Perennial herbs, clambering vines or erect subshrubs, often with thick rootstock, usually pubescent; stem branched, often with interpetiolar ridges and a tuft of hairs in leaf-axils. Leaves opposite, simple, entire, pinnately veined, subsessile to petiolate, exstipulate. Inflorescence of dense terminal heads, or flowers distant and borne singly in short spikes arranged in diffuse, axillary or terminal, long-pedunculate cymes or panicles. Flowers bisexual, sessile, subtended by 1 ventral bract and 2lateral bracteoles. Tepals 5, free, scarious, concave, subequal, outer 3 somewhat larger and distinctly longitudinally 3-ribbed (-veined), greenish-white, sometimes inside with a tuft of straight white hairs, exceeding perianth; stamens 5, filaments expanded and connate below in a cup, cup and filaments with or without apical appendages, anthers 2-locular, becoming 1-locular at anthesis, introrse, medifixed, pseudostaminodia absent; ovary ovoid or obovoid, ovule 1, campylotropous, placentation basal, with flattened, elongated funicle, stigma sessile, capitate or 2-lobed. Fruit a membranous, indehiscent capsule (sometimes designated a utricle) included in tepals; seed 1, lenticular, cochleate-orbicular, smooth.