The microscopic floating population of plants in the ocean are called phytoplankton. Diatoms are the dominant species of phytoplankton and are a major component in the food web. The above example (1500X) is a colorized example of a marine type, but diatoms can occur in fresh water as well is in moistened soil samples.

The example below shows several different examples of the two types of diatoms. The large circular examples s well as the surrounding circular forms are centric diatoms. Out at the edges of the image can be seen more narrow or pennate diatom examples. This slide was produced in the 19th. Century by pushing individual diatoms into the pattern shown using a single-haired brush. Then the specimens were mounted under a cover glass. The slides could then be put under a number of suitably ornate brass microscopes and viewed with great wonderment by your after dinner guests. (This is pre MTV)