Great importance in life land plants mechanical and conductive tissues play.
Mechanical fabrics
Everyone watched how a thin straw, supporting a heavy ear, swayed in the wind, but did not break.
Mechanical tissues give the plant strength. They serve as support for the organs in which they are located. Mechanical tissue cells have thickened membranes.
In leaves and other organs of young plants, cells mechanical fabric alive. This tissue is located in separate strands under the stem and petioles of leaves, bordering the veins of the leaves. Cells of living mechanical tissue are easily extensible and do not interfere with the growth of the part of the plant in which they are located. Thanks to this, plant organs act like springs. They are able to return to their original state after removing the load. Everyone has seen the grass rise again after a person has walked across it.
Mechanical tissue also serves as support for parts of the plant whose growth is complete, but the mature cells of this dead tissue. These include bast and wood cells - long thin cells collected in strands or bundles. The fibers give strength to the stem. Short dead cells of mechanical tissue (they are called stony cells) form the seed coat, nut shells, fruit seeds, and give the pear pulp its grainy character.
Conductive fabrics
All parts of the plant contain conductive tissues. They ensure the transport of water and substances dissolved in it.
Conductive tissues were formed in plants as a result of adaptation to life on land. The body of terrestrial plants is located in two environments of life - ground-air and soil. In this regard, two conductive fabrics arose - wood and bast. Water and mineral salts dissolved in it rise along the wood from bottom to top (from roots to). That's why wood is called a water-conducting fabric. Lub is inner part bark. Organic substances move along the bast from top to bottom (from leaves to roots). Wood and bast form a continuous branched system in the body of the plant, connecting all its parts.
The main conductive elements of wood are vessels. They are long tubes formed by the walls dead cells. At first, the cells were alive and had thin extensible walls. Then the cell walls became lignified and the living contents died. The transverse partitions between the cells collapsed, and long tubes formed. They consist of individual elements and are similar to bon din barrels and lids. Water with dissolved substances passes freely through the vessels of wood.
The conducting elements of the phloem are living elongated cells. They are connected at their ends and form long rows of cells - tubes. There are small holes (pores) in the transverse walls of the phloem cells. Such walls are similar to a sieve, which is why the tubes are called sieve-shaped. Solutions of organic substances move through them from the leaves to all organs of the plant.
25 ..CONDUCTIVE FABRICS.
Conductive tissues serve to transport dissolved substances in water throughout the plant. nutrients.
Rice. 43 Wood fibers of the meadow geranium leaf (transverse - A, B and longitudinal - C section of fiber groups):
1 - cell wall, 2 - simple pores, 3 - cell cavity
Like integumentary tissues, they arose as a consequence of the plant’s adaptation to life in two environments: soil and air. In this regard, it became necessary to transport nutrients in two directions.
An ascending, or transpiration, current of aqueous solutions of salts moves from the root to the leaves. The assimilation, downward flow of organic substances is directed from the leaves to the roots. The ascending current is carried out almost exclusively through the tracheal
Rice. 44 Sclereids of the stone of ripening cherry plum fruits with living contents: 1 - cytoplasm, 2 - thickened cell wall, 3-pore tubules
elements of xylem, a. descending - by sieve elements phloem.
A highly branched network of conducting tissues carries water-soluble substances and photosynthetic products to all plant organs, from the thinnest root endings to the youngest shoots. Conductive tissues unite all plant organs. In addition to long-distance, i.e. axial, transport of nutrients, short-range radial transport is also carried out through conducting tissues.
All conductive tissues are complex, or complex, that is, they consist of morphologically and functionally heterogeneous elements. Forming from the same meristem, two types of conducting tissues - xylem and phloem - are located nearby. In many plant organs, xylem is combined with phloem in the form of strands called vascular bundles.
There are primary and secondary conducting tissues. Primary tissues are formed in leaves, young shoots and roots. They differentiate from procambium cells. Secondary conducting tissues, usually more powerful, arise from the cambium.
