In Acanthaceae the filaments may be synorganized, dividing the corolla into two longitudinal compartments. The filament structure, here named a filament curtain, is a combination of fused and decurrent filaments, usually in association with sharply bent corolla traces. This complex structure is only found in the tribes Trichanthereae, Louteridieae and Ruellieae sine Barleriinae (sensu Bremekamp), and indicates a close relationship between these groups. The function of the filament curtain is probably to ensure nototribic (dorsal) pollen deposition, rationing the nectar bulk, protect the gynoecium, and in some cases form a barrier to the nectar. Reductions and modifications of the filament curtain as evolutionary responses to different pollinators are discussed.

Key words: Acanthaceae, corolla morphology, filaments, systematics