The 17th-century English physicist Robert Hooke was curious about the remarkable properties of cork—its ability to float, its springy quality, its usefulness in sealing bottles. Hooke investigated the structure of cork with a new scientific instrument he was very enthusiastic about: the microscope.
Hooke cut a thin slice of cork with a penknife, put it under his microscope, focused sunlight on it with a thick lens, and looked through the eyepiece.
What Hooke saw looked like a piece of honeycomb. The cork was full of small empty compartments separated by thin walls. He called the compartments "pores, or cells." He estimated that every cubic inch of cork had about twelve hundred million of these cells.
Robert Hooke had discovered the small-scale structure of cork. And he concluded that the small-scale structure of cork explained its large-scale properties.
Cork floats, Hooke reasoned, because air is sealed in the cells. That air springs back after being compressed, and that's why cork is springy. And that springiness, combined with the fact that the cells are sealed off from each other, explains why a piece of cork is so well suited for sealing a bottle.
Hooke's observation not only explained the properties of cork, but gave a hint that all living tissue might be made of small building blocks.
Our understanding of what those building blocks are has changed since Hooke's time. Today we'd say that what Hooke observed were dead walls that had been created by living cells when the cork was still part of the tree.
But we still use the word cell, and our usage can be traced back to the microscopic observations of cork made over 300 years ago by Robert Hooke.