Xylem - Overview, Definition and Functions

Last Updated : 05 Feb, 2024

Xylem is a vascular tissue, observed in all vascular plants. It helps in water transportation, provides strength, and stores fats and carbohydrates. Based on origin, xylem can be classified into primary xylem and secondary xylem. The xylem is composed of different cells such as tracheid, vessels, xylem fibers, and xylem parenchyma. In this article, we will learn about xylem types, functions, composition, characteristics, etc.

Table of Content

Define Xylem

Xylem is a plant vascular tissue that distributes minerals and water from the roots to leaves while also providing structural support. It is present in vascular plants' stems and leaves.

What is Xylem?

Xylem is a plant tissue that transports water and nutrients from roots to various parts of plants. The xylem has specialized cells known as tracheids and vessels. All vascular plants have xylems. It provides support and storage to plants. It helps in the transport of nutrients and minerals. The xylem conducts water through transpiration pull and capillary action. It helps in overall plant growth. The evaporation from leaves helps to build root pressure and transpiration. The term xylem was given by Carl Nageli in 1858. It is derived from the Greek word “xylon” which means “wood”.

Also Read: Difference Between Xylem And Phloem

Diagram of Xylem

The labelled diagram of xylem is given below:

Xylem

Composition of Xylem

The xylem is composed of four types of cells namely tracheid, xylem vessels, xylem fibers, and xylem parenchyma which provide strength to the plants and help in storage and transportation functions.

Tracheids

Tracheids are the xylem cells observed in vascular plants. It is said to be the plant's conductive element. It appears to be thin, and long and has tapered ends. The cell walls of tracheids are lignified and contain pits. The tracheids are involved in the transportation of inorganic salts and water. It also offers structural support to the plants. The presence of pits in their cell wall allows the flow of water between cells.

Xylem Vessels

Xylem vessels are another type of conductive element seen only in angiosperms and are absent in gymnosperms. It helps to distinguish between hardwood and softwood in plants. It is essential for the transport of minerals and water to the leaves from the roots. The cell walls are lignified and have pits on the side. Vessels have openings at both ends to form a continuous and tubular vessel.

Xylem fibers

Xylem fibers are made up of thick, lignified dead sclerenchyma cells. It is observed between vessels and tracheids of the plant. They are called wood fibers because of the hardness of their tissue. Xylem fibers are elastic and are known to provide strength to plants.

Xylem parenchyma

Xylem parenchyma cells are colorless, and have distinct nucleus, protoplast, and large vacuoles. mainly involved in the storage functions of the plants. It helps to store fats and carbohydrates. The plants need to carry out photosynthesis, secretion, respiration and assimilation. It gives rise to contact cells which are also called “tyloses”. Live parenchyma cells are seen in both primary and secondary xylems.

Characteristics of Xylem

The characteristics of xylem are stated below:

Xylem is a conductive tissue of the plant, which helps to move nutrients and water from roots to stems and leaves.
Xylem has special cells called trachea components that are used for water transport.
There are a number of plant species that use tracheids in water transport, such as gymnosperms and certain seedless plants.
Tracheids is the first component of xylem vessel members are the second type of tracheary component in xylem and are highly specialized cells, especially compared to tracheids.
Vessel elements carry water in angiosperms.
Gymnosperms do not contain elements of vessels.
The parenchyma tissue and fibre cells are also present in xylem.
The parenchyma is a type of storage cell composed of the simplest and thinwalled cells.
Xylem provides structural support to plants by the tough, lignifying fibres.

Types of Xylem

Xylem is classified into primary xylem and secondary xylem based on its origin.

Primary Xylem

The primary xylem consists of the protoxylem and metaxylem that emerge from the procambium present in the apical meristem. The primary xylem consists of a protoxylem and a metaxylem.
This arises during the primary development of the plants. The increase in length is observed during the primary growth.
It is observed in both monocots and dicots.
The pit formation is not observed.
In primary xylem, the xylem fibers are less in number, and xylem vessels are long and thin.
The tyloses are absent in the primary xylem.

Also Read: What is Monocot Root? – Definition, Structure, Characteristics, Examples

Secondary Xylem

The secondary xylem is formed from the vascular cambium present in the lateral meristem. It arises when the plant undergoes secondary growth.
It is observed in gymnosperms such as gnetophyta and ginkgophyta and in Angiosperms. Thus, it is said to be observed only in dicots.
The increase in diameter is observed during the secondary growth.
The pit formation is observed.
In the secondary xylem, xylem fibers occur more in number and the xylem vessels are short and wide.
The tyloses are present in the secondary xylem.

