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.