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Root and Its Parts: Root system, Structure of dicot and monocot root, Order, types, and functions of the root, Root modifications, Practice Problems, and FAQs

Have you heard about ‘Thimmamma Marrimanu’?. It is the big banyan tree of Andhra Pradesh. It is estimated to be around 550 years old and its canopy spans 19107 metres, which is almost equivalent to 300 cricket pitches! Think how enormous it is. Now tell me what causes this to happen?



                                                        Fig: Banyan tree

These banyan trees possess roots called prop roots. They are adventitious roots that sprout from the large horizontal branches of a banyan tree. These develop towards the soil and enter the soil. In this way they give additional support to the main stem and allow the banyan tree to expand across such a large area! That's how a single tree may grow to encompass 300 cricket pitches!

In the similar way there are different types of root modifications. Some are edible, some give additional support. So let’s take a deep dive into the details of root structure and root modifications in this article.

Table of contents

Roots

Roots are the most important subsurface component of all vascular plants. This part of the plant is in charge of anchoring it to the soil and absorbing vital mineral components, nutrients, and water. It also helps in storing food.

However, not all plants have roots that develop underground; some have roots that grow above ground. These are referred to as aerial roots. Like underground roots, aerial roots are also important for absorbing nutrients, anchoring and affixing the plant to structures such as surrounding walls, rocks, trellises, and so on. Examples of plants with aerial roots are banyan trees, and mangroves.



                                                  Fig: Buttress roots

Characteristics of roots

Some of the characteristics of roots are listed below:

  • They are non-green structures because of the absence of chlorophyll.
  • Roots do not possess nodes and internodes.
  • They do not possess leaves and buds.
  • They show positive geotropism i.e. the growth takes place towards the direction of gravity.
  • They show positive hydrotropism i.e. the growth takes place towards water.
  • They show negative phototropism i.e. the growth occurs away from the direction of light.
  • It possesses root hairs near the tip. 
  • The root tip is protected by a root cap. 

Root system

The component of the plant axis that descends is called the root system (grows downward). The radicle is the first organ to emerge from seed as it germinates. It grows longer to create the primary or tap root which then creates the root system by producing lateral branches (secondary and tertiary roots). It spreads out into huge, deep sections of the soil, firmly anchoring the plant. It also absorbs water and mineral salts from the Earth and transports them upwards, which is an important function of the root. The root system develops from the radicle of the seed. 



                                     Fig: Development of root system from the radicle

Structure of root

The following are the major five regions present in the root system:

  • Root cap region
  • Meristematic zone or growing point
  • Region of elongation
  • Region of root hair
  • Region of maturation

Root cap region

The meristematic (rapidly dividing) zone produces a thimble-like structure that protects the fragile apex (apical meristem) from harsh soil particles. It is called the root cap. The root cap wears down as the root goes deeper into the soil, yet it is regularly replaced. The cells present in the root cap produce mucilage that helps in lubrication while it penetrates through the soil. The root cap of aquatic plants (Pistia and water hyacinth) resembles a loose thimble termed the root pocket.

Region of meristematic cells

This region is present in the growing tip of the root. This is a small region of actively dividing cells. The region is protected by the root cap. It consists of various types of cells:

Dermatogen

It is the outermost layer. The epiblema and root cap develop from these cells.

Periblem

This layer is present inner to dermatogen. The cells of the periblem layer mature into the cortex.

Plerome

It is considered a central region and the cells of the plerome mature into the stele.

Calyptrogen

It is a separate set of rapidly dividing cells normally seen in monocots. It forms the root cap in them.

Region of elongation

This region is present next to the meristematic region. The cells of this region rapidly elongate and enlarge. As a result, the length of the roots increases. It is about 4 - 8mm in length. The cells here are able to absorb water and mineral salts from the soil. The cells of this region possess newly formed cells which lose the power of division. 

