11.3: Secondary Tissues in the Root - Biology

11.3: Secondary Tissues in the Root - Biology

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In roots, the formation of both secondary meristems involves the pericycle. The pericycle and some residual procambium join together to form the vascular cambium, a secondary meristem that produces vascular tissue. The other secondary meristem, the cork cambium, is initially formed solely from the pericycle.

Each of these secondary meristems divides in two directions to form a different secondary tissue to the inside and outside of the meristematic layer, respective to the center of the plant.

The vascular cambium produces secondary xylem to the inside of the root and secondary phloem to the outside.

The cork cambium produces phelloderm, a storage tissue, to the inside of the root and cork, a protective layer of dead, suberized cells, toward the outside. These three layers -- phelloderm, cork cambium, and cork -- are referred to as a periderm. As the first periderm layer is formed, it separates the epidermis, cortex, and endodermis from the conductive tissues of the root. The epidermis and cortex cells die and are shed as secondary growth proceeds.

The pine root below has one layer of periderm fully formed. Can you label the tissues and meristems described above? What are the large holes in the root?

Secondary Growth in Dicotyledonous Root (With Diagram)

The roots of some herbaceous dicotyledons and of all gymnosperms and woody dicotyledons show secondary increase in thickness, whereas most of the monocotyledonous roots, like those stems, are entirely primary.

The secondary tissues formed in the dicotyledonous roots are fundamentally similar to those of the stem, but the process is initiated in a different way (Figs. 645 to 647).

The dicotyledonous roots have limited number of radially arranged vascular bundles with exarch xylem. Pith is usually absent. A few parenchyma cells beneath each phloem group become meristematic and thus form strips of cambium, the number of strips being equal to the number of phloem groups present.

Cambial cells go on dividing and produce secondary tissues. The cells of the uniseriate pericycle against the protoxylem group now divide and form a few layers. The first- formed cambium now extends both ways and reaches the innermost derivatives of the xylem groups.

As a rule the cambial cells produce much more xylem than phloem and because the first-formed cambium started producing secondary xylem much earlier and produces secondary xylem much more rapidly, the wavy cambium cylinder is ultimately rendered circular.

As already stated, the secondary vascular tissues are fundamentally similar to those of the stem. They form a continuous cylinder and the primary xylem gets completely embedded in it.

At this stage the root structure is revealed only by the radially arranged exarch primary xylem located at the central region, the strands of secondary vascular tissues being collaterally arranged like those of the stem (Fig. 647). The sieve elements of the primary phloem often get crushed.

The cambial cells originating from the peri­cycle against protoxylem groups function as ray initials and produce broad bands of vas­cular rays. These rays running between xylem and phloem through the cambium are characteristic of the roots. They are also called main medullary rays.

Periderm is formed in the outer region. Phellogen arises in the outer cells of the peri­cycle (Figs. 646 & 647). It produces phellem or cork cells on the outer side, and probably some phelloderm on the inner.

The pressure caused by formation of secondary tissues inside ruptures the cortex with endodermis, which is ultimately sloughed off. Lenticels may be formed. They usually occur in pairs as transversely elongated rough areas, one on each side of a lateral root.

Secondary Tissues in Dicot Root of Plants

Conjunctive parenchyma cells lying on the inner edges of the primary phloem bundles become meristematic. They give rise to small quantity of secondary xylem on the inner side and secondary phloem on the outer side. In the process, these cambial strips and primary phloem bundles are pushed slightly to the outside.

Conjunctive parenchyma cells on the lateral sides of the phloem bundles as well as pericycle cells lying outside the protoxylem ends become brick-shaped and meristematic. This gives rise to a wavy band of a vascular cambium (Fig. 6.24 C).

The vascular cambium of the root is a complete secondary meristem. It continues to form secondary xylem on the inner side and secondary phloem on the outer side. Secondary phloem consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Secondary xylem is similarly made of vessels, xylem parenchyma and xylem fibres.

Initially vascular cambium derived from the pericycle gives rise to only ray cells. Hence wide multi-seriate primary vascular rays (also called medullary rays) are formed opposite the protoxylem points. However, the formation of ray cells is slower than the formation of secondary vascular tissues. As a result the depressed parts of vascular cambium move outwardly and ultimately the cambium becomes circular (Fig. 6.24 D).

The ring of vascular cambium produces secondary xylem on the inner side and secondary phloem to the outside. Both of them are in the form of rings (Fig. 6.24 D-E) (c.f, primary vascular bundles). The primary phloem gets crushed by the growth of secondary vascular tissues.

The older secondary phloem is also partially destroyed as the new phloem becomes func­tional. The primary and secondary xylems persist. Primary xylem is distinguishable by its exarch nature and central position.

