The majority of cells at the tip of a plant's root are under going mitosis because mitosis allows the root to grow longer. Cell division at the root tip continuously adds new cells to the end of the root causing elongation.
The ratio of cells undergoing mitosis to total number of cells (known as the mitotic index) is greatest at the tip of the root, and the ratio decreases closer to the plant. Mitosis continuously creates new cells in a healthy root. Root tip samples placed under a microscope are used to study mitosis because the samples are easy to obtain and contain a large portion of cells at varying phases.