Conductive tissue is one of the plant tissues that is necessary for movement nutrients throughout the body. This is an important structural component of generative and vegetative organs reproduction.
The conducting system is a collection of cells with intercellular pores, as well as parenchymal and transmitting cells, which together provide internal fluid transport.
Evolution of conductive tissues. Biologists suggest that the appearance of the vascular system of plants is due to the transition from water to land. At the same time, underground and above-ground parts were formed: the stem and leaves were in the air, and the root was in the soil. This is how the problem of transferring plastic and mineral compounds arose. Thanks to the appearance of conductive tissues, the circulation of fluid, minerals, and ATP throughout the body became possible.
Features of the structure of conducting plant tissue
The structure of plant conductive tissue is quite complex, as it contains different structural and functional elements. It includes xylem (wood) and phloem (bast), through which water moves in two directions.
Xylem (wood)
TO xylem The following fabrics include:
- Actually conductive (tracheids and tracheae);
- mechanical (wood fibers);
- parenchymatous.
Dead elements of plant conductive tissue can be vessels (tracheas) and tracheids, since they consist of dead cells.
Trachea- are tubes with thickened shells. They were formed from a series of elongated cells placed one above the other. The longitudinal cell membranes become lignified and their uneven thickening occurs, and the transverse walls are destroyed, forming through openings. The trachea are, on average, 10 cm long, but in some plants - up to 2 (oak) or 3-5 m (tropical vines).
Tracheids- single-celled spindle-shaped elements with pointed ends. Their length is about 1mm, but can be 4-7mm (pine). Just like the trachea, these are dead cells with lignified and thickened walls. The thickenings have the form of rings, spirals, and meshes. Tracheids differ from tracheas in the absence of openings, so the movement of fluid here occurs through the pores. They are highly permeable to minerals dissolved in water.
Phloem (bast)
Phloem also consists of three fabrics:
- Actually conductive (sieve system);
- mechanical (bast fibers);
- parenchymal.
The most important structural units of phloem are sieve tubes and cells, which are united into a single system through special fields and intercellular contacts.
Sieve tubes- oblong, living cells, their sizes range from 0.1 millimeter to 2 mm. Like the vessels, they are longest in vines. Their longitudinal walls are also thickened, but remain cellulose and do not become lignified. The transverse shells are perforated like a sieve and are called sieve plates.
Organic synthesis products ( ATP energy) move from the leaves to the underlying parts, along separated protoplasts (a mixture of vacuolar sap with cytoplasm).
The cytoplasm of the cells is preserved, and the nucleus is destroyed at the very beginning of tube formation. Even in the absence of a nucleus, the cells do not die, but their further activity depends on specific companion cells. They are located next to the sieve tubes. These are living, thin, elongated in direction sieve tube cells. The companion cells are a kind of storehouse of enzymes, which are released through the pores into the sieve tube segment and stimulate the movement of organic substances through them.
Companion cells and sieve tubes are closely interconnected and cannot function separately.
Sieve cells do not have special companion cells and do not lose their nuclei; sieve fields are randomly scattered on the side walls.
Conducting plant tissues, their structure and functions are briefly summarized in the table.
| Structure | Location | Meaning |
|---|---|---|
| Xylem is a conductive tissue, consisting of hollow tubes - tracheids and vessels with a compacted cell membrane. | Wood (xylem), inner part of a tree, which is closer to the axial part, in herbal plants - more in the root system and stem. | The upward movement of water and minerals from the soil to the roots, leaves, and inflorescences. |
| Phloem has companion cells and sieve tubes, which are built from living cells. | The bast (phloem) is located under the bark and is formed due to the division of cambium cells. | The downward movement of organic compounds from green parts capable of photosynthesis into the stem and root. |
Where is the conducting tissue found in plants?
If you make a cross section of wood, you can see several layers. Substances move along two of them: through wood and in bast.
The phloem (responsible for the downward movement) is located under the cortex and when the initial cells divide, the elements that are outside move to the phloem.
Wood is formed from cambium cells that move to the central part of the tree and provide an upward flow.
The role of conductive tissue in plant life
- Movement of mineral salts dissolved in water, absorbed from the soil into the stem, leaves, flowers.
- Transport of energy from the photosynthetic organs of the plant to other areas: root system, stems, fruits.
