1. Asclepias L. (milkweed)
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,
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.