Diatom. Coloured scanning electron micrograph of a freshwater diatom, Denticula tenuis, showing the exterior of the cell wall. Diatoms are unicellular algae. They consist of two valves, made of silica, which fit together in the manner of a tiny pill-box. Their exteriors are exquisitely sculpted with various projections, spikes or, as here, pores which are arranged with precise symmetry. Diatoms occur in large numbers in either sea, soil or freshwater and form the base of the food chain. Magnification: x3,800 at 6x7cm size. x13,000 at 7x7"~LANDSCAPE"