Within the Helobieae there has been a great deal of controversial opinion about the evaluation of the genera belonging to the Potamogetonaceae, among which Najas finds by almost unanimous opinion its closest relatives. Generally Najas has been accepted to represent a separate monotypic family on account of the basal ovule and the structure of the anther (with a thin, tight, 2-lipped envelope and apically escaping pollen). The closest allied genus among Potamogetonaceae seems to be Zannichellia, which is by HUTCHINSON (1934) accepted as a separate family, Zannichelliaceae, put together with Najadaceae in his order Najadales. Within the Helobieae some authors accept the structure of Najadaceae as primitive, notably CAMPBELL (1897) and RENDLE (1930), but others find it a derived, advanced state within the order, cf. HUTCHINSON (1934) and LAWRENCE (1951).