The carpology and taxonomic relationships of Bretschneidera (Bretschneideraceae)

Abstract

The fruit and seed anatomy and morphology of Bretschneidera sinensis Hemsl., the monotypic genus in the Bretschneideraceae, have been studied in order to clarify its phylogenetic relationships. The loculicidal trilocular syncarpous fruit has unusual unspecialised, aerenchymatous fibrous 12-14-layered endocarp and sclerified innermost part of the mesocarp with numerous radially elongated bundles pervading the parenchyma of mesocarp. Such a distribution of mechanical elements is indicative of the derivation of the Bretschneidera fruit from a drupaceous fruit of the «durian» type. In order to emphasise hard differences in pericarpic mechanical system of between drupe, capsule and this specialised type it is proposed to establish a new type of fruits - ascade (aseada, lat.) - to accommodate all fruits originated from the superior ovary with sclerified mechanical system located in the mesocarp. Thus the fruit of Bretschneidera is defined as a loculicidal ascade instead of loculicidal capsule. Seeds of Bretschneidera are relatively large, practically exalbuminous, with straight dicotyledonous embryo. The spermoderm of Bretschneidera is exomesotestal with unspecialised crushed tegmen, 40-45-layered, traversed by 8 postchalazal vascular bundles. Evidence mainly from seed morphology and anatomy of seed coats emphasises the somewhat anomaly of the traditional inclusion of Bretschneideraceae in the Sapindales, being quite distinct in spermoderm structure and origin from both Sapindaceae and Hippocastanaceae as well as from other families of the order, excepting anomalous exo-mesotestal Akaniaceae. Furthermore, seed anatomy does not confirm any relationships with Capparales. It is suggested that Bretschneideraceae with Akaniaceae constitute a distinct relict side-branch of connaraceous-sapindaceous ancestry. It can by no means be considered as a basal clade to Capparales. It is suggested to keep Bretschneideraceae with Akaniaceae separately from Sapindales and Capparales before more comprehensive studies of seedcoat anatomy of other orders of Rosidae.