Cosmarium tetragonum is a species of green algae in the family Desmidiaceae. It is a freshwater species with a worldwide distribution.
Description
The nominate variety (var. tetragonum) is about 35 to 48 üm long, and 20 to 26 üm wide, 14 to 18 üm thick, with an isthmus about 6 to 12 üm long. Semicells are roughly rectangular in outline, with undulate margins; each side has about four bumps and the top side has about two bumps.
Varieties
Cosmarium tetragonum is variable and several varieties have been described. They are:
- Variety lundelii differs from the nominate variety that the lateral bumps are unequal; the two median bumps are larger.
- Variety bipapillatum also has two median bumps that are larger, but also has a protuberance right at the base of the semicell, near the isthmus.
- Variety ornatum has a crease that follows the margin of the cell; the crease may also have projections near the apex of the semicell.
- Variety intermedium has stout cells that are 1.5 times as long as broad; semicells are more trapezoidal.
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