Dead elements of the conductive tissue of plants. The structure of a plant cell. Plant tissues. What is the difference between the xylem of different plants

In the process of evolution is one of the reasons that made possible the emergence of plants on land. In our article, we will consider the features of the structure and functioning of its elements - sieve tubes and vessels.

Conductive Fabric Features

When serious changes in climatic conditions occurred on the planet, plants had to adapt to them. Before that, they all lived exclusively in the water. In the ground-air environment, it became necessary to extract water from the soil and transport it to all plant organs.

There are two types of conductive tissue, the elements of which are vessels and sieve tubes:

1. Bast, or phloem - is located closer to the surface of the stem. Along it, organic substances formed in the leaf during photosynthesis move towards the root.
2. The second type of conductive tissue is called wood, or xylem. It provides an upward current: from the root to the leaves.

plant sieve tubes

These are the conductive cells of the bast. Between themselves they are separated by numerous partitions. Outwardly, their structure resembles a sieve. That's where the name comes from. The sieve tubes of plants are alive. This is due to the weak pressure of the downward current.

Their transverse walls are permeated with a dense network of holes. And the cells contain many through holes. All of them are prokaryotes. This means that they do not have a formalized core.

The living elements of the cytoplasm of sieve tubes remain only for a certain time. The duration of this period varies widely - from 2 to 15 years. This indicator depends on the type of plant and the conditions of its growth. Sieve tubes transport water and organic substances synthesized during photosynthesis from leaves to roots.

Vessels

Unlike sieve tubes, these elements of conductive tissue are dead cells. Visually, they resemble tubes. Vessels have dense shells. On the inside, they form thickenings that look like rings or spirals.

Thanks to this structure, the vessels are able to perform their function. It consists in the movement of soil solutions of minerals from the root to the leaves.

The mechanism of soil nutrition

Thus, the movement of substances in opposite directions is simultaneously carried out in the plant. In botany, this process is called the ascending and descending current.

But what forces cause water from the soil to move upwards? It turns out that this happens under the influence of root pressure and transpiration - the evaporation of water from the surface of the leaves.

For plants, this process is vital. The fact is that only in the soil are minerals, without which the development of tissues and organs will be impossible. So, nitrogen is necessary for the development of the root system. There is plenty of this element in the air - 75%. But plants are not able to fix atmospheric nitrogen, which is why mineral nutrition is so important for them.

Rising, the water molecules tightly adhere to each other and to the walls of the vessels. In this case, forces arise that can raise water to a decent height - up to 140 m. Such pressure causes soil solutions to penetrate through the root hairs into the bark, and further to the xylem vessels. On them, water rises to the stem. Further, under the action of transpiration, water enters the leaves.

In the veins next to the vessels are sieve tubes. These elements carry downward current. Under the influence of sunlight, the polysaccharide glucose is synthesized in the chloroplasts of the leaf. The plant uses this organic matter for growth and life processes.

So, the conductive tissue of the plant ensures the movement of aqueous solutions of organic and mineral substances throughout the plant. Its structural elements are vessels and sieve tubes.

