Conductive fabrics. Functions and structural features. Mechanical and conductive plant tissues Mineral conductive tissue

Different organs higher plants perform different functions. This is how the roots absorb water and minerals, and photosynthesis occurs in the leaves, as a result of which organic matter is formed. However, all plant cells need both water and organic matter. Consequently, a transport system is needed that ensures the delivery of the necessary substances to some organs from others. In plants (mainly angiosperms), this function is performed by conductive tissue.

Have woody plants conductive tissue is part of wood and bast... For wood is carried out upward current: water and minerals rise from the roots. By bast carried out downward current : there is an outflow of organic matter from the leaves. With all this, the concepts of "upward current" and "downward current" should not be understood quite literally, as if in conducting tissues water always goes up, and organic substances - down. Substances can move horizontally and sometimes in the opposite direction. For example, organic matter goes up to growing shoots that are above storage tissues or photosynthesizing leaves.

So, in plants, the movement of an aqueous solution and organic matter is separated. The composition of wood includes vessels, and in the bast - sieve tubes.

Vessels are a string of dead, long cells. An aqueous solution moves along them from the roots. Water rises due to root pressure and transpiration (evaporation of water from leaves). Gymnosperms and ferns instead of vessels have tracheids, along which the water moves more slowly. Hence it follows that the vessels have a more perfect structure. In another way, the vessels are called trachea.

The reason why the water in the vessels moves faster than in the tracheids lies in their slightly different structure. Tracheid cells have many pores at the points of contact with each other (above and below). The aqueous solution is filtered through these pores. Vessels, on the other hand, are essentially a hollow tube, their cells have large holes (perforations) at the junctions with each other.

The vessels have various thickenings in their longitudinal walls. This gives them strength. Through those places where there are no thickenings, water is transported in a horizontal direction. It enters the cells of the parenchyma and adjacent vessels (vessels are usually arranged in bundles).

Sieve tubes are formed by living elongated cells. Organic substances move along them. Above and below, vascular cells are connected to each other through numerous pores. This compound looks like a sieve, hence the name. It turns out a single long chain of cells. Although sieve tubes are living cells, they do not have a nucleus and some other structures and organelles necessary for life. Therefore, sieve tubes have so-called companion cells that support their livelihoods. The companions and tubes are interconnected through special pores.

Wood and bast are not only made of conductive fabrics. They also include parenchyma and mechanical tissues. Conductive fabrics together with mechanical fabrics form vascular fibrous bundles... The parenchyma often plays the role of storage tissue (especially in wood).

Wood is called differently xylem, and bast - phloem.

Plant tissues: conductive, mechanical and excretory

Conductive tissues are located inside the shoots and roots. Contains xylem and phloem. They provide the plant with two currents of substances: ascending and descending. Ascending the current is provided by xylem - mineral salts dissolved in water move to the aerial parts. Descending the current is provided by phloem - organic substances synthesized in leaves and green stems move to other organs (to the roots).

Xylem and phloem are complex tissues that are made up of three basic elements:

The parenchyma cells, which serve for the transport of substances between plant tissues, also perform a conducting function (for example, the core rays of woody stems provide the movement of substances in a horizontal direction from the primary cortex to the core).

Xylem

Xylem (from the Greek. xylon- a felled tree). Consists of the proper conductive elements and accompanying cells of the main and mechanical tissues. Ripe vessels and tracheids are dead cells that provide an upward current (movement of water and minerals). Xylem elements can also perform a supporting function. Through the xylem, in spring, the shoots receive solutions of not only mineral salts, but also dissolved sugars, which are formed as a result of starch hydrolysis in the storage tissues of roots and stems (for example, birch sap).

Tracheids Are the most ancient conductive elements of xylem. Tracheids are represented by elongated spindle-shaped cells with pointed ends located one above the other. They have lignified cell walls with varying degrees of thickening (annular, spiral, porous, etc.), which prevent them from disintegrating, stretching. The cell walls contain complex pores that are covered by a pore membrane through which water passes. Filtration of solutions takes place through the pore membrane. The movement of fluid along the tracheids is slow, since the pore membrane impedes the movement of water. In higher spore and gymnosperms, tracheids account for about 95% of the wood volume.

