TRACHEIDS: LOCATION, CHARACTERISTICS AND FUNCTIONS - BIOLOGY - 2025

The tracheids are elongated and at their ends pits cells, vascular plants, function as conduits for transporting water and mineral salts dissolved. The pit-pit contact areas between pairs of tracheids allow the passage of water. The rows of tracheids form a continuous conduction system through the plants.

As tracheids mature, they are cells with highly lignified cell walls, which is why they also provide structural support. Vascular plants have a great capacity to control their water content thanks to the possession of the xylem, of which the tracheids are part.

Source: Dr. phil.nat Thomas Geier, Fachgebiet Botanik der Forschungsanstalt Geisenheim.

Location on the plant

Plants have three basic types of tissue: the parenchyma, with unspecialized cells, with thin, non-lignified cell membranes; the collenchyma, with elongated support cells, with irregularly thickened cell walls; and sclerenchyma, with lignified cell wall support cells, devoid of living components at maturity.

The sclerenchyma can be mechanical, with sclereids (stone cells) and wood fibers, or conductive, with tracheids (without perforations, present in all vascular plants) and conductive vessels (with perforations at their ends, mainly present in angiosperms). Tracheids and elements of the conducting vessels are dead cells.

Plants have two types of conductive tissue: the xylem, which carries water and mineral salts from the soil; and phloem, which distributes the sugars produced by photosynthesis.

The xylem and phloem form parallel vascular bundles in the cortex of the plant. The xylem is made up of parenchyma, wood fibers, and conductive sclerenchyma. The phloem is made up of living vascular cells.

In some trees, annual growth rings are distinguished because the tracheids formed in spring are wider than those formed in summer.

characteristics

Cross section of a Elderberry plant (Sambucus sp.). Xylem vessels and trachedia. Taken and edited from: Berkshire Community College Bioscience Image Library.

The term "tracheid", coined by Carl Sanio in 1863, refers to a shape reminiscent of the trachea.

In ferns, cycads, and conifers, the tracheids are 1–7 mm. In angiosperms they are 1–2 mm or less. In contrast, conductive vessels (composed of numerous conductive vessel elements), unique to angiosperms, can be close to 1,000 mm in length.

Tracheid cells have a primary and a secondary cell wall. The secondary wall is secreted after the primary wall has formed. Therefore, the first is internal with respect to the second.

Cellulose fibers in the primary cell wall are randomly oriented, while those in the secondary cell wall are spirally oriented. Therefore, the former can more easily stretch as the cell grows. That is, the second is more rigid.

The lignified cell walls of tracheids possess scalariform, annular, helical (or spiral), reticulated, or libriform protrusions. This characteristic allows species to be identified by microscopic observation.

The lignin walls, an impermeable material, prevent the tracheids and conductive vessels from losing water or suffering from embolisms caused by the entry of air.

Transport function

The so-called "cohesion theory" is the most accepted explanation for the upward movement of water and salts in solution in the xylem. According to this theory, the loss of water due to foliar transpiration would produce tension in the liquid column that goes from the roots to the branches, passing through tracheids and conductive vessels.

The loss of water through transpiration would tend to reduce the pressure in the upper part of the plants, causing the water taken from the soil by the roots to rise through the xylem channels. In this way, the perspired water would be continuously replaced.

All of this would require sufficient tension to make the water rise, and for the cohesive force in the liquid column to support said tension. For a 100 m tall tree, a pressure gradient of 0.2 bar / m would be required, for a total cohesive force of 20 bar. Experimental evidence indicates that these conditions are met in nature.

Tracheids have a much greater interior surface-to-volume ratio than elements of conductive vessels. For this reason, they serve to conserve, by adhesion, water in the plant against gravity, regardless of whether or not there is perspiration.

Mechanical function

The lignification of the tracheids prevents their implosion due to the negative hydrostatic pressures of the xylem.

This lignification also causes the tracheids to provide most of the structural support of the wood. The larger the size of the plants, the greater the need for structural support. For this reason, the diameter of the tracheids tends to be larger in large plants.

The stiffness of the tracheids allowed the plants to acquire an erect terrestrial habit. This led to the appearance of trees and forests.

In large plants, the tracheids have a double function. The first is to bring water to the foliage (such as in small plants). The second is to structurally reinforce the foliage to resist the action of gravity, even if the reinforcement decreases the hydraulic efficiency of the xylem.

The environments subjected to strong winds or snowfall, as well as certain plant architectures, make the branches require greater resistance to fracture. Increased lignification of the wood due to tracheids can promote the longevity of the woody parts of these plants.

