What is "mesostigma"

Mesostigma viride is a species of freshwater green algae. It is now considered to be one of the earliest diverging members of the Streptophyta, one of the two lineages of green plants ( Viridiplantae).

Earlier studies were unable to resolve the position of the species, and it was often placed as a sister to all other green algae, as one of the basal members of the Streptophyta, or as close to Chaetosphaeridium. More recent studies agree that Mesostigma and Chlorokybus form a clade, being the earliest diverging streptophytes.