Light micrograph of different species of diatoms (Bacillariophyta), which are unicellular algae. They form a large proportion of the bottom flora of lakes, ponds and salt marshes. Diatoms are also abundant in marine and freshwater plankton. The feature that distinguishes them from all other algae is the siliceous wall which encloses them. When a diatom dies the siliceous wall sinks and gradually immense accumulations are formed at the bottom of the water. Diatoms are divided into two groups known as Centrales (radially symmetrical) and Pennales which comprises all the others. Magnification: x30 at 35mm size