Plant cells from an onion bulb lack chloroplasts because this part of the plant grows below ground, making it incapable of absorbing the light needed to photosynthesize. Most subterranean root and bulb structures in plants do not have chloroplasts for this reason.
Although cells at the very top of the bulb that peek through the soil may develop chloroplasts, the main function of these cells is to protect the plant against harmful pathogens in the soil and to store energy. These functions are made possible by the cell wall, cell membrane, cytoplasm, nucleus and large vacuum of the cell.
Light microscopy renders the nucleus of an onion cell visible at the edge of the cytoplasm. The vacuole is visible in the middle of the cell. This vacuole is transparent in the cells of white onions, making them invisible to light microscopy unless they are stained. Cells of red onions are naturally pigmented and do not require the addition of a staining agent.
The best samples for viewing through a light microscope are prepared from thin slices of small, firm onions. The orientation of the cells under a light microscope can increase or decrease their visibility. The more light that is allowed to pass through, the better the resolution becomes.