Wood and bark of Buddleja: Uniseriate phellem, and systematic and ecological patterns

Wood anatomy of Buddleja is well-explored but not in many southern African members, which form a grade of species and small clades that form successive sister groups to the rest of the genus, and its bark structure has not been studied at all. We provide new descriptions of wood anatomy for twelve species, including nearly all Buddleja from southern Africa and two species of Freylinia in the sister group of Buddleja. We also describe bark structure from fifteen species. To assess if wood anatomy provides phylogenetic and/or ecological signal, we compiled data on wood traits and climatic variables from the distributions of 53 species. Wood traits counteracting cavitation correlated with higher temperature and precipitation seasonality; simultaneously they were better expressed in species with smaller maximal plant height. It is likely that hotter and drier areas harbour smaller plants which have traits conveying higher conductance safety. Bark structure varies considerably. In bark of Buddleja section Gomphostigma, periderm is initiated in the outer cortex and develops thin-walled phellem, and sclerification of their phloem does not occur. This resembles bark in Freylinia, supporting the position of section Gomphostigma as sister to the rest of Buddleja. In the remaining Buddleja species, bark is characterised by formation of periderm with phelloid cells in the secondary phloem. The phellem is often uniseriate, a condition not reported elsewhere. Its formation occurs close in time to solid sclerification of the cut-off phloem, suggesting a possible novel ontogenetic mechanism.