The first organisms on Earth were single-celled. The entire body of the organism consisted of just one cell. Later appeared multicellular organisms, however, their bodies consisted of identical cells. And only then did organisms begin to consist not only of identical ones, but also of different cells. Identical cells in the same organism form tissues. IN complex organisms there can be a number of different tissues, so there are a number of different cells.
By the composition of plant tissues, you can determine which group they belong to - algae, mosses, ferns or seed plants.
Tissues contain cells that are similar in their structure and functions. Tissues can differ in the density of cells; in some they can be located very close to each other, forming rows of cells, in others they can lie as desired, not tightly to each other, loosely. The spaces between cells are called intercellular space, or intercellular spaces. The tissue also includes intercellular spaces.
Cells educational fabric divide throughout the life of the plant. The cells of the educational tissue lie tightly to each other; dividing, they form new cells and thereby ensure the growth of the plant not only in length, but also in thickness. In addition, cells of plant educational tissue are capable of transforming into cells of other tissues.
Responsible for the creation and accumulation of substances main fabric. It is in this tissue that chlorophyll is found, thanks to which inorganic substances organic is synthesized. The main tissue is mainly found in the leaves of plants.
However, the main tissues in which the reserve occurs nutrients, are found in seeds, modified roots (potato tuber), stems (bulb), etc.
Performs a protective function cover tissue . It protects all plant organs from the outside from drying out, damage, and overheating. In the skin of leaves and shoots, the cells of the integumentary tissue are tightly closed together; they have a transparent cell wall to allow light to pass through. In roots and stems, the covering tissue can become suberized, turning into a cork.
Thanks to conductive tissue substances can move throughout the plant. Substances move in aqueous solutions that flow through the cells of conducting tissues. U higher plants conducting tissue consists of vessels, tracheids and sieve tubes. Conducting tissues have pores and openings that allow the movement of substances between cells.
Conductive tissue is a branched network connecting all plant organs. Thus, all parts of the plant are combined into a single system.
Mechanical fabric allows plants to withstand various loads, such as wind. Mechanical tissue cells have very strong cell walls.
The existence of different tissues is due to the fact that plant cells on land have to perform different functions. The root is in the soil and absorbs water solution, also keeps the plant in the soil. The leaves are exposed to light and are responsible for the synthesis of organic substances. The stem connects different parts of the plant with each other.
Plant tissues are quite diverse. I wonder what morphological features of each such structure directly depend on the function it performs. It is customary to distinguish several types:
- educational;
- integumentary;
- mechanical;
- conductive;
- basic.
Each structure has certain features, which will be discussed below.
Educational plant tissue
Educational tissues are also called meristems. This structure consists of small, multifaceted cells with thin walls. They are tightly closed together. Under a microscope you can see that they have a large nucleus and many small vacuoles. A feature of this tissue is the ability of its cells to constantly divide. This is what ensures constant growth plants. It is customary to distinguish the following types:
- Primary meristem- In an adult plant, this tissue is preserved in the tips of the shoots and tips of the roots. It is thanks to it that the primary growth of the plant in length occurs.
- Secondary meristem - represented by cambium and phellogen. These tissues provide secondary growth of the stem and root in diameter. Based on their location, apical, lateral and intercalary secondary meristems are distinguished.
Plant integumentary tissues
The covering tissue is located on the surface of the plant body. Its main function is protection. Such structures are responsible for the plant’s resistance to mechanical impact, protect against sudden temperature fluctuations and excessive evaporation of moisture, protect against penetration of pathogenic microorganisms. Integumentary elements are usually divided into three main groups:
- The epidermis (skin) is a primary tissue that consists of small, transparent and tightly interconnected cells. Typically, this type of tissue covers the surface of leaves and young shoots. The epidermal layer of leaves also includes stomata - formations that are responsible for the processes of gas exchange and transpiration.
- Periderm is a secondary integumentary tissue that is located on the surface of the stem and root. It consists of pheloggen and is a dead layer of cells, the walls of which are impregnated with the waterproof substance suberin.
- Bark is a tissue that is characteristic of trees and some bushes. This layer of integumentary tissue is the outer part of the cork.
Conductive plant tissues
The main function of this group of tissues is the transport of water and minerals throughout the plant body. It is customary to distinguish the following types of conductive elements:
- Xylem - ensures the movement of water with dissolved minerals from the root system to the above-ground part of the plant. It consists of special vessels, the so-called trachea and tracheids.