As compared to the primary xylem, the vessels of the secondary xylem are broader and thinner. Annual rings are not very sharp because unlike aerial climate, the climate of the soil does not vary much during different seasons.

The secondary phloem is made up of sieve tubes, companion cells and phloem parenchyma. Sclerenchyma fibres are rare. The secondary xylem is formed of vessels, tracheids and xylem parenchyma.

At places the vascular cam­bium possesses ray initials. They produce vascular rays. The rays are made up of two parts, xylem or wood ray (present in secondary xylem) and phloem ray (present in sec­ondary phloem). They help in radial conduction of substances.

Type # 2. Secondary Ground Tissues or Periderm:

The pericycle layer, either directly or after a few divisions becomes converted into a secondary meristem called cork cambium or phellogen (Fig. 6.24 D-E). Rarely phellogen appears in the cortex.

The cells of phellogen divide both towards the outside as well as inside. The tissue formed towards the inner side is parenchymatous and known as secondary cortex or phelloderm. It is only a few layers in thickness.

The cells formed on the outside by the phellogen are rectangular and compactly ar­ranged. They soon become dead. Their cavities get filled up with tannin and their walls become suberized. They are described as cork cells. The tissue of cork cells is spoken as cork or phellem (Fig. 6.24 F). The cork is impervious to water.

It protects the interior from mechanical injury and entry of bacteria. Primary tissues present outside the cork undergo starvation and get shriveled. Under the impact of secondary growth in the interior, the outer layers of the cork are also peeled off occasionally.

The phellem, the phellogen and the phelloderm are collectively called secondary ground tissue or periderm. At places the phellem or cork bears lenticels for exchange of gases (Figs. 6.24 E, 6.25).

11.3: Secondary Tissues in the Root - Biology

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As a plant grows, the shoots and roots lengthen through primary growth and, in woody plants, thicken through secondary growth.

In stems, primary growth begins when undifferentiated cells in the shoot apical meristem divide. Some progeny cells differentiate into primary meristems&mdashthe protoderm, ground meristem, and procambium.

Ultimately, these primary meristems generate mature plant tissues: dermal, ground, and vascular tissues. Specifically, the protoderm produces the epidermis, a dermal tissue. The ground meristem forms ground tissues, including pith and cortex. The procambium generates vascular tissue: primary phloem and primary xylem.

In roots, on the other hand, primary growth occurs in three overlapping zones of cells called the zones of division, elongation, and differentiation.

The zone of cell division contains the root apical meristem. In the zone of elongation, new cells lengthen and extend the root tips. In the zone of differentiation, cells differentiate into distinct types.

The root and shoot apical meristems produce the same primary meristems, which generate the same tissue types. However, some primary tissues differ between roots and shoots. For example, root ground tissue consists mainly of cortex and lacks pith.

Branching is also part of primary growth. Lateral shoots&mdashor branches&mdashgrow from axillary bud meristems on the plant surface.

Hormonal signals inhibit axillary buds located near active apical meristems this is called apical dominance. Damage or removal of the apical meristem&mdashfor instance, by a feeding animal&mdashdisrupts apical dominance. This is also how pruning encourages growth.

Unlike lateral shoots, lateral roots originate in the pericycle, located near the center of the root, and disrupt outer tissues as they emerge.

Secondary growth is similar in roots and shoots. During secondary growth, the vascular cambium and cork cambium generate the mature tissues. The vascular cambium creates secondary xylem&mdashor wood&mdashand secondary phloem. The cork cambium originates from the pericycle in roots and in the cortex in shoots, and produces cork cells and phelloderm.

Primary and secondary growth contribute to the remarkable adaptability of plants, allowing them to thrive in changing environments despite their immobility.

34.8: Primary and Secondary Growth in Roots and Shoots

Vascular plants, which account for over 90% of the Earth&rsquos vegetation, all undergo primary growth&mdashwhich lengthens roots and shoots. Many land plants, notably woody plants, also undergo secondary growth&mdashwhich thickens roots and shoots.

Primary and secondary growth can occur simultaneously in a plant. While primary growth occurs in newer plant regions, secondary growth transpires in regions that have completed primary growth. There are overlaps and distinctions between root growth and shoot growth.

Apical meristems enable the primary growth of both roots and shoots - with primary shoot growth beginning in the shoot apical meristem and root primary growth starting in the root apical meristem.

Dividing cells in the root and shoot apical meristems differentiate into the same primary meristems&mdashthe protoderm, ground meristem, and procambium. In both roots and shoots, these primary meristems develop into the same tissue types the protoderm, ground meristem, and procambium respectively develop into dermal, ground, and vascular tissues.

However, there are differences between the specific tissues produced in roots and shoots. In roots, the epidermis contains roots hairs, which account for most of the root&rsquos surface area. Additionally, unlike the shoot ground tissue of eudicots&mdashthe most common flowering plants&mdashroot ground tissue is not divided into pith and cortex. Furthermore, the shoot apical meristem contains leaf primordia, which form leaves.

