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Plant Tissues: Types, Structure and Function, Practice Problems and FAQs

Plant Tissues: Types, Structure and Function, Practice Problems and FAQs

The oldest organisms were single celled. The single celled organisms evolved about 3.5 billion years ago. According to our scientists the oldest ancestor of all the organisms is LUCA (last universal common ancestor). But LUCA is not the first life on earth. So it is believed that the archaebacteria, eubacteria and eukaryotes share this common ancestor called LUCA. Now it is clear how eukaryotic cells have some similarities with the prokaryotic cells. According to Darwin, LUCA is the first organism who took a breath on the planet. It is believed that LUCA was a single celled organism which used RNA as genetic material and also for catalysing the reactions inside the cell. Later all the organisms evolved from this ancestor with higher levels of adaptations, so that their body can perform particular functions.

In simple terms we can say that multicellular organisms originated from unicellular organisms. In the case of plants, the unicellular algae later developed into multicellular algae. From those multicellular algae all the non vascular plants and then the vascular plants evolved. While transforming their habitat from water to land, plants have adapted many characters in their cells too such that they can perform different functions for their survival.

Have you ever observed a section of a plant under a microscope? The stained plant cells are very beautiful to observe. Do you think all the cells inside the plant are similar? No, we can observe many types of cells in a single cross section. Right? The types of cells change even in the different parts of the same plant. Do you know why? It is because different parts of the plant do different functions. So all types of cells present in the plant should be different. Those groups of cells which are similar are known as tissues. Here we are going to discuss more about different types of tissues, their structure and functions.

Table of contents:

  • Tissues
  • Meristematic tissues
  • Classification of meristematic tissues
  • Permanent tissues
  • Classification of permanent tissues
  • Practice Problems
  • FAQs

Tissues

Cells are the basic unit of life and they are the building blocks of all the living organisms. A group of similar cells which have the same origin and perform specific functions are called tissues.

Types of plant tissues

Growth of organisms occurs due to cell division and cell elongation. On the basis of the capacity to divide, the plant tissues can be classified into two major groups. They are:

  • Meristematic tissues
  • Permanent tissues

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Fig: Classification of plant tissues

Meristematic tissues

The regions of plants where active cell division takes place are called meristematic regions or meristems. Growth of a plant is restricted to the meristematic regions. The tissues of meristematic regions are called meristematic tissues. They are a group of actively dividing and immature cells with thin cell walls, large nucleus, dense cytoplasm and no intercellular space.

Fig: Meristematic tissue

Classification of meristematic tissues

Meristematic tissues can be classified according to their origin and their position.

Classification based on origin

On the basis of origin, meristematic tissues are classified into three types. These are:

  • Promeristems
  • Primary meristem
  • Secondary meristem

Pro meristems or primordial meristems or embryonic meristems

The meristematic tissues present in the germinating embryos or young seedlings are called pro meristems.

Fig: Promeristem

Primary meristems

Primary meristem originates from pro meristem. The tissue which has the capacity to divide throughout the whole life of plants are called primary meristems. Primary meristems are responsible for the primary growth of the plants. Examples are apical meristem, intercalary meristem and intrafascicular cambium present within vascular bundles.

Fig: Primary meristem

Secondary meristems

The dedifferentiation of permanent tissues results in the formation of secondary meristems. It is responsible for the secondary growth of the plants. Examples are interfascicular cambium, vascular cambium, cork cambium etc.

Fig: Secondary growth

Classification based on position

Meristematic tissues can be classified into three types based on their position in the plants. These are:

  • Apical meristems
  • Intercalary meristems
  • Lateral meristems

Apical meristems

Those meristems which can be observed at the apex or tip of the shoots and roots are called apical meristems. The meristem present at the shoot apex is called shoot apical meristem. The meristem present at the root apex is called root apical meristem. Since apical meristems are present at the apex, they help to increase the length of the plants. They are formed from promeristems.

Fig: Apical meristems

Intercalary meristems

Meristematic tissues present between the mature tissues which help in the elongation of plant organs are called intercalary meristems. They are the primary meristems which are present at the base of the leaves (below or above the nodes).

Fig: Intercalary meristems

Lateral meristems

The meristems found at the mature regions of the shoot and root are called lateral meristems.

They are called secondary meristems, since they are formed after primary meristems. So they are responsible for the increase in the girth of the plant. Intrafascicular cambium, interfascicular cambium, cork cambium, wound cambium etc are the examples of lateral meristems. They are also called cylindrical meristems, because of their shape.

