Tuber

Tubers are swollen plant parts—either from stems or roots—that store nutrients. They help plants survive winter or dry periods, supply energy and nutrients, and allow for asexual reproduction. The word comes from Latin *tuber*, meaning "lump, bump, or swelling." Some experts restrict the term "tuber" to stem-derived structures, while others also include root-derived ones.

Stem tubers develop from thickened underground stems (rhizomes) or horizontal stems (stolons). Their upper sides produce shoots that grow into stems and leaves, while the undersides grow roots. These tubers usually form at the sides of the parent plant, near the soil surface. They are short-lived storage and regenerative organs that sprout from a shoot branching off a mature plant. New tubers attach to a parent tuber or form at the end of a rhizome that starts below ground. In autumn, the parent plant dies, leaving the new tubers; each has one dominant bud that regrows a shoot in spring. By summer, the old tuber decays and new ones begin forming. Some plants also produce tiny tubers (tubercules) that act like seeds, yielding small plants that look like seedlings. Some stem tubers, like those of tuberous begonias, last for years, but many survive only until the plant fully leafs out, then shrivel into husks.

Stem tubers usually begin as enlargements of the seedling's hypocotyl, sometimes including the first node or two of the epicotyl and the upper root section. They grow vertically, with one or a few buds on top and fibrous roots at the base. Their shape is typically oblong and rounded. Common examples include tuberous begonias, yams, and cyclamens. Mignonette vine (*Anredera cordifolia*) produces aerial stem tubers on vines up to 7.5 meters tall; these tubers fall to the ground and grow. *Plectranthus esculentus* (mint family) forms underground tubers from the stem base, each weighing up to 1.8 kg, developing from axillary buds that produce short stolons. Though legumes rarely form stem tubers, *Lathyrus tuberosus* is a native of Asia and Europe once grown as a crop.

Potatoes are stem tubers—enlarged stolons that thicken into storage organs. They have all the parts of a normal stem, including nodes (the "eyes") and internodes. Each node has a leaf scar. The eyes spiral around the tuber, starting from the end opposite the stolon attachment. The terminal bud is at the farthest point from the stolon, showing apical dominance like a normal stem. Inside, the tuber is filled with starch stored in enlarged cells, and it has typical stem structures: pith, vascular zones, and cortex. A potato tuber forms in one growing season, helps the plant survive winter, and serves as a means of propagation. In autumn, the above-ground plant dies, but tubers stay underground until spring, when they regrow shoots using stored food. As the main shoot develops, the base near the tuber produces adventitious roots and lateral buds. The shoot also produces stolons—long, etiolated stems that elongate during long days with high auxin levels, which prevent root growth on the stolon. Before a new tuber forms, the stolon must reach a certain age. The enzyme lipoxygenase produces jasmonic acid, a hormone that controls potato tuber development. Stolons are easily seen when potatoes are grown from seeds; they form near the soil surface from nodes. When cultivated, tubers are cut into pieces and planted deeper, which gives more area for tuber formation and increases size. The pieces sprout rhizome-like shoots that grow to the surface, generating short stolons from nodes along the way. Once at the surface, these shoots produce roots and green stems.

Root tubers (also called tuberous roots or storage roots) are modified lateral roots enlarged to store nutrients. The swollen part may be at the root's tip, middle, or involve the entire root. They differ in origin from stem tubers but look and function similarly. Examples include sweet potato (*Ipomoea batatas*), cassava, dahlia, and *Sagittaria* (arrowhead) species. Root tubers are perennating organs—thickened roots that store nutrients during inactive periods, allowing the plant to survive from year to year. The massive enlargement of secondary roots, as in sweet potato, has the internal and external cell and tissue structure of a normal root; they produce adventitious roots and stems, which in turn produce more adventitious roots. Unlike stem tubers, root tubers have no nodes.