Compared to primary growth, secondary growth is more similar between roots and shoots. Secondary growth is enabled by two types of lateral meristems, which run along the lengths of roots and shoots.

The vascular cambium, between the primary xylem and primary phloem, creates a layer of secondary xylem (wood) and secondary phloem each year in roots and shoots.

The cork cambium, a component of bark, is located outside the vascular tissues. In both roots and shoots, it produces cork and phelloderm to form the periderm&mdashwhich replaces the epidermis.

Ichihashi, Yasunori, and Hirokazu Tsukaya. 2015. &ldquoBehavior of Leaf Meristems and Their Modification.&rdquo Frontiers in Plant Science 6 (January). [Source]

Pierre-Jerome, Edith, Colleen Drapek, and Philip N. Benfey. 2018. &ldquoRegulation of Division and Differentiation of Plant Stem Cells.&rdquo Annual Review of Cell and Developmental Biology 34 (1): 289&ndash310. [Source]

NCERT Solutions for Class 11 Biology Chapter 6 Anatomy of Flowering Plants

These Solutions are part of NCERT Solutions for Class 11 Biology. Here we have given NCERT Solutions for Class 11 Biology Chapter 6 Anatomy of Flowering Plants.

Question 1.
State the location and function of different types ofmeristems.

Question 2.
Cork cambium forms tissues that form the cork. Do you agree with this statement? Explain.
Sooner or later, another meristematic tissue called cork cambium or phellogen develops, usually in the cortex region. Cork cambium is a couple of layers thick. It is made of narrow, thin-walled, and nearly rectangular cells. Cork cambium cuts off cells on both sides. The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm. The cork is impervious to water due to suberin deposition in the cell wall. The cells of the secondary cortex are parenchymatous. Phellogen, phellem, and phelloderm are collectively known as periderm.

Question 3.
Explain the process of secondary growth in the stems of woody angiosperms with the help of schematic diagrams. What is its significance?

showing secondary growth.
Secondary growth in dicot stem:

  1. It is a “permanent increase in thickness due to the activity of vascular cambium and cork cambium in stellar and extrasolar regions”. In dicot stem intra fascicular cambium is present.
  2. The cells of the medullary ray become meristematic and form interfascicular cambium.
  3. These two cambiums unite and make a complete cambial ring.
  4. The cells of it divide and produce new cells both on its outer and inner sides.
  5. The cells formed on the outer side differentiate into secondary phloem while the cells of the inner side form secondary xylem.
  6. The epidermis is replaced by a secondary protective tissue by an increase in the growth of the stem of the plant. It is made of phellogen (cork cambium).
  7. It arises from the peripheral cells of the cortex. The phellogen forms new cells on the outer side which make phellem (cork) and phelloderm on its inner side also.
  8. Significance: Secondary growth increases the girth or thickness of the plant.
  9. Annual rings of woody angiosperms are very distinct and thus helps in determining the age of the plant.

Question 4.
Draw illustrations to bring out the anatomical difference between
(a) Monocot root and dicot root
(b) Monocot stem and dicot stem

Question 5.
Cut a transverse section of the young stem of a plant from your school garden and observe it under a microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.
After observing the transverse section of the stem we can differentiate that stem is monocot or dicot on the basis of the following characters:

Question 6.
The transverse section of a plant material shows the following anatomical features –

  1. the vascular bundles are conjoint, scattered, and surrounded by a sclerenchymatous bundle sheath,
  2. phloem parenchyma is absent What will you identify it as?

The transverse section of a typical young monocotyledonous stem shows that

  1. The vascular bundles are conjoint, scattered, and surrounded by sclerenchymatous bundle sheaths
  2. Phloem parenchyma is absent, and water containing cavities are present within the vascular bundles.

Question 7.
Why xylem and phloem are called complex tissues?
Xylem and phloem a composed of several types of cells and they work as a unit. Hence they are called complex tissues.

Question 8.
What is the stomatal apparatus? Explain the structure of stomata with a labelled diagram.

  1. Several minute openings or stomata are found on the epidermis of all the green aerial parts of plants but are abundant on the lower surface on the leaves as they regulate the process of transpiration.
  2. A large number of stomata occur on the upper surface of leaves of aquatic plants.
  3. Each stomata is surrounded by two cells known as the guard cells. In the dicotyledons plants these are bean-shaped, but in sedges and grasses these are dumb-bell-shaped.
  4. The guard cell is living. Their outer walls are thin where as the inner ones surrounding the aperture are highly thickened.
  5. Due to this variation in the thickening, the guard cell may become turgid and flaccid, depending upon the supply of water in them, which makes the opening and closing of stomata possible.
  6. Some times a few neighbouring epidermal cells in the vicinity of guard cells become specialized in their shape and size and contents. These are known as subsidiary cells.
  7. The stomatal aperture, guard cells and the surrounding subsidiary cell are together called stomatal apparatus.

