Matthias Schleiden was a German botanist born on April 5, 1804. He is best known for his role in developing the cell theory, which states that all living organisms are composed of cells. Schleiden's research focused on plant cells, and he emphasized the importance of the nucleus in cell function. In 1838, Schleiden published a paper that outlined his findings, collaborating with Theodor Schwann, who studied animal cells. Together, they established the foundational principles of cell theory, which later became a cornerstone of modern biology. Schleiden passed away on June 23, 1881, leaving a lasting impact on the scientific community.