The plant lineages grouped as 'algae' are polyphyletic. Each lineage has a distinct and unique evolutionary history. Some lineages have evolved as a consequence of primary endosymbiotic events (e.g., Chlorophyta). Others have evolved as the result of secondary endosym-biotic events involving eukaryotic heterotrophic host cells and eukaryotic photoautotrophic endosymbionts (e.g., Euglenophyta). Convergent morphological and anatomical evolution is evident within and across each of the various algal lineages and provides evidence for morphological adaptation to the physical settings in which phyletically dissimilar plants exist. The diversity of algal life-forms is so great and so convergent that no generally accepted life-form classification system exists. For this reason, this article adopts developmental criteria for sorting the algae into life-forms.
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