- Uniform distribution of phytohormones in the body, which contributes to the harmonious growth and development of the plant.
- Radial movement of substances into other tissues, for example, into cells educational fabric, where intensive division occurs. This type of transport also requires transfer cells with multiple projections in the membrane.
- Conductive fabrics make plants more flexible and resistant to external influences.
- Vascular tissue is a single system that unites all plant organs.
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.
Conductive Fabric Features
When the planet experienced major changes in climate conditions, plants had to adapt to them. Before that, they all lived exclusively in water. In the ground-air environment, it has become 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. WITH 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 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.
The function of conductive tissues is to conduct water with nutrients dissolved in it through the plant. Therefore, the cells that make up the conducting tissues have an elongated tubular shape, the transverse partitions between them are either completely destroyed or penetrated by numerous holes.
The movement of nutrients in a plant occurs in two main directions. Water and minerals rise from the roots to the leaves, which plants obtain from the soil through the root system. Organic substances produced during photosynthesis move from the leaves to the underground organs of plants.
Classification. Mineral and organic substances dissolved in water, as a rule, move along various elements conductive tissues, which, depending on the structure and physiological function performed, are divided into vessels (tracheas), tracheids and sieve tubes. Water rises through vessels and tracheids from minerals, through sieve tubes - various products of photosynthesis. However, organic substances move throughout the plant not only in a downward direction. They can rise up through the vessels, coming from underground organs to the above-ground parts of plants.
It is possible to move organic substances in an upward direction and through sieve tubes - from leaves to growing points, flowers and other organs located in the upper part of the plant.
Vessels and tracheids. The vessels consist of a vertical row of cells located one above the other, between which transverse partitions are destroyed. Individual cells are called vessel segments. Their shell becomes woody and thickens, the living contents in each segment die off. Depending on the nature of the thickening, several types of vessels are distinguished: annular, spiral, reticular, scalariform and porous (Fig. 42).
Ringed vessels have ring-shaped woody thickenings in the walls, but most of the wall remains cellulose. Spiral vessels have thickenings in the form of a spiral. Ringed and spiral vessels are characteristic of young plant organs, since, due to their structural features, they do not interfere with their growth. Later, reticular, scalariform and porous vessels are formed, with a stronger thickening and lignification of the membrane. The greatest thickening of the membrane is observed in porous vessels. The walls of all vessels are equipped with numerous pores, some of these pores have through holes - perforations. When vessels age, their cavity is often clogged with tills, which are formed as a result of neighboring parenchyma cells invaginating through the pores into the vessels and having the appearance of a bubble. The vessels in whose cavities tills appear cease to function and are replaced by younger ones. The formed vessel is a thin capillary tube (0.1...0.15 mm in diameter) and sometimes reaches a length of several tens of meters (some vines). Most often, the length of the vessels varies at different plants within 10...20 cm. The articulation between the segments of the vessels can be horizontal or oblique.
Tracheids differ from vessels in that they are individual closed cells with pointed ends. The movement of water and minerals occurs through various pores located in the shell of the tracheids, and therefore has a lower speed compared to the movement of substances through the vessels. Tracheids are similar in structure to vessels (thickening and lignification of the shell, death of the protoplast), but are a more ancient and primitive water-conducting element than vessels. The length of tracheids ranges from tenths of a millimeter to several centimeters.
Thanks to the thickening and lignification of the walls, vessels and tracheids perform not only the function of conducting water and minerals, but also mechanically, giving plant organs strength. The thickenings protect the water-conducting elements from being compressed by neighboring tissues.
In the walls of blood vessels and tracheids are formed various types pores - simple, bordered and semi-margined. Simple pores most often have a cross-section rounded shape and are a tubule passing through the thickness of the secondary membrane and coinciding with the pore tubule of the neighboring cell. Bordered pores are usually observed in the lateral walls of tracheids. They look like a dome rising above the wall of the water-conducting cage with a hole at the top. The dome is formed by the secondary membrane and its base borders on the thin primary cell membrane.
U coniferous plants in the thickness of the primary shell, directly below the opening of the bordered pore, there is a thickening - the torus, which plays the role of a two-way valve and regulates the flow of water into the cell. The torus is usually pierced with tiny holes. The bordered pores of adjacent vessels or tracheids, as a rule, coincide. If a vessel or tracheid borders on parenchyma cells, semi-bordered pores are obtained, since the border is formed only on the side of the water-conducting cells (see Fig. 21).
