Remarks on Myriophyllum sibiricum, a long-time unrecognised species of the flora of Lithuania

This research aimed to verify the identification and distribution of Myriophyllum sibiricum in Lithuania. The study was based on a revision of available herbarium specimens and information from archival materials and literature sources. A total of 134 herbarium specimens of Myriophyllum, mainly initially identified as Myriophyllum spicatum, were examined. After revision of herbarium specimens, 101 of these were identified as Myriophyllum sibiricum. The remaining 33 herbarium specimens were tentatively determined as hybrids Myriophyllum sibiricum × Myriophyllum spicatum. None of the specimens were identified as Myriophyllum spicatum. The distribution of Myriophyllum sibiricum is related mainly to standing waters of upland regions, while localities of putative hybrid scattered throughout the area and associated with upland lakes and river stretches. The morphological features of the leaves (number of segments pairs and its angle with the central axis, occurrence of glands at the base of young leaves and in axils of leaves segments) are helpful to distinguish typical specimens of Myriophyllum sibiricum from Myriophyllum spicatum, however useless for their separation from the hybrid. This study revealed that further research on Myriophyllum sibiricum, Myriophyllum spicatum and their putative hybrid must apply morphological and molecular methods.