Algae
Algae
comprise several different groups of organisms which produce food by
photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range
from large multicellular algae to single-celled organisms and are classified
into three groups, the brown, red and green
algae. There is good evidence that the brown algae evolved independently
from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from
cyanobacteria, and they are no longer classified as plants as defined here.[21][22]
The Viridiplantae, the green plants – green algae and land
plants – form a clade,
a group consisting of all the descendants of a common ancestor. With a few
exceptions, the green plants have the following features in common;
primary chloroplasts derived from cyanobacteria
containing chlorophylls a and b, cell
walls containing cellulose, and food stores in the form of starchcontained
within the plastids. 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.
Two
additional groups, the Rhodophyta (red algae)
and Glaucophyta (glaucophyte
algae), also have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria,
although they differ from Viridiplantae in the
pigments which are used in photosynthesis and so are different in colour. These groups also differ from green plants in that
the storage polysaccharide is floridean
starch and is stored in the cytoplasm rather than in the
plastids. They appear to have had a common origin with Viridiplantae
and the three groups form the clade Archaeplastida,
whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event. This is the broadest modern definition
of the term 'plant'.
In contrast,
most other algae (e.g. brown algae/diatoms, haptophytes, dinoflagellates,
and euglenids) not only have different pigments but also
have chloroplasts with three or four surrounding membranes. They are not close
relatives of the Archaeplastida, presumably having
acquired chloroplasts separately from ingested or symbiotic green and red
algae. They are thus not included in even the broadest modern definition of the
plant kingdom, although they were in the past.
The green
plants or Viridiplantae were traditionally divided
into the green algae (including the stoneworts) and
the land plants. However, it is now known that the land plants evolved from
within a group of green algae, so that the green algae by themselves are
a paraphyletic group,
i.e. a group that excludes some of the descendants of a common ancestor.
Paraphyletic groups are generally avoided in modern classifications, so that in
recent treatments the Viridiplantae have been divided
into two clades, the Chlorophyta and
the Streptophyta (including the land plants and Charophyta).[23][24]
The Chlorophyta (a name that has also been used for all green
algae) are the sister group to the Charophytes, from
which the land plants evolved. There are about 4,300 species,[25] mainly
unicellular or multicellular marine organisms such as the sea lettuce, Ulva.
The other
group within the Viridiplantae are the mainly
freshwater or terrestrial Streptophyta, which
consists of the land plants together with the Charophyta,
itself consisting of several groups of green algae such as the desmids and stoneworts.
Streptophyte algae are either unicellular or form
multicellular filaments, branched or unbranched.[24] The
genus Spirogyra is a filamentous streptophyte
alga familiar to many, as it is often used in teaching and is one of the organisms
responsible for the algal "scum" on ponds. The freshwater stoneworts strongly resemble land plants and are believed
to be their closest relatives.[citation needed] Growing
immersed in fresh water, they consist of a central stalk with whorls of branchlets.
