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30.3B: Root Modifications - Biology

30.3B: Root Modifications - Biology


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Plants have a wide variety of roots for functions as diverse as structural support, food storage, and parasitism.

Learning Objectives

  • Explain the reasons for root modifications

Key Points

  • Storage roots, which include a large number of edible vegetables such as potatoes and carrots, are some of the most commonly-known types of modified roots.
  • Aerial roots encompass a variety of shapes, yet function similarly as structural support for the plant.
  • Parasitic plants have special haustorial roots that allow the plant to absorb nutrients from a host plant.

Key Terms

  • succulent: having fleshy leaves or other tissues that store water
  • epiphyte: a plant that grows on another, using it as a physical support but neither obtaining nutrients from it nor causing it any damage if also offering no benefit

Root Modifications

Plants have different root structures for specific purposes. There are many different types of specialized roots, but two of the more familiar types of roots include aerial roots and storage roots. Aerial roots grow above the ground, typically providing structural support. Storage roots (for example, taproots and tuberous roots) are modified for food storage.

Aerial roots are found in many different kinds of plants, offering varying functions depending on the location of the plant. Epiphytic roots are a type of aerial root that enable a plant to grow on another plant in a non-parasitic manner. The banyan tree begins as an epiphyte, germinating in the branches of a host tree. Aerial prop roots develop from the branches and eventually reach the ground, providing additional support. Over time, many roots will come together to form what appears to be a trunk. The epiphytic roots of orchids develop a spongy tissue to absorb moisture and nutrients from any organic material on their roots. In screwpine, a palm-like tree that grows in sandy tropical soils, aerial roots develop to provide additional support that help the tree remain upright in shifting sand and water conditions.

Storage roots, such as carrots, beets, and sweet potatoes, are examples of roots that are specially modified for storage of starch and water. They usually grow underground as protection from plant-eating animals. Some plants, however, such as leaf succulents and cacti, store energy in their leaves and stems, respectively, instead of in their roots.

Other examples of modified roots are aerating roots and haustorial roots. Aerating roots, which rise above the ground, especially above water, are commonly seen in mangrove forests that grow along salt water coastlines. Haustorial roots are often seen in parasitic plants such as mistletoe. Their roots allow the plants to absorb water and nutrients from other plants.


Modification of Roots

In this article, we shall study the modification of roots for the purpose of food storage, respiration, support, etc. Roots in some plants change their shape and structure and become modified to perform functions other than absorption and conduction of water and minerals. They are modified for support, storage of food and respiration. We shall study the modification of roots for tap root system and adventitious system.

Modification of Tap Roots:

Modifications of Tap Root for Storage of Food:

Taproots of carrot, turnip and adventitious roots of sweet potato, get swollen and store food. The secondary roots remain thin. Hypocotyl, i.e. the embryonic region between cotyledons and radicle may also join the taproot in storing food. The stem is reduced and disc-shaped in the beginning and bears radical leaves. Depending upon their shapes they are further classified into four types.

Base is broad and tapers gradually towards the apex.

Swollen at middle and tapering at both the ends (spindle shaped). It gradually tapers towards the apex. Half of the fleshy part is derived from the hypocotyl

Radish (Raphanus sativus) (मूली),

Spherical at the base and sharply tapering towards the apex (top shaped or globular). It suddenly tapers towards the apex. The most of the fleshy part is derived from the hypocotyl

Beet (Beta vulgaris) (चुकंदर) , Turnip (Brassica rapa) (शलजम)

Thick and fleshy without any definite shape.

4 O’ clock plant (Mirabilis jalapa)

Modifications of Tap Root for Better Respiration:

The plants growing in saline, swamps, marshy places, and salt lakes are called halophytes. Such plants e.g. Rhizophora growing in swampy areas (mangroves), many roots come out of the ground (negatively geotropic) and grow vertically upwards. Such roots, called pneumatophores. They help to get oxygen for respiration. The roots appear like conical spikes coming out of the water. They occur in large numbers near the tree trunk. Exposed root tips possess Rhizophora minute pores (lenticels or pneumatothodes) through which roots respire. e.g. Rhizophora, Avicennia, Sonnerita, Heritiera (सुंद्री found in Sunderbans, Bengal).

Characteristics of Respiratory Roots:

  • These are modified tap roots.
  • These are non-green and non-photosynthetic
  • They are found in marshy habitats like swamp and mangroves near seashores
  • They grow vertically upward in response to gravity i.e. they are negatively geotropic.
  • These roots are covered with cork and the gaseous exchange takes place through pores called lenticels

Nodulated Roots:

In legumes (pea family), the secondary roots of primary tap root bear small tubercles or swellings which are called root nodules. These nodules shelter nitrogen-fixing bacteria Rhizobium leguminosarum. They help in fixing atmospheric nitrogen into nitrates which can be absorbed by the roots.

Modifications of Adventitious Roots:

Modifications of Adventitious Root for Storage of Food:

Simple Tuberous Roots:

These roots are creeping and become swollen and do not assume any shape. They are always borne singly. These roots arise from nodes of prostrate stem and enter in the soil. e.g. sweet potato (Ipomoea batatus) (शकरकंद).

