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

• Published in: Botanical journal of the Linnean Society 2000-06, Vol.133 (2), p.129-160
• Main Author: MANKTELOW, MARIETTE
• Format: Article
• Language: English
• Subjects: androecium; morphology; taxonomy
• ISSN: 0024-4074; 1095-8339
• DOI: 10.1111/j.1095-8339.2000.tb01539.x

The filament curtain is a complex structure found in Acanthaceae inside the corolla. It usually divides the corolla into two compartments, enclosing the main nectar bulk behind two adjacent filament bands. Four independent floral characters are involved in the structure of the filament curtain: a fusion of the filaments, decurrent filaments along the corolla wall, a slanting border between the synstapetal and apostapetal corolla regions (i.e. stamen corolla tube and corolla tube s.s.), and geniculate lower, lateral corolla lobe traces at this border. The distribution of a filament curtain within Acanthaceae suggests an enlarged tribe Ruellieae s.l., including Louteridieae and Trichanthereae, excluding subtribe Barleriinae, and raises a question mark as to the systematic position of the genera Calacanthus, Glossochilus, Lankesteria and Mhitfieldia. The evolutionary origin of the filament curtain is probably connected with its functions in pollination biology, which are proposed to be those of restricted nectar access, prevention of nectar evaporation, lever arm function facilitating dorsal pollen deposition, and stabilizing of posticous position of anthers and style. Four types of filament curtains are distinguished: phaulopsoid, corolla fold, reduced and strobilanthoid. Variation in the filament curtain structure appears to be related to different pollination syndromes of the flowers. Mapping of this variation on to an existing phylogeny shows an evolution from a phaulopsoid type diverging into the other three types, possibly a development responding to shifts of pollinators.