Description from Flora of China
Herbs, perennial with rhizomes or annuals. Roots glabrous. Stems erect, procumbent, or creeping, base sometimes woody. Leaves mostly opposite, rarely whorled or upper ones alternate. Inflorescences terminal or axillary racemes, sometimes spicate, long or short and capitate. Calyx 4-lobed, if 5-lobed upper lobe much smaller than other lobes. Corolla 4-sect or 4-lobed, rarely slightly 2-lipped; tube short and limb subrotate, rarely tube conspicuous, 1/3-1/2 of corolla length, usually hairy inside; lobes usually patent, unequal in width, upper lobe widest, lower lobe narrowest. Stamens 2; filaments adnate to posterior part of corolla tube; anther locules parallel or divergent, apically confluent. Style persistent; stigma capitate. Capsule diverse in shape, slightly to strongly compressed laterally, 1-grooved, loculicidal. Seeds 2-50 per capsule, flattened and convex on both sides, navicular or cupular.
About 250 species: cosmopolitan, mainly in Asia and Europe; 53 species in China.
(Authors: Hong De-yuan; Manfred A. Fischer)