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| Asclepias | |
|---|---|
 | |
| Asclepias syriaca showing flowers and latex like sap. | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Eudicots |
| (unranked): | Asterids |
| Order: | Gentianales |
| Family: | Apocynaceae |
| Subfamily: | Asclepiadoideae |
| Tribe: | Asclepiadeae |
| Subtribe: | Asclepiadinae |
| Genus: | Asclepias L.[1] |
| Type species | |
| Asclepias syriaca L. | |
| Species | |
See text. | |
| Synonyms | |
Acerates Elliott Anantherix Nutt. Asclepiodella Small Asclepiodora A.Gray Biventraria Small Oxypteryx Greene Podostemma Greene Schizonotus A.Gray Solanoa Greene[1] | |
Asclepias L. (1753), the milkweeds, is a 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 juice, which contains alkaloids, latex, 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, because of the many folk-medicinal uses for the milkweed plants.
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, pulling a pair of pollen sacs free when the pollinator flies off. Pollination is effected by the reverse procedure in which one of the pollinia becomes trapped within the anther slit.
Asclepias species produce their seeds in follicles. The seeds, which are arranged in overlapping rows, have white silky filament-like hairs known as pappus,silk, or floss. The follicles ripen and split open and the seeds, each carried by several dried pappus, are blown by the wind. They have many different flower colorations.
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[edit]Ecology
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.
Milkweeds use three primary defenses to limit damage caused by caterpillars: hairs on the leaves, cardenolide toxins, and latex fluids. Data from a DNA study indicate more recently evolved milkweed species use less of these preventative strategies, but grow faster than older species, potentially regrowing faster than caterpillars can consume them.[2]
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