What did Robert Hooke observe when he looked at a small slice of cork under the microscope in 1655?

Robert Hooke observed cell walls when he examined a small slice of cork under the microscope in 1655. He called the formations he saw "cells," but they were actually the nonliving cell walls of plant cells. 
The word "cell" originates from the Latin word "cellula," which means "a small room" or "a compartment.