Question 9.
Name the three basic tissue systems in the flowering plants. Give the tissue names under each system.
On the basis of their structure and location, there are three types of tissue systems. These are the

  1. Epidermal tissue system,
  2. The ground or fundamental tissue system and
  3. The vascular or conducting tissue system.

1. Epidermal tissue system The epidermal tissue system forms the outer-most covering of the whole plant body and comprises epidermal cells, stomata, and the epidermal appendages the trichomes, and hairs.
2. All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma, and sclerenchyma.
3. The vascular system consists of complex tissues, the phloem, and the xylem. The xylem and phloem together constitute vascular bundles.

Question 10.
How is the plant anatomy useful to us?
‘The study of plant anatomy is useful in many ways. First of all the study helps us understand the way a plant functions carrying out its routine activities like transpiration, photosynthesis, and growth and repair. Second, it helps botanists and agriculture scientists to understand the disease and cure for plants. Plants are important to maintain the ecological balance of the earth, so understanding plant anatomy is a way to understand the large system of the ecology on this planet.

Question 11.
What is periderm? How does periderm formation take place in the dicot stems?
Phellogen, phellem, and phelloderm are collectively known as periderm. Phellogen develops, usually in the cortex region. Phellogen is a couple of layers thick. It is made of narrow, thin-walled, and nearly rectangular cells. Phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm. The cork is impervious to water due to suberin deposition in the cell wall. The cells of the secondary cortex are parenchymatous.

Question 12.
Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.
Dorsiventral (dicotyledonous) leaf: The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll, and vascular system.
Epidermis: The epidermis which covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf has a conspicuous cuticle. The abaxial epidermis generally bears more stomata than the adaxial epidermis. The latter may even lack stomata.

  1. The tissue between the upper and the lower epidermis is called the mesophyll.
  2. It possesses chloroplasts and carries out photosynthesis, is made up of parenchyma.
  3. It has two types of cells – the palisade parenchyma and the spongy parenchyma.
  4. The adaxially placed palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other.
  5. The oval or round and loosely arranged spongy parenchyma is situated below the palisade cells and extends to the lower epidermis.

    6. There are numerous large spaces and air cavities between these cells.

Vascular system:

  • This includes vascular bundles, which can be seen in the veins and the midrib.
  • The size of the vascular bundles is dependent on the size of the veins.
  • The veins vary in thickness in the reticulate venation of the dicot leaves. The vascular bundles are surrounded by a layer of thick-walled bundle sheath cells.


Question 1.
Vascular bundles having cambium are known as.
Open, Vascular bundle

Question 2.
Name the two types of sclerenchyma.
Sclerenchyma fibers and stone cells.

Question 3.
From where do the secondary meristems originate?
Permanent tissue.

Question 4.
What does make the root apical meristem subterminal?
The presence of the root cap makes the root apical meristem subterminal.

Question 5.
Where are companion cells located in flowering plants? What are their functions?
Companion cells are located in phloem cells of vascular tissues, they support the sieve tubes in water conduction.

Question 6.
What is the advantage of lignocellulose in the wall of the xylem?
It provides rigidity, thickness, and resistance

Question 7.
A cross-section of a plant material shows the following features under the microscope: vascular bundles are radially arranged. These are found xylem strands showing exarch condition. What type of plant part of is this?
Dicot root.

Question 8.
Based on position, classify various types of meristems
Apical, intercalary and lateral meristems.

Question 9.
Name the various component cells of xylem. Which of them does not have a nucleus?
Tracheids, vessels, xylem parenchyma andxylem fibres. Only xylem parenchyma have nucleus and living.

Question 10.
Give an example of a secondary meristem.
Examples of secondary meristem are cork cambium and interfascicular cambium.

Question 11.
Name the tissue involved in linear and lateral growth in plants.
Linear growth is caused by apical meristem and lateral growth is caused by lateral meristem.

Question 12.
Heartwood is more durable than springwood. Why?
Heartwood is more durable than spring wood due to its little susceptibility to the attack of pathogens and insects.

Question 13.
Where these present:

  1. Hypodermis layer
  2. Mesophyll tissue
  3. Stomata
  4. Cambium
  1. Hypodermis layer – is found in stems
  2. Mesophyll tissue – in leaves
  3. Stomata – lower epidermis in leaves
  4. CambiumIn vascular bundles which are open


Question 1.
What are the differences between root hairs and stem hairs?
The main difference between stem hairs and root hairs are :

Question 2.
Draw well labelled diagrams of the T.S. of dicotyledonous leaf.

