Description from Flora of China
Herbs erect or procumbent, rarely suffruticose, glabrous or pubescent, often with internal glands appearing as pustulate dots or stripes. Leaves alternate, opposite, or whorled, usually entire. Flowers solitary in axils of upper leaves or in terminal and axillary panicles or racemes, often shortened into capitate clusters, with bracts. Calyx usually 5(or rarely 6--9)-parted. Corolla white or yellow, rarely pink, homomorphic, rarely heteromorphic, subrotate or campanulate, deeply 5(or rarely 6--9)-parted; lobes contorted in bud. Filaments free or connate into a ring or tube at base and ± adnate to corolla tube; anthers basifixed, dorsifixed or versatile opening by apical pores or by lateral slits. Capsule subglobose, usually dehiscing by valves, rarely indehiscent.
Approximately 180 species in temperate and subtropical parts of the northern hemisphere, but with a few species in Africa, Australia, and South America; 138 species occur in China.