The algae are oxygen-producing, mainly aquatic organisms possessing enormous morphological, cytological, molecular and reproductive diversity. Modern studies have led to the recognition that algae represent a number of evolutionary lines or lineages, almost all of which are represented in the British freshwater flora. Most of these lineages probably arose independently as a result of endosymbiosis. There has been a tendency by protozoologists to adopt what has been termed a ‘protistan’ view and to place these lineages into the Protista, a kingdom that itself is unnatural (Corliss, 1994). The majority of the lineages are eukaryotic, in which the cells have a double membrane surrounding the nucleus and most other organelles such as chloroplasts. Another evolutionary line, the Cyanobacteria (Cyanophyta or blue-green algae) are often considered together with other algae, although these organisms are prokaryotic (membrane-bound organelles absent) and more closely related to the bacteria.

A few algae have photosynthetic structures known as cyanelles, which in many ways are intermediate between a chloroplast and a free-living bluegreen alga. In most such cases, the rest of the cell resembles quite closely other species of algae with normal chloroplasts. It is likely that the cyanelles have evolved relatively recently from free-living cyanobacteria. However, in the case of Glaucocystis nostochinearum, which has conspicuous cyanelles, there is still doubt about its relationships and it is here classified in its own phylum, the Glaucophyta. Some other organisms possess no photosynthetic structure, but are otherwise quite similar to those that have chloroplasts. These are often treated as protozoans, but clearly belong in the same phyla as the related photosynthetic organisms. Most examples are in the flagellated phyla (Euglenophyta, Cryptophyta, Dinophyta and Chrysophyta). The first three, but not the Chrysophyta, are nowincluded in this edition.

What constitutes an algal species has been the subject of much debate (see John and Maggs, 1997, for a review). Most species are recognized by discontinuities in sets of morphological characters observed with the light microscope, so that algal systematics is largely based on what has been termed ‘the morphospecies concept’. Culture studies have frequently shown that species concepts based solely on characters observed in field-collected material are often too narrow and the taxonomic validity of many of the characters used is open to question (see Trainor, 1998, and his comments in a review of the genus Scenedesmus sensu lato).