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Tubers are a type of enlarged structure used as
storage organ A storage organ is a part of a plant specifically modified for storage of energy (generally in the form of carbohydrates) or water. Storage organs often grow underground, where they are better protected from attack by herbivores. Plants that have ...
s for
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s in some plants. They are used for the plant's
perennation In botany, perennation is the ability of organisms, particularly plants, to survive from one germinating season to another, especially under unfavourable conditions such as drought or winter. It typically involves development of a perennating orga ...
(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 Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...
. ''Stem tubers'' form thickened
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
s (underground stems) or
stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...
s (horizontal connections between organisms); well known species with stem tubers include the potato and
yam Yam or YAM may refer to: Plants and foods *Yam (vegetable), common name for members of ''Dioscorea'' * Taro, known in Malaysia and Singapore as yam * Sweet potato, specifically its orange-fleshed cultivars, often referred to as yams in North Amer ...
. Some writers also treat modified lateral roots (''root tubers'') under the definition; these are found in
sweet potato The sweet potato or sweetpotato (''Ipomoea batatas'') is a dicotyledonous plant that belongs to the Convolvulus, bindweed or morning glory family (biology), family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a r ...
es, cassava, and dahlias.


Terminology

The term originates from the Latin , 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., p. 124


Stem tubers

A stem tuber forms from thickened
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
s or
stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...
s. 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
begonia ''Begonia'' is a genus of perennial flowering plants in the family Begoniaceae. The genus contains more than 2,000 different plant species. The Begonias are native to moist subtropical and tropical climates. Some species are commonly grown ind ...
s, 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. The tuber has a vertical orientation, with one or a few vegetative buds on the top and fibrous roots produced on the bottom from a basal section. Typically the tuber has an oblong rounded shape. Tuberous begonias, yams, and cyclamens are commonly grown stem tubers. Mignonette vine (''
Anredera cordifolia ''Anredera cordifolia'', commonly known as the Madeira-vine or mignonette vine, is a South American species of ornamental succulent vine of the family Basellaceae. The combination of fleshy leaves and thick aerial tubers makes this a very heavy ...
'') produces aerial stem tubers on vines; the tubers fall to the ground and grow. '' Plectranthus esculentus'', of the mint family
Lamiaceae The Lamiaceae ( ) or Labiatae are a family of flowering plants commonly known as the mint, deadnettle or sage family. Many of the plants are aromatic in all parts and include widely used culinary herbs like basil, mint, rosemary, sage, savory ...
, produces tuberous underground organs from the base of the stem, weighing up to per tuber, forming from axillary buds producing short stolons that grow into tubers. Even though
legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock f ...
s are not commonly associated with forming stem tubers, '' Lathyrus tuberosus'' is an example native to Asia and Europe, where it was once grown as a crop.


Potatoes

Potatoes are stem tubersenlarged stolons thicken to develop into
storage organ A storage organ is a part of a plant specifically modified for storage of energy (generally in the form of carbohydrates) or water. Storage organs often grow underground, where they are better protected from attack by herbivores. Plants that have ...
s. The tuber has all the parts of a normal stem, including nodes and internodes. The nodes are the eyes and each has a leaf scar. The nodes or eyes are arranged around the tuber in a spiral fashion beginning on the end opposite the attachment point to the stolon. The terminal bud is produced at the farthest point away from the stolon attachment and tubers, and thus show the same apical dominance as a normal stem. Internally, a tuber is filled with starch stored in enlarged
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...
-like cells. The inside of a tuber has the typical cell structures of any stem, including a pith, vascular zones, and a cortex. The tuber is produced in one growing season and used to perennate the plant and as a means of propagation. When fall comes, the above-ground structure of the plant dies, but the tubers survive underground over winter until spring, when they regenerate new shoots that use the stored food in the tuber to grow. As the main shoot develops from the tuber, the base of the shoot close to the tuber produces adventitious roots and lateral buds on the shoot. The shoot also produces stolons that are long etiolated stems. The stolon elongates during long days with the presence of high auxins levels that prevent root growth off of the stolon. Before new tuber formation begins, the stolon must be a certain age. The enzyme lipoxygenase makes a hormone,
jasmonic acid Jasmonic acid (JA) is an organic compound found in several plants including jasmine. The molecule is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway. It was first isolat ...
, which is involved in the control of potato tuber development. The stolons are easily recognized when potato plants are grown from seeds. As the plants grow, stolons are produced around the soil surface from the nodes. The tubers form close to the soil surface and sometimes even on top of the ground. When potatoes are cultivated, the tubers are cut into pieces and planted much deeper into the soil. Planting the pieces deeper creates more area for the plants to generate the tubers and their size increases. The pieces sprout shoots that grow to the surface. These shoots are rhizome-like and generate short stolons from the nodes while in the ground. When the shoots reach the soil surface, they produce roots and shoots that grow into the green plant.


