Conductive fabrics. botany. Sieve tubes and vessels - elements of the conductive tissue of plants Wood fibers are classified as conductive tissues

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 organs of the plant.

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. 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 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.

6.1. 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) movement of water and mineral substances absorbed by the roots from the soil, as well as organic matter formed in the roots, in 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.

6.2. 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 have more ancient origins... 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 consist of elongated dead cells located one above the other and called the segments of the vessel. 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 overpressure 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.

6.3. Downdraft conductive tissue

Downdraft tissues include sieve cells and sieve tubes with companion cells. Sieve cells are of more ancient origin. 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.

6.4. 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 of educational tissue is used, as in monocots, they are called closed for 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 component parts 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.

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.

6.5. 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.

6.6. 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. Around the conductive tissues in one or two layers is the pericycle [K. Esau, 1969]. Haplostela was known among fossil rhinophytes and preserved among some psilotophytes (tmezipter).

A more developed type of protostela is the actinostela, in which the xylem in the cross section acquires the shape 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 the actinostela and plectostela, the phloem still surrounds the xylem from all sides.

In the course of evolution, a siphonostel arose from the protostela, distinctive feature which is a 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.

25.8.1. The excretory system of plants and its significance

Plant life is a genetically determined set of biochemical reactions, the speed and intensity of which is largely modified by the conditions of the growing environment. In these reactions, a wide variety of by-products are formed that are not used by the plant to build the body or to regulate the exchange of substances, energy and information with the environment. Such products can be removed from the plant different ways: during the dying off and separation of branches and areas of rhizomes, when leaves fall off and the outer layers of the crust peel off, as a result of the activity of specialized structures of external and internal secretion. Together, these adaptations form the excretory system of plants.

Unlike animals, the excretory system in plants is not aimed at removing nitrogen compounds, which can be reused in the process of life.

The excretory system of plants is multifunctional. In its structures are carried out: synthesis, accumulation, conduction and release of metabolic products. For example, in the secretory cells of the resin ducts in the leaves of conifers, resin is formed, which is released through the resin ducts. In the nectaries of linden flowers, sweet juice nectar is formed and secreted. Citrus fruits accumulate essential oils in special containers in the fruit shell.

The formation and excretion of metabolic by-products has a multifaceted adaptive meaning:

Attracting pollinating insects. In the flowers of apple, cucumber and other entomophilous cross-pollinators, nectar is formed, which attracts bees, and the fetid secretions of the rafflesia flower attract flies;

Scaring away herbivores (cumin, nettle, etc.);

Protection against bacteria and fungi that destroy wood (pine, spruce, etc.);

Release of volatile compounds into the atmosphere, which helps to cleanse the air from pathogenic bacteria;

Extracellular digestion of prey in insectivorous plants due to the release of proteolytic enzymes (sundew, aldrovanda, etc.);

Mineralization of organic residues in the soil due to the release of special soil enzymes by the roots;

Regulation water regime by means of water stomata - hydatodes located along the edge of the leaf blade (strawberry, cabbage, fat woman, etc.);

Regulation of water evaporation as a result of the release of volatile ether compounds, which reduce the transparency and thermal conductivity of the air near the surface of the sheet ( conifers);

Regulation of the salt regime of cells (marrow, quinoa, etc.);

Changes in the chemical and physical properties of the soil, as well as regulation of the species composition of soil microflora under the influence of root exudates;

Regulation of the interaction of plants in the phytocenosis by means of root, stem and leaf secretions, called allelopathy (onion, garlic, etc.).

The substances secreted by plants are very diverse. Their nature depends on the genotype of the plant.

Many species secrete water (strawberries, cabbage), salts (cheese, quinoa), monosaccharides and organic acids (dandelion, chicory), nectar (linden, buckwheat), amino acids and proteins (poplar, willow), essential oils (mint, rose) , balsam (fir), resins (pine, spruce), rubber (hevea, kok-sagyz), mucus (root cap cells, swelling seeds of various plants), digestive juices (sundew, fat), poisonous liquids (nettle, cow parsnip) and other connections.

26.1.1. Polarity

Polarity is the presence of biochemical, functional and structural differences in diametrically opposite parts of the organs of an integral plant organism. Polarity affects the intensity of biochemical processes in the cell and the functional activity of organelles, determines the design of anatomical structures. At the level of an integral plant organism, polarity is associated with the direction of growth and development under the conditions of the action of gravitational forces.

The phenomenon of polarity is observed at different levels of plant organization. At the molecular level, it manifests itself in the structure of molecules of organic substances, primarily nucleic acids and proteins. So, the polarity of DNA strands is determined by a special order of joining its nucleotides. The polarity of the polypeptides is associated with the presence of the amino group –NH 2 and the carboxyl group –COOH in the amino acid composition. The polarity of chlorophyll molecules is due to the presence of a tetrapyrrole porphyrin nucleus in it and residues of alcohols - methanol and phytol.

Cells and their organelles can have polarity. For example, at the regeneration pole of the Golgi complex, new dictyosomes are formed, and at the secretory pole, vesicles are formed, which are associated with the excretion of metabolic products from the cell.

Cell polarity arises in the course of their ontogenesis. Cell polarization occurs as a result of pH gradients, electric charges and osmotic potential, concentration of O 2 and CO 2, calcium cations, physiologically active substances and elements of mineral nutrition. It can also arise under the influence of mechanical pressure, surface tension forces, and the influence of neighboring cells.

