Comparative anatomy of stems and leaves of plectranthus L. (Lamiaceae) in Saudi Arabia and systematic implications.

A comparative anatomical study of the Plectranthus species growing in Saudi Arabia by using light microscopy method in order to evaluate their diagnostic value for systematic studies. Macro- and micromorphological characters, including stem, leaf, petiole anatomical, and trichome structures are presented. Descriptions of stem shape in cross-section, epidermal cell shape, cortex, pith, vascular bundles, leaf shape in cross-section, mesophyll, types and distribution of trichomes are presented. Taxonomic phylogenetic implications of stem, leaf, and trichomes anatomical micromorphology are also discussed in comparison with the available gross morphological and molecular data. Results of the present study offer useful data for evaluating the taxonomy of Plectranthus both at subgeneric and sectional levels. Moreover, our results indicate some degree of similarity among the species of subgenus Germanea and support the monophyly of this subgenus. A remarkable result from this study was identifying P. arabicus with distinctive characters and suggesting that it should be treated as a separate subgenus. Likewise, distinguishing differences between closely related endemic species P. asirensis and P. hijazensis are offered, and should be conserved as a different subgenus. A key for the identification of the investigated taxa based on studied characters is provided. Microsc. Res. Tech. 79:583-594, 2016. © 2016 Wiley Periodicals, Inc.