Plants
are a major group of life forms and include
familiar organisms such as trees, herbs, bushes,
grasses, vines, ferns, mosses, and green algae.
About 350,000 species of plants, defined as
seed plants, bryophytes, ferns and fern allies,
are estimated to exist currently. As of 2004,
some 287,655 species had been identified,
of which 258,650 are flowering and 15,000
bryophytes (see table below). Green plants,
sometimes called metaphytes, obtain most of
their energy from sunlight via a process called
photosynthesis.
Aristotle divided all living things between
plants (which generally do not move), and
animals (which often are mobile to catch their
food). In Linnaeus' system, these became the
Kingdoms Vegetabilia (later Metaphyta or Plantae)
and Animalia (also called Metazoa). Since
then, it has become clear that the Plantae
as originally defined included several unrelated
groups, and the fungi and several groups of
algae were removed to new kingdoms. However,
these are still often considered plants in
many contexts, both technical and popular.
Indeed, an attempt to perfectly match "plant"
with a single taxon is problematic, because
for most people the term "plant"
is only vaguely related to the phylogenic
concepts on which modern taxonomy and systematics
are based.
When the name Plantae or plants is applied
to a specific taxon, it is usually referring
to one of three concepts. From smallest to
largest in inclusiveness, these three groupings
are:
Land plants, also known as Embryophyta or
Metaphyta. As the narrowest of plant categories,
this is further delineated below.
Green plants - also known as Viridiplantae,
Viridiphyta or Chlorobionta - comprise the
above Embryophytes, Charophyta (i.e., primitive
stoneworts), and Chlorophyta (i.e., green
algae such as sea lettuce). It is this clade
which is mainly the subject of this article.
Archaeplastida - also known as Plantae sensu
lato, Plastida or Primoplantae - comprises
the green plants above, as well as Rhodophyta
(red algae) and Glaucophyta (simple glaucophyte
algae). As the broadest plant clade, this
comprises most of the eukaryotes that eons
ago acquired their chloroplasts directly by
engulfing cyanobacteria.
Informally, other creatures that carry out
photosynthesis are called plants as well,
but they do not constitute a formal taxon
and represent species that are not closely
related to true plants. There are around 375,000
species of plants, and each year more are
found and described by science.
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