Question 3.
Why is cambium considered to be a lateral meristem?
Cambium is responsible for the increase in the thickness of stems and roots as a result of the addition of secondary tissues (secondary cortex, secondary phloem and secondary xylem). They are located at the lateral position so-known as lateral meristems.

Question 4.
Name the plant part in which the endodermis is absent. Give one basic difference between the endodermis and epidermis.
The endodermis is absent in leaves. Cells of endodermis possess Casparian strips or bands in their radial and transverse walls which are not found in the epidermis.

Question 5.
What are Casparian strips?
These are thickenings of lignin and suberin formed around the lateral walls of the endodermis to prevent plasmolysis.

Question 6.
Which tissue is most abundantly found in plants? Where all is it present in plants?
The tissue most abundantly found in plants is parenchyma. It is found in pith, cortex, and in entire mesophyll of the leaves.

Question 7.
What is present in the phloem of leaves besides sieve elements and is it living or dead? How are these functional & used?
Besides sieve elements, in phloem parenchyma, living cells are present. These store food other cells are phloem fibres that are dead and provide mechanical strength. These are also used in making ropes and coarse textiles.


Question 1.
Describe the structure and functions of xylem tissues in angiosperm plants.

  • Xylem is a complex tissue. It forms a part of the vascular bundle.
  • It is mainly concerned with the conduction of water and minerals. It also provides mechanical support to the plant.
  • As a conducting strand, xylem forms a continuous channel through the roots, stem, leaves and other aerial parts.
  • It consists of four different types of cells—xylem vessels, trachieds, xylem fibres and xylem parenchyma.
  • Xylem vessels and tracheids are concerned with the conduction of water and minerals from roots to aerial parts of the plant.
  • Xylem fibres provide mechanical strength to the plant body. Xylem parenchyma are the only living components of xylem.
  • These are concerned with the storage of food and other vital functions.

Question 2.
What is collenchyma? Explain its structure and function in the plant body of a herbaceous angiosperm.

  • The cells of collenchyma are somewhat elongated with cellulose thickening, found as longitudinal strips.
  • These are usually confined to the comers of the cells.
  • Collenchyma cells appear circular, oval or angular in the transverse section. Internally, each cell possesses a large 4 central vacuole, peripheral cytoplasm and a nucleus.
  • Collenchyma is usually found beneath the epidermis in stem, petiole and leaves of herbaceous dicot plants. It is usually absent in monocot stems and monocot roots.
  • It provides tensile strength and rigidity to the plants due to thickening.
  • Chloroplasts containing collenchyma cells are responsible for photosynthesis.
  • Collenchyma also provides elasticity to the plant organs.
  • Collenchyma are alive and also stores food.

Question 3.
Explain sclerenchyma with a well labelled diagram.
Sclerenchyma is a simple permanent tissue. It consists of two types of cells. They are sclerenchyma fibres and sclereids.
(a) Sclerenchyma fibres –

  • These are much elongated fibers with tapering ends.
  • On ipaturity, they lose their protoplasm and become dead. Their cell wall is made up of cellulose or lignin, or both.
  • Central cavity of the cell is greatly reduced due to the formation of secondary thickening. Sclerenchyma provides mechanical strength to the plants.
  • They help in conduction when present in the secondary xylem.

(b) Sclereids –

  • They develop from ordinary parenchyma cells by the deposition of lignin.
  • These cells are thick-walled and highly lignified and become dead on maturity.
  • They are broader as compared to fibers and their cell lumen is veiy narrow.
  • Sclereids protect the plant from environmental forces like a strong wind.
  • They provide mechanical strength and rigidity to the plant.

Question 4.
Describe the structure of a monocotyledonous leaf.
Anatomy of Monocot/isobilateral leaf: The upper and lower surfaces are covered by a single-layered epidermis.

  • The upper epidermis has some cells larger than the others such large cells are known as bulliform/motor cells.
  • Stomata are found on both upper and lower epidermal layers. The mesophyll is not differentiated into palisade and spongy parenchyma.
  • Mesophyll cells are isodiametric and are arranged compactly they contain a number of chloroplasts. Since monocot leaves have parallel veins, a number of vascular bundles can be seen in a row in the section.
  • Each vascular bundle has sclerenchyma cells (caps) on its upper and lower edges.
  • The xylem is on the upper side and the phloem on the lower side. There is a parenchymatous bundle sheath, which often contains chloroplasts and performs the function of photosynthesis.

