Many plants were observed by Matthias Schleiden, a German botanist. He observed that all plants are made up of different types of cells that combine to form different types of tissues. Theodore Schwann, a British zoologist, observed that various types of animal cells have a thin outer layer. It is called the plasma membrane today. He also concluded that the cell wall is a unique feature of plants. Later Rudolf Virchow added the concept of 'Omnis cellula-e cellula'. It means that cells divide and new cells are formed from pre-existing cells.