Who proposed that all plants are composed entirely of cells?

The idea that all plants are composed of cells is attributed to Matthias Schleiden, making the chosen answer accurate. Schleiden, a German botanist, played a significant role in the development of the cell theory during the 19th century. He specifically focused on plant biology and concluded that all plant tissues are made up of cells, which helped to establish the foundation of the cell theory alongside Theodor Schwann, who later contributed similar ideas regarding animal cells.

This framework emphasizes that cells are the basic unit of life for all living organisms, both plants and animals, shaping our understanding of biology. Therefore, Schleiden’s contributions were crucial in recognizing the cellular composition of plants, integrating into the broader scope of the cell theory that was later expanded with the contributions of others in the field.