Re: Why do onions grow the way they do--in easily separated rings?

Rest of question:

Onions grow in generally concentric, nested spherical structures that easily separate into rings when cut in cross section. Why do they grow in this manner unlike other root crops such as turnips and beets, and how are the layers formed as the onion grows - do they start out with all layers or are they added as it grows?

Reply:

An onion is a bulb, which consists of a short compressed stem and fleshy leaves modified for storage. The concentric structures are the fleshy leaves. The stem is the circular area at the base of the bulb to which the leaves are attached. The stem height in a bulb is so short, it is much greater in diameter in order that all the leaves can be attached. In a normal, elongated stem, the leaves would attach at nodes with a substantial distance between nodes. More leaves are added as the bulb grows with the new leaves in the center of the bulb.

Turnips and beets are both swollen storage roots so have a single solid structure rather than many concentric leaves. Another type of underground storage stem is the tuber, such as a potato, which is all stem and no fleshy leaves.

References

Bulb Photo

Bulb