An extremely diverse group consisting predominantly of aquatic plants showing relatively little differentiation of tissues and organs as compared to bryo-phytes and tracheophytes. The group includes both prokaryotic and eukaryotic organisms ranging from unicells through colonial and filamentous forms to parenchymatous seaweeds over 50 m long. Some 23 500 species of eukaryotic algae and some 1450 species of prokaryotic algae have been described. Adaptation to an aquatic environment has led to the development of many distinctive biochemical traits. Algal sex organs are usually unicellular.
Formerly the algae were placed with the bacteria and fungi in the division *Thallophyta
. Increasing appreciation of the very basic differences between such organisms and between the algal groups themselves has led most systematists to consider each group as a separate class or division. The groups are classified according to their pigments, food reserves, cell-wall materials, number and types of flagella, and ultrastructural details. See Cyanophyta