- Phloem is the tissue that allows downward flow. Through sieve tubes, all organic nutrients that are synthesized by the leaves are carried to the rest of the plant organs, including the root system.
plants: parenchyma
This tissue consists of small living cells with thin walls. It is this that forms the basis of all organs. These include:
- Assimilation tissues - their cells contain a huge number of chloroplasts and are responsible for the formation of organic substances. Most of These tissues are found in the leaves.
- Storage tissues - deposited in cells useful material. This tissue is concentrated in fruits, roots and seeds.
- Aquiferous tissues serve to accumulate and preserve water. These tissues are characteristic of plants living in hot and dry climates, such as cacti.
- Air-bearing tissues - such tissues have huge intercellular cavities that are filled with air. Aerenchyma is characteristic of marsh and
Mechanical plant tissues
Responsible for creating a strong frame. They maintain the shape of the plant, making it more resistant to mechanical influence. This tissue consists of cells with thick membranes. They are most strongly developed in the stem of the plant.
Textile - a group of cells and non-cellular structures similar in origin and structure, forming a structural-functional complex and performing the same functions.
Typically, classification takes into account the function, structure, origin and location of tissues. There are six main groups (systems) of tissues:
· System of meristematic (educational) tissues:
Ø apical meristem;
Ø lateral meristem;
Ø intercalary meristem;
Ø wound meristem.
· System of integumentary (border) tissues:
Ø epidermis;
Ø periderm (plug);
Ø crust (rhytidome);
Ø epiblema.
· Main fabric system:
Ø assimilation (chlorophyll-bearing) parenchyma (chlorenchyma);
Ø storage parenchyma.
· Mechanical (reinforcement) fabric system:
Ø collenchyma;
Ø sclerenchyma.
· Conductive tissue system (complex tissues based on conductive elements):
Ø xylem;
Ø Phloem.
· System of excretory (secretory) tissues:
Ø external secretory structures;
Ø internal secretory structures.
4.1. Educational tissues (meristems)
Plants have unlimited growth due to the presence of educational tissues. They are formed by undifferentiated (parenchymal) round or polyhedral cells without intercellular spaces. The cell walls are thin, easily extensible, the cytoplasm is thick, viscous, the nucleus is large, occupies central position. Cells of educational tissues are capable of dividing quickly, so they contain many ribosomes and mitochondria.
By origin they distinguish:
· Primary meristems - meristems of the embryo. They determine the development of the seedling and the primary growth of organs.
· Secondary meristems . They arise on the basis of the primary ones. They ensure the growth of organs mainly in width.
By location there are:
· Apical (apical) meristems . Located at the ends of the main and lateral axes of the stem and root, they mainly determine the length growth of the organ.
· Lateral (lateral) meristems . They arise due to the activity of primary meristems. As a rule, they cause thickening of the axial organs.
· Intercalary meristems . Areas of intensively dividing cells, usually located at shoot nodes or at the bases leaf blades. They are remnants of the apical meristem. When the growth of internodes or leaves stops, the intercalary meristem turns into permanent tissues, that is, their activity is short-lived. But sometimes these meristems can function for quite a long time (for example, at the base of the internodes of horsetails and cereals).
· Wound (traumatic) meristems . They appear in places of mechanical destruction of tissue from living cells of various parenchymal tissues. They ensure healing of the wound and block access to pathogens.
4.2. Integumentary tissues
Integumentary tissues are permanent formations. Having arisen, the cells of these tissues no longer divide.
Usually, integumentary tissues called tissues that cover the body of a plant and interact with external environment. They protect inner fabrics from action unfavorable factors environment, regulate gas exchange and transpiration.
The integumentary tissues themselves include:
1. Epidermis
The main functions are protection of young organs from drying out, mechanical protection and gas exchange, sometimes secretory - the cells serve as a container for secretions. This is the primary integumentary tissue. Most often in the epidermis there are main cover cells, stomatal formations and various outgrowths ( trichomes). The main integumentary cells are usually represented by a single layer of tightly packed cells. The cell walls are usually tortuous, the outer walls are thicker than the others. These are living cells with large vacuoles, the cytoplasm looks like a thin wall layer. The ER and Golgi apparatus are usually well developed.
The covering tissue of the root absorption zone is called epibleme.