Cell polarity is symmetrical and asymmetrical. Symmetrical polarity is an indispensable condition for the division of the original and the formation of equivalent daughter cells. In particular, cytoskeleton microtubules move to the equatorial plane and participate in phragmoplast formation; chromosomes divide and their chromatids, using the pulling filaments of the achromatin spindle, diverge to the opposite poles of the cell. Here ribosomes and mitochondria begin to concentrate. Golgi complexes move towards the center and are specifically oriented in space. Their secretory poles are directed towards the equator of the dividing cell, which ensures the active participation of this organoid in the formation of the middle lamina and the primary cell membrane.

Asymmetrical polarity is characteristic of specialized cells. So, in a mature egg, the nucleus is displaced to the pole oriented to the chalase, the ovule, and a large vacuole is located at the micropillar pole.

The asymmetrical polarity is even more pronounced in the fertilized ovum - the zygote. It has a nucleus surrounded by tubules endoplasmic reticulum located on the chalazal side of the cell. Here, the elements of the cytoskeleton are more densely located, the optical density of the cytoplasm increases, an increased content of enzymes, phytohormones and other physiologically active substances is noted. Its functionally more active part becomes apical, and the opposite part becomes basal.

From the apical cell, during the development of the embryo of the seed, an embryonic stalk with a growing cone and primordial leaves is formed. A pendant is formed from the basal cell, and later - the spine root.

Due to the polarity of the cells of the developing embryo, the mature seed is also polarized. For example, in a wheat seed, on one side, there is an endosperm, and on the other, an embryo, in which the apical meristem of the stem and the tip of the root are at opposite poles.

In contrast to cells, the polarity of plant organs is more universal. It manifests itself in the structure of the shoot and root system, as well as their constituent parts. For example, the basal and apical parts of the shoot differ morphologically, anatomically, histologically, biochemically, and functionally. In many flowering plants, there is an apical bud at the top of the shoot, in which the most important morphogenetic center of the plant is located - the growth cone. Thanks to its activity, the rudiments of leaves, lateral axillary buds, nodes and internodes are formed. Differentiation of cells of the primary meristem leads to the emergence of primary integumentary tissues, histological elements of the primary cortex and the central cylinder.

The shoot tip is a powerful attracting center. This is where the main flow of water and dissolved elements of mineral nutrition and organic substances synthesized at the root is directed. The predominant supply of cytokinin to the apical kidney leads to apical dominance. The cells of the apical bud apex are actively dividing, ensuring the growth of the stem in length and the formation of new leaves and axillary buds on the main axis. In this case, there is a correlative inhibition of the development of axillary buds.

A good example of shoot morphological polarity is the structure of the wheat stem. As you move from bottom to top, from the basal part of the shoot to the apical, internodes become longer, their thickness in the middle part first increases and then gradually decreases. In the leaves of the upper layer, the ratio of the width of the leaf blade to its length is much greater than in the leaves of the lower layer. The position of the leaves in space also changes. In wheat, the leaves of the lower tier droop down, the leaves of the middle tier are located almost plagiotropically, i.e. parallel to the surface of the earth, and the flag (upper) leaves tend to an orthotropic - almost vertical, position.

Shoot polarity is well traced at the anatomical level. In wheat species, the poplar internode, in comparison with the lower located internodes, is distinguished by a smaller diameter, a smaller number of conducting bundles, and a better developed assimilatory parenchyma.

In dicotyledonous plants, the anatomical polarity of the shoot is enhanced by the emergence of secondary lateral educational tissues - cambium and phellogen. Pellogen generates a secondary covering tissue of phellem, which, being saturated with suberin, turns into a cork. In tree species, phellogen activity leads to the formation of a tertiary covering complex - a crust. Cambium provides a transition to the secondary anatomical structure of the stem below the level of the apical kidney.

The roots, as well as other plant organs, are also characterized by structural and functional polarity. Due to the positive geotropism, the basal part of the root is located at the soil surface. It is directly connected with the root collar - the place where the root passes into the stem. The apical part of the root is usually buried in the soil. Interconnected processes of growth and development take place in it. Cell division of the apex meristem provides linear root growth. And as a result of cell differentiation, qualitatively new structures are formed. Root hairs appear on the cells of the epiblema. The elements of the primary cortex are formed from the cells of the peribleme, and the pericycle and the conductive elements of the central cylinder are formed from the pleroma. This design of young root areas ensures active absorption of water and minerals, as well as their supply to the higher root areas. The functions of the basal part of the root are somewhat different. So, in perennial dicotyledons, the basal part of the root performs a transport, support and storage function. The anatomical structure corresponds to the fulfillment of these functions. Conductive tissues are better developed here, the cortex is formed due to the activity of the cambium, integumentary tissue represented by a cork. The polarity of the root structure ensures the diversity of its functions.

Polarity is also characteristic of the reproductive organs of plants. So, the flower, being a modified shortened shoot, retains the signs of shoot polarity. Parts of the flower are located on it in a regular sequence: calyx, corolla, androecium and gynoecium. This arrangement promotes better capture of pollen by the stigma of the pistil, and also protects the generative parts of the flower by vegetative ones. The polarity of the inflorescences is very indicative. In indeterminate inflorescences, the flowers of the basal part are laid first. They reach large sizes and produce fruit with well-developed seeds. In particular, larger seeds in sunflower are formed in the peripheral part of the basket, and in a complex ear of wheat, the best seeds are formed in the first flowers of the spikelets. Flowers of the apical part of inflorescences are formed later. The fruits and seeds obtained from them are smaller, and their sowing qualities are lower.

Thus, polarity is an important constructive, functional, and biochemical feature of plants that has an adaptive value and must be taken into account in agronomic practice.

