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FNA | Family List | FNA Vol. 14 | Apocynaceae

26. Asclepias Linnaeus, Sp. Pl. 1: 214. 1753; Gen. Pl. ed. 5, 102. 1754.

Milkweed [Greek Asklepios, mythological chief physician, perhaps alluding to therapeutic properties of Vincetoxicum or a related taxon of the Old World, applied to present taxon by Linnaeus]

Mark Fishbein

Acerates Elliott; Anantherix Nuttall; Asclepiodella Small; Asclepiodora A. Gray; Biventraria Small; Oxypteryx Greene; Podostemma Greene; Podostigma Elliott; Solanoa Greene

Herbs, subshrubs, or shrubs, rhizomatous or not; latex milky (clear). Stems not twining, unarmed, pubescent or glabrous. Leaves persistent (caducous), opposite, subopposite, alternate, or whorled, petiolate or sessile; stipular colleters interpetiolar (absent); laminar colleters present or absent. Inflorescences extra-axillary or terminal, umbelliform, pedunculate or sessile. Flowers: calycine colleters present or absent; corolla green, white, cream, pink, red, maroon, purple, orange, or yellow, rotate with lobes usually reflexed to spreading (campanulate, tubu­lar), aestivation valvate; coralline corona absent; androecium and gynoecium united into a gynostegium adnate to corolla tube; gynostegial corona of 1 whorl of 5 thick, cupulate, laminar, or clavate segments, often bearing an adaxial falcate, subulate, acicular, lingulate, or crestlike appendage; anthers adnate to style, locules 2; pollen in each theca massed into a rigid, vertically oriented pollinium, pollinia lachrimiform, joined from adjacent anthers by translators to a common corpusculum and together forming a pollinarium. Fruits follicles, usually solitary, erect (spreading) on upcurved or straight pedicels or pendulous, green to brown, fusiform, lance-ovoid, ovoid, or ellipsoid, smooth (ornamented), pubescent or glabrous. Seeds ovate or lanceolate to nearly orbiculate, flattened (naviculate in A. albicans, A. linaria; flat to somewhat naviculate in A. subulata), usually winged, not beaked, comose (coma absent in A. perennis), not arillate; comas white. x = 11.

Species ca. 400 (77 in the flora): North America, Mexico, Central America, South America, West Indies, Africa; introduced in Europe and pan-tropical areas.

Species of Asclepias are ecologically diverse, with habitats ranging from marshes and swamps to some of the driest regions in the North American and African deserts. They occur from sea level to over 3000 m elevation. However, the genus has diversified most dramatically in grasslands, savannas, woodlands, and forests, in seasonally dry, temperate and subtropical climates, and it is in these ecoregions that milkweeds are most conspicuous. All species are perennial, and the great majority are seasonally dormant herbs. Most species occur in environments subject to one or more of the following disturbances: extended freezing, drought, fire, or mammalian grazing. Nonetheless, the species are diverse in growth form and phenology—habit ranges from diminutive herbs less than 5 cm tall to herbs and shrubs exceeding 2 m in height, and flowering ranges from constrained two-week periods to year-round.

Species of Asclepias have been investigated for development of commercial products. Pro­duction of natural rubber from latex has not proved economical, even during the time of World War II shortages. There have been repeated efforts to develop an industry around harvesting seed comas as floss for stuffing and insulation. Mechanization has proved challenging, but a small industry is developing, based largely on A. syriaca because of its rapid growth and high seed production. Asclepias species have been used as a source of medicine by most Native American tribes for diverse ailments. One of the common names of A. tuberosa is pleurisy root, and the tuberous roots of this species have been included in herbal remedies for coughing and congestion. This seems to be the only commercialization of American milkweeds for medicinal purposes, and no clinical trials have taken place in North America involving milkweed-derived compounds. Despite toxicity, there is a tradition in native and immigrant American communi­ties, particularly in the Appalachian region, of using young shoots of A. syriaca as a vegetable, following boiling with two changes of water.

Species such as Asclepias curassavica, A. incarnata, and A. tuberosa are widely cultivated, and horticultural varietals are available. Recently, the popularity of diverse species of Asclepias as components of home and commercial gardens, as well as synthetic prairies, is skyrocketing, fueled in part by interest in growing food plants for monarch butterflies, or more generally as a nectar source for the conservation of pollinator communities. Surprisingly, A. subulata, found naturally on arid, sandy substrates, grows rapidly in standard potting media and is becoming ubiquitous in horticultural settings in Arizona. At least 25 species are available from nurseries across the flora area.

In its broad sense, Asclepias is the largest genus of Apocynaceae. Attempts have been made to segregate the African species into some 20 genera, but these attempts have left a residuum of African species in Asclepias that are more closely related to segregate African genera than to the type-bearing American species (D. J. Goyder et al. 2007; Goyder 2009; D. Chuba et al. 2017). Hence, the broader circumscription is adopted here, although the American species as a whole (ca. 130 species) are monophyletic, and circumscription does not affect the nomenclature of the species in the flora.

Two aspects of the ecology of Asclepias have received extensive study—reproductive and defensive strategies. As with Asclepiadoideae as a whole, Asclepias species have elaborate and precise insect pollination involving pollinia and coronas that mediate interactions with pollinators. Asclepias has been the most intensively studied genus in the subfamily with regard to the diversity of the pollinating fauna and forces that have shaped the evolution of floral traits. Whereas pollinator interactions in Asclepiadoideae genera vary widely from highly specialized to highly generalized (J. Ollerton et al. 2019), those species of Asclepias that have been studied quantitatively are moderately to highly generalized, including such species as A. tuberosa, commonly known as butterflyweed (M. Fishbein and D. L. Venable 1996). However, such studies are limited to fewer than a dozen species, and the existence of more specialized pollination seems likely for more rare and poorly known species. Species of Asclepias have also served as a model system for testing the hypothesis that the characteristically low fruit set in the genus is a consequence of sexual selection to increase flower number through competition among plants as pollen donors (M. F. Willson and B. J. Rathcke 1974; S. B. Broyles and R. Wyatt 1990; Fishbein and Venable 1996b).

The chemical ecology of Asclepias also has been widely studied because a class of secondary metabolites, cardiac glycosides (cardenolides), which is found in many Apocynaceae species, is sequestered by several specialist Asclepias-feeding insect herbivores and used as their own defense compounds (A. A. Agrawal et al. 2012). The interaction between Asclepias species and the monarch butterfly, Danaus plexippus, has been most intensively studied (L. P. Brower et al. 1967). Subsequently, the diversity of defensive traits of milkweeds (cardenolides, latex, trichomes, waxes, etc.) has been analyzed in an evolutionary context, considering also the functions of some of these traits as adaptations for drought resistance (Agrawal et al. 2009; M. Fishbein et al. 2018).

In the key and descriptions the following conventions are used. Leaf measurements pertain to midstem leaves, usually at the node below the first inflorescence. The lowest leaves and those in the flowering portion of the plant are commonly much smaller than midstem leaves and are less useful for identification. Vestiture of stems and leaves (unless otherwise specified) is described for mature structures below the inflorescences; many species possess trichomes on stems and leaves early in development that are lost with maturity but continue to be produced on new structures in the flowering portion of the plant. The gynostegial column refers to the cylindrical structure formed by the united filaments; its length is measured from its attachment to the corolla to the base of the anthers. The corona of Asclepias consists largely of five discrete segments that are usually cavitate and are often referred to as hoods. In some species, these segments are strongly compressed dorsally and the cavity is greatly reduced. In others, the margins of the segments are folded in, obscuring the cavity and creating a club-shaped form. The majority of species also have an adaxial appendage of each segment, often referred to as a horn, which arises from within the cavity and is often exserted. In some species, the appendage is merely a crest within the cavity, and it may be altogether absent. The diversity of corona forms is difficult to describe, but these features are highly useful for identification. They are ideally observed in fresh material, and photographs are invaluable.

A revised infrageneric classification of Asclepias is needed. The species in this treatment are arranged according to phylogenetic relationships obtained from analyses of plastid genomes (M. Fishbein et al. 2018) and a set of 768 nuclear loci (J. Boutte et al. 2019). These correspond largely to the major clades presented by Fishbein et al. (2011). Species 1–3 are early-diverging lineages, species 4–6 belong to the Sonoran Desert clade, species 7–16 belong to the Incarnatae clade, species 17–22 are not placed in a major clade, species 23–27 belong to the Mexican highland clade, and species 28–77 belong to the temperate North American clade. Within the temperate North American clade, several subclades are strongly supported: species 37 in the waxy clade, species 39–42 in the Podostemma clade, species 51–56 in the Colorado Plateau clade, species 57–60 in the dwarf clade, and species 61–77 in the eastern temperate clade.

SELECTED REFERENCES Fishbein, M. et al. 2011. Phylogenetic relationships of Asclepias (Apocynaceae) inferred from non-coding chloroplast DNA sequences. Syst. Bot. 36: 1008–1023. Goyder, D. J., A. Nicholas, and S. Liede-Schumann. 2007. Phylogenetic relationships in subtribe Asclepiadinae (Apocynaceae: Asclepiadoideae). Ann. Missouri Bot. Gard. 94: 423–434. Kephart, S. R., R. Wyatt, and D. Parella. 1988. Hybridization in North American Asclepias. I. Morphological evidence. Syst. Bot. 13: 456–473. Woodson, R. E. Jr. 1954. The North American species of Asclepias L. Ann. Missouri Bot. Gard. 41: 1–211. Woodson, R. E. Jr. 1962. Butterflyweed revisited. Evolution 16: 168–185.

