What did Robert Hooke observe in cork with a microscope?

In 1665, English physicist Robert Hooke published his seminal work, Micrographia, which detailed his observations using a microscope. When examining thin slices of cork, Hooke observed a honeycomb-like structure consisting of microscopic cavities surrounded by walls.1234

Because these small, empty compartments reminded him of the blocks of cells occupied by monks, he coined the term "cell" to describe them. While he initially believed these structures functioned to transport substances through the plant, his observation marked a foundational moment in the history of cell theory.534