Conductive tissues are complex, since they consist of several types of cells, their structures have an elongated (tubular) shape, and are penetrated by numerous pores. The presence of holes on the end (lower or upper) sections provide vertical transport, and the pores on the side surfaces contribute to the flow of water in the radial direction. Conductive tissues include xylem and phloem. They are found only in ferns and seed plants. Conductive tissue contains both dead and living cells.
Xylem (wood) is dead tissue. Includes the main structural components (tracheas and tracheids), wood parenchyma and wood fibers. It performs both a supporting and conductive function in the plant - water and mineral salts move up the plant along it.
tracheids - dead single cells of a spindle-shaped form. The walls are strongly thickened due to the deposition of lignin. A feature of tracheids is the presence of bordered pores in their walls. Their ends overlap, giving the plant the necessary strength. Water moves through the empty gaps of the tracheids, without encountering obstacles in the form of cellular contents on its way; from one tracheid to another, it is transmitted through the pores.
In angiosperms, tracheids have developed into blood vessels (trachea). These are very long tubes formed as a result of the “docking” of a number of cells; the remains of the end partitions are still preserved in the vessels in the form of perforated rims. Vessels vary in size from a few centimeters to several meters. In the first vessels of protoxylem formation, lignin accumulates in rings or in a spiral. This allows the vessel to continue to stretch during growth. In the vessels of the metaxylem, lignin is concentrated more densely - it is an ideal "water conduit" that acts over long distances.
?1. How are tracheas different from tracheids? (Answer at the end of the article)
?2 . How are tracheids different from fibers?
?3 . What do phloem and xylem have in common?
?4. How are sieve tubes different from tracheae?
Parenchymal xylem cells form peculiar rays connecting the core with the cortex. They conduct water in the radial direction, store nutrients. New xylem vessels develop from other parenchymal cells. Finally, wood fibers are similar to tracheids, but unlike it, they have a very small internal lumen, therefore, they do not conduct water, but give additional strength. And they also have simple pores, not bordered ones.
Phloem (bast)- this is a living tissue that is part of the bark of plants, a downward flow of water with assimilation products dissolved in it is carried out through it. The phloem is formed by five types of structures: sieve tubes, companion cells, bast parenchyma, bast fibers, and sclereids.
These structures are based on sieve tubes , formed as a result of the connection of a number of sieve cells. Their walls are thin, cellulose, the nuclei die off after maturation, and the cytoplasm is pressed against the walls, making way for organic substances. The end walls of the cells of the sieve tubes gradually become covered with pores and begin to resemble a sieve - these are sieve plates. To ensure their vital activity, satellite cells are located nearby, their cytoplasm is active, the nuclei are large.
?5 . Why do you think that when sieve cells mature, their nuclei die off?
ANSWERS
?1. Tracheas have multicellular structures and do not have end walls, while tracheids are unicellular, have end walls and bordered pores.
?2 . Tracheids have bordered pores and a well-defined lumen, while in fibers the lumen is very small and the pores are simple. They also differ in functions, tracheids perform a transport role (conductive), and mechanical fibers.
?3. Phloem and xylem are both conductive tissues, their structures are tubular in shape, they include cells of the parenchyma and mechanical tissues.
?4. Sieve tubes consist of living cells, their walls are cellulose, they carry out the downward transport of organic substances, and the trachea are formed by dead cells, their walls are strongly thickened with lignin, they provide an upward transport of water and minerals.
?5. Downward transport occurs along the sieve cells, and the nuclei, carried away by the current of substances, would cover a significant part of the sieve field, which would lead to a decrease in the efficiency of the process.

Plant tissues: conductive, mechanical and excretory

Types of plant tissues

Conductive tissues are located inside the shoots and roots. Contains xylem and phloem. They provide the plant with two currents of substances: ascending and descending. ascending the current is provided by xylem - mineral salts dissolved in water move to the aerial parts. descending the current is provided by the phloem - organic substances synthesized in leaves and green stems move to other organs (to the roots).

Xylem and phloem are complex tissues that consist of three main elements:

The conductive function is also performed by parenchyma cells, which serve to transport substances between plant tissues (for example, the core rays of woody stems ensure the movement of substances in a horizontal direction from the primary bark to the core).

Xylem

Xylem (from Greek. xylon- felled tree). Consists of actually conductive elements and accompanying cells of the main and mechanical tissues. Mature vessels and tracheids are dead cells that provide an upward current (the movement of water and minerals). Xylem elements can also perform a supporting function. Through the xylem in spring, the shoots receive solutions not only of mineral salts, but also dissolved sugars, which are formed due to the hydrolysis of starch in the storage tissues of roots and stems (for example, birch sap).

tracheids are the oldest conducting elements of the xylem. Tracheids are elongated spindle-shaped cells with pointed ends, located one above the other. They have lignified cell walls with varying degrees of thickening (annular, spiral, porous, etc.), which prevent them from disintegrating and stretching. Cell walls have complex pores covered by a pore membrane through which water passes. Solutions are filtered through the pore membrane. The movement of fluid through the tracheids is slow, since the pore membrane prevents the movement of water. In higher spore and gymnosperms, tracheids account for about 95% of the volume of wood.

