Perennial, sometimes annual herbs, unarmed; taprooted, or with a woody rootstock; stems decumbent or erect, somewhat 4-angled, glabrous or puberulent in lines, occasionally viscid-pubescent, nodes slightly enlarged. Leaves opposite; exstipulate, petiolate or sessile; entire or undulate. Inflorescences axillary or terminal, dense or open, paniculate or thyrsiform cymes; flowers sessile, 1-10 within an involucre; involucre campanulate, calyx-like, 5-lobed, lobes erect, imbricate, equal or unequal, only slightly accrescent, becoming rotate and conspicuously veined in fruit. Flowers bisexual; perianth corolla-like, brightly coloured, tubular, tubular-funnelform, campanulate or rotate, often oblique, deciduous after anthesis, tube slender, often elongate, constricted above ovary, limb 5-lobed, induplicate-plicate, often not broader than tube, lobes retuse or emarginate; stamens 3-5(-6), exserted, filaments capillary, incurved, free or connate at base into a short cup, pollen polyporate; ovary ellipsoid, ovoid, obovoid or subglobose, sessile, ovule anacampylotropous, style filiform, exserted, stigma capitate, papillose. Anthocarp coriaceous, 5-angled, -ribbed or -costate, more or less constricted at both ends, often tuberculate or rugulose, glabrous or pubescent, mucilaginous when wet; seed with testa adherent to pericarp, embryo uncinate, more or less folded, enclosing mealy endosperm, cotyledons foliaceous, unequal, radicle elongate, descending.
Guianas present, Northern America, southwestern United States present, tropical and temperate America present
Approximately 54 species, mostly in tropical and temperate America, with a center of diversity in the southwestern United States and Mexico, and with 1 species in the western Himalayan region; 1 species in the Guianas.