While there wasn't a single person who definitively "said" plants are made up of cells, the concept of cell theory was developed through the work of several scientists.

Here are some key figures and their contributions:

* Robert Hooke (1665): First observed cells (actually cell walls) in cork using a microscope and coined the term "cell".

* Anton van Leeuwenhoek (1674): Improved the microscope and observed living cells, including bacteria and red blood cells.

* Matthias Schleiden (1838): Concluded that all plants are made up of cells.

* Theodor Schwann (1839): Expanded the idea to include animals, stating that all living things are composed of cells.

* Rudolf Virchow (1855): Added the final piece to the theory by stating that all cells arise from pre-existing cells.

Therefore, Schleiden is considered the first to explicitly state that plants are made up of cells, but his work was built upon the observations of Hooke, Leeuwenhoek, and further developed by Schwann and Virchow.