Tribonema Derbes et Solier
	From Greek tribo, "to rub or wear down" + nema, "threads"
	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.   Tribonema has unbranched filaments composed of a single row of elongated, cylindrical cells. The thick cell wall is made up of open-ended double cylinders that overlap to enclose the cell contents. A new cell wall piece is created with each cell division. These wall pieces appear H-shaped and are very similar to those of the green alga Microspora. The two genera can be easily distinguished by a starch test. Microspora cells contain starch; Tribonema cells, like those of other tribophytes, do not. Tribonema also has two or more parietal, disk-like, pale green or golden colored chloroplasts with no pyrenoids that differ from the parietal, bright green chloroplasts of Microspora.     Filaments viewed under polarized light with a long exposure time. Note the crystals reflecting the light.