Xylem (wood). Water and dissolved minerals move through the xylem from the root to the leaves. Primary and secondary xylem contain the same types of cells. However, the primary xylem does not have medullary rays, differing in this from the secondary.
The composition of xylem includes morphologically various elements, performing the functions of both carrying and storing reserve substances, as well as purely support functions. Long-distance transport is carried out through the tracheal elements of xylem: tracheids and vessels, short-distance transport is carried out through parenchymal elements. Supporting and sometimes storage functions are performed by part of the tracheids and fibers of the mechanical tissue of the libriform, which are also part of the xylem.
Tracheids in a mature state are dead prosenchymal cells, narrowed at the ends and devoid of protoplast. The length of tracheids is on average 1-4 mm, while the diameter does not exceed tenths or even hundredths of a millimeter. The walls of the tracheids become lignified, thicken and bear simple or bordered pores through which solutions are filtered. Most of the bordered pores are located near the ends of the cells, that is, where solutions leak from one tracheid to another. All sporophytes have tracheids higher plants, and in most horsetails, lycophytes, pteridophytes and gymnosperms they are the only conducting elements of the xylem.
Vessels are hollow tubes consisting of individual segments located one above the other.
Between the segments of the same vessel located one above the other there are different types through holes - perforations. Thanks to the perforations along the entire vessel, liquid flows freely. Evolutionarily, vessels apparently originated from tracheids by destruction of the closing films of the pores and their subsequent fusion into one or more perforations. The ends of the tracheids, initially strongly beveled, took a horizontal position, and the tracheids themselves became shorter and turned into segments of blood vessels (Fig. 45).
Vessels appeared independently in different evolutionary lines of land plants. However, they reach their greatest development in angiosperms, where they are the main water-conducting elements of the xylem. The appearance of vessels is an important evidence of the evolutionary progress of this taxon, since they significantly facilitate the transpiration flow along the plant body.
In addition to the primary shell, vessels and tracheids in most cases have secondary thickenings. In the youngest tracheal elements, the secondary membrane may take the form of rings not connected to each other (ringed tracheids and vessels). Later, tracheal elements with spiral thickenings appear. These are followed by vessels and tracheids with thickenings, which can be characterized as spirals, the turns of which are interconnected (scalene thickenings). Ultimately, the secondary shell merges into a more or less continuous cylinder, forming inward from the primary shell. This cylinder is interrupted in certain areas by pores. Vessels and tracheids with relatively small rounded areas of the primary cell membrane, not covered from the inside by the secondary membrane, are often called porous. In cases where the pores in the secondary membrane form something like a mesh or ladder, they speak of reticulate or scalariform tracheal elements (scalene vessels and tracheids ).
Rice. 45 Changes in the structure of tracheal xylem elements during their evolution (the direction is indicated by an arrow):
1,2 - tracheids with rounded bordered pores, 3 - tracheids with elongated bordered pores, 4 - a vessel segment of a primitive type and its perforation formed by close pores, 5 - 7 - successive stages of specialization of vessel segments and the formation of a simple perforation
The secondary, and sometimes the primary shell, as a rule, is lignified, that is, impregnated with lignin, this gives additional strength, but limits the possibility of their further growth in length.
Tracheal elements, i.e. tracheids and vessels, are distributed in the xylem in different ways. Sometimes on a cross section they form well-defined rings (ring-vascular wood). In other cases, the vessels are scattered more or less evenly throughout the entire mass of xylem (disseminated vascular wood). Features of the distribution of tracheal elements in the xylem are used to identify wood of various tree species.
In addition to the tracheal elements, the xylem includes ray elements, i.e., cells that form the medullary rays (Fig. 46), most often formed by thin-walled parenchyma cells (radial parenchyma). Ray tracheids are less common in the rays of conifers. The medullary rays carry out short-range transport of substances in the horizontal direction. In addition to conducting elements, the xylem of angiosperms also contains thin-walled, non-lignified living parenchyma cells called wood parenchyma. Along with the core rays, short-range transport is partially carried out along them. In addition, the wood parenchyma serves as a storage site for reserve substances. Elements
medullary rays and wood parenchyma, like tracheal elements, arise from the cambium.
