Description from Flora of China
Annual or perennial herbs or shrubs; young branches 4-angled. Leaves opposite, alternate, or 3-whorled, sessile or subsessile. Flowers in terminal spikes or racemes, whorled in cymes, paired, or solitary in axils, 6-merous, [mono-, di-, or] trimorphic, shortly pedicellate. Floral tube elongate, cylindric [rarely broadly campanulate], 6-12-angled or -veined; sepals 6, generally short; epicalyx present, sometimes longer than sepals. Petals [absent to] 6, purple, rose, pink [or white]. Stamens [2-6 or]12, in two whorls of different lengths. Ovary 2-loculed; style of three lengths with capitate stigma below, above, or between two stamen whorls. Capsule elongated, included within persistent floral tube, 2-valved, valves often 2-lobed, dehiscence usually septicidal at apex. Seeds numerous, red-brown, elongate, bilaterally compressed, ca. 1 mm.
About 35 species: cosmopolitan; two species in China.
(Authors: Qin Haining (覃海宁); Shirley Graham)