The edible part of the onion plant is the bulb. This bulb is underground during growth, while green stalks that emerge from the bulb reach above the ground to gather sunlight and carbon dioxide.
The tops of onion stalks sometimes grow small, white flowers. The plant sprouts small black seeds that can be used to plant new onions. Additionally, onions can be grown by planting small onion bulbs or by grafting from one onion to create new ones.
Several foods related to the onion, such as chives, garlic and leeks, grow in similar ways, with an edible bulb below ground. Some foods, such as green onions, have edible bulbs and edible stalks.