The importance and variety of conductive tissues
Conductive tissues are the most important component of most higher plants. They are an obligatory 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 transmitting cells form a conducting system that ensures long-distance and radial transport of substances. Due to the special design of cells and their location in the plant body, 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, and reproductive organs;
2) movement of photosynthesis products from the green parts of the plant to places of their use and storage: roots, stems, fruits and seeds;
3) the movement of phytohormones throughout the plant, which creates a certain balance of them, which determines the rate of growth and development of the vegetative and reproductive organs of plants;
4) radial transport of substances from conducting tissues into nearby living cells of other tissues, for example, into assimilating leaf mesophyll cells and dividing meristem cells. Parenchyma cells of the medullary rays of wood and bark may also take part in it. Transmissive 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) conducting 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 onto land and the separation of their air and soil nutrition. The most ancient conducting tissues, tracheids, were found in fossil rhinophytes. They reached their highest development in modern angiosperms.
During the process of individual development, primary conducting tissues are formed from the procambium at the growth points of the seed embryo and renewal buds. Secondary conducting tissues, characteristic of dicotyledonous angiosperms, are generated by the cambium.
Depending on the functions performed, conducting tissues are divided into tissues of ascending current and tissues of descending current. The main purpose of ascending tissue is to transport water and minerals dissolved in it from the root to the higher above-ground organs. In addition, organic substances formed in the root and stem, such as organic acids, carbohydrates and phytohormones, move through them. However, the term “upward current” should not be taken unambiguously as movement from bottom to top. Ascending tissues ensure 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 located above or below the level of the roots. For example, in potatoes, water and mineral nutrition elements flow through ascending tissues into stolons and tubers formed in the soil, as well as into above-ground organs.
Downstream tissues ensure the outflow of photosynthetic products into the growing parts of plants and storage organs. In this case, the spatial position of photosynthetic organs does not matter. For example, in wheat, organic substances enter the developing grains from leaves of different tiers. Therefore, the names “ascending” and “descending” fabrics should be treated as nothing more than an established tradition.
Conductive tissues of ascending current
The ascending tissues include tracheids and vessels (tracheas), 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 influence 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 commonly in angiosperms. In angiosperms they are typical of the smallest branching of leaf veins. Tracheid cells are dead. They have an elongated, often spindle-shaped shape. Their length is 1 – 4 mm. However, in gymnosperms, for example in Araucaria, it reaches 10 mm. The cell walls are thick, cellulose, and often impregnated with lignin. The cell membranes have numerous bordered pores.
Vessels formed at later stages of evolution. They are characteristic of angiosperms, although they are also found in some modern representatives of the departments Mosses (genus Sellaginella), Horsetails, Ferns and Gymnosperms (genus Gnetum).
The vessels consist of elongated dead cells located one above the other and called vessel segments. In the end walls of the vessel segments there are large through holes - perforations, through which long-distance transport of substances occurs. Perforations arose during evolution from the bordered pores of tracheids. As part of the vessels they are ladder and simple. Numerous scalariform perforations are formed on the end walls of the vessel segments when they are laid obliquely. The openings 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 scalariform perforation are characteristic of plants of the Ranunculaceae, Limonaceae, Birch, Palm, and Chastukhova families.
Simple perforations are known in evolutionarily younger families, such as Solanaceae, Cucurbitaceae, Asteraceae, and Poaceae. They represent one large hole in the end wall of the joint, located perpendicular to the axis of the vessel. In a number of families, for example, Magnoliaceae, Roseaceae, Irisaceae, Asteraceae, both simple and scalariform perforations are found in vessels.
The side walls have uneven cellulose thickenings that protect the vessels from excess pressure created by nearby living cells of other tissues. There may be numerous pores in the side walls, allowing water to escape outside the vessel.
