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The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyte and Charophyte algae, which are now placed in separate Divisions. The land plants or Embryophytes (higher plants) are thought to have emerged from the Charophytes. As the embryophytes are not algae, and are therefore excluded, green algae are a paraphyletic group. However, the clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular seaweeds. In the Charales, the closest relatives of higher plants, full cellular differentiation of tissues occurs. There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds. A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids and chlorarachniophytes were acquired from ingested green algae,〔 and in the latter retain a nucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliate ''Paramecium'', and in ''Hydra viridissima'' and in flatworms. Some species of green algae, particularly of genera ''Trebouxia'' of the class ''Trebouxiophyceae'' and ''Trentepohlia'' (class Ulvophyceae), can be found in symbiotic associations with fungi to form lichens. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus. ''Trentepohlia'' is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae. ==Cellular structure== Green algae have chloroplasts that contain chlorophyll ''a'' and ''b'', giving them a bright green color, as well as the accessory pigments beta carotene and xanthophylls,〔Burrows 1991. ''Seaweeds of the British Isles.'' Volume 2 Natural History Museum, London. ISBN 0-565-00981-8〕 in stacked thylakoids.〔Hoek, C. van den, Mann, D.G. and Jahns, H.M. 1995. ''(Algae An introduction to phycology )''. Cambridge University Press, Cambridge. ISBN 0-521-30419-9〕 The cell walls of green algae usually contain cellulose, and they store carbohydrate in the form of starch.〔Judd, W.S., Campbell, C.S., Kellogg, E.A., Stevens, P.F. and Donoghue, M.J. Sinauer (2002) Plant systematics, a phylogenetic approach. Associates Inc., Sunderland Mass. ISBN 0-87893-403-0 , p156〕 All green algae have mitochondria with flat cristae. When present, paired flagella are used to move the cell. They are anchored by a cross-shaped system of microtubules and fibrous strands. Flagella are only present in the motile male gametes of charophytes〔Bhattacharya D, Medlin L (1998) Algal phylogeny and the origin of land plants" ''Plant Physiology'' 116, 9–15〕 and are absent from the gametes of Pinophyta and flowering plants. Members of the Chlorophyceae class undergo closed mitosis in the most common form of cell division among the green algae, which occurs via a phycoplast.〔J. Pickett-Heaps (1976) Cell division in eucaryotic algae. ''Bioscience'' 26 (7) 445-450〕 Like land plants (embryophytes), charophyte green algae undergo open mitosis without centrioles. Instead, a 'raft' of microtubules, the phragmoplast, is formed from the mitotic spindle and cell division involves the use of this phragmoplast in the production of a cell plate.〔P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): ''Biology of Plants'', 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「green algae」の詳細全文を読む スポンサード リンク
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