Conductive tissue: structural features. Conductive fabrics. Functions and structural features Which cells conduct water with minerals

Conductive tissue consists of living or dead elongated cells that look like tubes.

There are bundles of conductive tissue in the stem and leaves of plants. In the conducting tissue, vessels and sieve tubes are isolated.

Vessels- series-connected dead hollow cells, the transverse partitions between which disappear. Through the vessels, water and minerals dissolved in it from the roots enter the stem and leaves.

Sieve tubes - elongated non-nuclear living cells connected in series with each other. Through them, organic matter from the leaves (where they were formed) move to other plant organs.

Conductive fabric transports water containing dissolved minerals.

This fabric forms two transport systems:

• ascending(from roots to leaves);
• downward(from leaves to all other parts of plants).

The ascending transport system consists of tracheids and vessels (xylem or wood), and the vessels are more perfect conductive means than tracheids.

In descending systems, the flow of water with the products of photosynthesis passes through sieve tubes (phloem or bast).

Xylem and phloem form vascular-fibrous bundles - the "circulatory system" of the plant, which penetrates it completely, uniting it into one whole.

Scientists believe that the emergence of tissues is associated in the history of the Earth with the emergence of plants on land. When part of the plant was in the air, and the other part (root) in the soil, it became necessary to deliver water and mineral salts from the roots to the leaves, and organic matter from the leaves to the roots. So in the course of evolution flora there were two types of conductive fabrics - wood and bast.

Wood (tracheids and vessels) water with dissolved minerals rises from the roots to the leaves - this is a water-conducting, or ascending, current. Through the bast (through sieve tubes), the organic substances formed in green leaves enter the roots and other organs of the plant - this is downward current.

Educational fabric

Educational tissue is found in all growing parts of the plant. Educational tissue consists of cells that are capable of dividing throughout the life of a plant. The cells here lie very quickly to each other. Through division, they form many new cells, thereby ensuring the growth of the plant in length and thickness. The cells that appeared during the division of educational tissues are then transformed into cells of other plant tissues.

It is the primary tissue from which all other plant tissues are formed. It consists of special cells capable of multiple division. It is from these cells that the embryo of any plant consists.

This tissue is retained in an adult plant as well. It is located:

• at the bottom of the root system and at the tops of the stems (ensures the growth of the plant in height and the development of the root system) - the apical educational tissue;
• inside the stem (provides plant growth in width, its thickening) - lateral educational tissue.

Unlike other tissues, the cytoplasm educational tissue thicker and denser. The cell has well-developed organelles that provide protein synthesis. The nucleus is large in size. The mass of the nucleus and the cytoplasm is maintained in a constant ratio. The enlargement of the nucleus signals the beginning of the process of cell division, which occurs through mitosis for the vegetative parts of plants and meiosis for the sporogenic meristems.

Conductive fabric

The conductive tissue is responsible for the movement of dissolved nutrients through the plant. Many higher plants it is represented by conductive elements (vessels, tracheids and sieve tubes). The walls of the conductive elements have pores and through holes that facilitate the movement of substances from cell to cell. The conductive tissue forms a continuous branched network in the body of the plant, connecting all its organs in unified system- from the thinnest roots to young shoots, buds and leaf tips.

Origin

Scientists believe that the emergence of tissues is associated in the history of the Earth with the emergence of plants on land. When part of the plant was in the air, and the other part (root) in the soil, it became necessary to deliver water and mineral salts from the roots to the leaves, and organic matter from the leaves to the roots. Thus, in the course of the evolution of the plant world, two types of conductive tissues arose - wood and bast. Through the wood (along the tracheids and vessels), water with dissolved minerals rises from the roots to the leaves - this is a water-conducting, or ascending, current. Through the bast (through sieve tubes), the organic matter formed in green leaves is supplied to the roots and other organs of the plant - this is a descending current.

Meaning

The conductive tissues of plants are xylem (wood) and phloem (bast). An ascending stream of water with mineral salts dissolved in it goes along the xylem (from the root to the stem). On the phloem - weaker and slower flow of water and organic matter.

The value of wood

The xylem, through which a strong and fast ascending current flows, is formed by dead cells of various sizes. There is no cytoplasm in them, the walls are lignified and provided with numerous pores. They are chains of long dead water-conducting cells adjacent to each other. In places of contact, they have pores along which they move from cell to cell towards the leaves. This is how the tracheids are arranged. In flowering plants, more perfect conducting tissue-vessels appear. In vessels, the transverse walls of cells are destroyed to a greater or lesser extent, and are hollow tubes. Thus, the vessels are the junctions of many dead tubular cells called segments. Located one above the other, they form a tube. Solutions move even faster through such vessels. Besides flowering plants, other higher plants have only tracheids.

The meaning of bast

Due to the fact that the downward current is weaker, phloem cells can remain alive. They form sieve tubes - their transverse walls are densely pierced with holes. There are no nuclei in such cells, but they retain a living cytoplasm. Sieve tubes do not remain alive for long, usually 2-3 years, occasionally 10-15 years. They are constantly being replaced by new ones.

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Conductive tissues are used to move nutrients dissolved in water through the plant. They arose as a result of the adaptation of plants to life on land. In connection with life in two environments - soil and air, two conductive tissues have arisen, along which substances move in two directions.

Substances rise along the xylem from roots to leaves soil nutrition- water and mineral salts dissolved in it (ascending, or transpiration current).

