New data on the micromorphology, histochemistry, and fluorescence of colleters on leaf structures at different stages of development (leaf blade, stipules, and petiole) of Mespilus germanica L. are presented. Colleters are found on the tips of the teeth of both young and mature leaf blades and stipules, less often on the petioles. The leaf veins approach the leaf tooth, but no vascularization was found in the colleter. On leaf structures inside the bud, young colleters were observed in the form of finger-shaped or rounded outgrowths consisting of isodiametric cells. Mature colleters are multicellular secretory structures that have a head on a short stalk. The central part of the head consists of densely packed parenchymal cells, which are surrounded by radially elongated palisade-like secretory cells covered with a cuticle. The main secretion process of the colleter falls on the period of active growth of leaf structures. The secreted substances accumulated in the intercellular spaces of the palisade-like cells of the head and then were released outside in the form of translucent vesicles. The secretion products were released when the cuticle was ruptured and spread over the surface of the head and tooth of the leaf blade and stipules. After the end of secretion, the sizes of the head of the colleter decreased, and an abscission zone appeared in the cells of the colleter stalk, along the border of which a fracture occurred when the head fell off. Histochemical analysis of the contents of the colleter showed the presence of polysaccharides, especially at a young age, substances of a phenolic nature and lipids at a more mature age. In the fluorescence spectrum of young leaf colleter secretion, a peak at 671-672 nm was observed upon excitation at 405 and 473 nm. The obtained data on Mespilus germanica L. colleter can be used in the taxonomy of Pyrinae and Rosaceae.
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