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Published In: Species Plantarum 1: 214. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library

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1. Asclepias L. (milkweed)

(Woodson, 1954)

Plants with white latex and thus milky sap (except in A. tuberosa). Stems erect to spreading, not twining or climbing. Leaves alternate, opposite, or whorled. Calyces with the lobes reflexed at maturity (spreading in A. viridis). Corollas reflexed at maturity (spreading to loosely ascending in A. viridis), white, green, pink, purple, or orange. Gynostegium sessile or appearing short-stalked, the corona modified into 5 lateral segments (hoods, Pl. 220 c), these petaloid or fleshy, tubular to obconical, erect or ascending, often appearing curved or arched, sometimes with an erect or incurved, hairlike or linear appendage (horn, Pl. 220 c) extended from the open tip. Fruits single or in pairs, mostly erect or ascending (sometimes from a deflexed stalk), narrowly elliptic-lanceolate to ovate in outline, circular or slightly flattened in cross-section (not angled), the surface smooth or with warty tubercles. Seeds ovate to broadly ovate in outline, strongly flattened and usually winged, brown to dark brown, with a tuft of long silky hairs at the tip (except in A. perennis). About 150 species, mainly North America and Central America; also South America, Caribbean Islands, and Africa.

The genus Asclepias is distinct from other North American genera in its nontwining habit and unusual corona morphology. As with other groups of Asclepiadaceae, Asclepias in the broad sense apparently consists of several independent lineages whose interrelationships remain poorly understood. Most of the African species have been split into smaller genera, and segregate generic names are available for some North American species groups. The generic concept retained here is that of Woodson (1954), who combined the North American species into a single, broadly circumscribed genus divided into two subgenera and ten total series.

The latex of most species of milkweeds contains a mixture of chemicals, principally cardiac glycosides, that render the plants both unpalatable and poisonous to livestock and humans (although young leaves, flowers, and immature fruits of some species are eaten by wildlife and have been eaten by humans after boiling to leach toxic constituents). Some insects, notably the monarch butterfly (Danaus plexippus (L.), also called milkweed butterfly, whose caterpillars eat only Asclepiadaceae, principally species of Asclepias), use the plant as a larval food source and sequester the toxic compounds, rendering the larvae and adults unpalatable to predators like birds. Various species of Asclepias also have a long history of economic uses (Rosatti, 1989; Moerman, 1998). Native Americans used plants medicinally as an analgesic, cold remedy, respiratory aid, and emetic, among other uses. The latex was allowed to dry fresh or was boiled first for use as a chewing gum. The stem fibers have been used for cordage and weaving, although they are relatively brittle and subject to breakage. The silky hairs on the seeds have been used to stuff pillows and cushions and were harvested commercially during World War II as a substitute for kapok in life jackets. Additionally, a number of species have been investigated as potential sources of hydrocarbons and rubber. Several species also are cultivated as garden ornamentals.

The hoods of Asclepias flowers produce nectar, and several species have fragrant flowers. A number of different insects have been documented to visit flowers of various species. Pollination is accomplished mostly by species of wasps, bees, moths, and butterflies, but beetles and flies are important for some species (Woodson, 1954).

In addition to the species treated below, A. exaltata L. (poke milkweed) should be searched for in bottomland and mesic upland forests in southeastern Missouri. This northeastern species was mapped erroneously as present in this portion of Missouri by Broyles and Wyatt (1993), but it does occur in adjacent Illinois. Plants would key imperfectly to either A. amplexicaulis or A. incarnata in the key below, depending on which characters were emphasized. Asclepias exaltata tends to be a taller plant than A. amplexicaulis and has branched stems and sharply pointed leaf tips, but like A. amplexicaulis it has large flowers with more or less tubular hoods. Although similar to A. incarnata in general aspect, A. exaltata differs in its broader leaves that are glaucous beneath and more gradually tapered to the petiole, as well as its larger flowers with more tubular (vs. more conical) hoods.


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1 1. Many or all of the leaves alternate

2 2. Corollas bright orangish yellow to reddish orange; sap clear ... 13. A. TUBEROSA

Asclepias tuberosa
3 2. Corollas pale green to green or light yellow, sometimes purple-tinged; sap milky

4 3. Calyx lobes spreading; corolla lobes 10–17 mm long, ascending to spreading ... 17. A. VIRIDIS

Asclepias viridis
5 3. Calyx lobes reflexed; corolla lobes 4–8 mm long, reflexed

6 4. Corolla lobes 6–8 mm long; hoods 4.0–5.5 mm long ... 16. A. VIRIDIFLORA

Asclepias viridiflora
7 4. Corolla lobes 4–6 mm long; hoods 1.5–4.0 mm long

8 5. Gynostegium appearing stalked (visible between the corona and corolla); corona hoods without horns, the tip entire ... 2. A. HIRTELLA