Vessels or trachea , consist of elongated cells located one above the other. They form tubes during the fusion and death of individual cells - vascular segments. The cytoplasm dies. Between the cells of the vessels there are transverse walls that have large openings. In the walls of the vessels there are thickenings of various shapes (ringed, spiral, etc.). The ascending current occurs through relatively young vessels, which, over time, are filled with air, clogged with outgrowths of neighboring living cells (parenchyma) and then perform a supporting function. The fluid moves faster through the vessels than through the tracheids.

Phloem

Phloem (from Greek. floyos- bark) consists of conductive elements and accompanying cells.

sieve tubes - These are living cells that are sequentially connected by their ends, do not have organelles, a nucleus. They provide movement from leaves along the stem to the root (conduct organic substances, products of photosynthesis). They have an extensive network of fibrils, the internal contents are heavily watered. They are separated from each other by film partitions with a large number of small holes (perforations) - sieve (perforation) plates (reminds me of a sieve). The longitudinal membranes of these cells are thickened, but do not become woody. In the cytoplasm of sieve tubes, it breaks down tonoplast (vacuole membrane), and the vacuolar juice with dissolved sugars mixes with the cytoplasm. With the help of strands of cytoplasm, adjacent sieve tubes are combined into a single whole. The speed of movement through the sieve tubes is less than through the vessels. Sieve tubes function for 3-4 years.

Each segment of the sieve tube is accompanied by parenchyma cells - satellite cells , which secrete substances (enzymes, ATP, etc.) necessary for their functioning. Satellite cells have large nuclei filled with cytoplasm and organelles. They are not found in all plants. They are absent in the phloem of higher spore and gymnosperms. Satellite cells help carry out the process of active transport through the sieve tubes.

Phloem and xylem form vascular fibrous (conductive) bundles . They can be seen in the leaves, stems of herbaceous plants. In tree trunks, conducting bundles merge with each other and form rings. Phloem is part of the bast and is located closer to the surface. Xylem is part of the wood and is contained closer to the core.

Vascular-fibrous bundles are closed and open - this is a taxonomic feature. Closed the bundles do not have a cambium layer between the xylem and phloem layers, so the formation of new elements does not occur in them. Closed tufts are found predominantly in monocotyledonous plants. open vascular fibrous bundles between phloem and xylem have a layer of cambium. Due to the activity of the cambium, the bundle grows and thickening of the organ occurs. Open bundles are found mainly in dicots and gymnosperms.

Perform supporting functions. They form the skeleton of a plant, provide its strength, give elasticity, support organs in a certain position. Young areas of growing organs do not have mechanical tissues. The most developed mechanical tissues are in the stem. At the root, mechanical tissue is concentrated in the center of the organ. Distinguish between colenchyma and sclerenchyma.

Colenchyma

Colenchyma (from Greek. cola- glue and enchima- poured) - consists of living chlorophyll-bearing cells with unevenly thickened walls. There are angular and lamellar colenchymas. corner Colenchyma consists of cells that are hexagonal in shape. Thickening occurs along the ribs (at the corners). It occurs in the stems of dicotyledonous plants (mainly herbaceous) and leaf cuttings. Does not interfere with the growth of organs in length. lamellar colenchyma has cells with the shape of a parallelepiped, in which only a couple of walls are thickened, parallel to the surface of the stem. Found in the stems of woody plants.

Sclerenchyma

Sclerenchyma (from Greek. scleros- hard) is a mechanical tissue that consists of lignified (impregnated with lignin) predominantly dead cells, which have evenly thickened cell walls. The nucleus and cytoplasm are destroyed. There are two varieties: sclerenchymal fibers and sclereids.

