Viridiplantae (lit.'green plants')[6] is a clade of around 450,000–500,000 species of eukaryotic organisms, most of which obtain their energy by photosynthesis. The green plants are chloroplast-bearing autotrophs that play important primary production roles in both terrestrial and aquatic ecosystems.[7] They include green algae, which are primarily aquatic, and the land plants (embryophytes), which emerged within freshwater green algae.[8][9][10] Green algae traditionally excludes the land plants, rendering them a paraphyletic group, however it is cladistically accurate to think of land plants as a special clade of green algae that evolved to thrive on dry lands.[11] Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them.[11][12][13][14][15]
Viridiplantae species all 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. Corroborating this, a basal phagotrophArchaeplastida group has been found in the Rhodelphydia.[16] In some classification systems, the group has been treated as a kingdom,[17] 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. They rejected the name Viridiplantae on the grounds that some of the species are not plants as understood traditionally.[18] The Viridiplantae are made up of two clades: Chlorophyta and Streptophyta as well as the basal Mesostigmatophyceae and Chlorokybophyceae.[19][20] Together with Rhodophyta and glaucophytes, Viridiplantae are thought to belong to a larger clade called Archaeplastida or Primoplantae.