Dhristi NEET 2022-24

What is Xylem Tissue and its Functions?

To define Xylem, it is a transport tissue found in vascular plants along with phloem. The important function of xylem is to transport nutrients and water to leaves and stems from roots and also to provide support. The Xylem word was introduced by Carl Nageli in 1858. Now let us see the xylem diagram so that we can have a basic idea of xylem structure.


Xylem Diagram


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Types of Xylem

Xylem is mainly of two types: primary and secondary. They both perform the same function but they are categorized depending on the type of growth.

  • Primary Xylem:

It will be formed with the primary growth of the plant. This growth can be found in the tips of the stems, roots and flower buds. This will make the plants grow taller and roots to grow longer. This is known as primary growth because it occurs first in the growing season. 


  • Secondary Xylem: 

This will be formed with the plant’s secondary growth and this will allow the plant to grow wider by the time. For example, wide tree trunks which show a lot of secondary growth. It will occur every year after primary growth. Secondary xylem is responsible for tree trunks' dark rings which are used to determine the age of the tree. The two main groups of Plantae in which secondary xylem can be found are conifers and angiosperms.


What are the Components of Xylem?

Xylem is composed of mainly four types of cell in which some of them are living and some of them are dead.

  • Tracheids: 

These are the fundamental cells of the xylem, they are elongated with a tube-like structure having tapering in the end. A major portion of the tracheid's cell wall is perforated with pits. There are different patterns of secondary thickening in tracheids like annular thickening, Spiral thickening( helical thickening), scalariform thickening( ladder-like thickening), Pitted thickening and Reticulate thickening.


  • Vessels: 

These are also called the trachea and they are the second category of xylem elements which is composed of short and tube-like cells. Components of the vessels are Vessel segments or vessel elements.


  • Xylem Fibres:

Dead cells with lignified walls and a central lumen. Xylem fibres help in the transportation of water and provide mechanical support.


  • Xylem Parenchyma:

They are only living cells and it stores starch and fat. They also help in transportation water for short distances. We can see all the components in the xylem diagram.


Xylem Structure and Function

Xylem Structure

Xylem is composed of different types of cells. One of them is tracheids, having long cells helps to transport xylem sap and provide structural support. Another is the vessel element which is smaller compared to the tracheids but even this helps in water conduction. Vessel elements form one continuous vessel with perforated plates which connects all vessel elements. It also contains xylem parenchyma, this tissue helps to make the softest part of the plant and long fibres which support the plant. When xylem is watched under a microscope, it appears star-shaped. For the structure of xylem, the solution is almost the same. You can find more relevant information under the section named ‘what are the components of xylem?’.


Xylem Function

The most important function of xylem is to transport water and nutrients including minerals and inorganic nutrients from the roots of the plant to the other parts of the plant. The materials which flow through the xylem cell are called xylem sap. Substances are transported by passive transport, so it does not require any external energy. The phenomenon which allows the xylem sap to flow upward against gravity is called capillary action caused by the surface tension of the liquid.


The plant absorbs carbon dioxide from the atmosphere and water from the soil. Let's consider, when stomata in leaves open to absorb carbon dioxide, a lot of water evaporates from the leaves. So, xylem is necessary to transport water at the site of photosynthesis and other parts of the plants.


Xylem Development

Proto xylem is the first xylem which develops in the growing plants and its vessels are narrow as the plant is not grown yet.  Metaxylem is developed in the late stage where it has larger vessels and cells. Protoxylem and metaxylem can be arranged in the four ways depending on the centrach, exarch, endarch and mesarch.


Xylem vs. Phloem

Respectively, responsible for transporting water and food, xylem and phloem are vascular tissues. How is xylem different from phloem? Phloem is a living tissue whereas xylem is a dead tissue this is what creates the biggest difference in them.


What is the difference between Phloem and Xylem?

Phloem:

  • Nutrients such as glucose, proteins, and some other organic molecules are transported by phloem

  • Phloem transports the other parts of plants to leave synthesized food.

