Potamogeton ×fluitans (P. natans × P. lucens) in the Czech Republic I. Morphology and anatomy

Potamogeton ×fluitans Roth (P. natans L. × P. lucens L.) was discovered in NE Bohemia as a new taxon for the Czech Republic. In the site, the hybrid grew together with both its parents. The revision of herbarium specimens proved that all previous records of “P. fluitans” from the Czech Republic were erroneous. Morphological and anatomical characters of the hybrid are described, together with those of its parents and other morphologically similar taxa. P. ×fluitans differs from P. natans especially in having submerged leaves with a distinct lamina and petioles of floating leaves without a flexible section at the junction with the lamina. From P. lucens, it is distinguished by the capability of developing floating leaves and by long, narrowly oblong submerged leaves. While stem anatomical pattern of P. ×fluitans is intermediate between those of its parents, it is easily distinguishable from that of morphologically the most similar P. nodosus Poir. The hybrid is sterile and no trace of developed fruits has been found among the Czech material.