Mathias Schleiden was a German botanist who used the microscope in his scientific investigations. Based on his investigation, Mathias Schleiden stated that the plants are made of cells. His theory was sustained by the zoologist Theodor Schwann, after he has conducted some scientific researches on animal tissues. Their researches led to the conclusion that was emitted and published in 1839, in the treat called "Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals" that all living things are composed of cells.
The cell theory was later on established through the pasteurization experiment which Louis Pasteur undertook to contradict the theory of spontaneous generation. The Pasteur's experiment, not only set down the cell theory, but discovered the process,called pasteurization, through the germs responsible with fermentation and alteration of food are killed by the heat.