Fasciculated Tuberous Roots:

It is a cluster of adventitious roots for the storage of food. These roots have a definite shape. e.g. Dahlia, Asparagus (शतावरी)

Nodulose Roots:

Only apices of roots become swollen like single beads. e.g. mango, ginger, turmeric

Beaded or Moniliform Roots:

Roots alternately swollen and constricted which has beaded or moniliform appearance. e.g. Grasses, sedges, momordica.

Annulated Roots:

Looks like number of disc placed one above the other. e.g. Ipecac

Modifications of Adventitious Roots for Support:

Prop Roots:

The hanging structures that support a banyan tree are called prop roots. Roots develop from tree branches hang downwards and ultimately penetrate the ground, thus provide support to heavy branches. A banyan growing in Indian Botanical garden, Owrah (Kolkata) has nearly 1700 such prop roots and has a very large spread. The tree is about 200 years old. Another example is the mangrove plant Rhizophora.

Characteristics of Prop Roots:

  • They arise from the branches of the stem.
  • They hang down vertically and enter the soil.
  • They are quite long.
  • They behave like a pillar and give support to the plant.

Stilt Roots:

The stems of maize and sugarcane have supporting roots coming out of the lower nodes of the stem. These are called stilt roots. They are mainly found in monocots, shrubs and small trees. They grow obliquely downwards and penetrate the soil. Their primary function is to provide support to the plant. In plants like maize, bajra, sugarcane, jowar, they grow in whorls. In screwpine(केवडा) or Pandanus (a tropical palm-like tree) these roots arise only from the lower surface of the obliquely growing stem to provide support. Another example is bamboo.

Characteristics of Stilt Roots:

  • They develop from basal nodes of the stem.
  • They grow obliquely from the stem.
  • They are shorter in length
  • They provide support to the plant as the ropes provide to the tent.

Climbing Roots:

Such plants produce roots from their nodes, by which they attach themselves to some support and climb over it. Weak climbers twine around and clasp the support with the help of climbing roots arising from their nodes. e.g. Money plant, black pepper (kali mirch), betel (pan). In Ivy, adhesive disc grows from climbing roots.

Clinging Roots:

Special clinging roots arise, enter the crevices of support and fix the epiphyte. e.g. epiphytes orchids

Buttress Roots:

They are present at the basal part of the stem and spread in different directions in the soil. They are vertically elongated and horizontally compressed. They look like planks e.g. Ficus, Bombax, Terminalia.

Modifications of Adventitious Roots for Special Functions:

Epiphytic Roots:

Some plants like orchids grow on horizontal branches of big trees in the forest to get sunlight. They are autotrophic. These plants are called epiphytes. They develop special areal hanging roots called epiphytic roots. These roots are spongy. Due to the presence of velamen tissue are hygroscopic and have a porous wall. They absorb moisture from the atmosphere. e.g. vanda, dendrobium, etc. These roots are also called assimilatory roots due to their partial capacity of photosynthesis.

Sucking Roots or Haustoria or Parasitic Roots:

These are highly specialized and microscopic roots, developed by parasites to absorb nourishment from the host. In partial parasites penetrate only xylem element of the host and absorb water and minerals. E.g. Viscus album. In total parasites, they establish contact with both xylem and phloem of the host. Thus absorb water, minerals, and nutrients. e.g. Cuscuta, Orobanche, Viscum, Lorathus.

Floating Roots:

Spongy, floating roots filled with air, arise from nodes of some aquatic plants, and help in floating and respiration. eg. Jussiaea

Photosynthetic or Assimilatory Roots:

Roots which when exposed to sun develop chlorophyll, turn green and manufacture food. e.g. Tinospora (gilo) and orchids. In Tinospora, the roots arise as green hanging threads. Other examples are Taeniophyllum, Trapa ( Singhara), and Podostemon.

Characteristics of Assimilatory Roots:

  • These are modified adventitious roots.
  • These are green and photosynthetic
  • They are found in diverse habitats like aquatic, terrestrial and epiphytes.
  • They grow horizontally in response to gravity and hence referred as diageotropic similarly, they show branching pattern and hence they are also referred as plagiotropic.
  • Gaseous exchange takes place through the general surface.

Mycorrhizal Roots:

This is a symbiotic association between higher plants and fungus. In some plants, roots become associated with fungal hyphae. This association of a fungus with higher plants is called mycorrhiza. The fungus absorbs water and minerals from the soil, in turn, the plant provides organic food to the fungus. Example: Pinus, Monotropa

Reproductive Roots:

Adventitious roots of some plants develop buds that give rise to leafy shoots. These roots help propagation. Example: sweet potato

Contractile Roots:

These roots can be found on underground rhizome, bulb, tuber, corm, etc. of some plants. They maintain a proper level of the plant in the soil. Example: Cracus, Freesia, Canna.


Modifications of Roots for Physiological and Mechanical Functions

Storage roots become fleshy by the accumulation of stored up food. Very often the aerial parts of these plants die out during unfavourable seasons and, when the season becomes favourable again, new buds sprout either from the underground fleshy root itself or from a small bit of stem above.

Thus, they serve for perennation almost as well as the underground stems. In other cases, especially in the tropics, the reserve food is utilised by the plant during flowering and fruiting. These swollen roots serve as important vegetables and food reserve for the human race.

