Viridiplantae (literally "green plants")[5] are a clade of eukaryotic organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems.[6] They are made up of the green algae, which are primarily aquatic, and the land plants (embryophytes), which emerged from within them.[7][8][9] Green algae traditionally excludes the land plants, rendering them a paraphyletic group. But it is accurate to think of land plants as kind of alga.[10] Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them.[10][11][12][13][14] They have cells with cellulose in their cell walls, and primary chloroplasts derived from endosymbiosis with cyanobacteria that contain chlorophylls a and b and lack phycobilins.
In some classification systems, the group has been treated as a kingdom,[15] under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional plant kingdom to include the green algae. Adl et al., who produced a classification for all eukaryotes in 2005, introduced the name Chloroplastida for this group, reflecting the group having primary chloroplasts with green chlorophyll. They rejected the name Viridiplantae on the grounds that some of the species are not plants, as understood traditionally.[16] The Viridiplantae are made up of two clades: Chlorophyta and Streptophyta as well as the basal Mesostigmatophyceae and Chlorokybophyceae.[17][18] Together with Rhodophyta and glaucophytes, Viridiplantae are thought to belong to a larger clade called Archaeplastida or Primoplantae.
A taxonomic evaluation of eukaryotes based on myosin distribution showed the Viridiplantae lost class-I myosins.[19]
Below is a consensus reconstruction of green algal relationships, mainly based on molecular data.[22][20][21][23][12][24][25][18][26][27][28]