An onion root tip a good specimen to study mitosis:
Cells of the onion are keep on growing so it is easier to study the mitosis, as in the various stages in the cell we can see the occurrence of mitosis. This can be done by mounting a cover-slip on the cell with the help of a needle. With the help of a blunt end of the needle the cover slip is slowly tapped. So the meristematic tissues present in root tip under the cover slip will easily be smashed and spreads as a thin layer of the cell. Now the slide is ready to study the mitosis.