Question 5.
Give two examples & salient features of
(1) Simple Tissue
(2) Complex Tissue
(1) Simple Tissue:
(i) Parenchyma
(ii) Collenchyma
(2) Complex Tissue:
(i) Xylem
(ii) Phloem
(1) Simple Tissue:
(i) Parenchyma: These are living, thin-walled cells. It is used for storage of food, induction of substances, provides turgidity to softer parts of the plants
(ii) Collenchyma: These are longer than parenchyma. These are living mechanical tissue, it provides mechanical strength to organs and is present in peripheral position in plants to resist bending my the mind.
(2) Complex Tissue:
(i) Xylem: This is also called Hadrome, which is a water-conducting tissue. It is made up of cells like tracheids, xylem fibers, and xylem parenchyma only xylem parenchyma is living and all others are dead.
(ii) Phloem: This is also called Bast, which is a conducting tissue of food from leaves to all parts of the body. The parts of phloem are sieve elements, companion cells, phloem fibres, and phloem parenchyma. Phloem fibres are dead while parenchyma is living. Together these perform their function.

We hope the NCERT Solutions for Class 11 Biology at Work Chapter 6 Anatomy of Flowering Plants, help you. If you have any query regarding NCERT Solutions for Class 11 Biology at Work Chapter 6 Anatomy of Flowering Plants, drop a comment below and we will get back to you at the earliest.

Secondary Growth in Dicotyledonous Roots |Botany

Let us learn about Secondary Growth in Dicotyledonous Roots. After reading this article you will learn about: 1. Introduction to Secondary Growth 2. Formation of Cambium and Development of Secondary Tissues 3. Periderm.

Introduction to Secondary Growth:

The roots of gymnosperms and most dicotyledonous undergo secondary growth. Most of the dicotyledonous roots show secondary growth in thickness, similar to that of dicotyledonous stems. However, the roots of extant vascular cryptogams and most monocotyledons do not show any secondary growth they remain entirely primary throughout their life.

The secondary tissues developed in the dicotyledonous roots are fundamentally quite similar to that of dicotyledonous stems, but the process initiates in some different manner. Certain dicotyledonous roots do not show secondary growth. The secondary vascular tissues originate as a result of the cambial activity. The phellogen gives rise to the periderm.

Formation of Cambium and Development of Secondary Tissues:

The dicotyledonous roots posses a limited number of radial vascular bundles with exarch xylem. Normally the pith is very little or altogether absent. On the initiation of secondary growth, a few parenchyma cells beneath each group of phloem become meristematic and thus as many cambial strips are formed as the number of phloem groups.

The cambial cells divide tangentially again and again and produce secondary tissues. Thereafter some of the cells of single layered pericycle become meristematic lying against the protoxylem groups, which divide and form a few layers of cells. The first formed cambium now extends towards both of its edges and reaches the inner most derivatives of the pericycle, thus giving rise to a complete ring of cambium.

The cambium ring is wavy in outline, as it passes internal to phloem and external to xylem groups. The cambial cells produce more xylem elements than phloem. The first formed cambium produces secondary xylem much earlier, and the wavy cambium ring ultimately becomes circular.

Now whole of the cambium ring becomes actively meristematic, and behaves in the similar way as in the stem, giving rise to secondary xylem on its inner side and secondary phloem towards outside.

The secondary vascular tissues form a continuous cylinder and usually the primary xylem gets embedded in it. At this stage distinction can be made only by exarch primary xylem located in the centre. The primary phloem elements are generally seen in crushed condition.

The cambial cells that originate from the pericycle lying against the groups of protoxylem function as ray initials and produce broad vascular rays. These rays are traversed in the xylem and phloem through cambium this is characteristic feature of the roots. Normally, such rays are called medullary rays.


Simultaneously the periderm develops in the outer region of the root. The single layered pericycle becomes meristematic and divides, giving rise to cork cambium or phellogen. It produces a few brownish layers of cork cells or phellem towards outside, and the pheliodorm on the inside.

The phelloderm does not contain chloroplasts. The pressure caused by secondary tissues ruptures the cortex with endodermis, which is ultimately sloughed off. The epiblema dies out earlier. Lenticels may also be formed.

Atlas of plant and animal histology

S econdary growth leads to thicker roots and causes primary tissues like epidermis, hypodermis, cortex, and endodermis to be lost. Secondary roots develop a suberized cortical layer that prevents the entry of water, although it can enter through lenticels when they are present.

O nly the largest main and lateral roots of dicot plants and gymnosperm show a typical secondary growth. Secondary growth begins when the procambium meristem, between the xylem and phloem, becomes the vascular cambium meristem. Depending on the number of phloem bundles, there is initially formed a variable number of segments of vascular cambium (Figure 1). At the same time, the portions of the pericycle close to the xylem spoke poles divide periclinaly and the cells that locate inner become vascular cambium meristem too. Later, the vascular cambium originated between xylem and phloem and that originated from the pericycle are connected to form a continuous structure: the root vascular cambium, which is a cylinder extending along the mayor axis of the root. Short after the continuous vascular cambium is formed, it produces secondary phloem toward the outer part and secondary xylem toward the inner part. The new layers of secondary xylem push the vascular cambium toward the surface of the root. In this way the perimeter of vascular cambium increases in length and xylem progressively accumulates so that the root increases in thickness.

