^BTabellaria diatoms.^b Coloured scanning electron micrograph (SEM) of groups of ^ITabellaria sp.^i diatoms. The four groups each consist of several rod-like diatoms, or single-celled algae. They each have an intricately patterned cell wall, or frustule, made of silica and pectin. The wall is in two halves which fit neatly together. Diatoms can be elongated (pennate, as here) or circular (centric). They are found in both marine and freshwater, and are a major constituent of plankton. As such, they are of great importance to the food chains in their environment. Magnification: x200 at 5x7cm size. x700 at 6.5x9.5"