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The anatomy of the petiole, petiolule and leaf lamina of 20 Cussonia spp. is described and taxonomically evaluated. Comparisons are made with related taxa of Araliaceae: two Afro-Malagasy Schefflera spp., two Asian Schefflera spp. and Seemannaralia gerrardii. Transverse sections and epidermal peels (lamina only) were studied. The continuous collenchyma layer in the midrib, the absence of a hypodermis and papillate epidermal cells are characters shared between Afro-Malagasy Schefflera and Seemannaralia, and Cussonia and Seemannaralia share adaxial stomata, abaxial papillae on the lamina and similarly sized secretory canals in the petiole. Infrageneric groupings suggested by three types of petiolar vascular bundles correspond to one of the two existing infrageneric classification systems. Anatomical characters also contribute to a better understanding of species delimitation. The study has revealed six new generic apomorphies for Cussonia, namely the discontinuous, subepidermal collenchyma layer and the associated stomata in the petiole and petiolule, the adaxially invaginated and flattened ring of separate or connected bundles forming a narrow arc in the petiolules and the occurrence of arm palisade in the lamina. Leaf anatomical characters have greatly increased our understanding of relationships and circumscriptions at generic, infrageneric and specific levels in the taxa studied.

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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 temperat 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.

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