Figure 1. Root. From primary to secondary growth.

I n this way, the secondary root gets organized similarly to the secondary stem. Indeed, there is a continuity of the vascular bundles, as well as the vascular cambium, between the root and the stem. Unlike the primary growth, there is no transition zone between secondary stem and secondary root. The higher proportion of xylem and less delimited growth rings in the roots make possible to distinguish the root from the stem. In both, root and stem, the vascular cambium is composed of two types of cells: fusiform initial cells and radial initial cells. Fusiform initial cells differentiate in axially (vertical) oriented cells, whereas the radial initial cells give horizontally oriented cells.

T he periderm is the outer structure of the secondary root. Periderm is derived from the phellogen meristem (cork cambium), after the formation of the secondary vascular tissue has begun.

F rom the surface to the inner part, the following structures can be distinguished in a secondary root:

E pidermis/cortex/periderm. The secondary growth of the root may be more or less advanced. At the beginning of the secondary growth, roots show epidermis and cortex with parenchyma cells (cortical parenchyma). In more advanced secondary growth, the epidermis and cortex is replaced by the periderm, which is a protective layer derived from the cock cambium (phellogen), a lateral meristem differentiated from the pericycle.

S econdary phloem. It is differentiated from the vascular cambium meristem, and it is laid toward the outer part. The formation of the secondary phloem lets to the separation between the primary phloem and the vascular cambium.

V ascular cambium. It is a lateral meristem responsible for the growing in thickness of the root. It produces secondary phloem toward the outer part and secondary xylem toward the inner part. As the root is getting thicker, the vascular cambium is increasing in size and moves away from the center of the root.

S econdary xylem. It is produced by the vascular cambium. It forms the wood of the root, and it is dead tissue in the thicker roots . The most recent xylem is the most superficial.

P rimary xylem. It is generated during the primary growth and is found in the inner part of the root. It is death tissue in secondary roots.

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I. Select the correct answer from the following questions:

Question 1.
Intercalary meristem is derived from
(a) Apical meristem
(b) Protoderm
(c) Calyptrogen
(d) Lateral meristem

Question 2.
Secondary meristem develops from
(a) Apical meristem
(b) Permanent tissue
(c) Secondary tissue
(d) Vascular combium

Answer: (b) Permanent tissue

Question 3.
Cambium is considered to be a lateral meristem because it
(a) Gives rise to lateral branches
(b) Increase the girth of the plant
(c) Increase both length and girth of plant
(d) Increase the length of the plant.

Answer: (b) Increase the girth of the plant

Question 4.
Quiscent centre is located in
(a) Shoot apex
(b) Root apex
(c) Bud apex
(d) Leaf apex

Question 5.
Casparian strips occur in the cells of
(a) Exodermis
(b) Epiderms
(c) Hypodermis
(d) Endoderms

Question 6.
Lignified cells with narrow and pointed end wall are
(a) Chlorenchyma
(b) Parenchyma
(c) Sclerenchyma
(d) Endoderms

Question 7.
Nucleus is absent in
(a) Vessels
(b) Sieve tube elements
(c) Tracheid
(d) All of these

Question 8.
Age of a tree is calculated by its
(a) Girth
(b) Height
(c) Number of annual rings
(d) Number of branches

Answer: (c) Number of annual rings.

Question 9.
Youngest secondary xylem occurs
(a) Just outside the vascular cambium
(b) Just inside the vascular cambium
(c) Just outside the vascular cambium
(d) Just inside the cork cambium

Answer: (b) Just inside the vascular cambium.

Question 10.
Mesophll cells in a leaf are
(a) Sclerenchymatous
(b) Collenehymatous
(c) Parenchymatous
(d) Meristem

Question 11.
Healing of wounds occur due to the activity of
(a) Intercalary meristem
(b) Secondary meristem
(c) Primary meristem
(d) Apical meristem

Answer: (b) Secondary meristem

Question 12.
Lateral root arise from
(a) Cambium
(b) Pericycle
(c) Epidermis
(d) Endodermis

Question 13.
Vascular bundles are absent in
(a) Dicots
(b) Monocots
(c) Cambium
(d) Pteridophytes

Question 14.
Which one contain only living cells?
(a) Vessels
(b) Sclerenchyma
(c) Trachieds
(d) Parenchyma

Question 15.
Vascular bundle having cambium is
(a) Closed
(b) Open
(c) Colleral
(d) Conjoint

Question 16.
Lignified cell well occurs in
(a) Xylem cells
(b) Epidermal cells
(c) Cambial cells
(d) Phloem cells