Asclepias hirtella
9 5. Gynostegium appearing sessile (the corona base touching the corolla or nearly so); corona with a minute horn fused to each hood, the hood thus appearing 3-toothed or shallowly 3-lobed or at the tip ... 9. A. STENOPHYLLA

Asclepias stenophylla
10 1. Leaves opposite or whorled (rarely the uppermost nodes with only 1 leaf)

11 6. Leaves all or nearly all in whorls of 3–6, the blades 0.5–3.0(–4.0) mm wide, linear

12 7. Reduced branchlets bearing pairs or clusters of short leaves in the axils of many of the main leaves ... 10. A. SUBVERTICILLATA

Asclepias subverticillata
13 7. Axillary branchlets with pairs or clusters of leaves absent or rare ... 15. A. VERTICILLATA

Asclepias verticillata
14 6. Leaves all opposite, or, if some of the leaves are in whorls of 3 or 4, then the blades 7–80 mm wide, not linear

15 8. Corona hoods without horns ... 16. A. VIRIDIFLORA

Asclepias viridiflora
16 8. Corona hoods each with a hairlike or linear horn

17 9. Corona hoods shorter than to about as long as the tip of the anther/stigma head, the horns extended conspicuously beyond the tips of the hoods

18 10. Leaves mostly 2–5 pairs per stem, sessile or nearly so, the blade oblong-ovate, the base truncate to shallowly cordate and frequently overlapping that of the opposite leaf, the tip rounded, sometimes also with an abrupt, short, sharp point; corolla lobes 8–11 mm long ... 1. A. AMPLEXICAULIS

Asclepias amplexicaulis
19 10. Leaves 6 to numerous pairs per stem, mostly petiolate, the blade narrowly lanceolate or elliptic to narrowly oblong or less commonly ovate, the base rounded to tapered, occasionally shallowly cordate but not overlapping that of the opposite leaf, the tip narrowed or tapered to a sharp point; corolla lobes 3–6 mm long

20 11. Leaf blades abruptly narrowed or rounded at the base, occasionally shallowly cordate; seeds with a tuft of long silky hairs at the tip; corolla lobes 4–6 mm long ... 3. A. INCARNATA

Asclepias incarnata
21 11. Leaf blades gradually narrowed or tapered at the base; seeds without a tuft of hairs at the tip; corolla lobes 2.5–4.0 mm long ... 5. A. PERENNIS

Asclepias perennis
22 9. Corona hoods conspicuously longer than the tip of the anther/stigma head, the horns arched over the head but not extended past the tips of the hoods

23 12. Leaves moderately to densely hairy on the undersurface

24 13. Hoods 9–15 mm long, abruptly narrowed below the middle, the apical half narrowly lanceolate to lanceolate, tapered to the tip ... 8. A. SPECIOSA

Asclepias speciosa
25 13. Hoods 3.5–8.0 mm long, gradually narrowed from at or above the middle, the apical portion oblong to ovate

26 14. Hoods not noticeably lobed along the margins; corollas reddish purple to dark purple; fruits without warty tubercles on the surface ... 6. A. PURPURASCENS

Asclepias purpurascens
27 14. Hoods with a pair of sharply triangular, ascending and incurved lobes at about the middle of the margins; corollas green to lavender, usually tinged with pink and/or white; fruits with narrow, warty tubercles on the surface ... 12. A. SYRIACA

Asclepias syriaca
28 12. Leaves glabrous on the undersurface (except sometimes along the margins) or sparsely hairy, mostly along the midvein

29 15. Leaves sessile or nearly so, the blades rounded to shallowly cordate at the base

30 16. Leaves mostly in 2–5 pairs per stem, the blades with minute, rough hairs along the margins; corollas white to pale cream-colored or pale green ... 4. A. MEADII

Asclepias meadii
31 16. Leaves in 7–15 pairs per stem, the blades glabrous along the margins; corollas light pink to purplish-pink, sometimes tinged with green ... 11. A. SULLIVANTII

Asclepias sullivantii
32 15. Leaves mostly with noticeable petioles, the blades gradually narrowed or tapered at the base

33 17. Usually 1 or more nodes with whorls of 4 leaves, the leaf blades lanceolate to narrowly ovate-elliptic, the tip gradually narrowed or tapered to a sharply point; corolla lobes 4.5–6.0 mm long ... 7. A. QUADRIFOLIA

Asclepias quadrifolia
34 17. Leaves usually opposite, the blades ovate to broadly oblong-elliptic, the tip rounded or narrowed to a blunt point or rounded but often with an abrupt, short, sharp point; corolla lobes 7–9 mm long ... 14. A. VARIEGATA Asclepias variegata
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