Description from Flora of China
Herbs, base frequently woody. Leaves opposite or whorled, short petiolate. Cymes terminal and extra-axillary, erect, umbel-like, many flowered. Calyx glands 5-10. Corolla rotate, deeply parted; lobes reflexed, valvate or rarely overlapping to right. Corona lobes 5, inserted on gynostegium, erect, apex hooded, with an incurved, ligular-hornlike adaxial appendage. Stamens inserted at base of corolla tube; filaments connate into a tube; anther appendages incurved; pollinia 2 per pollinarium, pendulous. Stigma head conical. Follicles fusiform, apex acuminate. Seeds flat, with a long coma.
About 120 species: America, some naturalized in Old World; one species in China.