1 any of numerous plants of the genus Asclepias having milky juice and pods that split open releasing seeds with downy tufts [syn: silkweed]
2 annual Eurasian sow thistle with soft spiny leaves and rayed yellow flower heads [syn: Sonchus oleraceus]
Asclepias L. (1753), the milkweeds, is a genus of herbaceous perennial, dicotyledonous plants that contains over 140 known species. It used to belong to the family Asclepiadaceae, but this is now classified as a subfamily Asclepiadoideae of the dogbane family Apocynaceae.
Milkweeds are an important nectar source for bees and other nectar seeking insects, and a larval food source for monarch butterflies and their relatives, as well as a variety of other herbivorous insects (including numerous beetles, moths, and true bugs) specialized to feed on the plants despite their chemical defenses. Milkweed is named for its milky juice, which contains alkaloids, caoutchouc, and several other complex compounds including cardenolides. Some species are known to be toxic.
Carolus Linnaeus named the genus after Asclepius, the Greek god of healing, because of the many folk-medicinal uses for the milkweed plants.
Pollination in this genus is accomplished in an unusual manner, as the pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains, as is typical for plant pollen. The flower petals are smooth and rigid, and the feet of visiting insects (predominantly large wasps, such as spider wasps, which visit the plants for nectar) slip into notches in the flowers, where the sticky bases of the pollinia attach to the feet, pulling the pollen sacs free when the pollinator flies off. Bees, including honey bees only gather nectar from milkweed flowers, and are generally not effective pollinators despite the frequency of visitation.
Species in the Asclepias genus grow their seeds in pods. These seed pods contain soft filaments known as either silk or floss. The filaments are attached to individual seeds. When the seed pod ripens, the seeds are blown by the wind, each carried by several filaments.