Asclepias
Asclepias is a genus of herbaceous, perennial, flowering plants known as milkweeds, named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged. Most species are toxic to humans and many other species, primarily due to the presence of cardenolides. However, as with many such plants, some species feed upon milkweed leaves or the nectar from their flowers. A noteworthy feeder on milkweeds is the monarch butterfly, which uses and requires certain milkweeds as host plants for its larvae.
The Asclepias genus contains over 200 species distributed broadly across Africa, North America, and South America. It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae.
Etymology
In the first century AD, Dioscorides described a plant called ἀσκληπιάς in Greek and Pliny the Elder described a plant called asclepias in Latin. Though usually explained as being derived from the name of the Greek medical god Asklepios, neither author mentioned the god in connection with this plant. Their plant has since been commonly identified as the plant now known as Vincetoxicum hirundinaria. Some botanists have disagreed as, though the old descriptions are short and vague, Vincetoxicum hirundinaria does not have leaves like ivy.Linnaeus first described the genus Asclepias in his Genera Plantarum of 1737 without specifying any particular plants in the genus. Linnaeus published his Flora Suecica in 1745 and Materia Medica in 1749. The only Asclepias he mentioned in either book was the one he would later name as Asclepias vincetoxicum. Both books mentioned that in pharmacy this plant was called Hirundinariæ. In none of his works does Linnaeus give an origin for the name Asclepias, presumably because it was being used for these plants by some herbalists and botanists for centuries before. These publications were before his 1753 innovation of names that were simply two words - genus and species - rather than a much longer description.
When Linnaeus described the 18 species that he assigned to the genus Asclepias in his Species Plantarum of 1753, he included Asclepias vincetoxicum. That plant is now called Vincetoxicum hirundinaria. Of those 18, 7 have been assigned to other genera in the years since: Xysmalobium undulatum, Calotropis gigantea, Gymnema lactiferum, Vincetoxicum hirundinaria, Vincetoxicum nigrum, Gomphocarpus fruticosus and Cynanchum thesioides. The species that remain in Asclepias from those Linnaeus described are: A. syriaca, A. amoena and A. purpurascens both now regarded as A. purpurascens, A. variegata, A. nivea, A. incarnata, A. curassavica, A. decumbens and A. tuberosa both now regarded as A. tuberosa, A. verticillata and A. rubra.
Flowers
Members of the genus produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium surrounded by a five-membrane corona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn. Glands holding pollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genus Asclepias.Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures called pollinia, rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visiting insects, such as bees, wasps, and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force. Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit. Large-bodied hymenopterans are the most common and best pollinators, accounting for over 50% of all Asclepias pollination, whereas monarch butterflies are poor pollinators of milkweed.
Asclepias species produce their seeds in pods termed follicles. The seeds, which are arranged in overlapping rows, bear a cluster of white, silky, filament-like hairs known as the coma. The follicles ripen and split open, and the seeds, each carried by its coma, are blown by the wind. Some, but not all, milkweeds also reproduce by clonal reproduction.
