Milkweed

Milkweed, common name for a plant family, and for plants of the family's typical genus. Many of the 120 species of the genus are native to North America, and many are distributed widely. These perennial herbs have erect stems, opposite or whorled leaves, and small, unusual flowers borne in clusters at the top or along the stem of the plant. The colored parts of each flower are organized in two five-membered whorls. Milkweeds may be divided into two groups: those with broad leaves and those with narrow leaves. Both are characterized by copious milky sap, called latex, and by inflated fruit pods tightly packed with silky or cottony fuzz. At maturity the pod cover dries, opens, and releases many large, flat, brown seeds. Each is attached to a bit of the fuzz; thus the seed can drift for long distances in gentle currents of air.

Many species of milkweed are among the most dangerous of poisonous plants; others, such as the common broad-leaved milkweed, of eastern North America, have little if any toxicity. Butterfly weed and swamp milkweed are suspected of being toxic, especially the latter; these are among the more showy milkweeds and are sometimes cultivated. Some of the poisonous species of North America are the narrow-leaved labriform milkweed, which is limited to Utah; the narrow-leaved whorled milkweed, of dry plains in the Southwest; and the broad-leaved woolly-pod milkweed, of dry soils in California. The foliage of these poisonous milkweeds contains a complex resinous compound that produces acute muscle-spasm seizures, symptoms of profound depression, and weakness in animals that consume them. Members of another genus are called carrion flowers (see Carrion Flower).

Scientific classification: Milkweeds make up the family Asclepiadaceae. The typical genus is Asclepias. The common broad-leaved milkweed is classified as Asclepias syriaca, butterfly weed as Asclepias tuberosa, swamp milkweed as Asclepias incarnata, the narrow-leaved labriform milkweed as Asclepias labriformis, the narrow-leaved whorled milkweed as Asclepias subverticillata, and the broad-leaved woolly-pod milkweed as Asclepias eriocarpa. Carrion flowers are classified in the genus Stapelia.