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Aerial roots are Root growing above the ground. They are often adventitious, i.e. formed from nonroot tissue. They are found in diverse plant species, including such as orchids ( Orchidaceae), tropical coastal swamp trees such as , banyan figs ( Ficus subg. Urostigma), the warm-temperate rainforest rata ( Metrosideros robusta), and pōhutukawa trees of New Zealand ( Metrosideros excelsa). such as common ivy ( Hedera helix) and poison ivy ( Toxicodendron radicans) also have aerial roots.
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 differentiate the black mangrove and Avicennia marina from other mangrove species. Fishers in some areas of Southeast Asia make corks for by shaping the pneumatophores of mangrove apples ( Sonneratia caseolaris) into small floats.
Members of the subfamily Taxodioideae produce woody above-ground structures, known as , 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.
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 cloning of the parent.
Most aerial roots directly absorb the moisture from fog or humid air.
Some surprising results in studies on aerial roots of 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, have a physiology that collects water from humidity, and absorbs it directly.
In the Sierra Mixe (named after Sierra Mixe) variety of maize, aerial roots produce a sweet mucilage that supports nitrogen fixing bacteria, which supply 30–80 percent of the plant's nitrogen needs.
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.
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