1 Leaves alternate (sometimes a few subopposite).   (2)
+ Leaves opposite, subopposite, or whorled (rarely alternate in A. eriocarpa).   (31)
       
2 (1) Leaf blades filiform or needlelike to narrowly linear, 0.05–0.3 cm wide.   (3)
+ Leaf blades linear, elliptic, oblong, oval, lanceolate, ovate, oblanceolate, obovate, or falcate, 0.3–9 cm wide.   (15)
       
3 (2) Stems 30–160 cm.   (4)
+ Stems 5–30 cm.   (9)
       
4 (3) Stems woody, few–numerous; leaf blades needlelike, less than 4 cm; corona segments with rod-shaped, slightly exserted appendages   3 Asclepias linaria
+ Stems herbaceous, 1–3 (rarely more); leaf blades not needlelike, greater than 4 cm; corona segment appendages absent or at most a low crest.   (5)
       
5 (4) Stems glabrous; anthers 2–3 mm.   (6)
+ Stems puberulent with curved trichomes; anthers 1–2 mm.   (7)
       
6 (5) Gynostegial column 0.5–1.5 mm; fused anthers barrel-shaped, anther wings evenly crescent-shaped and separate throughout, apical anther appendages narrowly pandurate, conduplicate, not obscuring corpuscula   49 Asclepias engelmanniana
+ Gynostegial column 0–0.5 mm; fused anthers obconic, anther wings connivent, except at the wider base, apical anther appendages deltoid, obscuring corpuscula   50 Asclepias rusbyi
       
7 (5) Leaf blades conduplicate; corolla lobe apices minutely puberulent with curved trichomes abaxially; fused anthers obconic, 1.5–2 mm, anther wings narrowly crescent-shaped; corona segments 3–3.5 mm, appendage an internal crest, apex appearing 3-toothed; follicles erect on straight pedicels   48 Asclepias stenophylla (in part)
+ Leaf blades flat; corolla lobes glabrous; fused anthers cylindric, 1–1.5 mm, anther wings trapezoidal to triangular; corona segments 1.5–2.5 mm, appendage absent or obscure, apex not appearing toothed; follicles erect on upcurved pedicels.   (8)
       
8 (7) Corona segments 1.5–2 mm, not exceeding point of anther wings; pedicels and calyces hirtellous; follicles 1–2 cm wide; flowers 34–112 per umbel   65 Asclepias hirtella (in part)
+ Corona segments 2–2.5 mm, exceeding point of anther wings; pedicels and calyces puberulent with curved trichomes; follicles 0.7–1.2 cm wide; flowers 13–28(–36) per umbel   66 Asclepias longifolia (in part)
       
9 (3) Corona segments strongly dorsally compressed and laminar or chute-shaped, appendage absent or merely a low crest, wholly included.   (10)
+ Corona segments conduplicate, cupulate, or tubular, appendage lingulate, falcate or acicular, barely to well exserted.   (11)
       
10 (9) Leaf blades conduplicate; corolla lobe apices puberulent with minute, curved trichomes abaxially; fused anthers obconic, 1.5–2 mm, anther wings narrowly crescent-shaped; corona segments 3–3.5 mm, appendage an internal crest, apex appearing 3-toothed; follicles erect on straight pedicels   48 Asclepias stenophylla (in part)
+ Leaf blades flat; corolla lobes glabrous; fused anthers cylindric, 1–1.5 mm, anther wings trapezoidal to triangular; corona segments 2–2.5 mm, appendage absent or obscure, apex not appearing toothed; follicles erect on upcurved pedicels   66 Asclepias longifolia (in part)
       
11 (9) Corolla lobes 4.5–6 mm; corona segments 3.5–4.5 mm; follicles 1.5–2 cm wide, rugose   51 Asclepias involucrata (in part)
+ Corolla lobes 2.5–5 mm; corona segments 1–3.5 mm; follicles 0.5–1.5 cm wide, smooth.   (12)
       
12 (11) Corollas red-violet; corona segment appendages lingulate, barely exserted; follicles pendulous on spreading to declined pedicels or erect on upcurved pedicels.   (13)
+ Corollas pink, cream, green, or tan; corona segment appendages falcate or acicular, well exserted; follicles erect on straight pedicels.   (14)
       
13 (12) Stems erect to ascending; leaves sessile, blades strigose to glabrate; corona segments white; follicles pendulous on spreading to declined pedicels   4 Asclepias cutleri
+ Stems decumbent; leaves petiolate, petioles 1–5 mm, blades puber­ulent with curved trichomes on midvein; corona segments red-violet at base, white to orange at apex, follicles erect on upcurved pedicels   60 Asclepias uncialis (in part)
       
14 (12) Leaf blades needlelike to narrowly linear, 0.05–0.1 cm wide; anthers 1–1.5 mm; corona segment appendages acicular, arching towards style apex, follicles 2.5–9.5 cm   16 Asclepias pumila
+ Leaf blades narrowly linear, 0.1–0.4 cm wide; anthers 1.5–3 mm; corona segment appendages falcate, sharply inflexed over style apex; follicles 7.5–12.5 cm   69 Asclepias michauxii (in part)
       
15 (2) Corolla lobes ascending and exceeding corona segments, 7–15 mm; corona segments clavate-tubular, deflexed at base, ascending to incurved at apex, margins connivent.   (16)
+ Corolla lobes reflexed, sometimes with spreading tips, not overtopping corona segments, 3–9 mm; corona segments cupulate, tubular, conduplicate, chute-shaped, or laminar, margins separate.   (17)
       
16 (15) Leaf blade bases cuneate, apices acute to attenuate; peduncles not branched; corolla lobes 7–10 mm, puberulent abaxially at apex with curved trichomes; corona segments 4.5–7 mm, margins and appendage papillose   55 Asclepias asperula
+ Leaf blade bases rounded to subcordate, apices obtuse to rounded; peduncles usually branched; corolla lobes (9–)12–15 mm, glabrous; corona segments 3–5 mm, margins and appendage hirtellous   56 Asclepias viridis
       
17 (15) Corona segments strongly dorsally compressed and laminar or chute-shaped, appendage absent or merely a low crest.   (18)
+ Corona segments cupulate, tubular, or conduplicate, at most slightly flattened dorsally, appendage lingulate, falcate, subulate, or conical, barely to well exserted from cavity.   (21)
       
18 (17) Leaf blades conduplicate; corolla lobe apices puberulent abaxially with minute, curved trichomes; fused anthers obconic, anther wings narrowly crescent-shaped; corona segment appendages crestlike, segment apices appearing 3-toothed; follicles erect on straight pedicels   48 Asclepias stenophylla (in part)
+ Leaf blades flat; corolla lobes glabrous; fused anthers cylindric, anther wings trapezoidal to triangular; corona segment appendages absent, segment apices untoothed; follicles erect on upcurved pedicels.   (19)
       
19 (18) Stems 7–20 cm, hirtellous to pilose; leaf blades hirtellous; fused anthers 1.5–2 mm; follicles hirtellous to pilose; seeds 6–7 × 4–5 mm   47 Asclepias lanuginosa (in part)
+ Stems 25–125 cm, puberulent with curved trichomes; leaf blades puberulent with curved trichomes or scabridulous; fused anthers 1–1.5 mm; follicles puberulent with curved trichomes to pilosulous; seeds 10–12 × 7–8 mm.   (20)
       
20 (19) Flowers 34–112 per umbel; pedicels and calyces hirtellous; corona segments 1.5–2 mm, not exceeding point of anther wings; follicles 1–2 cm wide   65 Asclepias hirtella (in part)
+ Flowers 13–28(–36) per umbel; pedicels and calyces puberulent with curved trichomes; corona segments 2–2.5 mm, exceeding point of anther wings; follicles 0.7–1.2 cm wide   66 Asclepias longifolia (in part)
       
21 (17) Stems 15–100 cm; corolla lobes 4–9 mm.   (22)
+ Stems 5–15 cm; corolla lobes 3–6 mm.   (26)
       
22 (21) Corona segments 5–7 mm.   (23)
+ Corona segments 2.5–3.5 mm.   (24)
       
23 (22) Stems, leaves, and peduncles puberulent with curved trichomes, pilosulous, tomentulose, or glabrate; corollas pink, red, or green; coronas cream, segments with reddish or purplish dorsal stripes; anthers brown; follicles pilosulous; seeds 6–7 mm   45 Asclepias hallii (in part)
+ Stems, leaves, and peduncles hirsute; corollas and coronas red, orange, or yel­low; anthers yellow to yellowish green; follicles hirsute; seeds 8–9 mm   70 Asclepias tuberosa
       
24 (22) Pedicels 9–12 mm; corolla lobes 4–5 mm; corona segments cupulate, apex obtuse; fruiting pedicels straight, follicles 0.5–0.7 cm wide; se United States   69 Asclepias michauxii (in part)
+ Pedicels 15–50 mm; corolla lobes 6–9 mm; corona segments conduplicate, apex truncate; fruiting pedicels pendulous or upcurved, follicles 1.2–3 cm wide; California or Utah.   (25)
       
25 (24) Leaf blades 2–8 cm wide, tomentose to densely puberulent with curved trichomes; anthers brown, 2.5–3 mm; follicles erect on upcurved pedicels, 5–10 cm; seeds 7–9 × 4–6 mm   36 Asclepias eriocarpa (in part)
+ Leaf blades 0.5–2 cm wide, glabrous except for midvein; anthers green to yellowish green, 1.5–2 mm; follicles pendulous, 3.5–5.5 cm; seeds 12–13 × 8–9 mm   54 Asclepias labriformis
       
26 (21) Corollas green, sometimes tinged red or pink; corona segment appendages well exserted, sharply inflexed towards or over style apex; follicles rugose.   (27)
+ Corollas red-violet; corona segment appendages equaling or barely exserted from segment; follicles smooth.   (28)
       
