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 focusin ...

growing above the ground. They are often

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 ...

, i.e. formed from nonroot tissue. They are found in diverse plant species, including

epiphyte An epiphyte is a plant or plant-like organism that grows on the surface of another 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 epiphyt ...

s such as orchids (''

Orchidaceae Orchids are plants that belong to the family (biology), family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan distribution, cosmopolitan plants that ...

''), tropical coastal swamp trees such as

mangrove A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...

s, banyan figs ('' Ficus subg. Urostigma''), the warm-temperate rainforest rata ('' Metrosideros robusta''), and pōhutukawa trees of

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

(''

Metrosideros excelsa ''Metrosideros'' is a genus of approximately 60 trees, shrubs, and vines in the family Myrtaceae, mostly found in the Pacific region. Most of the tree forms are small, but some are exceptionally large, the New Zealand species in particular. The ...

'').

Vine A vine 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 themselves, for instance, when used in wicker work.Jackson; Benjamin; Da ...

s such as common ivy ('' Hedera helix'') and poison ivy (''

Toxicodendron radicans ''Toxicodendron radicans'', commonly known as eastern poison ivy or poison ivy, is a species of allergenic flowering plant. It has numerous subtaxons and forms both vines and shrubs. Despite its common name, it is not a true ivy, but rather a m ...

'') also have aerial roots.

Types

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 adjacent prop roots, allowing the tree to spread outwards indefinitely. This distinguishes banyans from other trees with a strangler habit that begin life as ...

trees are an example of a strangler fig that begins life as an

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, parti ...

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'') 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'', 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 with waterlogged soil. The roots may grow downward from the stem or upward from typical roots. Some botanists classify them as ''aerating'' roots rather than ''aerial'' roots if they emerge from the soil. The surface of these roots is covered with porous

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 (botany), bark of woody stems and roots of gymnosperms and dicotyledonous flowering plants. It func ...

s, which lead to air-filled spongy tissue called

aerenchyma Aerenchyma or aeriferous parenchyma or lacunae, is a modification of the parenchyma to form a spongy tissue that creates spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and ...

. This tissue facilitates the

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

of gases throughout the plant, as oxygen

diffusion coefficient Diffusivity, mass diffusivity or diffusion coefficient is usually written as the proportionality constant between the molar flux due to molecular diffusion and the negative value of the gradient in the concentration of the species. More accurate ...

in air is four orders of magnitude greater than in water. Pneumatophores of mangrove plant

Pneumatophores differentiate the black mangrove and grey mangrove from other

species. Fishers in some areas of

Southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

make corks for

fishing net A fishing net or fish net is a net (device), net used for fishing. Fishing nets work by serving as an improvised fish trap, and some are indeed rigged as traps (e.g. #Fyke nets, fyke nets). They are usually wide open when deployed (e.g. by cast ...

s by shaping the pneumatophores of mangrove apples ('' Sonneratia caseolaris'') into small floats. Members of the subfamily Taxodioideae produce woody above-ground structures, known as cypress knees, that project upward from their horizontal roots. One hypothesis suggests that these structures function as pneumatophores, facilitating gas exchange in waterlogged soils. However, modern research has largely discredited this idea, as the knees lack aerenchyma and gas exchange through them is not significant. Their true functions remain unclear, with alternative theories proposing roles such as nutrient acquisition or storage, structural support, or erosion prevention.

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 Parasite, parasitic plants develop a specialized organ ...

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 parasite, obligate parasitic plant, hemiparasitic plants in the Order (biology), order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they ...

is an example of this.

Propagative roots

Adventitious roots usually develop from plantlet nodes formed via horizontal, above ground stems, termed

stolon In biology, a stolon ( from Latin ''wikt:stolo, stolō'', genitive ''stolōnis'' – "branch"), also known as a runner, is a horizontal connection between parts of an organism. It may be part of the organism, or of its skeleton. Typically, animal ...

s, e.g.,

strawberry The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown Hybrid (biology), hybrid plant cultivated worldwide for its fruit. The genus ''Fragaria'', the strawberries, is in the rose family, Rosaceae. The fruit ...

runners, and spider plant. Some leaves develop adventitious buds, which then form adventitious roots, e.g. piggyback plant ('' Tolmiea menziesii'') and mother-of-thousands ('' Kalanchoe daigremontiana''). The adventitious plantlets then drop off the parent plant and develop as separate clones of the parent.

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 process by which air is circulated through, mixed with or dissolved in a liquid or other substances that act as a fluid (such as soil). Aeration processes create additional surface area in t ...

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. Orchids are cosmopolitan plants that are found in almost every habitat on Eart ...

s show that the velamen (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 plant, perennial flowering plants in the family (biology), family Bromeliaceae, native to the forests, mountains and deserts of the Neotropical realm, Neotropics, from northe ...

'', have a physiology that collects water from humidity, and absorbs it directly. In the

Sierra Mixe The Sierra Mixe or Mixes District is a district in the east of the Sierra Norte Region of the Mexican state of Oaxaca. It comprises 17 municipalities A municipality is usually a single administrative division having municipal corporation, corp ...

(named after the geographical area) variety of

maize Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...

, 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 fluid, serous and muc ...

that supports nitrogen fixing bacteria, which supply 30–80 percent of the plant's nitrogen needs.

On houseplants

Many plants that are commonly grown indoors can develop aerial roots, such as ''Monstera deliciosa'', Pothos (''Epipremnum aureum''), Rubber Tree (''Ficus elastica''), Fiddle Leaf Fig (''Ficus lyrata''), ''Thaumatophyllum bipinnatifidum'', many ''Philodendron'' and succulents such as ''Echeveria''. Aerial roots on houseplants do not serve as much of a purpose as on outdoor plants, as there is no rain indoors and indoor humidity is often low due to A/C and heating systems. However, studies have shown that increasing indoor humidity can result in houseplant aerial roots growing longer in length, resulting in lower levels of transpiration and more efficient intake of nitrogen than aroid houseplants grown in standard indoor humidity. Aerial roots on houseplant cuttings increase the chances of successful propagation. The presence of aerial roots is not an indicator of plant health. If a plant does not have aerial roots, that is no reason for concern.

References

{{Authority control Orchid morphology Plant roots