Phylogenetic significance of leaf micromorphology and anatomy in the tribe Mentheae (Nepetoideae: Lamiaceae)

A comparative micromorphological study of leaves was carried out on 102 species of Mentheae; 61 species were selected for the anatomical study. Mentheae possessed both amphistomatic and hypostomatic leaves. The diversity of leaf epidermal characteristics was based on the variation in morphology of epidermal cells, stomata types and trichome types. Although each characteristic on its own has rather limited systematic value, the combination of some of these features may be systematically relevant, especially for the identification of species. For example, branched multicellular nonglandular trichomes were a diagnostic characteristic for all genera investigated of the subtribe Salviinae; however, this trichome type was also observed in Hedeoma ciliolata and Neoeplingia leucophylloides of the subtribe Menthinae. Capitate glandular trichomes with pear-shaped heads were only observed in Salvia dorrii. Subsessile glandular trichomes with multicellular heads (more than ten cells) were an apomorphy for Perovskia. The anatomical leaf structure was consistent throughout the tribe. In some species, the vascular bundles in the midrib were modified into a mechanical tissue, which is an adaptation to xerophytic environments. The observed variations are discussed in an ecological context and their phylogenetic significance is evaluated. (C) 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 160, 211-231.