A Detailed Overview of Xylem

Key Concepts

• Vascular plants have xylem, a tissue that carries water and minerals from the soil to the leaves and stems. In 1858, Carl Nägeli invented the term “xylem”.
• Xylem offers a vital “supporting” role by giving tissues and organs strength, preserving the structure of the plant, and resistance to deformation.
• In most woody plants, secondary xylem and phloem are produced by the differentiation and division of cells in the vascular cambium, a bifacial lateral meristem.
• One of the world’s most essential and abundant renewable raw materials is wood, which is also xylem.
• The biological, physical, and chemical qualities of xylem cells, as well as the consequent properties of wood, are determined by their form, frequency, and distribution.
• The composition and interaction of the three polymers, cellulose, hemicellulose, and lignin, determine wood’s chemical and physical characteristics.
• A limited number of regulatory genes regulating the quantity and duration of xylem development determine whether a plant is woody or herbaceous.
• With the help of genetic engineering, desired properties for xylem can be developed for particular uses.

Table of Contents

• Xylem in Plants
  • Xylem Definition
• Structure of Xylem
• Characteristics of Xylem
• Functions of Xylem
• Xylems of Vascular Plants and Angiosperms
• Xylem in Monocots vs Dicots
• Development of Xylem
• Frequently Asked Questions (FAQs)

Xylem in Plants

Xylem Definition

Vascular plants are categorised by their vascular tissues, xylem and phloem, which carry nutrients throughout the plant. In vascular plants, the xylem is a type of tissue that transports water and nutrients from the roots to the leaves. The other form of transport tissue is the phloem, which carries nutrients like sucrose throughout the plant.

The xylem is a vascular tissue that transports water throughout a plant’s body. The complex processes and various cell types constitute xylem transfer water and dissolved nutrients to maintain and nourish plants.

Primary and Secondary Xylem

There are two different types of xylem cells based on their origin:

1. Primary Xylem: Primary development from procambium results in the formation of the primary xylem. It contains protoxylem and metaxylem. Protoxylem grows first, followed by metaxylem and then secondary xylem. Protoxylem lacks tracheids and has narrower vessels than metaxylem.
2. Secondary Xylem: During secondary growth, the secondary xylem develops from the vascular cambium. Although secondary xylem is also noticed in the gymnosperm communities Ginkgophyta and Gnetophyta and to a lesser extent in the Cycadophyta members, the two major categories in which secondary xylem can be found are conifers (Coniferae) and angiosperms (Angiospermae).

Structure of Xylem

Xylem in plants is composed of four different kinds of elements:

• Tracheids: Tracheids are tiny conductive elements linked to one another by bordered pits and openings in the secondary cell wall. Tracheids sustain the wood structure in conifers that lack the supporting cells and transport xylem sap. Gymnosperms (conifers) and angiosperms have wood containing a significant amount of tracheids.
• Vessels: The primary conductive cell type in angiosperms is known as a vessel element, which is typically broader in diameter than a tracheid and placed axially, one above the other, to form long tubes known as vessels. Xylem sap is transported by interconduit pits, which permit the lateral flow of solutes across neighbouring conductive elements and the axial transport in tracheary elements. Pits can also connect conduits to the nearby xylem parenchyma cells, which are non-tracheary elements.
• Xylem Parenchyma: Xylem parenchyma cells, which can be positioned either axially or radially, are the last type of wood cells. Although these cells usually have secondary cell walls that are relatively thin and commonly lignified, they conduct various vital functions that are essential for wood and trees.
• Xylem Fibre: Xylem fibres are dead cell with a central lumen and lignified walls and provides mechanical support in water transportation

Most of a plant’s soft tissues, including parenchyma and lengthy fibres that provide plant support, can also be found in the xylem. Xylem appears star-shaped when viewed via a microscope in a cross-section of a plant.

“Xylem parenchyma” refers to the parenchyma cells connected to the xylem.

In the secondary xylem, there are primarily two types of parenchyma cells.

• Axial parenchyma cells are organised around the axis.
• Radial parenchyma cells are organised in a ray pattern radiating from the common centre.

Characteristics of Xylem

• Xylem, one of the conducting tissues, is responsible for transferring nutrients and water from roots to the stem and leaves of the plant.
• It is composed of specialised cells called tracheary components that carry water.
• Tracheids are the first tracheary component discovered in the xylem.
• Some gymnosperms and other seedless plants only have tracheids as their main component for conducting water.
• Vessel members are the second tracheary component in the xylem. Compared to the tracheids, they are highly specialised cells.
• The main component, also known as the vessel element, transports water in angiosperms even when tracheids are present. It is absent in gymnosperms.
• In addition to the tracheary components, the xylem also has parenchyma tissue and fibre cells.
• The lignified fibre cells give the plants structural support. On the other hand, parenchyma consists of unspecialised, thin-walled cells that are used for storage.

Also Read:Xylem Parenchyma

Functions of Xylem

Transporting water and some soluble nutrients, such as minerals and inorganic ions, from the roots to the entire plant is the primary job of the xylem. Long tubes made of xylem cells carry nutrients, and the fluid that passes through the xylem cells is known as xylem sap. Passive transport is used to move these compounds; therefore, no energy is needed.