Substances formed in the process of photosynthesis, mainly sucrose (descending current), move along the phloem from leaves to roots. Since these substances are products of assimilation of carbon dioxide, the transport of substances through the phloem is called the current of assimilates.

Conducting tissues form a continuous branched system in the plant body, connecting all organs - from the thinnest roots to the youngest shoots. Xylem and phloem are complex tissues, they include heterogeneous elements - conductive, mechanical, storage, excretory. The most important are the conductive elements, they perform the function of carrying substances.

Xylem and phloem are formed from the same meristem and, therefore, are always located side by side in a plant. Primary conducting tissues are formed from the primary lateral meristem - procambium, secondary - from the secondary lateral meristem - cambium. Secondary conductive tissues have a more complex structure than primary ones.

Xylem (wood) consists of conductive elements - tracheids and vessels (trachea), mechanical elements - wood fibers (libriform fibers) and elements of the main tissue - wood parenchyma.

The conductive elements of the xylem are called tracheal elements. There are two types of tracheal elements - tracheids and vascular segments (Fig. 3.26).

The tracheid is a highly elongated cell with intact primary walls. The movement of solutions occurs by filtration through bordered pores. A vessel is made up of many cells called vascular segments. The segments are located one above the other, forming a tube. Between adjacent segments of the same vessel there are through holes - perforations. Solutions move through the vessels much easier than along the tracheids.

Rice. 3.26. Diagram of the structure and combination of tracheids (1) and vessel segments (2).

Tracheal elements in a mature, functioning state - dead cells without protoplasts. Preservation of protoplasts would hinder the movement of solutions.

Vessels and tracheids transfer solutions not only vertically, but also horizontally to neighboring tracheal elements and living cells. The lateral walls of the tracheids and blood vessels remain thin over a greater or lesser area. At the same time, they have secondary thickenings, which impart strength to the walls. Depending on the nature of the thickening of the lateral walls, the tracheal elements are called annular, spiral, reticulate, scaled and point-to-pore (Fig. 3.27).

Rice. 3.27. Types of thickening and porosity of the side walls of tracheal elements: 1 - annular, 2-4 - spiral, 5 - reticular thickening; 6 - ladder, 7 - opposite, 8 - regular porosity.

Secondary annular and spiral nubs are attached to the thin primary wall through a narrow protrusion. With the approach of the thickenings and the formation of bridges between them, a reticular thickening occurs, passing into the bordered pores. This series (Fig. 3.27) can be considered as a morphogenetic, evolutionary series.

Secondary thickening of the cell walls of tracheal elements is lignified (impregnated with lignin), which gives them additional strength, but limits the possibility of growth in length. Therefore, in the ontogeny of an organ, annular and spiral elements, still capable of stretching, first appear, which do not interfere with the growth of the organ in length. When the growth of an organ stops, elements appear that are incapable of longitudinal stretching.

In the process of evolution, tracheids were the first to appear. They are found in the first primitive land plants... Vessels appeared much later by transforming the tracheids. Almost all angiosperms have vessels. Spore and gymnosperms, as a rule, are devoid of blood vessels and have only tracheids. Only as a rare exception, vessels are found in such spore plants as selaginella, some horsetails and ferns, as well as in a few gymnosperms (oppressive). However, in these plants, the vessels arose independently of the angiosperm vessels. The emergence of vessels in angiosperms meant an important evolutionary achievement, since it facilitated the passage of water; angiosperms turned out to be more adapted to life on land.

Wood parenchyma and wood fibers perform storage and support functions, respectively.

Phloem (bast) consists of conducting - sieve - elements accompanying cells (companion cells), mechanical elements - phloem (bast) fibers and elements of the main tissue - phloem (bast) parenchyma.

In contrast to the tracheal elements, the conductive elements of the phloem remain alive even in the mature state, and their cell walls are primary, not lignified. On the walls sieve elements there are groups of small through holes - sieve fields, through which protoplasts of neighboring cells communicate and substances are transported. There are two types of sieve elements - sieve cells and segments of sieve tubes.

Sieve cells are more primitive, they are inherent in spore and gymnosperms. The sieve cell is one cell, strongly elongated in length, with pointed ends. Its sieve fields are scattered along the side walls. In addition, sieve cells have other primitive features: they are devoid of specialized accompanying cells and, in their mature state, contain nuclei.

In angiosperms, assimilates are transported by sieve tubes (Fig. 3.28). They consist of many separate cells - segments, located one above the other. The sieve fields of two adjacent segments form a sieve plate. Sieve plates have a more perfect structure than sieve fields (perforations are larger and there are more of them).

In the segments of the sieve tubes in a mature state, nuclei are absent, but they remain alive and actively conduct substances. An important role in carrying out assimilates through the sieve tubes belongs to the accompanying cells (companion cells). Each segment of the sieve tube and its accompanying cell (or two or three cells in the case of additional division) arise simultaneously from one meristematic cell. Companion cells have nuclei and cytoplasm with numerous mitochondria; there is an intensive metabolism in them. There are numerous cytoplasmic connections between the sieve tubes and the accompanying cells adjacent to them. It is believed that the companion cells, together with the segments of the sieve tubes, constitute a single physiological system that carries out the flow of assimilates.

Rice. 3.28. Pumpkin stem phloem on longitudinal (A) and transverse (B) sections: 1 - sieve tube segment; 2 - sieve plate; 3 - accompanying cage; 4 - bast (phloem) parenchyma; 5 - clogged sieve plate.

