Investigations of Melissa L. (Lamiaceae) from Istria: SEM and morphological characteristics

The genus Melissa consists of two taxa in Istria: M. officinalis ssp. officinalis and M. romana (syn. M. officinalis subsp. altissima). This study presents macro- and micromorphological characteristics of nutlets, pollen, leaves and calyx, including type of trichomes and exocarp pattern. Pollen and fruit macro- and micromorphological characteristics of the Istrian taxa were examined using scanning electron microscope (SEM). The results showed that in Melissa pollen grains are radially symmetrical, isopolar, hexacolpate and have a medium size. Pollen grain shape is elliptic to circular in equatorial view and circular in polar view. The exine ornamentation is bireticulate. Nutlets sculpturing pattern is psilate to reticulate. This study clarified that both pollen and nutlet micromorphological characters are significant features in taxonomic classification of the genus Melissa. Our research presents the Istrian distribution of Melissa along with ecological, chorological, macro- and micromorphological characteristics of vegetative and generative organs that differentiate the species.