An account of the structure of plants naturally begins with the cell which is the proximate unit of organic structure. The elementary unit of plant structure, as of animal structure, is the cell. Faunistically, although not geographically, the Nearctic and Palaearctic areas must form the two subdivisions of one great unit, for which the  Holarctic region  is now the generally accepted term.
Definition: a subclass considered a phylum of the kingdom Protista in some classifications; includes holozoic or saprozoic flagellates.
A flagellum (/flÉËdÊÉlÉm/ ; plural: flagella) is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells. The word flagellum in Latin means whip.rokaryotic flagella run in a rotary movement, while eukaryotic flagella run in a bending movement. The prokaryotic flagella uses a rotary motor,and the eukaryotic flagella uses a complex sliding filament system. Eukaryotic flagella is ATP driven, while prokaryotes are proton driven.