Protococcus

Protococcus (Gr., 'first-grain'), a genus of very simple unicellular green plants, one species of which (P. viridis) is everywhere abundant as a green film on tree-trunks and damp walls, or in stagnant rain-water. The colour is sometimes reddish, and the organism may be found passively encysted during drought, and at other times actively motile with a couple of cilia. See ALGÆ.