Plants perennial herbs (annual elsewhere), usually several-stemmed from the base, glabrous or minutely hairy. Leaves opposite, with bases usually more or less clasping.Inflorescences terminal clusters, often also solitary flowers or small clusters in the upper leaf axils. Flowers with parts in multiples of 5. Calyces tubular, with ovate to linear lobes. Corollas tubular to funnelform, with toothed or fringed, pleated, petaloid appendages between the lobes. Anthers of stamens often fused to each other. Ovaries short-stalked, with inconspicuous nectary glands at bases, the style short or absent, the stigmas 2, persistent. About 300 species, nearly worldwide (except Africa).