Herbs annual or perennial. Rhizomes absent. Stems creeping, ascending, or erect. Leaves alternate. Cincinni subtended by and included in spathelike involucral bracts; involucral bracts open or proximal margins coherent and becoming funnelform, often forming heads; cincinni dichotomously branched, flowers in 1 branch smaller, infertile, falling early, those in other branch fertile; bracts not falcate curved, usually extremely small or absent. Flowers zygomorphic. Sepals lanceolate or ovate-orbicular, sometimes shallowly boat-shaped, inner 2 often connate at base. Petals free, blue, spatulate or orbicular, inner (anterior) 2 larger and clawed. Fertile stamens 3, on 1 side, 2 inserted opposite sepals, 1 opposite petal; filaments glabrous; staminodes 3; antherodes 4-lobed, butterflylike. Ovary 2- or 3-loculed, posterior locule with 1 developed or abortive ovule or empty, anterior 2 locules each with 1 or 2 ovules. Capsule oblong, globose, or ellipsoid, 3- or most frequently 2-valved; posterior valve 1-seeded or seedless, often indehiscent; anterior 2 valves each seedless or 1- or 2-seeded. Seeds cylindric or ellipsoid, reticulate or subsmooth; hilum linear.

About 170 species: mainly in tropical and subtropical regions; eight species in China.