British scientist Robert Hooke first discovered cells in 1665. He was one of the earliest scientists to study living things under a microscope. He saw that cork was divided into many tiny compartments, like little rooms. He called the tiny compartments cells. Cork comes from trees, so what Hooke observed was dead plant cells.     Over the next century and a half, microscopes improved, and scientists observed cells in many different organisms.     In fact, every organism that was examined microscopically was found to consist of cells. These observations led German scientists Theodor Schwann... Show more

British scientist Robert Hooke first discovered cells in 1665. He was one of the earliest scientists to study living things under a microscope. He saw that cork was divided into many tiny compartments, like little rooms. He called the tiny compartments cells. Cork comes from trees, so what Hooke observed was dead plant cells.    
Over the next century and a half, microscopes improved, and scientists observed cells in many different organisms.    
In fact, every organism that was examined microscopically was found to consist of cells. These observations led German scientists Theodor Schwann and Matthias Schleiden to conclude in the early 1800s that cells are alive and that all living things are made of cells.    
Around 1850, a German doctor named Rudolf Virchow was observing living cells under a microscope. As he was watching, one of the cells happened to divide. Virchow realized that living cells produce new cells by dividing. He concluded that living cells arise from other cells.    
The work of Schwann, Schleiden, and Virchow led to the cell theory. This is one of the most important theories in life science. The cell theory can be summed up as follows:    
- All organisms consist of one or more cells.    
- Cells are alive and the site of all life processes.    
- All cells come from pre-existing cells.    

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