Sclerenchyma fibers

The cells are elongated with pointed ends and pore channels in the cell walls. The cell walls are thickened and very strong. The cells are tightly packed together. On the cross section - multifaceted.

In wood, sclerenchymal fibers are called woody . They are the mechanical part of the xylem, they protect the vessels from the pressure of other tissues, brittleness.

The sclerenchyma fibers of the bast are called bast. Usually they are non-lignified, strong and elastic (used in the textile industry - flax fibers, etc.).

Sclereids

They are formed from the cells of the main tissue due to the thickening of the cell walls, their impregnation with lignin. They have a different shape and are found in different organs of plants. Sclereids with the same cell diameter are called stony cells . They are the most durable. They are found in the stones of apricots, cherries, walnut shells, etc.

Sclereids can also have a stellate shape, extensions at both ends of the cell, and a rod-shaped shape.

excretory tissues plants

As a result of the metabolic process, substances are formed in plants that, for various reasons, are hardly used (with the exception of milky juice). Usually these products accumulate in certain cells. Excretory tissues are represented by groups of cells or single ones. They are divided into external and internal.

External excretory tissues

External excretory tissues are represented by modifications of the epidermis and special glandular cells in the main tissue inside plants with intercellular cavities and a system of excretory passages by which secrets are brought out. Excretory passages in different directions penetrate the stems and partially leaves and have a shell of several layers of dead and living cells. Modifications of the epidermis are represented by multicellular (rarely unicellular) glandular hairs or plates of various structures. External excretory tissues produce essential oils, balms, resins, etc.

There are about 3 thousand species of gymnosperms and angiosperms that produce essential oils. About 200 species (lavender, rose oils, etc.) are used as therapeutic agents, in perfumery, cooking, lacquer making, etc. Essential oils - These are light organic substances of different chemical composition. Their significance in plant life: they attract pollinators with a smell, repel enemies, some (phytoncides) kill or inhibit the growth and reproduction of microorganisms.

resins are formed in the cells that surround the resin passages, as the waste products of gymnosperms (pine, cypress, etc.) and angiosperms (some legumes, umbrellas, etc.) plants. These are various organic substances (resin acids, alcohols, etc.). Outside stand out with essential oils in the form of thick liquids, which are called balms . They have antibacterial properties. They are used by plants in nature and by humans in medicine for wound healing. Canadian balsam, which is obtained from balsam fir, is used in microscopic technology for the manufacture of micropreparations. The basis of coniferous balsams is turpentine (used as a solvent for paints, varnishes, etc.) and hard resin - rosin (used for soldering, making varnishes, sealing wax, rubbing the strings of bowed musical instruments). The fossilized resin of coniferous trees of the second half of the Cretaceous-Paleogene period is called amber (used as a raw material for jewelry).

Glands located in a flower or on different parts of the shoots, the cells of which secrete nectar, are called nectaries . They are formed by the main tissue, have ducts that open outward. Outgrowths of the epidermis that surround the duct give the nectary a different shape (hump-shaped, pit-shaped, corniculate, etc.). Nectar - this is an aqueous solution of glucose and fructose (concentration ranges from 3 to 72%) with impurities of aromatic substances. The main function is to attract insects and birds for pollination of flowers.

Thanks to guides - water stomata - occurs guttation - the release of drip water by plants (during transpiration, water is released in the form of steam) and salts. Guttation is a defense mechanism that occurs when transpiration fails to remove excess water. It is typical for plants that grow in a humid climate.

Special glands of insectivorous plants (more than 500 species of angiosperms are known) secrete enzymes that decompose insect proteins. Thus, insectivorous plants make up for the lack of nitrogenous compounds, since they are not enough in the soil. Digested substances are absorbed through the stomata. The most famous are pemphigus and sundew.

