All that shimmers is not the same: silvered leaves and micromorphological diversity in Timonius DC. (Rubiaceae)

The significance of leaf micromorphology in distinguishing species and higher taxa has been well established in several plant groups, but has not been well studied in Timonius. This deserves some attention because (1) leaves are most commonly represented in herbarium specimens; (2) leaf micromorphology may facilitate the recognition of male and female specimens of the same species in dioecious taxa with sexually dimorphic inflorescences and flowers, such as Timonius, and (3) silvery lower leaf lamina surfaces occur in several Timonius species. The micromorphology of the lower lamina surface of 12 Timonius species was examined using scanning electron microscopy to establish the taxonomic utility of lower lamina micromorphology and to investigate if species with silvery leaf undersides share similar leaf micromorphological traits. For all Timonius species examined, the leaf hairs are unbranched and non-glandular. Hair morphology and density were found to be uniform within species but vary substantially between species. The silvery leaved species are typically characterised by one or more of the following leaf traits—long hair length, high hair density, and the presence of lanate hairs. Nevertheless, different combinations of these three traits result in much leaf micromorphological diversity, enabling distinction among the silvery leaved species based on leaf hair characters.