Whorled Milkweed (Fourleaf Milkweed)

Family: 
Asclepiadaceae (milkweeds)
Description: 

A slender, single-stemmed perennial with round clusters of usually pink flowers. Flowers technically in loose umbels, either upright or drooping, from 1 to 3 umbels per plant, light pink or cream-colored, nicely fragrant. Blooms May–July. Leaves opposite or whorled. There are 3 or 4 sets of leaves, of which 1 or 2 of the upper sets has 4 leaves in a whorl, the other sets with 2 leaves. Leaves broadly lanceolate, pointed at both ends. Sap milky white.

Size: 
Height: normally 12–18 inches, but occasionally taller.
Habitat and conservation: 
Occurs in open, dry, or rocky woods, usually on upland slopes. Most of our more familiar milkweed species are more robust plants that favor prairies, pastures, and other more open places.
Distribution in Missouri: 
Scattered nearly statewide, but apparently absent from the Mississippi Lowlands of the Bootheel and from the western portion of the Glaciated Plains (northwestern Missouri).
Human connections: 
Milkweeds in this genus have a long history of medicinal use in Native American tradition and by white settlers. Various species of milkweeds have had a host of economic uses, with the silky hairs of the seeds used as pillow stuffing and the sap being explored for rubber production.
Ecosystem connections: 
The cardiac glycosides and other chemicals in the milky sap (latex) are unpalatable and toxic, so few herbivores eat milkweeds. The larvae of monarch butterflies, however, use milkweeds as a food plant. They store the toxins in their bodies, rendering them unpalatable to predators.