This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Erect or climbing herbs or shrubs with alternate entire or lobed leaves and solitary or clustered axillary hermaphrodite flowers. The unusual shape of the perianth in the genus Aristolochia is the most striking character, but in the only other hardy genus it is regular. Stamens 6 to 12, epigynous, free or adhering to the stigmas. Fruit inferior, capsular or baccate, 3- to 6-celled, many-seeded, splitting between the cells. The species are estimated at about 180, in eight genera. Most abundant in South America.