What are Tracheids?

Tracheids are elongated cells that transport water and mineral salts through the xylem of vascular plants. Tracheids are one of two groups of tracheary elements. The other is vessel elements. Tracheids do not have perforation plates, unlike vessel components.


The presence of tracheary elements is a distinguishing feature of vascular plants that distinguishes them from non-vascular plants. Tracheids have two main functions: contributing to the transportation system and providing structural support. The secondary walls have thickenings in a variety of shapes and sizes, including annular rings, continuous helices (known as helical or spiral), a network (known as reticulate), and transverse thickenings.


What are Tracheids and Vessel Elements?

The primary xylem is composed of protoxylem and metaxylem. Secondary growth in thickness of the stem and root of gymnosperms and dicotyledons is accompanied by the formation of secondary xylem. The structural elements of the xylem are tracheids, vessels or tracheae, xylem fibres, xylem parenchyma and rays.


Structure of Tracheid Cells

Tracheids are the xylem's most basic cell type. They have a chisel-like look and are elongated tube-like cells with tapering ends. At maturity, the cells are no longer alive, and the mature cells are devoid of protoplast. The secondary cell wall is heavily lignified, and the cells are angular and polygonal in cross-section. The tracheid is 5–6 mm long on average.


Pits perforate a large portion of the cell wall of tracheids. They also have pit pairs on their common walls between two neighbouring tracheids. Simple circular pits or advanced bordered pits are both possible.


Pteridophytes have only one xylem element: tracheids. Tracheids make up the majority of the secondary xylem in Gymnosperms. Tracheids coexist with other xylem elements in Angiosperms. The xylem of certain primitive Angiosperms, such as Drimys, Trochodendron, and Tetracentron, consists solely of tracheids (vessels absent).


Patterns of Secondary Thickening in Tracheids:

The secondary cell wall materials are laid down in complex patterns on the lateral walls of the tracheids. The following are the most common patterns:

  1. Annular Thickening: Secondary wall thickening appears as a series of rings stacked on top of each other. The most primitive form of wall thickening is annular thickening.

  2. Spiral Thickening (Helical Thickening): The secondary wall materials are accumulated in spirals along the inner wall of the tracheids at this location.

  3. Scalariform Thickening (Ladder-like Thickening): The wall materials are laid down in transverse bands along the length of the wall. There are few interconnections between the bands.

  4. Reticulate Thickening (Net-like Thickening): The pattern of wall thickening here is net-like (reticulate).

  5. Pitted Thickening: In tracheids, it is the most advanced method of secondary wall thickening. The secondary wall materials are uniformly distributed in the inner portion of the cell, and the cell wall thickness appears to be more or less uniform. Pits can be found all over the cell wall. In different plant classes, the nature and structure of the pits differ. The pits may be circular or elongated with a border. Scalariform pitted thickening is a type of advanced pitting pattern in which elongated bordered pits are arranged in a ladder-like pattern.


Xylem Tracheids Function

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Tracheids' Structural Advancement in Relation to their Functions: Tracheids have been specifically adapted to perform functions such as water and mineral conduction and mechanical support in plants. The following are the tracheid structural innovations that better fit these functions-

  • Tracheid cells are long and tapered at the ends.

  • When cells reach maturity, they are devoid of protoplasts (ensure easy flow of water)

  • Secondary cell wall with a thick lignified layer (provide mechanical support)

  • Pit pairs are supported on the lateral and end walls (facilitate lateral conduction of water)

  • The long axis of the organ in which they occur is lined up with cells.

  • The pit membrane allows water and minerals to move through.

  • The torus of the pit acts as a valve that regulates the flow of water.

Vessels

Vessels (also known as the trachea) are the second type of xylem element, and they are made up of short, tube-like cells. Vessels are arranged in an end-to-end pattern along the long axis of the organ in which they are found. Vessel segments or vessel elements are the vessel's components. The length of each vessel feature is shorter than that of tracheids, but the diameter of the vessel lumen is much larger than that of tracheids. The cells are dead and devoid of protoplast as they reach maturity. The lateral walls of the vessels have several pits for contact.


Vessel Structure in Relation to Its Functions:

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The vessel system is made up of a long tube-like structure made up of a series of cells positioned end to end. The term "vessel member" or "vessel element" refers to each cell. Perforations (large openings) in the end walls of each vessel member allow water and minerals to flow freely between the cells. Vessel members are typically shorter than tracheids. Vessels, on the other hand, have a much greater diameter than tracheids. This allows water to flow more quickly and efficiently through the vessel lumen.


The vessel cells in advanced forms have a shorter length and a larger diameter, and they behave as drum-shaped structures (as in Quercus alba). Either vessel member's end wall is oblique or transverse. Vessels with oblique ends are regarded as primitive, whereas those with transverse ends are considered advanced. Some species, such as Malus, have a tail-like tip that extends beyond the end wall. Perforations are the openings or pores in each vessel's end wall (Perforation plate: the region of the vessel with perforation occurs). Perforations are most often seen on the end wall, but lateral perforations may also occur.


