The bicolor unit -: homology and transformation of an inflorescence structure unique to core Malvales

A broad comparative analysis reveals that the inflorescences of core Malvales, families Sterculiaceae, Tiliaceae, Bombacaceae and Malvaceae, include characteristic repeating units. The basic repeating unit is called bicolor unit (after Theobroma bicolor, where it was first observed). It is determinate and bears three bracts, one of which is invariably sterile, whereas the others subtend lateral cymes or single flowers. Through the demonstration of intermediate steps in closely related taxa the triad of bracts within a bicolor unit and the trimerous malvalean epicalyx are shown to be homologous. Various possibilities for an origin of the bicolor unit are discussed. Bicolor units are variously arranged to form complete inflorescences. In many taxa they are terminal on modules that comprise two (or fewer) prophylls. These modules may be arranged in elongated anthocladia or condensed sympodia, which in turn may constitute components of higher order inflorescence structures. The presence of the bicolor unit or its derivatives links Sterculiaceae, Tiliaceae, Bombacaceae and Malvaceae. It is absent from all other families included in a broader defined order Malvales and represents one of the rare morphological synapomorphies of core Malvales. Furthermore, inflorescence morphology provides characters of systematic significance for various taxa within core Malvales.