Wood anatomy of Cussonia and Seemannaralia (Araliaceae) with systematic and ecological implications

The wood structure of two related African genera, Cussonia Thunb. (15 of 21 species) and the monotypic Seemannaralia R. Vig. (Araliaceae) is examined. The considerable diversity in wood anatomical characters within these taxa is mostly related to environmental factors; taxonomic groupings or phylogenetic relationships seem to be less important. The shortening of vessel elements and fibres, an increase in vessel number per group, a decrease in vessel diameter and a reduction in the number of bars of perforation plates, are associated with the more temperate species. The changes in vessel grouping show a significant correlation with rainfall. The placement of the simple-leaved Cussonia species in the subgenus Protocussonia and the isolated position of C.paniculata Eckl. & Zeyh., the only member of the subgenus Paniculatae, are supported. Many Cussonia species share a very low fibre to vessel element length ratio. Despite the basal position of Seemannaralia relative to Cussonia revealed by molecular data (Plunkett et al. 2004), its wood structure is more specialised in terms of the Baileyan major trends in wood evolution. This discrepancy may be the effect of a long-term adaptation of tropical ancestors of Seemannaralia to drier biomes.