This swelling and storage may involve the taproot (with or without its branches) or adventitious roots. Many common vegetables are instances of storage in taproots. Such swollen taproots are named according to the shape assumed.

It is fusiform (swollen in the middle and gradually tapering towards both ends) in radish (Raphanus sativus ), conical (broadest on top and gradually tapering towards the lower end) in carrot (Daucus carota) and napiform (very much swollen above, abruptly tapering towards the lower end) in turnip (brassica campestris var.rapa ) or beet (Beta vulgaris).

It should be noted in this connection that I he underground swollen part of these vegetables is not usually formed by the root alone but the hypocotyl (in radish and turnip the top part is hypocotyl, in carrot the hypocotyl part is negligible) is also incorpo­rated within it.

It should also be remembered that the termi­nologies are for particular shapes only and this is not always uniform. Thus, one must not take it for granted that every carrot in the market is conical.

Similar swellings are found in the adventitious routs of many plants. In sweet potato (Ipomoea batatas ) adventitious roots grow from the nodes of the running stem. These become swollen and form the edible root-tubers. Sweet potato tubers do even give rise to adventitious buds but should be distinguished from the ordinary potato which is a stem-tuber.

Tuberous roots are to be found in many common plants like Dahlia, Tapioca (Manihot esculenta), Ruellia tuberosa, etc., and may involve both adventitious and taproots. In some Dahlias, Asparagus, etc., the adventitious roots occur in clusters and all these are swollen. These are called fasciculated roots. An interesting tuberous root is that of some orchids.

In Orchis there is a pair of succulent tuberous roots one of which perishes every year while another new one is formed by its side. Such orchid roots may sometimes be of a palmate shape when they may be described as palmate tuberous.

In the mango-ginger (Curcuma amada ) and in Costus speciosus, only the apices of the adventitious roots become swol­len like single beads. This condition is termed as nodulose. In Dioscorea alata, many grasses and sedges and in some Vitis, some roots are alternately swollen and constricted presenting a beaded (moniliform) appearance.

In the valuable medicinal plant ipecac— Cephaelis (= Psychotna) ipecacuanha yielding emetine and now planted on the Himalayas, the roots look as if formed by a number of discs placed one above another. This is described as annulated.

2. Epiphytic or Aerial Absorbing Roots:

Epiphytic or Aerial absorbing roots are the peculiar common roots seen hanging from the orchids . Epiphytes are provided with these aerial roots in addition to the clinging roots .

The aerial roots are greenish and covered with a spongy velamen tissue with which they absorb atmospheric moisture. Their function is, therefore, absorptive. They also carry on a little carbon assimilation because of the chlorophyll. Such roots are also found in some aroids.

3. Assimilatory Roots:

Assimilatory roots are green while ordinary roots are not so. In some trees, when surface roots get exposed, they turn green by the development of chlorophyll and carry on some carbon assimilation. Epiphytic roots also are assimilatory.

But, certain plants like Tinospora cordifolia (Menispermaceae) have long green aerial roots whose main function is carbon assimilation. The peculiar family of plants, Podostemaceae , which is quite common in places like Cherrapunji, grows in shallow water on the hills.

The roots cannot penetrate rocks and are exposed. They are green and assimilatory as well as shoot-forming. These roots often assume a flattened thalloid structure and remain fixed on the rock by means of holdfasts called haptera resemb­ling some algae.

4. Reproductive Roots:

Adventitious buds sometimes grow on roots and serve as means of reproduction. Plantlings arise from the base of many garden plants in this way. Root cuttings are the main mode of reproduction in Trichosanthes dioica, sweet potato and ipecac. Podostemaceae propagates through roots. Such roots are, therefore, reproductive.

5. Respiratory Roots or Pneumatophores:

The roots of some trees and shrubs grow­ing in saline marshes (mangrove plants) suffer from the lack of oxygen. To cope with this situation some root branches grow vertically up (i.e., negatively geotropic) into the air from the horizontal secondary roots. Atmospheric air enters these roots through the minute pores of special lenticels (pneumathodes) on the exposed root tips.

They are, therefore, respiratory roots. Common examples of plants showing res­piratory roots are Heritiera (Bengali—sundri which has given the name to the Sunderbans), Rhizophora, etc.

6. Sucking Roots or Haustoria:

These are the sucking roots of parasites discussed . The parasites penetrate the small adventitious rootlets into the tissues of the host so that there is connection between the conducting systems of the two plants .

This is the channel through which food is sucked in by the parasite from

the host. Such sucking roots or haustoria are found in many total and partial parasites .

7. Mycorrhizal Roots:

These are the roots of the mycorrhizal saprophytes, discussed . They usually grow in humus and roots are infested with some fungal mycelia. The mycelia form a mantle on the root which absorbs food solutions (mostly organic) utilised both by the host plant and the mycorrhizal fungus.

The growth of this beneficial fungus on the root atrophies the root which becomes somewhat stunted and often looks abnormal . The tips do not grow and root-hairs are not formed. Such mycorrhizal roots are to be found in trees like pine and birch and in sap­rophytes like Monotropa, etc. .

