Describe the structure of an onion bulb.

An onion bulb is a modified underground stem designed for food storage and vegetative propagation. It consists of concentric fleshy scales, which are thickened leaf bases, surrounding a short, flattened stem called a basal plate. 

The basal plate gives rise to fibrous roots below and the shoot above. Each fleshy scale stores carbohydrates, mainly in the form of fructans. The outer dry papery scales protect the bulb from injury and water loss. In the center lies the bud, which later grows into the green shoot. 

The layers we see when cutting an onion are overlapping leaf bases formed during successive growth cycles. The compact design allows the onion to survive unfavorable seasons underground and regrow when conditions improve. Structurally, the bulb combines both stem and leaf modifications to store food efficiently.