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A rhizome is a root-like, underground stem, growing horizontally on or just under the surface of the ground, and capable of producing shoots and roots from its nodes. Rhizomes are most commonly produced by perennial, herbaceous species of plants, that die back to the ground at the end of the growing season, and must grow a new shoot at the beginning of the next season. Rhizomes are capable of storing energy, usually as starch, which is used to fuel the regeneration of new shoots. Rhizomes are also sometimes called rootstocks.

Plant species that have well developed rhizomes often rely on these organs as a means of propagation. However, the regeneration of plants through the spreading of rhizomes and development of new shoots is a type of non-sexual, vegetative propagation, because the progeny are genetically identical to the parent. Horticulturalists take advantage of the ease of propagation of certain plants with rhizomes by using bud-containing segments of these organs to grow new plants. This is the major method by which many ornamental species, such as iris (Iris spp.), are propagated. Some agricultural plants are also propagated in this way, such as sugarcane (Saccharum officinarum), arrowroot (Canna edulis), ginger (Zingiber officinale), and potato (Solanum tuberosa). In the case of some agricultural species, the rhizome is also the harvested part of the plant. The potato, for example, has discrete, modified sections of its rhizomes, called tubers, that are modified to store starch. Potato tubers are, of course, an important agricultural product.

Some species of tree can regenerate extensively by issuing new vegetative shoots from their underground rhizomes, after damages caused by disturbance by fire or harvesting. In North America, trembling aspen (Populus tremuloides) can regenerate very effectively in this way, and stands dominated by genetically identical "trees" of this species can sometimes occupy an area of several to many hectares (up to 40 ha). These stands may represent the world's largest "individual" organisms, in terms of biomass.

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