Edible Roots, Stems, and Bulbs

A diet that includes tubers, rhizomes, corms, bulbs, and flower buds might seem primitive, but examples of these plant structures are found in common foods like potatoes, ginger, water chestnuts, onions, broccoli, and carrots. These structures are necessary for the plant's survival and reproduction.

While we usually think roots grow below ground and stems are above ground, it's not quite that simple. Underground structures can either be roots or stems. Roots anchor and support a plant, but they also absorb water and minerals from the soil, make plant growth hormones, and move these substances to the stems. They also interact with soil organisms and store nutrients. Stems, on the other hand, act as a highway; they carry water and soil nutrients from the roots to the rest of the plant, transport sugars and other compounds from photosynthesis down to roots, and support leaves, flowers, and fruits. If you look closely at a plant stem, you'll see that it consists of nodes, where leaves and branches develop, and internodes, where vertical growth occurs.

"Root vegetable" is a kitchen category, not a botanical one. Celeriac, turnips, beets, and jicama are taproots. Sweet potatoes are tuberous roots, but these are lateral roots that are thickened for storing food during non-growing periods. Root tubers are called perennating organs, meaning that they help the plant to survive (usually over the winter) until the next season and produce a new plant without going through seed production. Even though they're not edible, dahlias are another example of a plant with tuberous roots that grow in clusters.

Watch out for the term "tuber," though. People often use tuber to refer to any plant structure with rounded or wart-like prominences, but botanically, true tubers are actually modified plant stems, not roots, even though they're found underground. Potatoes are the most common example of true tubers. The "eyes" of a potato are buds that can develop into new plants. Tuberous begonias and daylilies are examples of non-edible tubers that can persist for years when cared for correctly.

Like tubers, true bulbs are stems; an example is the onion. Bulbs have a papery or skin-like covering called a tunic and a modified stem on the lower surface of the bulb called a basal plate from which roots grow. Bulbs add new layers (scales) from the inside and reproduce by offsets (lateral bulblets) from the basal plate. Daffodils, tulips, and snowdrops are also true bulbs.

Corms, like bulbs, are compressed stems with a papery exterior and basal plate with rootlets. Unlike bulbs, though, corms are made of solid tissue, not layers. New corms grow from either from a bud on top or from little cormels that form around the basal plate. Water chestnuts, taro, crocuses, crocosmia, gladioli, and jack-in-the-pulpits are corms.

A rhizome is a branching horizontal stem growing on or just under the soil surface. Unlike a root, it has nodes. From its' nodes, a rhizome can grow roots, branch to create other plantlets, and send up shoots. Some plants use thin rhizomes to spread and colonize an area (like lily-of-the-valley or Japanese knotweed), while other plants develop thick, fleshy rhizomes that they use both to spread and to store nutrients. Examples of "storage" rhizomes are ginger, turmeric, and irises.

Finally, let's take a quick look at one above-ground edible plant structure. Buds are at the nodes and tips of stems where they form flowers, leaves, and branches. Broccoli is made up of clusters of flower buds, which is apparent if you've ever planted broccoli and allowed it to bloom. Cabbage is another form of edible bud – in this case, it's a large terminal bud, which is a bud found at the very tip of a stem.

Examples of these diverse structures can be found among all the plants that we value as vegetables, herbs, and ornamentals. The next time you eat a vegetable or divide a plant from your garden, take a few seconds to think about what type of structure it is and how it helped that plant survive. You might be surprised!