Asclepias

Milkweeds
Asclepias syriaca showing flowers and latex like sap.
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Gentianales
Family:	Apocynaceae
Subfamily:	Asclepiadoideae
Genus:	Asclepias L.
Type species
Asclepias syriaca L.
Species
See text.

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 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, latex, 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 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.

Species in the Asclepias genus grow their seeds in follicles. These follicles contain soft filaments known as either silk or floss. The filaments are attached to individual seeds. When the follicle ripens, the seeds are blown by the wind, each carried by several filaments.

Milkweeds use three primary defenses to limit damage caused by caterpillars: hairs on the leaves, cardenolid toxins, and latex fluids. Data from a DNA study indicates that more recently evolved milkweed species utilize less of these preventative strategies, but grow faster than older species; potentially regrowing faster than caterpillars can consume them.^[1]

Species

Some Asclepias species:

	Asclepias albicans	Whitestem milkweed
	Asclepias amplexicaulis	Blunt-leaved milkweed
	Asclepias asperula	Antelope horns
	Asclepias californica	California milkweed
	Asclepias cordifolia	Heart-leaf milkweed
	Asclepias cryptoceras	Pallid milkweed
	Asclepias curassavica	Scarlet milkweed, Tropical milkweed, Bloodroot, Bloodflower, Bastard Ipecacuanha
	Asclepias eriocarpa	Woollypod milkweed
	Asclepias erosa	Desert milkweed
	Asclepias exaltata	Poke milkweed
	Asclepias fascicularis	Narrow leaf milkweed
	Asclepias fruticosa syn. Gomphocarpus fruticosus	swan plant, African milkweed
	Asclepias humistrata	Sandhill milkweed
	Asclepias incarnata	Swamp milkweed
	Asclepias lanceolata	Lanceolate milkweed
	Asclepias linaria	Pine needle milkweed
	Asclepias linearis	Slim milkweed
	Asclepias meadii	Mead's milkweed
	Asclepias nyctaginifolia	Mojave milkweed
	Asclepias obovata	Pineland milkweed
	Asclepias physocarpa	Gomphocarpus physocarpus, commonly balloonplant, balloon cotton-bush, giant swan plant, testicle tree, bishop's balls or swan plant
	Asclepias purpurascens	Purple milkweed
	Asclepias quadrifolia	Four-leaved milkweed
	Asclepias rubra	Red milkweed
	Asclepias solanoana	Serpentine milkweed
	Asclepias speciosa	Showy milkweed
	Asclepias subulata	Rush milkweed(Leafless milkweed)
	Asclepias sullivantii	Sullivant's milkweed
	Asclepias syriaca	Common milkweed
	Asclepias tuberosa	Butterfly weed, Pleurisy root
	Asclepias variegata	White milkweed
	Asclepias verticillata	Whorled milkweed
	Asclepias vestita	Woolly milkweed
	Asclepias vincetoxicum
	Asclepias viridiflora

Uses

Asclepias syriaca seed pods
Baldwinsville, New York

The milkweed filaments from the follicles are coated with wax, and have good insulation qualities. Tests have shown them to be superior to down feathers for insulation. During World War II, over 11 million pounds (5000 t) 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.^[2]

Seeds.

In the past, the high dextrose content of the nectar led to milkweed's use as a source of sweetener for Native Americans and voyageurs.

The bast fibers of some species were also used for cordage.

Milkweed latex contains about 1 to 2% caoutchouc, and was attempted as a natural source for rubber by both Germany and the United States during World War II. No record has been found of large-scale success.

Milkweed is a common folk remedy used for removing warts. Milkweed sap is applied directly to the wart several times daily until the wart falls off. Dandelion sap is often used in the same manner.

Milkweed is beneficial to nearby plants, repelling some pests, especially wireworms.

Milkweed also contains cardiac glycoside poisons which 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 1/10 its body weight in any part of the plant. Milkweed also causes mild dermatitis in some who come in contact with it.

Milkweed sap is also externally used as a natural remedy for Poison Ivy.

Being the sole food source of Monarch Butterfly larva, the plant is often used in Butterfly gardening.

See also

• List of beneficial weeds
• List of companion plants

References

• Everitt, J.H.; Lonard, R.L., Little, C.R. (2007). Weeds in South Texas and Northern Mexico. Lubbock: Texas Tech University Press.  ISBN 0-89672-614-2

External links

• Media related to Asclepias at Wikimedia Commons
• Medical research
• Milkweed test-cultivated for the insulation value of floss
• Milkweed in Handbook of Energy Crops
• Common milkweed production research at Western Illinois University
• UVSC Herbarium - Asclepias
• Milkweed Photo Gallery