kingdom

Primary division in biological classification. At one time, only two kingdoms were recognized: animals and plants. Today most biologists prefer a five-kingdom system, even though it still involves grouping together organisms that are probably unrelated. One widely accepted scheme is as follows: Kingdom Animalia (all multicellular animals); Kingdom Plantae (all plants, including seaweeds and other algae, except blue-green); Kingdom Fungi (all fungi, including the unicellular yeasts, but not slime moulds); Kingdom Protista or Protoctista (protozoa, diatoms, dinoflagellates, slime moulds, and various other lower organisms with eukaryotic cells); and Kingdom Monera (all prokaryotes - the bacteria and cyanobacteria, or blue-green algae, and the archaea). The first four of these kingdoms make up the eukaryotes.

When only two kingdoms were recognized, any organism with a rigid cell wall was a plant, and so bacteria and fungi were considered plants, despite their many differences. Other organisms, such as the photosynthetic flagellates (euglenoids), were claimed by both kingdoms. The unsatisfactory nature of the two-kingdom system became evident during the 19th century, and the biologist Ernst Haeckel was among the first to try to reform it. High-power microscopes have revealed more about the structure of cells; it has become clear that there is a fundamental difference between cells without a nucleus (prokaryotes) and those with a nucleus (eukaryotes). However, these differences are larger than those between animals and higher plants, and are unsuitable for use as kingdoms. At present there is no agreement on how many kingdoms there are in the natural world.

Although the five-kingdom system is widely favoured, some schemes have as many as 20.