Root hair region

It lies above the region of elongation. This region possesses a cluster of very fine tubular outgrowths called the root hairs. They are produced from the outermost layer of the root called epiblema. They increase the surface area for absorption. Root hairs help in the absorption of water and minerals normally from the soil. Different types of primary tissues differentiate or mature in this region, hence this region is called zone of differentiation or maturation. 

Region of maturation

This region forms the major part of the root. The cells in the elongation area mature and differentiate as they grow older. The freshly formed meristematic tissues push them upwards. Some cells form a permanent region that is hairless and lies behind the root hair zone. It develops lateral roots and anchors the plant in the ground. This region has thick walled cells hence does not help in absorption of water from the soil. 



                                                Fig: Structure of root

Order of roots

Roots normally develop in the following order in the root system of a plant:

Primary roots

They are the direct elongation from the radicle. It is the most prominent part of the root system which lies in the centre. It thrives on the soil. The tap root system is made up of primary roots and their branches.

Secondary roots

The secondary roots are also known as the lateral roots. They emerge from the primary roots of a plant as side branches.

Tertiary roots

The branches of the secondary root are known as tertiary roots. They will branch to rootlets. The tips of the rootlets are covered with root caps. Behind the root tips root hairs are present. 



                                Fig: Order of formation of roots

Types of roots

Although all roots perform the same activities, their structure differs. Based on this, the root system is divided into three categories as follows:

  • Tap root system
  • Fibrous root system
  • Adventitious root system

Tap root system

A tap root system consists of one primary root, which is a thick, cylindrical main root. The primary roots give rise to secondary and tertiary roots, which are branches of the primary roots.

So taproots have a primary central root to which root hairs, or little lateral roots, are attached. The radicle of a developing embryo gives rise to taproots. In many plant species radicles die shortly after reaching maturity; hence the taproot eventually transforms into a fibrous root system. Mustards, carrots, beetroots, parsleys, china roses, and all dicotyledons have taproot systems. 



                                              Fig: Taproots in mustard

Taproot modifications

Tap roots can be transformed into a range of shapes and sizes to accomplish a variety of tasks. Tap roots become fleshy and swollen for the storage of food. Different types of tap root modifications for food storage are as follows:

  • Conical root
  • Fusiform root
  • Napiform root
  • Tuberous root

Conical root

The base is wide and gradually narrows as it approaches the tip in conical roots. An example of conical roots is the carrot.



                            Fig: Conical root

Fusiform root

The middle of this root is bulbous, and the ends taper in fusiform root. An example of a fusiform root is radish.



                       Fig: Fusiform root

Napiform root

The middle of this sort of root is inflated, and the ends taper. A turnip is an example of a napiform root.



                     Fig: Napiform root

Tuberous root

This type of root is thick and fleshy with no definite shape. An example of a tuberous root is the 4 O'clock plant or Mirabilis jalapa



                                                         Fig: Tuberous root

Fibrous roots

They are bushy roots that sprout from the stem with thin, modestly branched roots. It consists of many fine hair-like roots which form a thick mat below the soil surface. This root system is effective in absorbing water and minerals. It helps the plant to be strongly attached to the substratum also. It will not penetrate deeply inside the soil. For example, rice, wheat, maize, marigold, banana, and all monocotyledonous plants.



                Fig: Fibrous roots in wheat

Adventitious roots

Adventitious roots are a special type of root that arises from parts other than the radicle. A mass of adventitious roots along with their branches are called the adventitious root system. The plants with this particular type of root system will have primary roots also. The adventitious roots are found in Monstera



                                    Fig: Adventitious roots in Monstera

Adventitious root modifications

Adventitious roots are altered for a variety of purposes as follows:

  • Modification for food storage
  • Modification for photosynthesis
  • Modification for absorbing atmospheric moisture
  • Modification for better gaseous exchange
  • Modification for sucking nutrition from the host
  • Modification for strong support
  • Modification for buoyancy and respiration

Modification for food storage

Adventitious roots are modified to store food in the following cases:

Tuberous root

These are a type of swollen roots that develop from nodes of the prostrate stem. An example of a tuberous root is a sweet potato.