The duration of the functioning of the sieve tubes is short. In annuals and in aerial shoots of perennial grasses - no more than one growing season, in shrubs and trees - no more than three to four years. When the living contents of the sieve tube die off, the companion cell also dies off.

The bast parenchyma consists of living thin-walled cells. Its cells often accumulate storage substances, as well as resins, tannins, etc. Bast fibers play a supporting role. They are not present in all plants.

In the body of the plant, xylem and phloem are located side by side, forming either layers or separate strands, which are called conductive bundles. There are several types of conducting beams (Fig. 3.29).

Closed bundles consist only of primary conductive tissues, they do not have a cambium and do not thicken further. Closed bunches are characteristic of spore and monocotyledonous plants. Open bundles have cambium and are capable of secondary thickening. They are characteristic of gymnosperms and dicotyledons.

Depending on the relative position of the phloem and xylem in the bundle, the following types are distinguished. The most common are collateral bundles in which the phloem lies on one side of the xylem. Collateral bundles can be open (stems of dicotyledonous and gymnosperms) and closed (stems of monocotyledonous plants). If with inside from the xylem, an additional phloem strand is located, such a bundle is called bicollateral. Bicollateral bundles can only be open; they are characteristic of some families of dicotyledonous plants (pumpkin, nightshade, etc.).

There are also concentric bundles in which one conductive tissue surrounds another. They can only be closed. If the phloem is in the center of the bundle, and the xylem surrounds it, the bundle is called centrofloem, or amphivasal. Such bunches are often found in the stems and rhizomes of monocotyledonous plants. If the xylem is located in the center of the bundle, and the phloem is surrounded by it, the bundle is called centroxylem, or amphycribral. Centroxylem bundles are common in ferns.

Rice. 3.29. Types of conducting bundles: 1 - open collateral; 2 - open bicollateral; 3 - closed collateral; 4 - concentric closed centrofloem; 5 - concentric closed centroxylem; K - cambium; Ks - xylem; F - phloem.

5.Mechanical, storing, airy tissue. Structure, functions

Mechanical fabric- a type of tissue in a plant organism, fibers from living and dead cells with a strongly thickened cell wall, giving mechanical strength the body. It arises from the apical meristem, as well as as a result of the activity of the procambium and cambium.

The degree of development of mechanical tissues largely depends on the conditions; they are almost absent in plants of humid forests, in many coastal plants, but they are well developed in most plants in arid habitats.

Mechanical tissues are present in all organs of the plant, but they are most developed along the periphery of the stem and in the central part of the root.

The following types of mechanical fabrics are distinguished:

collenchyma is an elastic supporting tissue of the primary bark of young stems of dicotyledonous plants, as well as leaves. Consists of living cells with unevenly thickened non-lignified primary membranes, elongated along the axis of the organ. Creates support for the plant.

sclerenchyma is a strong tissue of rapidly dying cells with lignified and uniformly thickened membranes. Provides the strength of the organs and the whole body of plants. There are two types of sclerenchymal cells:

fibers - long thin cells, usually collected in strands or bundles (for example, bast or wood fibers).

sclereids are round dead cells with very thick lignified membranes. They form the seed coat, nut shell, cherry, plum, apricot pits; they give the pears a characteristic gritty character. They are found in groups in the crust of conifers and some deciduous species, in the hard shells of seeds and fruits. Their cells are round in shape with thick walls and a small nucleus.

Mechanical tissues provide strength to plant organs. They constitute a framework that supports all plant organs, resisting their break, compression, rupture. The main characteristics of the structure of mechanical tissues, which ensure their strength and elasticity, are the powerful thickening and lignification of their membranes, close closure between cells, and the absence of perforations in the cell walls.

Mechanical tissues are most developed in the stem, where they are represented by bast and wood fibers. In the roots, mechanical tissue is concentrated in the center of the organ.

Depending on the shape of the cells, their structure, physiological state and the method of thickening of the cell membranes, two types of mechanical tissue are distinguished: collenchyme and sclerenchyme (Fig. 8.4).

Rice. 8.4. Mechanical tissues: a-angular collenchyma; 6- sclerenchyme; c - sclereids from cherry plum fruits: 1 - cytoplasm, 2 - thickened cell wall, 3 - pore tubules.

Collenchyma is represented by living parenchymal cells with unevenly thickened membranes, making them especially well suited for strengthening young growing organs. Being primary, collenchyma cells are easily stretched and practically do not interfere with the elongation of the part of the plant in which they are located. Usually collenchyma is located in separate strands or in a continuous cylinder under the epidermis of the young stem and leaf petioles, and also borders the veins in the leaves of dicotyledons. Sometimes collenchyme contains chloroplasts.

Sclerenchyma consists of elongated cells with uniformly thickened, often lignified membranes, the contents of which die off in the early stages. The shells of sclerenchymal cells have a high strength, close to that of steel. This tissue is widely represented in the vegetative organs of terrestrial plants and constitutes their axial support.

There are two types of sclerenchymal cells: fibers and sclereids. Fibers are long, thin cells, usually bundled in strands or bundles (for example, bast or wood fibers). Sclereids are round, dead cells with very thick lignified membranes. They form the seed coat, nut shell, cherry, plum, apricot pits; they give the pears a characteristic gritty character.