1.2. Symmetry

The world around us is characterized by integrity and harmonious orderliness. In the nearest outer space, the position, mass, shape and trajectory of movement of objects are harmonious. Solar system... On Earth, seasonal and daily changes in the most important physical parameters of the conditions of existence of living organisms are harmonious, which are also characterized by harmonious coordination of structure and functions. The sages of the Pythogor school believed that harmony is "a way of coordinating many parts, with the help of which they unite into a whole." Symmetry is a reflection of harmony in nature. According to Yu.A. Urmantsev symmetry is a category that denotes the preservation of the features of objects in relation to their changes. In utilitarian terms, symmetry is the uniformity of the structure and the mutual arrangement of the same type of constituent parts of a single whole. Symmetry is inherent in both minerals and wildlife. However, the forms of symmetry and the degree of their manifestation in different objects differ significantly.

An essential feature of the symmetry of objects of different origins is its relational character. Comparison of structures is carried out on several characteristic points. The points of the figure that give the same picture when looking at the figure with different sides are called equal. These can be equidistant points on a straight line, points of intersection of the sides of an isosceles triangle and a square, faces of a polyhedron, points on a circle or on the surface of a ball, etc.

If there is no other equal point in the figure for some point X, then it is called special. Figures that have one special and several equal points are called rosettes. Shapes that do not contain equal points are considered asymmetric.

The axes and planes of symmetry pass through the singular points. In an isosceles triangle, three can be drawn, in a square - four, in an equilateral pentagon - five planes of symmetry. Accordingly, two beams of symmetry are formed on a straight line, three-beam symmetry is characteristic of a triangle, four-beam symmetry for a square, and multi-beam symmetry for a circle.

The movements of a figure, as a result of which each point is replaced by an equal point, and each singular point remains in place, are called symmetry transformations, and figures for which a symmetry transformation is allowed are considered symmetric.

The most common forms of symmetry transformation are:

1. REFERENCE - movement of each point located at some distance from the fixed plane, in a straight line perpendicular to this plane, at the same distance on the other side of it (for example, mirror symmetry of a zygomorphic pea flower);

2. P about in about t - the movement of all points at a certain angle around a fixed axis (for example, the multi-beam symmetry of an actinomorphic cherry blossom);

3. Paralle lny feathers, for example, the location of metameres on the shoot.

Symmetry transformation forms are not identical and do not reduce to one another. Thus, in the case of reflection, the plane remains stationary; when turning - straight line (axis); and with parallel translation, no point remains in place.

Symmetrical shapes with several equal points can have a different number of special points. So, the rosette has one particular point. The main form of symmetry transformation for it is rotation. For figures that do not have singular points, a parallel transfer, or shift, is characteristic. Such figures are conventionally called endless.

2.2.1. Features of the manifestation of symmetry in plants. The symmetry of plants differs from the symmetry of crystals in a number of features.

1) The symmetry of plants is determined not only by the symmetry of the molecules that form their cells, but also by the symmetry of conditions environment in which the development of plants takes place.

Particular cases of the symmetry of the habitat can be considered the homogeneity of the addition of the root layer of the soil, the uniformity of distribution in the soil of water and mineral nutrients, and equal remoteness of plants growing nearby. These circumstances must be taken into account when growing cultivated plants.

2) Plants, like other living organisms, do not have absolute identity of the elements of their constituent parts. This is primarily determined by the fact that the conditions for the formation of these parts are not absolutely identical. On the one hand, this is due to the difference in timing of the segregation of the growth cone of different metameres, including the rudiments of leaves, buds, nodes, and internodes. The same applies to the generative organs. Thus, the marginal tubular flowers in the sunflower basket are always larger than the flowers in the central part of the inflorescence. On the other hand, during the growing season, both regular and random changes in development conditions are observed. For example, the temperature of air and soil, illumination and spectral composition of light change statistically regularly during the growing season. Local changes in stock can be accidental nutrients in soil, exposure to diseases and pests.

The absence of absolute identity of the constituent parts is of great importance in the life of plants. The resulting heterogeneity is one of the mechanisms for the reliability of ontogenesis.

3) Plants are characterized by substantial, spatial and temporal symmetry.

A. Substantial measurement consists in the exact repetition of the shape and linear parameters of the structures, as well as in the exact change of these parameters. It is characteristic of crystals of inorganic substances, molecules of organic compounds, cell organelles, anatomical structures and plant organs. High level DNA molecules differ in substantial symmetry. When the moisture content of the preparations is close to physiological, the DNA molecule is in the B-form and is characterized by a clear repetition of the parameters of its constituent elements. Each turn of the helix of the DNA molecule contains 10 nucleotides; the projection of the coil onto the axis of the molecule is 34.6 Å (1 Å = 1 · 10 –10 m); the projection distance between adjacent nucleotides is 3.4 Å, and the linear distance is 7 Å; the diameter of the molecule when oriented along the phosphorus atoms is close to 20 Å; the diameter of the large groove is approximately 17 Å, and the diameter of the small one is 11 Å.

The substantial symmetry of plants is characterized not only by the exact repetition of the parameters of the structures, but also by their regular change. For example, in spruce, the decrease in the diameter of the trunk per 1 m of length, when moving from its basal part to the top, is relatively constant. In purple canna (Canna violacea), the flower is traditionally considered asymmetrical. He has sepals, petals and staminodes have different sizes... However, the change in the linear dimensions of these members of the flower remains relatively constant, which is a sign of symmetry. Only here, instead of the mirror one, other forms of symmetry develop.