Glandular hairs accumulate and bring out, for example, essential oils (mint, etc.), enzymes and formic acid, which cause pain and lead to burns (nettle), etc.

Internal excretory tissues

Internal excretory tissues are receptacles of substances or individual cells that do not open outside during the life of a plant. This, for example, milkers - a system of elongated cells of some plants through which the juice moves. The juice of such plants is an emulsion of an aqueous solution of sugars, proteins and minerals with drops of lipids and other hydrophobic compounds, called latex and has a milky white (euphorbia, poppy, etc.) or orange (celandine) color. The milky juice of some plants (for example, Hevea brazilian) contains a significant amount of rubber .

To the internal excretory tissue belong idioblasts - individual scattered cells among other tissues. They accumulate calcium oxalate crystals, tannins, etc. Cells (idioblasts) of citrus fruits (lemon, tangerine, orange, etc.) accumulate essential oils.

Histology (the study of tissues).

The transition of plants from relatively monotonous living conditions in the aquatic environment to terrestrial ones was accompanied by an intensive process of dismemberment of a homogeneous vegetative body into organs - a stem, leaves and roots. These organs are composed of structurally diverse cells that form easily distinguishable groups. Groups of cells that are homogeneous in structure, performing the same function and having a common origin, are called tissues. Often several tissues of the same origin form a complex that functions as a whole.

There are six main groups of tissues: meristematic (educational), integumentary, basic, mechanical, conductive and excretory.

conductive tissues.

The plant has two nutritional poles: leaves, providing air nutrition, and roots, providing soil nutrition. Accordingly, there are two main routes for the transit of nutrients: the route by which water and mineral salts rise from the root along the stem to the leaves, and the route by which organic matter from the leaves is sent to all other plant organs, where they are consumed or deposited. in stock.

Vessels (tracheas) and tracheids- conductive tissues through which the movement of water and mineral salts is carried out. Vessels (tracheas) - tubes consisting of segments. They differentiate from a vertical row of procambial or cambial cells, in which the side walls thicken and lignify, the contents die off, and one or more perforations form in the transverse walls. The average length of the vessels is 10 cm.

Tracheids, like vessels, are dead formations, but unlike the latter, they are not tubes, but prosenchymal cells, in the walls of which there are bordered pores. The length of the tracheids is on average 1 - 10 mm.

Depending on the shape of the thickening of the wall, the vessels and tracheids are annular, spiral, mesh, etc. Annular and spiral vessels have a small diameter. They are characteristic of young organs, since their walls have non-woody areas and are capable of stretching. Mesh and porous vessels of much larger diameter, their walls are completely lignified. They usually form later than the annular and spiral vessels from the cambium. Vessels and tracheids also perform a mechanical function, giving strength to the plant. They function for several years until they are blocked by the surrounding living cells of the parenchyma. The outgrowths of the latter, penetrating through the pores into the cavity of the vessel, are called tills.

Sieve tubes are a conductive tissue through which the movement of organic substances synthesized in the leaves is carried out. This is a vertical row of living cells (segments), in which the transverse walls are pierced by perforations (sieve plates). The wall of the segment of the sieve tube is cellulose, the nucleus is destroyed, most of the cytoplasmic organelles degrade. Fibrillar structures of a protein nature (phloem protein) appear in the protoplast. Next to the segment of the sieve tube, one or more so-called accompanying cells (companion cells) with a nucleus are usually located. The presence of a large number of mitochondria in accompanying cells suggests that they provide energy for the movement of organic substances through sieve tubes.

The segment of the sieve tube and the accompanying cell adjacent to it are formed from one cell of the meristem due to its division by a vertical septum. Sieve tubes function most often for one year. In autumn, sieve plates become impermeable to plastic substances due to clogging of perforations with a polysaccharide close to cellulose, callose.