The main tissue, or parenchyma, consists of living, usually thin-walled cells that form the basis of organs (hence the name of the tissue). It houses mechanical, conductive and other permanent tissues. The main tissue performs a number of functions, in connection with which there is a distinction between assimilation (chlorenchyme), storing, airborne (aerenchyma) and aquiferous parenchyma (Fig. 8.5).

Fig 8.5. Parenchymal tissues: 1-3 - chlorophyll-bearing (columnar, spongy and folded, respectively); 4-storage (cells with starch grains); 5 - airborne, or aerenchyme.

Proteins, carbohydrates and other substances are deposited in the cells of the storage parenchyma. It is well developed in the stems woody plants, in roots, tubers, bulbs, fruits and seeds. Plants of desert habitats (cacti) and salt marshes have an aquiferous parenchyma in their stems and leaves, which serves to accumulate water (for example, large specimens of cacti from the genus Carnegia contain up to 2-3 thousand liters of water in their tissues). In aquatic and marsh plants, a special type of basic tissue develops - the air parenchyma, or aerenchyma. Aerenchyma cells form large air intercellular spaces, through which air is delivered to those parts of the plant, the connection of which with the atmosphere is difficult.

Aerenhima (or Ehrenhima) is an airborne tissue in plants, built of cells interconnected so that large air-filled voids (large intercellular spaces) remain between them.

In some manuals, aerenchyma is considered as a type of the main parenchyma.

Aerenchyma is constructed either from ordinary parenchymal cells, or from stellate cells, connected to each other by their spurs. It is characterized by the presence of intercellular spaces.

Purpose. Such an airborne tissue is found in aquatic and marsh plants, and its purpose is twofold. First of all, it is a repository of air reserves for the needs of gas exchange. In plants completely submerged in water, the conditions for gas exchange are much less convenient than in terrestrial plants. While the latter are surrounded on all sides by air, aquatic plants are at best found in environment very small reserves; these reserves are already absorbed by the surface cells, and no longer reach the depths of the thick organs. Under these conditions, a plant can provide itself with a normal gas exchange in two ways: either by increasing the surface of its organs with a corresponding decrease in their massiveness, or by collecting air reserves inside its tissues. Both of these methods are observed in reality.

Gas exchange: On the one hand, in many plants, the underwater leaves are extremely strongly dissected, as, for example, in the water buttercup. (Ranunculus aquatilis), Ouvirandrafene s tralis, etc.

On the other hand, in the case of massive organs, they represent a loose, spongy mass filled with air. During the day, when, due to the assimilation process, the plant releases oxygen many times more than is necessary for the purpose of respiration, the released oxygen is collected in reserve in the large intercellular spaces of the aerenchyme. V sunny weather significant amounts of released oxygen do not fit in the intercellular spaces and go out through various random openings in the tissues. With the onset of night, when the assimilation process stops, the stored oxygen is gradually consumed for the respiration of the cells, and instead of it, carbon dioxide is released by the cells into the airways of the aerenchyma, in order to in turn go to the needs of assimilation during the day. So, day and night, the waste products of the plant, due to the presence of aerenchyma, are not wasted in vain, but are left in reserve to be used in the next period of activity.

As for marsh plants, their roots are in especially unfavorable conditions in terms of respiration. Under a layer of water, in water-soaked soil, different kinds fermentation and decay processes; oxygen in the uppermost layers of the soil has already been completely absorbed, then conditions for anaerobic life are created, proceeding in the absence of oxygen. The roots of marsh plants could not exist under such conditions if they did not have a supply of air in the aerenchem at their disposal.

The difference between marsh plants and not completely submerged aquatic plants from completely submerged is that the renewal of gases inside the aerenchyme occurs not only due to the vital activity of tissues, but also with the help of diffusion (and thermal diffusion); in terrestrial organs, the intercellular system opens outward with a mass of tiny openings - stomata, through which, by diffusion, the air between the intercellular spaces is equalized in composition with the surrounding air. However, with a very large plant size, this path of air renewal in the root aerenchyme would not be fast enough. Correspondingly, for example, in mangrove trees growing on sea coasts with a muddy bottom, some root branches grow up from the silt and carry their tops into the air, above the surface of the water, the surface of which is penetrated by numerous holes. Such "respiratory roots" aim at a more rapid air renewal in the aerenchyme of the feeding roots, which are branched in the anoxic sludge of the seabed.

Reduction in specific gravity

The second task of aerenchyme is to reduce the specific gravity of the plant. The body of a plant is heavier than water; aerenchyma plays the role of a swim bladder for the plant; thanks to its presence, even thin organs, poor in mechanical elements, are kept directly in the water, and do not fall in disorder to the bottom. The maintenance of organs, mainly leaves, in a position favorable for the vital functions of the plant, achieved in terrestrial plants at the dear price of the formation of a mass of mechanical elements, is achieved here in aquatic plants simply by overfilling the aerenchyma with air.

This second task of aerenchyma is especially clearly expressed in floating leaves, where the demands of respiration could be satisfied even without the aid of aerenchyma. Due to the abundance of intercellular air passages, the leaf not only floats on the surface of the water, but is also able to withstand some weight. The giant leaves of Victoria regia are especially famous for this property. Aerenchyma, which plays the role of swim bladders, often forms bubble-like swellings on the plant. Such bubbles are found both in flowering plants (Eichhornia crassipes, Trianea bogotensis) and in higher algae: Sargassum bacciferum. Fucus vesiculosus and other species have well-developed swim bladders.