(B) Modifications to Carry on Special Mechanical Functions:

1. Prop Roots:

In some tropical trees like Ficus bengahtensis (banyan), the horizontal aerial branches give rise to aerial roots which are provided with root-caps and hang vertically down from the boughs like so many strings. These grow down and, on reaching the soil and becoming anchored, they begin to stouten and ultimately become almost as strong as the main trunk. They support or prop up the hori­zontal branches like so many pillars .

Ultimately, the main trunk may die when the prop roots fully replace it. Long living banyan trees cover large areas by’ their spreading branches supported on prop roots.

The banyan tree at the Indian Botanical Garden, Shibpur, Calcutta is about 200 years old, covers a large area and has produced more than 900 prop roots. A similar tree in the Theosophical Society compound Adyar, Madras and another in the Buitenzorg Botanical Garden, Java, are equally remarkable.

2. Stilt Roots:

Certain shrubs and small trees like screw-pine (Pandanus foetidus ) grow on the edges of tanks, marshes, etc., where the anchorage is not very strong. In these cases, short roots grow obliquely downwards from near the base of the stem and act like stilts providing additional support as well as anchorage to the stem.

The adventitious roots growing from the lower nodes of maize plants act in a similar way. Such stilt roots are also seen in mangrove plants like Rhizophora.

3. Root Buttresses:

In some large trees, instead of stilt roots, there are great plank-like roots radiating from the base of the tree like wings . These are called buttresses and are actually partly root and partly stem. They may be seen in old trees of Bombax ceiba, Terminalia catappa, Ficus sp., etc.

4. Climbing Roots:

Some climbers can climb up their supports as adventitious roots growing from the nodes of the frail climber twine round and clasp the support as if the climber has been tied to the support at those points. Common examples are to be found in the aroid Scindapsus officinalis , the betel vine etc.

5. Clinging Roots:

Epiphytes like orchids cling to their support by means of special clinging roots which enter the crevices of the support and fix the epiphyte . Clinging roots may carry on some absorp­tion but are different from the specialised aerial absorbing roots.

6. Haptera or Holdfasts:

The thalloid bran­ching root of Podostemaceae , which is their main plant body carrying on assimilation as well as reproduction, is stuck up to the rock by means of special root processes which may be called holdfasts or haptera as in some algae.

The adhesive discs of adhesive climbers like Ficus repens and Ivy described on page 60 are also types of holdfasts.

It may be noted that all the three types of roots, climbing, clinging and hold­fasts, may behave in the same way by becoming flattened and disc-like and sticking to their supports or to the substratum.

7. Contractile or Pull Roots:

In plants with underground stems (rhizome, bulb, tuber, corm), there are certain roots which are different from the others as they are found to contract or swell so that the aerial shoot or the underground parts arc kept at a proper level in the soil.

These roots are, therefore, called pull or contractile roots. Those may be seen in Canna, Crocus, Allium, Lilium, etc.

8. Floating Roots:

Certain aquatic floating plants like fussiaea repens (Onagraceae) have special roots growing from the nodes in addition to the ordinary adventitious roots. These roots look like masses of white cotton and are extremely spongy in nature.

Their buoyancy helps to keep the plant floating. If taken out of water they dry and shrivel up quickly. The large amount of air contained in the floating roots may also be of some use in the aeration (‘breathing’) of the submerged organs.

9. Root-Thorns:

In many palms (e.g., Acanthorhiza and Iriartea), some aroids (Pothos armatus) and others, adventitious roots from the base of the stem become hard pointed and thorn-like. These are called root-thorns.

They are of indirect help as an Armature to the plants in driving out marauding animals. Old adventitious roots of coconut, etc., also serve the same purpose.


Adventitious Root System: Types, Modifications and Examples

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.

Index of Article (Click to Jump)

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:

  1. Store Food
  2. Provide Support
  3. 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.

Example- Dahlia

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.

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.

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 (Piperbetle):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.


30.3B: Root Modifications - Biology

Sometimes in addition to the normal functions of absorption, conduction and anchorage roots may perform storage of food materials, mechanical support, and vital functions. For this, they get modified structurally/ physiologically which is known as a modification of root. This can be found in both tap root and adventitious root.

Modification of Taproot

Fig:Modification of Taproot

In some plants with tap root system, the extra food materials are stored by the primary root due to which the root gets swollen into different shapes on the basis of which they are categorized into four types:

Fusiform root

Root is swollen into a spindle-shaped structure with a broad middle portion that tapers towards upper and lower end. Eg Raphanus sativus (Radish)

Napiform root

In this type, the root is broad on the upper part that suddenly tapers towards the lower end. Eg: Brassica rapa (Turnip)

Conical root

The root is swollen into a cone-shaped structure with a broad upper part that gradually tapers towards the lower end. Eg Daucos carota(carrot).

Tuberous root

It is the type of root which is swollen into indefinite shape structure. Eg Mirabilis jalapa( 4 o'clock plant)

Modification of Adventitious root

For storage of food materials:

Fig:Modification of Adventitious root

In some plants, the extra food materials are stored by adventitious root due to which they become swollen into thick and fleshy structure arising from different parts of the plant body.

Tuberous root

The modified root arises from the node of the prostate stem which occurs singly as a swollen root tube. Eg Ipoemoeabatatas (Sweet potato).

