Adventitious roots are those roots that are generally found to be growing above the ground’s surface. They perform special functions and play an important role in the life cycles of certain plants/trees. This Gardenerdy article studies the formation, types, and functions of these unusual roots.
Did You Know
In his 1962 award-winning science-fiction novel ‘Hothouse’, author Brian Aldiss describes a future where humans are forced to live under the canopy of a colossal Banyan tree which covers half the planet, and is supported by a large number of thick, tree-like adventitious prop roots dropping down from its branches.
The roots of a plant do more than just anchor it to the ground. They also provide it with sustenance by sucking up nutrients and moisture from the ground. In general terms, roots represent the portion of a plant’s anatomy that grows under the ground. However, in actuality, roots can be found growing above the earth’s surface as well. These are classified as adventitious roots.
Adventitious roots are out-of-the-way growth in plants in response to various environmental factors. They serve several important functions and help the plant survive. In the upcoming sections, we shall explore the reasons for the formation of these roots, and also discuss their types and functions.
An adventitious occurrence is one which happens as a result of extrinsic factors. It is a divergence from the normal ‘expected’ sequence of events, occurring unexpectedly or by accident. In botany, it refers to an abnormal anatomical development seen in plants, such as roots growing from stems, shoots growing from roots, etc. It is seen as an adaptive measure incorporated by the plant to help allow it to survive.
Adventitious roots are modifications in the structure of a plant in response to external stimulus. While in the normal chain of events, root tissue grows only from another root tissue, certain special circumstances allow roots to develop from non-root tissues too, such as stems, branches, and older roots.
Causes of Adventitious Roots
- The development of adventitious roots is seen as a measure for survival and/or propagation of the species. For instance, in mangrove trees, which are found in saline coastal habitats, adventitious root growth is a means of propagation. They descend laterally, and on reaching a certain distance away from the parent plant, send up new stems.
- In areas plagued with floods, adventitious roots are developed to help the plant survive. While the main root is submerged, these grow on aerial parts of the plant, and actively absorb minerals and water to keep it alive.
- They also grow in response to injury. If the main root of the plant is injured and unable to fully perform the task of absorbing minerals and water, they grow to supplement it.
- Horticulturists, for years, have been successfully inducing the growth of adventitious roots by using various techniques such as wounding, hormone treatment, etc. Some plant stems, such as those of the willow or the poplar, develop them upon being placed in moist soil.
- Atmospheric conditions too can play an important role in the formation of such roots. A typical example of this can be seen in the swamp mahogany. In the drier climate of Australia, this species doesn’t develop adventitious roots. However, they can be seen growing freely amongst the members of the species thriving in the humid climate of Hawaii.
In some cases, such roots also form in response to bacterial infection or due to ants or other insects. For example, tomato stems develop roots because of bacterial infection.
Functions of Adventitious Roots
- Adventitious roots are still roots, and therefore, their main function is the absorption of minerals and water. Their formation is basically a survival mechanism, and therefore, their primary duty is to supplement the main root when its functioning is impaired due to disease, damage, or otherwise.
- They are also quite important for the propagation of the species. For example, mangroves propagate by having their adventitious roots shoot new trunks away from the parent plant.
- In flood-lands, small adventitious roots absorb oxygen and make it available to the submerged roots.
- In large trees, such as the banyan, aerial hanging roots act as pillars providing structural support to the tree. In corn, these accomplish nutrient and water absorption, and provide support as well.
- Horticulturists use the property of adventitious root development for propagating plants by using their cuttings. Specific hormone-based treatment can be used to induce growth of such roots on the cuttings, enabling their re-plantation.
Types and Examples of Adventitious Roots
Adventitious roots are developed as a means of adjusting to special circumstances. As such, often, distinct structural and functional modifications develop in them. The following are a few of the notable ones:
In larger trees, they develop as a means of structural support. The following are the main types of modifications observed:
These roots enter cracks and crevices and provide support to the plant/tree. e.g. banyan tree.
These roots act as pillars. They grow from the branches downwards and take root in the soil below providing a steady support to the plant. e.g. mangrove.
When the stem is weak, roots develop from its lower portion and spread laterally, strengthening the base of the plant. e.g. palm plant.
Climbing plants develop climbing roots to assist them in getting a firm hold on the structure that they are climbing. e.g. betel.
As the name suggests, these grow from the base of the stem, and provide support to the plant, like a buttress does to a wall. e.g. fig trees.
In some plants, these roots get modified to allow them to store nutrition. The following are the types most commonly seen:
Annulated: These roots have swollen ring-like modifications on them, appearing one next to the other. e.g. ipecacuanha.
Fasciculated: Cluster-like development of roots for food storage. e.g. dahlia.
Moniliform: A similar form of swelling observed at regular intervals. e.g. grasses.
Nodulose: These have swelling towards the tips of the roots, appearing like nodes. e.g. ginger.
Tuberous: These types have random swellings with no definite shape. e.g. sweet potato.
In many plants, these roots undergo distinct modification, enabling them to carry out several specialized functions. The following are a few examples:
Aerial Absorption: In a few plants, these roots absorb nutrition and moisture from the air. e.g. mangroves.
Parasitic Roots: Parasitic plants develop adventitious roots capable of sucking nutrients from the host plant. e.g. cuscuta
Food Assimilation: Some roots have chlorophyll, and are capable of photosynthesis. Thus, these roots can prepare the food from the plant from the absorbed minerals and water. e.g. tinospora.
Buoyancy: In aquatic plants, they develop the capability of storing air. Thus, they help the plant float above the water surface. e.g. jussiaea
The formation of adventitious roots is an important adaptive phenomenon indicative of, not just the plant’s ability to evolve and survive, but also of the environmental changes occurring in its habitat. Studying them will help us in learning more about the plant as well as its ecosystem.