In 1838,schleiden  a german botanist examined a large no. of plants and observed that all plants  are  composed of different kinds of cells which form tissues.
In 1839,schwann a german zoologist studied about animal cells and reported that all cells possess outer layer called plasma membrane.he also told that animal cells  have cell wall.
Both schleiden and schwann proposed cell theory but they failed to explain how new cells are formed .
In 1855 Rudolf virchow explained that cells divide and new cells form from pre-existing cells.He modified cell theory.

cell theory states that
--- all cells arise from pre-existing cells
---all living organisms are composed of cells .