CONDUCTIVE FABRICS.
Conductive tissues serve to transport dissolved substances in water throughout the plant. nutrients.
Rice. 43 Wood fibers of a 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, there was a need 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 - along the sieve elements of the 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 lines of evolution 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 - primitive type vessel segment and its perforation formed by close pores, 5 - 7 - successive stages of specialization of vessel segments and the formation of 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.
Conductive tissues serve to move nutrients dissolved in water throughout the plant. They arose as a consequence of plants adapting to life on land. In connection with life in two environments - soil and air, two conductive tissues arose, through which substances move in two directions.
Substances rise along the xylem from roots to leaves soil nutrition– water and mineral salts dissolved in it (ascending, or transpiration current).
Substances formed during photosynthesis, mainly sucrose, move through the phloem from leaves to roots. downward current). Since these substances are assimilation products carbon dioxide, the transport of substances through the phloem is called the flow of assimilates.
Conductive tissues form a continuous branched system in the plant body, connecting all organs - from the thinnest roots to the youngest shoots. Xylem and phloem are complex tissues; they include heterogeneous elements - conductive, mechanical, storage, excretory. The most important are the conductive elements; they perform the function of conducting substances.
Xylem and phloem are formed from the same meristem and, therefore, are always located side by side in the plant. Primary conducting tissues are formed from the primary lateral meristem - procambium, secondary - from the secondary lateral meristem - cambium. Secondary conducting tissues have a more complex structure than primary ones.
Xylem (wood) consists of conductive elements - tracheids and vessels (tracheas), mechanical elements-wood fibers (libriform fibers) and elements of the main tissue - wood parenchyma.
The conducting elements of xylem are called tracheal elements. There are two types of tracheal elements – tracheids and vascular segments (Fig. 3.26).
The tracheid is a highly elongated cell with intact primary walls. The movement of solutions occurs by filtration through bordered pores. A vessel consists of many cells called vessel segments. The segments are located one above the other, forming a tube. Between adjacent segments of the same vessel there are through holes - perforations. Solutions move through vessels much more easily than through tracheids.
Rice. 3.26. Diagram of the structure and combination of tracheids (1) and vessel segments (2).
Tracheal elements in a mature, functioning state – dead cells that do not have protoplasts. Preservation of protoplasts would impede the movement of solutions.
Vessels and tracheids transmit solutions not only vertically, but also horizontally to neighboring tracheal elements and to living cells. The lateral walls of tracheids and vessels remain thin over a larger or smaller area. At the same time, they have secondary thickenings that give the walls strength. Depending on the nature of the thickenings of the side walls, the tracheal elements are called annular, spiral, reticular, scalariform and punctate (Fig. 3.27).
Rice. 3.27. Types of thickening and porosity of the side walls of the tracheal elements: 1 – ring-shaped, 2-4 – spiral, 5 – mesh thickening; 6 – ladder, 7 – opposite, 8 – regular porosity.
Secondary annular and spiral thickenings are attached to the thin primary wall by means of a narrow projection. When the thickenings come together and bridges form between them, a mesh thickening appears, turning into bordered pores. This series (Fig. 3.27) can be considered as a morphogenetic, evolutionary series.
Secondary thickenings of the cell walls of the tracheal elements become lignified (impregnated with lignin), which gives them additional strength, but limits the possibility of growth in length. Therefore, in the ontogenesis of an organ, ring-shaped and spiral elements that are still capable of stretching first appear, which do not interfere with the growth of the organ in length. When the growth of an organ stops, elements appear that are incapable of longitudinal stretching.
In the process of evolution, tracheids appeared first. They are found in the first primitive land plants. Vessels appeared much later by transforming tracheids. Almost all angiosperms have vessels. Spore and gymnosperm plants, as a rule, lack blood vessels and have only tracheids. Only as a rare exception, vessels were found in such spores as Selaginella, some horsetails and ferns, as well as in a few gymnosperms (Gnetaceae). However, in these plants, vessels arose independently of the vessels of angiosperms. The appearance of vessels in angiosperms marked an important evolutionary achievement, as it facilitated the conduction of water; Angiosperms turned out to be more adapted to life on land.
Wood parenchyma and wood fibers perform storage and support functions, respectively.
