its a bulb
A onion is a modified stem
Onion is a modified stem because it is a stem which grows underground
The carrot, radish, onion and sugar beet have stems that are above ground and the roots and edible tuber/bulb is underground. The stem is green and the carrot is orange; the beet is red, the onion is white/red; the radish is red/white.
Onion roots emerge from the base of the condensed stem.
Onions are alliums and they are bulbs that grow underground.
An onion, strictly speaking, is a 'bulb' and is a stem.
Oh, dude, that thing you're talking about is called a "bulb." It's like the round base of an onion where all the layers hang out. So, yeah, next time you're chopping onions, just remember you're slicing through those bulb buddies.
stem
white
Onion is an underground stem called a bulb. It grows horizontally below the surface of the soil, from which roots grow downward and shoots emerge above ground.
Onion is not an underground stem; it is a bulb vegetable. Carrot, sugarcane, and turnip have underground stems that store nutrients and help the plants grow.