One of the basic tenets of biology is that all living things are composed of one or more cells. Some organisms consist of a single cell, while others have multiple cells organized into tissues, and tissues organized into organs. In many living things, organs function together as an organ system. However, even in these complex organisms, the basic biology revolves around the activities of the cell.
One of the first scientists to observe cells was Englishman Robert Hooke. In the mid-1600s, Hooke examined a thin slice of cork through the newly developed microscope. The microscopic compartments in the cork impressed him and reminded him of rooms in a monastery, known as cells. He therefore referred to the units as cells. Later in that century, Anton Van Leeuwenhoek, a Dutch merchant, made further observations of plant, animal, and microorganism cells. In 1838, German botanist Matthias Schleiden proposed that all plants are composed of cells. A year later, his colleague, anatomist Theodor Schwann, concluded that all animals are also composed of cells. In 1858, biologist Rudolf Virchow proposed that all living things are made of cells and that all cells arise from preexisting cells. These premises have come down to us as the cell theory.