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.
Flagellates as specialized cells or life cycle stages [edit]. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla, or the choanocytes of sponges. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants do.he word flagellate also describes a particular construction (or level of organization) characteristic of many protists (eukaryotic organisms) and their means of motion. The term presently does not imply any specific relationship or classification of the organisms that possess flagellae.