The value and variety of conductive tissues

Conductive tissues are the most important component of most higher plants. They are an indispensable structural component of the vegetative and reproductive organs of spore and seed plants. Conducting tissues, together with cell walls and intercellular spaces, some cells of the main parenchyma and specialized transmission cells, form a conducting system that provides long-distance and radial transport of substances. Due to the special structure of cells and their location in the body of plants, the conducting system performs numerous but interrelated functions:

1) the movement of water and minerals absorbed by the roots from the soil, as well as organic substances formed in the roots, into the stem, leaves, reproductive organs;

2) the movement of products of photosynthesis from the green parts of the plant to the places of their use and storage: in the roots, stems, fruits and seeds;

3) the movement of phytohormones in the plant, which creates a certain balance of them, which determines the rates of growth and development of vegetative and reproductive organs of plants;

4) radial transport of substances from conducting tissues to nearby living cells of other tissues, for example, to assimilating cells of the leaf mesophyll and dividing cells of the meristems. Parenchymal cells of the pith rays of wood and bark can also take part in it. Transmission cells with numerous protrusions of the cell membrane located between the conducting and parenchymal tissues are of great importance in radial transport;

5) conductive tissues increase the resistance of plant organs to deforming loads;

6) conductive tissues form a continuous branched system that connects plant organs into a single whole;

The emergence of conductive tissues is the result of evolutionary structural transformations associated with the emergence of plants on land and the separation of their air and soil nutrition. The most ancient conducting tissues, tracheids, are found in fossil rhinophytes. They reached the highest development in modern angiosperms.

In the process of individual development, primary conductive tissues are formed from procambium at the growth points of the embryo of the seed and of the buds of renewal. Secondary conductive tissues, characteristic of dicotyledonous angiosperms, are generated by cambium.

Depending on the functions performed, the conductive tissues are subdivided into the tissues of the ascending current and the tissues of the descending current. The main purpose of the tissues of the ascending current is to transport water and minerals dissolved in it from the root to the above-located above-ground organs. In addition, organic substances formed in the root and stem move along them, for example, organic acids, carbohydrates and phytohormones. However, the term "upward current" should not be taken unambiguously as movement from bottom to top. The tissues of the ascending current provide the flow of substances in the direction from the suction zone to the shoot apex. In this case, the transported substances are used both by the root itself and by the stem, branches, leaves, reproductive organs, regardless of whether they are above or below the level of the roots. For example, in potatoes, water and elements of mineral nutrition pass through the tissues of the ascending current to the stolons and tubers formed in the soil, as well as to the aboveground organs.

Downdraft tissues provide the outflow of photosynthetic products to the growing parts of plants and to storage organs. In this case, the spatial position of the photosynthetic organs does not matter. For example, in wheat, organic substances enter the developing caryopsis from the leaves of different tiers. Therefore, the names “ascending” and “descending” fabrics should be regarded as nothing more than an established tradition.

Ascending conductive tissue

The tissues of the ascending current include tracheids and vessels (trachea), which are located in the woody (xylem) part of plant organs. In these tissues, the movement of water and substances dissolved in it occurs passively under the action of root pressure and evaporation of water from the surface of the plant.

Tracheids are of more ancient origin. They are found in higher spore plants, gymnosperms, and less often in angiosperms. In angiosperms, they are typical of the smallest branching of leaf veins. The tracheid cells are dead. They have an elongated, often fusiform shape. Their length is 1 - 4 mm. However, in gymnosperms, for example, in araucaria, it reaches 10 mm. The cell walls are thick, cellulosic, often impregnated with lignin. The cell membranes contain numerous bordered pores.

Vessels were formed at later stages of evolution. They are characteristic of angiosperms, although they are also found in some modern representatives of the divisions Plauna (genus Sellaginella), Horsetails, Ferns and Gymnosperms (genus Gnetum).

Vessels are made up of elongated dead cells, one above the other, called vascular segments. In the end walls of the segments of the vessel there are large through holes - perforations, through which long-distance transport of substances is carried out. Perforations have arisen in the course of evolution from the bordered pores of the tracheids. In the composition of the vessels, they are ladder and simple. Numerous scaled perforations are formed on the end walls of the segments of the vessel when they are obliquely laid. The holes of such perforations have an elongated shape, and the partitions separating them are located parallel to each other, resembling the steps of a staircase. Vessels with ladder perforation are characteristic of plants of the families Buttercup, Lemongrass, Birch, Palm, Chastukhovye.

Simple perforations are known in evolutionarily younger families, such as Solanaceae, Pumpkin, Aster, Bluegrass. They represent one large opening in the end wall of the segment, located perpendicular to the axis of the vessel. In a number of families, for example, among Magnolia, Rose, Iris, Astrov, both simple and ladder perforations are found in vessels.

The side walls have uneven cellulose thickenings that protect the vessels from excess pressure created by adjacent living cells of other tissues. Numerous pores may be present in the side walls to allow water to escape from the vessel.

Depending on the nature of the thickening, the types and nature of the location of the pores, the vessels are subdivided into annular, spiral, bispiral, reticular, ladder and point-pore. In annular and spiral vessels, cellulose thickenings are arranged in the form of rings or spirals. Through non-thickened areas, the transported solutions are diffused into the surrounding tissues. The diameter of these vessels is relatively small. In reticular, scalene, and punctate-pore vessels, the entire lateral wall, with the exception of the locations of simple pores, is thickened and often impregnated with lignin. Therefore, the radial transport of substances in them is carried out through numerous elongated and pinpoint pores.

