Viridiplantae

Viridiplantae (literally "green plants") constitute a clade of eukaryotic organisms that comprises approximately 450,000–500,000 species that play important roles in both terrestrial and aquatic ecosystems. They include the green algae, which are primarily aquatic, and the land plants (embryophytes), which emerged from within them. Green algae traditionally excludes the land plants, rendering them a paraphyletic group. However it is accurate to think of land plants as a kind of alga. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. 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. Corroborating this, a basal phagotroph archaeplastida group has been found in the Rhodelphydia.

In some classification systems, the group has been treated as a kingdom, 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. The Viridiplantae are made up of two clades: Chlorophyta and Streptophyta as well as the basal Mesostigmatophyceae and Chlorokybophyceae. Together with Rhodophyta and glaucophytes, Viridiplantae are thought to belong to a larger clade called Archaeplastida or Primoplantae.

Phylogeny and classification
Simplified phylogeny of the Viridiplantae, according to Leliaert et al. 2012.
 * Viridiplantae
 * Chlorophyta
 * core chlorophytes
 * Ulvophyceae
 * Cladophorales
 * Dasycladales
 * Bryopsidales
 * Trentepohliales
 * Ulvales-Ulotrichales
 * Oltmannsiellopsidales
 * Chlorophyceae
 * Oedogoniales
 * Chaetophorales
 * Chaetopeltidales
 * Chlamydomonadales
 * Sphaeropleales
 * Trebouxiophyceae
 * Chlorellales
 * Oocystaceae
 * Microthamniales
 * Trebouxiales
 * Prasiola clade
 * Chlorodendrophyceae
 * Chlorodendrales
 * Pedinophyceae
 * prasinophytes (paraphyletic)
 * Pyramimonadales
 * Mamiellophyceae
 * Pycnococcaceae
 * Nephroselmidophyceae
 * Prasinococcales
 * Palmophyllales
 * Streptophyta
 * Charophytes
 * Mesostigmatophyceae
 * Mesostigmatales
 * Chlorokybales
 * Klebsormidiophyceae
 * Phragmoplastophyta
 * Charophyceae
 * Coleochaetophyceae
 * Zygnematophyceae
 * Embryophyta (land plants)

Cladogram
In 2019, a phylogeny based on genomes and transcriptomes from 1,153 plant species was proposed. The placing of algal groups is supported by phylogenies based on genomes from the Mesostigmatophyceae and Chlorokybophyceae that have since been sequenced. Both the "chlorophyte algae" and the "streptophyte algae" are treated as paraphyletic (vertical bars beside phylogenetic tree diagram) in this analysis. The classification of Bryophyta is supported both by Puttick et al. 2018, and by phylogenies involving the hornwort genomes that have also since been sequenced.

Ancestrally, the green algae were flagellates.