27 (26) Leaf blades 0.2–0.8 cm wide; anthers 1.5–2 mm; corona segments cream, usually with a pink or red dorsal stripe, conduplicate, 3.5–4.5 mm; seeds 7–8 × 5–6 mm   51 Asclepias involucrata (in part)
+ Leaf blades 0.5–2 cm wide; anthers 1–1.5 mm; corona segments yellow to ochroleucous, tubular, 2–3 mm; seeds 8–12 × 6–8 mm   52 Asclepias macrosperma (in part)
       
28 (26) Corona segment appendages falcate, not or barely exserted; Colorado Plateau, ne Arizona, nw New Mexico, Utah.   (29)
+ Corona segment appendages lingulate or conical, barely exserted; Arizona, Colorado, Nevada, New Mexico, Oklahoma, Texas, not on the Colorado Plateau.   (30)
       
29 (28) Leaf blades persistently pilosulous, more densely so on veins; pedicels and calyces densely pilosulous to tomentulose; corona segment appendages barely exserted; Utah, n of Colorado River and w of Green River, from San Rafael Reef to Waterpocket Fold   58 Asclepias ruthiae (in part)
+ Leaf blades sparsely pilosulous to glabrate, persistently hairy on veins; pedicels and calyces pilosulous; corona segment appendages included; Arizona, New Mexico, just entering Utah in Monument Valley   59 Asclepias sanjuanensis (in part)
       
30 (28) Leaf blades lanceolate to ovate, 0.6–3 cm wide, strigulose or puberulent on midvein to glabrate; flowers 2–25 per umbel; anthers 1.5–2 mm; corona appendages conical; Nevada   57 Asclepias eastwoodiana (in part)
+ Leaf blades linear to lanceolate, 0.2–1 cm wide, puberulent with curved trichomes on midvein; flowers 3–7 per umbel; anthers 1–1.5 mm; corona appendages lingulate; Arizona, Colorado, New Mexico, Oklahoma, Texas   60 Asclepias uncialis (in part)
       
31 (1) One or more nodes with leaves whorled or (in A. quadrifolia) apparently whorled because of a reduced internode.   (32)
+ Leaves opposite or subopposite, not whorled.   (38)
       
32 (31) Shrubs, 50–400 cm; leaves ephemeral; stems often leafless or nearly so, glaucous; follicles spreading to pendulous; Sonoran Desert and se Mohave Desert, sw Arizona, se California, s Nevada.   (33)
+ Herbs; stems 20–150 cm; leaves persistent; stems leafy, not glaucous; follicles erect on straight or upcurved pedicels; widespread, approaching but not entering the Sonoran Desert.   (34)
       
33 (32) Stems 5–40 (usually 1–few), 140–400 cm; pedicels 8–16 mm; corolla lobes 4.5–6 mm; corona segments 2–3 mm, conduplicate, not exceeding style apex   5 Asclepias albicans (in part)
+ Stems 3–100 (usually few–numerous), 50–175 cm; pedicels 11–21 mm; corolla lobes 7–12 mm; corona segments 7–9 mm, tubular, greatly exceeding style apex   6 Asclepias subulata (in part)
       
34 (32) At least some leaves on main stem alternate or opposite, blades elliptic or lanceolate to ovate, 1–8 cm wide; corona lobes 2.5–4 mm.   (35)
+ All leaves whorled (rarely opposite on vegetative branches), blades linear to linear-lanceolate or narrowly elliptic, 0.1–1.8 cm wide; corona lobes 1.5–2 mm.   (36)
       
35 (34) Plants 1–10-stemmed; whorled leaves, when present, at upper nodes, leaf blades tomentose to densely puberulent with curved trichomes; pedicels and calyces densely tomentose; corolla lobes 7–9 mm; follicles erect on upcurved pedicels, lance-ovoid, 1.5–3 cm wide; California   36 Asclepias eriocarpa (in part)
+ Plants single-stemmed; whorled leaves, when present, at a single, mid-stem node; leaf blades inconspicuously puberulent only on veins; pedicels puberlent on 1 side; calyces glabrous; corolla lobes 4–5 mm; follicles erect on straight pedicels, narrowly fusiform, 0.4–1 cm wide; e North America   73 Asclepias quadrifolia (in part)
       
36 (34) Leaf blades 0.2–1.8 cm wide; pedicels 9–14 mm; corollas pink (rarely green with pink tinge); California, Idaho, Nevada, Oregon, Washington   13 Asclepias fascicularis
+ Leaf blades 0.1–0.4 cm wide; pedicels 5–12 mm; corollas green to cream, sometimes tinged pink or tan; widespread, s or e of Idaho, Nevada.   (37)
       
37 (36) Stems 1–8, vegetative branches often present in 1+ leaf axils; leaf blades 0.1–0.4 cm wide, glabrous; anthers 1.5–2 mm; corona segment margins entire; w United States, including w Kansas, w Oklahoma, w Texas   14 Asclepias subverticillata
+ Stems solitary (rarely 2–3), vegetative branches absent; leaf blades 0.1–0.2 cm wide, puberulent with curved trichomes adaxially, especially on midvein to glabrate; anthers 1.2–1.5 mm; corona segment margins shallowly lobed to sharply toothed; e North America, e of Colorado, Idaho, New Mexico   15 Asclepias verticillata
       
38 (31) Leaf blades filiform to narrowly linear, 0.05–0.3 cm wide.   (39)
+ Leaf blades linear, elliptic, oblong, oval, lanceolate, ovate, deltate, oblanceolate, obovate, oblate, or orbiculate, 0.1–15 cm wide.   (56)
       
39 (38) Shrubs, 50–400 cm; leaves ephemeral; stems often leafless or nearly so, glaucous; follicles spreading to pendulous; Sonoran Desert and se Mohave Desert, sw Arizona, se California, s Nevada.   (40)
+ Subshrubs or herbs; stems 4–125 cm; leaves persistent; stems leafy, not glaucous; follicles erect on straight or upcurved pedicels; approaching but not entering the Sonoran Desert.   (41)
       
40 (39) Stems 5–40 (usually 1–few), 140–400 cm; pedicels 8–16 mm; corolla lobes 4.5–6 mm; corona segments 2–3 mm, conduplicate, not exceeding style apex   5 Asclepias albicans (in part)
+ Stems 3–100 (usually few–numerous), 50–175 cm; pedicels 11–21 mm; corolla lobes 7–12 mm; corona segments 7–9 mm, tubular, greatly exceeding style apex   6 Asclepias subulata (in part)
       
41 (39) Stems prostrate to decumbent.   (42)
+ Stems ascending to erect.   (45)
       
42 (41) Stems 4–18 cm; petioles 1–5 mm; umbels sessile; follicles ovoid, 3–5.5 cm; Arizona, Colorado, Kansas, New Mexico, Oklahoma, Texas, Wyoming.   (43)
+ Stems 15–70 cm; petioles 0–2 mm; peduncles 0–6 cm; follicles narrowly fusiform, 7.5–13.5 cm; se United States.   (44)
       
43 (42) Corollas green, sometimes tinged pink or red, lobes 4.5–6 mm; corona segments 3.5–4.5 mm; follicles 1.5–2 cm wide, rugose   51 Asclepias involucrata (in part)
+ Corollas red-violet, lobes 3–5 mm; corona segments 1–2 mm; follicles 0.8–1.5 cm wide, smooth   60 Asclepias uncialis (in part)
       
44 (42) Leaf blades scabridulous to puberulent with curved trichomes; corona segments laminar, 2–2.5 mm, appendage absent or obscure; fruiting pedicels upcurved; follicles 0.7–1.2 cm wide; seeds 11–12 × 7–8 mm   66 Asclepias longifolia (in part)
+ Leaf blades glabrous; corona segments cupulate, 2.5–3.5 mm, appendage falcate, well exserted from cavity; fruiting pedicels straight; follicles 0.5–0.7 cm wide; seeds 7–8 × 5–6 mm   69 Asclepias michauxii (in part)
       
45 (41) Subshrubs, usually persistently woody at base (sometimes herbaceous to base), cespitose; stems 5–20+, sparsely to densely branched, at least at base.   (46)
+ Herbs, not woody at base; stems 1(–5), rarely branched (often branched in A. linearis).   (48)
       
46 (45) Stems 20–60 cm; petioles 2–4 mm; flowers 3–15 per umbel; corollas cream, sometimes tinged or striped pink; corona segments tubular, glabrous at apex, appendage acicular, arching over style apex, glabrous; fruiting pedicels straight   8 Asclepias angustifolia (in part)
+ Stems 5–35 cm; leaves sessile; flowers 1–7 per umbel; corollas greenish cream or greenish yellow, sometimes tinged red or purple; corona segments conduplicate or chute-shaped, hirtellous at apex, appendage falcate, arching over style apex, or an included crest, hirtellous; fruiting pedicel upcurved.   (47)
       
47 (46) Flowers 2–7 per umbel; anthers 1–1.5 mm; corona segments conduplicate, 4–5 mm, apex long-caudate, appendage falcate, exserted   1 Asclepias macrotis
+ Flowers 1–2 per umbel; anthers 1.5–2 mm; corona segments chute-shaped, sharply inflexed, 2–3 mm, apex subcaudate, appendage an included crest   2 Asclepias sperryi
       
48 (45) Corona segments strongly dorsally compressed or chute-shaped, margins incurved, appendages absent or a low, inlcuded crest; follicles 1–2 cm wide.   (49)
+ Corona segments conduplicate or cupulate (may be slightly flattened dorsally, but not compressed), margins not incurved, appendages evident and exserted from cavity; follicles 0.3–1 cm wide.   (50)
       