Vessels or trachea , consist of elongated cells located one above the other. They form tubes during the fusion and death of individual cells - vascular segments. The cytoplasm dies off. There are transverse walls between the cells of the vessels, which have large holes... In the walls of blood vessels there are thickenings of various shapes (annular, spiral, etc.). The ascending current occurs through relatively young vessels, which over time are filled with air, clogged with outgrowths of neighboring living cells (parenchyma) and then perform a supporting function. The fluid moves through the vessels faster than through the tracheids.

Phloem

Phloem (from the Greek. phloyos- bark) consists of conductive elements and accompanying cells.

Sieve tubes - these are living cells that are sequentially connected by their ends, do not have organelles, nuclei. Provide movement from the leaves along the stem to the root (carry out organic matter, photosynthetic products). They have a branched network of fibrils, the inner contents are heavily watered. Separated from each other by film partitions with a large number of small holes (perforations) - sieve (perforation) plates (resemble a sieve). The longitudinal membranes of these cells are thickened, but not woody. In the cytoplasm of the sieve tubes is destroyed tonoplast (shell of vacuoles), and the vacuolar juice with dissolved sugars is mixed with the cytoplasm. With the help of strands of cytoplasm, adjacent sieve tubes are combined into a single whole. Travel speed on sieve tubes less than the vessels. Sieve tubes have been functioning for 3-4 years.

Each segment of the sieve tube is accompanied by cells of the parenchyma - satellite cells that secrete substances (enzymes, ATP, etc.) necessary for their functioning. Satellite cells have large nuclei, filled with cytoplasm with organelles. They are not inherent in all plants. They are absent in the phloem of higher spore and gymnosperms. Satellite cells help to carry out the process of active transport through the sieve tubes.

Phloem and xylem form vascular fibrous (conducting) bundles ... They can be seen in leaves, stems herbaceous plants... In tree trunks, conducting beams merge with each other and form rings. Phloem is part of the bast and is located closer to the surface. Xylem is part of the wood and is found closer to the core.

Vascular fibrous bundles are closed and open - this is a taxonomic feature. Closed the bundles do not have a cambium layer between the xylem and phloem layers; therefore, the formation of new elements in them does not occur. Closed bunches are found mainly in monocotyledonous plants. Open the vascular fibrous bundles between the phloem and xylem have a cambium layer. Due to the activity of the cambium, the bundle grows and the organ thickens. Open bundles are found mainly in dicotyledonous and gymnosperms.

They perform supporting functions. They form the skeleton of the plant, provide its strength, give elasticity, support the organs in a certain position. Young areas of growing organs do not have mechanical tissues. The most developed are mechanical tissues in the stem. At the root, mechanical tissue is concentrated in the center of the organ. Distinguish between colenchyma and sclerenchyma.

Colenchyma

Colenchyma (from the Greek. cola- glue and enchyma- poured) - consists of living chlorophyll-bearing cells with unevenly thickened walls. Distinguish between angular and lamellar klenchyma. Corner collenchyma consists of cells that are hexagonal in shape. Thickening occurs along the ribs (at the corners). Found in the stems of dicotyledonous plants (mainly herbaceous) and leaf cuttings. Does not interfere with the growth of organs in length. Lamellar collenchyma has cells with the shape of a parallelepiped, in which only a pair of walls parallel to the surface of the stem are thickened. Found in the stems of woody plants.

Sclerenchyma

Sclerenchyma (from the Greek. sclerosis- hard) is a mechanical tissue that consists of lignified (lignin-impregnated) predominantly dead cells that have uniformly thickened cell walls. The nucleus and cytoplasm are destroyed. There are two types: sclerenchymal fibers and sclereids.

