Contributions to the wood anatomy of the Rubioideae (Rubiaceae)

The secondary xylem of Craterispermeae, Coussareeae, Morindeae s.str., Prismatomerideae, Pauridiantheae, Urophylleae, and Triainolepideae (Rubiaceae, Rubioideae) is described and illustrated in detail. Genera that were previously placed in the Morindeae or Psychotrieae such as Damnacanthus, Lasianthus, Saldinia, and Trichostachys are also included. Wood anatomical characters are compared with recent phylogenetic insights into the study group on the basis of molecular data. The observations demonstrate that the delimitation and separation of several taxa from the former Coussareeae/Morindeae/Prismatomerideae/ Psychotrieae aggregate is supported by wood anatomical data. The Coussareeae can be distinguished from the other Rubioideae by their scanty parenchyma, septate libriform fibres, and the combination of uniseriate and very high multiseriate rays with sheath cells. Axial parenchyma bands and fibre-tracheids characterise Gynochtodes and some species of Morinda (Morindeae sstr.), but the latter genus is variable with respect to several features (e.g. vessel groupings and axial parenchyma distribution). Wood data support separation of Rennellia and Prismatomeris from Morindeae s.str.; vessels in both genera are exclusively solitary and axial parenchyma is always diffuse to diffuse-in-aggregates. Damnacanthus differs from the Morindeae alliance by the occurrence of septate fibres, absence of axial parenchyma, and the occasional presence of fibre wall thickenings. There are interesting similarities between members of the Lasianthus clade and the Pauridiantheae/ Urophyleae group such as the sporadic occurrence of spiral thickenings in axial parenchyma cells.