Capillary action is the process that enables xylem sap to move upward against gravity. Surface tension causes the liquid to rise in this condition. Water is also facilitated in moving upward through the xylem by strict adherence to the xylem cells. However, it gets tough to work against gravity to carry components as a plant gets taller, so xylem sets a maximum limit on the growth of tall trees.

Three phenomena cause xylem sap to flow:

• Root Pressure: If the water potential of root cells is more negative than the soil’s, water can move by osmosis from the soil into the root, typically due to high solute concentrations. As a result, positive pressure forces the sap up the xylem and toward the leaves. Before the stomata open and enable transpiration to start, root pressure is maximum in the morning. Even in a similar habitat, different plant species can have varying root pressures; examples include up to 145 kPa in Vitis riparia but zero in Celastrus orbiculatus.
• Pressure Flow Hypothesis: The phloem system maintains the sugars made in the leaves and other green tissues, whereas the xylem system carries much lighter solutes, water and minerals. Phloem pressure, much greater than air pressure, can increase to several MPa. This high solute content in the phloem permits xylem fluid to be drawn higher by negative pressure due to the selective interconnection between both systems.
• Transpirational Pull: A similar negative pressure is produced at the apex of a plant by water evaporating from the mesophyll cell surfaces and entering the atmosphere. Surface tension, as a result, creates a negative pressure or stress in the xylem that draws water away from the soil and roots. Hence, the mesophyll cell wall experiences the formation of millions of tiny menisci.

Plants must take in water from the soil and carbon dioxide from the air to produce food through photosynthesis. However, a lot of water evaporates when a plant’s stomata, or tiny openings in its leaves that allow CO₂ to enter, evaporates considerably more than the CO₂ that is taken in. Over 400 million years ago, plants began to develop xylem. The chances of survival were higher for plants that evolved methods to deliver water to the photosynthetic sites on leaves.

Xylems of Vascular Plants and Angiosperms

One major category of vascular plants is the angiosperms, also known as flowering plants. The other two are pteridophytes and naked seed-producing gymnosperms, for example, ferns. These groups can be differentiated based on their xylem tissues. For example, xylem vessels are present in the xylem tissues of flowering plants but not in gymnosperms or ferns. They only have tracheids, not xylem vessels. The primary conductive component in the majority of angiosperms is provided by the xylem vessels.

However, xylem vessels and tracheids lose their protoplasts at maturity and turn hollow and non-living. Lignin is deposited as a polymer to create a secondary cell wall. However, the secondary walls of the xylem vessels are thinner than those of the tracheids. The lateral walls of both of them eventually develop pits.

Xylem in Monocots vs Dicots

Angiosperms can be divided into two main categories: monocots (which include plants like orchids, bamboos, bananas, palm trees, grasses, and other similar species) and dicots (For example, magnolias, roses, sunflowers, strawberries, oaks, sycamores, maples, etc.). The primary difference between the two groups is the presence of cotyledons: monocots have one, while dicots have two. In addition to the cotyledons, they can be identified by their xylem tissues.

The xylem of a dicot root resembles a star (3 or 4-pronged). Phloem is located between the “prongs” of the xylem. While monocot roots have oval or rounded xylem vessels, dicot roots contain angular or polygonal vessels. Dicot roots usually have 2 to 6 fewer xylem-phloem components than monocot roots (8 or more).

Development of Xylem

The formation of the secondary xylem by the vascular cambium and bifacial lateral meristem cells characterises the development of the xylem, as well as the secondary phloem. In addition, xylem development shifts from one form to another. The growth of the xylem is described in various ways: Exarch, Endarch, Mesarch, and Centrarch.

• Exarch: When there is more than one primary xylem in roots or stems, the xylem develops from the outside inward. As a result, the protoxylem develops close to the boundary, whereas the metaxylem is located close to the centre. For example, the xylem of vascular plants develops as an exarch form.
• Endarch: The protoxylem formed near the centre, and the metaxylem develops close to the boundary because the xylem develops from the inner part of the plant and extends outward. For example, the stems of the seed plant exhibit an endarch form of development.
• Mesarch: From the middle of the primary xylem strand, the xylem develops in each direction. For example, the stems and leaves of ferns grow in a mesarch form. However, the metaxylem filled the core and boundary regions, leaving the protoxylem in between.
• Centrarch: Metaxylem surrounding the protoxylem as the primary xylem grows outward from the cylinder formed in the middle of the stem. For example, many terrestrial plants develop in a centrarchid form.

There are two different phases to the growth and development of xylem tissues. The primary growth, also referred to as the first phase, is the process by which cells originating from the procambium differentiate into the primary xylem. For example, meristem cells from the procambium and vascular cambium are used to form the xylem tissue. A lateral meristem produces secondary xylem in the second stage, also called secondary growth. According to research, genetic engineering can be used to improve xylem development and produce the desired results.

Related Links:

• Vascular Tissue: Xylem and Phloem
• Difference between Xylem and Phloem
• The Plant Tissue System
• What are the components of xylem?
• Explain all the elements of xylem and phloem.