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 18,000
bryophytes .Green plants, sometimes called metaphytes, obtain most of their energy from
sunlight via a process called
photosynthesis.Definition
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.
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.
AlgaeAlgae
Green algae from
Ernst Haeckel's
Kunstformen der Natur, 1904.
Main article:
AlgaeMost
algae are no longer classified within the Kingdom Plantae.
[2][3] The algae comprise several different groups of organisms that produce energy through photosynthesis, each of which arose independently from separate non-photosynthetic ancestors. Most conspicuous among the algae are the
seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the
green,
red, and
brown algae. Each of these algal groups also includes various microscopic and single-celled organisms.
Only two groups of algae are considered close relatives of land plants (
embryophytes). The first of these groups is the
Charophyta (
desmids and stoneworts), from which the embryophytes developed.
[4][5][6] The
sister group to the combined embryophytes and charophytes is the other group of green algae (
Chlorophyta), and this more inclusive group is collectively referred to as the green plants or
Viridiplantae. The Kingdom Plantae is often taken to mean this
monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing
cellulose, have
chloroplasts containing
chlorophylls a and b, and store food in the form of
starch. They undergo closed
mitosis without
centrioles, and typically have
mitochondria with flat cristae.
The
chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic
cyanobacteria. The same is true of two additional groups of algae: the
Rhodophyta (red algae) and
Glaucophyta. All three groups together are generally believed to have a common origin, and so are classified together in the taxon
Archaeplastida. In contrast, most other algae (e.g.
heterokonts,
haptophytes,
dinoflagellates, and
euglenids) have chloroplasts with three or four surrounding membranes. They are not close relatives of the green plants, presumably acquiring chloroplasts separately from ingested or symbiotic green and red algae.
Fungi
Main article:
FungiFungi are no longer considered to be plants, though they were previously included in the plant kingdom. Unlike embryophytes and algae, fungi are not photosynthetic, but are
saprotrophs: obtaining food by breaking down and absorbing surrounding materials. Fungi are not plants, but were historically treated as closely related to plants, and were considered to be in the purview of botanists. It has long been recognized that fungi are evolutionarily closer to animals than to plants, but they still are covered more in depth in introductory botany courses and are not necessarily touched upon in introductory zoology courses. Most fungi are formed by microscopic structures called
hyphae, which may or may not be divided into cells but contain
eukaryotic nuclei. Fruiting bodies, of which
mushrooms are most familiar, are the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of
animals. Therefore, the
fungi are in a kingdom of their own.
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