a. Rudolf Virchow first explained that the cells divide and new cells are formed from the pre-existing cells. He gave the modern cell theory by modifying the old one given by Schleiden and Schwann –
• all living organisms are composed of cells and products of cells.
• all cells arise from pre-existing cells.
b. • In 1838, Schleiden conducted tests on different plants and concluded that all the plant are made up of different types of cells which together form plant tissues.
• In 1839, Schwann studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’.
• He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.
• So the cell theory with the contribution of the two stated that the bodies of animals and plants are composed of cells and products of cells.
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