Evolution

The evolutionary process of tracheids, which spans more than 400 million years, is well documented because the hardness of these vascular cells, caused by lignification, favors their preservation as fossils.

As terrestrial flora evolved in geologic time, tracheids experienced two adaptive trends. First, they gave rise to conductive vessels to increase the efficiency of water and nutrient transport. Second, they were transformed into fibers to give structural support to larger and larger plants.

The elements of the conducting vessels acquire their characteristic perforations late in the course of ontogeny. During the early stages of their development they resemble tracheids, from which they evolved.

In fossil and living gymonosperms, and in primitive dicotyledons (Magnoliales), tracheids have pits with scalariform edges. During the evolution to more advanced plant groups, tracheids with scalariform edges gave rise to those with circular edges. In turn, the latter gave rise to libriform fibers.

Xylem

The xylem together with the phloem constitute the tissues that make up the vascular tissue system of vascular plants. This system is quite complex and is responsible for the conduction of water, minerals and food.

While the xylem carries water and minerals from the root to the rest of the plant, the phloem carries the nutrients made during photosynthesis, from the leaves to the rest of the plant.

The xylem is constituted in many cases by two types of cells: the tracheids, considered the most primitive, and the elements of the vessel. However, the most primitive vascular plants only present tracheids in the xylem.

The flow of water through the tracheids

The way in which the tracheids are placed inside the plant is such that their pits are perfectly aligned between the neighboring tracheids, allowing flow between them in any direction.

Some species present thickening of the cell wall at the edges of the pits that decrease the diameter of their opening, thus reinforcing the union of the tracheids and also reducing the amount of water and minerals that can pass through them. These types of pits are called areolate pits.

Some species of angiosperms, as well as conifers, have an additional mechanism that allows regulating the flow of water through the areolate pits, such as the presence of a structure called torus.

A torus is nothing more than a thickening of the membrane of the pit at the level of the central area of ​​the same and that acts as a control valve for the passage of water and minerals between the cells.

When the bull is in the center of the pit, the flow between tracheids is normal; but if the membrane moves to one of its sides, the torus blocks the opening of the hole, slowing the flow or obstructing it completely.

Types of pits

Simple

They do not present thickenings at their edges

Areolated

They present thickenings at the edges of the pits of both a tracheid and the adjacent tracheid.

Semiareoladas

The edges of one cell's pits are thickened, but the adjacent ones are not.

Areoladas with bull

As already mentioned, conifers and some angiosperms have a central torus in the areolate pit that helps regulate the flow of water and minerals.

Blind

Eventually the pit of a tracheid does not coincide with that of the adjacent cell, for which the flow of water and minerals is interrupted in this area. In these cases we speak of a blind or non-functional pit.

Tangential section of softwood from a conifer (Pinus sp.). Trachedia and other structures. Taken and edited from: Berkshire Community College Bioscience Image Library.

In gymnosperms

The gymnosperms of the phylum Gnetophyta are characterized, among other aspects, by presenting a xylem made up of tracheids and vessels or tracheas, but the rest of the gymnosperms only have tracheids as conduction elements.

Gymnosperms tend to have longer tracheids than angiosperms, and they also tend to be of the areolate type with a torus. More than 90% of the weight and volume of the secondary xylem of conifers is made up of tracheids.

The formation of tracheids in the secondary xylem of conifers occurs from the vascular cambium. This process can be divided into four phases.

Cellular division

It is a mitotic division in which after nuclear division into two daughter nuclei, the first structure to form is the primary wall.

Cell elongation

After complete cell division, the cell begins to grow in length. Before this process has concluded, the formation of the secondary wall begins, which starts from the center of the cell and increases towards the apex.

Cellulose matrix deposition

The cellulose and hemicellulose matrix of the cell is deposited in different layers.

Lignification

The cellulose and hemicellulose matrix is ​​impregnated by lignin and other materials of a similar nature in what constitutes the final stage of the maturation phase of the tracheids.

In angiosperms

Tracheids are present in the xylem of all vascular plants, however in angiosperms they are less important than in gymnosperms because they share functions with other structures, known as elements of the vessels or tracheas.

Angiosperm tracheids are shorter and thinner than gymnosperm tracheids and also never have bull-pits.

Angiosperm tracheae, like tracheids, have pits in their walls, die upon reaching maturity, and lose their protoplast. These cells, however, are shorter and up to 10 times wider than tracheids.

The tracheas lose most of their cell wall at their apices, leaving perforation plates between adjacent cells, thereby forming a continuous conduit.

Tracheae can transport water and minerals much faster than tracheids. However, these structures are more susceptible to being blocked by air bubbles. They are also more susceptible to frostbite in winter seasons.

References

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