Depending on the nature of the thickenings, the types and nature of the location of the pores, the vessels are divided into annular, spiral, bispiral, reticular, scalariform 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 diffuse into the surrounding tissues. The diameter of these vessels is relatively small. In reticulate, scalariform, and punctate-pore vessels, the entire side wall, with the exception of the locations of simple pores, is thickened and often impregnated with lignin. Therefore, their radial transport of substances occurs through numerous elongated and pinpoint pores.
Vessels have a limited lifespan. They can be destroyed as a result of blockage by tills - outgrowths of neighboring parenchyma cells, as well as under the influence of centripetal pressure forces of new wood cells formed by the cambium. During evolution, blood vessels undergo changes. The vessel segments become shorter and thicker, oblique transverse partitions are replaced by straight ones, and scalariform perforations become simple.
Conductive tissues of descending current
Descending tissues include sieve cells and sieve tubes with companion cells. Sieve cells have more ancient origin. They are found in higher spore plants and gymnosperms. These are living, elongated cells with pointed ends. In the mature state, they contain nuclei as part of the protoplast. In their side walls, in the places 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 from each other by transverse walls called sieve plates, in which sieve fields are located. If a sieve plate has one sieve field, it is considered simple, and if it has several, it is considered complex. Sieve fields are formed by numerous through holes - sieve perforations of small diameter. Plasmodesmata pass through these perforations from one cell to another. Callose polysaccharide is placed on the walls of the perforations, which reduces the lumen of the perforations. As the sieve tube ages, callose completely plugs the perforations and the tube stops working.
When a sieve tube is formed, a special phloem protein (F-protein) is synthesized in the cells that form them and a large vacuole develops. It pushes the cytoplasm and nucleus towards the cell wall. The vacuole membrane then breaks down and inner space The cells are filled with a mixture of cytoplasm and cell sap. The F protein bodies lose their distinct outlines and merge, forming strands near the sieve plates. Their fibrils pass through perforations from one segment of the sieve tube to another. One or two companion cells, which have an elongated shape, thin walls and living cytoplasm with a nucleus and numerous mitochondria, are tightly adjacent to the segments of the sieve tube. Mitochondria synthesize ATP, which is necessary for the transport of substances through sieve tubes. In the walls of the companion cells there are a large number of pores with plasmadesmata, which is almost 10 times greater than their number in other mesophyll cells of the leaf. The surface of the protoplast of these cells is significantly increased due to numerous folds formed by the plasmalemma.
The speed of movement of assimilates through sieve tubes significantly exceeds the speed of free diffusion of substances and reaches 50–150 cm/hour, which indicates the active transport of substances using ATP energy.
The duration of operation of sieve tubes in perennial dicotyledons is 1–2 years. To replace them, the cambium constantly forms new conducting elements. In monocots lacking a cambium, sieve tubes last much longer.
Conductive bundles
Conductive tissues are located in plant organs in the form of longitudinal cords, forming conductive bundles. There are four types of vascular bundles: simple, general, complex and fibrovascular.
Simple bundles consist 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 the flowering shoots of lilies - from only sieve tubes.
Common bundles are formed by tracheids, vessels and sieve tubes. Sometimes the term is used to refer to the metamer bundles that run through the internodes and are leaf trails. The complex bundles include conductive and parenchymal tissues. The most advanced, diverse in structure and location are the vascular-fibrous 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. The xylem transports water and substances dissolved in it from the root system to the leaf and other organs. The volume of the xylem part is several times greater than the volume of the phloem part, since the volume of water entering the plant exceeds the volume of assimilates formed, since a significant part of the water is evaporated by the plant.
The diversity of vascular-fibrous bundles is determined by their origin, histological composition and location in the plant. If the bundles are formed from the procambium and complete their development as the cell supply is used up educational fabric, like monocots, they are called closed for growth. In contrast, in dicotyledons, open tufts are not limited in growth, since they are formed by the cambium and increase in diameter throughout the life of the plant. In addition to conductive tissues, vascular-fibrous bundles may include basic and mechanical tissues. For example, in dicotyledons, phloem is formed by sieve tubes (ascending tissue), bast parenchyma (ground tissue), and bast fibers (mechanical tissue). The xylem consists of vessels and tracheids (conductive tissue of descending current), wood parenchyma (ground tissue) and wood 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 component parts of the bunches can change under the influence of plant growth conditions.
