Stamens 5; filaments filiform, flattened, fused only at the extreme base without intermediate pseudostaminodes; anthers narrow, bilocular.
Perianth segments 5, obviously narrower from the outer to the inner.
The sterile flowers (which develop so rapidly as to be difficult to detect in young flowers) consist of a single finally hard and gibbous-based spine, the entire partial inflorescence thus falling as a 3-spined burr complete with bracteoles; bracts persistent on the inflorescence axis.
Partial inflorescences normally consisting of two (occasionally one in the upper part of the thyrse) fertile flowers; one of these is subtended dorsally by a single sterile flower (which is thus contained in a partial inflorescence bracteole), the other by a sterile flower on each side, each of which is contained in a bracteole of its own.
Flowers in pedunculate (rarely some sessile) spike-like bracteate thyrses, each bract subtending a bibracteolate partial inflorescence.
Leaves entire, opposite on mostly branched stems.
Erect perennial herbs or low shrubs.
Utricle scarcely compressed, delicate save for a small, firm area about the base of the style, irregularly ruptured by the developing seed; seed with abundant endosperm.