Description from Flora of China
Herbs stoloniferous, perennial, gynodioecious or gynomonoecious. Stems ascending to prostrate. Leaves long petiolate, base cordate. Verticillasters axillary, 2-6-(to many) flowered. Calyx tubular to campanulate, slightly curved near throat, 15-veined, indistinctly 2-lipped, adaxial lip 3-toothed, abaxial lip 2-toothed. Corolla tubular, dilated at apex, 2-lipped; adaxial lip straight, emarginate or 2-lobed; abaxial lip spreading, 3-lobed. Stamens 4, anterior 2 inserted below lateral lobes of abaxial lip, posterior 2 inserted near throat below adaxial lip; filaments glabrous, undeveloped in pistillate flowers; anther cells oblong, parallel or divergent. Ovary glabrous. Style slender, apex subequally 2-cleft. Nutlets dark brown, oblong-ovoid, smooth or impressed-punctate, glabrous.
About eight species: Asia, Europe, cultivated in North and South America; five species in China.