The filament curtain: a structure important to systematics and pollinationbiology in the Acanthaceae

The filament curtain is a complex structure found in canthaceae inside the corolla. It usually divides the corolla into two compartments, enclosing th e main nectar bulk behind two adjacent filament bands. Four independent flo ral characters are involved in the structure of the filament curtain: a fus ion of the filaments, decurrent filaments along the corolla wall, a slantin g border between the synstapetal and apostapetal corolla regions (i.e. stam en corolla tube and corolla tube s.s.), and geniculale lower, lateral corol la lobe traces at this border. The distribution of a filament curtain withi n Acanthaceae suggests an enlarged tribe Ruellieae s.l., including Louterid ieae and Trichanthereae, excluding subtribe Barleriinae, and raises a quest ion mark as to the systematic position of the genera Calacanthus, Glossochi lus, Lankesteria and Whitfieldia. The evolutionary origin of the filament c urtain is probably connected with its functions in pollination biology, whi ch are proposed to be those of restricted nectar access, prevention of nect ar evaporation, lever arm function facilitating dorsal pollen deposition, a nd stabilizing of posticous position of anthers and style. Four types of fi lament curtains are distinguished: phaulopsoid, corolla fold, reduced and s trobilanthoid. Variation in the filament curtain structure appears to be re lated to different pollination syndromes of the flowers. Mapping of this va riation on to an existing phylogeny shows an evolution from a phaulopsoid t ype diverging into the other three types, possibly a development responding to shifts of pollinators. (C) 2000 The Linnean Society of London.