepias | ||||||||
Sibling taxa | ||||||||||
Subordinate taxa | ||||||||||
Name authority | Linnaeus: Sp. Pl. 1: 217. (1753) | S. Watson: Proc. Amer. Acad. Arts 24: 59. (1889) | ||||||||
Web links |