Green alga
The green algae arelarge groupalgae from whichembryophytes (higher plants) emerged. As suchformparaphyletic group, variously included amongPlantae or withProtista. The green algae include unicellularcolonial flagellates, usually but not alwaystwo flagella per cell, as well as various colonial, coccoid,filamentous forms. InCharales,closest relativeshigher plants, full differentiationtissues occurs.Almost all forms have chloroplasts. These contain chlorophylls ab, giving thembright green colour,have stacked thylakoids. Theybound bydouble membrane, so presumably were acquired by direct endosymbiosiscyanobacteria. A numbercyanobacteria show similar pigmentation, but this appearshave arisen more than once, andchloroplastsgreen algaeno longer considered closely relatedsuch forms. The only other groupsprimary chloroplasts arered algaeglaucophytes,it may be thatgreen algae sharecommon originthem. The euglenidschlorarachniophytes also have green chloroplasts, which were presumably acquired from ingested green algae, inlatter case retainingvestigial nucleus (nucleomorph).
All green algae have mitochondriaflat cristae. When present flagellatypically anchored bycross-shaped systemmicrotubules, but theseabsent amonghigher plantscharophytes. They usually have cell walls containing cellulose,undergo open mitosis without centrioles. Sexual reproduction varies from fusionidentical cells (isogamy)fertilization oflarge non-motile cell bysmaller motile one (oogamy). However, these traits show some variation, most notably amongbasal green algae, called prasinophytes. The remaining formsusually classified as follows:
- Chlorophyta
- Chlorophyceae
- Ulvophyceae
- Trebouxiophyceae
- Chlorokybales
- Klebsormidiales
- Zygnematales
- Desmidales
- Coleochaetales
- Charales (stoneworts)
