Perennial herbs, more commonly woody at the base, undershrubs or shrubs, erect, scrambling or scandent, sometimes high lianas. Rhizome not rarely tuberous. Branches often slightly swollen and jointed at nodes. Hairs simple, uni- or multicellular, short ones often with a hooked apex. Leaves simple, spiral or alternate, petioled (without an abscission zone), exstipulate; midrib usually prominent beneath, elevated or flat above; nervation commonly palmate, or pinnate, nerves often obliquely extending towards the margin. Flowers bisexual, actinomorphic or zygomorphic, solitary, fasciculate, or in axillary or cauligerous, racemose, paniculate or cymose inflorescences, usually only one or two flowers open at a time; bracts present and often persistent; pedicel often hardly distinct from the ovary. Calyx petaloid, gamosepalous, 3- (or 6-) lobed or 1-lipped; lobes valvate or induplicate. Petals (in Mal.) absent. Disk (?) 0, rarely present (e.g. a few Thottea spp.). Stamens 6 (4 or 5 in some extra-Mal. Aristolochia spp.) or 6—c. 36 (—46), in 1 whorl or in 2 (3 or 4) whorls (Thottea); filaments free or slightly mutually united at the base, and/or almost completely adnate to the style column to form a gynostemium; anthers free (Thottea) or dorsally united with the style column (Aristolochia), each consisting of 2 thecae with 4 pollen sacs, extrorse, rarely introrse (extra-Mal. spp.), dehiscing longitudinally. Ovary inferior (rarely half-inferior in extra-Mal. genera), 4—6-carpellate, 4—6-celled, syncarpous (or ± apocarpous in extra-Mal. Saruma); placentae parietal (distinct when young, then intruding and connivent axially, thus often seemingly axile); ovules usually many, anatropous, in 1 or 2 vertical rows in each locule of the ovary, horizontal or pendulous; style-column 3—many-lobed, sometimes some of the lobes redivided; stigmas or stigmatic tissue apical, lateral, or on the surface of style lobes. Fruits capsular or siliquiform (follicular or cocci in extra- Mal. genera), 4—6-celled; dehiscing apically towards the base (basipetal, e.g. Thottea) or basally towards the apex (acropetal, e.g. most Aristolochia); septicidal, rarely septifragal (some extra-Mai. Aristolochia) or bursting irregularly (extra-Mal. Asarum); rarely indehiscent (W. African Pararistolochia). Seeds many in each locule (1-seeded in extra-Mal. Euglypha), often coated with remains of placental tissue (membranous when dry), horizontal or pendulous, variously shaped; ovate, deltoid or triangular, flat, convex-concave, or longitudinally curved, or oblong (and triangular in cross-section), rugose, finely verrucose, or smooth, immarginate (Thottea; Aristolochia, p.p.) or winged (Aristolochia, p.p.); albumen fleshy, copious; embryo minute, cotyledons two, distinct. Distribution. There are 7 genera, Aristolochia worldwide, Asarum over the northern hemisphere, Thottea in continental Southeast Asia and Malesia, Pararistolochia in tropical Africa, and 3 monotypic genera, viz. Saruma in China, Holostylis and Euglypha in South America. As to number of species, Aristolochia is by far the largest with some 300 spp., largely concentrated in the New World, especially in Central and South America, in Malesia with 28 spp.; Asarum (incl Hexastylis and Heterotropa) with possibly some 70 spp. in northern temperate regions, Thottea with 26 spp., of which 22 in Malesia, and Pararistolochia with 12 spp. in West Africa.