The structure of conducting tissues can be used to judge the evolutionary level of a plant. Tracheids are more primitive formations than vessels. Among the vessels, the more primitive ones will be those in which the ends of the segments are beveled and have several perforations. One large perforation is a progressive sign. Sieve tubes with obliquely placed plates with many sieve fields are considered primitive, and those with horizontal sieve plates and a small number of sieve fields are considered progressive.

Vessels, tracheids and sieve tubes are located in plants, as a rule, not randomly, but are collected in special complexes - xylem and phloem.

Xylem(wood) consists of vessels and tracheids, wood parenchyma and (not always) wood fibers (libriform). The xylem moves water and minerals. Secondary xylem is called wood.

Phloem consists of sieve tubes and accompanying cells, bast parenchyma and (also not always) bast fibers. Organic matter moves through the phloem. The secondary phloem is called bast.

Xylem and phloem, in turn, are often (but not always) located inside the plant organs in the form of vascular-fibrous, or conductive, bundles.

If there is a cambium between the phloem and xylem, then such bundles are called open. Thanks to the activity of the cambium, new elements of xylem and phloem are formed, so the bundle grows over time. Open bundles are characteristic of dicots. There is no cambium in the closed bundles between the phloem and xylem, so no overgrowth occurs. Closed bundles have monocots and, as an exception, some dicots, in which the cambium ceases to function very early (for example, in species of the genus Ranunculus).

The bundles are also classified according to the relative position of the phloem and xylem.

Collateral - phloem and xylem are located side by side, with the phloem facing the periphery of the axial organ, and the xylem facing the center.

Bicollateral - the phloem is adjacent to the xylem on both sides, the outer part of the phloem is larger than the inner one; characteristic of pumpkin, nightshade, bindweed.

There are two types of concentric: xylem surrounds the phloem - amphivasal (mainly in monocots); the phloem surrounds the xylem - amphicribral (in ferns).

Radial - xylem is located in the center, forms radial protrusions to the periphery, alternating with areas of phloem, occurs only in the roots during the primary structure. According to the number of protrusions of xylem, radial bundles are distinguished diarch (2 protrusions), triarch (3 protrusions), tetrarch (4 protrusions) and polyarch (more than 4 protrusions).

Bibliography:

Abstract of lectures of the candidate of biological sciences Surkov Viktor Aleksandrovich

Just like in animals, plants have separate transport mechanisms that are responsible for delivering nutrients to individual cells and tissues. Today we will discuss the structural features of plants.

What it is?

Conductive tissues are those through which the movement of nutrient solutions necessary for the growth and development of the plant organism takes place. The reason for their occurrence is the emergence of the first plants on land. From the root to the leaves, as you might guess, there is an upward flow of solutions of salts and other nutrients. Accordingly, the downward current flows in the opposite direction.

The ascending transport is carried out through vessels in the woody tissue (xylem), while the downward delivery is carried out with the help of sieve structures in the bast of the bark (phloem). In general, the shape of xylem resembles that of animal vessels. Their cells are elongated, have a pronounced oblong shape. What other features of the structure of the conductive

What are they like?

You should know that there are primary and secondary tissues of this type. Let's give their standard classification, since the visibility of the material improves its absorption. So, here is the simplest structure of the conductive tissue of plants, presented in the form of a table.

As you can already understand, xylem and phloem belong to a complex variety, since due to their heterogeneous structure they are able to perform such a wide range of functions.

As you can see, the structure of the conductive tissue of plants is not distinguished by some kind of supernatural complexity. In any case, it is much simpler than in the cells of higher mammals.

Xylem. Conductive elements

The most ancient elements of the entire conducting system are tracheids. This is the name of cells of a specific shape, having characteristic, pointed ends. It was from them that the usual fibers of wood fabric subsequently originated. They have a stiff wall of considerable thickness. The shape of tracheids can be very different:

• Ring-shaped.
• Spiral.
• In the form of dots.
• Sporiform.

It should be remembered that along the way, nutrient solutions are filtered through multiple pores, and therefore their speed of movement is quite low. These important features of the structure of the conducting tissue of plants are often forgotten.

