Asclepias L.

Asclepias syriaca seed pods, Baldwinsville, New York

Milkweed sprout, a few days after sowing

Chemical structure of oleandrin, one of the cardiac glycosides

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 sap, which consists of a latex containing alkaloids and several other complex compounds including cardenolides.^[2] 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, 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).^[3] 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^[4] (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.

Ecology[edit]

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,^[3][5] 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.^[6]

Uses[edit]

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.^[7][8] As of 2007, milkweed is grown commercially as a hypoallergenic filling for pillows.^[9] 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.^[10] Milkweed fibers are used to clean up oil spills.^[11]

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 can be used for cordage.

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

Milkweed also contains cardiac glycoside poisons that inhibit animal cells from maintaining a proper K⁺, Ca⁺ concentration gradient.^{[citation needed]} 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.^{[citation needed]} 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.^{[citation needed]}

Species[edit]

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 curtissii	Curtiss' milkweed, endemic to sandy areas of Florida
	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[12]
	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 viridiflora
	Asclepias viridis	Green milkweed
	Asclepias welshii	Welsh's milkweed

Formerly placed here[edit]

Some species formerly classified under the Asclepias genus include:

• Calotropis gigantea (L.) W.T.Aiton (as A. gigantea L.)
• Calotropis procera (Aiton) W.T.Aiton (as A. procera Aiton)
• Cynanchum louiseae Kartesz & Gandhi (as A. nigra L.)
• Cynanchum thesioides (Freyn) K.Schum. (as A. sibirica L.)
• Funastrum clausum (Jacq.) Schltr. (as A. clausa Jacq.)
• Gomphocarpus cancellatus (Burm.f.) Bruyns (as A. cancellatus Burm.f. or A. rotundifolia Mill.)
• Gomphocarpus fruticosus (L.) W.T.Aiton (as A. fruticosa L.)
• Marsdenia macrophylla (Humb. & Bonpl. ex Willd.) E.Fourn. (as A. macrophylla Humb. & Bonpl. ex Schult.)
• Marsdenia tenacissima (Roxb.) Moon (as A. tenacissima Roxb.)
• Matelea maritima (Jacq.) Woodson (as A. maritima Jacq.)
• Sarcostemma acidum (Roxb.) Voigt (as A. acida Roxb.)
• Sarcostemma viminale (L.) R.Br. (as A. viminalis (L.) Steud.)
• Telosma cordata (Burm.f.) Merr. (as A. cordata Burm.f.)
• Telosma pallida (Roxb.) Craib (as A. pallida Roxb.)
• Tylophora indica (Burm.f.) Merr. (as A. asthmatica L.f.)
• Vincetoxicum hirundinaria Medik. (as A. vincetoxicum L.)
• Vincetoxicum pycnostelma Kitag. (as A. paniculata Bunge)
• Xysmalobium undulatum (L.) R.Br. (as A. undulata L.)^[13]

References[edit]

• 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