49 (48) Flowers 9-28 per umbel; pedicels puberulent with curved trichomes to pilosulous, straight in fruit; corona segments equaling style apex   18 Asclepias stenophylla (in part)
+ Flowers 34-112 per umbel; pedicels hirtellous, upcurved in fruit; corona segments greatly exceeded by style apex   65 Asclepias hirtella (in part)
       
50 (48) Corona segment appendages acicular, subulate, or falcate, exserted and inflexed towards or over style apex.   (51)
+ Corona segment appendages crestlike, included, or absent.   (53)
       
51 (50) Stems 60–125 cm; leaf blades 7–25 cm; corollas red, lobes 9–10 mm; corona segments yellow to reddish orange, 5–6 mm; fruiting pedicels upcurved   67 Asclepias lanceolata (in part)
+ Stems 25–75 cm; leaf blades 3–9 cm; corollas green, tan, or pink, sometimes tinged brown or red, lobes 3–5 mm; coronas cream, sometimes tinged brown or green, segments 1.5–4 mm; fruiting pedicels straight.   (52)
       
52 (51) Plants rhizomatous; interpetiolar ridges present; pedicels 6–8 mm; corona segments 1.5 mm; follicles 5–8.5 cm; Texas   12 Asclepias linearis
+ Plants not rhizomatous; interpetiolar ridges absent; pedicels 7–13 mm; corona segments 3–4 mm; follicles 9–12.5 cm; Alabama, Florida   31 Asclepias viridula
       
53 (50) Corollas and coronas yellowish green to green; corolla lobes erect, concealing gynostegium and partially obscuring corona; corona appendages absent; follicles densely puberulent with curved trichomes   33 Asclepias pedicellata (in part)
+ Corollas green, white, or cream, sometimes tinged gray, pink, red, lavender, or purple, lobes reflexed or spreading; coronas red, pink, violet, white, cream, gray, or a combination of these, appendage crestlike, included; follicles glabrous, glabrate, or pilosulous.   (54)
       
54 (53) Corollas green, sometimes tinged red, lobes pilose abaxially; anthers tan to brown; coronas red, pink, purple, or red-violet, sometimes white at segment apices; fruiting pedicels upcurved, follicles pilosulous to glabrate; s Arizona, s New Mexico   25 Asclepias quinquedentata (in part)
+ Corollas cream or white, tinged gray, pink, lavender, or purple, lobes glabrous; anthers green or lavender; corona segments cream or white, sometimes tinged gray, pink, or purple; fruiting pedicels straight, follicles glabrous; se United States.   (55)
       
55 (54) Flowers erect; corollas white, sometimes tinged lavender, lobes spreading, 7–10 mm; anthers lavender; corona segments white, cupulate, apex edentate   29 Asclepias feayi
+ Flowers spreading to pendent; corollas and coronas cream, tinged gray, pink, or purple; corolla lobes reflexed, 4–5 mm; anthers green; corona segments conduplicate, apex toothed laterally   30 Asclepias cinerea
       
56 (38) Both stems and leaf blades completely glabrous or glabrate, except for ciliate leaf margins in some species or abaxial midvein (A. cryptoceras).   (57)
+ Either stems and/or leaf blades bearing trichomes, sometimes restricted to veins.   (78)
       
57 (56) Stems and/or leaf blades glaucous.   (58)
+ Neither stems nor leaf blades glaucous.   (66)
       
58 (57) Corona segment appendages merely low, included crests or absent; w United States, w of Rocky Mountains plus Arizona, New Mexico, w Texas.   (59)
+ Corona segment appendages exserted, falcate, subulate, or ensiform; e United States, w to Arizona, Colorado, Kansas, Nebraska, New Mexico, Texas, Utah.   (61)
       
59 (58) Corollas red-violet, lobes 6–7 mm; corona segments tubular, 2–3 mm; n California, extreme w Nevada, sw Oregon   17 Asclepias cordifolia
+ Corollas green to yellowish green, sometimes tinged red abaxially, lobes 8–14 mm; corona segments conduplicate, 4–8 mm; Arizona, e California, w Colorado, Idaho, Nevada, s New Mexico, e Oregon, w Texas, Utah, e Washington, s Wyoming.   (60)
       
60 (59) Stems prostrate to decumbent, 8–25 cm; umbels sessile; corolla lobe apices minutely hirtellous adaxially; anthers brown (sometimes with green apex), 1.8–3 mm; corona segments red-violet to pinkish purple, apex with recurved, papillose teeth, appendage absent; follicles 4.5–6 cm, glabrous, glaucous   18 Asclepias cryptoceras
+ Stems erect, 30–80 cm; peduncles 5–20 cm; corolla lobes glabrous; anthers green, 3–3.5 mm; corona segments white, yellow to tan dorsally, apex edentate, appendage a low, included crest; follicles 8–13 cm, pilosulous, not glaucous   37 Asclepias elata
       
61 (58) Peduncles, pedicels, calyx lobes, and developing leaf blades tomentose, leaves becoming glabrate with age; w United States, plains e of Rocky Mountains to c Kansas, c Nebraska, c Oklahoma, w Texas   44 Asclepias latifolia (in part)
+ Peduncles, pedicels, calyx lobes, and developing leaf blades glabrous, puberulent with curved trichomes, or pilosulous; e United States and Canada, w to e Kansas, e Nebraska, c Oklahoma, and e Texas.   (62)
       
62 (61) Stems decumbent to erect; corolla lobes 5.5–6 mm; anthers brown; corona segments 3–3.5 mm, appendage ensiform, only slightly exserted   19 Asclepias humistrata
+ Stems erect; corolla lobes 7–12 mm; anthers green; corona segments 4–7 mm, appendage falcate or subulate, well exserted.   (63)
       
63 (62) Leaf blades 1.5–9 cm wide, apex obtuse, rounded, truncate, or emarginate (acute); pedicels 20–55 mm; anthers 2.5–3.5 mm; corona segments tubular, apex broadly obtuse and oblique or truncate and erose; follicles 1–3 cm wide.   (64)
+ Leaf blades 1–5 cm wide, apex acute to attenuate; pedicels 11–20 mm; anthers 2–2.5 mm; corona segments conduplicate, apex acute to obtuse and emarginate; follicles 0.9–1.6 cm wide.   (65)
       
64 (63) Peduncles 5–40 cm; flowers 18–53 per umbel; pedicels puberulent with curved trichomes; corollas green, often tinged reddish purple, or bronze; coronas reddish purple to cream, segments stipitate, apex truncate, erose; follicles smooth, 9–16 cm; seeds 9–10 × 6–7 mm   61 Asclepias amplexicaulis
+ Peduncles 1–6 cm; flowers 9–29 per umbel; pedicels glabrous, glaucous; corollas dark pink, pale at base of lobes; coronas pale to dark pink, segments subsessile, apex broadly obtuse, oblique; follicles sparsely muricate, 7–11 cm; seeds 7–9 × 5–6 mm   62 Asclepias sullivantii
       
65 (63) Leaves sessile, blade base rounded to obtuse; flowers pendent; corollas green to greenish cream, sometimes tinged red; coronas green to yellowish green or greenish cream, segments sessile, apex obtuse, emarginate, appendage falcate, sharply inflexed towards style apex; seeds 6–7 × 4–5 mm; Illinois, Indiana, Iowa, Kansas, Missouri, Wisconsin   63 Asclepias meadii
+ Petioles 1–2 mm; leaf blade bases cordate; flowers erect; corollas pink to reddish purple; coronas pink to lavender, segments stipitate, apex acute, appendage subulate, arching above style apex; seeds 7–9 × 5–7 mm; coastal plain, e United States to e Texas   68 Asclepias rubra (in part)
       
66 (57) Leaf blades 0.1–2 cm wide.   (67)
+ Leaf blades 2–15 cm wide.   (72)
       
67 (66) Stems 10–60 cm; corolla lobes 1.5–6 mm; corona segments 1.5–4 mm.   (68)
+ Stems 25–125 cm; corolla lobes 7–13 mm; corona segments 5–10 mm.   (70)
       
68 (67) Leaf blades linear, 0.2–0.6 cm wide; flowers 4–10 per umbel; corollas green, sometimes tinged red, lobes pilose abaxially; corona segment apices truncate, appendage crestlike, barely exserted; fruiting pedicels upcurved; seeds 4–5 × 3–4 mm; s Arizona, s New Mexico   25 Asclepias quinquedentata (in part)
+ Leaf blades linear to elliptic, oval, oblong, or ovate, 0.3–6 cm wide; flowers 7–31 per umbel; corollas white, cream, or pink, lobes glabrous abaxially; corona segment apices obtuse to acute, appendage acicular or subulate, well exserted; fruiting pedicels straight or declined; seeds 6–15 × 4–14 mm; e United States and Canada to e Texas.   (69)
       
69 (68) Subshrubs or herbs; stems cespitose; interpetiolar ridges present; petioles 10–12 mm; pedicels 7–13 mm; flowers erect to spreading; corolla lobes 3–4 mm; corona segments tubular, rounded dorsally, 1.5–2.5 mm, appendage acicular, arching above style apex; fruiting pedicels declined, follicles pendulous, 4–7 × 1–2.5 cm; seeds 12–15 × 11–14 mm, coma absent   10 Asclepias perennis (in part)
+ Herbs; stems solitary; interpetiolar ridges absent; petioles 2–7 mm; pedicels 17–28 mm; flowers spreading to pendent; corolla lobes 4–5 mm; corona segments conduplicate, flattened dorsally, 2.5–4 mm, appendage subulate, inflexed over style apex; fruiting pedicels straight, follicles erect, 8–16 × 0.4–1 cm; seeds 6–7 × 4–5 mm, coma present   73 Asclepias quadrifolia (in part)
       
