The theory that was born of the findings of Matthias Schleiden in 1838 and Theodor Schwann in 1839, who postulated, respectively, that plants and animals were made up of cells and that these units were basic to the structure and function of all organisms. Previously, in 1665, Robert Hooke (1635–1703), while examining cork under the microscope, had observed that its structure consisted of hollow boxlike units, which he called ‘cells’. At the time, however, he did not realize the significance of these units, which were in fact dead cells.