Fasciculated root

The modified root occurs as a cluster of swollen root tubers arising on the base of the stem. Eg Dahlia, Asparagus.

Moniliform (Beaded root)

The modified root has alternate swollen and constricted regions giving it a chain of beads like appearance. Eg: Momordica indica(tete karela)

Annulated root

The swollen root consists of a series of annulis or ring-like structures. Eg: Ipecacuanha

Nodulose root

In this type, the tips of roots become swollen into beads like structure.Eg Curcuma amada.

For mechanical support:

In the plants like Zea mays(Maize), Saccharum officinarum( Sugarcane) obliquely downward growing structures arise from the lower nodes known as stilt root which provides mechanical support against bending force of the wind.

In the tropical tree like Ficus benghalensis vertically downward growing structures arise from the aerial branches which after reaching the soil surface start to function as the normal root and provide mechanical support to the aerial branches. This is known as prop roots.

Climbing root

In the woody climbers or vines like Piper betel (Betel vine)(Paan), Pothos species(Money plant) etc hanging roots arise from the nodes or internodes or both parts which twine around and clasp the support to provide mechanical strength to the plant.

For vital functions

Assimilatory roots

In the plants like Trapa natans(Water chestnut) (Panifall), Tinospora cordifolia(gurja) etc green hanging roots arise from the aerial parts which due to the presence of chlorophyll pigments can perform photosynthesis and are known as assimilatory roots.

Epiphytic roots

In the epiphytes like orchids, special types of roots arise from the stem which is covered by the spongy tissue known as velamen that can absorb atmospheric moisture. This moisture is used by the plant to carry on photosynthesis process and is known as the epiphytic roots.

Haustorium or sucking roots

In parasitic plants like Cuscuta small peg-like outgrowths are given off which penetrate the host epidermics and get branched. These branches get inserted in the conducting tissue i.e. xylem and phloem. Thus, the parasitic plant can obtain necessary water minerals and food materials from the host plant. This is known as the hostoria or sucking roots.

Respiratory roots

In the plants growing in saline regions( Mangrove plants) vertically upward growing structure arise from the secondary roots which occur above the soil surface and are known as respiratory roots or pneumatophores. These are provided with minute pores known as pneumathodes through which atmospheric oxygen can be taken in to fulfil the oxygen requirements. Eg Rhizophora

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Things to remember
  • Sometimes in addition to the normal functions of absorption, conduction and anchorage roots may perform storage of food materials, mechanical support and vital functions. For this, they get modified structurally/ physiologically which is known as a modification of root.
  • In some plants with tap root system, the extra food materials are stored by the primary root due to which the root gets swollen into different shapes.
  • In some plants, the extra food materials are stored by adventitious root due to which they become swollen into thick and fleshy structure arising from different parts of the plant body.
  • In the tropical tree like Ficus benghalensis vertically downward growing structures arise from the aerial branches which after reaching the soil surface start to function as the normal root and provide mechanical support to the aerial branches. This are known as prop roots.
  • In the tropical tree like Ficus benghalensis vertically downward growing structures arise from the aerial branches which after reaching the soil surface start to function as the normal root and provide mechanical support to the aerial branches. This are known as prop roots.
  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.

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11th Class Biology Morphology of Flowering Plants (Root, Stem And Leaf) Modification Of Roots

Sometimes the root performs other functions other than fixation, absorption and conduction so get modified structurally. Both tap roots and adventitious roots may undergo such modifications. There are many types of root modifications.

Modification of tap roots

(1) Storage roots : In some plants, the primary tap roots are modified for storing reserve food materials. The secondary roots remain thin and they are absorptive in function. The storage roots are usually swollen and assume various forms:

(i) Conical : The swollen root is broad at the base and tapers gradually towards the apex giving a shape of cone, e.g., Carrot.

(ii) Fusiform : The root is swollen in the middle and narrow towards both its base e.g., Radish (Raphanus sativus).

(iii) Napiform : The root is nearly globular or spherical in shape. The basal portion of root is much swollen which suddenly tapers towards the apex giving a top-shaped appearance, e.g., Turnip (Brassica napus, vern, Shalgam) and Beet (Beta vulgaris, vern. Chukandar).

(iv) Tuberous : The storage root having no definite shape is called tuberous, .e.g., Mirabilis jalapa (4 O&rsquoclock plant),

(2) Branched roots

(i) Nodular roots (Tuberculated roots) : The primary tap roots and its branches of leguminous plants, i.e., plants belonging to sub-family papilionatae of the family leguminosae (e.g., Pea, Gram, Ground nut, Beans etc.), bear nodule like swellings, called root nodules.

They are red in colour due to the presence of leg-haemoglobin. The nodules are inhabited by nitrogen fixing bacteria called Rhizobium leguminosarum. It converts atmospheric nitrogen into nitrates and supply them to the plant. In turn Rhizobium gets nutrients and shelter from the plant.

(ii) Pneumatophores or Respiratory roots : The roots of some plants growing in saline marshes (mangrove plants) suffer from the lack of oxygen. This is due to the water logged condition of the soil. To cope with this situation some root branches grow vertically upwards.

