In 1838, Matthais Schleiden proposed that cells and cellular products comprise all elements of a plant. He was mistaken, however, about the method of cell production.
The work of Marcello Malpighi and Nehemiah Grew preceded that of Schleiden. They uncovered the presence of cells throughout the body of the plant and suggested cells perform some greater function for the plant.
Robert Hooke, using the newly invented microscope, first described the cellular structures of plants in 1665. He did not correlate the plant's cellular features to functions they are known to perform today, but saw the plant cells as a potential fluid transport system.