The Archaeplastida (or Plantae sensu lato) are a major group of eukaryotes, comprising the red algae (Rhodophyta), the green algae and the land plants, together with a small group of freshwater unicellular algae called glaucophytes. The chloroplasts of all these organisms are surrounded by two membranes, suggesting that they developed directly from endosymbiotic cyanobacteria. In all other groups besides the amoeboid Paulinella chromatophora, the chloroplasts are surrounded by three or four membranes, suggesting that they were acquired secondarily from red or green algae.