In the process of evolution, there was a gradual improvement in the water-conducting elements of plants. Tracheids as a primitive type of conductive tissue are characteristic of more ancient representatives flora(mosses, gymnosperms), although they are sometimes found in highly organized plants.
The initial type should be considered ring-shaped vessels, from which further development proceeded to the most advanced vessels - porous. There was a gradual shortening of the vascular segments with a simultaneous increase in their diameter. The transverse partitions between them acquired a horizontal position and were pierced with holes, which ensured better movement of water. Subsequently, complete destruction of the partitions occurred, from which a small ridge sometimes remains in the cavity of the vessel.
Vessels and tracheids, in addition to water with minerals dissolved in it, sometimes also carry organic substances, the so-called sap. This is usually observed in the spring, when fermented organic substances are directed from the places of their deposition - roots, rhizomes and other underground parts of plants - to above-ground organs - stems and leaves.
Sieve tubes. Organic substances dissolved in water are transported through sieve tubes. They consist of a vertical row of living cells and contain well-defined cytoplasm. The nuclei are very small and are usually destroyed during the formation of the sieve tube. There are also leucoplasts. The transverse partitions between the cells of the sieve tubes are equipped with numerous openings and are called sieve plates. Plasmodesmata extend through the holes. The shells of the sieve tubes are thin, cellulose, and have simple pores on the side walls. In most plants, during the development of sieve tubes, satellite cells adjacent to them are formed, with which they are connected by numerous plasmodesmata (Fig. 43). Companion cells contain dense cytoplasm and a well-defined nucleus. Companion cells were not found in conifers, mosses and ferns.
The length of the sieve tubes is significantly shorter than that of the vessels, and ranges from fractions of a millimeter to 2 mm with a very small diameter, not exceeding hundredths of a millimeter.
Sieve tubes usually function for one growing season. In autumn, the pores of the sieve plates become clogged, and a corpus callosum is formed on them, consisting of a special substance - calloses. In some plants, such as linden, the corpus callosum resolves and the sieve tubes resume their activity, but in most cases they die off and are replaced by new sieve tubes.
Living sieve tubes resist the pressure of neighboring tissues due to the turgor of their cells, and after dying they flatten and dissolve.
Lacteal vessels (lacteals). Lactifers, found in many flowering plants, can be classified as both conducting and excretory tissues, since they perform diverse functions - conducting, excreting and accumulating various substances. Lacteal vessels contain cell sap of a special composition, called milky sap, or latex. They are formed by one or more living cells that have a cellulose membrane, wall layers of cytoplasm, a nucleus, leukoplasts and a large central vacuole with milky juice, which occupies almost the entire cell cavity. There are 2 types of laticifers - articulated and non-articulated (Fig. 44).
Articulated lacticifers, like vessels and sieve tubes, consist of a longitudinal row of elongated cells. Sometimes the transverse partitions between them dissolve, and continuous thin tubes are formed, from which numerous lateral outgrowths extend, connecting individual laticifers with each other. Articulated laticifers include plants from the families Compositae (Asteraceae), Poppy, Campanaceae, etc.
Unsegmented lacticifers consist of a single cell, which grows as the plant grows. Branching out, they permeate the entire body of the plant, but the individual laticifers never connect. Their length can reach several meters. Unsegmented lacticifers are observed in plants of the nettle, euphorbia, kutraceae, and other families.
Milkers are usually short-lived and, having reached a certain age, die off and flatten. At the same time, rubber plants the latex coagulates, resulting in a mass of hardened rubber.
Excretory tissues(excretory system)
Functions and structural features. Excretory tissues serve to accumulate or secrete final metabolic products (catabolites) that are not involved in further metabolism and are sometimes harmful to plants. Their accumulation can occur both in the cavity of the cell itself and in the intercellular spaces. The elements of excretory tissues are very diverse - specialized cells, canals, glands, hairs, etc. The combination of these elements represents the excretory system of plants.
Classification. There are excretory tissues of internal secretion and excretory tissues of external secretion.
Excretory tissues of internal secretion. These include various containers of secretions in which metabolic products such as essential oils, resins, tannins, and rubber accumulate. However, in some plants, resins can also be released outside.