For gas exchange and transpiration, the epidermis contains special education -stomata. They are a group of highly specialized cells. The stomata is a slit-like opening in the epidermis, bounded by two bean-shaped cells. This guard cells. Unlike other epidermal cells, they contain chloroplasts. The walls of the guard cells facing the stomatal fissure are thickened. The epidermal cells surrounding the guard cells are called collateral or adjacent. Below the stomata is gas-air chamber. Guard and side cells, stomatal fissure and ha-
the air chamber form stomatal apparatus. Stomata are most often located on the underside of the leaf.
2. Cuticle
The protective function of the epidermis can be enhanced by the presence of a cuticle and waxy coating. Cuticle is a cell-free formation. It is a product of protoplast activity and consists of a special substance - cutin and wax-like substances. Wax-like substances can be part of the cuticle or located on its surface. Cuticle and waxy coating are found on fruits, leaves, stems, and flower parts. The cuticle and waxy coating are impermeable to water and almost impermeable to gases.
3. Periderm (cork)
Secondary integumentary tissue consisting of phellems- the actual traffic jams, phellogen- cork cambium phelloderms- cork parenchyma. It replaces the epidermis, which gradually dies and sloughs off. It is formed mainly in stems and roots. The cork consists of regular radial rows of densely spaced cells with suberized walls. The contents of the cell die. There are no intercellular spaces. Cork is impermeable to water and gases. Lentils are formed in the plug for gas exchange and transpiration.
4. Ritide, or crust
In most woody plants, the cork is replaced by a crust, which is sometimes called tertiary integumentary tissue. When a crust forms new layer phellogen and periderm are laid down in the main tissue lying deeper than the first outer periderm. The newly formed layers of the plug separate to the periphery of the organ not only the periderm, but also part of the underlying parenchyma of the cortex. This is how a thick multicellular and dead formation arises. Since the crust cannot stretch, when the trunk thickens, it bursts and cracks form.
4.3. Mechanical (reinforcing) fabrics
Intensively developed in land plants. The main purpose is to prevent rupture of tissues and organs. The stems are located alongperiphery, in the roots - in the center. They consist of cells with thick walls, often lignified.
1. Collenchyma
Developed mainly in the stems, petioles and leaves of dicotyledonous plants. As a rule, it is found in the peripheral part of organs directly under the epidermis or a little deeper. Formed by living, elongated cells, often containing chloroplasts. Cell walls are unevenly thickened.
2. Sclerenchyma
The most important mechanical tissue of higher plants. By origin it can be primary (if it was formed from the procambium or from the pericycle) and secondary (if it was formed from the cambium). Formed by cells with uniformly thickened, often lignified walls. The protoplast dies early, and the supporting function is performed by dead cells which are called fibres. 1. Xylem (wood)
It consists of vessels and tracheids that carry out the upward flow of water and minerals, as well as wood fibers and wood parenchyma.
2. Tracheids
Cells elongated along the axis of the organ with strongly beveled end walls. Penetration of the solution from one tracheid to another occurs through the pores. More often found in higher spores and gymnosperms.
3. Vessels
Formed from individual segments that were previously cells. These are long microscopic tubes. The end walls of the vessel segments almost completely dissolve andthrough holes (perforations) appear. The lumen of the vessels is wider than that of the tracheids. This is a more advanced conductive fabric, reaching greatest development in angiosperms.
It consists of sieve elements, accompanying companion cells, bast parenchyma and phloem (bast) fibers.
4. Sieve elements
The most important part of phloem. Due to the presence of sieve elements, phloem provides a downward flow of water and organic matter. The cells of the sieve elements have a living protoplast, through which the movement of water and organic substances occurs. Protoplasts of neighboring cells communicate with each other through special small holes - perforation. Perforations are collected in groups - sieve fields. There are sieve cells and sieve tubes.
5. Sieve cells
Characteristic of higher spore and gymnosperm plants. They are highly elongated cells with pointed ends. Sieve fields are scattered along the side walls. Mature cells retain their nucleus. Sieve cells lack accompanying cells.
6. Sieve tubes
Characteristic of angiosperms (Fig. 7). Perforations are collected in groups and form sieve plates, which are located at the end ends of the cells. In mature segments of sieve tubes, the nucleus is absent, the central vacuole dissolves, the cell sap combines with the cytoplasm. However, the cell remains alive. The protoplast takes the form of long strands passing through perforations from segment to segment.
Next to each segment of the sieve tube there are satellite cells. They take part in the transport of substances through sieve tubes.
