He was the first to discover the existence of cells. He examined very thin slices of cork and saw a multitude of tiny pores that he remarked looked like the walled compartments a monk would live in. Because of this association, Hooke called them cells, the name they still bear. However, Hooke did not know their real structure or function. Hooke's description of these cells was published in Micrographia. His cell observations gave no indication of the nucleus and other organelles found in most living cells
He described cells in a drop of pond water using a microscope and was considered the first to observe single-celled organisms known as Prokaryotes.
He observed that all plants seemed to be composed of cells and he declared that the cell is the basic building block of all plant matter. This statement of Schleiden was the first generalizations concerning cells.
He extended Schleiden's cell theory to animals, stating that all living things are composed of cells. He believed that new cells form principally outside pre-existing cells. He demonstrated that the cell is the basis of animal as well as of plant tissue.
He was the first scientist to prove that cells can only form from pre-existing cells by creating an experiment that showed cells would only grow in broth if air was exposed.
Rudolf Virchow was the first to demonstrate that the cell theory applies to diseased tissue as well as to healthy tissue-that is, that diseased cells derive from the healthy cells of normal tissue and completed cell theory when he determined that cells only come from other pre-existing cells.
Found strong evidence of common ancestry between free-living bacteria and the organelles of living eukaryotic cells