Essential oils most often accumulate in the receptacles of secretions. These receptacles are usually located among the cells of the main tissue near the surface of the organ. According to their origin, secretion receptacles are divided into schizogenic and lysigenic (Fig. 45). Schizogenic spaces arise as a result of the accumulation of substances in the intercellular space and the subsequent separation and death of neighboring cells. Similar channel-shaped excretory passages containing essential oil are characteristic of the fruits of plants of the umbelliferous (celery) family - dill, coriander, anise, etc. Resin passages in the leaves and stems of coniferous plants can serve as an example of containers of schizogenic origin.
Lysigenic receptacles arise as a result of the accumulation of the excretory product inside the cells, after which the dissolution of the cell membranes occurs. Lysigenic receptacles are widely known essential oils in citrus fruits and leaves.
Excretory tissues of external secretion. They are less diverse than endocrine tissues.
Of these, the most common are glandular hairs and glands, adapted to secrete essential oils, resinous substances, nectar and water. The glands that secrete nectar are called nectaries. They have a varied shape and structure and are mainly found in flowers, but are sometimes formed on other plant organs. The glands that secrete water play the role of hydathodes. The process of releasing water in a drop-liquid state is called guttation. Guttation occurs under conditions high humidity air that prevents transpiration.
This type belongs to complex tissues and 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 conducting tissues: xylem and phloem (Greek xylon - tree; phloios - bark, bast). They have both structural and functional differences.
The conducting elements of xylem are formed dead cells. They carry out long-distance transport of water and substances dissolved in it from the root to the leaves. The conducting elements of the phloem preserve the living protoplast. They carry out long-distance transport from photosynthetic leaves to the root.
Typically, xylem and phloem are located in the plant body in a certain order, forming layers or vascular bundles. Depending on the structure, there are several types of vascular bundles, which are characteristic of certain groups of plants. The collateral open bundle between the xylem and phloem contains the cambium, which provides secondary growth. In a 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. You can find two more types of concentric bundles, where either phloem surrounds xylem, or xylem surrounds phloem.
Xylem (wood). Xylem development in higher plants associated with ensuring water exchange. Since water is constantly removed 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 taken into account that the presence of a living protoplast in water-conducting cells would greatly slow down transport; dead cells here turn out to be more functional. However, a dead cell does not have turgidity; therefore, the membrane must have mechanical properties. Note: turgescence - states plant cells, tissues and organs, for which? they become elastic due to the pressure of the cell contents on their elastic membranes. Indeed, the conducting elements of the xylem consist of dead cells elongated along the axis of the organ with thick lignified shells.
Initially, xylem is formed from the primary meristem - procambium, located at the tops of the axial organs. First, protoxylem is differentiated, then metaxylem. Three types of xylem formation are known. In the exarch type, protoxylem elements first appear at 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 endarchic type. In the mesarchic type, xylem is formed 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 exarchal type of xylem formation, while the stems are characterized by an endarchic 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 conducting tissues that are not capable of secondary thickening. In other forms (for example, woody ones), lateral meristems (cambium) remain between the xylem and phloem. These cells are able to divide, renewing the xylem and phloem. This process is called secondary growth. In many, growing in relatively stable climatic conditions, plants, growth is constant. 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 them from stretching during the growth of the organ. The protoplast then begins to lay down a secondary shell. But this process has distinct features. The secondary shell 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 young cells, the rings or turns of the helix are located close to each other. In mature cells, cells diverge as a result of cell elongation. The ringed and spiral thickenings of the shell do not interfere with growth, but mechanically they are inferior to the shells, where the secondary thickening forms a continuous layer. In this regard, after growth ceases, elements with a continuous lignified shell (metaxylem) are formed in the xylem. It should be noted that the secondary thickening here is not ringed or spiral, but dotted, scalariform, mesh-shaped. Its cells are not capable of stretching and die within a few hours. This process occurs in a coordinated manner in nearby cells. A large number of lysosomes appear in the cytoplasm. Then they disintegrate, and the enzymes contained in them destroy the protoplast. When the transverse walls are destroyed, the cells located in a chain above each other form a hollow vessel. Most angiosperms and some? pteridophytes have blood vessels.