B. The space dimension consists in a regularly repeated spatial arrangement of the same type of constituent parts in plants. Spatial symmetry is widespread in the plant world. It is typical for the location of buds on the shoot, flowers in the inflorescence, flower members on the receptacle, pollen in the anther, scales in gymnosperms cones and many other cases. A special case of spatial symmetry is longitudinal, radial and mixed symmetry of the stem.

Longitudinal symmetry arises during parallel transfer, i.e. spatial repetition of metameres in the shoot structure. It is determined by the constructive similarity of the constituent parts of the shoot - metameres, as well as the correspondence of the length of internodes to the rule of the "golden section".

A more complex case of spatial symmetry is the arrangement of leaves on the shoot, which can be whorled, opposite, and alternate (spiral).

Radial symmetry arises when the structure axis is combined with symmetry planes passing through it. Radial symmetry is widespread in nature. In particular, it is characteristic of many species of diatoms, a cross-section of the stem of higher spore plants, gymnosperms and angiosperms.

The radial symmetry of the shoot is multi-element. It characterizes the location of the constituent parts and can be expressed by different dimensional indicators: the distance of the conducting bundles and other anatomical structures from a particular point through which one or more planes of symmetry pass, the rhythm of the alternation of anatomical structures, the angle of divergence, which shows the displacement of the axis of one structure in relation to axis of the other. For example, in a cherry blossom, five corolla petals are arranged spatially symmetrically. The place of attachment of each of them to the receptacle is at the same distance from the center of the flower, and the divergence angle will be equal to 72º (360º: 5 = 72º). For a tulip flower, the divergence angle of each of the six petals is 60º (360º: 6 = 60º).

C. The temporal dimension of plants is expressed in the rhythmic repetition in time of the processes of morphogenesis and other physiological functions. For example, the segregation of the growth of leaf primordia by a cone is carried out at more or less equal intervals of time, called a plastochron. Seasonal changes in the processes of vital activity in perennial polycarpic plants are very rhythmically repeated. Temporal symmetry reflects the adaptation of plants to daily and seasonal changes in environmental conditions.

4) Plant symmetry develops dynamically during ontogenesis and reaches its maximum expression during sexual reproduction. In plants, an example of the formation of the radial symmetry of the shoot is indicative in this respect. Initially, the cells of the growth cone are more or less homogeneous, not differentiated. Therefore, it is morphologically difficult to distinguish a cell or a group of cells through which a plane of symmetry could be drawn.

Later, the cells of the tunic form the protoderm, from which the epidermis is formed. Assimilating and storing tissues, as well as primary mechanical tissues and core, develop from the main meristem. In the peripheral zone, strands of narrow and long procambium cells are formed, from which conducting tissues will develop. With a continuous formation of procambium, continuous layers of phloem and xylem are formed from it. If the procambium is laid in the form of strands, then separate conducting bundles are formed from it. In woody plants, the perennial, seasonally changing functional activity of the cambium will lead to the formation of secondary bark and annual rings of wood, which will enhance the radial symmetry of the stem.

5) The symmetry of plants changes during evolution. Evolutionary transformations of symmetry are of great importance in the development of the organic world. Thus, the emergence of bilateral symmetry was a major morphophysiological adaptation (aromorphosis) that significantly raised the level of organization of animals. In the plant kingdom, a change in the number of basic types of symmetry and their derivatives is associated with the appearance of multicellularity and the emergence of plants on land. In unicellular algae, in particular in diatoms, radial symmetry is widespread. In multicellular organisms, various forms of parallel transfer appear, as well as mixed forms of symmetry. For example, in charoh algae, the radial symmetry of the thallus cross section is combined with the presence of a metameric transfer axis.

The evolution of the conducting system was of great importance in the formation of the symmetry of higher plants. The radially symmetric stem protostele of primitive forms was replaced by a complex of bilaterally symmetric (monosymmetric) collateral bundles that form the eustela and ensure the radial symmetry of the stem in angiosperms.

The general trend in the evolution of symmetry in living organisms, including plants, is a decrease in the level of symmetry. This is due to a decrease in the number of basic types of symmetry and their derivatives. So, evolutionarily earlier flowering plants (family Magnoliaceae, family Buttercup, etc.) are characterized by polynomial, free, spirally located parts of the flower. In this case, the flower turns out to be actinomorphic, i.e. polysymmetric. The formulas of such flowers tend to the expression: Å Ca ¥ Co ¥ A ¥ G ¥ ... In the course of evolution, a reduction in the number of flower members is observed, as well as their accretion, which invariably leads to a decrease in the number of symmetry planes.

In evolutionarily young families, for example, in lamines or bluegrass, flowers become zygomorphic (monosymmetric). The dissymmetrization of flowers has clearly an adaptive meaning associated with the improvement of pollination methods. This was often facilitated by the coupled evolution of the flower and pollinators - insects and birds.

The specialization of the symmetry of flowers in the inflorescence has acquired great importance. So, in a sunflower basket, sterile marginal zygomorphic flowers have a large false tongue of yellow color, formed by three fused petals. The fertile flowers of the central part of the inflorescence are actinomorphic, they are formed by five small equal-sized petals accreted into a tube. Another example is the representatives of the Celery family. In them, in a complex umbrella, the marginal flowers are weakly zygomorphic, while other flowers remain typically actinomorphic.

The evolution of plant structure and symmetry is not straightforward. The dissymmetrization, predominant in terms of its role, is replaced and supplemented at certain stages of evolution by symmetrization.

Thus, the symmetry of plants and their constituent parts is very multifaceted. It is associated with the symmetry of its constituent elements at the molecular, cellular, histological-anatomical and morphological levels. Symmetry develops dynamically in the course of ontogeny and phylogeny and provides a connection between plants and the environment.