Phloem (bast) consists of conductive - sieve - elements, accompanying cells (companion cells), mechanical elements - phloem (bast) fibers and elements of the main tissue - phloem (bast) parenchyma.
In contrast to the tracheal elements, the conducting elements of the phloem remain alive even in the mature state, and their cell walls remain primary, non-lignified. On the walls sieve elements There are groups of small through holes - sieve fields, through which protoplasts of neighboring cells communicate and transport of substances occurs. There are two types of sieve elements - sieve cells and sieve tube segments.
Sieve cells are more primitive; they are characteristic of spore and gymnosperm plants. A sieve cell is a single cell, very elongated in length, with pointed ends. Its sieve fields are scattered along the side walls. In addition, sieve cells have other primitive characteristics: they lack specialized accompanying cells and in the mature state contain nuclei.
In angiosperms, the transport of assimilates is carried out by sieve tubes (Fig. 3.28). They consist of many individual cells - segments, located one above the other. The sieve fields of two adjacent segments form a sieve plate. Sieve plates have a more perfect structure than sieve fields (the perforations are larger and there are more of them).
In the mature state, the segments of sieve tubes lack nuclei, but they remain alive and actively conduct substances. An important role in carrying out assimilates on sieve tubes belongs to accompanying cells (companion cells). Each sieve tube segment and its accompanying cell (or two or three cells in the case of additional division) arise simultaneously from one meristematic cell. Companion cells have nuclei and cytoplasm with numerous mitochondria; intensive metabolism occurs in them. There are numerous cytoplasmic connections between the sieve tubes and the accompanying cells adjacent to them. It is believed that companion cells, together with segments of sieve tubes, constitute a single physiological system that carries out the flow of assimilates.
Rice. 3.28. Phloem of a pumpkin stem on a longitudinal (A) and transverse (B) section: 1 – sieve tube segment; 2 – sieve plate; 3 – accompanying cell; 4 – phloem parenchyma; 5 – clogged sieve plate.
The duration of operation of sieve tubes is short. For annuals and above-ground shoots of perennial grasses - no more than one growing season, for shrubs and trees - no more than three to four years. When the living contents of the sieve tube die, the companion cell also dies.
Bast parenchyma consists of living thin-walled cells. Its cells often accumulate reserve substances, as well as resins, tannins, etc. Bast fibers play a supporting role. They are not present in all plants.
In the plant body, xylem and phloem are located side by side, forming either layers or separate strands, which are called vascular bundles. There are several types of conductive bundles (Fig. 3.29).
Closed bundles consist only of primary vascular tissues; they do not have a cambium and do not thicken further. Closed bunches are characteristic of spore-bearing and monocotyledonous plants. Open bunches have a cambium and are capable of secondary thickening. They are characteristic of gymnosperms and dicotyledonous plants.
Depending on relative position phloem and xylem in a bundle are distinguished following types. The most common are collateral bundles, in which the phloem lies on one side of the xylem. Collateral bundles can be open (stems of dicotyledons and gymnosperms) and closed (stems of monocots). If with inside An additional strand of phloem is located from the xylem; such a bundle is called bicollateral. Bicollateral bundles can only be open; they are characteristic of some families of dicotyledonous plants (pumpkin, nightshade, etc.).
There are also concentric bundles in which one conducting tissue surrounds another. They can only be closed. If there is phloem in the center of the bundle and xylem surrounds it, the bundle is called centrifloem or amphivasal. Such bundles are often found in the stems and rhizomes of monocots. If xylem is located in the center of the bundle and is surrounded by phloem, the bundle is called centoxylem, or amphicribral. Centoxylem bundles are common in ferns.
Rice. 3.29. Types of conduction bundles: 1 – open collateral; 2 – open bicollateral; 3 – closed collateral; 4 – concentric closed centrifloem; 5 – concentric closed centroxylem; K – cambium; X – xylem; F – phloem.
5.Mechanical, storage, air-bearing tissue. Structure, functions
Mechanical fabric- a type of tissue in a plant organism, fibers from living and dead cells with a highly thickened cell wall, giving mechanical strength body. It arises from the apical meristem, as well as as a result of the activity of the procambium and cambium.
