Towards an integrative understanding of stamen–corolla tube modifications and floral architecture in Boraginaceae s.s. (Boraginales)

Abstract

Abstract Morphological studies rarely address floral organ modifications or their integration into floral architecture and floral function. Boraginaceae show two prominent types of stamen–corolla tube modifications: faucal and basal scales. Both types, especially faucal scales, are widely used in classification. Here, the ontogeny and morphology of faucal and basal scales are studied in 29 species from eight tribes and all three subfamilies of Boraginaceae s.s. (=Boraginoideae sensu APG IV) using scanning electron microscopy. Integration into floral architecture is visualized with micro-computed tomography (µCT). Faucal and basal scales are present in 18 and 27 species, respectively. Both types of scales develop late in flower ontogeny, but with variable timing. Faucal scales are morphologically far more variable than basal scales. Faucal scales are located close to the anthers and sometimes are involved in anther cone formation. Basal scales cover either the gynoecial disc nectary or the entire ovary. The different scale morphologies identified here enclose complex internal spaces, but they show no obvious phylogenetic patterns. This probably indicates strong functional constraints and adaptive pressures. This is a first in-depth study of the morphology and development of stamen–corolla tube modifications in Boraginaceae s.s., demonstrating that in situ three-dimensional visualization of floral architecture with µCT provides unprecedented insights into the evolution and functional integration of stamen–corolla tube modifications in Boraginales.