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Aerial roots are
roots A root is the part of a plant, generally underground, that anchors the plant body, and absorbs and stores water and nutrients. Root or roots may also refer to: Art, entertainment, and media * ''The Root'' (magazine), an online magazine focusing ...
above the ground. They are almost always
adventitious Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant a ...
. They are found in diverse plant species, including
epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...
s such as orchids (''
Orchidaceae Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Along with the Asteraceae, they are one of the two largest families of flowering ...
''), tropical coastal swamp trees such as
mangrove A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evoluti ...
s, banyan figs ('' Ficus subg. Urostigma''), the warm-temperate rainforest rata (''
Metrosideros robusta ''Metrosideros robusta'', the northern rātā, is a forest tree endemic to New Zealand. It grows up to or taller, and usually begins its life as a hemiepiphyte high in the branches of a mature forest tree; over centuries the young tree sends d ...
''), and pohutukawa trees of
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
(''
Metrosideros excelsa ''Metrosideros excelsa'', commonly known as pōhutukawa ( mi, pōhutukawa), New Zealand Christmas tree, New Zealand Christmas bush, and iron tree, is a coastal evergreen tree in the Myrtus, myrtle family, Myrtaceae, that produces a brilliant di ...
'').
Vine A vine (Latin ''vīnea'' "grapevine", "vineyard", from ''vīnum'' "wine") is any plant with a growth habit of trailing or scandent (that is, climbing) stems, lianas or runners. The word ''vine'' can also refer to such stems or runners themselv ...
s such as common ivy (''
Hedera helix ''Hedera'', commonly called ivy (plural ivies), is a genus of 12–15 species of evergreen climbing or ground-creeping woody plants in the family Araliaceae, native to western, central and southern Europe, Macaronesia, northwestern Africa and ...
'') and poison ivy (''
Toxicodendron radicans ''Toxicodendron radicans'', commonly known as eastern poison ivy or poison ivy, is an allergenic Asian and Eastern North American flowering plant in the genus ''Toxicodendron''. The species is well known for causing urushiol-induced contact derm ...
'') also have aerial roots.


Types of aerial roots

This plant organ that is found in so many diverse plant-families has different specializations that suit the plant-habitat. In general growth-form, they can be technically classed as '' negatively gravitropic'' (grows up and away from the ground) or ''positively gravitropic'' (grows down toward the ground).


"Stranglers" (prop-root)

Banyan A banyan, also spelled "banian", is a fig that develops accessory trunks from adventitious prop roots, allowing the tree to spread outwards indefinitely. This distinguishes banyans from other trees with a strangler habit that begin life as a ...
trees are an example of a strangler fig that begins life as an
epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...
in the
crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
of another tree. Their roots grow down and around the stem of the host, their growth accelerating once the ground has been reached. Over time, the roots coalesce to form a pseudotrunk, which may give the appearance that it is strangling the host. Another strangler that begins life as an epiphyte is the
Moreton Bay fig ''Ficus macrophylla'', commonly known as the Moreton Bay fig or Australian banyan, is a large evergreen banyan tree of the family Moraceae native to eastern Australia, from the Wide Bay–Burnett region in the north to the Illawarra in New So ...
(''Ficus macrophylla'') of tropical and subtropical eastern Australia, which has powerfully descending aerial roots. In the subtropical to warm-temperate rainforests of northern New Zealand, ''
Metrosideros robusta ''Metrosideros robusta'', the northern rātā, is a forest tree endemic to New Zealand. It grows up to or taller, and usually begins its life as a hemiepiphyte high in the branches of a mature forest tree; over centuries the young tree sends d ...
'', the rata tree, sends aerial roots down several sides of the trunk of the host. From these descending roots, horizontal roots grow out to girdle the trunk and fuse with the descending roots. In some cases, the "strangler" outlives the host tree, leaving as its only trace a hollow core in the massive pseudotrunk of the rata.


Pneumatophores

These specialized aerial roots enable plants to breathe air in habitats that have waterlogged soil. The roots may grow down from the stem, or up from typical roots. Some botanists classify these as ''aerating'' roots rather than ''aerial'' roots, if they come up from soil. The surface of these roots are covered with
lenticel A lenticel is a porous tissue consisting of cells with large intercellular spaces in the periderm of the secondarily thickened organs and the bark of woody stems and roots of dicotyledonous flowering plants. It functions as a pore, providing a ...
(small pores) which take up air into spongy tissue, which in turn uses
osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...
pathways to spread oxygen throughout the plant as needed. Pneumatophores differentiate the black mangrove and grey mangrove from other
mangrove A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evoluti ...
species. Fishers in some areas of
Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...
make corks for
fishing net A fishing net is a net used for fishing. Nets are devices made from fibers woven in a grid-like structure. Some fishing nets are also called fish traps, for example fyke nets. Fishing nets are usually meshes formed by knotting a relatively thin ...
s by shaping the pneumatophores of mangrove apples (''
Sonneratia caseolaris ''Sonneratia caseolaris'', commonly known as mangrove apple, is a species of plant in the family Lythraceae. The fruit is noted for its outward similarity to the persimmon fruit. This tree is a type of mangrove growing up to 20 m in height and ...
'') into small floats. Members of the subfamily
Taxodioideae Taxodioideae is a subfamily in Cupressaceae. Genera See also *Taxodiaceae Taxodiaceae is a formerly recognized coniferous plant family comprising the following ten genera: *''Athrotaxis'' *''Cryptomeria'' *''Cunninghamia'' *†'' Cunningham ...
produce woody above ground structures, known as
cypress knee A cypress knee is a distinctive structure forming above the roots of a cypress tree of any of various species of the subfamily Taxodioideae, such as the bald cypress. Their function is unknown, but they are generally seen on trees growing in swa ...
s, that project upward from their roots. These structures were initially thought to function as pneumatophores, but recent experiments have failed to find evidence for this hypothesis.