In plant organs, xylem and phloem are located mainly in the composition complex formations - conductive bundles.
4.5. Main fabrics
They form the basis of organs, filling the spaces between other tissues, and provide all aspects of internal metabolism in plants. They are called parenchymal or parenchyma. In typical cases, intercellular spaces are well developed.
Most typical for leaves and green assimilating stems. Contains chloroplasts and performs the function of photosynthesis. Cells round or slightly elongated oval shape. Their walls are thin, never lignified, and sometimes folded. The cells are almost completely filled with chloroplasts, only in the center there is a vacuole. The nucleus and cytoplasm occupy a wall position. Divided into columnar, or palisade, And spongy chlorenchyma. Columnar chlorenchyma cells are arranged in one or several layers. The end walls face the outside and inside of the organ. The longitudinal walls are in close contact with each other. The cells of spongy chlorenchyma are arranged loosely, with large intercellular spaces.
1. Storage parenchyma
Mainly developed in axial organs, reproductive and vegetative reproduction organs. Serve to preserve nutrients. Formed by thin-walled cells. There are no chloroplasts. The nucleus, cytoplasm and other organelles first occupy a wall position, and then may disappear altogether, while the cells remain alive. In arid areas, plants have water-storing tissues. The cells of such tissue contain a lot of mucus, which helps retain water.
4.6. Excretory tissues
Release or accumulate various substances. The cells of excretory tissues are thin-walled. Depending on the nature of the secreted substances, the smooth endoplasmic reticulum or Golgi apparatus. Excretory tissues are divided into external and internal.
Evolutionarily associated with integumentary tissues. They highlight different chemical substances playing specific value in the life of plants: some attract pollinating insects, others are metabolic products, etc. Such fabrics include:
· nectaries-specialized glandular outgrowths that produce nectar;
· hydathodes-multicellular formations that secrete droplet-liquid water and salts dissolved in it;
· osmophores-specialized cells of the epidermis or special glands that secrete aromatic substances.
Tissues are groups of cells that are similar in origin, shape, structure and function.
Based on the shape of the constituent cells, tissues are distinguished between parenchymal tissues, composed of isodiametric (with approximately equal internal diameters) cells, and prosenchymal tissues, which consist of elongated cells having different “length” and “width” from each other.
Tissues are usually classified according to the main function they perform.
Educational fabrics, or meristems, ensure continuous growth of plants. Meristem cells long time retain the ability to divide without turning into permanent tissues. The cells of educational tissues are thin-walled, filled with dense cytoplasm, small, and a significant part of the cell volume is occupied by the nucleus. In higher plants, from the first stages of embryo development, apical (apical) meristems are formed at the top of the stem and at the tip of the root. As they grow and branch, each lateral shoot and each lateral root develop their own apical meristems. They ensure the plant grows in length. Lateral meristems (cambium) may appear in stems and roots. In dicotyledonous plants, the division of cambium cells ensures the growth of stems and roots in thickness. From permanent parenchymal tissues, a secondary meristem sometimes arises - phellogen (cork cambium), which forms the integumentary tissue - cork.
At the base of the internodes of the stem and at the base of young growing leaves there is an intercalary meristem. At the end of the growth of the stem section or leaf, the intercalary meristem ceases to function, turning into permanent tissues.
IN assimilation tissues photosynthesis occurs. Assimilation tissues are located mainly in the leaf and stem under the epidermis and consist of thin-walled parenchyma cells containing chloroplasts.
IN storage tissues reserve substances are deposited. These tissues consist of living parenchyma cells. In perennial plants, storage tissues are found in stems, roots, rhizomes, tubers, and bulbs. Storage substances can be carbohydrates, proteins and fats.
Air tissue (aerenchyma) - tissue with very large intercellular spaces, the main function of which is ventilation. Intercellular systems are connected to the external environment through openings in the integumentary tissues (stomata, lentils). Through the intercellular spaces, oxygen from the leaves, where it accumulates as a result of photosynthesis, can penetrate to the tips of the roots, which is very important for marsh, aquatic and other plants living in conditions of difficult gas exchange.
Integumentary tissues protect higher plants from drying out, sudden temperature fluctuations, from excess radiant energy, from mechanical damage, from excessive insolation.