Fig: Lateral meristems

Permanent tissues

Now that we know about the meristematic tissues, which are constantly dividing, let us focus on the cells which arise due to division of meristematic tissues and then lose their ability to divide. These cells become specialised to perform specific functions and the tissues formed by them are called permanent tissues.

Classification of permanent tissues

There are two types of permanent tissues on the basis of the cells present in the tissues. They are:

  • Simple permanent tissues
  • Complex permanent tissues

Fig: Types of permanent tissues

Simple permanent tissues

If the permanent tissues are made up of similar types of cells, then they are called simple permanent tissues. All cells of simple permanent tissue have the same structure and function.

There are three types of simple permanent tissues. They are:

  • Parenchyma
  • Collenchyma
  • Sclerenchyma

Fig: Types of simple tissues

Parenchyma

The most common and abundant tissue in a plant body is parenchyma. They are living tissues. The cells are isodiametric and the shapes of parenchyma cells can be spherical, oval, round, polygonal or elongated. They have thin cell walls with small intercellular spaces. Cell walls are made up of cellulose. The major functions of parenchyma cells are photosynthesis, storage of food and conduction of materials.

Fig: Parenchyma

Types of parenchyma cells

There are different types of parenchyma cells based on their functions. They are:

  • Chlorenchyma
  • Aerenchyma
  • Prosenchyma
Chlorenchyma

Parenchyma cells with chloroplast which perform photosynthesis are called chlorenchyma cells. Leaf mesophyll tissue is an example of chlorenchyma.

Fig: Chlorenchyma

Aerenchyma

If the parenchyma tissues are composed of gas conducting intercellular spaces, then it is called aerenchyma. The major functions of aerenchyma tissues are to provide oxygen under emergency situations, provide buoyancy to hydrophytes (Eg: water lily) and also to facilitate the movement of gases.

Fig: Aerenchyma

Prosenchyma

The specialised fibre-like and elongated parenchyma tissues are called prosenchyma. Their function is to provide strength and rigidity.

Collenchyma

The simple living tissues with thickened corners are called collenchyma. The thickening is due to the deposition of cellulose, hemicellulose and pectin. So they provide mechanical support to the plant. They can be seen in oval, spherical and polygonal shapes. They have chloroplast and help in photosynthesis.

Fig: Collenchyma

Sclerenchyma

The long and narrow simple tissues are called sclerenchyma. They are devoid of protoplast, hence they are dead cells with thick lignin depositions on their walls. They help the plants during bending and to resist compression and shearing forces.

Fig: Sclerenchyma

Types of sclerenchyma cells

Sclerenchyma cells can be classified into two on the basis of their structure, origin and development. They are:

  • Fibres
  • Sclereids
Fibres

The thick walled, elongated sclerenchyma cells are called fibres. These cells have pointed ends and they will be interlocked with adjacent fibres.

Fig: Fibre

Sclereids

The short and thick walled, spherical or cylindrical sclerenchyma cells are called sclereids. They can be irregularly shaped too. They have narrow cavities or lumen. They are usually seen in the walls of fruits like nuts, pear and sapota.

Fig: Sclereid

There are different types of sclereids on the basis of their size and shape. They are:

  • Brachysclereids
  • Macrosclereids
  • Osteosclereids
  • Astrosclereids
  • Filiform sclereids
  • Trichosclereids

Complex permanent tissues

The tissues which are made up of more than one type of cells are called complex permanent tissues. All the cells in a complex tissue function together as a unit. The complex permanent tissues in plants are of two types. They are:

  • Xylem
  • Phloem

Both these tissues are called the vascular tissue or conductive tissue, because the major function of them is to transport the water, minerals and food.

Xylem

It consists of both living and non living cells which helps in the transport of sap (water and minerals) from root to different parts of the plant. They can also provide mechanical support to the plants. Xylem is composed of four different components like:

  • Tracheids
  • Vessels
  • Xylem parenchyma
  • Fibres

All the components of xylem are dead at maturity except xylem parenchyma.

Fig: Components of xylem

Tracheids

The elongated, tube-like xylem cells with tapering ends are called tracheids. They are dead cells, because they are devoid of protoplasm. Since it is a component of xylem, the major function is to transport sap. Based on the thickening of tracheids they are classified as annular, spiral, reticulate, scalariform and pitted.

Fig: Tracheids

Vessels

The long and cylindrical, tube-like cells of xylem are called vessels. They also have lignified walls. They are made up of vessel members with perforated walls and their function is to transport sap.

Fig: Vessels

Xylem parenchyma

The living cells of xylem with thin walled cells made up of cellulose are called xylem parenchyma. Their function is to store food materials (starch, fat or tannins). The radial conduction of water is also done by xylem parenchyma cells.

