The Aristolochiaceae () are a family, the birthwort family, of flowering plants with seven genera and about 400 known species belonging to the order Piperales. The type genus is Aristolochia L. Description They are mostly perennial, herbaceous plants, shrubs, or lianas. The membranous, cordate simple leaves are spread out, growing alternately along the stem on leaf stalks. The margins are commonly entire. No stipules are present. The bizarre flowers are large to medium-sized, growing in the leaf axils. They are bilaterally or radially symmetrical. Classification Aristolochiaceae are magnoliids, a basal group of angiosperms which are not part of the large categories of monocots or eudicots. As of APG IV (2016), the former families Hydnoraceae and Lactoridaceae are included, because exclusion would make Aristolochiaceae in the traditional sense paraphyletic. Some newer classification schemes, such as the update of the Angiosperm Phylogeny Group, place the family Aristolochiaceae in the order Piperales, but it is still quite common, though superseded, for the Aristolochiaceae to be assigned, sometimes with some other families, their own order (Aristolochiales). Phylogeny Four assemblages can be distinguished in the genus-level cladogram of Aristolochiaceae: Aristolochia is closely related to Thottea. Hydnora is closely related to Prosopanche. Lactoris occupies an isolated position. Asarum is closely related to Saruma, and both genera display a deep-branching position in the family. Phytochemistry Many members of Aristolochia and some of Asarum contain the toxin aristolochic acid, which discourages herbivores and is known to be carcinogenic in rats. Aristolochia species are carcinogenic to humans. Genomics The complete plastid genome sequence of one species of Aristolochiaceae, Hydnora visseri, has been determined. As compared to the chloroplast genome of its closest photosynthetic relatives, the plastome of Hydnora visseri shows extreme reduction in both size (ca. 27 kilo base pairs) and gene content (24 genes appear to be functional). This Aristolochiaceae species therefore possesses one of the smallest plastid genomes among flowering plants. Ecology Pipevine swallowtail butterflies lay their eggs on pipevine (Aristolochia species), and the larvae feed on the plant, but are not affected by the toxin, which then offers the adult butterfly protection against predators. Fossil record The oldest fossil leaf remains are of †Aristolochites dentata from the Late Cretaceous of Nebraska, United States. Pollen record of †Aristolochiacidites viluiensis has been described from Upper Cretaceous sediments of Siberia. Fossil wood is known from the Deccan Traps of India some 66 million years ago. Leaf fossils of Aristolochia