Sclerenchymal fibers

The cells are elongated with pointed ends and pore channels in the cell walls. The cell walls are thickened and very strong. The cells are tightly adjacent to one another. On the cross section - multifaceted.

In wood, sclerenchymal fibers are called woody ... They are a mechanical part of the xylem, protect the vessels from the pressure of other tissues, fragility.

The sclerenchymal fibers of the bast are called bast fibers. Usually they are not lignified, strong and elastic (used in the textile industry - flax fibers, etc.).

Sclereids

Formed from the cells of the main tissue due to the thickening of the cell walls, their impregnation with lignin. Have different shape and are found in various plant organs. Sclereids with the same cell diameter are called stony cells ... They are the most durable. Found in pits of apricots, cherries, shells walnuts etc.

Sclereids can also have a stellate shape, expansion at both ends of the cell, rod-shaped.

Excretory tissue plants

As a result of the metabolic process in plants, substances are formed that different reasons almost never used (with the exception of milky juice). Usually, these foods accumulate in certain cells. The excretory tissues are represented by groups of cells or single ones. They are divided into external and internal.

External excretory tissues

External The excretory tissues are represented by modifications of the epidermis and special glandular cells in the main tissue inside plants with intercellular cavities and a system of excretory passages, by which secretions are excreted. The excretory passages in different directions penetrate the stems and partially the leaves and have a shell of several layers of dead and living cells. Modifications of the epidermis are represented by multicellular (less often unicellular) glandular hairs or plates of various structures. External excretory tissues produce essential oils, balms, resins, etc.

About 3 thousand species of gymnosperms and angiosperms are known to produce essential oils. About 200 types (lavender, rose oils, etc.) of them are used as medicinal products, in perfumery, cooking, making varnishes, etc. Essential oils Are light organic substances of different chemical composition... Their importance in plant life: they attract pollinators by smell, scare off enemies, some (phytoncides) - kill or suppress the growth and reproduction of microorganisms.

Resins are formed in the cells that surround the resin passages, as waste products of gymnosperms (pine, cypress, etc.) and angiosperms (some legumes, umbrella plants, etc.). These are various organic substances (resin acids, alcohols, etc.). Outwardly excreted with essential oils in the form of thick liquids, which are called balms ... They have antibacterial properties. Used by plants in nature and by humans in medicine for wound healing. Canadian balsam, which is obtained from balsam fir, is used in microscopic technique for the manufacture of microslides. The basis of coniferous balsams is turpentine (used as a solvent for paints, varnishes, etc.) and solid resin - rosin (used for soldering, making varnishes, sealing wax, rubbing bowed strings musical instruments). Fossilized resin conifers the second half of the Cretaceous-Paleogene period is called amber (used as a raw material for jewelry).

The glands located in the flower or on different parts of the shoots, the cells of which secrete nectar, are called nectaries ... They are formed by the main tissue and have ducts that open outward. The outgrowths of the epidermis that surround the duct give the nectary a different shape (hump-shaped, pit-shaped, horn-shaped, etc.). Nectar Is an aqueous solution of glucose and fructose (concentration ranges from 3 to 72%) with impurities of aromatic substances. The main function is to attract insects and birds to pollinate flowers.

Thanks to hydathodes - water stomata - occurs gutting - the release of droplet water by plants (during transpiration, water is released in the form of steam) and salts. Gutting is a defense mechanism that occurs when transpiration fails to remove excess water. Typical for plants that grow in humid climates.

Special glands of insectivorous plants (more than 500 species of angiosperms are known) secrete enzymes that decompose insect proteins. Thus, insectivorous plants make up for the lack of nitrogenous compounds, since there are not enough of them in the soil. Digested substances are absorbed through the stomata. The most famous are pemphigus and sundew.

The glandular hairs accumulate and excrete, for example, essential oils (mint, etc.), enzymes and formic acid, which cause pain and lead to burns (nettle), etc.

