The Epic Interactive Encyclopedia 1997/The_Epic_Interactive_Encyclopedia_97.iso/k/kingdom/infotext

Text File | 1992-09-02 | 2KB | 42 lines

The 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, all
seaweeds and other algae, including
unicellular algae); 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). 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.