- In 1838, Matthias Schleiden, a German botanist observed that all plants are composed of different kinds of cell which form the tissues of the plant. Theodore Schwann (1838), a British zoologist, reported that animal cells had a thin outer layer, which is now known as plasma membrane.
- Based on Schwann's studies of the plant cells, he concluded that the presence cell wall is a unique character of the plant cells. He proposed the hypothesis that the bodies of animals and plants are composed of cells and product of cells.
- Schleiden and Schwann together formulated cell theory. Rudolf Virchow (1855), explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula e cellula). He modified the cell theory and gave a final shape to it.
Features of Cell Theory
(i) All living organisms are composed of cells and products of cells.
(ii) All cells arise from pre-existing cells. Viruses are exception to cell theory as they do not have cellular organisation.
