Pennate diatoms. Light micrograph of a group of diatoms Pleurosigma angulatum. The blue colour is a diffraction effect. Diatoms have an intricately patterned, glass-like cell wall, or frustule. The frustule consists of 2 halves which fit together like the lid and bottom of a box. The frustule is often decorated with rows of tiny holes, known as striae, which are arranged radially (in centric diatoms) or on the 2 sides of a central axis (in pennate diatoms). Diatoms are a distinctive group of single-celled algae containing about 10,000 species. They form a large part of the plankton in marine and freshwater food chains. Magnification: x153 at 6x7cm size. Magnificatiom: x260 at 4x5 inch size.