Conductive fabric. Plant cell structure. Plant tissue Conductive plant tissue is composed of dead cells

CONDUCTIVE FABRICS

Conductive tissues transport nutrients in two directions. Upward (transpiration) current liquids (aqueous solutions and salts) goes along vessels and tracheids xylem (Fig. 32) from the roots up the stem to the leaves and other organs of the plant. Downward current(assimilation) organic matter is carried from the leaves along the stem to the underground organs of the plant along

special sieve tubes phloem (Fig. 33). The conductive tissue of a plant is somewhat reminiscent of the human circulatory system, since it has an axial and radial highly branched network; nutrients enter every cell of a living plant. In each organ, xylem and phloem plants are located side by side and are presented in the form of strands - conducting bundles.

There are primary and secondary conductive tissues. The primary ones differentiate from the procambium and are laid in the young organs of the plant, the secondary conducting tissues are more powerful, they are formed from the cambium.

Xylem (wood) presented tracheids and trachea, or vessels.

Tracheids- elongated closed cells with obliquely cut serrated ends, in the mature state are represented by dead prosenchymal cells. The length of the cells is on average 1 - 4 mm. Communication with neighboring tracheids occurs through simple or bordered pores. The walls are unevenly thickened, according to the nature of the thickening of the walls, the tracheids are annular, spiral, scaled, reticulate and porous (Fig. 34). Porous tracheids always have bordered pores (Fig. 35). Sporophytes of all higher plants have tracheids, and in most horsetails, lycopods, ferns and gymnosperms, they serve as the only conductive elements of the xylem. Tracheids

perform two main functions: water conduction and mechanical strengthening of the organ.

Trachea, or vessels, - the main water-conducting elements of xylem of angiosperms. Tracheas are hollow tubes composed of separate segments; there are holes in the septa between the segments - perforations, thanks to which the fluid flow is carried out. Tracheas, like tracheids, are closed system: the ends of each trachea have beveled transverse walls with bordered pores. The segments of the trachea are larger than the tracheids: in diameter they are different types plants from 0.1 - 0.15 to 0.3 - 0.7 mm. The length of the trachea is from several meters to several tens of meters (in lianas). Tracheas are made up of dead cells, although they are alive in the initial stages of formation. It is believed that the trachea evolved from tracheids.

Vessels and tracheids, in addition to the primary membrane, in most cases have secondary thickenings in the form of rings, spirals, ladders, etc. Secondary thickenings are formed on the inner wall of blood vessels (see Fig. 34). So, in an annular vessel, the internal thickening of the walls in the form of rings located at a distance from each other. The rings are located across the vessel and slightly obliquely. In a spiral vessel, the secondary membrane is layered from the inside of the cell in the form of a spiral; in the reticular vessel, the non-thickened places of the shell look like cracks, resembling mesh cells; in the ladder vessel, thickened places alternate with non-thickened ones, forming a kind of stairs.

Tracheids and vessels - tracheal elements - are distributed in the xylem in different ways: on a cross section in solid rings, forming ring-vascular wood, or scattered more or less evenly throughout the xylem, forming vascular wood... The secondary coat is usually impregnated with lignin, giving the plant additional strength, but at the same time limiting its growth in length.

In addition to vessels and tracheids, xylem includes beam elements composed of cells that form core rays. The medullary rays consist of thin-walled living parenchymal cells, along which nutrients flow in a horizontal direction. The xylem also contains living cells of the woody parenchyma, which function as a near transport and serve as a storage site for reserve substances. All xylem elements originate from cambium.

Phloem- conductive tissue, through which glucose and other organic substances are transported - products of photosynthesis from leaves to places of their use and deposition (to growth cones, tubers, bulbs, rhizomes, roots, fruits, seeds, etc.). Phloem is also primary and secondary.

