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• Leaf anatomy in six annual plant taxa of the genus Polygonum s. str. was analyzed. • The first study of this kind in the Balkan Peninsula and the Pannonian Plain. • Anatomical characteristics for two neglected polygonum s. str. taxa were considered. • Identified qualitative characters proved to be important in taxonomic delimitation. Leaf blade anatomy in six annual plant taxa of the genus Polygonum from the western and central regions of the Balkan Peninsula and the southern region of the Pannonian Plain was analyzed. The aim was to compare the anatomical characteristics of the leaf blades and identify the diagnostic significance of these traits in the taxonomy of the studied taxa. The analyses included 32 populations, and focused on 18 quantitative (14 basic and 4 derived) and 5 qualitative traits. A detailed description of the leaf anatomy was also provided for all examined taxa. Based on the statistical analysis and tests conducted on the quantitative leaf blade anatomical traits, it is possible to distinguish P. albanicum from the remaining taxa (P. arenarium subsp. arenarium, P. arenarium subsp. pulchellum, P. arenastrum, P. aviculare , and P. graminifolium). However, the analyzed quantitative characteristics did not contribute to the delimitation of the remaining taxa or to the formation of new groups within this genus. Consequently, they are insufficient for the separation of the examined taxa, but can provide important additional taxonomic information. On the other hand, the results yielded by nonparametric statistical analyses confirm that the analyzed qualitative traits, the midrib protrusion in particular, and the presence and level of protrusion of the lateral ribs, are valuable in taxa delimitation. [ABSTRACT FROM AUTHOR]