What plants can have this structural element?

Tracheids can be found in almost all higher sporophytes. The lower gymnosperms, for the most part, also have these structural elements in their structure, and even in them they play a very important role. The fact is that the strong walls of the tracheids, which we already wrote about above, allow them to perform not only a directly conducting function, but also be a supporting, mechanical structure. These are the most important features of the structure of the conducting tissue of plants, on which a lot depends.

Often, only they are the only supporting structure that gives the body of the plant the necessary strength. Curiously, all (!) Coniferous plants in wood completely lack any special ones, and strength is ensured solely by the tracheids we are discussing. The length of these amazing conductive elements can range from a few millimeters to a couple of centimeters.

In general, these features of the structure of the conductive tissue of plants are studied by the 5th grade of any general education school, but often the question of the longest vessels in plants perplexes even students of biological faculties.

Characteristics of the vessels

They are a very characteristic element in the xylem of angiosperms. They look like long and hollow tubes. Each of them is formed as a result of the fusion of elongated cells according to the "butt-to-butt" pattern. Each cell is called a vessel segment, which, in its functional structure, repeats that of the tracheid. Note, however, that the segments are much wider and shorter than them.

What category of students should know these features of the structure of the conducting tissue of plants? Grade 5, who began to study botany and the structure of a plant organism, can already navigate the simplest issues of this topic.

The process of vessel formation

The xylem that first appears in the process of plant development is called primary. Its bookmark occurs in the roots and tops of young shoots. In this case, the divided segments of the xylem vessels grow at the distal ends of the procambial cords. The vessel itself appears after their merger, due to the destruction of the internal partitions. You can verify this if you look at their section through a microscope: the rims are preserved inside, which are precisely the remains of a destroyed partition.

Let's remember what structural elements form the conductive tissue of plants, and which of them are in the root of the plant:

• epidermal membrane.
• Bark.
• Protoderma, which constantly renews the layers above.
• Apical meristem, which is the main growth zone of the plant root.
• The root cap protects more delicate tissues from damage.
• Inside the root are the familiar tissues: xylem and phloem.
• They are formed, respectively, from the protophloem and protoxylem.
• Endoderm.

Protoxylem (that is, the first vessels formed in the plant) appears at the very top of all young axial organs. Formation occurs directly under the meristem layer, that is, where the cells surrounding the vessels continue to grow and stretch intensively. It should be noted that even mature protoxylem vessels do not lose their ability to stretch, since their walls have not yet undergone lignification.

As a rule, the conductive tissues of flowering plants undergo such compaction quite early, since the stem needs to support a rather massive and vulnerable flower.

Recall what is responsible for the hardening process? Lignin. And it is just the same deposited in the walls of the “blanks” of the vessels either in a spiral or in an annular direction. This position of its layers does not prevent the vessel from stretching. At the same time, this lignin provides quite a decent strength of young vessels in the plant, which prevents their destruction under mechanical stress.

That is why the conductive tissue of plants is so important. The drawing that is available on the pages of this article will surely help you better understand this issue, as it clearly demonstrates the main components of the mentioned fabric.

Metaxylem formation

In the process of growth, new vessels appear, which undergo the process of lignification much earlier. When their formation in the mature parts of the plant ends, the process of metaxylem growth is completed. How should a school biology course consider the structure of the conducting tissue of plants? Grade 5 is usually limited to the mere fact that vessels exist. Further study is included in the curriculum for older students.

At the same time, the first vessels formed from the protoxylem first stretch and then collapse completely. Mature structural formations, which arose from the metaxylem, are in principle incapable of stretching and growth. In fact, these are dead, very rigid and hollow tubes.

It is easy to think about the biological expediency of the flow of this process in this direction. If these vessels appeared immediately, they would greatly interfere with the formation of all surrounding tissues. As with tracheids, thickening of the vessel walls can be divided into the following groups (depending on their shape):

• Ring-shaped.
• Spiral.
• Stair form.
• Mesh.
• Porous.