In the first year of life, the leaves and young stems of the plant are covered with a layer of tightly packed cells - the epidermis (skin). Epidermal cells are living, containing a nucleus, dense cytoplasm and small leukoplasts. As cells grow, vacuoles form in them, often with dissolved in cell sap anthocyanin pigment, which causes the purple-red color of leaves and stems.
Epidermal cells have devices that prevent the plant from losing water: their outer walls are greatly thickened and covered with a thin waterproof film - cuticle (cuticle). Some plants have a waxy coating over the cuticle. Sometimes the walls of epidermal cells are impregnated with silica (horsetails, sedges, cereals). Often, epidermal cells form outgrowths - hairs, covering leaves and stems in a thick layer. The hairs can be very diverse in size and shape (filamentous, branched, star-shaped, etc.). If the hairs end in swelling and are capable of accumulating and secreting mucus, essential oils etc., they are called glandular.
The entry of air into the plant, the release of oxygen and water vapor is carried out by special formations present in the epidermis - stomata. Typically, in land plants, stomata are located on the underside of the leaf, near aquatic plants with floating leaves - on the top. Each stoma consists of a pair of bean-shaped guard cells and a stomatal fissure, which is a pronounced intercellular space. The guard cells of the stomata, unlike other cells of the epidermis, always contain chloroplasts and actively undergo photosynthesis. The stomatal fissure can expand and contract due to changes in osmotic pressure within the guard cells.
Through open stomata, intense diffusion of water vapor, oxygen and carbon dioxide. When the stomata are closed, transpiration and gas exchange are sharply reduced. In plants temperate zone the number of stomata varies depending on the type of plant and living conditions from 100 to 700 per 1 mm 2 of leaf surface.
In perennial terrestrial plant organs, the primary integumentary tissue (epidermis) is after some time replaced by a secondary, more reliably protecting plant tissue - a plug (periderm). The periderm arises as a result of the activity of the cork cambium (phellogen), which is formed under the epidermis. When phellogen cells divide, they lay out layers of cork cells, which perform protective functions. The rows of cells in the cork are tightly adjacent to each other; there are no intercellular spaces in the cork. Phellogen deposits layers of phelloderm cells inside, which consists of living cells and provides nutrition to the phellogen cells.
As the stem thickens and the periderm forms, the epidermis is shed and the stem turns from green to brown. Stems with a formed periderm are capable of overwintering.
To ensure gas exchange of internal tissues, so-called lentils are formed in the cork. In lentils, cork cells and living parenchyma cells are loosely connected to each other. Gas exchange occurs through intercellular spaces. Lentils on young stems look like small tubercles.
Most tree species After a few years, a tertiary covering tissue - a crust - is formed. In apple trees it occurs at 6-8 years, in pine trees. - at 8-10, for oak - at 25-30, for hornbeam - at 50 years. The crust is formed as a result of repeated deposition of new layers of periderm in increasingly deeper layers of the cortex. As the trunk thickens and the phellogen is repeatedly laid down and active, the peripheral dead tissue of the crust cracks and the surface of the tree trunk becomes uneven. Crusted trunks are protected from abrupt changes temperatures, from ground forest fires, from damage by animals.
Excretory tissues accumulate or release substances that are excluded from metabolism. Internal excretory tissues accumulate metabolic waste within the body. They consist of individual isolated cells among cells of other tissues. These are the so-called idioblasts. Crystals of calcium oxalate, essential oils, tannins, etc. accumulate in them. Receptacles for secretions (for example, in citrus fruits) and excretory passages (resin passages of conifers, tubules of umbrellas, etc.) can form in the intercellular spaces. Living cells that accumulate milky sap in vacuoles are called laticifers. The milky juice contains resins, rubber, essential oils, protein compounds, and alkaloids.
External excretory tissues are also diverse. The glandular hairs that form the protrusion of some plants release essential oils, salts and other substances. Hydathodes are groups of cells associated with the conducting tissues of the leaf and ending with water stomata; they secrete water and salts dissolved in it. External excretory tissues include nectaries, located in flowers and secreting a sugary liquid (nectar), which attracts pollinating insects; as well as the digestive glands of carnivorous plants (in sundew, dew leaf, butterwort, etc.), which secrete enzymes and acids necessary for digesting the tissues of captured invertebrate animals.
Mechanical (reinforcing) fabrics perform a supporting function. In young organs of higher plants, cell walls and turgor provide sufficient strength and a certain form organ. However, in land plants, as they grow, there is a need to form specialized mechanical tissues with thickened cell walls. Depending on the shape of the cells and the method of thickening their walls, three types of mechanical tissues are distinguished: collenchyma, sclerenchyma (fibers) and sclereids (stony cells).