The degree of development of mechanical tissues largely depends on conditions; they are almost absent in plants rain forests, many coastal plants, but are well developed in most plants of arid habitats.
Mechanical fabrics are present in all plant organs, but they are most developed along the periphery of the stem and in the central part of the root.
The following types of mechanical fabrics are distinguished:
collenchyma is an elastic supporting tissue of the primary bark of young stems of dicotyledonous plants, as well as leaves. It consists of living cells with unevenly thickened, non-lignified primary membranes, elongated along the axis of the organ. Provides support for the plant.
sclerenchyma is a durable tissue made of rapidly dying cells with lignified and evenly thickened membranes. Provides strength to organs and the entire body of plants. There are two types of sclerenchyma cells:
fibers are long thin cells, usually collected in strands or bundles (for example, bast or wood fibers).
sclereids are round dead cells with very thick lignified membranes. They form the seed coat, nut shells, seeds of cherries, plums, apricots; they give the flesh of pears their characteristic coarse character. They are found in groups in the crust of coniferous and some deciduous trees, in the hard shells of seeds and fruits. Their cells round shape with thick walls and a small core.
Mechanical tissues provide strength to plant organs. They form a frame that supports all plant organs, resisting their fracture, compression, and rupture. The main characteristics of the structure of mechanical tissues, ensuring their strength and elasticity, are the powerful thickening and lignification of their membranes, close closure between cells, and the absence of perforations in the cell walls.
Mechanical tissues are most developed in the stem, where they are represented by bast and wood fibers. In roots, mechanical tissue is concentrated in the center of the organ.
Depending on the shape of the cells, their structure, physiological state and the method of thickening of the cell membranes, two types of mechanical tissue are distinguished: collenchyma and sclerenchyma (Fig. 8.4).
Rice. 8.4. Mechanical tissues: a - angular collenchyma; 6- sclerenchyma; c -- sclereids from cherry plum fruits: 1 - cytoplasm, 2 - thickened cell wall, 3 - pore tubules.
Collenchyma is represented by living parenchyma cells with unevenly thickened membranes, making them especially well suited for strengthening young growing organs. Being primary, collenchyma cells easily stretch and practically do not interfere with the elongation of the part of the plant in which they are located. Collenchyma is usually located in separate strands or a continuous cylinder under the epidermis of the young stem and leaf petioles, and also borders the veins in dicotyledonous leaves. Sometimes collenchyma contains chloroplasts.
Sclerenchyma consists of elongated cells with uniformly thickened, often lignified membranes, the contents of which die in the early stages. The membranes of sclerenchyma cells have high strength, close to the strength of steel. This fabric is widely available in vegetative organs land plants and forms their axial support.
There are two types of sclerenchyma cells: fibers and sclereids. Fibers are long, thin cells, usually collected in strands or bundles (for example, bast or wood fibers). Sclereids are round, dead cells with very thick, lignified membranes. They form the seed coat, nut shells, seeds of cherries, plums, and apricots; they give the flesh of pears their characteristic coarse character.
Ground tissue, or parenchyma, consists of living, usually thin-walled cells that form the basis of organs (hence the name tissue). It houses mechanical, conductive and other permanent tissues. The main tissue performs a number of functions, and therefore they distinguish between assimilation (chlorenchyma), storage, pneumatic (aerenchyma) and aquiferous parenchyma (Fig. 8.5).
Figure 8.5. Parenchyma tissues: 1-3 - chlorophyll-bearing (columnar, spongy and folded, respectively); 4-storage (cells with starch grains); 5 - pneumatic, or aerenchyma.
Proteins, carbohydrates and other substances are deposited in the cells of the storage parenchyma. It is well developed in the stems of woody plants, in roots, tubers, bulbs, fruits and seeds. Plants in desert habitats (cacti) and salt marshes have aquiferous parenchyma in their stems and leaves, which serves to accumulate water (for example, large specimens of cacti from the genus Carnegia contain up to 2-3 thousand liters of water in their tissues). Aquatic and marsh plants develop a special type of basic tissue - air-bearing parenchyma, or aerenchyma. Aerenchyma cells form large air-bearing intercellular spaces, through which air is delivered to those parts of the plant whose connection with the atmosphere is difficult.