The primary phloem is formed from the procambium, the secondary (bast) - from the cambium. The primary phloem lacks core rays and a less powerful system sieve elements than in tracheids. In the process of formation sieve tube in the protoplast of cells - the segments of the sieve tube, mucus bodies appear, which take part in the formation of a mucus cord around the sieve plates (Fig. 36). This completes the formation of the sieve tube segment. Sieve tubes function in most herbaceous plants one growing season and up to 3-4 years in arboreal and shrubby plants. Sieve tubes consist of a series of elongated cells that communicate with each other through perforated septa - strainer... The membranes of the functioning sieve tubes do not become lignified and remain alive. Old cells become clogged with the so-called corpus callosum, and then die off and, under the pressure on them, younger functioning cells are flattened.

Phloem refers to bast parenchyma, consisting of thin-walled cells in which reserve nutrients are deposited. By core rays secondary phloem is also carried out by short-term transportation of organic nutrients- products of photosynthesis.

Conducting beams- cords, usually formed by xylem and phloem. If strands adjoin the conducting beams

mechanical tissue (more often sclerenchyma), then such bundles are called vascular fibrous... Other tissues can also be included in the vascular bundles - living parenchyma, lactic acidae, etc. The vascular bundles can be complete when both xylem and phloem are present, and incomplete, consisting only of xylem (xylem, or woody, conducting bundle) or phloem (phloem , or bast, conducting beam).

The conducting bundles were originally formed from the procambium. There are several types of conducting beams (Fig. 37). Part of the procambium can remain and then turn into cambium, then the bundle is capable of secondary thickening. it open beams (Fig. 38). Such vascular bundles predominate in most dicotyledonous and gymnosperms. Plants with open bunches are able to grow in thickness due to the activity of the cambium, and the woody areas (Fig. 39, 5) are about three times larger than the bast areas (Fig. 39, 2) ... If, during the differentiation of the conducting bundle from the procambial cord, the entire educational tissue is completely spent on the formation of permanent tissues, then the bundle is called closed(fig. 40). Closed

conductive bundles are found in the stems of monocotyledonous plants. Wood and bast in bundles can have different relative positions. In this regard, several types of conducting beams are distinguished: collateral, bicollateral (Fig. 41), concentric and radial. Collateral, or lateral, - bundles in which xylem and phloem are adjacent to each other. Bicolateral, or double-sided, - bunches in which two phloem strands adjoin the xylem side by side. V concentric in bundles, the xylem tissue completely surrounds the phloem tissue or vice versa (Fig. 42). In the first case, such a bundle is called centrofloemic. Centrofloemic bundles are found in the stems and rhizomes of some dicotyledonous and monocotyledonous plants (begonia, sorrel, iris, many sedges and liliaceae). Ferns possess them. There are also

intermediate conducting bundles between closed collateral and centrofloemic. In the roots they meet radial beams, in which the central part and rays along the radii are left by wood, and each ray of wood consists of central larger vessels, gradually decreasing along the radii (Fig. 43). The number of rays in different plants is not the same. Bast areas are located between the wood beams. The types of conducting beams are schematically shown in Fig. 37. Conductive bundles stretch along the entire plant in the form of strands that start at the roots and pass along the entire plant along the stem to the leaves and other organs. In the leaves, they are called veins. Their main function is to conduct the descending and ascending currents of water and nutrients.

Conductive fabric

The conductive tissue is responsible for the movement of dissolved nutrients through the plant. In 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 plant's body, connecting all its organs into a single 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. So in the course of evolution flora there were two types of conductive fabrics - wood and bast. Through wood (along 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 the sieve tubes), the organic matter formed in the green leaves flows 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 also 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 connections 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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The function of the conductive tissues is to conduct water with nutrients dissolved in it through the plant. Therefore, the cells that make up the conductive tissues have an elongated tubular shape, the transverse partitions between them either completely collapse, or are penetrated by numerous holes.

The movement of nutrients in a plant is carried out in two main directions. From the roots to the leaves, water and minerals rise, which plants receive from the soil using the root system. Organic substances produced in the process of photosynthesis move from the leaves to the underground organs of plants.

