While it's a fundamental principle of biology, there isn't a single individual credited with definitively stating "all plants are made of cells". The concept developed gradually over time thanks to the work of several scientists.

Here's a breakdown of how the idea emerged:

* Robert Hooke (1665): He was the first to observe and name "cells" using a microscope, though he was looking at the cell walls of cork, not living cells.

* Matthias Schleiden (1838): He proposed that all plants are made of cells, based on his extensive observations of plant tissues.

* Theodor Schwann (1839): He extended Schleiden's ideas and proposed the cell theory, stating that all living things are composed of cells.

Therefore, while Schleiden is often credited with the first formal statement regarding plants being made of cells, it was a collective effort by numerous scientists that led to the development of this fundamental biological principle.