70 (67) Leaf blades 2.5–8 cm; flowers spreading to pendent; corollas green, sometimes tinged brown at lobe tips, lobes 10–13 mm; anthers 3–3.5 mm; corona segments cream to pale green, 8–10 mm, clavate, incurved, appendage an included crest; follicles 11.5–15 cm   32 Asclepias connivens (in part)
+ Leaf blades 5–25 cm; flowers erect; corollas red, pink, or reddish purple, lobes 7–10 mm; anthers 2–2.5 mm; corona segments yellow, reddish orange, pink, or lavender, 5–7 mm, tubular or conduplicate, appendage subulate, well exserted; follicles 5.5–12 cm.   (71)
       
71 (70) Petioles 0–1 mm; leaf blades linear to linear-lanceolate, 0.2–1.7 cm wide, base cuneate; flowers 4–16 per umbel; corollas red, lobes 9–10 mm; corona segments yellow to reddish orange, tubular, apex obtuse, 5–6 mm, appendage sharply inflexed over style apex; follicles 0.8–1 cm wide   67 Asclepias lanceolata (in part)
+ Petioles 1–2 mm; leaf blades narrowly lanceolate to ovate, 1–4.5 cm wide, base cordate; flowers 9–20 per umbel; corollas pink to reddish purple, lobes 7–9 mm; corona segments pink to lavender, conduplicate, apex acute, 6–7 mm, appendage arching above style apex; follicles 1–1.5 cm wide   68 Asclepias rubra (in part)
       
72 (66) Leaf blade bases cuneate.   (73)
+ Leaf blade bases truncate or rounded to cordate.   (75)
       
73 (72) Petioles 0–1 mm; leaf blades linear to narrowly elliptic or oblanceolate, 0.3–2.5 cm wide; flowers 4–8 per umbel; pedicels 10–20 mm; corolla lobes 10–13 mm; corona segments 8–10 mm, clavate, appendage an included crest; coastal plain, s Alabama, Florida, s Georgia, se South Carolina   32 Asclepias connivens (in part)
+ Petioles 2–15 mm; leaf blades ovate to elliptic or oblong, 1–11 cm wide; flowers 7–41 per umbel; pedicels 17–45 mm; corolla lobes 4–12 mm; corona segments 2.5–5 mm, tubular or conduplicate, appendage subulate or falcate, well exserted; inland e North America, including n Alabama, n Georgia, w South Carolina.   (74)
       
74 (73) Stems 25–60 cm; petioles 2–7 mm; leaf blades 2.5–12 × 1–6 cm; pedicels 17–28 mm; corollas pink or cream, lobes 4–5 mm; anthers tan to brown, 1–1.5 mm; corona segments conduplicate, apex obtuse, appendage subulate; follicles 0.4–1 cm wide; seeds 6–7 mm   73 Asclepias quadrifolia (in part)
+ Stems 65–150 cm; petioles 5–15 mm; leaf blades 10–24 × 2–11 cm; pedicels 25–45 mm; corollas green (rarely pink-tinged), lobes 6–12 mm; anthers green, 2.5–3.5 mm; corona segments tubular, apex truncate, appendage falcate; follicles 1.5–2 cm wide; seeds 8–10 mm   74 Asclepias exaltata (in part)
       
75 (72) Peduncles and pedicels puberulent with curved trichomes in 1 line; flowers 9–20 per umbel; corollas pink to reddish purple; coronas pink to lavender, 6–7 mm, segment apices acute; follicles fusiform, 1–1.5 cm wide; coastal plain, e United States to e Texas   68 Asclepias rubra (in part)
+ Peduncles tomentose, puberulent with curved trichomes, or pilosulous; pedicels tomentose or densely pilose to glabrate; flowers 12–80 per umbel; corollas green, ochroleucous, tan, or red-violet; coronas cream to ochroleucous, 2.5–5.5 mm, segment apices truncate; follicles lance-ovoid to ovoid or ellipsoid, 1.7–3.5 cm wide; w United States.   (76)
       
76 (75) Stems 40–250 cm; leaf blades 7.5–25 cm, apex attenuate to acuminate, margins erose; umbels terminal and extra-axillary; corolla lobe apices tomentose abaxially; seeds 8–13 mm; Sonoran and Mohave deserts and surrounding areas, w Arizona, s California, s Nevada, sw Utah   35 Asclepias erosa (in part)
+ Stems 25–100 cm; leaf blades 4.5–14.5 cm, apex truncate to rounded (sometimes acute), margins entire; umbels extra-axillary; corolla lobes densely tomentose throughout abaxially or glabrous; seeds 7–8 mm or 18–20 mm; e of the Sonoran and Mohave deserts, ne Arizona, s Utah to the Great Plains.   (77)
       
77 (76) Peduncles 0–2.5 cm, puberulent with curved trichomes or pilosulous; pedicels 15–35 mm; corollas green, lobes 7–9 mm, glabrous abaxially; anthers 3–3.5 mm; corona segments 3–5.5 mm, apex papillose; fruiting pedicels upcurved; seeds 7–8 × 5–6 mm; ne Arizona, s Utah to the Great Plains, substrates various including sand   44 Asclepias latifolia (in part)
+ Peduncles 2.5–6 cm, tomentose; pedicels 10–15 mm; corollas ochroleucous or tan to red-violet, lobes 5.5–6.5 mm, tomentose abaxially; anthers 1.5 mm; corona segments 2.5–3.5 mm, apex glabrous; fruiting pedicels spreading; seeds 18–20 × 9–10 mm; local and rare in n Arizona, s Utah on unstabilized sand dunes   53 Asclepias welshii (in part)
       
78 (56) Stems prostrate or decumbent.   (79)
+ Stems spreading to erect.   (98)
       
79 (78) Stems 5–15 cm.   (80)
+ Stems 15–250 cm.   (86)
       
80 (79) Leaf blades 1.7–5 cm; corollas green, sometimes tinged red, pink, or brown; corona segments 2–10 mm; follicles 4.5–9.5 × 1.2–2.5 cm.   (81)
+ Leaf blades 1.5–12 cm; corollas red-violet; corona segments 1–2 mm; follicles 3–6 × 0.5–1.5 cm.   (83)
       
81 (80) Leaf blades 1.2–6.5 cm wide, apex obtuse to rounded; umbels extra-axillary; corolla lobes 7–12 mm; anthers 2–2.5 mm; corona segments green, sometimes tinged bronze, apex white or cream, sinuous-tubular, 5–10 mm   40 Asclepias oenotheroides (in part)
+ Leaf blades 0.2–2 cm wide, apex acute to attenuate; umbels terminal; corolla lobes 4.5–6.5 mm; anthers 1–2 mm; corona segments cream with pink or red dorsal stripe or yellow to ochroleucous, conduplicate or tubular, but not sinuous, 2–4.5 mm.   (82)
       
82 (81) Leaf blades 0.2–0.8 cm wide; anthers 1.5–2 mm; corona segments cream, usually with a pink or red dorsal stripe, conduplicate, 3.5–4.5 mm; seeds 7–8 × 5–6 mm   51 Asclepias involucrata (in part)
+ Leaf blades 0.5–2 cm wide; anthers 1–1.5 mm; corona segments yellow to ochroleucous, tubular, 2–3 mm; seeds 8–12 × 6–8 mm   52 Asclepias macrosperma (in part)
       
83 (80) Corona segment appendages falcate, not or barely exserted; Colorado Plateau, ne Arizona, nw New Mexico, Utah.   (84)
+ Corona segment appendages lingulate or conical, barely exserted; Arizona, Colorado, Nevada, New Mexico, w Oklahoma, w Texas, not on the Colorado Plateau.   (85)
       
84 (83) Leaf blades persistently pilosulous, more densely so on veins; pedicels and calyces densely pilosulous to tomentulose; corona segment appendages included; Utah, n of Colorado River and w of Green River, from San Rafael Reef to Waterpocket Fold   58 Asclepias ruthiae (in part)
+ Leaf blades sparsely pilosulous to glabrate, persistently and densely hairy on veins and margins; pedicels and calyces pilosulous; corona segment appendages barely exserted; ne Arizona, nw New Mexico, just entering Utah in Monument Valley   59 Asclepias sanjuanensis (in part)
       
85 (83) Leaf blades lanceolate to ovate, 0.6–3 cm wide, strigulose or pilosulous on midvein to glabrate; flowers 2–25 per umbel; anthers 1.5–2 mm; corona appendages conical; Nevada   57 Asclepias eastwoodiana (in part)
+ Leaf blades linear to lanceolate, 0.2–1 cm wide, puberulent with curved trichomes on midvein; flowers 3–7 per umbel; anthers 1–1.5 mm; corona appendages lingulate; Arizona, Colorado, New Mexico, w Oklahoma, w Texas   60 Asclepias uncialis (in part)
       
86 (79) Leaf blades 0.1–1 cm wide.   (87)
+ Leaf blades 1–15 cm wide.   (91)
       
87 (86) Corolla lobes 8–11 mm, pilosulous abaxially; corona segments 5–7 mm, appendage papillose; follicles muricate ridged; s Texas   39 Asclepias prostrata (in part)
+ Corolla lobes 3–6 mm, glabrous abaxially; corona segments 1.5–4.5 mm, appendage glabrous; follicles smooth or rugose; widespread, but not s Texas.   (88)
       
88 (87) Stems 25–70 cm; corona segments laminar, margins incurved, append­age absent or obscure; follicles 8–13.5 × 0.7–1.2 cm   66 Asclepias longifolia (in part)
+ Stems 5–30 cm; corona segments cupulate or conduplicate, margins not incurved, appendage conical or falcate, barely to well exserted; follicles 3.5–6 × 0.5–2 cm or 7.5–12.5 × 0.5–0.7 cm.   (89)
       