Perforation plates in vessels come in a variety of shapes and sizes-

  1. Simple perforation plate

  2. Multiple perforation plate

  3. Scalariform perforation plate

  4. Reticulate perforation plate

  5. Forminate type perforation

Vessels' primary role is to transport water and nutrients. Aside from that, vessels provide mechanical assistance. These two roles are better served by the vessel's structure. End-to-end, the vessel components are arranged to form long tube-like channels. This is ideal for the continuous flow of water and minerals. Mechanical support is provided by a thick lignified cell wall.


Difference Between Tracheids and Vessels

One noteworthy distinction between tracheids and vessels is that tracheids can hold water due to their ability to withstand gravity, while vessels cannot. This is due to the fact that tracheids have a greater surface-to-volume ratio than vessel cells. Tracheids, on the other hand, do not have perforated endplates, while vessels do. This is a significant distinction between tracheids and vessels.


Tracheids can be found in all vascular plants, but vessel cells are unique to angiosperms. Tracheids, on the other hand, are single cells with openings on both ends (hence the name "syncytes"), while vessels are formed by the joining of several cells in various arrangements (thus are syncytes).


The differences between Xylem vessels and tracheids have been summarized in the following table:



Tracheids

Vessels

Definition

Tracheids are elongated, narrow tube-like cells of the vascular plants that transport water and minerals within the plant.

Vessels are wider, cylindrical-shaped tube-like cells of angiosperms that transport water and minerals within plants.

Presence

Found in all vascular plants

Limited to angiosperms

Lumen

Narrow lumen

Wider lumen

Cell wall thickness

Highly thickened

Comparatively less thickened

Cells

Elongated thin cells with tapering ends

Elongated cylindrical cells which are wider

End Plates

Absent

Perforated endplates present


Similarities Between Tracheids and Vessels

  • Xylem is made up of two groups of cells: tracheids and vessels.

  • Both are non-living cells that help the plant transport water and minerals.

  • Both have thickened cell walls that are heavily lignified.

  • Furthermore, both are elongated tube-like cells.

  • The tracheary elements are made up of both of them.

  • Vessels and tracheids are also highly specialised cells.

  • When they reach adulthood, they are devoid of protoplast.

  • The plant is assisted mechanically by tracheids and vessels.

FAQs (Frequently Asked Questions)

1. What are Xylem Fibres?

Ans: Xylem fibres, also known as xylary fibres, are the third portion of the xylem. They are also dead cells, including tracheids and vessels, and do not contain protoplast at maturity. The secondary cell wall of the cells is very dense and lignified. The primary purpose of this component is to provide mechanical support. Xylary fibres are divided into two types:

  • Fibre tracheids

  • Libriform fibres

Fibre tracheids have apical invasive development and are longer than tracheids. The bordered pits on fibre tracheids are less established.

Libriform fibres are extremely specialised. Their walls are adorned with plain pits. Gelatinous fibres are a type of xylem fibre that can be found in tension wood (a reaction wood in Angiosperms). Gelatinous fibres have a cellulosic cell wall instead of lignin in their secondary cell wall. In cross-section, this portion of the cell wall appears gelatinous. Gelatinous fibres are extremely hygroscopic, meaning they can absorb and retain a lot of moisture.

2. How are Xylems Classified?

Ans: Xylem is divided into two categories based on its origin-

  • Primary Xylem: The primary xylem of a plant is produced during its initial development. It is made up of two components, Protoxylem and Metaxylem, and is derived from procambium (a meristem). The protoxylem is the first xylem to develop, and it contains fewer tracheary elements and more parenchyma. During the maturation of the plant, proto-xylem is usually killed. In the vascular bundles, metaxylem is generated or distinguished after protoxylem. More tracheary elements are found in metaxylem than in parenchyma. Metaxylem is a functional xylem component in plants that have not undergone secondary thickening. The secondary replaces certain plants with secondary thickening of the metaxylem.

  • Secondary Xylem: The xylem produced during the plant's secondary growth is known as secondary xylem. The vascular cambium gives rise to it (a lateral meristem). Conduction of water and minerals in the secondary plant body is the primary feature. They also provide mechanical assistance.

3. What are the Functions of Xylem?

Ans: The xylem performs the following functions- 

  • Conduction of water from roots to leaves.

  • Conduction of minerals and nutrients from roots to leaves.

  • Provide mechanical assistance.

  • Tyloses are formed by the ray parenchyma and are used to store ergastic substances.

  • These ergastic substances are responsible for the wood's distinctive colour and odour.

  • Tyloses contain ergastic substances that protect the wood from termites and mites.