Question 17.
Bordered pits are more common in
(a) Gymmosperms
(b) Monocots
(c) Dicots
(d) All of these

Question 18.
Both apical meristems and intercalary meristem are also called
(a) Intercalary meristems
(b) Lateral meristems
(c) Primary meristems
(d) Meristems

Answer: (c) Primary meristems

Question 19.
Complex tissues also provides mechanical strength to the plant parts. It is composed elements
(a) Trachieds
(b) Vessels
(c) Xylem fibres
(d) Xylem parenchyma
(e) All of these

Question 20.
The first formed xylem elements are called
(a) Metaxylem
(b) Endarch
(c) Protoxylem
(d) Exarch

Question 1.
The plant is made up of Cells which are organised into ………… and the tissues into …………..

Question 2.
Plants have different kinds of ………..

Question 3.
Fascicular vascular cambium, interfascicular cambium and cork- cambium are examples of ………… meristems.

Answer: parenchyma, collenchyma, sclerenchyma

Answer: cortex., pit, mesophyll, leaves, floral

Answer: Collenchyma, hypodermis

Question 7.
…………. consists of long, narrow cells with thick and lignified i cell walls having a few or numerous pits.

Question 8.
………….. is cbmposed of four different kinds of elements, namely, trachieds, vessels, xylem fibres and xylem paranchyma.

Question 9.
…………… are long, tube-like structures, arranged longitudinally and are associated with the companion cells.

Answer: Sieve tube elements

Question 10.
Each stoma is composed of two bean-shaped cells known as the …………….

Question 11.
The cells of epidermis bear a number of hair known as …………..

Question 13.
The …………. consists of several layers of thin-walled parenchymata with intercellular spaces.

Answer: vascular, conjoint, collateral endarch, open

Question 15.
When the ………….. cells in the leaves are turgid, the leaf surface is exposed.

III. Mark the statements True (T) or False (F)

Question 1.
The spring wood is lighter in colour and has a low density whereas the autumn wood is darker and has a higher density.

Question 2.
The wood formed during spring season is called autumn wood or late wood.

Question 3.
Phellogen, phellem, and phelloderm are collectively known as lenticel.

Question 4.
The peripheral region of the secondary xylem, is lighter in colour and is known as the sapwood.

Question 5.
Secondary growth also occur in stems and roots of gymnosperms. However, secondary growth does not occur in monocotyledons.

Question 6.
All the dead cells lying outside the active cork cambium constitute the bark.

Question 7.
Lenticels permit the exchange of gases between the outer at-mosphere and the internal tissue of the stem.

Question 8.
The two kinds of woods appear as alternate concentric rings, constituting an annual ring.

Question 9.
Vascular system includes vascular bundles, which can be seen in the veins.

Question 10.
When xylem and phloem within a vascular bundle are arranged in an alternate manner on different radii as in the roots are called radial as in the roots.

IV. Match the items or column I with the items of column II

Column I Column II
(i) Heart wood (a) spring wood
(ii) Spring season (b) innermost layer of the cortex
(iii) Secondary growth (c) unicellular
(iv) Isobilateral leaf (d) tapering cylindrical cells
(v) Endodermis (e) comprises dead elements with highly lingnifled walls.
(vi) Root hairs (f) thick-walled, elongated and pointed cells.
(vii) Phloem paranchyma (g) two lateral mertstems vascular cambium and cork cambium.
(viii) Primary xylem (h) they are mostly dead and without protoplasts
(ix) Xylem parenchymatous (i) bulliform cells.
(x) Sclereids (j) walls are made up of cellulose
(xi) Collenchyma (k) hypodermis below the epidermis.
(xii) Parenchymatous (l) cortex, pith, mesophyll of leaves, floral parts.
(xiii) Fibres (m) protoxylem and metaxylem
(xiv) Sclerenchyma (n) spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities.

Column I Column II
(i) Heart wood (e) comprises dead elements with highly lingnifled walls.
(ii) Spring season (a) spring wood
(iii) Secondary growth (g) two lateral mertstems vascular cambium and cork cambium.
(iv) Isobilateral leaf (i) bulliform cells.
(v) Endodermis (b) innermost layer of the cortex
(vi) Root hairs (c) unicellular
(vii) Phloem paranchyma (d) tapering cylindrical cells
(viii) Primary xylem (m) protoxylem and metaxylem
(ix) Xylem parenchymatous (j) walls are made up of cellulose
(x) Sclereids (n) spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities.
(xi) Collenchyma (k) hypodermis below the epidermis.
(xii) Parenchymatous (l) cortex, pith, mesophyll of leaves, floral parts.
(xiii) Fibres (f) thick-walled, elongated and pointed cells.
(xiv) Sclerenchyma (h) they are mostly dead and without protoplasts

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