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"Pneumatophore" redirects here. It is also a name for the air bladder of the Portuguese Man o' War.

The Grey Mangrove (Avicennia marina)'s pneumatophorous aerial roots
A Schefflera arboricola indoor bonsai soon after branch pruning to show extensive aerial roots.
Banyan tree of undetermined species in Fort Myers, Florida
European Beech with aerial roots in a wet Scottish Glen.

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 like Common Ivy (Hedera helix) and irritating poison ivy (Toxicodendron radicans).

Contents

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).

Aerial roots as supports

Non-parasitic ivy are vines that use their aerial roots to cling to host plants, rocks, or houses. Prop roots form on aerial stems and grow down into the soil to brace the plant, e.g. maize and screw pines (Pandanus species).

Stranglers

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, eventually strangling and killing 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 down 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. Eventually the host tree dies, leaving as its only trace a hollow core in the massive pseudotrunk of the rātā.

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 lenticels which take up air into spongy tissue which in turn uses osmotic pathways to spread oxygen throughout the plant as needed.

Black mangrove is differentiated from other mangrove species by its pneumatophores.

See also Cypress knee

Haustorial roots

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.

Propagative roots

Horizontal, aboveground stems, termed stolons or runners, usually develop plantlets with adventitious roots at their nodes, e.g. strawberry 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.

See also

References

  1. UCLA Botany glossary page: Roots

Simple English


Aerial roots are roots that are above ground. Different plants have grown them. They can serve different purposes:

  • For support: Sometimes such roots are grown for support. Ivy plants have them to cling to structures, such as stone walls, or other trees. The Banyan tree (also known as strangler fig) grows them downward from the place where it is. Eventually, it will strangle the host.
  • Roots for breathing: Some plants grow in environments where there is much water in the soil. They may then grow such roots (upwards) to breathe air. Black mangroves have such roots, for example.
  • As a parasite. Parasites sometimes grow these, into the host to get nutritients. One example of this is the Mistletoe.
  • Roots to propagate: Some plants have aboveground roots. In some places, little plants (with leaves and roots) develop. They will drop off at some point, and develop into new plants (that are clones of the parent). Examples for this are strawberries or the spider plant.

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