The taxonomic significance of leaf anatomical characters in Cussonia and related genera (Araliaceae)

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.