Description from Flora of China
Herbs with stolons and watery juice. Leaves opposite, entire, short petiolate, intra- and interpetiolar glands present. Flowers solitary or rarely in 2-flowered cymes, axillary. Calyx small, without glands. Corolla violet, funnelform, tube cylindric, hairy or with scales at throat; lobes obliquely obovate, spreading, shorter than tube, overlapping to left. Stamens inserted just below middle of corolla tube. Disc glands 2, ligulate, alternating with ovaries. Ovules 6-many. Style filiform; pistil head ringlike, apex densely hairy. Follicles 2, erect or spreading, cylindric, striate. Seeds glabrous.
About five species: W Asia, Europe; two species cultivated in China.