1. Over 300 years ago, Robert Hooke observed cork cells under the microscope. He saw little boxlike structures as pictured above. Cork does not contain living tissue and therefore only the cell walls can be observed. He called these structures cells. Robert Hooke gave the basic units of life their name. The basic units of life are cells.
By the 19th century, it was accepted that all living things are made of cells. It was during this time that the cell theory began to take shape. The three parts of the cell theory are: Cell are the basic units of life, all organisms are composed of cells, and the cells come from existing cells.

2. Cork cells

3. Onion Cells

5. Human Cheek Cell