Onions, like carrots and potatoes, have specialized storage structures that remain underground. In the case of onions, the specialized storage structures are leaves. The shoot systems of onions are called bulbs. They have a short, compact stem that bears a cluster of broad, fleshy leaves. If we look at onions over the course of a growing season, we find that they produce three forms of leaves: (1) cylinder-like photosynthetic leaves that extend above ground, (2) broad, thick leaves in which sugar products are stored, and (3) thin, scale leaves around outside of the bulb.

Green onion, showing young buld and cylinder-like photosynthetic leaves.

Onion

Section of an onion buld showing its fleshy storage leaves.

The cylinder-like photosynthetic leaves are most prominent during the early part of the growing season when they extend above ground and gather sunlight for photosynthesis. These are the leaves of so-called "green onions" that we chop into small pieces for use in salads and soups.

The sugars produced in the cylinder-like photosynthetic leaves are transported underground and through the stem to the broad, thick storage leaves. Toward the end of the growing season, the storage leaves thicken noticeably, and it is at this point that we harvest onion bulbs to obtain the nutrient-filled leaves.

Garlic is a close relative of onion. As with onions, we eat fleshy storage leaves produced underground by garlic. It is interesting to compare the shoot systems of garlic and onions. If we remove the thin scale leaves that surround a garlic, we find the so-called "cloves." Each of these cloves is a branch of the garlic shoot. Moreover, each clove has the form of a bulb: each has short, compact stem that bears a cluster of broad, fleshy leaves. Thus, we can think of garlic has having a shoot system that consists of a set of tightly aggregated bulbs.

Garlic

Garlic and some "cloves" that have been removed.