Fig: Parenchyma cells

Xylem fibres

The sclerenchymatous and highly thick walled components of xylem are called xylem fibres. They have an obliterated central lumen. They provide mechanical support. They can be septate or aseptate.

Fig: Xylem fibre

Types of xylem

There are different types of xylem tissues and they can be divided into primary and secondary xylem on the basis of their origin. Xylem can be divided into endarch, exarch and mesarch on the basis of their arrangement.

Based on origin
  • Primary xylem: It is formed from the procambium of apical meristem during the primary growth of a plant. The first formed primary xylem is called protoxylem and later formed primary xylem is called metaxylem.
  • Secondary xylem: It is formed from the vascular cambium during secondary meristem.
Based on the arrangement
  • Endarch: Protoxylem towards the centre and metaxylem towards the periphery. Eg: stems.
  • Exarch: Protoxylem towards the periphery and metaxylem towards the centre. Eg: roots.
  • Mesarch: Protoxylem in the middle, surrounded by metaxylem. Eg: ferns.

Phloem

The complex permanent tissue which helps in the transport of food materials from the leaves to several other plant parts is called phloem. Phloem components of gymnosperms are different from angiosperms.

Phloem components in angiosperms

There are major four components of phloem in the angiosperms. They are:

  • Sieve tube elements
  • Companion cells
  • Phloem parenchyma
  • Phloem fibres

All the phloem components except phloem fibre are living.

Fig: Phloem components of angiosperms

Sieve tube elements

The long, longitudinally placed tube-like structures which are associated with the companion cells are called sieve tube elements. Large pores are present on the end walls or sieve plates of sieve tubes and it is known as sieve pits. Peripheral cytoplasm and large vacuoles can be observed on a mature sieve tube and they lack nuclei.

Fig: Sieve tubes

Companion cells

The specialised parenchyma cells connected to sieve tube elements are called companion cells. They are non-conducting cells of phloem tissues. The longitudinal walls of companion cells and sieve tube elements are connected through sieve pits. Pits are abundant with plasmodesmata, which is the cytoplasmic bridge which connects the two adjacent cells. They can maintain the pressure gradient in the sieve tubes.

Fig: Companion cell

Phloem parenchyma

The elongated and cylindrical cells in phloem with tapering ends are the phloem parenchyma cells. They have a nucleus and dense cytoplasm. They have walls made up of cellulose which have pits. Phloem parenchyma is the cell which stores food materials (resins, latex and mucilage) in phloem. They cannot be observed in most monocots.

Fig: Phloem parenchyma

Phloem fibres

The sclerenchymatous phloem cells are called phloem fibres or bast fibres. They are elongated and unbranched cells with needle-like apices. Their function is to provide mechanical strength to the plants since they have thick walls. At maturity phloem fibres lose protoplasm and they become dead cells.

Fig: Phloem fibre

Phloem components in gymnosperms

The phloem of gymnosperms lack companion cells and sieve tubes. Then the components of phloem in a gymnosperm includes:

  • Albuminous cells (specialised parenchyma cells found in association with sieve cells)
  • Sieve cells
  • Phloem parenchyma
  • Phloem fibres

Types of phloem

There are two types of phloem based on their origin. They are primary and secondary phloem.

  • Primary phloem: The phloem formed from apical meristem. On the basis of the period of their formation there are two types of primary phloem.
    • Protophloem: First formed phloem.
    • Metaphloem: Later formed phloem.
  • Secondary phloem: The phloem formed from vascular cambium.

Fig: Types of phloem

Practice Problems

1. Which of the following is incorrect about the meristematic tissue?

I) Meristematic tissues are a group of similar and immature cells.

II) The meristematic tissues present in the germinating embryos or young seedlings are called primary meristems.

III) Interfascicular cambium, vascular cambium, cork cambium etc. are secondary meristems.

IV) Lateral meristems are responsible for the increase in the girth of the plant.

  1. I
  2. II
  3. III
  4. IV

Solution: The regions of plants where active cell division takes place are called meristematic regions or meristems. The tissues of meristematic regions are called meristematic tissues. They are a group of similar and immature cells. On the basis of origin, meristematic tissues are classified into three types. They are promeristems, primary meristems, and secondary meristems. The meristematic tissues present in the germinating embryos or young seedlings are called pro meristems, not primary meristems. Hence statement II is incorrect. The dedifferentiation of permanent tissues results in the formation of secondary meristems. It is responsible for the secondary growth of the plants. Examples are interfascicular cambium, vascular cambium, cork cambium etc.

Hence the correct option is b.