89 (88) Corollas red-violet; corona segments 1.5–2 mm, appendage conical, barely exserted, follicles spreading to pendulous, strigose   57 Asclepias eastwoodiana (in part)
+ Corollas green to tan, sometimes tinged pink or red; corona segments 2.5–4.5 mm, appendage falcate, well exserted, sharply inflexed over style apex; follicles erect, puberulent with curved trichomes to pilosulous.   (90)
       
90 (89) Stems 5–18 cm; anthers brown; corona segments conduplicate, apex truncate, 3.5–4.5 mm; fruiting pedicels upcurved, fol­licles ovoid, rugose, 4.5–5.5 × 1.5–2 cm; c United States   51 Asclepias involucrata (in part)
+ Stems 15–30 cm; anthers green; corona segments cupulate, apex obtuse, 2.5–3.5 mm; fruiting pedicels straight, follicles narrowly fusiform, smooth, 7.5–12.5 × 0.5–0.7 cm; se United States   69 Asclepias michauxii (in part)
       
91 (86) Corona segments 5–11 mm, greatly exceeding style apex, tubular, sinuous-tubular, or conduplicate-tubular, appendage papillose (rarely glabrous or glabrate).   (92)
+ Corona segments 1.5–6 mm, shorter than to slightly exceeding style apex, cupulate or conduplicate, appendage glabrous.   (94)
       
92 (91) Stems, leaf blades, peduncles, and pedicels pilosulous to tomentulose; petioles 2–3 mm, leaf blades 1.8–5 × 0.4–1.8 cm; flowers 3–8 per umbel; pedicels 8–14 mm; gynostegial column 3–3.5 mm; follicles pendent, muricate-ridged, tomentulose   39 Asclepias prostrata (in part)
+ Stems, leaf blades, peduncles, and pedicels puberulent with curved trichomes or hirtellous; petioles 2–25 mm; leaf blades 4–15 × 1.2–7.5 cm; flowers 5–32 per umbel; pedicels 10–30 mm; gynostegial column 0.3–1.5 mm; follicles erect, smooth, puberulent with curved trichomes or hirtellous.   (93)
       
93 (92) Leaf blades ovate or lanceolate to oblong or elliptic, base cuneate to obtuse, apex obtuse to rounded, sometimes conduplicate; pedicels 10–20 mm; gynostegial column 1–1.5 mm; anthers 2–2.5 mm; corona segments relatively slender, green, sometimes tinged bronze, apex white or cream, deeply emarginate; se Arizona, sw Colorado, w Louisiana, New Mexico, Oklahoma, Texas   40 Asclepias oenotheroides (in part)
+ Leaf blades ovate to lanceolate, base cuneate or obtuse to truncate or sub­cordate, apex obtuse to acute, rarely conduplicate; pedicels 17–30 mm; gyno­stegial column 0.3–0.5 mm; anthers 1.7–2 mm; corona segments relatively stout, cream or green with cream apex, apex truncate; Arizona, se California, s Nevada, sw New Mexico   42 Asclepias nyctaginifolia (in part)
       
94 (91) Corolla lobes 4–6 mm; corona segments 1.5–3 mm.   (95)
+ Corolla lobes 6–11 mm; corona segments 3–6 mm.   (96)
       
95 (94) Leaf blades ovate to nearly orbiculate, 3–4 cm wide, base obtuse to cordate; flowers 20–55 per umbel; corolla lobes 5–6 mm; corona segments conduplicate, 2–3 mm, appendage absent; follicles 6–10 × 2–3 cm   28 Asclepias solanoana
+ Leaf blades narrowly lanceolate to ovate, 0.6–3 cm wide, base cuneate to obtuse; flowers 2–25 per umbel; corolla lobes 4–5 mm; corona segments cupulate, 1.5–2 mm, appendage conical, barely exserted; follicles 3.5–6 × 0.5–1 cm; Nevada   57 Asclepias eastwoodiana (in part)
       
96 (94) Flowers 5–21 per umbel; corolla lobes pilose at base adaxially; gynostegial column 2–3.5 mm; corona segments red-violet, margins connivent, papillose, appendage absent   21 Asclepias californica (in part)
+ Flowers 12–50 per umbel; corolla lobes papillose or glabrous at base adaxially; gynostegial column 1–1.5 mm; corona segments cream to ochroleucous or dark pink, margins separate, glabrous, appendage falcate.   (97)
       
97 (96) Petioles 3–10 mm; leaf blade margins entire; peduncles 0–3.5 cm; corolla lobes densely tomentose abaxially throughout; anthers dark brown, 1.5–2 mm; corona segments sessile, apex obtuse, oblique; follicles 5–6.5 cm   22 Asclepias vestita (in part)
+ Petioles 0–6 mm; leaf blade margins erose; peduncles 2–10 cm; corolla lobes tomentose abaxially only at tips; anthers green, 2.5–3 mm; corona segments stipitate, apex truncate; follicles 6.5–10 cm   35 Asclepias erosa (in part)
       
98 (78) Corolla lobes 3–5 mm.   (99)
+ Corolla lobes 5–15 mm (rarely 4.5 mm in A. incarnata, with corolla and corona pink to white, corona segments 2–2.5 mm, and fruiting pedicels straight).   (112)
       
99 (98) Corona segments laminar or chute-shaped, margins incurved, tightly appressed to gynostegium; appendage absent or obscure.   (100)
+ Corona segments conduplicate, cupulate, or tubular, margins not incurved, not appressed to gynostegium; appendage falcate, acicular, subulate, conical, lingulate, or crestlike, barely to well exserted.   (102)
       
100 (99) Stems 7–20 cm; leaf blades oblong or lanceolate to narrowly lanceolate, 4–8 cm, hirtellous; umbels terminal; anthers 1.5–2 mm; seeds 6–7 × 4–5 mm   47 Asclepias lanuginosa (in part)
+ Stems 25–70 cm; leaf blades linear to linear-lanceolate, 4–18 cm, scabrid­ulous to puberulent with curved trichomes; umbels terminal and/or extra-axillary; anthers 1–1.5 mm; seeds 10–12 × 7–8 mm.   (101)
       
101 (100) Flowers 34–112 per umbel; pedicels and calyces hirtellous; corona segments 1.5–2 mm, not exceeding point of anther wings; follicles 1–2 cm wide   65 Asclepias hirtella (in part)
+ Flowers 13–28(–36) per umbel; pedicels and calyces puberulent with curved trichomes; corona segments 2–2.5 mm, exceeding point of anther wings; follicles 0.7–1.2 cm wide   66 Asclepias longifolia (in part)
       
102 (99) Stems, leaf blades, and peduncles tomentose, at least initially.   (103)
+ Stems, leaf blades, peduncles, pedicels, and calyces puberulent with curved trichomes, pilose, strigulose, hirtellous, or glabrous.   (104)
       
103 (102) Stems 6–15 cm; leaf blades orbiculate to obovate or oblate, 1.5–3.2 × 1.7–4 cm; peduncles 2.7–6 cm; corolla lobes glabrous abaxially; anthers 1–1.5 mm; corona segments conduplicate, appendage fal­cate; follicles smooth   24 Asclepias nummularia
+ Stems 20–40 cm; leaf blades linear-lanceolate, 5–15 × 0.3–1.3 cm; peduncles 0.2–1.5 cm; corolla lobes minutely pilosulous abaxially; anthers 2–2.5 mm; corona segments tubular, appendage lingulate; follicles ribbed   26 Asclepias brachystephana (in part)
       
104 (102) Leaf blades linear, 0.1–0.8 cm wide.   (105)
+ Leaf blades narrowly elliptic, oval, oblong, lanceolate, ovate, oblanceolate, or obovate, 0.3–6 cm wide.   (107)
       
105 (104) Flowers pendent to spreading; corolla lobes pilose abaxially; corona segments conduplicate, sessile, apex truncate, append­age crestlike, barely exserted; fruiting pedicels upcurved   25 Asclepias quinquedentata (in part)
+ Flowers erect; corolla lobes glabrous abaxially; corona seg­ments tubular or cupulate, stipitate, apex obtuse, appendage falcate or acicular, well exserted; fruiting pedicels straight.   (106)
       
106 (105) Stems few–numerous, 20–60 cm; leaf blades spreading, not secund, midvein puberulent with curved trichomes; umbels all extra-axillary; corollas cream, sometimes tinged or striped pink; anthers brown; corona segments tubular, appendage acicular, arching above style apex; follicles glabrous; se Arizona   8 Asclepias angustifolia (in part)
+ Stems 1–4 (rarely more), 15–30 cm; leaf blades ascending, appearing secund, midvein glabrous; umbels terminal, some­times also extra-axillary; corollas green to tan, tinged pink or red; anthers green; corona segments cupu­late, appendage fal­cate, sharply inflexed over style apex; follicles minutely puber­ulent with curved trichomes; se United States   69 Asclepias michauxii (in part)
       
107 (104) Corollas white, pink, or cream; corona segment appendages acicular or subulate.   (108)
+ Corollas green, sometimes tinged red, purple, or bronze, or red-violet; corona segment appendages falcate or conical.   (110)
       
108 (107) Interpetiolar ridges absent; petioles 2–7 mm; leaf blades 1–6 cm wide; umbels terminal (sometimes also extra-axillary); flowers spreading to pendent; pedicels 17–28 mm; corona segments conduplicate, sessile, appendage subulate   73 Asclepias quadrifolia (in part)
+ Interpetiolar ridges present; petioles 5–20 mm; leaf blades 0.3–3 cm wide; umbels extra-axillary (may appear terminal); flowers erect to spreading; pedicels 7–14 mm; corona segments cupulate or tubular, stipitate, appendage acicular.   (109)
       
