Concept
Tubers are a type of enlarged structure used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing season, and as a means of asexual reproduction. Stem tubers form thickened rhizomes (underground stems) or stolons (horizontal connections between organisms); well known species with stem tubers include the potato and yam. Some writers also treat modified lateral roots (root tubers) under the definition; these are found in sweet potatoes, cassava, and dahlias. The term originates from the Latin tuber, meaning "lump, bump, swelling". Some writers define the term "tuber" to mean only structures derived from stems; others use the term for structures derived from stems or roots. A stem tuber forms from thickened rhizomes or stolons. The top sides of the tuber produce shoots that grow into typical stems and leaves and the undersides produce roots. They tend to form at the sides of the parent plant and are most often located near the soil surface. The underground tuber is normally a short-lived storage and regenerative organ developing from a shoot that branches off a mature plant. The offspring or new tubers are attached to a parent tuber or form at the end of a hypogeogenous (initiated below ground) rhizome. In the autumn the plant dies, except for the new offspring tubers, which have one dominant bud that in spring regrows a new shoot producing stems and leaves; in summer the tubers decay and new tubers begin to grow. Some plants also form smaller tubers and/or tubercules that act like seeds, producing small plants that resemble (in morphology and size) seedlings. Some stem tubers are long-lived, such as those of tuberous begonias, but many plants have tubers that survive only until the plants have fully leafed out, at which point the tuber is reduced to a shriveled-up husk. Stem tubers generally start off as enlargements of the hypocotyl section of a seedling, but sometimes also include the first node or two of the epicotyl and the upper section of the root.
