Abstract Light, fluorescence and electron microscopical analysis of the rooted freshwater plant Ranunculus trichophyllus revealed a peculiar anatomical feature. In addition to the true endodermis encircling the root stele, endodermis-like sheaths occurred around each vascular bundle of the leaf segments and of the eustelic stem with its large central cavity, which assumed an anatomical feature resembling that of some pteridophyte stems. These impermeable sheaths, whose cells differentiate suberized walls, can play a major role in hampering the apoplastic leakage of the pressurized water solution which flows throughout the plant in xylem vessels and contains the mineral nutrients taken up by the roots from the sediment. Moreover, these sheaths can function in preventing flooding of the aerenchymatic cavities of the submerged organs. In this way the endodermis-like sheaths preserve the correct circulation of gas and nutrient solution through the entire organism and assume great significance as a structural mechanism evolved by this species to survive and grow underwater.