Selected species
| Image | Scientific name | Common name | Native distribution |
| Asclepias albicans | Whitestem milkweed | Native to the Mojave and Sonoran deserts | |
| Asclepias amplexicaulis | Blunt-leaved milkweed | Native to central and eastern United States | |
| Asclepias asperula | Antelope horns | Native to American southwest and northern Mexico | |
| Asclepias californica | California milkweed | Native to central and southern California | |
| Asclepias cordifolia | Heart-leaf milkweed | Native to the Sierra Nevada and Cascade Range up to. | |
| Asclepias cryptoceras | Pallid milkweed | Native to the western United States. | |
| Asclepias curassavica | Scarlet milkweed, tropical milkweed, bloodflower, bastard ipecacuanha | Native to the American tropics, introduced to other continents | |
| Asclepias curtissii | Curtiss's milkweed | Endemic to sandy areas of Florida | |
| Asclepias eriocarpa | Woollypod milkweed | Native to California, Baja California, and Nevada | |
| Asclepias erosa | Desert milkweed | Native to California, Arizona, and Baja California | |
| Asclepias exaltata | Poke milkweed | Native from Quebec to Ontario and from Maine to Minnesota, south to Georgia and Mississippi | |
| Asclepias fascicularis | Narrow-leaf milkweed | Native from northeastern Washington and Idaho, south to Baja California and northern Arizona | |
| Asclepias hirtella | Tall green milkweed | Native to Canada and the Midwestern and Upper South United States | |
| Asclepias humistrata | Sandhill milkweed | Native to southeastern United States | |
| Asclepias incarnata | Swamp milkweed | Native from Manitoba to Quebec and Nova Scotia, from New England south to Georgia, west to Louisiana and Texas, and north to North Dakota. | |
| Asclepias lanceolata | Lanceolate milkweed | Native to coastal plain of eastern United States from Texas to New Jersey | |
| Asclepias linaria | Pine needle milkweed | Native to Mojave and Sonoran deserts | |
| Asclepias meadii | Mead's milkweed | Native to midwestern United States | |
| Asclepias nyctaginifolia | Mojave milkweed | native to the American southwest | |
| Asclepias purpurascens | Purple milkweed | Native to eastern, southern, and midwestern United States | |
| Asclepias prostrata | Prostrate milkweed | Native to Texas and northern Mexico | |
| Asclepias quadrifolia | Four-leaved milkweed | Native to eastern United States and Canada | |
| Asclepias rubra | Red milkweed | Native to New York, south to Florida, and west to Arizona and Texas | |
| Asclepias solanoana | Serpentine milkweed | Native to northern California | |
| Asclepias speciosa | Showy milkweed | Native to western United States and Canada | |
| Asclepias subulata | Rush milkweed | Native to southwestern North America | |
| Asclepias subverticillata | Horsetail milkweed | Native to southwestern United States and Mexico | |
| Asclepias sullivantii | Sullivant's milkweed | Native to the midwestern United States and Canada, ranging north to Minnesota, east to southern Ontario and Ohio, west to Nebraska, Kansas, and south to Oklahoma. It is considered rare in Minnesota, Wisconsin, and Ontario, and is known only from historical records in North Dakota. | |
| Asclepias syriaca | Common milkweed | Native to southern Canada and much of the United States east of the Rocky Mountains, excluding Florida and the drier parts of the prairies. | |
| Asclepias texana | Texas milkweed | Native from central Texas west to the Edwards Plateau and south into Coahuila and the Chihuahuan Desert as far south as Durango. | |
| Asclepias tuberosa | Butterfly weed, pleurisy root | Native from Ontario to Newfoundland and New England, south to Florida, west to Texas, and north through Colorado to Minnesota. | |
| Asclepias uncialis | Wheel milkweed | Native to Arizona, Colorado, New Mexico, Nevada, Oklahoma, Texas, Utah and Wyoming. | |
| Asclepias variegata | White milkweed | Native to New York and Connecticut, south to Florida, west to Texas and Oklahoma, and northeast to Illinois and Ohio. | |
| Asclepias verticillata | Whorled milkweed | Native from Massachusetts to Montana, south to Florida and eastern Texas; also southern Canada | |
| Asclepias viridiflora | Green milkweed | Native from Ontario to British Columbia and from Massachusetts and New York to Montana, south to Florida, Texas, and Arizona | |
| Asclepias viridis | Green antelopehorn, spider milkweed | Native from West Virginia to Nebraska, south to Florida and Texas | |
| Asclepias welshii | Welsh's milkweed | Native to southern Utah and northern Arizona |
There are also 12 species of Asclepias in South America, among them: A. barjoniifolia, A. boliviensis, A. curassavica, A. mellodora, A. candida, A. flava, and A. pilgeriana.