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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. Because Tracheids are single-celled, their maximal capacity is potentially limited.


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).The ferns are one of the oldest Tracheophytic plant lineages, and they can be found in a variety of environments, from arctic to deserts and the tropical tropics. Modern ferns have Tracheid-based Xylem, like their coniferous ancestors, but the structure–function links of fern Xylem are poorly known.


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.On the inner side of the major wall, there are ring-like thickenings. The remainder of the wall is rather thin.

  2. Spiral Thickening (Helical Thickening): The secondary wall materials are accumulated in spirals along the inner wall of the Tracheids at this location.Spiral or helical thickening of secondary wall materials is what these are. There might be several helixes. ProtoXylem is an example.

  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.Like the rungs of a ladder, the thickenings appear as parallel transverse bands.

  4. Reticulate Thickening (Net-like Thickening): The pattern of wall thickening here is net-like (reticulate).Because the meshes are narrow, the secondary wall looks like a network.

  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.

Primary Pit Fields

Without secondary walls, Meristematic cells and their descendants have several deeply sunken patches on their walls. Primary pit fields are these depressions in the primary wall. They are also known as primary pits or Primordial Pits because they contain Plasmodesmata.


Meaning of Pit

All of those are tiny, finely defined, more or less circular spots on the cell wall that look like depressions in the wall when viewed from the surface. Secondary wall materials are not deposited in these regions. The secondary wall layers are not continuous at the pit location, unlike the primary pit, and the primary wall is not covered. Pits can be built on top of or below the principal pit field, i.e. above the primary wall. Pit chamber, pit aperture, and pit membrane are the three components of a pit. The pit void, also known as the pit chamber, is a section of the secondary wall that has been interrupted. The mouth or entrance of the pit chamber, which faces the cell lumen, is called the pit aperture.


Types of Pit

Simple pit: When the secondary wall does not arch over the pit chamber and the rim of the pit aperture has no boundary, the pit is considered to be simple.


Ramiform pit: The simple pit appears as a channel in the cell wall in the transverse section of exceptionally thick-walled brachysclereids.


Bordered pit: In lignified fibres, Tracheids, and trachea, it can be discovered. The pit cavity is partly contained in these pits by over-arching of the secondary cell wall, which may be seen in the longitudinal section.


Bordered Pit Ultrastructure

The structure of bordered pits is convoluted. Pit chamber refers to the pit cavity that is encircled by the overhanging borders. In bordered pits, the pit opening might be circular, linear, oval, or irregular in shape. A pit canal emerges as the pit's border becomes substantially thicker, forming a route between the pit chamber and the cell lumen. An exterior aperture faces the pit chamber, whereas an inner aperture faces the cell lumen. The inner aperture is usually big and lenticular, whereas the outer aperture is usually small and circular. The inner aperture is often big and lenticular, with a tiny and circular exterior aperture. These holes resemble compressed funnels in appearance.


Elements of Xylem include Tracheids, Vessels, Xylem fibres, Xylem Parenchyma. 


Xylem Tracheids Function

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

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.


Xylem Parenchyma 

Xylem Parenchyma is a type of Parenchyma that is one of the components of the Xylem. Primary and secondary Xylem both have Xylem parenchyma, which comes from the procambium and the cambium, respectively. Xylem-parenchyma, also known as wood parenchyma, is found in the secondary Xylem and is divided into axial and radial parenchyma, which run parallel and perpendicular to the organ's long axis, respectively. The fundamental function of it is to store starch, fat, and orgastic chemicals, among other things. Minerals, solutes, and water, among other things, are transported via it. When turgid, it provides mechanical support. The plant's mechanical strength is provided by the thick-walled Parenchyma.


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 end plates, 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:


Comparison between Xylem Vessels and Tracheids


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.

Although both conifers and ferns contain Xylem based on the Tracheid, important distinctions in Xylem architecture have a significant impact on the overall structure of both plants, as well as the physical considerations that dictate the shape and size of the Xylem conduits.

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