Matthias Jakob Schleiden was the first person to conclude that all plants are made of living cells in 1838. His work, along with parallel work on animal cells by Theodor Schwann, formed the basis for cell theory.
Schleiden devoted his life to the study of botany with a focus on studying plant structures magnified with a microscope. In addition to his conclusions about the cellular structure of plants, he promoted the importance of the cell nucleus, which he observed in his work. He also promulgated the ideas that plant cells reproduce by dividing at the nucleus, and that each plant grows from a single embryo cell.