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Central area of a diatom (Pinnularia) frustule, x2000.
The Varieties of Algae.
When the term Algae is used today, it refers to simple eukaryotic photosynthetic organisms, unicellular or multicellular, which have their pigments localized in membrane-bounded intracellular bodies (plastids), have no vascular system, and do not develop from an embryo.
Whilst they are extremely numerous, they do not exist as a taxonomic group.
Formerly, they were placed mainly within the Plant Kingdom as a diverse group which included microscopic unicells at one end of the size scale, and giant ocean kelp with fronds extending a hundred metres at the other. The current practice is to regard many of the groups of the former classification as sufficiently distinct to be treated as separate phyla within the Kingdom Protoctista -- a large group which includes all the microscopic unicells having a nucleus.
Within the new classificationary boundaries, there is enormous diversity. The grouping still includes microscopic unicells at one end of the scale and giant ocean kelp at the other.
The former blue-green algae are now classified amongst the Bacteria (or Monera in earlier systems).
In fact, the separation of the bacteria and the blue-green algae from the other micro organisms marks the most important shift in the last hundred years in terms of the way life on Earth is viewed. It is in effect the division of all living creatures into two major groups:
1. The Prokaryotes -- those having no nucleus (Kingdom Bacteria: the traditional bacteria and blue-green algae), and
2. the Eukaryotes, those which have a nucleus. The Eukaryotes are therefore all living creatures except the bacteria and blue-green algae. They comprise the remaining four Kingdoms: Protoctista, Fungi, Plants and Animals.
It is certain that as our knowledge of the genetic relationships between creatures advances, the kingdom most likely to undergo radical rearrangement and subdivision is the Protoctista.
The algal phyla which most concern the freshwater biologist are:
The micrographs of algae in these galleries are presented under the group names by which microscopists and naturalists have known them for the last century or so, and by which they will probably continue to be known amongst non-experts.