2. Assertion: Collenchyma cells help the plants to store food, photosynthesise or provide buoyancy to aquatic plants.

Reason: Corners of collenchyma are composed of cellulose, hemicellulose and pectin.

Which of the following is correct about the assertion and reason given above?

  1. Both assertion and reason are true and reason is the correct explanation of assertion.
  2. Both assertion and reason are true and reason is not the correct explanation of assertion.
  3. The assertion is true and the reason is false.
  4. The assertion is false and the reason is true.

Solution: If the permanent tissues are made up of similar types of cells, then it is called simple permanent tissue. All cells of simple permanent tissue have the same structure and function.

There are three types of simple permanent tissues. They are parenchyma, collenchyma and sclerenchyma. The simple living tissues with thickened corners are called collenchyma. The thickening is due to the deposition of cellulose, hemicellulose and pectin. So they provide mechanical support to the plant. Hence the assertion is false but the reason is true. The most common and abundant tissue in a plant body is parenchyma. They are living tissues which have cells with thin cell walls and small intercellular spaces. The major functions of parenchyma cells are photosynthesis, storage of food, and conduction of materials. Specialised parenchyma tissues known as aerenchyma are composed of gas conducting intercellular spaces and help to provide buoyancy to aquatic plants such as water lilies.

Hence the correct option is d.

3. Which is the incorrect match in the following?

  1. Living cells: Parenchyma, collenchyma, xylem vessels, companion cells
  2. Non living cells: Sclerenchyma, xylem tracheids, phloem fibres
  1. A
  2. B
  3. Both A and B
  4. Both A and B are correct

Solution: The permanent tissues made up of similar types of cells are called simple permanent tissues. The three types of simple permanent tissue are parenchyma, collenchyma and sclerenchyma. The most common tissue in a plant body is parenchyma. They are living tissues. The simple living tissues with thickened corners are called collenchyma. The long and narrow simple tissues are called sclerenchyma. They are devoid of protoplast, hence they are dead cells with thick lignin depositions on their walls. Xylem consists of both living and non living cells which helps in the transport of sap from root to different parts of the plant. It is composed of four different components: tracheids, vessels, xylem parenchyma and fibres. All the components of xylem are dead at maturity except xylem parenchyma. Hence xylem tracheids, xylem vessels and xylem fibres are dead. So here A is not correct. The complex permanent tissue which helps in the transport of food materials from the leaves to several other plant parts is called phloem. The four components of phloem in the angiosperms are sieve tube elements, companion cells, phloem parenchyma and phloem fibres. All the phloem components except phloem fibre are living. Hence sieve tube, companion cells and phloem parenchyma are living.

Hence the correct option is a.

4. What is the difference between angiosperms and gymnosperms with respect to their phloem components?

Answer: The complex permanent tissue which helps in the transport of food materials from the leaves to several other plant parts is called phloem. Phloem components of gymnosperms are different from angiosperms. There are major four components of phloem in the angiosperms. They are:

  • Sieve tube elements
  • Companion cells
  • Phloem parenchyma
  • Phloem fibres

The phloem of gymnosperms lack companion cells. They have albuminous cells which are the modified parenchyma cells. Then the components of phloem in a gymnosperm includes:

  • Albuminous cells
  • Sieve cells
  • Phloem parenchyma
  • Phloem fibres

FAQs

1. What is the difference between pit and plasmodesmata?

Answer: The thin regions of the plant cell wall which facilitate the exchange and communication of substances with the adjacent cells are called pits. They are composed of microscopic intercellular bridges that connect the cytoplasm of adjacent cells and are known as plasmodesmata. Plasmodesmata also helps to transport the minerals from one cell to another.

2. Which are the commercially important phloem components?

Answer: Phloem fibres or bast is a common source of commercial fibres. The commercially important phloem fibres are obtained from jute plant, flax and hemp. The jute plant is called Corchorus capsularis from which we extract the jute. Flax is a flowering plant called Linum usitatissimum. It is used in the form of oil and ground seed. Hemp is also known as industrial hemp and the scientific name is Cannabis sativa. It is used in the industrial and medical field.

3. What type of tissues are present in a coconut husk?

Answer: Coconut husk is made up of the permanent tissue called sclerenchyma. The dead cells and thickened walls of the sclerenchymatous tissue make the husk hard and stiff. The thickening is due to the presence of lignin.

4. Who all discovered parenchyma, sclerenchyma and collenchyma?

Answer: Robert Hooke discovered parenchyma in the early 17th century. Sclerenchyma cells were discovered by Mettenius and he is the one who coined the name too. Schleiden discovered the collenchyma.

YOUTUBE LINK: https://www.youtube.com/watch?v=ISN_MHEe4Yc

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