Matthias Schleiden was a botanist who first proposed the idea that:

Matthias Schleiden, a German botanist, was one of the pioneers of cell biology. In 1838, he proposed the idea that all plant tissues are composed of cells, and that an individual plant cell is a basic unit of life. Schleiden's proposal was a major contribution to the development of the cell theory, which states that all living things (plants, animals, bacteria, fungi, etc.) are composed of cells.