Aerial roots are roots above the ground. They are almost always adventitious. They are found in diverse plant species, including epiphytes such as orchids, tropical coastal swamp trees such as mangroves, the resourceful banyan trees, the warm-temperate rainforest rātā (Metrosideros robusta) and pōhutukawa (M. excelsa) trees of New Zealand and vines such as Common Ivy (Hedera helix) and poison ivy (Toxicodendron radicans).
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).
The Banyan tree (Ficus sp.) is an example of a strangler fig that begins life as an epiphyte in the crown of another tree. Its 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 rātā 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 rātā.
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 lenticels which take up air into spongy tissue which in turn uses osmotic pathways to spread oxygen throughout the plant as needed. Pneumatophores differentiate the Black mangrove and Grey mangrove from other mangrove species.
Members of the subfamily Taxodioideae produce woody aboveground structures, known as cypress 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.
Now the question is why are this pneumatophores even form when the shoot of all these plants have arrangements to respirate quite efficiently? The answer to this question is that the soil on which this halophytic plants grow are saline and largely anaerobic. It is important to mention here that even in other plants the gaseous exchange that is done at leaves is of minimal work for roots which are a lot far away. So roots absorb their own, dissolved, oxygen from the soil. However, since saline soil is largely anaerobic it becomes impossible for the roots to do gaseous exchange through soil and hence form pneumatophores that can absorb oxygen directly from air.
These roots are found in parasitic plants, where aerial roots become cemented to the host plant via a sticky attachment disc before intruding into the tissues of the host. Mistletoe is a good example of this.
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.
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 aerial roots, are used for aeration 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.
Some surprising results in studies on aerial roots of Orchids show that the 'Velamen' - the white spongy envelop of the aerial roots, are actually totally water proof, 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.
- UCLA Botany glossary page - Roots
- "Epiphytes - adaptations to an aerial habitat". Royal Botanic Gardens, Kew.
- Wild Singapore - Berembang Sonneratia caseolaris
- "Physiological and Anatomical Responses to Water Deficits in the Cam Epiphyte Tillandsia Ionantha (Broeliaceae)".
- Roots absorb moisture from humid air: orchids (AskNature.org website)
- [The role of the Velomen of Aerial Roots of the Orchid] Botanical Gazette, Chicago, 1957
- See articles: Leaves of Collospermum epiphytes capture water due to their fan-shaped arrangement and The leaves of some bromeliads capture water and nutrients in a storage tank via hydrophobic leaf surfaces and hydrophilic hairs (AskNature.org website)
- The stems of cushion plants reabsorb nutrients from dead leaves by sending out lateral rootlets (AskNature.org website)
- For example Ribbon-like roots absorb moisture: Taeniophyllum orchid (AskNature.org website)