Root tubers

A tuberous root or storage root is a modified lateral root, enlarged to function as a
storage organ A storage organ is a part of a plant specifically modified for storage of energy (generally in the form of carbohydrates) or water. Storage organs often grow underground, where they are better protected from attack by herbivores. Plants that have ...
. The enlarged area of the tuber can be produced at the end or middle of a root or involve the entire root. It is thus different in origin, but similar in function and appearance, to a stem tuber. Plants with tuberous roots include the
sweet potato The sweet potato or sweetpotato (''Ipomoea batatas'') is a dicotyledonous plant that belongs to the Convolvulus, bindweed or morning glory family (biology), family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a r ...
(''Ipomoea batatas''), cassava, and dahlia. Root tubers are
perennating organ A perennial plant or simply perennial is a plant that lives more than two years. The term (''wikt:per-#Prefix, per-'' + ''wikt:-ennial#Suffix, -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annual pl ...
s, thickened roots that store nutrients over periods when the plant cannot actively grow, thus permitting survival from one year to the next. The massive enlargement of secondary roots typically represented by sweet potato have the internal and external cell and tissue structures of a normal root; they produce adventitious roots and stems, which again produce adventitious roots. In root tubers, there are no nodes and internodes or reduced leaves. The proximal end of the tuber, which was attached to the old plant, has crown tissue that produces buds which grow into new stems and foliage. The distal end of the tuber normally produces unmodified roots. In stem tubers the order is reversed, with the distal end producing stems. Tuberous roots are biennial in duration: the plant produces tubers the first year, and at the end of the growing season, the shoots often die, leaving the newly generated tubers; the next growing season, the tubers produce new shoots. As the shoots of the new plant grow, the stored reserves of the tuber are consumed in the production of new roots, stems, and reproductive organs; any remaining root tissue dies concurrently to the plant's regeneration of the next generation of tubers. The ''
Hemerocallis fulva ''Hemerocallis fulva'', the orange day-lily, tawny daylily, corn lily, tiger daylily, fulvous daylily, ditch lily or Fourth of July lily (also railroad daylily, roadside daylily, outhouse lily, and wash-house lily), is a species of daylily native ...
''
daylily A daylily or day lily is a flowering plant in the genus ''Hemerocallis'' , a member of the family Asphodelaceae, subfamily Hemerocallidoideae. Despite the common name, it is not in fact a lily. Gardening enthusiasts and horticulturists have long ...
and a number of daylily hybrids have large root tubers; ''H. fulva'' spreads by underground stolons that end with a new fan that grows roots that produce thick tubers and then send out more stolons.http://sain.utk.edu/invasives/species32.shtml Root tubers, along with other storage tissues that plants produce, are consumed by animals as a rich source of nutrients. The root tubers of arrowhead plants of the genus '' Sagittaria'' are eaten by ducks. Plants with root tubers are propagated in late summer to late winter by digging up the tubers and separating them, making sure that each piece has some crown tissue for replanting.


See also

*
Bulb In botany, a bulb is structurally a short stem with fleshy leaves or leaf basesBell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. that function as food storage organs duri ...
, modified stems with a short fleshy vertical stem, covered by thick fleshy modified leaves that enclose a bud for the next season's growth *
Caudex A caudex (plural: caudices) of a plant is a stem, but the term is also used to mean a rootstock and particularly a basal stem structure from which new growth arises.pages 456 and 695 In the strict sense of the term, meaning a stem, "caudex" is m ...
, a form of stem modification similar in appearance to a tuber * Corm, modified stems covered by dry scale-like leaves called a tunic, differing from true bulbs by having distinct nodes and internodes * Taproot, the largest, most central, and most dominant root of some plants


References


External links

{{commons category, Tubers
Cook's Thesaurus
has a good inventory of tuber varieties.
CGIAR Research Program on Roots, Tubers and Bananas
Plant morphology Plant reproduction Plant anatomy *