                                   Fig: Tuberous root

Fasciculated root

The stem produces a cluster of swollen roots here. Dahlia is an example of a fasciculated root.

Nodulose root

Only the apices of the roots swell like single beads in a nodulose root. An example of nodulose root is mango-ginger.

Moniliform root

Roots alternately swell and constrict, giving them a beaded or moniliform appearance here. Examples of moniliform roots include grasses and sedges.

Annulated root

Annulated root appears to be made up of a series of discs stacked one on top of the other. An example of an annulated root is Ipecac.



                  Fig: Annulated root

Modifications for photosynthesis

The roots are modified into assimilatory roots for photosynthesis here. These roots develop chlorophyll when exposed to sunlight. As a result, it turns green and manufactures food. Examples of assimilatory roots are Tinospora and orchid.



                           Fig: Assimilatory root

Modification for absorbing atmospheric moisture

The roots are modified into epiphytic roots for absorbing atmospheric moisture. Epiphytes have greenish aerial roots that are covered with spongy tissue called velamen that absorbs atmospheric moisture. An example of an epiphytic root is seen in an orchid.

Modification for better gaseous exchange

The roots are modified into pneumatophores or respiratory roots for better gaseous exchange. They appear like conical spikes coming out of water. These roots are negatively geotropic roots that bear pores called pneumatothodes. These minute pores help the roots to respire. They are found in plants growing in mangroves, swamps, and salt lakes. An example of a plant with pneumatophores is seen in Rhizophora.



                                                 Fig: Pneumatophores

Modification for sucking nutrition from the host

Sucking roots, also known as haustoria, are produced by parasitic plants that enter the living host plant and suck nutrients from the phloem. An example for the plant that contains sucking roots is Cuscuta.



                                        Fig: Haustoria

Modification for strong support

The roots modify to provide strong support to the plants:

Prop roots

Prop roots grow from horizontal tree branches, hang downward, and eventually penetrate the ground, providing support for heavy branches. The prop roots are present in a banyan tree.



                 Fig: Prop roots in banyan tree

Stilt roots

Stilt roots are extra roots that grow obliquely downwards from nodes at the stem's base. They penetrate the Earth and provide a strong anchoring to the plant. This type of root is found in sugarcane, maize, and screwpine.



                                  Fig: Stilt roots in maize

Climbing roots

Weak climbers use climbing roots emerging from their nodes to coil around and grab the support. The climbing roots are found in money plants and betel.

Clinging roots

They are special adhering roots emerging from the nodes, internodes or both. They enter support fissures, and anchor the epiphytes. These roots are found in epiphytes and orchids.



                   Fig: Clinging roots in epiphytes

Modification for buoyancy and respiration

The roots were modified into floating roots to provide buoyancy and respiration here. Some aquatic plants have spongy, air-filled floating roots that assist in flotation and breathing. These roots are found in Jussiaea

Difference between the root systems - Tap root system and Fibrous root system

Tap root system

Adventitious root system

It arises from the radicle 

It arises from any part of the plant other than radicle

It has a single main root

A number of roots arise in clusters

The main root is thick 

All roots are generally fibrous

A characteristic of dicot plants

A characteristic of monocot plants

There are distinct primary roots, secondary roots, tertiary roots and rootlets present

There is no such distinction

It penetrate deep inside the soil

It is usually present just below the surface

It is always seen underground

It can be underground or above ground

Primary root persists throughout the life

Primary root is short lived

Examples include root system of mango tree and jackfruit tree

Examples include root system of rice and wheat

Functions of roots

Roots perform a range of functions that are critical to the survival of the plant. They are part of a bigger system that helps the plant to survive as follows: 

Anchoring

 Plants cling to the Earth with the help of roots. They help the plant to stand erect by providing support for its body.

Absorption

The main job of the root is to take in water and dissolved minerals from the soil. This is important because it facilitates photosynthesis and survival.