1.3. Metamerism

An important morphophysiological adaptation of plants is metamerism, which is the presence of repeating elementary finite structures, or metameres, in the system of a whole organism. The metameric construction provides multiple repetition of the constituent parts of the shoot and, therefore, is one of the mechanisms for the reliability of ontogenesis. The metameric construction is characteristic of various taxonomic groups of plants. It is known in charove algae, horsetails and other higher spore plants, gymnosperms, terrestrial and aquatic angiosperms. Functionally different parts of plants are metameric - vegetative and generative. With the participation of metamerically arranged organs in plants, a metameric system is formed. Branching is a special case of the formation of metameric systems.

The metamer of the vegetative zone of the shoot of angiosperms includes a leaf, a node, an internode and a lateral axillary bud, which is located at the base of the internode on the side opposite to the place of leaf attachment. This bud is covered by a sheet of the previous metamer. The metameres of the generative zone are very diverse. For example, in wheat, metameres of a compound spike consist of a segment of the spikelet and a spikelet attached to it. Sometimes plants have a transitional shoot zone. In some species and varieties of wheat, it can be represented by scales of underdeveloped spikelets.

Metamericity of plants is a morphological expression of the specificity of their growth and morphogenesis, which proceed rhythmically, in the form of repeated subordinate cycles localized in the foci of the meristem. The rhythm of the formation of metamers is inextricably linked with the periodicity of growth processes characteristic of plants. The formation of metameres is the primary morphogenetic process in plant development. It forms the basis of the complication of organization in ontogeny and reflects the process of polymerization, which is one of the mechanisms of the evolution of higher plants.

The formation and development of metameres is primarily provided by the function of the apical and intercalary meristems.

In the vegetative bud, as well as in the embryo of the germinating seed, as a result of mitotic division, an increase in the volume of the growth cone of the embryonic stalk occurs. The subsequent active division of cells in the peripheral zone of the growth cone leads to the formation of a leaf primordium - a leaf primordium and an insertion disc. At this time, the meristematic activity of the tunic and the central meristematic zone is somewhat reduced, but the cells of the insertion disc are actively dividing. The upper part of the disc is the place of attachment of the leaf primordium, and when it grows in thickness, a node is formed from it. An internode develops from the lower part of the insertion disc. Here, the rudimentary tubercle of the lateral axillary bud is formed on the side opposite to the midrib of the primordial leaf. Taken together, the leaf bud, the insertion disc, and the bud bud are an embryonic metamere.

As the formation of the embryonic metamer is completed, the activity of cell division in the tunic and the central meristematic zone increases again. The volume of the smooth part of the growth cone increases again, reaching a maximum before the formation of the next leaf primordium. Thus, a new one begins to form on the apical part of the previous embryonic metamere. This process is genetically determined and rhythmically repeated many times. In this case, the metamer formed first will be located in the basal part of the shoot, and ontogenetically the youngest - in the apical part. Accumulation of the number of metameric primordia in T.I. Serebryakova called it ripening. She coined the term "kidney capacity" to denote the maximum number of metameres that can be deposited in the kidney.

The growth and development of internodes as constituent parts of the metamer are largely due to the activity of the intercalary meristem. The division of the cells of this meristem and the elongation of their derivatives leads to the elongation of the internodes.

Metamers have a number of characteristic features that allow them to ensure the structural and functional integrity of the plant organism.

1) Polarity of metamers. Each metamere has a basal and apical portion. The term "apical part" suggests that the upper part of the metamere either contains the apical meristem or is oriented towards the apex.

The basal and apical parts differ in morphological, histological-anatomical and physiological-biochemical characteristics. For example, in wheat, when moving from the basal to the apical part of the internode, the thickness of the stem, the diameter of the medullary lacuna, and the thickness of the peripheral ring of the sclerenchyma first increase and then gradually decrease; the number of cells in the cords of the assimilation parenchyma increases significantly; the radial diameter of the conducting bundles decreases, as well as the number of vessels in the xylem.

2) Symmetry of metameres. The symmetry of plant organs is ensured by the symmetry of the metamers that form them, which arises as a result of a specific sequence of cell division of the growth cone.

3) Heterochronism of metamer formation. Metameres at different times, alternately isolated by a growth cone. Therefore, the first metameres of the basal part of the shoot are ontogenetically older, and the last metameres of the apical part are younger. Morphologically, they differ in sheet parameters, length and thickness of internodes. For example, in cereals, the leaves of the upper tier are wider than the leaves of the lower tier, and the under-spike internodes are longer and thinner than the lower ones.

4) Variability of metamer traits. Ontogenetically younger metameres have a lower amplitude of variability of characters. Therefore, the anatomical features of the poplar internode can be used with greater accuracy to identify varieties of cereal crops and to compile breeding programs.

5) Optimality of the design of metamers. The optimal biological design is one that requires a minimum amount of organic matter to build and maintain. Since the structure of plants is inextricably linked with their function, the criterion of optimality can be the adaptability of plants to growing conditions and the ratio of their seed productivity to the mass of vegetative organs. The optimality of the design of an integral plant is ensured by the optimal design of its constituent metamers.

6) A whole plant is a polymeric system formed by a set of metamers. Polymerity manifests itself at all levels of the organization of the plant organism. So, in the structure of DNA there are many repetitions of genes; the karyotype of many plants is represented by a diploid or polyploid set of chromosomes; Plastids, mitochondria, ribosomes and other organelles are found in large numbers in the cell. Adult plant is a subordinate set of shoots, roots and reproductive organs with a metameric structure. The metameric system ensures high productivity of plants and significantly increases the reliability of their ontogenesis.

