milkweed, common name for members of the Asclepiadaceae, a family of mostly perennial herbs and shrubs characterized by milky sap, a tuft of silky hairs attached to the seed (for wind distribution), and (usually) a climbing habit. Forms of this primarily tropical family are especially abundant in South America and in Africa, where many are succulents. Only a few genera are temperate; those species native to the United States are mostly of the genus Asclepias, the milkweeds, or silkweeds. The common milkweed, a plentiful roadside and field plant of the eastern and central states, is A. syriaca. A number of western species are poisonous to livestock, especially sheep. The milkweeds have been utilized as food (particularly the young shoots and buds), masticatory, medicament, and fiber. Some species yield an excellent bast fiber, like flax, but are difficult to cultivate and refine. The readily obtainable seed hairs from wild plants were sometimes used as a rather inferior substitute for kapok. Several species have been examined as potential sources of natural rubber; Palay rubber comes from a species of Crypostegia native to Madagascar. Among the milkweeds grown as ornamentals, the showy-blossomed butterfly weed or pleurisy root ( A. tuberosa ), native to the United States, was eaten by the Native Americans for lung and throat ailments. Hoya is an Old World genus that includes the wax plant ( H. carnosa ), a tropical climbing shrub cultivated as a pot plant for its fleshy leaves and fragrant waxy flowers. The milkweed family is classified in the division Magnoliophyta, class Magnoliopsida, order Gentianales.