We draw your attention to the fact that long and hollow xylem tubes with sufficient mechanical strength are an ideal system for delivering water and mineral salt solutions over long distances. The movement of fluid through their cavities is not hindered by anything, there is practically no loss of water and nutrients. What other features of the structure of the conductive tissue of plants are there? Biology (grade 6 of a secondary educational institution) also considers the mutual conductivity of xylem walls. Let's explain.

Similar in this regard to tracheids, xylems allow water to flow through pores in their walls. Since they contain a lot of lignin, they have high mechanical strength, and therefore do not deform, in addition, there is almost no risk of rupture under the pressure of the nutrient fluid. However, we have already spoken about the supreme importance of this distinctive feature of xylem, due to which the wood of many types of trees is highly durable and resilient.

It is the strong and at the same time elastic xylems that ancient ships owe their strength to. The inconspicuous but strong conductive fabric of the plants provided high resistance to long pine masts, which rarely broke even in the most severe storms.

Conductive structures of the phloem

Consider the conductive matter that is present in the tissues of the phloem.

First, sieve structures. The material of their origin is procambium localized in the primary phloem. It should be noted that with the growth of the tissues surrounding it, the protophloem quickly stretches, after which part of its structures dies off and completely ceases to function. The metaphloem finishes its maturation after (!) the growth of the plant stops.

Other features

So what other features of the structure of the conductive tissue of plants should be known? Grade 7 of a general education school should study, in addition to all of the above, also the characteristics of sieve structures, as well as their companion cells. Let's write this question a little more in detail.

Segments of sieve structures have a particularly characteristic structure. Firstly, they are extremely thin, which include quite a lot of cellulose and pectin. In this they strongly resemble parenchyma cells. Important! Unlike the latter, during maturation, the nucleus of these cells completely dies off, and the cytoplasm "dries out", being distributed in a thin layer along the inner side of the cell membrane. Oddly enough, they remain alive, but at the same time dependent on satellite cells (reminiscent of the relationship of neurons and astrocytes in the brain of animals).

Of course, class 6 usually does not consider these structural features of the conductive tissue of plants, but it is useful to know them. At least in order to imagine the essence of the processes occurring in the plant organism.

and companion cells

So. The segments of the sieve structure form one whole, being closely interconnected. The satellite cell is unique in its cytoplasm: it is extremely dense, contains a huge number of mitochondria and ribosomes. You could guess that they provide nutrition not only for the "companion" itself, but also for the sieve segment. If the satellite cell dies for some reason, the entire structure that is associated with it dies as well.

The sieve tubes themselves are easy to distinguish by the sieve plates they contain. Even when using a weak light microscope, they can be easily seen. It arises in the place where the articulation of the end ends of the two segments was formed. It is logical that these plates are located exactly in the course of growth of these same segments.

Types of conductive beams

Are there any other features of the structure of the conductive tissue of plants? Biology considers as such some aspects of the structure of conducting bundles, which we will briefly discuss.

In any higher plant, these structures can be found. They are a specific type of cord, located in the roots, young shoots and other parts that are constantly growing. These bundles include vessels and the mechanical supporting elements we have already discussed. Each such structural unit consists of two parts:

• Wood department. Consists of vessels and stiff fibers.
• Bast area. It consists of sieve structures and

Very often, a protective layer is formed around the bundles, which consists of living or dead parenchymal cells. In addition, according to their structure, they are divided into two types:

• Complete - contain xylem and phloem.
• Incomplete - only one of these tissues is included in their structure.

Classification of conducting beams according to Lotova

Currently, the standard classification of Lotova is quite common, which subdivides conducting bundles into the following varieties:

• Closed, collateral type.
• Closed, bicollateral variety.
• Concentric type - xylem is located outside.
• A variation of the previous species, in which the xylem is inside.
• Radial bundles.

In general, this is almost all the information that you should know when studying the conductive tissues of a plant as part of the school curriculum.