Leaf anatomy of the genus Passerina (Thymelaeaceae): taxonomic and ecological significance

Abstract

A comparative anatomical study was made of the genus Passerina comprising 20 species and four subspecies, most of which are endemic to southern Africa. It showed that anatomical variation is useful in species recognition and classification. Anatomical characters typical of Thymelaeaceae and displayed in Passerina include isobilateral leaves, a papillate cuticular membrane, mucilaginous epidermal cell walls, a parenchymatous bundle sheath and extraxylary sclerenchyma fibres. Vascular bundles of the leaf lack intraxylary phloem. Characters common to  Passerina are inverse-dorsiventral and epis- tomatic leaves, inverted palisade parenchyma and an abaxial hvpodermal sclerenchymatous sheath. Orientation of the main vascular bundle in relation to the epidermis and mesophyll allows the recognition of four leaf structural types and ten states, according to which all species can be characterized and grouped. Functionally many anatomical features of the leaf in Passerina are interpreted as adaptations to the Mediterranean climate of the Cape Floristic Region, where most species occur.