In 1838, Schleiden, a German Botanist, concluded that, despite differences in the structure of various tissues, plants were made of cell. In 1839, a German zoologist. T. Schwann, published a comprehensive report on the cellular basis of animals. Both of them proposed the cell theory which state that:
- All organism are composed of one or more cells.
- The cell is the structural unit of life.
Schleiden and Schwann's ideas on the origin of cell was not convincing; both agreed that cell is a "denovo" structure, could arise non-cellular materials. In 1855 Rudolf Virchow a German pathologist, had made a convincing case and added a third point to the cell theory.
- Cells can arise only by division of pre-existing cell.
Cell theory brought a great revolution in the field of biology which established that the function of an organism is the result of activities and interaction of the cell units.
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