They become aerial and negatively geotropic. These roots bear many minute pores called pneumathodes (lenticels) towards their upper ends. Gaseous exchange takes place through pneumathodes. Such aerial, porous, roots which help in gaseous exchange are called breathing roots. e.g., Sonneratia, Heritiera, Rhizophora, Avicennia and Ceriops etc. and are found in sundarbans of West Bengal.

For physiological or Vital functions

(1) Storage roots : The roots where adventitious roots become swollen to store food. They are of following types :

Tuberous roots : These adventitious roots are swollen without any definite shape e.g., Ipomoea batata or (Sweet potato).

Fasciculated roots : These are tuberous roots arising in cluster from the base of the stem. e.g., Dahlia, Ruellia (Menow weed), Asparagus etc.

Nodulose roots : These roots become swollen at their tips due to accumulation of food e.g., Maranta sp. (Arrowroot), Curcuma amanda (Mango &ndash ginger).

Moniliform or Beaded roots : These adventitious roots are swollen at frequent intervals. This gives the root a beaded appearance. e.g., Portulaca (Rose moss), Momordica (Bitter gourd), Cyperus (Guinea rush).

Palmate tuberous roots : In Orchis there is a pair of succulent tuberous root, one of which perishes every year while another new one is formed by its side. Such orchid roots may sometimes be of palmate shape, therefore, called palmate roots.

Annulated roots : The roots of a medicinal plant, Cephaelis ipecacuanha (Ipecac) yields emetine that looks like discs placed one above another, therefore, called annulated.

(2) Epiphytic roots : These roots are also called &lsquohygroscopic roots&rsquo. These roots develop in some orchids which grow as epiphytes upon the trunks or branches of trees. They hang freely in the air and absorb moisture with the help of special sponge like tissue called velamen. Velamen is modification of epidermis. e.g., Vanda, Dendrobium etc.

(3) Parasitic or Haustorial or Sucking roots : The roots of parasitic plants, which penetrate into the host tissues to absorb nourishment, are called haustorial roots. e.g., Cuscuta (Dodder, Vern, Amarbel).

(4) Saprophytic roots (Mycorrhizal roots) : The roots are associated with fungal hyphae either superficially (ectomycorrhizae) or internally (endomycorrhizae) for absorption of water and minerals. e.g., Monotropa and Sarcodes.

(5) Photosynthetic or Assimilatory roots : These are green, aerial, adventitious roots which prepare food materials by photosynthesis are called photosynthetic roots or assimilatory roots e.g., Taeniophyllum, Trapa and Tinospora. In some epiphytes like Taeniophyllum, the stem and leaves are absent. The entire plant is represented by thin green, ribbon like roots which contain velamen. These roots absorb moisture from the atmosphere and manufacture food materials by photosynthesis.

(6) Reproductive roots : Some fleshy adventitious root develop buds which can grow in to new plants. These are called reproductive root. These roots serve as means of vegetative propagation. e.g., Sweet potato, Dahlia etc.

For mechanical function

(1) Stilt roots (Brace roots) : The aerial, adventitious obliquely growing roots that develop from the lower nodes of the stem to give additional support are called stilt roots. These roots bear several large overlapping root caps called multiple root caps. e.g., Sugarcane, Pandanus, Rhizophora, Sorghum and Maize. Pandanus (screw pine) is a common sea shore plant.

(2) Prop roots or Pillar or Columnar roots : These adventitious aerial roots arise from horizontal aerial branches of the trees like Ficus benghalensis (Banyan). They grow vertically downward, penetrate the soil, become thick and assume the shapes of pillars. They provide support to the spreading branches of tree. Prop roots possess lenticels for aeration.

In India, the biggest banyan tree having large number of prop roots are found at Indian Botanical Gardens, Kolkata and Kadiri (Andhra Pradesh).

(3) Buttress roots : The horizontal plank like aerial, adventitious roots that develop at the base of the stem to give additional support are called buttress roots or ballast roots, e.g., Terminalia and Salmalia.

(4) Climbing roots : The aerial adventitious roots that arise from the nodes or internodes of weak stemmed plants to climb up their support are called climbing roots, e.g., Pothos, Piper betel, Vanilla and Hedera. In Pothos and Hedera, climbing roots develop all over the stem. In Vanilla, single tendril like root arise at each node. Hence they are called tendrillar roots. In Piper betel, many short branched, adventitious roots arise at each node. These roots are called clinging roots.

(5) Floating roots : These roots develop from the nodes of floating aquatic plants like Jussiaea (=Ludwigia). They store air, become inflated and spongy, project above the level of water, make the plant light and function as floats.

(6) Contractile or Pull roots : Some roots of plants with underground stems contract or swell so that the aerial shoots are kept in a proper depth in the soil. These roots are called contractile or pull roots, e.g., Canna, Crocus, Allium, Lilium, Freesia, etc.

(7) Root thorns : In aroids like Pothos and many palms (Acanthorhiza and Iriartea) the adventitious roots become hard and pointed hence called root thorns.


