Systematics and Taxonomy
Under the old five kingdom system of classification, all the algae were placed within the Kingdom Protista. However, the molecular research of the last decades has resulted in major reclassifications, and at least for now, some uncertainty and disagreement. There is no debate about the placement of the algae in the Domain Eukarya – they have eukaryote cells. And, for the most part, there seems to be agreement that the green algae are members of the plant kingdom. However, classification into lower taxa is less clear.
- Volvocales – can be unicellular or multicellular; the vegetative cells are flagellated & motile
- Tetrasporales – non-filamentous colonies with immobile vegetative cells capable of cell division; pseudocilia may be present
- Schizogoniales – foliose marine algae with stellate choroplasts
- Chlorococcales – unicellular or non-filamentous colonial algae; if colonial, daughter colonies formed as coenobia; vegetative cells non-motile
- Sphaerophleales – unbranched filaments with new walls formed inside the old filament walls, resulting in H-shaped wall pieces
- Chlorosarcinales – daughter cells retained within parent cell wall; no plasmodesmata present
- Chaetophorales – branched or unbranched filaments; plasmodesmata present
- Oedogoniales – uninucleate filamentous freshwater algae with a unique type of cell division; motile spores and gametes with a whorl of flagella at one pole
In Graham and Wilcox (2000), the Chlorophyceans are a lineage of green algae, encompassing the following clades:
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