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.[3][4][5] Most species are toxic to humans and many other species, primarily due to the presence of cardenolides, although, as with many such plants, there are species that feed upon them (e.g. their leaves) and from them (e.g. their nectar). Most notable are monarch butterflies, who use and require certain milkweeds as host plants for their larvae.
The genus contains over 200 species distributed broadly across Africa, North America, and South America.[6] It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae.
The genus was formally described by Carl Linnaeus in 1753,[7] who named it after Asclepius, the Greek god of healing.[8]
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.[9]
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).[10] 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,[11] whereas monarch butterflies are poor pollinators of milkweed.[4]
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[12] (often referred to by other names such as pappus, "floss", "plume", or "silk"). 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 (or vegetative) reproduction.