Background

Plants have developed a broad range of defense strategies and a complex network of signal transduction pathways [1, 2]. Calcium (Ca 2+ ) serves as a ubiquitous secondary messenger that is involved in multiple physiological and developmental processes in plants [3, 4]. Unfavorable environmental conditions, such as drought, salt, extreme temperatures, and pathogen infection are all known to trigger discreet spatial and temporal changes in the concentration of [Ca 2+ ]cyt in plant cells, leading to specific cellular responses [5]. Ca 2+ sensor proteins, including Ca 2+ -dependent protein kinases (CDPKs), calmodulins (CAMs), and calcineurin B-like proteins (CBLs), decode these specific signatures and interact with targeted proteins to relay signals [6,7,8]. Plants have evolved complex Ca 2+ -decoding mechanisms. The CBL-CIPK network is an example of a significantly diverged Ca 2+ -decoding system in plants [9]. CBLs show significant similarity with both the regulatory β subunit of calcineurin (CNB) and the neuronal calcium sensors (NCS) of animals [10]. CBLs harbor four elongation factor (EF) hand motifs as the structural basis for Ca 2+ binding. These EF hands specifically target a group of CDPKs designated as the CBL-interacting protein kinases (CIPKs) [9]. CIPKs consist of an N-terminal kinase catalytic domain and a C-terminal regulatory domain. The N-terminal kinase catalytic domain is related to sucrose non-fermenting kinase (SNF1) and AMP-activated protein kinase (AMPK) [10, 11]. The C-terminal regulatory domain contains a conserved NAF (Asn-Ala-Phe)/FISL (Phe-Ile-Ser-Leu) motif, consisting of 24 unique amino acid residues, that is essential and sufficient for interaction of CIPK proteins with CBLs [6, 12]. The protein phosphatase interaction (PPI) domain, containing 37 unique amino acid residues, is conserved in Arabidopsis protein kinase S (PKS) and in the DNA damage repair and replication block checkpoint kinase, Chk1, from various organisms including humans [13, 14]. The PPI motif, adjacent to the NAF/FISL motif, is necessary for interaction with abscisic acid-insensitive (ABI) protein phosphatases [14].

Comparative genomic analysis of CBL-CIPK genes in plants provides details about the functions, complexity, and conservation of the CBL/CIPK family and the evolution of the CBL-CIPK signaling network [15]. To date, a total of 10 CBLs and 26 CIPKs in Arabidopsis and 10 CBLs and 31 CIPKs in rice have been identified [16,17,18]. Although the specificity of the interactions of most of the CBLs and CIPKs has been confirmed using yeast two-hybrid assay experiments in Arabidopsis, the function of CBLs and CIPKs remains elusive [19, 20]. CBL4 interacts with CIPK24 to form specific complexes that function in activating plasma membrane-localized Na + /H + antiporters and vacuolar H + -ATPases to promote salt tolerance [11]. CIPK21 is involved in the regulation of osmotic stress response in Arabidopsis through interaction with the vacuolar Ca 2+ sensors CBL2 and CBL3 under salt stress conditions [21]. CBL1 and/or CBL9 interact with CIPK23, and control activation of the inward K + channel AKT1, thereby regulating K + uptake under low-K + conditions [22]. The CBL-CIPK signaling pathway plays important roles in plant responses to environmental stresses [23]. To date, investigations of the CBL-CIPK network have mainly concentrated on how ion channels are involved in the influx or efflux of various ions. However, how the CBL-CIPK network participates in drought stress responses in plants have not been extensively reported [22, 24].

Bread wheat (Triticum avestivum L.) is one of the global staple crops and is mainly grown in arid and semi-arid regions. Serious water scarcity can cause dramatic yield losses in wheat production systems. Although extensive studies have been made that elucidate the role of the CBL-CIPK signaling pathway in Arabidopsis, wheat CIPKs remain poorly investigated likely owing to the polyploid nature of the bread wheat genome and absence of a complete genome sequence [24, 25]. In the present study, a drought-responsive gene, TaCIPK23, was isolated based on a wheat drought de novo transcriptome sequencing experiment [26]. Over-expression of TaCIPK23 conferred drought tolerance in transgenic wheat and Arabidopsis. Further, the TaCIPK23 transgenic lines were more sensitive to ABA than the wild-type plants. TaCIPK23 enhanced the expression of a group of drought- and ABA-responsive genes under drought stress conditions. These results reveal a positive role for TaCIPK23 in conferring drought tolerance and regulating ABA signaling in plants.


30.3B: Root Modifications - Biology

The stem is the main axis of the plant body arises from the plumule of an embryo. It bears branches, leaves, and flowers. The stem is characterized by its positively phototropic nature and the presence of nodes, internodes, and buds. On the basis of position, buds can be categorized into two types:

  • Apical or Terminal bud occurring at the apex or growing tip
  • Auxiliary or lateral bud occurring on the axil of leaf

Modifications of stem

Underground modifications of stem

In some herbaceous plants, the aerial part die off during unfavourable conditions and the reserve food materials are stored in their underground portion due to which they become thick and fleshy. These structures are although underground are considered as the modified stems because they have distinct nodes, internodes, scaly leaves and buds on them.

These underground modified stems can grow into the new plant with the coming of favourable conditions. Thus, they are also known as organs of perennation. Its types are discussed below:

Rhizome is underground, branched, dorsiventral horizontally growing modified stem having distinct nodes and internodes. Thin and membranous scaly leaves arise on the nodes. Apical as well as auxiliary bud arise on the rhizome which grows into new aerial shoots. Adventitious root arises from the lower surface of a rhizome. Eg Zingiber officinate (Ginger).