Several types of vascular-fibrous bundles are known.
Closed collateral vascular bundles are characteristic of the leaves and stems of monocot angiosperms. They lack cambium. Phloem and xylem are located side-by-side. They are characterized by certain design features. Thus, in wheat, which differs in the C 3 -pathway of photosynthesis, bundles are formed from procambium and have primary phloem and primary xylem. In the phloem, there is an earlier protophloem and a later in time formation, but larger cell metaphloem. The phloem part lacks bast parenchyma and bast fibers. In the xylem, smaller protoxylem vessels are initially formed, located in one line perpendicular to the internal border of the phloem. The 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. The V-, Y- and È-shaped arrangement of vessels is also known. Between the metaxylem vessels, in 1–2 rows, there is small-celled sclerenchyma with thickened walls, which become saturated with lignin as the stem develops. This sclerenchyma separates the xylem zone from the phloem. On both sides of the protoxylem vessels there are wood parenchyma cells, which probably play a transfusion role, since during the transition of the bundle from the internode to the leaf pad of the stem node, they participate in the formation of transfer cells. Around the vascular bundle of the wheat stem there is a sclerenchyma sheath, better developed on the protoxylem and protophloem side; near the lateral sides of the bundle, sheath cells are arranged in one row.
In plants with the C 4 type of photosynthesis (corn, millet, etc.), in the leaves around the closed vascular bundles there is a lining of large chlorenchyma cells.
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 sclerenchyma 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 bundles primarily formed by the procambium. Then, from the cells of the main parenchyma, the cambium develops in them, producing secondary elements of phloem and xylem. As a result, the bundles 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 bunched type of stem structure (legumes, Rosaceae, etc.).
Secondly, open collateral bundles can be formed only by the 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 (asteraceae, etc.), as well as for root crops such as beets.
In the stems of plants of a number of families (Pumpkin, Solanaceae, Campanaceae, etc.) there are open bicollateral bundles, where the xylem is surrounded on both sides by phloem. In this case, the outer section of the phloem, facing the surface of the stem, is better developed than the inner one, and the cambium strip, as a rule, is located between the xylem and the outer section of the phloem.
Concentric beams come in two types. In amphicribral bundles, characteristic 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.). Concentric bundles are less common in dicotyledons (castor beans).
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 has the appearance of 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 plants. For example, in carrots, beets, cabbage and other dicotyledons, the xylem of the radial bundle has only two rays. Apple and pear trees can have 3–5 of them, pumpkins and beans have four-rayed xylem, and monocots have multi-rayed xylem. The radial arrangement of xylem rays has adaptive significance. 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, such as flax, vascular tissues are located in the stem without forming clearly defined vascular bundles. Then they talk about the non-tufted type of stem structure.
Tissues regulating radial transport of substances
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 forms directly under the primary covering tissue epiblema in the zone of root hairs and consists of one or several layers of tightly closed cells with thickened cellulose membranes. In the exodermis, water entering the root through the root hairs experiences resistance from the viscous cytoplasm and moves into the cellulose membranes of the exodermal cells, and then leaves them into the intercellular spaces of the middle layer of the primary cortex, or mesoderm. This ensures efficient flow of water into the deeper layers of the root.
In the zone of conduction in the root of monocots, where epiblema cells die and slough off, the exodermis appears on the surface of the root. Its cell walls are saturated with suberin and prevent the flow of water from the soil into the root. In dicotyledons, the exoderm in the primary cortex is sloughed off during root molting and replaced by periderm.