Classification. Mineral and organic substances dissolved in water, as a rule, move along various elements of conductive tissues, which, depending on the structure and the physiological function performed, are subdivided into vessels (trachea), tracheids and sieve tubes. Water with minerals rises through the vessels and tracheids, and various products of photosynthesis through the sieve tubes. However, organic matter moves through the plant not only in a downward direction. They can rise up the vessels, coming from underground organs to the aboveground parts of plants.

The movement of organic matter in the ascending direction and along the sieve tubes is possible - from leaves to points of growth, flowers and other organs located in the upper part of the plant.

Vessels and tracheids. Vessels consist of vertical row cells located one above the other, between which the transverse partitions are destroyed. Individual cells are called vascular segments. Their shell becomes woody and thickens, the living content in each segment dies off. Depending on the nature of the thickening, several types of vessels are distinguished: annular, spiral, mesh, ladder and porous (Fig. 42).

Annular vessels have annular woody thickenings in their walls, while most of the wall remains cellulose. Spiral vessels have spiral-like thickenings. Ringed and spiral vessels are characteristic of young plant organs, since, due to the structural features, they do not interfere with their growth. Later, reticular, scalene and porous vessels are formed, with a stronger thickening and lignification of the shell. The greatest thickening of the membrane is observed in porous vessels. The walls of all vessels are provided with numerous pores, some of these pores have through holes - perforations. With aging of the vessels, their cavity is often clogged with tills, which are formed as a result of invagination through the pores into the vessels of neighboring parenchymal cells and have the appearance of a bladder. Vessels, in the cavity of which tills appear, cease to function and are replaced by younger ones. The formed vessel is a thin capillary tube (0.1 ... 0.15 mm in diameter) and sometimes reaches a length of several tens of meters (some vines). Most often, the length of the vessels varies in different plants within 10 ... 20 cm. The articulation between the segments of the vessels can be horizontal or oblique.

Tracheids differ from vessels in that they are separate closed cells with pointed ends. The movement of water and mineral substances is carried out through a variety of pores located in the tracheid membrane, and therefore has a lower speed compared to the movement of substances through the vessels. Tracheids are similar in structure to vessels (thickening and lignification of the shell, dying off of the protoplast), but they are an older and more primitive water-conducting element than vessels. The length of the tracheids ranges from tenths of a millimeter to several centimeters.

Due to the thickening and lignification of the walls, the vessels and tracheids perform not only the function of carrying water and minerals, but also mechanical, giving the plant organs strength. The thickening protects the water-conducting elements from being squeezed by adjacent tissues.

In the walls of blood vessels and tracheids are formed of various kinds pores are simple, bordered and semi-bordered. Simple pores are most often rounded in cross-section and represent a tubule passing through the thickness of the secondary membrane and coinciding with the pore tubule of a neighboring cell. Bordered pores are usually observed in the lateral walls of the tracheids. They look like a dome that rises above the wall of a water-conducting cage with a hole at the top. The dome is formed by a secondary membrane and is bordered by its base with a thin primary cell membrane.

Have conifers in the thickness of the primary membrane directly under the hole of the bordered pore there is a thickening - the torus, which plays the role of a two-way valve and regulates the flow of water into the cell. The torus is usually riddled with tiny holes. The edged pores of adjacent vessels or tracheids, as a rule, coincide. If a vessel or tracheid is adjacent to parenchymal cells, semi-bordered pores are formed, since bordering is formed only on the side of the water-conducting cells (see Fig. 21).

In the course of evolution, there was a gradual improvement in the water-conducting elements of plants. Tracheids as a primitive type of conducting tissue are characteristic of more ancient representatives of the plant world (mosses, gymnosperms), although they are sometimes found in highly organized plants.

The initial type should be considered annular vessels, from which further development proceeded to the most perfect vessels - porous. There was a gradual shortening of the vascular segments with a simultaneous increase in their diameter. The transverse partitions between them acquired a horizontal position and were pierced with holes, which ensured better water movement. Later it happened complete destruction partitions, from which a small ridge sometimes remains in the vessel cavity.

