Leaf epidermal features of Quercus Group Ilex (Fagaceae) and their application to species identification

Leaf epidermal features are useful for elucidating the taxonomy and systematics of oak species. Quercus Group Ilex has a wide distribution mainly in subtropical areas of Eurasia with rich fossil records since the Oligocene, but no comprehensive study has ever been conducted on its diversity of leaf epidermal features. We compared the leaf epidermal features of 37 species of Quercus Group Ilex and Q. suber from Group Cerris using light microscopy (LM) and scanning electron microscopy (SEM). A total of five trichome types (including three glandular and two non-glandular types) and two trichome base types were found. Leaf epidermal features of Group Ilex show typical xeromorphic features. The stomatal complexes are stephanocytic. The leaf trichome types and trichome base types are stable at inter- and intra-species levels. The trichomes, stomatal sizes and density and the ray arm lengths show a wide variation during developing stages within species. The leaf epidermal features show a highly convergent pattern in the genus Quercus based on tree-mapping analysis, except for the multiple epidermal layers on the adaxial surface supports the monophyletic origin of section Heterobalanus. Cluster analysis of leaf epidermal characters infers two morphological subgroups within Group Ilex: (1) the capitate and (2) the branched uniseriate. The trichomes and trichome base types, and their density are useful for species identification, which can improve the accuracy of assigning fossil leaves to their nearest living relatives in Group Ilex.