Also Read: What is Dicot Root? – Definition, Structure, Characteristics, Examples

Protoxylem and Metaxylem

The two developmental phases of the xylem, a vascular tissue in plants that is in charge of moving nutrients and water from the roots to other sections of the plant, are protoxylem and metaxylem.

Protoxylem
- Location: When plant organs like roots, stems, and leaves are developing, the earliest xylem tissue to form is called protoxylem.
- Characteristics: Protoxylem cell walls are thin and elongated. Although they can expand as the plant grows, they are frequently connected by primary growth.
Metaxylem
- Location: When a plant grows and develops, its xylem tissue divides into metaxylem following protoxylem.
- Characteristics: Compared to protoxylem, metaxylem cells frequently have stronger cell walls. They could also have pits or holes that are more pronounced and bigger.

Development of Xylem

In roots and stems, the protoxylem and metaxylem are arranged in different patterns. The main patterns of arrangement are centrarch, exarch, endarch, mesarch.

Centrarch: In the centrarch pattern, protoxylem is seen at the center while metaxylem is arranged as a cylindrical structure around the protoxylem.
Exarch: In this condition, the xylem develops from outside towards the center and the growth is said to occur in centripetal fashion. The metaxylem is seen close to the center of the root or stem while protoxylem is seen close to the periphery. It is commonly observed in the vascular plant’s root.
Endarch: In this condition, the xylem develops from the inner part towards the outside and the growth is said to occur in centrifugal fashion. The protoxylem is seen close to the center of the root or stem while metaxylem is seen close to the periphery. It is commonly observed in the seed plant’s stem.
Mesarch: In this condition, the xylem is developed in both directions from the center of the strand. The metaxylem is present near the center and the periphery while protoxylem is seen between metaxylem.

Xylems of Vascular Plants and Angiosperms

There are different characteristics of xylem of vascular and angiosperm plants.

Characteristics	Vascular Plants Xylem	Angiosperms Xylem
Vascular Tissue Components	Vessels Tracheids	Parenchyma cells Tracheids Fibers Vessels
Vessel Elements	Lacks vessel elements	Vessel elements have perforation plates
Fiber Cells	Less specialized but present	Specialized fiber cells for support
Parenchyma Cells	Present in few cases	Parenchyma cells are involved in metabolism and storage
Lignin Deposition	Not highly deposited	Well deposited lignin for structural support
Life Cycle Dominance	Found in both vascular and non-vascular	Predominant in the vascular tissue of the flowering plant.
Secondary Growth	Lacks secondary growth	Capable of extensive secondary growth

Xylem in Monocots vs Dicot

Two main groups of angiosperms or flowering plants include monocot and dicot. The difference between xylem of monocot and dicot is as follows:

Characteristic	Monocot Plant	Dicot Plant
Shape	Oval or rounded	Star-shaped
Vessel Shape	Oval or rounded	Angular
Number of Xylem Components	8 or more	2-6
Presence of Phloem	In between xylem Vessels	In between prongs of xylem Vessels

Factors Affecting Xylem Functions

The factors affecting xylem Functions are as follows:

The availability of water can affect the pressure required to transport water in xylem.
If the transpiration rate is high, it will create negative pressure.
Root health is also an important factor
Wider xylem vessels help in faster water movement and narrower movement provides more resistance.
Warm temperature increases the transpiration rate and affects the viscosity of water. Both of them help in more water movement through the xylem.

Functions of Xylem

Xylem helps in the minerals and other stems from the root of the plants to the leaves.
Xylem enables the upward movement of water in plant by capillary action.
It provides strength to the whole plant mainly through the presence of xylem fibers.
It helps to store fats, carbohydrates and other substances.
It plays a crucial role for the plants to carry out respiration, photosynthesis, secretion, storage and transportation.

Conclusion - Xylem

Xylem not only helps in storage, transportation and providing mechanical support it helps to differentiate primary growth and secondary growth of plants by identifying the presence of primary xylem and secondary xylem. The arrangement of xylem may be different in stem and root and may follow patterns such as centrach, exarch, endarch and mesarch. It plays a crucial role for the plants to carry out respiration, photosynthesis, secretion, storage and transportation.

Also Read:

Difference between Xylem and Phloem
Why are Xylem and Phloem called Complex Tissues?
Means of Transportation In Plants
Long Distance Transport Of Water
Plant Tissue System

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