Vessels and tracheids, in addition to water with minerals dissolved in it, sometimes carry organic substances, the so-called sap. This is usually observed in spring, when fermented organic matter is directed from the places of their deposition - roots, rhizomes and other underground parts of plants - to the aboveground organs - stems and leaves.

Sieve tubes. The organic substances dissolved in water move through the sieve tubes. They consist of a vertical row of living cells and contain a well-defined cytoplasm. The nuclei are very small and usually disintegrate during the formation of the sieve tube. There are also leukoplasts. The transverse partitions between the cells of the sieve tubes are provided with numerous holes and are called sieve plates. Plasmodesmata stretch through the holes. The shells of the sieve tubes are thin, cellulosic; there are simple pores on the side walls. In most plants, during the development of sieve tubes, adjoining companion cells are formed, with which they are connected by numerous plasmodesmata (Fig. 43). Companion cells contain dense cytoplasm and a well-defined nucleus. Companion cells were not found in conifers, mosses, and ferns.

The length of sieve tubes is much less than that of vessels, and ranges from fractions of a millimeter to 2 mm with a very small diameter, not exceeding hundredths of a millimeter.

Sieve tubes usually function for one growing season. In the fall, the pores of the sieve plates become clogged, and a corpus callosum is formed on them, consisting of a special substance - calla. In some plants, such as linden, the corpus callosum dissolves and the sieve tubes resume their activity, but in most cases they die off and are replaced by new sieve tubes.

Living sieve tubes resist the pressure of neighboring tissues due to the turmoil of their cells, and after death they flatten and dissolve.

Milky vessels (lactariuses). Millers, found in many flowering plants, can be attributed to both conductive and excretory tissues, since they perform heterogeneous functions - conducting, excreting and accumulating various substances... The milky vessels contain a special composition of cell sap called milky sap, or latex. They are formed by one or more living cells that have a cellulose membrane, the wall layers of the cytoplasm, a nucleus, leukoplasts and a large central vacuole with milky juice, which occupies almost the entire cell cavity. There are 2 types of milkmen - articulated and non-articulated (Fig. 44).

Articulated lactifiers, like vessels and sieve tubes, consist of a longitudinal row of elongated cells. Sometimes the transverse partitions between them dissolve, and continuous thin tubes are formed, from which numerous lateral outgrowths extend, connecting the individual lactarias to each other. Articulated lactic acid plants have plants from the families Compositae (Aster), poppy, bellflower, etc.

Non-segmental lactarias consist of a single cell that grows as the plant grows. Branching out, they permeate the entire body of the plant, but at the same time the individual lactates never connect. Their length can reach several meters. Non-articular lactarius are observed in plants of the families nettle, euphorbia, kutrovy, etc.

Millers are usually short-lived and, upon reaching a certain age, die off and flatten. At the same time, in rubber plants, latex coagulates, as a result of which a mass of hardened rubber is formed.

Excretory tissue (excretory system)

Functions and structural features. Excretory tissues are used to accumulate or excrete end products of metabolism (catabolites), which are not involved in further metabolism, and sometimes are harmful to plants. Their accumulation can occur both in the cavity of the cell itself and in the intercellular spaces. The elements of excretory tissues are very diverse - specialized cells, canals, glands, hairs, etc. The combination of these elements is excretory system plants.

Classification. Distinguish between excretory tissues of internal secretion and excretory tissues of external secretion.

Excretory tissues of internal secretion. These include the various receptacles of secretions in which metabolic products such as essential oils, resins, tannins, rubber. However, in some plants, resin can also be released to the outside.

