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The green algae are the large group of eukaryotes? from which the higher Plantae emerged. As such they form a [paraphyletic grouping]?, variously classed with their kin or with the Protista. In the latter case they are often put into a single phylum Chlorophyta, but sometimes separated out into several different groups. |
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The green algae are the large group of eukaryotes? from which the higher plants emerged. As such they form a paraphyletic grouping, i.e. one including some but not all descendants of a particular form, classed variously with the plants or with the Protista. |
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There is considerable diversity among the basal green algae, called Prasinophytes?. All have platycristate mitochondria and most contain green chloroplasts, with chlorophylls a and b, and bound by a double membrane. The flagella? have some ultrastructure in common but vary in number and covering, and there is even some differences in nuclear mitosis. Other green algae are divided into these classes: * Chlorophyceae * Ulvophyceae * Trebouxiophyceae? * Charophyceae? (paraphyletic to land plants) All of these groups have cellulose cell walls. Flagellate cells occur among the first three and typically have two flagella. Colonial, coccoid, and filamentous forms all occur, and sexual reproduction ranges from isogamy? all the way to oogamy? with internal development. In the last group, from which higher plants evolved, there are even a few forms (Charales? - stoneworts) that show true differentiation of tissues. |
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Almost all forms have green chloroplasts, with chlorophylls a and b, and bound by a double membrane. These are believed to have been aquired from direct symbiosis of cyanobacteria like Prochloron, and other groups with chloroplasts of similar pigmentation probably aquired them secondarily from green algae. All have mitochondria with flat cristae. Otherwise there is some variation in the biochemistry and ultrastructure, including even some differences in nuclear mitosis. |
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Most of this diversity is found among the basal green algae, flagellates collectively referred to as the Prasinophyta?. The classification of the remaining forms varies, but one version is the following: |
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* Chlorophyta ** Chlorophyceae ** Ulvophyceae ** Trebouxiophyceae? * Charophyta - paraphyletic to higher plants, together comprising the Streptophyta ** Chlorokybales? ** Klebsormidiales? ** Zygnematales? ** Desmidales? ** Coleochaetales? ** Charales? (stoneworts) |
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Sometimes the Zygnematales and Desmidales are given their own division, the Gamophyta, but in that case the Charophyta become paraphyletic to two groups instead of becoming monophyletic. The exact circumscription of the Chlorophyta varies, and sometimes all green algae are included there, but as listed above they probably comprise a monophyletic group. |
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Flagellate cells can be found among the Chlorophyta as given here, each cell usually but not always having two flagella, and colonial, coccoid, and filamentous forms also occur. Sexual reproduction varies from fusion of identical cells (isogamy) to fertilization of a large non-motile cell by a smaller motile one (oogamy). In the Charales?, the closest relatives of higher plants, full differentiation of tissues occurs. |
Almost all forms have green chloroplasts, with chlorophylls a and b, and bound by a double membrane. These are believed to have been aquired from direct symbiosis of cyanobacteria like Prochloron, and other groups with chloroplasts of similar pigmentation probably aquired them secondarily from green algae. All have mitochondria with flat cristae. Otherwise there is some variation in the biochemistry and ultrastructure, including even some differences in nuclear mitosis.
Most of this diversity is found among the basal green algae, flagellates collectively referred to as the Prasinophyta?. The classification of the remaining forms varies, but one version is the following:
Sometimes the Zygnematales and Desmidales are given their own division, the Gamophyta, but in that case the Charophyta become paraphyletic to two groups instead of becoming monophyletic. The exact circumscription of the Chlorophyta varies, and sometimes all green algae are included there, but as listed above they probably comprise a monophyletic group.
Flagellate cells can be found among the Chlorophyta as given here, each cell usually but not always having two flagella, and colonial, coccoid, and filamentous forms also occur. Sexual reproduction varies from fusion of identical cells (isogamy) to fertilization of a large non-motile cell by a smaller motile one (oogamy). In the Charales?, the closest relatives of higher plants, full differentiation of tissues occurs.
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