The endoderm, or inner layer of the primary root cortex, is located around a central cylinder. It is formed by one layer of tightly closed cells of unequal structure. Some of them, called permeable, have thin shells and are easily permeable to water. Through them, water from the primary cortex enters the radial vascular bundle of the root. Other cells have specific cellulose thickenings of the radial and internal tangential walls. These thickenings, saturated with suberin, are called Casparian belts. They are impermeable to water. Therefore, water enters the central cylinder only through passage cells. And since the absorbing surface of the root significantly exceeds the total cross-sectional area of the endodermal passage cells, 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 bark of the young stem. In some herbaceous angiosperms, it, like the root, may have Casparian belts. In addition, in young stems the endodermis can be represented by a starch-bearing sheath. In this way, the endodermis can regulate water transport in the plant and store nutrients.
The concept of the stele and its evolution
Much attention is paid to the emergence, development in ontogenesis and evolutionary structural transformations of the conducting system, since it ensures the interconnection of plant organs and the evolution of large taxa is associated with it.
At the suggestion of the French botanists F. Van Tieghem 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 stele may 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 unified. The study of stele in representatives of different divisions of higher plants led to the formation of the stele theory.
There are two main types of stele: protostele and eustele. The most ancient is the protostele. Its conducting tissues are located in the middle of the axial organs, with xylem in the center, surrounded by a continuous layer of phloem. There is no pith or cavity in the stem.
There are several evolutionarily related types of protostele: haplostele, actinostele and plectostele.
The original, primitive type is the haplostele. It has xylem rounded shape cross section and is surrounded by an even, continuous layer of phloem. A pericycle is located around the conductive tissues in one or two layers. The haplostele was known from fossil rhyniophytes and is preserved in some psilotophytes (tmesipterus).
A more developed type of protostele is the actinostele, in which the xylem in cross section takes the shape of a multi-rayed star. It is found in fossil Asteroxylon and some primitive lycophytes.
Further separation of the xylem into separate sections located radially or parallel to each other led to the formation of a plectostele, characteristic of lycophyte stems. In actinostele and plectostele, phloem still surrounds the xylem on all sides.
During evolution, a siphonostele emerged from the protostele, the distinctive feature of which is its tubular structure. In the center of such a stele there is a core or cavity. Leaf slits appear in the conducting part of the siphonostele, thanks to which a continuous connection between the core and the bark occurs. Depending on the method relative position xylem and phloem siphonostele is ectophloic and amphiphloic. In the first case, the phloem surrounds the xylem on one, outer side. In the second, phloem surrounds the xylem on both sides, external and internal.
When the amphiphloic siphonostele is divided into a network or rows of longitudinal strands, a dissected stele, or dictyostele, appears, characteristic of many fern-like plants. Its conducting part is represented by numerous concentric conducting bundles.
In horsetails, an arthrostele arose from the ectophloic siphonostele, which has a segmented structure. It is distinguished by the presence of one large central cavity and separate vascular bundles with protoxylem cavities (carinal canals).
In flowering plants, on the basis of the ectophloic siphonostele, an eustele, characteristic of dicotyledons, and an ataxostele, typical of monocotyledons, were formed. In the eustela, the conducting part consists of separate collateral bundles that have a circular arrangement. In the center of the stele in the stem there is a core, which is connected to the bark with the help of medullary rays. In the ataxostele, the vascular bundles have a scattered arrangement; between them there are parenchyma cells of the central cylinder. This arrangement of the bundles hides the tubular structure of the siphonostele.
Emergence various options siphonosteles are an important adaptation of higher plants to increase the diameter of the axial organs - root and stem.
Different organs of higher plants perform different functions. So the roots absorb water and minerals, and photosynthesis occurs in the leaves, as a result of which organic substances are formed. However, all plant cells need both water and organic matter. Therefore, a transport system is needed to ensure the delivery of necessary substances to one organ from another. In plants (mainly angiosperms) this function is performed conductive fabrics.
In woody plants, conductive tissues are part of wood And bast. For wood it is carried out rising current: Water and minerals rise from the roots. On bast it is carried out downward current : There is an outflow of organic matter from the leaves. With all this, the concepts of “upward current” and “downward current” should not be understood quite literally, as if in conducting tissues water always goes up and organic substances always go down. Substances can move horizontally and sometimes in the opposite direction. For example, organic matter goes up to growing shoots that are above storage tissue or photosynthetic leaves.
