Microrelief of the leaf epidermis and stomatal polymorphism of Malus orientalis, Pyrus caucasica and Mespilus germanica in mountains and plains

•Large primary stomata and numerous small secondary stomata were observed.•The size of secondary stomata linearly depended on the number of cuticular folds.•The length of secondary stomata correlated inversely with their density.•The stomata of pear and medlar were reduced in size under high-altitude stress.•The size of abaxial and adaxial epidermal leaf cells did not depend on the altitude.