Permanent Tissues: Xylem, its Components and its Types

Do you know how blood is flowing through our body?

Yes, you are right . It passes through veins and arteries.

What is the role of blood in our body?

Of course, the transportation of materials like carbon dioxide, oxygen, minerals etc.

Just like the blood flows in our body, plants also need a similar system to transport the food and water. Complex permanent tissues are meant for that. Just like the name indicates, complex tissues are made up of more than one type of cells but together function as a unit.

Two types of complex tissues can be seen in plants. They constitute the vascular or conductive tissue. It includes both Xylem and Phloem. 

Let's discuss more about xylem tissues.

Table of Contents

• Xylem
• Components of xylem
• Types of xylem
• Types of arrangement of xylem
• Practice Problems
• FAQs

Xylem

Xylem is the complex conducting tissue which helps in the transport of sap from root to stem and leaves. It consists of living and non-living cells. Xylem can provide mechanical support to the plants. It is also called wood. It forms a continuous channel through roots, stems and leaves. 

Components of Xylem

It is composed of four kinds of elements as follows:

• Tracheids
• Vessels
• Fibres
• Xylem parenchyma

Tracheids

Tracheids are elongated or tube-like cells. They have thick and lignified walls. They possess tapering ends. Cells are dead without protoplasm. It is one of the main transporting elements in xylem.

Types of Tracheids

Tracheids are of different types based on thickening as follows:

• Annular - The thickening occurs in the form of rings.
• Spiral - Thickening occurs in the form of spiral.
• Reticulate - Thickening occurs in the form of a network.
• Scalariform - Thickening occurs in the form of ladder-like appearance.
• Pitted - Small unthickened areas called pits present. 

Vessels

Vessels are long and cylindrical cells. They are tube-like structures. They form one of the transporting elements of xylem. Vessels are made up of vessel members. They have lignified walls. Vessel members are joined end to end and have perforated walls. The perforated end walls are called perforation plates. Xylem vessels are absent in gymnosperms but present in angiosperms.

Xylem fibres

Xylem fibres are sclerenchymatous. Their walls are highly thick. They possess obliterated central lumens. They provide mechanical support. They are septate or aseptate.

Xylem Parenchyma

Xylem parenchyma are living cells. Walls are thin and made up of cellulose. They store food materials like starch, fat or tannins. Xylem parenchyma helps in the radial conduction of water . Examples include ray parenchyma cells. 

Types of Xylem

Based on origin

Xylem can be divided into two types according to their origin as Primary xylem and Secondary xylem.

Primary Xylem

Primary xylem is the xylem formed during the primary growth. It is formed from the procambium of the apical meristem. 

Types of Primary Xylem

Primary xylem can be divided into two types according to their period of formation and internal structure. They are as follows:

• Protoxylem
• Metaxylem

Differences between Protoxylem and Metaxylem

Secondary xylem

It is the xylem formed during the secondary growth from the vascular cambium of the lateral meristem.

Types of Arrangement of Xylem

According to the arrangement of protoxylem in relation with metaxylem, there are three types of xylem.

• Endarch - In this type of xylem arrangement protoxylem occurs towards the inner side of the metaxylem. Examples include stem.

• Exarch - In this type of arrangement the protoxylem lies towards the outside of the metaxylem. Examples include roots.

• Mesarch: In this type of arrangement, protoxylem is present in the middle of the metaxylem. Examples include ferns.

Practice Problems of Xylem

Q1. Find out which is the mismatched pair.

• Tracheids - These are dead and without protoplast
• Xylem parenchyma - Here the food materials are stored in the form of resin, mucilage and latex
• Vessels - These are devoid of protoplasm.
• Xylem fibres - Walls are highly thickened and obliterated central lumens.

Solution : Xylem parenchyma cells are living cells with a thin cellulosic cell wall. These cells store the food materials like sugars, fats, and water during their conduction. Hence the correct option is b.

Fill in the blanks with correct option:

Q2. In _____1___ the protoxylem lies towards the periphery and the metaxylem lies towards the centre. Such arrangement of the primary xylem is called _____2_____.

• 1 - roots, 2 - endarch
• 1 - roots, 2 - exarch
• 1 - stems, 2 - endarch
• 1 - stems,2 - exarch

Solution: In roots, the matured, first formed xylem lies at the periphery whereas the youngest cells of the xylem lie toward the centre, called exarch. Hence the correct option is b.

Q3. Which one among the following is correct?

• Tracheids are unicellular with a wide lumen
• Tracheids are multicellular with narrow lumen
• Vessels are unicellular with narrow lumen
• Vessels are multicellular with a wide lumen

Solution: Vessels are multicellular, long cylindrical tube-like structures. Vessels have highly lignified walls with pits and a large central cavity. Hence the correct option is d.

Q4. The root of the fairly old dicotyledonous plant is anatomically distinguished from its stem by :

• absence of secondary xylem
• absence of secondary phloem
• position of protoxylem
• presence of cortex

Solution: In stem, the protoxylem is placed towards the centre. This is known as an endarch condition. The protoxylem is arranged towards the periphery normally in roots. This condition is known as the exarch condition. Hence the correct option is c.

FAQs of Xylem

Question1.- What is a complex tissue?

 Anstion. Complex tissues are made up of more than one type of cells that function together. Examples of complex tissue are xylem and phloem. In plants complex tissues are meant for the transportation of food, water and minerals. Xylem is the complex conducting tissue which helps in the transport of sap from root to stem and leaves. Phloem is for the transport of food from leaves to all parts of the plant.

Question2.- What are the types of xylem based on the origin?

Anstion. Xylem can be divided into two types according to their origin as primary xylem and secondary xylem. The xylem formed during the primary growth normally is called the primary xylem. It is formed from the procambium of the apical meristem. Primary xylem can be divided into two types according to their period of formation and internal structure. And they are protoxylem and metaxylem. The xylem formed during the secondary growth from the vascular cambium of the lateral meristem is called secondary xylem.

Question3.- What is the difference between endarch, exarch and mesarch xylem?

Anstion. The endarch, exarch and mesarch xylem are classified according to the arrangement of protoxylem in relation with metaxylem. Endarch xylem is the type of xylem arrangement where protoxylem occurs towards the inner side of the metaxylem. Examples include stem. Exarch xylem is the type of arrangement the protoxylem lies towards the outside of the metaxylem. Examples include roots. Mesarch xylem is the type of arrangement where protoxylem is present in the middle of the metaxylem. Examples include ferns. 

Question4.- What are the different components of xylem?

Anstion. Xylem is composed of four kinds of elements. They are tracheids, vessels, fibres, xylem parenchyma. The elongated or tube-like cells with tapering ends are the tracheids. Vessels are long and cylindrical cells. Vessels are made up of vessel members and are joined end to end and have perforated walls. Xylem fibres are sclerenchymatous with obliterated central lumens. Xylem parenchyma are living cells that help in the radial conduction of water.

Related Concepts to Xylem in Biology

• Phloem
• Permenant Tissues

NCERT Class 11 Biology Chapters