So, in plants, the movement of aqueous solution and organic substances are separated. The composition of the wood includes vessels, and in the composition of the bast - sieve tubes.
The vessels are a chain of dead long cells. An aqueous solution moves along them from the roots. Water rises due to root pressure and transpiration (evaporation of water from leaves). U gymnosperms and there are ferns instead of vessels tracheids, along which water moves more slowly. It follows that the vessels have a more perfect structure. The vessels are called differently trachea.
The reason why water moves faster in vessels than in tracheids is due to their slightly different structure. Tracheid cells have many pores at the points of contact with each other (above and below). The aqueous solution is filtered through these pores. The vessels are essentially a hollow tube; their cells have large holes (perforations) at the points of connection with each other.
The vessels have various thickenings in their longitudinal walls. This gives them strength. Through those places where there are no thickenings, water is transported horizontally. It enters the parenchyma cells and neighboring vessels (vessels are usually arranged in bundles).
Sieve tubes are formed by living elongated cells. Organic substances move through them. At the top and bottom, the vascular cells are connected to each other due to numerous pores. This connection is similar to a sieve, hence the name. It turns out to be a single long chain of cells. Although sieve tubes are living cells, they do not have a nucleus and some other structures and organelles necessary for life. Therefore, sieve tubes have so-called companion cells that support their life. The satellites and tubes are connected to each other through special pores.
Wood and bast consist of more than just conductive tissues. They also include parenchyma and mechanical tissues. Conductive tissues together with mechanical ones form vascular-fibrous bundles. Parenchyma often plays the role of storage tissue (especially in wood).
Wood has another name xylem, and bast - phloem.
Rice. Cellular structure of an annual linden stem. Longitudinal and transverse sections: 1 - system of integumentary tissues (from the outside to the inside; one layer of the epidermis, cork, primary cortex); 2-5 - bast : 2 - bast fibers, 3 - sieve tubes, 4 - satellite cells, 5 - bast parenchyma cells; 6 - cambium cells, stretched and differentiated in the outer layers; 7-9 cellular elements of wood: 7 - vascular cells, 8 - wood fibers, 9 - wood parenchyma cells ( 7 , 8 And 9 shown also large); 10 - core cells.
Water and minerals supplied through the root must reach all parts of the plant, while the substances produced in the leaves during photosynthesis are also intended for all cells. Thus, a special system must exist in the plant body to ensure the transport and redistribution of all substances. This function is performed in plants conductive fabrics. There are two types of conductive fabrics: xylem (wood) And phloem (bast). Along the xylem it is carried out rising current: movement of water with mineral salts from the root to all organs of the plant. It goes along the phloem downward current: transport of organic substances coming from leaves. Conducting tissues are complex tissues because they consist of several types of differently differentiated cells.
Xylem (wood). Xylem consists of conducting elements: vessels, or trachea, And tracheid, as well as from cells that perform mechanical and storage functions.
Tracheids. These are dead elongated cells with obliquely cut pointed ends (Fig. 12).
Their lignified walls are greatly thickened. Typically, the length of tracheids is 1-4 mm. Arranged in a chain one after another, tracheids form the water-conducting system in ferns and gymnosperms. Communication between neighboring tracheids occurs through pores. By filtration through the pore membrane, both vertical and horizontal transport of water with dissolved minerals. The movement of water through the tracheids occurs at a slow speed.
Vessels (trachea). The vessels form the most perfect conductive system characteristic of angiosperms. They are a long hollow tube consisting of a chain of dead cells - vessel segments, in the transverse walls of which there are large holes - perforations. These holes allow rapid flow of water. Vessels are rarely single; they are usually located in groups. The diameter of the vessel is 0.1 - 0.2 mm. At an early stage of development, cellulose thickenings are formed from the xylem procambium on the inner walls of the vessels, which subsequently become lignified. These thickenings prevent the vessels from collapsing under the pressure of neighboring growing cells. First are formed ringed And spiral thickenings that do not prevent further cell elongation. Later, wider vessels appear with staircases thickenings and then porous vessels that are characterized by the largest thickening area (Fig. 13).
