Milkweeds
| Asclepias | |
|---|---|
 |
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| Asclepias syriaca showing flowers and latex. | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Eudicots |
| (unranked): | Asterids |
| Order: | Gentianales |
| Family: | Apocynaceae |
| Subfamily: | Asclepiadoideae |
| Tribe: | Asclepiadeae |
| Subtribe: | Asclepiadinae |
| Genus: |
Asclepias L. |
| Type species | |
|
Asclepias syriaca L. |
|
| Species | |
|
See text. |
|
| Synonyms | |
See text.
Asclepias L. (1753), the milkweeds, is an American genus of herbaceous perennial, dicotyledonous plants that contains over 140 known species. It previously belonged to the family Asclepiadaceae, but this is now classified as the subfamily Asclepiadoideae of the dogbane family Apocynaceae.
Milkweed is named for its milky sap, which consists of a latex containing alkaloids and several other complex compounds including cardenolides. Some species are known to be toxic.
Carl Linnaeus named the genus after Asclepius, the Greek god of healing.
Asclepias species produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium (fused stamen filamens and styles) surrounded by a five-membered 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 (or "pollen sacs"), 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 (if not, the insect may become trapped and die). Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit.
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