The yellow-green or tribophyte algae are named for the genus Tribonema. The group includes about 100 genera, many of which are rare. The cell walls are made up mainly of cellulose, and sometimes silica. The cytoplasm contains numerous lipid droplets, but no starch. The chloroplasts of the tribophytes are green or yellow-green, so the cells appear very similar to those of the green algae. The tribophyte algae are usually found in cooler waters of lower pH, with some exceptions.