Aerial root
Aerial rootsarerootsgrowing above the ground. They are oftenadventitious,i.e. formed from nonroot tissue. They are found in diverse plant species, includingepiphytessuch as orchids (Orchidaceae), tropical coastal swamp trees such asmangroves,banyan figs (Ficus subg. Urostigma), the warm-temperate rainforest rata (Metrosideros robusta), and pohutukawa trees ofNew Zealand(Metrosideros excelsa).Vinessuch as common ivy (Hedera helix) and poison ivy (Toxicodendron radicans) also have aerial roots.
Types
[edit]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 asnegatively gravitropic(grows up and away from the ground) orpositively gravitropic(grows down toward the ground).[1]
"Stranglers" (prop-root)
[edit]Banyantrees are an example of astrangler figthat begins life as anepiphytein thecrownof 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 theMoreton 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
[edit]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 asaeratingroots rather thanaerialroots, if they come up from soil. The surface of these roots is covered withlenticel(small pores) which take up air into spongy tissue, which in turn usesosmoticpathways to spread oxygen throughout the plant as needed. Pneumatophores differentiate theblack mangroveandgrey mangrovefrom othermangrovespecies.
Fishers in some areas ofSoutheast Asiamake corks forfishing netsby shaping the pneumatophores of mangrove apples (Sonneratia caseolaris) into small floats.[2]
Members of the subfamilyTaxodioideaeproduce woody above ground structures, known ascypress knees,that project upward from their roots. These structures were initially thought[by whom?]to function as pneumatophores, but recent experiments have failed to find evidence for this hypothesis.[citation needed]
Haustorial roots
[edit]These roots are found inparasitic plants,where aerial roots become cemented to the host plant via a sticky attachment disc before intruding into the tissues of the host.Mistletoeis an example of this.
Propagative roots
[edit]Adventitious roots usually develop fromplantletnodes formed via horizontal, above ground stems, termedstolons,e.g.,strawberryrunners, andspider 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 separateclonesof the parent.
Pumping and physiology
[edit]Aerial roots may receive water and nutrient intake from the air. There are many types of aerial roots; some, such as mangrove, are used foraerationand 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 oforchidsshow that thevelamen(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 genusTillandsia,have a physiology that collects water from humidity, and absorbs it directly.
In theSierra Mixe(named afterthe geographical area)[3]variety ofmaize,aerial roots produce a sweetmucusthat supportsnitrogen fixing bacteria,which supply 30–80 percent of the plant's nitrogen needs.[4]
On houseplants
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Many plants that are commonly grown indoors can develop aerial roots, such asMonstera deliciosa,Pothos (Epipremnum aureum), Rubber Tree (Ficus elastica), Fiddle Leaf Fig (Ficus lyrata),Thaumatophyllum bipinnatifidum,many 'xPhilodendronand succulents such asEcheveria.
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.[5]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.
See also
[edit]
References
[edit]- ^"UCLA Botany glossary page - Roots".Archived fromthe originalon 2005-09-06.Retrieved2005-10-10.
- ^"BerembangSonneratia caseolaris".Wild Singapore. 2017-05-31.Retrieved2019-04-12.
- ^Pskowski, Martha (July 16, 2019)."Indigenous Maize: Who Owns the Rights to Mexico's 'Wonder' Plant?".Yale E360.
- ^Daley, Jason (August 10, 2018)."The Corn of the Future Is Hundreds of Years Old and Makes Its Own Mucus".Smithsonian Magazine.ISSN0037-7333.
- ^Sheeran, Laura; Rasmussen, Amanda (14 February 2023)."Aerial roots elevate indoor plant health: Physiological and morphological responses of three high-humidity adapted Araceae species to indoor humidity levels".Plant, Cell & Environment.1(12).doi:10.1111/pce.14568.