109 (108) Leaf blades narrowly elliptic to oval or oblong, 5–14 cm; corona segments tubular, 1.5–2.5 mm; follicles pendulous on deflexed pedicels, lance-ovoid, 4–7 × 1–2.5 cm; seeds 12–15 × 11–14 mm, coma absent; bottomlands and ditches, e United States to e Texas   10 Asclepias perennis (in part)
+ Leaf blades ovate to lanceolate or elliptic, 2–7 cm; corona segments cupulate, 2–3 mm; follicles erect on straight pedicels, narrowly fusiform, 9–13 × 0.5–1 cm; seeds 6–7 × 4–5 mm, coma present; canyons and slopes; c, w Texas   11 Asclepias texana
       
110 (107) Corollas red-violet; corona segments cupulate, 1.5–2 mm, appendage conical, barely exserted; fruiting pedicels spreading, follicles spreading to pendulous, lance-ovoid, 3.5–6 cm, strigose; Nevada   57 Asclepias eastwoodiana (in part)
+ Corollas green, tinged purple or bronze, rarely reddish purple; corona segments tubular or conduplicate, 2–6 mm, appendage falcate, well exserted; fruiting pedicels upcurved, follicles erect, fusiform, 5–10.5 cm, pilosulous or puberulent with curved trichomes; Florida or w Texas.   (111)
       
111 (110) Stems 15–20 cm, pilosulous; leaf blades 6–8 cm, sparsely hirtellous, mar­gins crisped; umbels terminal; peduncles 7–17 cm; pedicels 15–20 mm, pilose; anthers brown, 1.5–2 mm; corona segments tubular, 2–3 mm, apex truncate; w Texas   20 Asclepias scaposa
+ Stems 15–100 cm, puberulent with curved trichomes; leaf blades 1.8–5 cm, sparsely puberulent with curved trichomes on midvein only, margins entire; umbels terminal and extra-axillary; peduncles 0–4 cm; pedicels 10–14 mm, sparsely puberulent with curved trichomes; anthers green, 1–1.5 mm; corona segments conduplicate, 5–6 mm, apex attenuate; Florida   34 Asclepias curtissii
       
112 (98) Stems, leaf blades, peduncles, or pedicels persistently tomentose or tomentulose.   (113)
+ Stems, leaf blades, peduncles, and pedicels variously indumented, sometimes ephemerally tomentose or tomentulose.   (126)
       
113 (112) Corona segments 7–15 mm.   (114)
+ Corona segments 2–7 mm.   (115)
       
114 (113) Petioles 2–6 mm; leaf blade surfaces adaxially pilosulous or tomentulose to glabrate, abaxially tomentose; corolla lobes deep maroon to green adaxially, glabrous adaxially; anthers 2–2.5 mm; coronas deep maroon to yellowish green, segments 7–9 mm, appendage absent or merely a low crest; follicles fusiform, 1.2–1.4 cm wide; extreme se Arizona and sw New Mexico   27 Asclepias hypoleuca
+ Petioles 4–12 mm; leaf blade surfaces uniformly green, tomentose to pilose; corolla lobes dark pink (rarely pale), hirtellous at base adaxially; anthers 2.5–3 mm; coronas pale pink to nearly cream, segments 9–15 mm, appendage subulate, sharply inflexed over style apex; follicles lance-ovoid, 2–3 cm wide; widespread in w United States and Canada, but not in se Arizona and sw New Mexico   76 Asclepias speciosa
       
115 (113) Corolla lobes glabrous abaxially.   (116)
+ Corolla lobes pilose, pilosulous, or tomentose abaxially, at least at tips.   (119)
       
116 (115) Corollas pink, red green, or pinkish green, or reddish green; anthers brown; corona segments cream with red, pink, or purple dorsal stripe, apex acute, appendage glabrous; Rocky Mountains and associated alluvial plains, 1700–3000 m   45 Asclepias hallii (in part)
+ Corollas green to yellowish green, sometimes tinged red or purple; anthers green; corona segments green with cream apex, sometimes tinged pink or purple, or cream, greenish cream, or ochroleucous, not striped red, pink, or purple, apex truncate to rounded, appendage papillose; se United States, Great Plains, or Colorado Plateau, to 2300 m.   (117)
       
117 (116) Corona segments green with cream apex, sometimes tinged pink or purple; corona segment apices glabrous; follicles fusiform to narrowly lance-ovoid, 1–1.5 cm wide; se United States and e Texas   43 Asclepias tomentosa (in part)
+ Corona segments cream to greenish cream or ochroleucous, apices papillose; follicles lance-ovoid to ovoid, 2–3 cm wide; Great Plains and Colorado Plateau to e Texas.   (118)
       
118 (117) Petioles 7–17 mm; leaf blade bases rounded, truncate, or subcordate; anthers 2–2.5 mm; corona segment apices truncate to rounded, emarginate, not oblique, appendage apex upturned; seeds 9–12 × 6–8 mm   38 Asclepias arenaria
+ Petioles 0–4 mm; leaf blade bases cordate; anthers 3–3.5 mm; corona segment apices truncate, not emarginate, oblique, appendage apex not upturned; seeds 7–8 × 5–6 mm   44 Asclepias latifolia (in part)
       
119 (115) Leaf blades linear-lanceolate, 0.3–1.3 cm wide; corolla lobes 4–6 mm; corona segments 1.5–2 mm; follicles conspicuously striate   26 Asclepias brachystephana (in part)
+ Leaf blades ovate or oval to lanceolate, elliptic, oblong, or obovate, 1.5–15 cm wide; corolla lobes 5–11 mm; corona segments 2.5–6 mm; follicles not striate.   (120)
       
120 (119) Corolla lobes pilose at base adaxially; gynostegial column 2–3.5 mm; corona segments red-violet, dorsally convex from base to apex, margins connivent, appendage absent   21 Asclepias californica (in part)
+ Corolla lobes glabrous, minutely hirtellous, or papillose at base adaxially; gynostegial column 0.2–1.5 mm; corona segments cream or ochroleucous to dark pink (sometimes reddish purple in A. syriaca), dorsally straight or concave from base to apex, margins separate, appendage falcate or acicular.   (121)
       
121 (120) Leaf blades 3–9 × 1.5–4.5 cm, pilosulous; flowers 5–21 per umbel; s Canada (Alberta to Ontario), n United States (Montana and Wyoming e to Illinois and Michigan)   75 Asclepias ovalifolia (in part)
+ Leaf blades 6–30 × 2–15 cm, tomentose or puberulent with curved trichomes to glabrate (sometimes pilosulous in A. syriaca); flowers 12–113 per umbel; Arizona, California, Nevada, and Utah, or widespread in e United States and Canada, w to Manitoba, North Dakota, and South Dakota (A. syriaca).   (122)
       
122 (121) Pedicels 10–15 cm; corona appendages acicular; fruiting pedicels spreading, follicles pendulous; extreme n Arizona and s Utah   53 Asclepias welshii (in part)
+ Pedicels 15–50 cm; corona appendages falcate; fruiting pedicels upcurved, follicles erect; w Arizona, California, s Nevada, and sw Utah, or e United States and Canada.   (123)
       
123 (122) Pedicels pilose to puberulent with curved trichomes; corona segments slightly flattened dorsally, apex obtuse and usually flared, not oblique; follicles muricate (less commonly smooth); e United States and Canada   77 Asclepias syriaca (in part)
+ Pedicels tomentose to glabrate; corona segments rounded dorsally, apex truncate or obtuse and oblique, not flared; follicles smooth or longitudinally ridged; Arizona, California, Nevada, and Utah.   (124)
       
124 (123) Peduncles 0–3.5 cm; corolla lobes tomentose abaxially throughout; anthers 1.5–2 mm; corona segments sessile, apex obtuse; follicles ovoid, 5–6.5 cm, ridged   22 Asclepias vestita (in part)
+ Peduncles 1–10 cm; corolla lobes tomentose abaxially only at tips; anthers 2.5–3 mm; corona segments stipitate, apex truncate; follicles lance-ovoid to ovoid, 5–10 cm, smooth.   (125)
       
125 (124) Petioles 0–6 mm; leaf blade margins erose; corollas green; anthers green; coronas cream to ochroleucous, segment apices and appendages glabrous; follicles thinly tomentose; seeds 8–13 × 5–10 mm   35 Asclepias erosa (in part)
+ Petioles 4–15 mm; leaf blade margins entire or undulate; corollas greenish cream, ochroleucous, pink, tan, or pinkish cream; anthers dark brown; coronas cream to dark pink, segment apices and appendages papillose; follicles densely tomentose, seeds 7–9 × 4–6 mm   36 Asclepias eriocarpa (in part)
       
126 (112) Stems, leaf blades, peduncles, and pedicels hirsute or hirtellous.   (127)
+ Stems, leaf blades, peduncles, and pedicels variously indumented, but trichomes not straight and spreading.   (131)
       
127 (126) Stems 100–150 cm; leaf blades 3–14 cm wide; umbels terminal and extra-axillary; peduncles 6–13 cm; flowers 21–53 per umbel; corollas cream to greenish cream or ochroleucous, sometimes tinged pink; corona segments conduplicate; se Arizona   23 Asclepias lemmonii
+ Stems 6–70 cm (rarely to 200 cm in A. obovata); leaf blades 0.4–7.5 cm wide; umbels extra-axillary (sometimes appearing terminal); peduncles 0–2 cm; flowers 4–32 per umbel; corollas green, sometimes tinged red, purple, brown, or bronze; corona segments tubular; s United States, including se Arizona.   (128)
       
