Robert Hooke discovered cells in 1665 when he observed cork through his microscope and saw tiny boxlike cavities, which he illustrated and described as cells. He coined the term "cells" to describe the boxlike cells of cork. Hooke believed that the cells had served as containers for the "noble juices" or "fibrous threads" of the once-living cork tree. Hookes discovery of plant cells led to the understanding of cells as the smallest units of life and contributed to the development of cell theory.