Trichome micromorphology and its systematic significance in Asian Leucas (Lamiaceae)

Trichome micromorphology of 37 taxa of the genus Leucas s.s. included under three infrageneric sections and two related species of 'African Leucas' found in Asia were studied using light microscopy and scanning electron microscopy. Two basic types of trichomes can be observed: non-glandular and glandular trichomes. The non-glandular trichomes show variations in shape and cell number, accordingly four forms (unicellular, two-celled, three-celled and more than three cells) were distinguished. Glandular trichomes can in turn be subdivided into two types: peltate and capitate trichomes. Trichomes showed considerable variability among different species, but exhibited similarity in their structure and distribution in different populations of a particular species, and therefore contribute significant characters in the delimitation of sections and species. The results of our study show that the presence of capitate trichomes with distinct morphology and absence of non-glandular trichomes with one cell and more than three cells imparts an additional morphological character to support the segregation of African Leucas from Asian Leucas. Within Asian Leucas, unicellular non-glandular trichomes, non-glandular trichomes with more than three cells and capitate trichomes are significant characters for the separation of three formerly recognized sections, sect. Astrodon Benth., sect. Plagiostoma Benth., and sect. Leucas (Ortholeucas) Benth. This data on trichome micromorphology supports the view of Ryding and Scheen and Albert, and rejects the sectional treatment of Singh. Moreover, this study helps to solve problems regarding the sectional treatment of two African species of Leucas found in Asia.