That's one more reason to eat organically grown food when you can. If you took a regular, aboveground, single, straight stem with its various nodes, and, keeping it standing vertically, squeezed it downward until it became wider than tall, and buried it under the ground, you'd have something like a corm. Corms, then, are unlike stolons and rhizomes because they usually grow vertically, instead of lying horizontally. They're unlike tubers in that they are so compressed. Gladiolus, crocus, and tuberous begonias all arise from corms.
Onion rings are layered-bulb cross sections.
The picture below shows an onion bulb sprouting a green shoot.