Annual or perennials, erect or prostrate herbs, rarely shrubby; branches sometimes rooting at the nodes. Leaves entire, opposite, rarely alternate. Flowers yellow, pink, white or red, axillary solitary or two, or in racemes, spikes, panicles or paniculate-corymbs. Calyx and corolla 5(-6-7)-partite, the latter more or less rotate, infundibuliform or campanulate. Stamens 5, attached at the base of the corolla or about the middle, often monadelphous to form a tube-like membrane, glandular or not and adnate to the corolla throat; anthers basifixed or versatile, dehiscing longitudinally or by pores. Ovary subglobose, style slender, stigma capitate. Capsule subglobose, dehiscing by 5 valves or irregularly. Seeds many, small.