The algae are an extensive group of plants, which show a great diversity in their range of forms. Some taxonomists consider them to be a separate division in the plant kingdom, while others place them at a lower rank of classification, labeling them as a class within the division Thallophyceae.

All algae are primitive plants, and are taxonomically united by the presence of unicellular organs of reproduction. The algae are comprised of representatives from two different groups of living organism, the prokaryotes (1,450 algal species) and the eukaryotes (23,500 algal species). The thallus (plant body) of algae can show a wide diversity of forms, ranging from unicellular to multi-cellular, filamentous, flattened or ribbon-like. The smallest, diatoms (which range in size from 5 to 200 microns), are visible only through microscopic observation, but the largest, seaweeds, can be in excess of 150 feet (50m) long.

Algae are aquatic, either marine or freshwater, although some...