Vessels have a limited lifetime. They can be destroyed as a result of blockage by tills - outgrowths of neighboring parenchymal cells, as well as under the action of centripetal forces of pressure of new wood cells formed by cambium. In the course of evolution, the vessels undergo changes. The segments of the vessels become shorter and thicker, the oblique transverse partitions are replaced by straight ones, and the ladder perforations are simple.

Downdraft conductive tissue

The downstream tissues include sieve cells and sieve tubes with companion cells. Sieve cells have more ancient origins... They are found in higher spore plants and gymnosperms. They are living, elongated cells with pointed ends. In a mature state, they contain nuclei as part of the protoplast. In their side walls, at the points of contact of adjacent cells, there are small through perforations, which are collected in groups and form sieve fields through which substances move.

Sieve tubes consist of a vertical row of elongated cells, separated by transverse walls and called sieve plates, in which the sieve fields are located. If a sieve plate has one sieve field, it is considered simple, and if several, then complex. Sieve fields are formed by numerous through holes - small diameter sieve perforations. Plasmodesmata pass through these perforations from one cell to another. The polysaccharide callose is placed on the walls of the perforations, which reduces the lumen of the perforations. As the sieve tube ages, the callose completely clogs the perforations and the tube stops working.

During the formation of a sieve tube, a special phloem protein (F-protein) is synthesized in the cells forming them, and a large vacuole develops. It pushes the cytoplasm and nucleus towards the cell wall. Then the vacuole membrane collapses and inner space cells are filled with a mixture of cytoplasm and cell sap. F-protein bodies lose their distinct outlines, merge, forming strands near sieve plates. Their fibrils pass through perforations from one segment of the sieve tube to another. One or two companion cells are tightly attached to the segments of the sieve tube, which have an elongated shape, thin walls and a living cytoplasm with a nucleus and numerous mitochondria. In the mitochondria, ATP is synthesized, which is necessary for the transport of substances through the sieve tubes. In the walls of the companion cells, there are a large number of pores with plasmasdesmata, which is almost 10 times higher than their number in other cells of the leaf mesophyll. The surface of the protoplast of these cells is significantly increased due to the numerous folds formed by the plasmalemma.

The speed of movement of assimilates through the sieve tubes significantly exceeds the speed of free diffusion of substances and reaches 50 - 150 cm / h, which indicates the active transport of substances using the energy of ATP.

The duration of the work of sieve tubes in perennial dicotyledons is 1 - 2 years. To replace them, cambium constantly forms new conducting elements. In monocots lacking cambium, sieve tubes last much longer.

Conducting beams

Conductive tissues are located in plant organs in the form of longitudinal strands, forming conductive bundles. There are four types of vascular bundles: simple, general, complex and fibrous vascular.

Simple bundles are composed of one type of conductive tissue. For example, in the marginal parts of the leaf blades of many plants, there are small-diameter bundles of vessels and tracheids, and in flowering shoots of liliaceae, from only sieve tubes.

Common bundles are formed by tracheids, vessels and sieve tubes. Sometimes this term is used to refer to metameric bundles that pass in internodes and are leaf traces. The complex bundles include conductive and parenchymal tissues. The most perfect, diverse in structure and location are fibrous vascular bundles.

Vascular fibrous bundles are characteristic of many higher spore plants and gymnosperms. However, they are most typical of angiosperms. In such bundles, functionally different parts are distinguished - phloem and xylem. Phloem ensures the outflow of assimilates from the leaf and their movement to places of use or storage. Through the xylem, water and substances dissolved in it move from the root system to the leaf and other organs. The volume of the xylem part is several times larger than the volume of the phloem part, since the volume of water entering the plant exceeds the volume of formed assimilates, since a significant part of the water is evaporated by the plant.

The variety of vascular fibrous bundles is determined by their origin, histological composition and location in the plant. If bundles are formed from procambium and complete their development as the supply of cells of educational tissue is used, as in monocots, they are called closed for growth. In contrast, in dicots, open bundles are not limited in growth, since they are formed by a cambium and increase in diameter throughout the life of the plant. In addition to the conductive bundles, the vascular fibrous bundles may include basic and mechanical tissues. For example, in dicots, phloem is formed by sieve tubes (ascending conductive tissue), bast parenchyma (main tissue), and bast fibers (mechanical tissue). The xylem consists of vessels and tracheids (conductive tissue of the descending current), woody parenchyma (main tissue) and woody fibers (mechanical tissue). The histological composition of xylem and phloem is genetically determined and can be used in plant taxonomy to diagnose different taxa. In addition, the degree of development of the constituent parts of the beams can change under the influence of the growing conditions of plants.

Several types of vascular fibrous bundles are known.

