Prostrate to erect, annual or perennial herbs, becoming suffruticose near base. Leaves opposite, entire, petiolate, rhombic-ovate. Inflorescence sometimes capitate, usually comprised of long, terminal spikes and subglobose or cylindrical axillary spikes; fertile and sterile flowers clustered in shortly pedunculate glomerules along major axes; glomerules comprised of bracteate triads of 1 central bisexual fertile and 2 modified sterile flowers formed of uncinate, straight or glochidiate spines, all falling together as fruit; bracts ovate, oblong, acuminate; unisexual flowers reduced to tepals with 5 rigid, hooked awns or spines (glochidia), developing in axils of bracteoles after bisexual flowers. Flowers becoming reflexed early in development; tepals 5, free, often hairy; stamens 5, united at base into a tube, anthers 2-locular, oblong, pseudostaminodia 5, fringed, alternating with fertile stamens; ovary obovoid, ovule 1, style filiform, stigma capitate, glandular. Fruit a thin-walled, indehiscent capsule; seed compressed, ellipsoid.