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|Asclepias syriaca showing flowers and latex.|
|Asclepias syriaca showing flowers and latex.|
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.
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, 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.
Asclepias species produce their seeds in follicles. The seeds, which are arranged in overlapping rows, bear a cluster white, silky, filament-like hairs known as the coma (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.
They have many different flower colorations, depending on species.
Milkweeds are an important nectar source for native bees, wasps, and other nectar-seeking insects, though non-native honey bees commonly get trapped in the stigmatic slits and die, 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 that more recently evolved milkweed species use these preventative strategies less but grow faster than older species, potentially regrowing faster than caterpillars can consume them.
The milkweed filaments from the coma (the "floss") are hollow and coated with wax, and have good insulation qualities. During World War II, over 5,000 t (5,500 short tons) of milkweed floss were collected in the United States as a substitute for kapok. As of 2007, milkweed is grown commercially as a hypoallergenic filling for pillows. A study of the insulative properties of various materials found that milkweed floss was outperformed by other materials in insulation, loft, and lumpiness, but scored well on various metrics when mixed with down feathers.
Milkweed latex contains about 1 to 2% latex, and was attempted as a source of natural rubber by both Germany and the United States during World War II. No record has been found of large-scale success.
Milkweed also contains cardiac glycoside poisons that inhibit animal cells from maintaining a proper K+, Ca+ concentration gradient. As a result, many natives of South America and Africa used arrows poisoned with these glycosides to fight and hunt more effectively. Milkweed is toxic and may cause death when animals consume 10% of their body weight in any part of the plant. Milkweed also causes mild dermatitis in some who come in contact with it.
The leaves of Asclepias species, and of some species formerly classified as Asclepias such as Gomphocarpus physocarpus, are the only food source for monarch butterfly larvae and other milkweed butterflies. These plants are therefore often used in butterfly gardening.
Some Asclepias species:
|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 2000 m.|
|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 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 to eastern North America|
|Asclepias fascicularis||Narrow-leaf milkweed, native to Western United States|
|Asclepias humistrata||Sandhill milkweed, native to southeastern United States|
|Asclepias incarnata||Swamp milkweed, native to wetlands of North America|
|Asclepias lanceolata||Lanceolate milkweed (Cedar Hill 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 linearis||Slim milkweed|
|Asclepias longifolia||Longleaf milkweed|
|Asclepias meadii||Mead's milkweed, native to midwestern United States|
|Asclepias nyctaginifolia||Mojave milkweed, native to the American southwest|
|Asclepias obovata||Pineland milkweed|
|Asclepias purpurascens||Purple milkweed, native to eastern, southern, and midwestern United States|
|Asclepias quadrifolia||Four-leaved milkweed, native to eastern United States and Canada|
|Asclepias rubra||Red milkweed|
|Asclepias solanoana||Serpentine milkweed, native to northern California|
|Asclepias speciosa||Showy milkweed, native to western United States and Canada|
|Asclepias subulata||Rush milkweed, leafless milkweed, native to southwestern North America|
|Asclepias subverticillata||Horsetail milkweed|
|Asclepias sullivantii||Sullivant's milkweed|
|Asclepias syriaca||Common milkweed|
|Asclepias tuberosa||Butterfly weed, pleurisy root|
|Asclepias uncialis||Wheel milkweed|
|Asclepias variegata||White milkweed|
|Asclepias verticillata||Whorled milkweed|
|Asclepias vestita||Woolly milkweed|
|Asclepias viridis||Green milkweed|
|Asclepias welshii||Welsh's milkweed|
Some species formerly classified under the Asclepias genus include:
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