What plant organ is modified to produce onion bulbs?

Here's why:

* Bulbs are underground storage structures: They are composed of layers of modified leaves (the fleshy scales) that store food.

* Stems are modified: The stem of the onion plant is shortened and thickened, forming the base of the bulb.

* Growth and development: The stem's growing point (apical bud) produces the layers of leaves that make up the bulb.

So, while the bulb itself looks like a large root, it's actually a modified stem.