Aerenchyma (or Erenchyma) is an air-bearing tissue in plants, built from cells interconnected so that large air-filled voids (large intercellular spaces) remain between them.
In some manuals, aerenchyma is considered a type of main parenchyma.
Aerenchyma is constructed either from ordinary parenchyma cells or from stellate-shaped cells connected to each other by their spurs. Characterized by the presence of intercellular spaces.
Purpose: Such air-bearing tissue is found in aquatic and marsh plants, and its purpose is twofold. First of all, it is a container for air reserves for the needs of gas exchange. In plants that are completely immersed in water, the conditions for gas exchange are much less convenient than in terrestrial plants. While the latter are surrounded on all sides by air, aquatic plants in best case scenario found in environment very small reserves; These reserves are already absorbed by the superficial cells, but no longer reach the depths of the thick organs. Under these conditions, a plant can ensure normal gas exchange in two ways: either by increasing the surface of its organs with a corresponding decrease in their massiveness, or by collecting air reserves inside its tissues. Both of these methods are observed in reality.
Gas exchange. On the one hand, in many plants the underwater leaves are extremely strongly dissected, as, for example, in the water buttercup (English) Russian. (Ranunculus aquatilis), Ouvirandrafene s tralis, etc.
On the other hand, in the case of massive organs, they represent a loose, air-filled spongy mass. During the day, when, thanks to the assimilation process, the plant releases oxygen many times more than is necessary for respiration purposes, the released oxygen is collected in reserve in the large intercellular spaces of the aerenchyma. In sunny weather, significant amounts of released oxygen do not fit into the intercellular spaces and exit through various random openings in the tissues. With the onset of night, when the assimilation process stops, the stored oxygen is gradually consumed for cell respiration, and in return, carbon dioxide is released into the air-bearing cavities of the aerenchyma by the cells, which in turn is used during the day for the needs of assimilation. So, day and night, the waste products of the plant, thanks to the presence of aerenchyma, are not wasted, but are left in reserve to be used in the next period of activity.
As for swamp plants, their roots are in particularly unfavorable conditions in terms of respiration. Under a layer of water, in soil saturated with water, various kinds processes of fermentation and decay; oxygen in the most upper layers soil has already been completely absorbed, then conditions for anaerobic life are created, occurring in the absence of oxygen. The roots of marsh plants could not exist under such conditions if they did not have a supply of air in the aerenchyma.
The difference between marsh plants and not completely submerged aquatic plants from completely submerged ones is that the renewal of gases inside the aerenchyma occurs not only due to the vital activity of tissues, but also with the help of diffusion (and thermal diffusion); in terrestrial organs, the system of intercellular spaces opens outwards with a mass of tiny openings - stomata, through which, through diffusion, the composition of the air in the intercellular spaces is equalized with the surrounding air. However, with very large plant sizes, such a way of renewing air in the aerenchyma of the roots would not be fast enough. Accordingly, for example, in mangrove trees growing along sea shores with a muddy bottom, some branches of the roots grow upward from the mud and carry their tops into the air, above the surface of the water, the surface of which is pierced by numerous holes. Such “breathing roots” aim to more quickly renew the air in the aerenchyma of the feeding roots, branched in the oxygen-free silt of the seabed.
Decrease specific gravity
The second task of aerenchyma is to reduce the specific gravity of the plant. The body of the plant is heavier than water; aerenchyma plays the role of a swim bladder for the plant; thanks to its presence, even thin organs, poor in mechanical elements, are held directly in the water, and do not fall in disarray to the bottom. Maintaining organs, mainly leaves, in a position favorable for the vital functions of the plant, achieved in terrestrial plants at the high cost of forming a mass of mechanical elements, is achieved here in aquatic plants simply by overflowing the aerenchyma with air.
This second task of aerenchyma is especially clearly expressed in floating leaves, where the demands of respiration could be satisfied without the help of aerenchyma. Thanks to the abundance of intercellular air passages, the leaf not only floats on the surface of the water, but is also able to withstand some weight. The giant leaves of Victoria regia are especially famous for this property. Aerenchyma, which acts as swim bladders, often actually forms bubble-like swellings on the plant. Such bubbles are found both in flowering plants (Eichhornia crassipes, Trianea bogotensis) and in higher algae: Sargassum bacciferum. Fucus vesiculosus and other species are equipped with well-developed swim bladders.