  • This food is transported in both downward and upward directions.

  • ATP or Adenosine Triphosphate a form of energy is required for the food conduction in phloem

  • Phloem tissues are elongated with tubular-shaped structures and have walls made up of thin sieve tubes.

  • Phloem has large fibers and is present near the periphery of the vascular bundle.


Xylem:

  • Dissolved minerals and water are transported through the xylem.

  • To other parts of the plants from roots, it conducts water flow.

  • Only in an upward direction the conduction or transportation of water occurs in the xylem.

  • Water in the xylem is conducted through a physical force that pulls from the roots known as transpiration pull.

  • Xylem tissues have tubular or star-shaped structures and they do not have cross walls in them.

  • Xylem has smaller spindle fibers and is present in the middle of the vascular bundle.


Characteristics Of Xylem Parenchyma

The main characteristics of xylem parenchyma are as follows:


  • Xylem parenchyma's main characteristics are as follows:

  • They are the only cells of the xylem which live.

  • Always thin and made up of cellulose, the cell wall of xylem parenchyma.

  • Contains protoplasm and nucleus which is prominent.

  • Have large vacuoles and cell walls are colourless.

  • Living parenchyma cells are contained in both primary and secondary xylem.

  • Fats and proteins the components of parenchyma cells vary seasonally.

  • They are composed of crystals containing parenchyma cells that have lignified walls and may have subdivided septa.

  • Chloroplasts that are present in angiosperm, woody plants, and herbaceous plants are what xylem parenchyma consist of.

  • Tyloses are the vessels from outgrowth beside both axial and ray parenchyma cells.

  • Store a variety of substances that may develop tyloses.

  • Sclereids might get differentiated into tyloses.

  • The cell migrates into tyloses of the nucleus and cytoplasm.

  • Which parenchyma cells give rise to tyloses are termed as contact cells.


This is all about xylem, its characteristics, features, and types. Focus on its functions and learn how it functions like a continuous connection channel for all the parts of a plant.

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FAQs (Frequently Asked Questions)

1. Where is the Xylem?

Xylem can be found in the non-woody plants and the parts of the non-woody plants. It is also found in the secondary xylem formed by the meristem called vascular cambium in woody plants. Xylems are dead cells found in stems, leaves and roots of the plants. In trees, it is found inside cambium the woody part of the tree extends through the trunk centre or to the pith and is transported to branches, foliage, etc. Xylem consists of xylem fiber cells, xylem parenchyma cells, and treachery elements.

2. Why are Xylem Cells Dead?

Xylem is made up of two types of cell tracheids and vessel elements. Both of the cells are dead when they are used in the xylem. Dead cells, which don’t have organelle, help to transport more capacity of water.  As we know that all the components except xylem parenchyma are dead, they are called non-living tissues. The xylem cells are dead as when the central vacuoles burst it triggers the autolytic hydrolysis in the content of cells which results in the dead cells. Xylem cannot perform photosynthesis as they are dead at the time of maturity.

3. Brief the process of Xylem.

Tracheids and vessel members are both typically narrow, elongated and hollow; this is what xylem treachery cell consists of. The only type of water-conducting cells in most gymnosperms and seedless vascular plants are tracheids which are less specialized than the other members. Water must move through a thin modified primary cell wall from tracheids to tracheids known as pit membranes. This membrane serves to keep the passage of damaging air bubbles safe. Through these perforations, though disruptions and fractures of air bubbles are more likely to flow unimpeded from vessel to vessel.

4. How does the process of xylem begin?

When the actively dividing shoot tips and growing cells of the root of apical meristem begin, the formation of xylem by giving rise to xylems primary. Secondary xylem contains in woody plants expands upto girth and builds a ring around the original primary xylem tissue of the new xylem. After this the primary cells of xylem cells lose their conducting function, forming a harder Skelton that serves support only to the plants and die. Thus, the inner part of the xylem is composed of dead structurally strong primary xylem and on the outer part which secondary xylem serves water transportation and conduction.

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