An order of flowering plants, division Magnoliophyta (Angiospermae), in the subclass Magnoliidae of the class Magnoliopsida (dicotyledons). It contains only the family Aristolochiaceae, with 8 or 10 genera and about 600 species, most of them in tropical and subtropical regions. Within its subclass the order is marked by the presence of ethereal oil cells, by its uniaperturate or nonaperturate pollen, and especially by its strongly perigynous to epigynous flowers, usually with united carpels, that typically lack petals and have the sepals joined into a highly irregular, corolloid calyx. Many of the species are climbing vines. Aristolochia (birthwort or Dutchman's pipe) and Asarum are well-known genera of the order.