ANATOMICAL CHANGES AND EVOLUTIONARY TRENDS IN THE FOLIAR EPIDERMIS OFEXTANT AND FOSSIL EURO-MEDITERRANEAN OAKS (FAGACEAE)

The differentiation of foliar epidermis structures, cuticles, stomata, and indumentum of most of the extant Euro-Mediterranean taxa of Querc us have been studied by light and scanning electron microscopy, and co mpared with other genera and fossil material. Results allow the recogn ition of the subgg. Sclerophyllodrys, Cerris and Quercus, and of vario us sections and even individual species. The cyclocytic stomata type, regarded as plesiomorphic, is found in the geologically oldest represe ntatives of the family (Dryophyllum, resp. Eotrigonobalanus) and in th e relatively old, evergreen to semi-evergreen species of Quercus subgg . Sclerophyllodrys and Cerris. These taxa also have relatively thick a nd smooth cuticles. Anomocytic stomata, thinner cuticles and the prese nce of crystalline wax flakes are regarded as apomorphic features and characterize the deciduous taxa of subg. Quercus. These changes are in terpreted as ecological adaptations to the changing climates from the Tertiary to the present. An analysis of the various character combinat ions found in macrofossils and the often poor species separation in Qu ercus suggest that hybridization has played a major role in the evolut ion of the Euro-Mediterranean oaks.