It is spherical, an underground structure having reduced conical stem known as disc in which the internodes are highly compressed and nodes closely arranged. Thick and fleshy scaly leaves arise on the nodes which store reserved food materials. Apical bud is enclosed by the scaly leaves and auxiliary buds are sometimes developed. Adventitious root arises from the lower surface of the stem. Eg Allium cepa(Onion), Allium sativum (Garlic).

It is underground swollen vertically growing modified stem having circular nodes and internodes. Thin membranous and brownish scaly leaves arise on the nodes. Single apical bud is prominent and auxiliary buds are sometimes developed. Adventitious roots arise normally all over the body of the corm. Eg Colocasia, Croccus sativus(Safron).

Tubers are swollen terminal portions of an underground stem which are covered with a thin corky sheath having lenticels(pores). Many depression known as eyes are present which represent nodes. Thin scaly leaves and buds arise on the eyes or nodes. Adventitious roots are absent. Eg Solanum tuberosum (Potato).

Sub-aerial modification of stem

The sub-aerial modifications are found in many herbaceous plants with a thin, delicate and weak stem. In such plants, a part of the stem lives underground whereas remaining part of the stem is aerial. These plants bear adventitious roots and aerial branches at nodes. Based on type of growth and part of plant that provides them, it is divided into four types:

The runner is a specialized weak stem that has long and thin internodes and the branches creep over the surface of the soil. The branches develop adventitious roots from the lower sides of each node. The aerial branches develop from the axil of the scale leaves at the nodes. The runner gives rise to new plants either from axillary or terminal buds. Eg Cynodon dactylon (Doobo), mint.

It is a thin layered branch which grows horizontally outward and bears nodes and internodes. It produces adventitious root at the point of contact with the soil. Leaves are distributed all over the stolon due to normal apical growth. Eg Strawberry

It is the lateral branch which develops from the axillary bud of an underground part of the stem. It grows obliquely and gives rise to leafy shoot. The adventitious root develops at the base of nodes of the underground part of a sucker. Eg Mentha arvensis (Mint), Chrysanthemum.

The offset is also known as condensed aquatic runners. It is weak, elongated, horizontal branches at one internode that arise in the axil of a leaf. An offset produces a tuft of leaves above and a cluster of the roots below. The offset may break off from the parent plant and acts as an independent plant. Eg Pistia(Water lettuce).

Aerial modifications of stem

Phylloclade:

It is the characteristic feature of some xerophytic plants. Phylloclade can be defined as green, swollen, cylindrical or flattened aerial modified stem of unlimited growth. It has many depressions known as Aereoles representing on the nodes on which reduced leaves or spines and buds are developed. Phylloclade store water in the form of mucilage and is covered by thick cuticle so that the plant can survive in a dry habitat. Eg Opuntia(Nagphani), Euphorbia royleana(Siundi)

Cladode is short, green aerial modified stem or branch of limited growth having single internode only. It can perform photosynthesis on itself because the leaves are modified into thin membranous scales. Eg Asparagus racemoscus (Kurilo).

Tendrils are thin, elongated, thread-like spirally coiled modified stem which may be branched or unbranched and provide support to the climbers. Tendrils arising from the axils of leaves are known as axillary tendrils. Eg: Passiflora (Passion flower). Tendrils arising on the internodes are known as extra-axillary tendrils. Eg Cucurbita (Gourd). Tendrils arising on the opposite of leaf are known as leaf-opposed tendrils. Eg Vitis vinifera(Grapevine).

Thorns are short pointed hard often straight aerial, modified stem arising from the axil of the leaf which may be branched or unbranched and may bear leaves or flowers. Eg Durant repens, Punica granatum(Pomegranate), Bougainvillea.

Bulbils are modified axillary buds which are meant for vegetative propagation of the plant. They develop from axillary buds, which become swollen and fleshy with food and drop off to form new plants. Eg Agave.

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Leave A Comment :
Things to remember
  • The stem is the main axis of the plant body arises from the plumule of an embryo.
  • The stem is characterized by its positively phototropic nature and the presence of nodes, internodes, and buds.
  • Rhizome is underground, branched, dorsiventral horizontally growing modified stem having distinct nodes and internodes.
  • Phylloclade can be defined as green, swollen, cylindrical or flattened aerial modified stem of unlimited growth.
  • Cladode is short, green aerial modified stem or branch of limited growth having single internode only.
  • Phylloclade store water in the form of mucilage and is covered by thick cuticle so that the plant can survive in the dry habitat.
  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.

Stay connected with Kullabs. You can find us in almost every social media platforms.


AUTHOR CONTRIBUTIONS

Z. Shi and S. Xu carried out most of the experiments and bioinformatics analyses S. Xing contributed to the SAM-binding assay and assisted in several experiments L. Xue carried out part of molecular cloning L. Zhang, G. Yan, and P. Yang carried out mass spectrometry analyses J. Liu provided the screening reporter system and constructive insights in ESC biology P. Zhou and M.W. carried out the GoldCLIP experiment and analyses K. Yao and Z. Hu carried out the metabolome analyses and F. Lan conceived the project and cowrote the manuscript with Z. Shi and S. Xu.

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.



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