Robert Hooke observed plant cells in 1665 using an early microscope and coined the term "cell" to describe the box-like structures he saw. Matthias Schleiden and Theodor Schwann established in 1838-1839 that all living things are composed of one or more cells, laying the foundations for the Cell Theory. Rudolf Virchow added in 1855 that new cells are produced from pre-existing cells, completing the Cell Theory.