|MadSci Network: Cell Biology|
While most plant cells are typically green, the epidermal cells are rarely green. Plant cells are green because of the presence of chloroplasts and the only epidermal cells having chloroplasts are the guard cells of the stomatal apparatus. The cells beneath the epidermis are green and you can see them through the colorless epidermis. Plant cells not exposed to light, such as root cells, do not develop chloroplasts. While an onion bulb is not a root, it shares a similar environment with roots. The rings of an onion are actually modified leaves. They are surrounding the stem so, when you slice an onion you may notice a green spot in the center. This green spot is the stem. The main function of the onion bulb is food storage for the plant. The food being stored comes from photosynthesis.
Try the links in the MadSci Library for more information on Cell Biology.