In the process of evolution is one of the reasons that made possible exit plants on land. In our article we will look at the features of the structure and functioning of its elements - sieve tubes and vessels.
Features of conductive fabric
When the planet experienced major changes climatic conditions, plants had to adapt to them. Before that, they all lived exclusively in water. In ground- air environment It became necessary to extract water from the soil and transport it to all plant organs.
There are two types of conductive tissue, the elements of which are vessels and sieve tubes:
- Bast, or phloem, is located closer to the surface of the stem. Along it, organic substances formed in the leaf during photosynthesis move towards the root.
- 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 conducting cells of the phloem. They are separated from each other by numerous partitions. Externally, their structure resembles a sieve. This is where the name comes from. The sieve tubes of plants are living. This is explained by the weak pressure of the downward current.
Their transverse walls are penetrated by a dense network of holes. And the cells contain many through holes. All of them are prokaryotic. This means that they do not have a formal 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 its growing conditions. Sieve tubes transport water and organic matter synthesized during photosynthesis from the leaves to the roots.
Vessels
Unlike sieve tubes, these conductive tissue elements are dead cells. Visually they resemble tubes. The vessels have dense membranes. On the inside they form thickenings that look like rings or spirals.
Thanks to this structure, the vessels are able to perform their function. It involves the movement of soil solutions of mineral substances from the root to the leaves.
Mechanism of soil nutrition
Thus, the plant simultaneously transports substances in opposite directions. In botany, this process is called ascending and descending current.
But what forces cause water to move upward from the soil? It turns out that this occurs 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 the soil contains minerals, without which the development of tissues and organs will be impossible. Thus, 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.
As they rise, water molecules adhere tightly to each other and to the walls of the vessels. In this case, forces arise that can raise water to a decent height - up to 140 m. Such pressure forces soil solutions to penetrate through the root hairs into the bark, and then to the xylem vessels. Water rises along them to the stem. Further, under the influence of transpiration, water enters the leaves.
In the veins next to the vessels there are also sieve tubes. These elements carry out a downward current. When exposed to sunlight, the polysaccharide glucose is synthesized in leaf chloroplasts. The plant uses this organic matter to carry out growth and vital processes.
So, the conductive tissue of the plant ensures the movement of aqueous solutions of organic and mineral substances throughout the plant. Its structural elements are vessels and sieve tubes.
Conductive tissues serve to move nutrients dissolved in water throughout the plant. They arose as a consequence of plants adapting to life on land. In connection with life in two environments - soil and air, two conductive tissues arose, through which substances move in two directions. By xylem Soil nutrition substances - water and mineral salts dissolved in it - rise from the roots to the leaves ( ascending, or transpiration current). By phloem substances formed during photosynthesis, mainly sucrose ( downward current). Since these substances are products of carbon dioxide assimilation, the transport of substances through the phloem is called current of assimilates.
Conductive tissues form a continuous branched system in the plant body, connecting all organs - from the thinnest roots to the youngest shoots. Xylem and phloem are complex tissues; they include heterogeneous elements - conductive, mechanical, storage, excretory. The most important are the conductive elements; they perform the function of conducting substances.
Xylem and phloem are formed from the same meristem and, therefore, are always located nearby in the plant. Primary conducting tissues are formed from the primary lateral meristem - procambia, secondary– from the secondary lateral meristem – cambium. Secondary conducting tissues have a more complex structure than primary ones.
Xylem (wood) consists of conductive elements - tracheid And vessels (tracheas), mechanical elements - wood fibers (libriform fibers) and elements of the main fabric - wood parenchyma.
The conducting elements of xylem are called tracheal elements. There are two types of tracheal elements - tracheids And vascular segments(rice. 3.26).
Tracheid It is a highly elongated cell with intact primary walls. The movement of solutions occurs by filtration through bordered pores. Vessel consists of many cells called members vessel. The segments are located one above the other, forming a tube. Between adjacent segments of the same vessel there are through holes - perforation. Solutions move through vessels much more easily than through tracheids.
