Leaf undulation is used as a diagnostic feature in the taxonomy of different taxa of the Epipactis (Orchidaceae) genus. It is considered a specifically important trait, apart from the column (gynostemium) structure, in the identification of E. muelleri. However, leaf undulation is a common developmental phenomenon in various plants, including orchids, thus the main goal of the research presented was to validate the usefulness of this feature in Epipactis taxonomy. In the course of this study, the leaf structure of six Epipactis species was analyzed, i.e., E. albensis, E. atrorubens, E. helleborine, E. muelleri, E. palustris and E. purpurata. The leaf margin outline and the cellular pattern of the leaf surface, as well as the presence of papillae, were examined in detail. The results obtained showed that in all taxa analyzed, representing different groups of the Epipactis genus, leaf undulation was present. The phenomenon was related to leaf blade development and the local differential growth of leaf sectors rather than to the taxonomic position of the species. Therefore, leaf undulation studied in Helleborines does not have a diagnostic value as a non-programmed intrinsic feature and should not be applied to taxa identification. The valid trait in Epipactis classification (including E. muelleri) is the column structure.
