The root tips of onion plants are commonly used for observing cell division because: The onion plants are easy to grow in large numbers.
The cells at the tip of the onion roots are actively dividing, so many of the cells will be in stages of mitosis.
The root tips can be prepared in a way that allows them to be flattened on the microscope slide so that the chromosomes of individual cells can be observed.
This species has a relatively small number of chromosomes, and they are fairly large and distinct.
There are three cellular regions near the tip of an onion root.

The root cap contains cells that cover and protect the underlying growth region as the root pushed through the soil.
The region of cell division is where cells are actively dividing but not increasing significantly in size.
In the region of cell elongation, cells are increasing in size but not dividing.
We will be observing cells in the region of cell division to ensure that we observe all stages of mitosis and the cell cycle.

Onion root tip, region of cell elongation, followed by region of cell division, ending in a protective root cap
For this lab activity, you will be examining a section of an onion root tip under a compound light microscope. These slides were prepared by slicing the roots into thin sections, mounting them on glass microscope slides, staining so that the chromosomes are more visible, and then covering the specimen with a cover slip.

The images below give examples of what you might see in each of the stages of the cell cycle.

illustration of the stages of mitosis