Onion root tips are often used to study mitosis because they grow very rapidly, which means that many cells are dividing. The very tip of the onion root ends in a structure called the root cap. The other end of the root attaches to the onion. A scientist is interested in determining whether an onion root tip grows from the base of the root or from the tip near the root cap. She counts the number of cells that are in one of the phases of mitosis at different distances from the root cap. What is the dependent variable in her experiment