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.
Photosynthetic Protists Pyrrhophyta: TheDinoflagellates The dinoflagellates consist of about2100 known species of primarily uni-cellular, photosynthetic organisms,most of which have two flagella. Amajority of the dinoflagellates are ma-rine, and they are often abundant inthe plankton, but some occur in freshwater.hrysophyta: The Diatoms and Golden Algae The Diatoms. Diatoms, members of the phylum Chrys-ophyta, are photosynthetic, unicellular organisms withunique double shells made of opaline silica, which are oftenstrikingly and characteristically marked.