Diatoms are brown microalgae that can be found in all freshwater and marine environments. They are an important link in the ecological cycle. In streams and rivers you often see stones covered with a brown layer that is formed by diatoms.

Stomata consist of specialised cells that mainly occur on the underside of leaves. They regulate the gas exchange between the plant and it’s environment, the plant is 'breathing' through them, as it were. Stomata are very recognizable by the two bean-shaped guard cells that regulate the size of the opening.

The inside of the oral cavity is covered with a layer of squamous epithelial cells. These are relatively large, flat cells that are constantly being produced and rejected by the oral mucosa. It is quite easy to examine these cells microscopically.