This document provides instructions to estimate the percentage of cells in different stages of the cell cycle by examining 20 cells from an onion root tip under a microscope. Students are asked to determine what stage each cell is in - interphase, prophase, metaphase, anaphase, telophase, or cytokinesis - and record their findings in a table. They then calculate the percentage of cells in each stage and rank the mitotic stages from least abundant to most abundant.