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Along with two other guys, Theodor Schwann and Rudolf Virchow, cell theory.
He did not invent anything... As TranceNova said, he contributed in developing the cell theory. Schleiden was a German Botanist who viewed plant parts under a microscope and discovered that plant parts are made of cells. He is considered to be the co-founder of cell theory together with Schwann, with whom he consulted. While professor of botany at the University of Jena, he wrote “Contributions to Phytogenesis” (1838), in which he stated that the different parts of the plant organism are composed of cells or derivatives of cells. Thus, Schleiden became the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry. He also recognized the importance of the cell nucleus, discovered in 1831 by the Scottish botanist Robert Brown, and sensed its connection with cell division. Schleiden was one of the first German biologists to accept Darwin’s theory of evolution.