Yes, an onion is a modified underground stem, specifically a tunicate bulb.
Understanding Onion Structure
The onion's structure isn't simply a root or leaf; it's a complex organ resulting from stem modification. The fleshy layers we see and eat are actually modified leaves, densely packed around a short, flattened stem at the base. This stem is where the roots emerge, appearing as thread-like structures. The entire bulb – the onion itself – functions as a storage organ, accumulating nutrients in its modified leaves for the plant's survival.
Several sources confirm this:
- Multiple references (Byju's, Testbook, Vedantu) explicitly state that the onion is a modified underground stem, a tunicate bulb with numerous fleshy, scaly leaves emerging from a central thread-like structure.
- Biology LibreTexts notes that bulbs are modified short stems with numerous fleshy leaves, explicitly including onions and shallots as examples.
- Wikipedia similarly describes the onion bulb as composed of "shortened, compressed, underground stems surrounded by fleshy modified scale leaves."
- Quora identifies the onion as an underground stem modification called a bulbil.
While some sources may mention the leaf-like structure of the fleshy layers, the overarching structure and function of the entire bulb are undeniably derived from a modified stem. The fleshy leaves are an adaptation on the modified stem, not the defining characteristic.
Distinguishing Onion from Other Plant Structures
It is important to distinguish between the modified leaves within the onion bulb and the bulb itself. While the layers are modified leaves storing energy, the structure that holds and supports them is the modified stem.
