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.
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:
- Charophyta - paraphyletic to higher plants, together comprising the Streptophyta
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.