LM of mixed diatoms, showing centric & pennate types. Diatoms are a distinctive group of single- celled algae with 10,000 species, & they form an important part of the plankton at the base of the marine & freshwater food chains. The typical feature of diatoms is their intricately patterned, glass-like cell wall, or frustule. The frustule consists of 2 halves which fit together like the lid & 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 (pennate diatoms). Nomarski illumination. Magnification: x65 at 35mm, x130 at 6x7cm size. Ref: MICROCOSMOS, fig. 5.26, page 102