27.4.1. Primary anatomical structure of the root

The features of the primary structure are clearly manifested in the longitudinal and transverse sections of the root tip.

On a longitudinal section of the root tip, four zones can be distinguished

The root cap zone covers the apical root meristems. It is made up of living cells. Their surface layer is constantly sloughing off and lines the passage along which the root moves. The sloughing cells also produce mucus, which makes it easier for the root tip to move in the soil. The cells of the central part of the cap, or columella, contain starch grains, which contributes to the geotropic growth of the root. The cells of the cap are constantly renewed due to the division of cells of a special educational tissue - caliptrogen, which is characteristic of monocots.

I - zone of the root cap; II - growth zone; III - zone of root hairs; IV - the zone of the conduction. 1 - epiblema, 2 - pericycle, 3 - endoderm, 4 - primary cortex, 5 - exoderm, 6 - central cylinder, 7 - root hair; 8 - lateral root formation.

The growth zone consists of two subzones. In the subzone of division, root growth is carried out due to active mitotic cell division. For example, in wheat, the proportion of dividing cells (mitotic index) is 100-200 ppm. The division subzone is a valuable material for cytogenetic studies. Here it is convenient to study the number, macro- and microstructure of chromosomes. In the elongation subzone, the meristematic activity of cells decreases, but due to a certain balance of phytohormones, primarily auxins and cytokinins, root growth occurs due to axial elongation of young cells.

The suction zone can rightfully be called the zone of root hairs, as well as the zone of differentiation, since the epiblema, the primary cortex and the central cylinder are formed here.

Epiblema is a special, constantly renewed integumentary tissue, consisting of two types of cells. Thin-walled root hairs 1 - 3 mm long develop from trichoblasts, due to which water and substances dissolved in it are absorbed. Root hairs are short-lived. They live for 2 to 3 weeks and then slough off. Atrichoblasts do not form root hairs and perform an integumentary function.

The primary cortex regulates the flow of water into the conductive tissues of the central cylinder. During the formation of the central cylinder, protofloem tissues are the first to form, through which organic substances necessary for its growth enter the root tip. Above the root, at the level of the first root hairs, vascular elements of the protoxylem appear. The original vascular tissues develop into the primary phloem and primary xylem of the radial vascular bundle.

The conduction area occupies the largest part of the root. It performs numerous functions: transport of water and dissolved mineral and organic substances, synthesis of organic compounds, storage of nutrients, etc. The zone of conduction ends with a root collar, ie. the place of transition of the root to the stem. In dicotyledonous angiosperms, at the beginning of this zone, there is a transition from the primary to the secondary anatomical structure of the root.

On a cross section in the zone of root hairs, structural and topographic features of the primary anatomical structure of the root are revealed (Fig. 2). On the surface of the root there is an epiblem with root hairs, with the help of which water and substances dissolved in it are absorbed. Unlike the epidermis, the epiblema is not covered with cuticles and has no stomata. Under it is the primary cortex, which consists of exoderm, mesoderm and endoderm.

1 - root tip zones; 2 - 6 - cross sections at different levels; Vks - secondary xylem; Vf - secondary phloem; K - cambium; Ms, metaxylem; Mf - metaphloem; Pd - protoderm; Pcs - protoxylem; Prd - periderm; Pf - protofloem; PC - pericycle; Рд - rhizoderm; Ex - exoderm; En - endoderm; H - boot.

The cells of the exoderm are densely folded and have thickened membranes. The exoderm ensures the apoplastic flow of water into the deeper layers of the cortex, and also gives the root strength from the surface.

The middle layer of the cortex, the mesoderm, consists of thin-walled parenchymal cells and has a loose structure due to the presence of numerous intercellular spaces along which water moves to the exoderm.

The endoderm, or the inner layer of the primary cortex, consists of a single row of cells. It includes cells with Caspari belts and access cells. Cells with Caspari belts have thickened lateral (radial) and tangential (end) shells facing the central cylinder. These cellulosic nubs are impregnated with lignin and therefore cannot pass water. In contrast, the passage cells are thin-walled and are located opposite the xylem rays. It is through the passage cells that water enters the radial conducting bundle of the central cylinder.

The central cylinder of the root, or stele, consists of several layers of cells. One or more rows of pericycle cells are located immediately under the endoderm. Of these, during the transition to the secondary structure of the root in dicotyledonous angiosperms and gymnosperms, an interfundus cambium and phellogen (cork cambium) are formed. In addition, pericycle cells are involved in the formation of lateral roots. The cells of the radial conducting beam are located behind the pericycle.

The conducting bundle is formed from procambium. First, protofloem cells are formed, and then at the level of the first root hairs - protoxylem cells. The cells of the conductive tissues arise exarchically, i.e. from the surface of the beam, and further develop in the centripetal direction. In this case, the very first, larger vessels are located in the center of the xylem, and the younger ones, of smaller diameter, are at the periphery of the xylem ray. Phloem cells are located between the xylem rays.

The number of xylem rays depends on the systematic position of the plants. For example, some ferns may have only one ray of xylem and one patch of phloem. Then the bundle is called monarch. For many dicotyledonous angiosperms, diarchic bundles with two xylem rays are characteristic. In addition, they have plants with tri-, tetra- and pentarchy bundles. Monocotyledonous angiosperms are characterized by polyarchic vascular bundles with multi-beam xylem.

The structure of the radial conducting beam affects the way the lateral roots are laid. In the diarchic root, they are formed between the phloem and the xylem, in the triarchic and tetrarchous, opposite the xylem, and in the polyarchic, opposite the phloem.