Closed collateral vascular bundles are characteristic of leaves and stems of monocotyledonous angiosperms. They lack cambium. Phloem and xylem are positioned side-by-side. They are characterized by some design features. So, in wheat, which differs by the C 3 -way of photosynthesis, the bundles are formed from procambium and have a primary phloem and primary xylem. In the phloem, an earlier protofloem and a later in the time of formation, but a larger-cell metaphloem, are distinguished. The phloem part lacks bast parenchyma and bast fibers. In the xylem, initially, smaller vessels of protoxylem are formed, located in one line perpendicular to the inner border of the phloem. Metaxylem is represented by two large vessels located next to the metaphloem perpendicular to the chain of protoxylem vessels. In this case, the vessels are arranged in a T-shape. V-, Y- and È-shaped arrangement of vessels is also known. Between the vessels of the metaxylem in 1 - 2 rows, there is a small-cell sclerenchyma with thickened walls, which are impregnated with lignin as the stem develops. This sclerenchyma separates the xylem zone from the phloem. On both sides of the vessels of the protoxylem, the cells of the woody parenchyma are located, which probably play a transfusion role, since during the transition of the bundle from the internode to the leaf cushion of the stem node, they participate in the formation of transfer cells. Around the conductive bundle of the wheat stem is the sclerenchymal sheath, which is better developed from the side of the protoxylem and protofloem; near the lateral sides of the bundle, the cells of the sheath are arranged in one row.

In plants with the C 4 -type of photosynthesis (corn, millet, etc.), a sheath of large chlorenchyme cells is located in the leaves around the closed vascular bundles.

Open collateral bundles are characteristic of dicotyledonous stems. The presence of a cambium layer between the phloem and xylem, as well as the absence of a sclerenchymal sheath around the bundles, ensures their long-term growth in thickness. In the xylem and phloem parts of such bundles, there are cells of the main and mechanical tissues.

Open collateral bundles can be formed in two ways. Firstly, these are the bundles primarily formed by the procambium. Then, cambium develops in them from the cells of the main parenchyma, producing secondary elements of phloem and xylem. As a result, the beams will combine histological elements of primary and secondary origin. Such beams are characteristic of many herbaceous flowering plants of the Dicotyledonous class, which have a beam type of stem structure (legumes, rosaceae, etc.).

Second, open collateral bundles can be formed only by cambium and consist of xylem and phloem of secondary origin. They are typical for herbaceous dicotyledons with a transitional type of anatomical structure of the stem (aster, etc.), as well as for root crops such as beets.

In the stems of plants of a number of families (Pumpkin, Solanaceae, Kolokolchikovye, etc.), there are open bicollateral bundles, where the xylem is surrounded by phloem on both sides. In this case, the outer part of the phloem facing the surface of the stem is better developed than the inner one, and the strip of cambium, as a rule, is located between the xylem and the outer part of the phloem.

Concentric beams are of two types. In amphivasal bundles, typical of fern rhizomes, the phloem surrounds the xylem, in amphivasal bundles, the xylem is located in a ring around the phloem (rhizomes of iris, lily of the valley, etc.). Less often, concentric bundles are found in dicotyledons (castor oil plant).

Closed radial vascular bundles are formed in areas of the roots that have a primary anatomical structure. The radial tuft is part of the central cylinder and passes through the middle of the root. Its xylem looks like a multi-rayed star. Phloem cells are located between the xylem rays. The number of xylem rays largely depends on the genetic nature of the plant. For example, in carrots, beets, cabbage, and other dicotyledons, the xylem of the radial bundle has only two rays. An apple tree and a pear may have 3 - 5, pumpkins and beans have a four-rayed xylem, and monocots have a multi-rayed one. The radial arrangement of the xylem rays is adaptive. It shortens the path of water from the suction surface of the root to the vessels of the central cylinder.

In perennial woody plants and some herbaceous annuals, for example, in flax, conductive tissues are located in the stem without forming distinct conductive bundles. Then they talk about a non-bunchy type of stem structure.

Radial transport tissue

The specific tissues that regulate the radial transport of substances include exoderm and endoderm.

The exoderm is the outer layer of the primary root cortex. It is formed directly under the primary integumentary tissue epiblema in the zone of root hairs and consists of one or more layers of tightly closed cells with thickened cellulose membranes. In the exoderm, water entering the root along the root hairs experiences resistance from the viscous cytoplasm and moves into the cellulose membranes of the exoderm cells, and then leaves them in the intercellular spaces of the middle layer of the primary cortex, or mesoderm. This allows water to flow efficiently into the deeper layers of the root.

In the zone of conduction in the root of monocots, where the cells of the epibleme die off and slough off, the exoderm appears on the surface of the root. Its cell walls are impregnated with suberin and prevent the flow of water from the soil to the root. In dicotyledons, the exoderm in the primary cortex sloughs off during root molting and is replaced by the periderm.

The endoderm, or the inner layer of the primary root cortex, is located around the central cylinder. It is formed by one layer of tightly closed cells of unequal structure. Some of them, called permeable ones, have thin shells and are easily permeable to water. Through them, water from the primary crust enters the radial conducting bundle of the root. Other cells have specific cellulosic thickenings of the radial and internal tangential walls. These nubs impregnated with suberin are called Caspari belts. They are impervious to water. Therefore, water enters the central cylinder only through the passage cells. And since the absorbing surface of the root significantly exceeds the total cross-sectional area of ​​the passage cells of the endoderm, then root pressure arises, which is one of the mechanisms for the flow of water into the stem, leaf and reproductive organs.

Endoderm is also part of the young stem bark. In some herbaceous angiosperms, it, like the root, may have Caspari belts. In addition, in young stems, endoderm can be represented by a starchy sheath. Thus, the endoderm can regulate the transport of water in the plant and store nutrients.

The concept of the stele and its evolution

Much attention is paid to the emergence, development in ontogeny and evolutionary structural transformations of the conducting system, since it provides the interconnection of plant organs and the evolution of large taxa is associated with it.

