Adventitious roots are the type of roots that arise from parts of the plant other than the radicle. These roots can arise from the injured root, nodes of the stem, internodes, branches, or any other tissue. A mass of adventitious roots along with its branches constitutes an ‘adventitious root system.’
The adventitious root system is different from the taproot system in such a way that instead of having one primary root from which the branches arises, there are numerous morphologically similar roots arising from the same node in adventitious roots.
Adventitious roots are generally seen growing from aerial parts of the plants. Adventitious roots can grow from the leaf and stem cuttings when placed in the soil.
They are formed from the root primordial cells and found in monocotyledonous plants.
In this lesson, we will learn about the types (modifications) of the adventitious root system with suitable examples.
Modifications of Adventitious Root System
Adventitious roots are modified for mechanical support, climbing, clinging and perform other vital functions. The adventitious roots are modified so as to:
- Store Food
- Provide Support
- Carry Out Additional Functions
1. Storage of Food
A. Tuberous Roots
Tuberous roots are the modification of adventitious roots. They are fleshy, do not have any particular shape and often get swollen. In case of tuberous roots, shoots sprout from one end while roots arise from the other end.
Example- Sweet Potato (Ipomoea batatas)
B. Fasciculated Roots
Fasciculated roots occur in the form of clusters. These clusters arise from the base of the stem.
C. Moniliform Roots
Another modification of the adventitious roots is the moniliform roots. These roots are also swollen. However, the swelling in moniliform roots occurs at regular intervals which renders a beaded appearance to the roots.
Example- Rose moss (Portulaca grandiflora)
D. Annulated Roots
In this type of modified adventitious roots, a series of outgrowths, which resemble the shape of rings, are present on the body. The series of these outgrowths looks like discs stacked one above the other.
Example- Ipecac (Cephaelis ipecacuanha)
E. Nodulose Roots
Nodulose roots are the modification of adventitious roots which are swollen at the apex or tips. They possess a characteristic shape.
Example- Turmeric (Curcuma longa)
2. Mechanical Strength
A. Prop or Pillar Adventitious Roots
- These are the type of adventitious roots which grow downward from the branches of the trees. These roots are modified to support the thick and heavy branches.
- The aerial roots are hygroscopic, that is, they absorb water (present in the form of moisture) from the air. Due to moisture absorption, these roots turn red in colour.
- Prop roots have root caps on their tips. As the prop roots reach the soil, they become thick and pillar-like. At this point, it becomes difficult to distinguish between the trunk and prop roots.
- An interesting fact is that even if the trunk dies, the tree as whole remains alive because the prop roots of the tree are supporting and nourishing the crown.
- Banyan Tree (Ficus benghalensis)
The Great Banyan growing in Acharya Jagadish Chandra Bose Indian Botanic Gardens, Howrah is nearly 250 years old and has 1775 prop roots. The main trunk of this Banyan tree decayed years ago. The circumference of the crown of this tree is around 404 m.
The largest tree specimen in the Guinness Book of World Records is found in Thimmamma Marrimanu village of Anantapur district in Andhra Pradesh, India. It is spread in the area of 5.2 acres.
- Mangrove plant (Rhizophora) )
B. Stilt Roots
These roots arise obliquely from the basal node of the stem and then penetrate the soil. These roots are short but thick and are modified to support the plant. After penetrating the soil, they get modified into the fibrous roots and help in water and mineral absorption.
- Sugarcane (Saccharum officinarum)
- Maize (Zea mays)
- Fountaingrass (Pennisetum setaceum)
- Sorghum (Sorghum bicolor)
- Screwpine (Pandanus odoratissimus0
C. Climbing Roots
- These roots are found in climbers (plants climbing on various structures). They are non-absorptive kind of adventitious roots that help the plant to remain adhered to the structure.
- Climbing roots penetrate the cracks or fissures of the support and help the plant to climb. These roots form claw, swell, or secrete sticky juice from their tip to hold the support firmly.
- A tendril-like root arises from the node in case of Vanilla (Vanilla planifolia) plant. Therefore, they are known as tendrillar roots.
- The climbing adventitious roots may also sprout from each node and get branched. Such types of climbing adventitious roots are known as clinging roots.
- Tecoma (Tecoma stans) and Betel (Piper betle): roots arise from the nodes.
- Climbing Fig (Ficus pumila): roots arise from the internodes.
- Ivy (Hedera): roots arise from the nodes and internode. The tip of the roots secrete sticky juice to hold the structure firmly
- Tecoma (Tecoma stans): roots form claws for climbing.
D. Buttress Roots
- Buttress roots develop at the base of the stem and help in maintaining the structural integrity of the plant.
- The basal part of the stem, which is vertically elongated, spreads in different directions in the soil.
- This type of modified adventitious roots gives an appearance of planks.
- Arjuna Tree (Terminalia arjuna)
- Cotton Tree (Bombax ceiba)
- Moreton Bay Fig Tree in Santa Barbara, California (Ficus macrophylla)
3. Additional Functions
A. Assimilatory Roots
- These roots are modified for carrying out vital functions of the plants.
- These are green roots that are capable of performing photosynthesis due to the presence of chloroplast in their cells.
- Assimilatory roots are highly branched to increase their surface area so that maximum amount of sunlight is absorbed by them.
- Water Chestnut (Trapa natans): their roots are submerged like normal roots.
- Moonseed (Tinospora cordifolia): its assimilatory roots, arising from stem nodes, sprout during the rainy season and wrinkle during drought
- Ribbon Roots (Taeniophyllum)
B. Epiphytic or Aerial Roots
- These are the type of adventitious roots that are present in epiphytes, that is, on those plants that live on other plants for shelter and nutrition.
- Epiphytic roots are irregular in shape and hang down the surface of the other plants. These roots do not possess a root cap and, instead, have a covering of a dead spongy tissue known as velamen.
- Epiphytic roots are hygroscopic in nature, that is, they absorb water, present in the form of moisture, from the air with the help of velamen.
Epiphytic plants possess two types of roots; for clinging (or fixation) and for the absorption of mineral salts and moisture from dust collected on the bark.
- Orchid (Orchidaceae)
- Dendrobium (Dendrobium aggregatum)
C. Floating Roots
- This type of adventitious root modification is found in aquatic plants.
- These roots arise from the node of the horizontal floating stem.
- Some of these adventitious roots store water and become inflated. The inflated roots come out of the water surface and help the plant to float.
- They also help in gaseous exchange, therefore, they are also called respiratory roots.
D. Mycorrhizal Roots
- Mycorrhizae refer to the symbiotic association of a fungus with a higher plant.
- The mycorrhizal roots present a classic example of a mutual association between a fungus and roots.
- The nutrients from the soil are absorbed by the fungus. The plant, in turn, support the fungus with organic food.
- Pine (Pinus sabiniana)
- Snow Plant(Sarcodes)
E. Sucking Roots or Haustoria
- In order to absorb nutrients from the host, parasites develop microscopic roots which are known as sucking roots.
- Since they are also found in non-green parasitic plants, they are also known as Haustoria.
- Sucking roots sprout from the nodes and penetrate deep into the conducting tissue of the host to obtain nutrients.
- Dodder (Cuscuta)
- Mistletoe (Viscum album)
F. Reproductive Roots
- Reproductive roots are modified adventitious roots which help in reproduction.
- Such kind of adventitious roots give rise to buds which further develop into shoots.
- A root cutting can be planted in the soil from which a new plant grows.
- The reproductive roots form one of the ways of vegetative propagation.
- Sweet Potato (Ipomoea batatas)