Aerial root, an example of the adventitious root, is defined as the type of root which grows above the surface of the ground. These are the roots that do not develop from root tissue, instead, growth takes place from nodes, internodes, stems, leaves, or branches. Aerial roots absorb water and nutrient intake from the air. All epiphytes (plants that grow above the ground) have aerial roots, but not all plants that have aerial roots are epiphytes for eg. mangroves. Mangrove and banyan figs, having aerial roots, are tropical coastal swamp trees and not epiphytes. There are various types of aerial roots, namely, strangler roots, pneumatophores, haustorial roots, and propagative roots.
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EXAMPLES OF AERIAL ROOTS
1. Epiphytes
Epiphytes, also known as air plants, are mostly found in moist tropical areas, and they are non-parasitic in nature as they do not cause any harm to the host plant. Epiphytes rely on their aerial roots for their survival as these roots absorb nutrients and moisture from the humid air, rain, or debris around them. The tree only provides anchorage and structural support to epiphytes. The majority of epiphytic plants are angiosperms (flowering plants)which include various species of orchids, tillandsias, and other members of the pineapple family (Bromeliaceae). Common epiphytes found in both tropical and temperate regions include mosses, ferns, and liverworts. There are numerous examples of epiphytes. Some of the common examples are explicated as follows-
Tillandsia recurvata
Epiphytes are uncommon in arid regions, but tillandsia recurvata is a notable exception and is found in coastal deserts in Mexico receiving moisture from marine fog.
Orchids
Orchids are the largest flowering plants available in various shapes, colors, and sizes which belong to the Orchidaceae family. Orchids have the ability to conserve water in their thick stems. They are found everywhere except the Antarctica region. For eg, orchids are found as an epiphyte on a branch of a mango tree.
Ferns
Fern epiphytes include the staghorn, strap, and blue star ferns. Ferns use aerial roots to cling to trees, walls, and other surfaces. They mostly develop in moist areas and some common examples of ferns include Asplenium (birds nest ferns) and Platycerium.
Bromeliaceae
Bromeliads can be both terrestrial and epiphytic bromeliads. Epiphytic bromeliads are the most common example of aerial roots belonging to the Bromeliaceae family. They store rainwater in their leaves by leaves overlapping. Water and nutrient uptake is done by the leaves using hair-like structures called trichomes. Aerial roots of epiphytic bromeliads are found at the base of the plant and are usually thin, brown, and some inches long.
2. Tropical coastal swamp trees
Tropical coastal swamp trees involve mangroves and banyan trees.
Mangroves
These are tropical trees that grow in swamps, where either the soil is soft or there is no oxygen present in the soil for the roots. There are two types of mangroves, red and black. Red mangroves (Rhizophora mangle), belonging to the Rhizophoraceae family, bulge out thick woody aerial prop roots from the trunk and branches to support the plant in the soft swamp soil. Instead of having prop roots, black mangroves (usually Avicennia nitida, sometimes A. marina) from the Acanthaceae family have small vertical air roots that can be seen around the trunk of the tree and are called pneumatophores.
Banyan fig trees
Ficus benghalensis(scientific name), commonly known as the banyan fig, belongs to the Moraceae family. These trees have prominent aerial roots that grow downward from underneath the branches like lianas. As soon as these roots reach the ground, they take root and become woody trunks and supportive. It is an evergreen, fast-growing tree found mainly in monsoons and rainforests that grow up to 30 meters. The figs of these trees are eaten by birds known as Indian myna.
3. Warm-temperate rainforest rata
Metrosideros robusta (northern rata), which belongs to the Myrtle family, can be easily distinguished from other metrosiderous species by its small, leathery, dark green leaves that are 25–50mm long by 15–25mm wide, and have distinct notch at the tip. It is a slow-growing species that produces masses of brilliant orange-red flowers which open up in the summer.
4. Pohutukawa trees
Metrosideros excelsa (New Zealand’s Christmas tree), commonly known as pohutukawa, is a coastal evergreen tree that belongs to the Myrtaceae family, which produces red (sometimes, orange, yellow or white) flowers. They usually grow as multi-trunked spreading trees, reaching an average height of 20 meters.