Collenchyma is a mechanical tissue made up of living cells, usually parenchymal or slightly elongated. It is found in petioles and leaf blades, in growing parts of stems. Depending on the characteristics of cell thickening, angular and lamellar collenchyma are distinguished. The most common is angular collenchyma, which is characterized by partial thickening of the walls at the corners of cells in contact with each other. In general, a kind of strong reinforcement is created in the fabric. The cell walls in such tissue consist of fiber. In lamellar collenchyma, partial thickenings of the membranes are arranged in parallel rows.
Sclerenchyma is very different from collenchyma. It usually consists of prosenchymal (elongated) cells called fibers. The shells in them are evenly thickened, often lignified. The contents of the cells die and mechanical function perform thickened membranes of tissue cells. The fibers that make up wood are called wood fibers, or libriform. The fibers that make up the bast (phloem) are called bast fibers. In the textile industry, bast non-lignifying fibers consisting of pure fiber (linen) are used.
Sclereids are cells with highly thickened lignified membranes that do not have a fiber shape. Such cells consist, for example, of nut shells, seeds of juicy drupes, etc. Stony cells in the pulp of pear or quince fruits are parenchyma cells with highly thickened lignified walls.
Mechanical tissues create a strong frame of the plant body, which is filled with an elastic mass of living cells. Mechanical tissue in the stem is located along the periphery and thereby increases resistance to bending and fracture. In the root, the mechanical elements are concentrated mainly in the center, which prevents the organ from rupturing.
Conductive fabrics carry out the function of conducting water and nutrients into plant bodies. The plant has two types of conducting tissues: xylem and phloem. An upward flow occurs through the xylem (wood): the movement of water and minerals dissolved in it from the root to all plant organs. Xylem is a complex tissue consisting of actual conductive elements (vessels or tracheids), which determine the nature of the tissue, as well as cells that perform mechanical and storage functions.
Tracheids are dead, elongated, sometimes pointed at the ends, cells with intact primary walls. Through these walls (in the bordered pores), water penetrates from one tracheid to another by filtration. The primary shell of the tracheid thickens and becomes lignified, only numerous pores remain unthickened. Tracheids are characteristic of the wood of ferns and gymnosperms, where they perform both conductive and mechanical functions.
The most perfect conducting system is found in flowering plants. Their xylem is represented by more advanced water-conducting elements - vessels. The vessel consists of a number of segments, in the transverse walls of which holes are formed. Thanks to them, the unhindered movement of solutions through the long and narrow capillary-like tube of the vessel is carried out. The membranes of the vessels, like those of tracheids, are unevenly thickened and impregnated with lignin. The thickenings give the vessels mechanical strength. Through non-thickened areas of vessels (pores), solutions can also flow horizontally into neighboring vessels and parenchyma cells. According to the nature of the thickening of the membranes, vessels are distinguished into annular, spiral, scalariform and point-pore. Due to the fact that in regions with a seasonal climate in the spring the cambium deposits tracheids or vessels in the wood with wide openings and a relatively thin wall, and in the autumn - with narrow openings and a thicker wall, the so-called growth rings differ. The mechanical elements of wood - libriforms - as well as vessels, evolved from tracheids by enhancing the mechanical rather than conductive function: the libriform fibers developed a thickened secondary shell and the number and size of pores decreased.
The downward flow of dissolved organic substances coming from the leaves is carried out by the phloem. The phloem consists of sieve tubes through which assimilates move, companion cells, mechanical cells (bast fibers), and parenchyma cells.
Sieve tubes are made up of a chain of living cells. Between adjacent cells (segments) of the tube there are sieve-like plates pierced by holes (perforations), through which the protoplasts of neighboring segments communicate with cytoplasmic cords. The sieve plate is thus a transverse wall of two adjacent cells that make up the sieve tube. In the cells that form sieve tubes, as a rule, there are no nuclei, but the protoplasts remain alive and actively conduct organic matter. The accompanying cells, or satellite cells, are located near the sieve tubes. They have nuclei and cytoplasm with numerous mitochondria; intensive metabolism occurs in them. The sieve tube segments and companion cells are structurally and functionally closely related to each other; the nuclei of satellite cells coordinate the functions of the cytoplasm of sieve tube cells.