Cellular structure the epiblema and primary cortex provides the root water pressure. Water is absorbed by root hairs, from which it enters the cell membranes of the exoderm, then enters the intercellular spaces of the mesoderm, and from them, through thin-walled passage cells, into the vessels of the radial conducting beam. Since root hairs have a larger total surface than permeable cells, the speed of movement of water, and therefore its pressure, increases as it approaches the vessels. The difference in the arising pressures is the root pressure, which is one of the mechanisms for the flow of water into the stem and other plant organs. The second important mechanism for the movement of water is transpiration.

28.4.2. Secondary anatomical structure of the root

In dicotyledonous angiosperms and gymnosperms, the primary anatomical structure of the root in the conduction zone is supplemented by structures of secondary origin, which are formed due to the emergence and meristematic activity of secondary lateral educational tissues - cambium and phellogen (cork cambium) (Fig. 3). During the transition to the secondary structure, the following significant changes occur at the root.

A. The appearance in the central cylinder of the cambium root and the secondary xylem and secondary phloem generated by it, which bandwidth far exceed the elements of the original radial conducting beam.

This type refers to complex tissues, consists of differently differentiated cells. In addition to the conductive elements themselves, the tissue contains mechanical, excretory and storage elements. Conductive tissues unite all plant organs into a single system. There are two types of conductive tissues: xylem and phloem (Greek xylon - tree; phloios - bark, bast). They have both structural and functional differences.

The conductive elements of the xylem are formed by dead cells. Long-distance transport of water and substances dissolved in it from the root to the leaves is carried out along them. Phloem conductive elements preserve living protoplast. Long-distance transport from photosynthetic leaves to the root is carried out along them.

Usually, xylem and phloem are located in the body of the plant in a certain order, forming layers or conductive bundles. Depending on the structure, several types of conducting beams are distinguished, which are characteristic of certain groups plants. In the collateral open bundle between the xylem and phloem, there is a cambium, which provides secondary growth. In the bicollateral open bundle, the phloem is located relative to the xylem on both sides. Closed bundles do not contain cambium, and hence are not capable of secondary thickening. Two more types of concentric bundles can be found, where either the phloem surrounds the xylem, or the xylem is the phloem.

Xylem (wood). The development of xylem in higher plants is associated with the provision of water exchange. Since water is constantly excreted through the epidermis, the same amount of moisture must be absorbed by the plant and added to the organs that carry out transpiration. It should be borne in mind that the presence of a living protoplast in the cells conducting water would greatly slow down the transport; dead cells here are more functional. However, the dead cell does not possess turgidity; therefore, the membrane must have mechanical properties. Note: turgence is the state of plant cells, tissues and organs, at which? they become elastic due to the pressure of the contents of the cells on their elastic membranes. Indeed, the conductive elements of the xylem consist of dead cells elongated along the axis of the organ with thick lignified membranes.

Initially, xylem is formed from the primary meristem, the procambium, located at the tops of the axial organs. First, protoxylem differentiates, then metaxylem. Three types of xylem formation are known. In the exarch type, protoxylem elements first appear on the periphery of the procambium bundle, then metaxylem elements appear in the center. If the process goes in the opposite direction (i.e. from the center to the periphery), then this is an endarchy type. In the mesarch type, the xylem is laid in the center of the procambial bundle, after which it is deposited both towards the center and towards the periphery.

The root is characterized by an exarch type of xylem initiation, while the stems are characterized by an endarch type. In low-organized plants, the methods of xylem formation are very diverse and can serve as systematic characteristics.

Some? In plants (for example, monocots), all procambium cells differentiate into conductive tissues that are not capable of secondary thickening. In other forms (for example, arboreal), lateral meristems (cambium) remain between the xylem and phloem. These cells are able to divide to renew the xylem and phloem. This process is called secondary growth. Many plants growing in relatively stable climatic conditions grow constantly. In forms adapted to seasonal climate changes - periodically.

The main stages of differentiation of procambium cells. Its cells have thin membranes that do not prevent their elongation during organ growth. Then the protoplast begins to deposit a secondary membrane. But this process has distinct features. The secondary membrane is not deposited in a continuous layer, which would not allow the cell to stretch, but in the form of rings or in a spiral. Elongation of the cell is not difficult in this case. In young cells, the rings or coils of the spiral are located close to each other. In mature cells, cells diverge as a result of stretching. The annular and spiral thickening of the envelope does not interfere with growth, but mechanically they yield to the envelope, where the secondary thickening forms a continuous layer. In this regard, after the cessation of growth, elements with a continuous lignified shell (metaxylem) are formed in the xylem. It should be noted that the secondary thickening here is not annular or spiral, but point, scaled, reticular. Its cells are not capable of stretching and die off within several hours. This process in nearby cells occurs in a coordinated manner. A large number of lysosomes appear in the cytoplasm. Then they disintegrate, and the enzymes in them destroy the protoplast. When the transverse walls are destroyed, the cells arranged in a chain one above the other form a hollow vessel. Most angiosperms and some? ferns possess vessels.

A conductive cell that does not form through perforations in its wall is called a tracheid. The movement of water along the tracheids is at a lower speed than through the vessels. The fact is that in tracheids the primary membrane is not interrupted anywhere. The tracheids communicate with each other through pores. It should be clarified that in plants the pore is only a depression in the secondary membrane up to the primary membrane and there are no through perforations between the tracheids.

Bordered pores are most common. In them, the channel facing the cell cavity forms an extension - the pore chamber. The pores of most conifers on the primary membrane have a thickening - the torus, which is a kind of valve and is able to regulate the intensity of water transport. Moving, the torus blocks the flow of water through the pore, but after that it can no longer return to its previous position, performing a one-time action.