Haustorial roots

These roots are found in
parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the ...
s, where aerial roots become cemented to the host plant via a sticky attachment disc before intruding into the tissues of the host.
Mistletoe Mistletoe is the common name for obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. ...
is a example of this.


Propagative roots

Adventitious roots usually develop from
plantlet A plantlet is a young or small plant."Plantlet", Merriam-Webster' Many plants such as spider plants naturally create stolons with plantlets on the ends as a form of asexual reproduction. Vegetative propagules or clippings of mature plants may ...
nodes formed via horizontal, above ground stems, termed
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, e.g.
strawberry The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown hybrid species of the genus '' Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The fruit is widely ap ...
runners, and spider plant. Some leaves develop adventitious buds, which then form adventitious roots, e.g. piggyback plant (''
Tolmiea menziesii ''Tolmiea menziesii'' () is a species of flowering plant in the family Saxifragaceae. It is known by the common names youth on age, pick-a-back-plant, piggyback plant, and thousand mothers. It is a perennial plant native to the West Coast of Nort ...
'') and mother-of-thousands (''
Kalanchoe daigremontiana ''Kalanchoe daigremontiana'', formerly known as ''Bryophyllum daigremontianum'' and commonly called mother of thousands, or Mexican hat plant, is a succulent plant native to Madagascar. Like other members of ''Bryophyllum'' (now included in th ...
''). The adventitious plantlets then drop off the parent plant and develop as separate clones of the parent.


Aerial root pumping and physiology

Aerial roots may receive water and nutrient intake from the air. There are many types of aerial roots; some, such as mangrove, are used for
aeration Aeration (also called aerification or aeriation) is the Systems engineering process, process by which air is circulated through, mixed with or solvation, dissolved in a liquid or other substances that act as a fluid (such as soil). Aeration proces ...
and not for water absorption. In other cases they are used mainly for structure, and in order to reach the surface. Many plants rely on the leaf system for gathering the water into pockets, or onto scales. These roots function as terrestrial roots do. Most aerial roots directly absorb the moisture from fog or humid air. Some surprising results in studies on aerial roots of
orchid Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Along with the Asteraceae, they are one of the two largest families of flowering ...
s show that the
velamen Velamen or velamen radicum is a spongy, multiple epidermis that covers the roots of some epiphytic or semi-epiphytic plants, such as orchid and ''Clivia'' species. The velamen of an orchid is the white or gray covering of aerial roots (when dry, ...
(the white spongy envelope of the aerial roots), are actually totally waterproof, preventing water loss but not allowing any water in. Once reaching and touching a surface, the velamen is not produced in the contact area, allowing the root to absorb water like terrestrial roots. Many other epiphytes - non-parasitic or semi-parasitic plants living on the surface of other plants - have developed cups and scales that gather rainwater or dew. The aerial roots in this case work as regular surface roots. There are also several types of roots, creating a cushion where a high humidity is retained. Some of the aerial roots, especially in the genus ''
Tillandsia ''Tillandsia'' is a genus of around 650 species of evergreen, perennial flowering plants in the family Bromeliaceae, native to the forests, mountains and deserts of northern Mexico and south-eastern United States, Mesoamerica and the Caribbean to ...
'', have a physiology that collects water from humidity, and absorbs it directly. In the Sierra Mixe (named after the geographical area) variety of
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
, aerial roots produce a sweet
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
that supports
nitrogen fixing bacteria Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh ...
, which supply 30–80 percent of the plant's nitrogen needs.


See also

*
Adventitious Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant a ...
*
Root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
*
Vegetative reproduction Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or spec ...
*
Vine A vine (Latin ''vīnea'' "grapevine", "vineyard", from ''vīnum'' "wine") is any plant with a growth habit of trailing or scandent (that is, climbing) stems, lianas or runners. The word ''vine'' can also refer to such stems or runners themselv ...
*
Aeroponics Aeroponics is the process of growing plants in the air or mist environment without soil or an aggregate medium. The word "aeroponic" is derived from the Greek meanings of ''aer'' ("air") and ''ponos'' ("labour"). It is a subset of hydroponic ...


References

{{Authority control Plant roots Orchid morphology