Internal excretory tissues

Internal excretory tissues are receptacles of substances or individual cells that do not open outward during the life of a plant. This, for example, milkmen - a system of elongated cells of some plants, along which the sap moves. The sap of such plants is an emulsion of an aqueous solution of sugars, proteins and minerals with drops of lipids and other hydrophobic compounds, called latex and has a milky white (spurge, poppy, etc.) or orange (celandine) color. The milky juice of some plants (for example, Brazilian Hevea) contains a significant amount rubber .

To the inner excretory tissue belong idioblasts - separate scattered cells among other tissues. Crystals of calcium oxalate, tannins, etc. accumulate in them. Cells (idioblasts) of citrus fruits (lemon, tangerine, orange, etc.) accumulate essential oils.

In the process of evolution, with the emergence of higher plants on land, they developed tissues that reached their greatest specialization in flowering plants. In this article, we will consider in more detail what plant tissues are, what types of them exist, what functions they perform, as well as the structural features of plant tissues.

Cloth call groups of cells that are similar in structure and perform the same functions.

The main plant tissues are shown in the figure below:

Types, functions and structure of plant tissues.

The integumentary tissue of plants.

Covering tissue of plants - crust

Conductive tissue of plants.

Mechanical tissue of plants.

CONDUCTIVE FABRICS.

Conductive tissues are used to move around the plant dissolved in water nutrients.

Rice. 43 Wood fibers of a leaf of meadow geranium (transverse - A, B and longitudinal - C section of a group of fibers):
1 - cell wall, 2 - simple pores, 3 - cell cavity

Like integumentary tissues, they arose as a result of the plant's adaptation to life in two environments: soil and air. In this regard, it became necessary to transport nutrients in two directions.

An ascending, or transpirational, current of aqueous solutions of salts moves from the root to the leaves. The assimilative, downward flow of organic matter is directed from the leaves to the roots. The ascending current is carried almost exclusively through the tracheal

Rice. 44 Sclereids of the seeds of ripening cherry plum fruits with live contents: 1 - cytoplasm, 2- thickened cell membrane, 3-pore tubules
elements of xylem, and. descending - by sieve elements phloem.

A highly branched network of conductive tissues carries water-soluble substances and products of photosynthesis to all plant organs, from the thinnest root ends to the youngest shoots. Conductive tissues unite all organs of the plant. In addition to the long-distance, i.e., axial, transport of nutrients, short-range, radial transport is carried out along the conductive tissues.

All conductive tissues are complex, or complex, that is, they consist of morphologically and functionally dissimilar elements. Forming from the same meristem, two types of conductive tissues - xylem and phloem - are located side by side. In many plant organs, xylem is combined with "phloem" in the form of strands called vascular bundles.

There are primary and secondary conductive tissues. Primary tissues are laid in leaves, young shoots and roots. They differentiate from procambium cells. Secondary conductive tissues, usually more powerful, arise from the cambium.

Xylem (wood). Water and minerals dissolved in it move along the xylem from the root to the leaves. Primary and secondary xylems contain cells of the same types. However, the primary xylem does not have core rays, which differs from the secondary one.

The xylem includes morphologically various elements, carrying out the functions of both carrying and storing spare substances, as well as purely supporting functions. Long-distance transport is carried out along the tracheal elements of the xylem: tracheids and blood vessels, the short-distance transport is carried out along the parenchymal elements. Supporting and sometimes storage functions are performed by part of the tracheids and fibers mechanical tissue libriform, also part of the xylem.

Tracheids in a mature state are dead prosenchymal cells, narrowed at the ends and devoid of a protoplast. The length of the 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 are lignified, thickened and have simple or bordered pores through which solutions are filtered. Most of the bordered pores are located near the ends of the cells, i.e., where solutions seep from one tracheid to another. Tracheids are found in sporophytes of all higher plants, and in most horsetails, lymphoids, ferns and gymnosperms, they are the only conductive elements of the xylem.

