WHAT ARE CELLS? 
6 
n 1665, the English 
physicist Robert Hooke 
looked at a sliver of 
cork through a micro- 
scope lens and noticed 
some pores or cells m it. 
Hooke believed the cells had 
served as containers for the 
“noble juices” or “fibrous 
threads” of the once-living 
cork tree. He thought these 
cells existed only in plants, 
since he and his scientific 
contemporaries had observed 
the structures only in plant 
material. 
Nearly two centuries later, 
scientists began to develop 
the idea that every living 
thing is made up of cells. In 
1838, during a now-famous 
dinner conversation, two 
German scientists — the 
botanist Matthias 
Schleiden, who had 
been studying plant 
cells, and the 
zoologist Theodor 
Schwann, who had 
been examining the 
nervous tissue 
of animals — realized that the 
similarities between the 
structures they had been 
investigating were too strong 
to be accidental. In 1847, 
Schwann wrote a paper 
describing how all animal tissue, 
including bone, blood, skin, 
muscle, and glands, is com- 
posed of cells. Even sperm 
and eggs are cells. Schleiden 
elaborated on this idea as it 
applied to plants. A German 
pathologist, Rudolph Virchow, 
is given credit for being the 
first to state, in 1858, what 
became known as the cell 
theory: “Every animal appears 
as a sum of vital units, each 
of which bears in itself the 
complete characteristics of life.” 
The cell theory united plant 
and animal sciences by recog- 
nizing that the cell is the 
fundamental component of all 
living organisms, from orchids 
and earthworms to human 
beings. It provided an intel- 
lectual framework that revealed 
the hidden similarities of form 
and function in extremely 
diverse organisms, and it gave 
scientists a way of making 
sense out of the bewildering 
array of living creatures. But 
what is a cell? 
Obviously, there are major 
differences among cell types. 
Muscle cells, which can 
contract, have to be quite 
different from liver or bone 
cells. Nerve cells have 
long, thin fibers that, in 
humans, may extend 
more than 3 feet from 
the spinal cord to the 
toes, while blood cells 
have no projecting 
This dra wing of cork tissue, as seen 
under a simple microscope, appeared 
in Robert llooke's l(>(>7 book. 
Microscopy. Ilooke named the 
compartments “cells. ” 
