Description from Flora of China
Herbs, annual or perennial, erect, diffusely prostrate, or creeping, glabrous or glandular hairy, sometimes viscid. Stems simple or branched, terete or quadrangular and winged. Leaves opposite, margin entire to toothed. Flowers axillary and solitary or in terminal racemes. Bracteoles present or absent. Calyx tubular to campanulate, often inflated in fruit, glabrous to villous, 5-ribbed, with veins sometimes narrowly winged, 5-lobed; lobes short, regular or irregular. Corolla 2-lipped; tube cylindric, longer than calyx, palate with 2 bulging plaits; lower lip 3-lobed, lobes often spreading flat; upper lip 2-lobed, lobes erect or reflexed. Stamens 4, didynamous, inserted on corolla tube, included. Stigma 2-lamellate. Capsule compressed, loculicidal. Seeds numerous, ovoid to oblong, minute; seed coat smooth or reticulate.
About 150 species: worldwide; five species in China.