Asclepias cordifolia
Asclepias prostrata
heart-leaf milkweed, purple milkweed
prostrate milkweed
1–20, ascending to spreading, unbranched, 25–100 cm, glabrous, glaucous, rhizomes absent.
2–7, prostrate to decumbent, sometimes branched, 15–30 cm, pilosulous to tomentulose, not glaucous, rhizomes absent.
opposite, sessile, stipular colleters absent;
blade ovate to lanceolate, 3.5–11 × 1.8–8 cm, chartaceous, base cordate, clasping, margins entire, apex acute, mucronate, venation brochidodromous to eucamptodromous, surfaces glabrous, glaucous, margins minutely ciliate, laminar colleters absent.
opposite, petiolate, with 1 stipular colleter on either side of petiole;
petiole 2–3 mm, pilosulous to tomentulose;
blade linear-lanceolate to deltate, 1.8–5 × 0.4–1.8 cm, chartaceous, base truncate to rounded or subcordate, margins crisped, apex acute, mucronate, venation eucamptodromous, surfaces pilosulous to tomentulose, margins ciliate, laminar colleters absent.
terminal, branched, sometimes also extra-axillary at upper nodes, pedunculate, 5–20-flowered;
peduncle 0.3–6.5 cm, apically sparsely pilose to glabrate, with few bracts.
extra-axillary, pedunculate, 3–8-flowered;
peduncle 0.4–2 cm, densely pilosulous to tomentulose, with 1 caducous bract at the base of each pedicel.
16–37 mm, sparsely pilose.
8–14 mm, densely pilosulous to tomentulose.
erect to pendent;
calyx lobes elliptic to lanceolate, 3–5 mm, apex acute, pilose;
corolla red-violet, lobes reflexed with spreading tips, oval, 6–7 mm, apex acute, glabrous;
gynostegial column 1–1.5 mm;
fused anthers brown, cylindric, 1.5–2 mm, wings right-triangular, open at tip, apical appendages deltoid;
corona segments white, tinged pink to red-violet at base, sessile, tubular, 2–3 mm, exceeded by style apex, base saccate, apex truncate, oblique, glabrous, internal appendage absent;
style apex shallowly depressed, pink to red-violet.
erect;
calyx lobes lanceolate, 3–4 mm, apex acute, pilosulous;
corolla green, lobes reflexed, elliptic, 8–11 mm, apex acute, pilosulous abaxially, minutely hirtellous at base adaxially;
gynostegial column 3–3.5 mm;
fused anthers brown, obconic, 2–2.5 mm, wings right-triangular, closed, apical appendages ovate, erose;
corona segments cream, tinged dorsally yellow, green, or pinkish, subsessile, conduplicate-tubular, 5–7 mm, greatly exceeding style apex, apex truncate, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, papillose;
style apex shallowly depressed, green to yellowish.
ovate, 8–9 × 5–7 mm, margin not winged, faces reticulate-rugulose;
coma 3–3.5 cm.
broadly ovate, 7–8 × 5–6 mm, margin corky-winged, erose, faces very sparsely papillose;
coma 1–1.8 cm.
erect on upcurved pedicels, lance-ovoid, 7.5–10.5 × 1.5–1.8 cm, apex long-acuminate, smooth, glabrous, glaucous.
pendent on lax pedicels, ovoid, 3.5–5.5 × 1–1.5 cm, apex acuminate, muricate-ridged, tomentulose.
= 22.
Asclepias cordifolia
Asclepias prostrata
Fresh leaves and stems of Asclepias cordifolia are often slightly or strongly colored bluish, grayish, or purplish. This is one of the few American species of Asclepias with cavitate corona segments that lack adaxial appendages. Such species were segregated along with diverse African species in Gomphocarpus R. Brown, a polyphyletic segregate (M. Fishbein et al. 2011; D. Chuba et al. 2017). Asclepias cordifolia is a distinctive species unlike any other within its range. It is phylogenetically and geographically isolated, although not highly derived morphologically. In Nevada, A. cordifolia is restricted to the Sierra Nevada, in Carson City, Douglas, and Washoe counties. Its range extends to northern California and southwestern Oregon.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Asclepias prostrata is one of the most unusual and poorly known milkweeds in the flora. It was first collected by A. Schott in 1853 during the United States-Mexico border survey, along the Rio Grande (Río Bravo) between Laredo and Ringgold barracks (near Rio Grande City). However, it was not described until much later, from a collection made in Tamaulipas. The species remains rarely collected in both the United States and Mexico, and it is considered extremely rare in Texas (Starr and Zapata counties) and of conservation concern. Many historically known populations in the lower Rio Grande valley have not been relocated in recent years and are presumed extirpated (A. Strong, Texas Parks and Wildlife Department, pers. comm.). Several populations are known to have been eliminated by the widening of highways; others are thought to have been impacted by the spread of the invasive grass, Cenchrus ciliaris Linnaeus. The prostrate habit of A. prostrata cannot be confused with any other species of Asclepias. However, A. prostrata exhibits a remarkable similarity in all vegetative traits, including habit, to two co-occurring asclepiads, Matelea brevicoronata and M. parvifolia, as well as species of Acleisanthes (Nyctaginaceae), particularly the ubiquitous Acleisanthes longiflora. These species form a rather curious assemblage for which there is no hypothesized explanation involving convergent evolution. Since the description of Asclepias prostrata, prostrate species of Matelea occasionally have been misidentified as this species, even far outside its range.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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