128 (127) Stems 40–70(–200) cm; flowers erect to pendent; anthers 2.5–4 mm; corona segments bronze or yellow, often tinged red, sometimes paler or cream at apex, appendage falcate, glabrous; seeds 8–9 × 6–7 mm; s Canada (Alberta to Ontario), n United States (Montana and Wyoming e to Illinois and Michigan)   64 Asclepias obovata
+ Stems 6–50 cm; flowers erect to ascending; anthers 1–2.5 mm; corona segments cream or green with cream or white at apex, sometimes tinged bronze, appendage lingulate, papil­lose or glabrous; seeds 6–8 × 4.5–6.5 mm; California to w Louisiana, n to c Oklahoma.   (129)
       
129 (128) Leaf blades 0.4–2.5 cm wide, base cuneate; flowers 4–8 per umbel; corolla lobes 5–7 mm; anthers green, 1–1.5 mm; corona segments 3.5–5.5 mm, appendage flush with apex and obscuring cavity   41 Asclepias emoryi
+ Leaf blades 1.2–7.5 cm wide, base cuneate to subcordate; flowers 5–32 per umbel; corolla lobes 7–13 mm; anthers brown, sometimes green at base, 1.7–2.5 mm; corona segments (5–)7–11 mm, appendage exserted below apex, exposing cavity.   (130)
       
130 (129) Leaf blades ovate or lanceolate to oblong or elliptic, base cuneate to obtuse, apex obtuse to rounded, sometimes conduplicate; pedicels 10–20 mm; gynostegial column 1–1.5 mm; anthers 2–2.5 mm; corona segments green, sometimes tinged bronze, white or cream at apex, relatively slender, apex deeply emarginate; se Arizona, sw Colorado, w Louisiana, New Mexico, Oklahoma, Texas   40 Asclepias oenotheroides (in part)
+ Leaf blades ovate to lanceolate, base cuneate or obtuse to truncate or subcordate, apex obtuse to acute, rarely con­du­plicate; pedicels 17–30 mm; gynostegial column 0.3–0.5 mm; anthers 1.7–2 mm; corona segments cream or green with cream apex, relatively stout, apex truncate; Arizona, se California, s Nevada, sw New Mexico   42 Asclepias nyctaginifolia (in part)
       
131 (126) Corona appendages crestlike, included or barely exserted from segment, or absent.   (132)
+ Corona appendages falcate or acicular, well exserted.   (135)
       
132 (131) Corolla lobes 10–13 mm; corona segments clavate, 8–10 mm, greatly exceeding style apex, segment apices sometimes connivent   32 Asclepias connivens (in part)
+ Corolla lobes 4–10 mm; corona segments conduplicate or laminar with incurved margins, 1.5–4 mm, barely exceeding to greatly exceeded by style apex, segment apices separated by gynostegium.   (133)
       
133 (132) Leaf blades linear to broadly oval or nearly orbiculate, 0.8–6 cm wide; flowers 22–60 per umbel; anthers 3–4 mm; corona segments laminar, appressed to gynostegium, margins incurved; follicles fusiform to lance-ovoid, 1.5–2 cm wide   46 Asclepias viridiflora
+ Leaf blades linear to narrowly elliptic or narrowly lanceolate, 0.1–0.8 cm wide; flowers 2–10 per umbel; anthers 1–2 mm; corona conduplicate, margins not incurved, not appressed to gynostegium; follicles fusiform to lance-ovoid, 0.3–1 cm wide.   (134)
       
134 (133) Leaf blades linear, 6–14 cm; peduncles 1.1–2.7 cm; flowers pendent to spreading; pedicels 14–21 mm; corolla lobes reflexed, exposing corona, 4–6 mm, pilose abaxially; anthers tan to brown; corona segments red, pink, red-violet, or purple at base, white at apex, 3–4 mm; appendage crestlike, barely exserted; fruiting pedicels upcurved, follicles 0.5–1 cm wide, pilosulous to glabrate   25 Asclepias quinquedentata (in part)
+ Leaf blades linear to narrowly elliptic or narrowly lanceolate, 1.5–6 cm; peduncles 0.15–1 cm; flowers erect; pedicels 5–13 mm; corolla lobes erect, mostly concealing corona, 7–10 mm, glabrous abax­ially; anthers green; corona segments yellowish green to green, 1.5–2.5 mm, appendage absent; fruiting pedicels straight, follicles 0.3–0.5 cm wide, puberulent with curved trichomes   33 Asclepias pedicellata (in part)
       
135 (131) Corona segment apices truncate.   (136)
+ Corona segment apices acute to obtuse.   (140)
       
136 (135) Petioles 10–25 mm; corollas white with red-violet at base of lobes; anthers brown, 1.5–2 mm; seeds 5–7 × 3–5 mm   71 Asclepias variegata
+ Petioles 0–15 mm; corollas green, sometimes tinged pink, red, or purple; anthers green, 2.5–4 mm; seeds 6.5–13 × 4–10 mm.   (137)
       
137 (136) Petioles 5–15 mm; leaf blades membranous, base cuneate; corona segments tubular, appendage arching above style apex; ne United States to n Alabama, n Georgia, w North Carolina, w South Carolina   74 Asclepias exaltata (in part)
+ Petioles 0–9 mm; leaf blades chartaceous, coriaceous, or succulent, base obtuse to cordate; corona segments conduplicate, appendage sharply inflexed over or towards style apex; w United States or s Alabama, Florida, s Georgia, e North Carolina, e South Carolina, and e Texas.   (138)
       
138 (137) Leaf blade apices attenuate to acuminate, margins erose; umbels terminal and extra-axillary; peduncles 2–10 cm; pedicels 20–45 mm; corolla lobes tomentose abaxially at tips; anthers 2.5–3 mm; corona segment appendages glabrous; seeds 8–13 × 5–10 mm; n Arizona, s California, s Nevada, sw Utah   35 Asclepias erosa (in part)
+ Leaf blade apices acute to truncate, sometimes emargin­ate, margins entire or crisped; umbels only extra-axillary; peduncles 0–2.5 cm; pedicels 12–35 mm; corolla lobes gla­brous abaxially; anthers 3–4 mm; corona seg­ment appendages papillose; seeds 6.5–8 × 4.5–6 mm; se United States or c United States to n Arizona, s Utah.   (139)
       
139 (138) Petioles 2–9 mm; leaf blades 1–5 cm wide; peduncles 0–0.3 cm; pedicels 12–19 mm; corona segments green with cream apex, often tinged pink or purple; follicles fusiform to narrowly lance-ovoid, 9–18 × 1–1.5 cm; se United States and e Texas   43 Asclepias tomentosa (in part)
+ Petioles 0–4 mm; leaf blades 3–14 cm wide; peduncles 0–2.5 cm; ped­icels 15–35 mm; corona segments cream, sometimes yellow dorsally; follicles ovoid, 6.5–9.5 × 2–3 cm; c United States   44 Asclepias latifolia (in part)
       
140 (135) Stems often branched, at least in inflorescence; interpetiolar ridges present; corona segment appendages arching above style apex; fruiting pedicels straight.   (141)
+ Stems unbranched (rarely branched in A. syriaca, branched only in the inflorescence in A. hallii, A. purpurascens); interpetiolar ridges absent; corona segment appendages sharply inflexed over style apex (sometimes ascending in A. ovalifolia); fruiting ped­icels upcurved.   (142)
       
141 (140) Corollas red, sometimes yellow in throat (may be uniformly yellow or orange in cultivars), lobes 6–9 mm; gynostegial column 2–2.5 mm; corona segments yellow to orange, 3.5–4 mm, appendage falcate; follicles glabrous; seeds 6–7 × 4–5 mm; California, Florida, Louisiana, Texas   7 Asclepias curassavica
+ Corollas pink to white, lobes 5–6 mm; gynostegial column 1.2–1.5 mm; corona segments pink to white, 2–2.5 mm, appendage acicular; follicles sparsely puberulent with curved trichomes to pilose or pilosulous; seeds 8–9 × 5–6 mm; widespread in United States and Canada   9 Asclepias incarnata
       
142 (140) Stems 30–70 cm; flowers 5–29 per umbel; gynostegial column 0.2–1 mm; follicles 0.7–2 cm wide; seeds 5–7 mm.   (143)
+ Stems 50–200 cm; flowers 17–113 per umbel; gynostegial column 1–1.5 mm; follicles 1.5–4 cm wide; seeds 7–8 mm.   (144)
       
143 (142) Petioles 10–20 mm; corollas pale pink to red or green, tinged pink or red, lobes 7–8 mm, glabrous abaxially; anthers brown, 2–2.5 mm; corona seg­ments cream with red, pink, or purple dorsal stripe, 5–6.5 mm; follicles 8–12 cm; Rocky Mountains and associated alluvial plains   45 Asclepias hallii (in part)
+ Petioles 3–10 mm; corollas cream to yellowish, lobes 5–7 mm, pilosulous abaxially at apices; anthers green, 1.5–2 mm; corona segments cream to yellowish, 3–4 mm; follicles 5–8 cm; s Canada (Alberta to Ontario), n United States (Montana and Wyoming e to Illinois and Michigan)   75 Asclepias ovalifolia (in part)
       
144 (142) Adaxial leaf blades sparsely pilosulous; umbels terminal and extra-axillary; flowers erect to spreading; corollas reddish purple, lobes glabrous abaxially; anthers brown; corona segments reddish purple, stipitate, conduplicate, apex acute, 5–6 mm; follicles 10–16 × 1.5–2.5 cm, smooth, puberulent with curved trichomes   72 Asclepias purpurascens
+ Adaxial leaf blades tomentose to glabrate; umbels only extra-axillary; flowers erect to pendent; corollas dark to pale pink or green, lobes pilose abaxially; anthers green; corona segments reddish purple to cream, sessile, tubular, apex obtuse, flared, 4–5 mm; follicles 7–12 × 2–4 cm, muricate (sometimes smooth), tomentose   77 Asclepias syriaca (in part)

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