Rice. 3.26. Diagram of the structure and combination of tracheids (1) and vessel segments (2).
Tracheal elements in a mature, functioning state are dead cells that do not have protoplasts. Preservation of protoplasts would impede the movement of solutions.
Vessels and tracheids transmit solutions not only vertically, but also horizontally to neighboring tracheal elements and to living cells. The lateral walls of tracheids and vessels remain thin over a larger or smaller area. At the same time, they have secondary thickenings that give the walls strength. Depending on the nature of the thickening of the lateral walls, the tracheal elements are called ringed, spiral, mesh, staircases And point-pore (rice. 3.27).
Rice. 3.27. Types of thickening and porosity of the lateral walls of tracheal elements: 1 – ringed, 2-4 – spiral, 5 – mesh thickening; 6 – ladder, 7 – opposite, 8 – regular porosity.
Secondary annular and spiral thickenings are attached to the thin primary wall by means of a narrow projection. When the thickenings come together and bridges form between them, a mesh thickening appears, turning into bordered pores. This series ( rice. 3.27) can be considered as a morphogenetic, evolutionary series.
Secondary thickenings of the cell walls of the tracheal elements become lignified (impregnated with lignin), which gives them additional strength, but limits the possibility of growth in length. Therefore, in the ontogenesis of an organ, ring-shaped and spiral elements that are still capable of stretching first appear, which do not interfere with the growth of the organ in length. When the growth of an organ stops, elements appear that are incapable of longitudinal stretching.
In the process of evolution, tracheids appeared first. They are found in the first primitive land plants. Vessels appeared much later by transforming tracheids. Almost all angiosperms have vessels. Spore and gymnosperm plants, as a rule, lack blood vessels and have only tracheids. Only as a rare exception, vessels were found in such spores as Selaginella, some horsetails and ferns, as well as in a few gymnosperms (Gnetaceae). However, in these plants, vessels arose independently of the vessels of angiosperms. The appearance of vessels in angiosperms marked an important evolutionary achievement, as it facilitated the conduction of water; Angiosperms turned out to be more adapted to life on land.
Wood parenchyma And wood fibers perform storage and support functions, respectively.
Phloem (bast) consists of conductive sieve- elements, accompanying cells (companion cells), mechanical elements – phloem (bast) fibers and elements of the main fabric - phloem (bast) parenchyma.
In contrast to the tracheal elements, the conducting elements of the phloem remain alive even in the mature state, and their cell walls remain primary, non-lignified. On the walls of the sieve elements there are groups of small through holes - sieve fields, through which protoplasts of neighboring cells communicate and transport of substances occurs. There are two types of sieve elements - sieve cells And sieve tube segments.
Sieve cells are more primitive, they are inherent in spore and gymnosperm plants. A sieve cell is a single cell, very elongated in length, with pointed ends. Its sieve fields are scattered along the side walls. In addition, sieve cells have other primitive characteristics: they lack specialized accompanying cells and in the mature state contain nuclei.
In angiosperms, assimilates are transported sieve tubes(rice. 3.28). They are made up of many individual cells - members, located one above the other. The sieve fields of two adjacent segments form sieve plate. Sieve plates have a more perfect structure than sieve fields (the perforations are larger and there are more of them).
In the mature state, the segments of sieve tubes lack nuclei, but they remain alive and actively conduct substances. An important role in carrying assimilates through sieve tubes belongs to accompanying cells (companion cells). Each sieve tube segment and its accompanying cell (or two or three cells in the case of additional division) arise simultaneously from one meristematic cell. Companion cells have nuclei and cytoplasm with numerous mitochondria; intensive metabolism occurs in them. There are numerous cytoplasmic connections between the sieve tubes and the accompanying cells adjacent to them. It is believed that companion cells, together with segments of sieve tubes, constitute a single physiological system that carries out the flow of assimilates.
Rice. 3.28. Phloem of a pumpkin stem on a longitudinal (A) and transverse (B) section: 1 – segment of the sieve tube; 2 – sieve plate; 3 – accompanying cell; 4 – phloem parenchyma; 5 – clogged sieve plate.
The duration of operation of sieve tubes is short. For annuals and above-ground shoots of perennial grasses - no more than one growing season, for shrubs and trees - no more than three to four years. When the living contents of the sieve tube die, the companion cell also dies.
