Adventitious roots are specialized roots that develop from a non-root tissue part of the plant, such as the stem, branch or leaves, rather than the root. Adventitious roots are formed either as part of the plant's normal development or in response to stressful conditions. These roots are ecologically important as they allow plants to adapt to environmental stress, and humans utilize adventitious roots for food production and the vegetative propagation of plants. Adventitious roots can be differentiated into various types, which are described below.
1. Adventitious Roots As Storage Organs
Adventitious roots as storage organs are specialized to perform the function of food storage. These types of roots are thick and fleshy, and can further be subdivided into different categories based on the location and shape of the swollen parts.
a) Tuberous Roots/Single Root Tuber
An enlarged fleshy root that is modified to store food is called a tuberous root. It is not a true tuber, which is a modified stem tissue. In such plants, only a single such root exists.
Example: sweet potato
b) Fasciculated Fleshy Roots
When swollen or fleshy adventitious roots occur in a cluster, they are called fasciculated fleshy roots.
c) Palmate Roots
As the name suggests, such fleshy roots appear like the palm.
d) Nodulose Roots
Nodulose root swellings occur at the tips of the root and appear like nodules.
e) Beaded Roots
Just like a beaded necklace, this type of fleshy roots exhibit swellings or nodules at definite intervals.
Example: Indian spinach
f) Annulated Roots
In annulated roots, the fleshy root has a series of ring-like grooves.
2. Adventitious Roots That Provide Mechanical Support
Adventitious roots of some plants are modified to provide structural support to the plant so that it can grow and spread further. The different types of such supporting roots are as follows:
a) Prop Roots
Prop roots are adventitious roots that arise from the aerial horizontal branches of certain trees. They function to provide extra support to the plant. Initially, these roots are aerial and hygroscopic in function, with root caps protecting their tips. However, they grow vertically downward to touch the ground and penetrate the soil, and become thick and pillar-like. The death of such roots does not result in the death of the plant, as other roots are supporting the plant as well. In such plants, the main stem often becomes indistinguishable from the rest of the plant. Occasionally, the main trunk of a tree will die but the rest of the tree continues to survive with the help of prop roots.
Example: banyan tree
b) Stilt Roots
Stilt roots grow obliquely from the basal nodes of the stem, and then touch the ground and penetrate the soil. In monocots, such roots arise in whorls from a few basal nodes on the stem. Stilt roots provide mechanical support to the plant, allowing rapid vertical growth. They also enhance the absorption of water and minerals from the soil by increasing the surface area for absorption.
Example: sugarcane, Rhizophora Pandanus
c) Buttress Roots
In certain large trees, plank-like roots develop at the stem base on the surface of the soil. These roots are also called ballast roots, as they act like ballasts by supporting the plant.
Example: Terminalia and Salmalia.
d) Climbing Roots
Some plants with weak stems often use other plants or objects as support to reach greater heights so that their leaves are exposed to sufficient sunlight. These plants often develop adventitious roots that grow aerially from the nodes or internodes of the stem, and then twine and clasp around a support to allow the plant to reach greater heights.
Example: Betel wine
e) Floating Roots
Floating roots are adventitious roots that develop from the nodes of aquatic plants that float. These roots act as a floating device that keeps the plant afloat. They are filled with air, and are therefore spongy and inflated. They also allow gaseous exchange and are called respiratory roots.
f) Contractile Roots
Contractile roots are adventitious roots that develop from the base of a bulb or corm, or other underground and subaerial modified stems. These roots swell or contract to push or pull the modified stems to the correct depth in the soil.
Examples: Crocus, Lilium
g) Clinging Roots
Like climbing roots, clinging roots also allow plants with frail stems to attain greater heights. Such roots enter the crevices of the support to fix the plant to it.
3. Adventitious Roots Performing Other Specialized Functions
a) Reproductive Roots
In some plants, adventitious roots can be used in the vegetative propagation of plants. Such roots bear adventitious buds that can give rise to new plants when conditions are favorable.
Examples: Sweet potato, Dahlia
b) Assimilatory Roots
Roots modified to perform photosynthesis are called assimilatory roots. They are green roots that are often highly branched to increase the photosynthetic area.
Examples: Water chestnut, Tinospora
c) Saprophytic Roots
Saprophytic adventitious roots are associated with fungal hyphae, either ectomycorrhizae or endomycorrhizae. Such plants usually grow in humus when roots are infested by fungal mycelia, which form a mantle on the root. The mycelia aid in the absorption of food solutions from the soil that is utilized both by the host plant and the mycorrhizal fungus. Roots in such plants are usually underdeveloped. Tips stop growing and root hairs are mostly absent.
Examples: Sarcodes, Monotropa
d) Parasitic/Haustorial Roots
Some plants live on other plants for their nutrients and water supply. The roots of parasitic plants penetrate the stems or roots of the host plant, either only up to the xylem or even up to the phloem, in order to absorb the necessary water, minerals, and organic food.
e) Epiphytic Roots
As the name suggests, epiphytic roots are found on epiphytes or plants that grow on other plants. In such plants, the roots are hygroscopic and hang freely in the air. They have modified epidermal tissue called velamen that performs the specialized function of absorbing moisture from the air. Some of these roots are also greenish due to the presence of chlorophyll and perform carbon assimilation.
Examples: Dendrobium, Venda
f) Root Thorns
In some plants, adventitious roots are modified to form thorns that are hard and pointed. Such roots might function to defend the plant from being uprooted by animals.
Examples: Acanthorhiza, Iriartea