Conductive tissues, together with mechanical tissue fibers, are organized into special structures - conductive, or vascular-fibrous bundles. These bundles penetrate all plant organs, uniting into a single conducting system.
Vascular-fibrous bundles differ in the location of xylem and phloem relative to each other; There are concentric, collateral, bicollateral and radial bundles.
In concentric bundles, conducting tissue of one kind surrounds conducting tissue of another kind: phloem-xylem or xylem-phloem.
In collateral bundles, xylem and phloem are located side by side. In such bundles, the xylem region often faces the axis of the organ, and the phloem region often faces the periphery. Such bunches are characteristic of the stems and leaves of most modern plants.
In bicollateral bundles, two strands of phloem adjoin the xylem: one is closer to the axis of the organ, the other is closer to the outside.
Complex radial tufts are characteristic of plant roots. In them, the xylem is located along the radii of the organ, between them there are strands of phloem.
Closed fibrovascular bundles are those in which there are no cambial cells and they are not capable of secondary thickening due to the formation of new cells.
In addition to conductive and mechanical elements, open vascular-fibrous bundles also contain cambial tissue. The bundle cambium is a single layer of constantly longitudinally dividing cells that develop into new conductive and mechanical elements, thereby providing secondary thickening of the bundles in particular and the stem in general.
Textile- a group of cells that are structurally and functionally interconnected with each other, similar in origin, structure and performing certain functions in the body.
Tissues arose in higher plants in connection with their access to land and reached the greatest specialization at angiosperms, in which up to 80 species are distinguished. The most important plant tissues:
Educational,
Integumentary,
Conductive,
Mechanical
Basic.
Fabrics can be simple and complex. Plain fabricsconsist of one type of cell (for example, collenchyma, meristem), andcomplex- from cells of different structures, performing, in addition to the main and additional functions(epidermis, xylem, phloem, etc.).
Educational fabrics, or meristems, are embryonic tissues. Thanks to their long-lasting ability to divide (some cells divide throughout life), meristems participate in the formation of all permanent tissues and thereby form the plant, and also determine its long-term growth.
The cells of educational tissue are thin-walled, multifaceted, tightly closed, with dense cytoplasm, a large nucleus and very small vacuoles. They are capable of dividing in different directions.
According to the origin of meristems, there are primary and secondary. The primary meristem is the embryo of the seed, and in an adult plant it remains at the tip of the roots and tips of the shoots, which makes it possible for them to grow in length. Further growth of the root and stem in diameter ( secondary growth) is provided secondary meristems- cambium and phellogen. Based on their location in the plant body, apical (apical), lateral (lateral), intercalary (intercalary) and wound (traumatic) meristems are distinguished.
Integumentary tissues located on the surface of all plant organs. They mainly perform protective function- protect plants from mechanical damage, penetration of microorganisms, sudden temperature fluctuations, excessive evaporation, etc. Depending on their origin, three groups of integumentary tissues are distinguished - epidermis, periderm and crust.
Epidermis (epidermis, skin)- primary integumentary tissue located on the surface of leaves and young green shoots (Fig. 8.1). It consists of a single layer of living, tightly packed cells that do not have chloroplasts. The cell membranes are usually tortuous, which ensures their strong closure. The outer surface of the cells of this tissue is often covered with a cuticle or waxy coating, which is an additional protective device. The epidermis of leaves and green stems contains stomata that regulate transpiration and gas exchange in the plant.
Periderm- secondary integumentary tissue of stems and roots, replacing the epidermis in perennial (less often annual) plants (Fig. 8.2.). Its formation is associated with the activity of the secondary meristem - phellogen (cork cambium), the cells of which divide and differentiate in the centrifugal direction (outward) into the cork (phellema), and in the centripetal direction (inward) - into a layer of living parenchyma cells (phelloderm). Cork, phellogen and phelloderm make up the periderm.
Rice. 8.1. Leaf epidermis various plants: A-chlorophytum; 6 - common ivy: in - fragrant geranium; G - white mulberry; 1- epidermal cells; 2 - stomatal guard cells; 3 - stomatal fissure.
Figure 8.2. Periderm of elderberry stem (a - cross section of a shoot, b - lentils): I-performing fabric; 2 - remnants of the epidermis; 3 -cork (phellema); 4 - phellogen; 5 - phelloderm.
