Diatoms are delicate unicellular organisms that have a yellow-brown chloroplast that enables them to photosynthesize. Their cell walls are made of silica almost like a glass house. The construction of the cell wall, called the frustule, consists of two valves that fit into each other like a little pill box.

The colour of the chloroplast is yellow-brown instead of the green we know of other creatures that use light as a source for energy.

There are two different groups of diatoms, the pennates which are pen-shaped and the centric which are like a cylinder. In fresh water most diatoms you will see are of the pennate type.

At the end of the winter they are most numerous in fresh water. They will cover surfaces of aquatic plants or poles and wooden borders of ponds. If you like to study them you can scrape the brown growth with a flat piece of plastic. You can also use a sponge.

For the free living (plankton) species a special fine mesh plankton net is very useful.

Many species of diaom stay connected after the cells divide. They form colonies, long chains. Sometimes only the tips are connected and they form a zig zag pattern.

There are two different groups of diatoms, the pennates which are pen-shaped and the centric which are like a cylinder. In fresh water most diatoms you will see are of the pennate type.

In marine waters the variety of body shapes is much greater. In the ocean they form the main part of phytoplankton, the photosynthetic organisms that float with the current. This image shows the top view of a large Coscinodiscus, a marine species that can just be seen with the naked eye.

Clearly visible is the fine net-like structure of the siliceous cell wall. The yellow-brown chloroplasts used for photosynthesis are also easy to see.