Through non-thickened areas of vessels (pores), horizontal transport of water occurs into neighboring vessels and parenchyma cells. The appearance of vessels in the process of evolution provided angiosperms with high adaptability to life on land and, as a result, their dominance in the modern vegetation cover of the Earth.
Other xylem elements. In addition to conducting elements, xylem also includes wood parenchyma And mechanical elements - wood fibers, or libriform. Fibers, like vessels, arose in the process of evolution from tracheids. However, unlike vessels, the number of pores in the fibers decreased and an even thicker secondary shell formed.
Phloem (bast). Phloem carries out a downward flow of organic substances - products of photosynthesis. Phloem contains sieve tubes, satellite cells, mechanical (bast) fibers and bast parenchyma.
Sieve tubes. Unlike the conducting elements of xylem, sieve tubes are a chain of living cells (Fig. 14).
The transverse walls of two adjacent cells that make up the sieve tube are penetrated a large number through holes forming a structure resembling a sieve. This is where the name sieve tubes come from. The walls that support these holes are called sieve plates. Through these openings the transport of organic substances from one segment to another occurs.
The segments of the sieve tube are connected by peculiar pores to companion cells (see below). The tubes communicate with parenchyma cells through simple pores. Mature sieve cells lack a nucleus, ribosomes and the Golgi complex, and their functional activity and vital activity are supported by companion cells.
Companion cells (accompanying cells). They are located along the longitudinal walls of the sieve tube segment. Companion cells and sieve tube segments are formed from common mother cells. The mother cell is divided by a longitudinal septum, and from the two resulting cells, one turns into a segment of the sieve tube, and from the other one or more companion cells develop. Companion cells have a nucleus, cytoplasm with numerous mitochondria, active metabolism occurs in them, which is associated with their function: to ensure the vital activity of nuclear-free sieve cells.
Other elements of phloem. The composition of phloem, along with conducting elements, includes mechanical bast (phloem) fibers And phloem parenchyma.
Conductive bundles. In a plant, conducting tissues (xylem and phloem) form special structures - conducting bundles. If the bundles are partially or completely surrounded by strands of mechanical tissue, they are called vascular-fibrous bundles. These bundles penetrate the entire body of the plant, forming a single conducting system.
Initially, conducting tissues are formed from cells of the primary meristem - procambia. If, during the formation of a bundle, the procambium is completely spent on the formation of primary conducting tissues, then such a bundle is called closed(Fig. 15).
It is not capable of further (secondary) thickening because it does not contain cambial cells. Such bunches are characteristic of monocotyledonous plants.
In dicotyledons and gymnosperms, a part of the procambium remains between the primary xylem and phloem, which later becomes fascicular cambium. Its cells are capable of dividing, forming new conductive and mechanical elements, which ensures secondary thickening of the bundle and, as a consequence, growth of the stem in thickness. The vascular bundle containing the cambium is called open(see Fig. 15).
Depending on the relative position of xylem and phloem, several types of vascular bundles are distinguished (Fig. 16)
Collateral bundles. Xylem and phloem are adjacent to each other side by side. Such bunches are characteristic of the stems and leaves of most modern seed plants. Typically, in such bundles, the xylem occupies a position closer to the center of the axial organ, and the phloem faces the periphery.
Bicollateral bundles. Two strands of phloem adjoin the xylem side by side: one - with inside, the other - from the periphery. The peripheral strand of phloem mainly consists of secondary phloem, the internal strand consists of primary phloem, as it develops from procambium.
Concentric beams. One conducting tissue surrounds another conducting tissue: xylem - phloem or phloem - xylem.
Radial beams. Characteristic of plant roots. The xylem is located along the radii of the organ, between which there are strands of phloem.