Bast parenchyma consists of living thin-walled cells. Its cells often accumulate reserve substances, as well as resins, tannins, etc. Bast fibers play a supporting role. They are not present in all plants.
In the plant body, xylem and phloem are located side by side, forming either layers or separate strands, which are called conducting beams. There are several types of conductive bundles ( rice. 3.29).
Closed bundles consist only of primary conducting tissues, they do not have a cambium and do not thicken further. Closed bunches are characteristic of spore-bearing and monocotyledonous plants. Open Bundles have a cambium and are capable of secondary thickening. They are characteristic of gymnosperms and dicotyledonous plants.
Depending on the relative position of phloem and xylem in the bundle, the following types are distinguished. Most common collateral bundles in which the phloem lies on one side of the xylem. Collateral bundles can be open (stems of dicotyledons and gymnosperms) and closed (stems of monocotyledons). If there is an additional strand of phloem on the inner side of the xylem, such a bundle is called bicollateral. Bicollateral bundles can only be open; they are characteristic of some families of dicotyledonous plants (pumpkin, nightshade, etc.).
There are also concentric bundles in which one conducting tissue surrounds another. They can only be closed. If there is phloem in the center of the bundle and xylem surrounds it, the bundle is called centrifloem, or amphivasal. Such bundles are often found in the stems and rhizomes of monocots. If the xylem is located in the center of the bundle and is surrounded by phloem, the bundle is called centroxylem, or amphicribral. Centoxylem bundles are common in ferns.
Rice. 3.29. Types of conductive bundles: 1 – open collateral; 2 – open bicollateral; 3 – closed collateral; 4 – concentric closed centrifloem; 5 – concentric closed centroxylem; TO– cambium; KS– xylem; F– phloem.
Many authors highlight radial bunches. The xylem in such a bundle is located in the form of rays from the center along radii, and the phloem is located between the xylem rays. Radial beam – characteristic feature root of the primary structure.
Conducting tissue consists of living or dead elongated cells that look like tubes.
The stems and leaves of plants contain bundles of conductive tissue. The conducting tissue contains vessels and sieve tubes.
Vessels- sequentially connected dead hollow cells, the transverse partitions between them disappear. Through vessels, water and minerals dissolved in it from the roots enter the stem and leaves.
Sieve tubes - elongated, nuclear-free living cells connected in series to each other. Through them, organic substances from the leaves (where they were formed) move to other organs of the plant.
Conductive fabric ensures the transport of water with minerals dissolved in it.
This tissue forms two transport systems:
- upward(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 vessels are more advanced conductors than tracheids.
In descending systems, the flow of water with photosynthesis products passes through sieve tubes (phloem or phloem).
Xylem and phloem form vascular-fibrous bundles - the “circulatory system” of the plant, which penetrates it completely, connecting 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 found itself in the air, and the other part (the root) in the soil, it became necessary to deliver water and mineral salts from the roots to the leaves, and organic substances from the leaves to the roots. So in the course of evolution flora Two types of conductive fabrics arose - wood and bast.
Through the wood (through tracheids and vessels) water with dissolved minerals rises from the roots to the leaves - this is a water-conducting, or ascending, current. Through the phloem (through sieve tubes) the organic substances formed in the green leaves flow to the roots and other organs of the plant - this is a 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 the plant. The cells here lie very quickly to each other. Through division, they form many new cells, thereby ensuring the plant grows in length and thickness. The cells that appear during the division of educational tissues are then transformed into cells of other plant tissues.
This primary tissue, from which all other plant tissues are formed. It consists of special cells capable of multiple divisions. It is these cells that make up the embryo of any plant.
This tissue is retained in the adult plant. It is located:
- at the bottom of the root system and at the tops of the stems (ensures plant growth in height and development of the root system) - apical educational tissue;
- inside the stem (ensures the plant grows in width and thickens) - lateral educational tissue.
Unlike other tissues, the cytoplasm educational fabric thicker and denser. The cell has well-developed organelles that provide protein synthesis. The core is characterized by large size. The mass of the nucleus and cytoplasm is maintained in a constant ratio. 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 sporogenic meristems.
