The tracheids They are elongated cells with pits at their ends that, in vascular plants, function as conduits to transport water and dissolved mineral salts. The pit-pit contact areas between pairs of tracheids allow the passage of water. The rows of tracheids form a continuous conduction system throughout the plants.
When maturing, tracheids 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.
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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, lacking living components at maturity.
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. 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..
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 be more easily stretched 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 feature allows species to be identified by microscopic observation..
The lignin walls, a waterproof material, prevent the tracheids and conductive vessels from losing water or suffering from embolisms caused by the entry of air..
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, crossing 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 higher interior surface area to volume ratio than conductive vessel elements. For this reason, they serve to conserve, by adhesion, water in the plant against gravity, regardless of whether or not there is perspiration..
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. Therefore, the diameter of the tracheids tends to be larger in large plants..
The rigidity of the tracheids allowed the plants to acquire an erect terrestrial habit. This led to the appearance of trees and jungles.
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..
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.
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 underwent 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 towards more advanced plant groups, tracheids with scalariform edges gave rise to those with circular edges. In turn, the latter gave rise to libriform fibers.
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 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, present 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 torus 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 pit, slowing the flow or obstructing it completely.
They do not have thickening at their edges
They present thickenings at the edges of the pits of both a tracheid and the adjacent tracheid.
The edges of the pits of one cell are thickened, but those of the adjacent cell are not..
As already mentioned, conifers and some angiosperms have a central torus in the areolate pit that helps regulate the flow of water and minerals..
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.
The gymnosperms of the phylum Gnetophyta are characterized, among other aspects, by presenting a xylem made up of tracheids and vessels or tracheae, 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.
It is a mitotic division in which after nuclear division into two daughter nuclei, the first structure to form is the primary wall.
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..
The cellulose and hemicellulose matrix of the cell is deposited in different layers.
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..
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 tracheae.
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 the tracheids..
The tracheas lose most of their cell wall at their apices, leaving perforation plates between adjacent cells and thus 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..
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