European and Mediterranean oaks (Quercus L.; Fagaceae): SEM characterization of the micromorphology of the abaxial leaf surface.

This paper examines the micromorphological characteristic features (trichomes, waxes, stomata) of the abaxial leaf surface of European and Mediterranean oaks. Observations were performed by scanning electron microscopy, and the purpose of the investigation was to ascertain whether it is possible to distinguish the different species taxonomically by means of these features. Several species, as well as some subspecific entities and taxa considered doubtful, were taken into consideration. Subgenera appear to be well-defined, in that the subgenus Quercus is characterized by waxes arranged in vertical scales, while the subgenera Cerris and Sclerophyllodris are characterized by smooth waxes. They differ because the former has markedly elliptical stomata, while the latter has roundish stomata. The different species are also fairly easy to distinguish, and this fact confirms the taxonomical validity of the parameters we have taken into consideration. In some cases subspecific entities and doubtful species do not differ at all from the related species, but in others they present such marked variations that it is even difficult to classify them with certainty. The most critical taxonomic groups (i.e. those which include a number of specific and subspecific entities often to difficult to distinguish from each other) are the series Quercus robur - Quercus petraea - Quercus pubescens and the Quercus faginea - Quercus lusitanica - Quercus canariensis group.