Why is onion considered as a stem?

Onion is not considered a stem; it is actually a modified underground bulb. The onion plant, Allium cepa, belongs to the family Amaryllidaceae. It has a unique structure consisting of concentric layers of modified leaves, which we commonly refer to as onion layers or onion bulbs.

The confusion might arise because onions are often used as an example when explaining the concept of stem, root, and leaves in plants. In these explanations, the onion bulb is used to represent the underground stem.

Stems are the part of a plant that typically grow above ground and provide support to the leaves, flowers, and fruits. They also transport water, minerals, and nutrients between the roots and leaves. Onions do have a stem, but it is a short, compressed, and underground stem called a basal plate. This basal plate gives rise to the roots at the bottom and produces a shoot that eventually emerges above the ground as a new onion plant.

In summary, while onions have a stem, they are not classified as stems themselves. They are modified underground bulbs made up of layers of modified leaves.