The phylogenetic significance of vestured pits in Boraginaceae

The bordered pit structure in tracheary elements of 105 Boraginaceae species is studied using scanning electron microscopy to examine the systematic distribution of vestured pits. Forty-three species out of 16 genera show a uniform presence of this feature throughout their secondary xylem. Most vestures are small, unbranched and associated with the outer pit aperture of bordered intervessel pits. The feature is likely to have originated independently in the distantly related subfamilies Boraginoideae (tribe Lithospermeae) and Ehretioideae. The distribution of vestures in Ehretia agrees with recent molecular phylogenies: (1) species with vestured pits characterise the Ehretia I group (incl. Rotula), and (2) species with non-vestured pits belong to the Ehretia II group (incl. Carmona). The occurrence of vestured pits in Hydrolea provides additional support for excluding this genus from Hydrophylloideae, since Hydrolea is the only species of this subfamily with vestured pits. Functional advantages of vestured pits promoting parallel evolution of this conservative feature are suggested. The hydraulic benefits of poorly developed vestures remain underinvestigated.