The cells of the cork are impregnated with a fat-like substance - suberin - and do not allow water and air to pass through, so the contents of the cell die and it fills with air. The multilayer cork forms a kind of stem cover that reliably protects the plant from adverse influences. environment. For gas exchange and transpiration of living tissues lying under the plug, the latter contains special education -lentils; These are gaps in the plug filled with loosely arranged cells.
Crust formed in trees and shrubs to replace cork. In the deeper tissues of the cortex, new areas of phellogen are laid down, forming new layers of cork. As a result, the outer tissues are isolated from the central part of the stem, deformed and die. On the surface of the stem, a complex of dead tissues gradually forms, consisting of several layers of cork and dead sections of bark. A thick crust provides more reliable protection for the plant than cork.
Conductive fabrics ensure the movement of water and nutrients dissolved in it throughout the plant. There are two types of conductive tissue - xylem (wood) and phloem (bast).
Xylem is the main water-conducting tissue of higher vascular plants, ensuring the movement of water with minerals dissolved in it from the roots to the leaves and other parts of the plant (ascending current). It also performs a supporting function. The xylem consists of tracheids and tracheae (vessels) (Fig. 8.3), wood parenchyma and mechanical tissue.
Tracheids They are narrow, highly elongated dead cells with pointed ends and lignified membranes. The penetration of solutions from one tracheid into another occurs by filtration through pores - recesses covered with a membrane. Liquid flows through the tracheids slowly, since the pore membrane prevents the movement of water. Tracheids are found in all higher plants, and in most horsetails, club mosses, ferns and gymnosperms they serve as the only conducting element of the xylem. U angiosperms Along with tracheids there are vessels.
Figure 8.3. Elements of xylem (a) and phloem (6): 1-5 - ringed, spiral, scalariform and porous (4, 5) trachea, respectively; 6 - ringed and porous tracheids; 7 - sieve tube with companion cell.
Trachea (vessels)- these are hollow tubes consisting of individual segments located one above the other. In the segments, through holes are formed on the transverse walls - perforations, or these walls are completely destroyed, due to which the speed of the flow of solutions through the vessels increases many times over. The shells of the vessels are impregnated with lignin and give the stem additional strength. Depending on the nature of the thickening of the membranes, tracheas are distinguished as ringed, spiral, scalariform, etc. (see Fig. 8.3).
Phloem conducts organic substances synthesized in the leaves to all plant organs (downward current). Like xylem, it is a complex tissue and consists of sieve tubes with companion cells (see Fig. 8.3), parenchyma and mechanical tissue. Sieve tubes are formed by living cells located one above the other. Their transverse walls are pierced small holes, forming like a sieve. The cells of the sieve tubes are devoid of nuclei, but contain cytoplasm in the central part, strands of which pass through through holes in the transverse partitions into neighboring cells. Sieve tubes, like vessels, stretch along the entire length of the plant. Companion cells are connected to the segments of the sieve tubes by numerous plasmodesmata and, apparently, perform some of the functions lost by the sieve tubes (enzyme synthesis, ATP formation).
Xylem and phloem are in close interaction with each other and form special complex groups- conductive bundles.
Mechanical fabrics ensure the strength of 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 method of thickening cell membranes There are two types of mechanical tissue: collenchyma and sclerenchyma (Fig. 8.4).
Rice. 8.4. Mechanical fabrics: a -angular collenchyma; 6- sclerenchyma; V -- 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 adapted 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 evenly thickened, often lignified membranes, the contents of which die off early stages. The membranes of sclerenchyma cells have high strength, close to the strength of steel. This tissue is widely represented in the vegetative organs of land plants and forms their axial support.
There are two types of sclerenchyma cells: fibers and sclereids. Fibers- these are long thin cells, usually collected in strands or bundles (for example, bast or wood fibers). Sclereids - these 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.
Main fabric, 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. Parenchymal tissues: 1-3 - chlorophyll-bearing (columnar, spongy and folded, respectively); 4-storage (cells with starch grains); 5 - pneumatic, or aerenchyma.
Cells assimilation tissues contain chloroplasts and perform the function of photosynthesis. The bulk of this tissue is concentrated in the leaves, a smaller part in young green stems.
In cells storing proteins, carbohydrates and other substances are deposited in the parenchyma. It is well developed in the stems of woody plants, in roots, tubers, bulbs, fruits and seeds. Plants of desert habitats (cacti) and salt marshes have aquifer parenchyma, 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 special type main fabric - pneumatic 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
Onion scales under a microscope