At the suggestion of the French botanists F. Van Thiegem and A. Dulio (1886), the set of primary conducting tissues, together with other tissues located between them and the pericycle adjacent to the bark, was called a stele. The composition of the stele can also include a core and a cavity formed in its place, as, for example, in bluegrass. The concept of "stele" corresponds to the concept of "central cylinder". The stele of the root and stem is functionally the same. The study of the stele in representatives of different divisions of higher plants led to the formation of the stele theory.

There are two main types of stele: protostela and eustela. The most ancient is the protostela. Its conductive tissues are located in the middle of the axial organs, with a xylem in the center surrounded by a continuous layer of phloem. There is no core or cavity in the stem.

There are several evolutionarily related types of protostela: haplostela, actinostela and plectostela.

The original, primitive species is the haplostela. Her xylem has a rounded cross-sectional shape and is surrounded by an even continuous layer of phloem. The pericycle is located around the conductive tissues in one or two layers. Haplostela was known among fossil rhinophytes and preserved among some psilotophytes (tmezipter).

More developed species protostela is an actinostela, in which xylem on cross section takes the form of a multibeam star. It is found in the fossil asteroxylon and some primitive lycopods.

Further separation of the xylem into separate areas, located radially or parallel to each other, led to the formation of a plectostela, which is characteristic of lymphoid stems. In actinostela and plectostela, the phloem still surrounds the xylem from all sides.

In the course of evolution, a siphonostel arose from the protostela, a distinctive feature of which is its tubular structure. In the center of such a stele is a core or a cavity. In the conductive part of the siphonostela, leaf slits appear, due to which a continuous connection of the core with the bark occurs. Depending on the method of mutual arrangement of the xylem and phloem, the siphonostel can be ectofloid and amphifloic. In the first case, the phloem surrounds the xylem on one outer side. In the second, the phloem surrounds the xylem from two sides, from the outside and from the inside.

When dividing the amphifloous siphonostela into a network or rows of longitudinal strands, a dissected stele, or dictyostela, characteristic of many fern-like ones, appears. Its conductive part is represented by numerous concentric conductive bundles.

In horsetails from the ectofloic siphonostela, an arthrostele has arisen, which has a jointed structure. It is distinguished by the presence of one large central cavity and separate conducting bundles with protoxylem cavities (carinal canals).

In flowering plants, on the basis of an ectofloid siphonostela, an eustela, characteristic of dicotyledons, and an atactostela, typical of monocots, were formed. In the eustela, the conductive part consists of separate collateral bundles with a circular arrangement. In the center of the stele in the stem is the core, which is connected to the bark with the help of core rays. In the atactostelle, the conducting beams have a scattered arrangement, between them are the parenchymal cells of the central cylinder. This arrangement of the beams hides the tubular structure of the siphonostela.

Emergence different options siphonostels is an important adaptation of higher plants to an increase in the diameter of the axial organs - the root and stem.

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 in the end (bottom or top) sections provide vertical transport, and the pores on the side surfaces facilitate the flow of water in the radial direction. The 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. It includes the main structural components (trachea and tracheids), woody parenchyma and woody fibers. It performs both a supporting and a conducting function in the plant - water and mineral salts move up the plant along it.
Tracheids - dead single fusiform cells. The walls are greatly thickened due to the deposition of lignin. A feature of the tracheids is the presence of bordered pores in their walls. Their ends overlap, giving the plant the necessary strength. Water moves through the empty lumens of the tracheids, without encountering obstacles in its path in the form of cellular contents; from one tracheid to another, it is transmitted through the pores.
In angiosperms, tracheids have developed into vessels (trachea)... These are very long tubes formed as a result of "docking" of a number of cells; the remains of the end septa are still preserved in the vessels in the form of rims - perforations. The sizes of the vessels vary from a few centimeters to several meters. In the earliest protoxylem vessels, lignin accumulates in rings or in a spiral. This allows the vessel to continue to stretch as it grows. In the vessels of metaxylem, lignin is concentrated more densely - this is an ideal "aqueduct" acting 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 trachea?
The parenchymal cells of the xylem form a kind of rays that connect the core with the bark. They conduct water in a radial direction, store nutrients. New vessels of the xylem develop from other cells of the parenchyma. 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. They also have simple pores, not bordered ones.
Phloem (bast)- this is living tissue, which is part of the bark of plants, a downward flow of water with the products of assimilation dissolved in it is carried out through it. 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, cellulosic, the nuclei die off after maturation, and the cytoplasm is pressed against the walls, clearing the way for organic substances. The end walls of the cells of the sieve tubes are gradually covered with pores and begin to resemble a sieve - these are sieve plates. To ensure their vital activity, companion 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 are multicellular structures and have no end walls, and tracheids are unicellular, have end walls and bordered pores.
?2 ... The tracheids have bordered pores and a well-defined lumen, while the fibers have a very small lumen and the pores are simple. They also differ in functions, the tracheids perform a transport role (conductive), and the fibers are mechanical.
?3. Phloem and xylem are both conductive tissues, their structures are tubular, they include cells of the parenchyma and mechanical tissues.
?4. Sieve tubes consist of living cells, their walls are cellulose, carry out the downward transport of organic substances, and the trachea are formed by dead cells, their walls are strongly thickened with lignin, provide the upward transport of water and minerals.
?5. Downward transport occurs along the sieve cells and the nuclei, carried away by the flow of substances, would close a significant part of the hundred-like field, which would lead to a decrease in the efficiency of the process.