Wood anatomy of the tribe Diosmeae, a large Cape lineage of Rutaceae

The tribe Diosmeae (Rutaceae) encompasses circa 278 species classified in 11 genera: ten of them are shrubs endemic to the Cape Floristic Region (South Africa), while the only arborescent genus (two species) extends from South Africa to Tanzania. We examined the wood structure of 21 species representing nine genera of Diosmeae and analysed it with respect to their life form and climatic niches. Studied taxa share a suite of traits typical for the whole family: radial vessel multiples, simple perforation plates, minute alternate intervessel pits, distinctly bordered vessel-ray pits, non-septate libriform fibres, and — except for Empleurum — marginal axial parenchyma. However, unlike most Rutaceae, they also occasionally feature vessel-ray pits with reduced borders. Most of the studied species are very similar to each other in their wood structure and habit. The two exceptions are Calodendrum capense (the only canopy tree under study) and Empleurum unicapsulare (a large riparian shrub). The first differs from other species in having wider vessels, longer libriform fibres, prismatic crystals in ray cells and winged-aliform axial parenchyma (which may be plesiomorphic for the tribe); the latter is distinctive because it lacks growth rings and banded axial parenchyma. Analysing anatomy–climate relationship, we found that wood anatomy among shrubby Diosmeae relates to their size and environmental factors, rather than taxonomy. Particularly, the vessel grouping weakly increases with stricter water limitation along the gradient from a semi-arid winter-dry climate to a Mediterranean winter-wet climate.