Storage

Starch is used by plants to generate food, which they store in their leaves, branches, and roots. Carrots, radishes, and beetroot are just a few examples of storage roots.

Reproduction

Despite the fact that roots are not reproductive, they are vegetative parts of the plant. Some plants use their roots as a way of reproduction. New plants arise from spreading horizontal stems called runners (stolons) in jasmine, grass, and other plants. This type of reproduction is termed as vegetative propagation. 

Ecological function

They prevent soil erosion, supply food, and serve as a habitat for a variety of creatures. Examples include birds, insects and bacteria. 

Practice Problems

Q 1. Which of the following roots grows in the opposite direction of gravity?

a. Prop roots
b. Stilt roots
c. Buttress roots
d. Pneumatophores

Answer: Roots are plant structures that extend downward from the radicle to the soil. Geotropism is the characteristic of plant roots. They grow towards the soil or the growth of the roots takes place towards the direction of gravity. Roots are thus positively geotropic. Plants that flourish in salt lakes, mangroves, or swamps, on the other hand, suffer from oxygen deficit. As a result, they have horizontal cable roots that sprout vertically upward aerial roots at regular intervals. Pneumatophores, or respiratory roots, are negatively geotropic roots. Pneumatophores are present in Rhizophora. Hence, the correct option is d.

Q 2. The root cap is not utilised in water absorption because of ___________________________.

a. The presence of root hairs
b. The absence of root hairs
c. Its presence in the elongation zone
d. None of the above

Answer: The root cap is the area of the root that surrounds the root apex. It is a thimble-shaped structure that shields the root's sensitive apex as it travels through the soil. Root hairs, which are thin and delicate thread-like structures, are required for water absorption by roots. The root hairs are only found in the maturation region of the root. Just below this region is the elongation region, which is responsible for the root's lengthening. Water cannot be absorbed by the root cap because there are no root hairs in the root cap region. Hence, the correct option is b.

Q 3. Adventitious roots are absent in which plant?

a. Banyan tree
b. Monstera 
c. Asparagus
d. Mustard

Answer: The mustard plant is a dicot with a tap root system. It does not have any adventitious roots. Hence, the correct option is d.

Q 4. Match the correct options:

Column I

Column II

A. Conical root

I. Rhizophora

B. Fusiform root

II. Turnip

C. Napiform root

III. Radish

D. Pneumatophore

IV. Carrot


a. A - I, B - III, C - IV, D - II
b. A - I, B - II, C - III, D - IV
c. A - IV, B - II, C - III, D - I
d. A - IV, B - III, C - II, D - I

Answer: The base is wide and gradually narrows as it approaches the tip in conical roots. An example of conical roots is the carrot. The Fusiform type of root is swollen in the middle and tapers towards both ends. Radish is considered as an example of a fusiform root. The napiform type of root is swollen in the middle and tapers towards both ends. An example of a napiform root is a turnip. The roots are modified into pneumatophores or respiratory roots for better gaseous exchange. They resemble conical spikes emerging from the water. These roots are negatively geotropic roots that bear pores called pneumatothodes. These minute pores help the roots to respire. They are found in plants growing in mangroves, swamps, and salt lakes. An example of a pneumatophore is the plant Rhizophora. Hence, the correct option is d.

FAQs

Q 1. Which root system is more effective at anchoring, and why?
Answer: The tap root system is deeply rooted and has numerous branches that ramify across wide amounts of soil. It provides superior anchoring.

Q 2. Dermatogen and plerome differentiate into which tissues?
Answer: Plerome is the central core of the primary meristem of a plant which evolves into the stele. Dermatogen is the outer primary meristem of a plant which differentiates into epiblema and cap.

Q 3. Which mechanism do roots use to absorb water?
Answer: Roots absorb water through two mechanisms namely active and passive transport.

Q 4. What are root hairs?
Answer: Root hairs are cylindrical extensions of epidermal cells called epiblema.

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