A conducting cell that does not form through perforations in its wall is called a tracheid. The movement of water through tracheids occurs at a lower speed than through vessels. The fact is that in tracheids the primary shell 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 shell up to the primary shell and there are no through perforations between the tracheids.
The most common are bordered pores. In them, a channel facing the cell cavity forms an extension - a pore chamber. The pores of most coniferous plants on the primary shell have a thickening - a torus, which is a kind of valve and is able to regulate the intensity of water transport. By shifting, 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 round, elongated perpendicular to the elongated axis (a group of these pores resembles a ladder; therefore, such porosity is called staircase). Through the pores, transport occurs both in the longitudinal and transverse directions. Pores are present not only in tracheids, but also in individual vascular cells that form the vessel.
From the point of view evolutionary theory tracheids are the first and main structure that conducts water in the body of higher plants. It is believed that the vessels arose from tracheids due to lysis of the transverse walls between them. Most pteridophytes and gymnosperms do not have vessels. Their movement of water occurs through tracheids.
In the process of evolutionary development, vessels arose in different groups 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 advanced transport mechanism helped them not only survive, but also achieve a significant variety of forms.
Xylem is a complex tissue; in addition to water-conducting elements, it also contains others. Mechanical functions are performed by libriform fibers (Latin liber - bast, forma - form). The presence of additional mechanical structures is important because, despite the thickening, the walls of the water-conducting elements are still too thin. They are not able to support large masses on their own. perennial plant. The fibers developed from tracheids. They are characterized by smaller sizes, lignified (lignified) shells and narrow cavities. Pores without borders can be found on the wall. These fibers cannot conduct water; their main function is support.
The xylem also contains living cells. Their mass can reach 25% of the total volume of wood. Since these cells are round in shape, they are called wood parenchyma. In the plant body, parenchyma is located in two ways. In the first case, the cells are arranged in the form of vertical strands - this is strand parenchyma. In another case, the parenchyma forms horizontal rays. They are called pith rays because they connect the pith and cortex. The core performs a number of functions, including storing substances.
Phloem (bast). This is a complex tissue, as it is formed by different types of cells. The main conducting cells are called sieve elements. The conducting elements of the xylem are formed by dead cells, while in the phloem they retain a living, albeit highly modified, protoplast during the period of functioning. The phloem carries out the outflow of plastic substances from photosynthetic organs. All living plant cells have the ability to conduct organic substances. And hence, if xylem can be found only in higher plants, then the transport of organic substances between cells is also carried out in lower plants.
Xylem and phloem develop from apical meristems. At the first stage, protophloem is formed in the procambial cord. As surrounding tissues grow, it stretches, and when growth is complete, metaphloem is formed instead of protophloem.
In different groups of higher plants two types can be found sieve elements. In pteridophytes and gymnosperms it is represented by sieve cells. Sieve fields in the cells are scattered along the side walls. The protoplast retains a somewhat destructed nucleus.
In angiosperms, sieve elements are called sieve tubes. They communicate with each other through sieve plates. Mature cells lack nuclei. However, next to the sieve tube there is a companion cell, 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 influence the functional activity of anucleate tube sieve cells.
The structure of mature sieve cells has some peculiarities. There is no vacuole, and therefore the cytoplasm is greatly liquefied. The nucleus may be absent (in angiosperms) or in a wrinkled, 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 found in abundance.
Between cells, substances are transported through holes located on the cell membranes. Such openings are called pores, but unlike the pores of tracheids, they are through. It is assumed that they represent greatly expanded plasmodesmata, on the walls, which? callose polysaccharide 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 advanced angiosperms, they are located at the ends of adjacent cells adjacent to each other, forming a sieve plate. If there is one sieve field on it, it is called simple, if there are several, it is called complex.
The speed of movement of solutions through sieve elements is up to 150 cm per hour. This is a thousand times faster than the speed of free diffusion. Active transport probably takes place, and numerous mitochondria of sieve elements and companion cells supply the necessary ATP for this.
The duration of activity of the phloem sieve elements 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.
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 is carried out. The movement of water through the tracheids occurs at a slow speed.
Vessels (trachea). The vessels form the most perfect conducting 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 of 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 cords mechanical fabric, 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 relative position xylem and phloem there are several types of vascular bundles (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 on the inside, the other on 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.