Essential oils most often accumulate in the receptacles of secretions. These receptacles are usually located among the cells of the underlying tissue near the surface of the organ. By their origin, the receptacles of secretions are subdivided into schizogenic and lysigenic (Fig. 45). Schizogenic receptacles arise as a result of the accumulation of substances in the intercellular space and the subsequent separation and death of neighboring cells. Such canal-like excretory passages containing essential oil are characteristic of the fruits of plants of the umbrella family (celery) - dill, coriander, anise, and others. Resin passages in the leaves and stems of coniferous plants can also serve as an example of containers of schizogenic origin.

Lysigenic receptacles arise as a result of the accumulation of the excretion product inside the cells, after which the cell membranes dissolve. Lysigenic receptacles of essential oils in citrus fruits and leaves are widely known.

Excretory tissues of external secretion. They are less diverse than endocrine tissues.

Of these, the most common are glandular hairs and glands, adapted to the release of essential oils, resinous substances, nectar and water. The glands that produce nectar are called nectaries. They have a varied shape and structure and are mainly found in flowers, but sometimes they are formed on other plant organs. The water-secreting glands play the role of hydathodes. The process of separating water in a liquid-droplet state is called guttation. Gutting occurs in conditions high humidity air, preventing transpiration.

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 transfer 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) movement of phytohormones through the plant, which creates a certain balance of them, which determines the growth and development rates 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, were 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 ensure the outflow of photosynthetic products into the growing parts of plants and into storage organs. In this case, the spatial position of the photosynthetic organs does not matter. For example, in wheat, organic matter enters 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 typical for 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

Downdraft 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. On the walls of the perforations, the polysaccharide callose is placed, 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 companion cells, there are a large number of pores with plasmadesmata, 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. 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 is used educational tissue like monocots, they are called closed to growth. In contrast, in dicotyledons, 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 dicotyledons, 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. Thus, in wheat, which differs by the C 3 -way of photosynthesis, bundles are formed from procambium and have 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 perform 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 transmission 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 bunches are characteristic of many herbaceous flowering plants of the Dicotyledonous class, which have a bunchy 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 bundle 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 can 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.

For 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 ensures that water flows 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 cortex 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 departments 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 multi-rayed 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 the 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.

Plant tissues: conductive, mechanical and excretory

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 phloem - organic substances synthesized in leaves and green stems move to other organs (to the roots).

Xylem and phloem are complex tissues that are made up of three basic elements:

The parenchyma cells, which serve for the transport of substances between plant tissues, also perform a conducting function (for example, the core rays of woody stems provide the movement of substances in a horizontal direction from the primary cortex to the core).

Xylem

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

Tracheids Are the most ancient conductive elements of xylem. Tracheids are represented by 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, stretching. The cell walls contain complex pores that are covered by a pore membrane through which water passes. Filtration of solutions takes place through the pore membrane. The movement of fluid along the tracheids is slow, since the pore membrane impedes the movement of water. In higher spore and gymnosperms, tracheids account for about 95% of the wood volume.

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 off. There are transverse walls between the cells of the vessels, which have large openings. In the walls of blood vessels there are thickenings of various shapes (annular, 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 through the vessels faster than through the tracheids.

Phloem

Phloem (from the Greek. phloyos- 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, nuclei. Provide movement from the leaves along the stem to the root (carry out organic matter, photosynthetic products). They have a branched network of fibrils, the inner contents are heavily watered. Separated from each other by film partitions with a large number of small holes (perforations) - sieve (perforation) plates (resemble a sieve). The longitudinal membranes of these cells are thickened, but not woody. In the cytoplasm of the sieve tubes is destroyed tonoplast (shell of vacuoles), and the vacuolar juice with dissolved sugars is mixed with the cytoplasm. With the help of strands of cytoplasm, adjacent sieve tubes are combined into a single whole. The speed of movement along the sieve tubes is less than along the vessels. Sieve tubes have been functioning for 3-4 years.

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

Phloem and xylem form vascular fibrous (conducting) bundles ... They can be seen in the leaves and stems of herbaceous plants. In tree trunks, conducting beams 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 found 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; therefore, the formation of new elements in them does not occur. Closed bunches are found mainly in monocotyledonous plants. Open the vascular fibrous bundles between the phloem and xylem have a cambium layer. Due to the activity of the cambium, the bundle grows and the organ thickens. Open bundles are found mainly in dicotyledonous and gymnosperms.

