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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.
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.
Stamens 5; filaments filiform, flattened, fused only at the extreme base without intermediate pseudostaminodes; anthers narrow, bilocular.
Flowers in pedunculate (rarely some sessile) spike-like bracteate thyrses, each bract subtending a bibracteolate partial inflorescence.
Ovary uniovulate, somewhat compressed, delicate; style slender, with two short or elongate stigmatal branches.
Perianth segments 5, obviously narrower from the outer to the inner.
Leaves entire, opposite on mostly branched stems.
Erect perennial herbs or low shrubs.