Pleustophyte

The pleustophyte (Luther 1949) life form does not have root or rhizoid contact with the bottom (although benthopleustophytes rest on the bottom).

Source: wiktionary

phosphate-enrichment of both sediment and water leads to luxurious growth of pleustophytes such as Riccia fluitans L. and Lemna minor L. in small, shallow waters

Source: wiktionary

The planktophytes and smaller pleustophytes (i.e. the Lemnids and Ricciellids in the sense of DEN HARTOG & SEGAL 1964) differ from all other water plants by forming mobile synusiae, able to shift from one phytocoenose to another. Before going into detail it is useful to distinguish with LUTHER (1949) between haptophytes (adnate or non-rooting hydrophytes), rhizophytes (radicant or rooted hydrophytes), planktophytes, and pleustophytes (i.e. microscopic and macroscopic errant or mobile hydrophytes respectively).

Source: wiktionary