The pores are more or less rounded, elongated perpendicular to the elongated axis (a group of these pores resembles a ladder, in this regard, such porosity is called ladder). Through the pores, transport is carried out both in the longitudinal and transverse directions. Pores are present not only in tracheids, but also in individual vascular cells that form a vessel.

From the point of view of evolutionary theory, the tracheids are the first and main structure that carries out the conduction of water in the body of higher plants. It is believed that the vessels arose from the tracheids as a result of lysis of the transverse walls between them. Most fern-like and gymnosperms do not have vessels. The movement of water in them occurs through the tracheids.

In the process of evolutionary development, vessels appeared in different groups of plants repeatedly, but they acquired the most important functional significance in angiosperms, in which? they are present along with tracheids. It is believed that the possession of a more perfect transport mechanism helped them not only to survive, but also to achieve a significant variety of forms.

Xylem is a complex tissue, in addition to water-conducting elements, it contains others. Mechanical functions perform libriform fibers (Latin liber - bast, forma - form). The presence of additional mechanical structures is important, since, despite the thickening, the walls of the water-conducting elements are still too thin. They are not able to independently hold a large mass of a perennial plant. Fibers developed from the tracheids. They are characterized by smaller sizes, lignified (lignified) shells and narrow cavities. On the wall, you can find pores devoid of bordering. These fibers cannot conduct water, their main function is supporting.

The xylem also contains living cells. Their mass can reach 25% of the total wood volume. Since these cells are round in shape, they are called wood parenchyma. In the body of a plant, the parenchyma is located in two ways. In the first case, the cells are arranged in the form of vertical strands - this is a heavy parenchyma. In another case, the parenchyma forms horizontal rays. They are called core rays because they connect the core and bark. The core performs a number of functions, including storage of substances.

Phloem (bast). This is a complex tissue, as it is formed by cells of different types. The main conductive cells are called sieve cells. The conductive elements of the xylem are formed by dead cells, and in the phloem they retain a living, albeit strongly altered, protoplast during the period of functioning. Through the phloem, there is an outflow of plastic substances from the photosynthetic organs. All living plant cells have the ability to conduct organic matter. And hence, if xylem can be found only in higher plants, then the transport of organic matter between cells is also carried out in lower plants.

Xylem and phloem develop from apical meristems. At the first stage, a protofloem is formed in the procambial mass. As the surrounding tissues grow, it stretches, and when growth is completed, a metafloem is formed instead of a protofloem.

In various groups of higher plants, two types of sieve elements can be found. In ferns and gymnosperms, it is represented by sieve cells. Sieve fields in cells are scattered along the side walls. A somewhat destructed nucleus remains in the protoplast.

In angiosperms, the sieve elements are called sieve tubes. They communicate with each other through sieve plates. In mature cells, nuclei are absent. However, next to the sieve tube is a companion cell, which is formed together with the sieve tube as a result of mitotic division of the common mother cell (Fig. 38). The companion cell has a denser cytoplasm with a large number of active mitochondria, as well as a fully functioning nucleus, a huge number of plasmodesmata (ten times more than other cells). Companion cells affect the functional activity of the anucleated sieve cells of the tubes.

The structure of mature sieve cells has some peculiarities. There is no vacuole, in this regard, the cytoplasm is greatly diluted. The nucleus may be absent (in angiosperms) or be in a shriveled, functionally inactive state. Ribosomes and the Golgi complex are also absent, but the endoplasmic reticulum is well developed, which not only penetrates the cytoplasm, but also passes into neighboring cells through the pores of the sieve fields. Well-developed mitochondria and plastids are abundant.

Between cells, the transport of substances goes through holes located on the cell membranes. Such holes are called pores, but unlike tracheid pores, they are through. It is assumed that they are highly expanded plasmodesmata, on the walls of which? the polysaccharide callose is deposited. The pores are arranged in groups, forming sieve fields. In primitive forms, sieve fields are randomly scattered over the entire surface of the shell, in more perfect angiosperms they are located at the ends of adjacent cells adjoining each other, forming a sieve plate. If there is one sieve-like field on it, it is called simple, if several - complex.

The speed of movement of solutions through sieve elements is up to 150 cm per hour. This is a thousand times the rate of free diffusion. Probably, active transport takes place, and numerous mitochondria of sieve elements and companion cells supply the necessary ATP for this.

The duration of the activity of the sieve elements of the phloem depends on the presence of lateral meristems. If they are present, then the sieve elements work throughout the life of the plant.

In addition to sieve elements and companion cells, the phloem contains bast fibers, sclereids, and parenchyma.

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. Thus, in the course of the evolution of the plant world, two types of conductive tissues arose - 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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In the process of evolution, it is one of the reasons that made it possible for plants to land on land. In our article, we will consider the features of the structure and functioning of its elements - sieve tubes and vessels.

Features of conductive fabric

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

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

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

Sieve tubes of plants

These are the conductive cells of the bast. They are separated from each other by numerous partitions. Outwardly, their structure resembles a sieve. This is where the name comes from. The sieve tubes of plants are living. This is due to the weak downward current pressure.

Their transverse walls are pierced with a dense network of holes. And the cells contain many through holes. They are all prokaryotic. This means that they do not have a formalized core.

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

Vessels

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

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

Soil nutrition mechanism

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

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

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

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

Sieve tubes are also located in the veins next to the vessels. These elements carry a downward current. Under the influence of sunlight, the polysaccharide glucose is synthesized in the chloroplasts of the leaf. The plant spends this organic matter on growth and vital processes.

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