Description from Flora of China
Herbs [or shrubs], perennial, with cystoliths, often anisophyllous. Leaves opposite; leaf blade margin entire or crenate-serrate. Inflorescences dense spikes or cymes condensed into globose heads, sometimes clustered; bracts and bracteoles present. Calyx 5-lobed; lobes heteromorphic; posterior lobe usually largest; lateral 2 lobes smallest; anterior 2 lobes usually connate to a greater degree than other lobes. Corolla small; tube usually expanded into a distinct throat near middle; limb 2-lipped; lower lip 3-lobed, patent, usually spotted, lobes subequal or middle one slightly larger; upper lip erect or slightly patent, entire to 2-lobed; lobes quincuncial in bud. Stamens 4, didynamous [or with one pair reduced or absent], usually included in corolla tube; filaments short; anthers 2-thecous [or with one pair of anthers sometimes 1-thecous]; thecae oblong, parallel or one lower and somewhat divergent, base muticous or spurred. Ovary with 2 ovules per locule; stigma subcapitate, entire or slightly 2-cleft. Capsule subellipsoid to oblong to linear-conical, lacking a stalk, to 4-seeded; retinacula present. Seeds discoid, pubescent with hygroscopic trichomes.
About 100 species: tropical to subtropical regions worldwide (mostly paleotropical); seven species (three endemic) in China.
(Authors: Hu Jiaqi (胡嘉琪 Hu Chia-chi), Deng Yunfei (邓云飞); Thomas F. Daniel)