5. Vines
Some plants always grow as vines, while a few of them grow as vines only for a specific period or short time. For example, poison ivy and bittersweet grow as low shrubs in absence of support and can grow as vines when support is available. Certain vines having aerial roots, like Toxicodendron radicans (poison ivy), Hedera helix (common ivy), Baltic ivy, Virginia creeper, and climbing hydrangea, use these roots to cling onto surfaces as they climb. In this, the aerial roots grow from the side of the stem, appearing as tiny greenish, yellow, or brown tendrils along the climbing stem, giving the stem a hairy appearance. Some examples of vines having aerial roots are explained as follows-
Swiss Cheese Plant
The Swiss Cheese Plant, also known as, Monstera Deliciosa, is a usual houseplant grown for its large green foliage/leafage. This plant develops prominent aerial roots which can reach up to a few meters in length. The aerial roots in this plant are easily noticeable because of their brown color. They develop aerial roots to attach to tree trunks for support, to collect moisture and nutrients, and to grow higher up to reach brighter light. These roots can attach to trees, walls, rocks, and other structures outdoors.
Hoya
Hoya, also known as waxplants/waxvine, belongs to the Apocynaceae family. These plants grow as vines and develop aerial roots, which are used for anchoring purposes and to collect extra water and nutrients.
6. Christmas cactus
Schlumbergera is the scientific name for the Christmas cactus/Thanksgiving cactus which belongs to the Cactaceae family. This epiphytic plant mainly grows on trees or rocks in its natural habitat. These plants contain aerial roots that help them to cling/attach to their host. Aerial roots in these plants help to get sunlight and absorb necessary moisture and nutrients from leaves, humus, and other plant debris that surround the plant. Improper lightening and nutrient deficiency are the two main factors for the outgrowth of aerial roots in such plants.
7. Philodendron
Philodendron belongs to the genus of flowering plants. Numerous varieties of Philodendrons produce aerial roots. When grown in outdoors, these aerial roots are used for climbing trees, receiving water and nutrients from the air, and for anchoring the plant. One can prune/cut these aerial roots, only when grown indoors, if they become too large. It will not cause any harm to the plant.
8. Pothos
Pothos, scientifically known as Epipremnum aureum, trailing vine with heart-shaped leaves that is commonly grown as a houseplant. Epipremnum aureum belongs to the Araceae family. Pothos uses aerial roots for support purposes as well as for hanging on trees, rocks, or anything. They are also used to consume additional nutrients and water from the air.
9. Jade plant
The Jade plant, also known as the lucky plant, is a succulent plant with thick leaves and stems. They are about 3 to 8 feet tall in height and belong to the Crassulaceae family. Underwatering is the most common reason for the presence or outgrowth of aerial roots on the jade plant. Jade plants prefer slightly moist but not wet soil. Jade plants in low temperatures will not produce aerial roots and will also cause harm to the plant. In jade plants, the outgrowth of aerial plants depends upon various factors such as, high-temperature conditions, insufficient nutrient supply, and unsuitable weather conditions.
10. Kalanchoe
Kalanchoe plants belong to the Crassulaceae family. Aerial roots on the Kalanchoe plant are a proof that the plant is not doing well. Aerial roots outgrow on this plant only under unsuitable conditions like insufficient water and nutrient supply. A thorough and deep watering should be beneficial under such conditions. Other factors which lead to the aerial roots in these plants include, improper lightening, high-temperature conditions, traumatic conditions (breaking stem, branch, or foliage injury), overfertilizing, and using softened tap water.
11. Rubber plant
The rubber plant/rubber fig, scientifically known as Ficus elastica, is a flowering plant species that belongs to the Moraceae family. These plants are mainly known for popping out aerial roots toward the base of their trunk. The major roles of aerial roots in such epiphytic plants involve climbing, gaining nutrients, and keeping the tree stable and upright in the wild.
