Final answer:
Mitosis is a process of cell division that includes stages such as prophase, metaphase, anaphase, and telophase. To identify the stages in the photomicrograph, one must observe the state of the chromosomes and the spindle apparatus within the onion root tip cells.
Step-by-step explanation:
During mitosis, a eukaryotic cell's chromosomes are copied and distributed to the two daughter nuclei in a series of stages: prophase, prometaphase, metaphase, anaphase, and telophase. In prophase, the chromosomes condense and the mitotic spindle begins to form. During metaphase, chromosomes line up at the metaphase plate. In anaphase, the sister chromatids are pulled apart to opposite ends of the cell. Finally, in telophase, nuclear envelopes re-form around the separated chromosomes.
Now, to determine the phases occurring in cells X and Y from a photomicrograph of onion root tip cells during mitosis, one would look for the unique characteristics of each phase. If, for instance, Cell X shows condensed chromosomes and the beginning formation of the spindle, it would be in prophase. If Cell Y shows chromosomes that are separated and moving towards the poles, it would be in anaphase.