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

Colenchyma

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

Sclerenchyma

Sclerenchyma (from the Greek. sclerosis- hard) is a mechanical tissue that consists of lignified (lignin-impregnated) predominantly dead cells that have uniformly thickened cell walls. The nucleus and cytoplasm are destroyed. There are two types: sclerenchymal fibers and sclereids.

Sclerenchymal 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 adjacent to one another. On the cross section - multifaceted.

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

The sclerenchymal fibers of the bast are called bast fibers. Usually they are not lignified, strong and elastic (used in the textile industry - flax fibers, etc.).

Sclereids

Formed from the cells of the main tissue due to the thickening of the cell walls, their impregnation with lignin. They have different shapes and are found in different plant organs. Sclereids with the same cell diameter are called stony cells ... They are the most durable. Found in pits of apricots, cherries, shells walnuts etc.

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

Excretory tissue plants

As a result of the metabolic process in plants, substances are formed that, for various reasons, are almost never used (with the exception of milky juice). Usually, these foods accumulate in certain cells. The excretory tissues are represented by groups of cells or single ones. They are divided into external and internal.

External excretory tissues

External The 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 secretions are excreted. The excretory passages in different directions penetrate the stems and partially the leaves and have a shell of several layers of dead and living cells. Modifications of the epidermis are represented by multicellular (less often unicellular) glandular hairs or plates of various structures. External excretory tissues produce essential oils, balms, resins, etc.

About 3 thousand species of gymnosperms and angiosperms are known to produce essential oils. About 200 types (lavender, rose oils, etc.) of them are used as medicinal products, in perfumery, cooking, making varnishes, etc. Essential oils Are light organic substances of different chemical composition... Their importance in plant life: they attract pollinators by smell, scare off enemies, some (phytoncides) - kill or suppress the growth and reproduction of microorganisms.

Resins are formed in the cells that surround the resin passages, as waste products of gymnosperms (pine, cypress, etc.) and angiosperms (some legumes, umbrella plants, etc.). These are various organic substances (resin acids, alcohols, etc.). Outwardly excreted with essential oils in the form of thick liquids, which are called balms ... They have antibacterial properties. 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 technique for the manufacture of microslides. The basis of coniferous balsams is turpentine (used as a solvent for paints, varnishes, etc.) and solid resin - rosin (used for soldering, making varnishes, sealing wax, rubbing bowed strings musical instruments). Fossilized resin conifers the second half of the Cretaceous-Paleogene period is called amber (used as a raw material for jewelry).

The glands located in the flower or on different parts of the shoots, the cells of which secrete nectar, are called nectaries ... They are formed by the main tissue and have ducts that open outward. The outgrowths of the epidermis that surround the duct give the nectary a different shape (hump-shaped, pit-shaped, horn-shaped, etc.). Nectar 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 to pollinate flowers.

Thanks to hydathodes - water stomata - occurs gutting - the release of droplet water by plants (during transpiration, water is released in the form of steam) and salts. Gutting is a defense mechanism that occurs when transpiration fails to remove excess water. Typical for plants that grow in humid climates.

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 there are not enough of them in the soil. Digested substances are absorbed through the stomata. The most famous are pemphigus and sundew.

The glandular hairs accumulate and excrete, 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 outward during the life of a plant. This, for example, milkmen - a system of elongated cells of some plants, along which the sap moves. The sap 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 (spurge, poppy, etc.) or orange (celandine) color. The milky juice of some plants (for example, Brazilian Hevea) contains a significant amount rubber .

To the inner excretory tissue belong idioblasts - separate scattered cells among other tissues. Crystals of calcium oxalate, tannins, etc. accumulate in them. Cells (idioblasts) of citrus fruits (lemon, tangerine, orange, etc.) accumulate essential oils.