Vessels are hollow tubes consisting of separate 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. Due to the perforations, fluid flows freely along the entire vessel. Evolutionarily, the vessels appear to have evolved from the tracheids by the destruction of the closing pore films and their subsequent fusion into one or more perforations. The ends of the tracheids, initially strongly oblique, took a horizontal position, and the tracheids themselves became shorter and turned into vascular segments (Fig. 45).

Vessels appeared independently in different evolutionary lines land plants... However, they reach the greatest development in angiosperms, where they are the main water-conducting elements of the xylem. The emergence 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 membrane, vessels and tracheids in most cases have secondary thickenings. In the youngest tracheal elements, the secondary membrane may be in the form of rings that are not connected to each other (annular tracheids and vessels). Later, tracheal elements with spiral thickenings appear. 3 $ that are followed by vessels and tracheids with thickenings, which can be characterized as spirals, the coils of which are interconnected (ladder thickenings). Ultimately, the secondary envelope merges into a more or less solid cylinder that forms inward from the primary envelope. This cylinder is interrupted at selected sites pores. Vessels and tracheids with relatively small rounded sections of the primary cell membrane, not covered from the inside by the secondary membrane, are often called porous. ).

Rice. 45 Changes in the structure of tracheal elements of the xylem in the course of their evolution (direction indicated by an arrow):
1,2 - tracheids with rounded bordered pores, 3 - tracheids with elongated bordered pores, 4 - a primitive type of vessel segment and its perforation formed by contiguous pores, 5 - 7 - successive stages of specialization of vascular segments and the formation of a simple perforation

The secondary and sometimes the primary shell, as a rule, are lignified, that is, they are 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 mass of xylem (scattered vascular wood). The features of the distribution of tracheal elements in the xylem are used to determine the woods of various tree species.

In addition to tracheal elements, xylem includes ray elements, that is, cells that form pith rays (Fig. 46), formed most often by thin-walled parenchymal cells (ray parenchyma). Less commonly, ray tracheids are found in the rays of conifers. The core beams are used for short-range transport of substances in the horizontal direction. In the xylem of angiosperms, in addition to conducting elements, there are also thin-walled non-lignified living parenchymal cells called woody parenchyma. Along with the core rays, short-range transport is partly carried out along them. In addition, the woody parenchyma serves as a storage site for reserve substances. The elements
the medullary rays and woody parenchyma, like the tracheal elements, arise from the cambium.

Mechanical and conductive tissues are of great importance in the life of terrestrial plants.

Mechanical fabrics

Everyone watched as the thin straw, supporting the heavy ear, swayed in the wind, but did not break.

Strength is given to the plant by mechanical tissues They serve as a support for the organs in which they are located. The cells of mechanical tissues have thickened membranes.

In the leaves and other organs of young plants, the cells of the mechanical tissue are living. Such tissue is located in separate strands under the stem and leaf petioles, borders the veins of the leaves. The cells of living mechanical tissue are easily stretchable and do not interfere with the growth of the part of the plant in which they are located. Thanks to this, the organs of plants act like springs. They are able to return to their original state after removing the load. Everyone saw the grass rise again after a man walked over it.

Mechanical tissue also supports the parts of a plant whose growth is complete, but the mature cells of this tissue are dead. These include bast and woody - long thin cells, collected in strands or bundles. The fibers give the stem strength. Short dead cells of mechanical tissue (they are called stony) form a seed coat, a shell of nuts, seeds of fruits, give the pulp of pears a granular character.

Conductive fabrics

All parts of the plant contain conductive tissues. They provide the transfer of water and substances dissolved in it.

Conductive tissues were formed in plants as a result of adaptation to life on land. The body of terrestrial plants is in two environments of life - ground-air and soil. In this regard, two conductive fabrics arose - wood and bast. Water and mineral salts dissolved in it rise along the wood in the direction from bottom to top (from roots to). Therefore, wood is called a water-conducting fabric. Lub is inner part bark. Organic substances move along the bast in the direction from top to bottom (from leaves to roots). Wood and bast form a continuous branched system in the body of the plant, connecting all its parts.

The main conductive elements of wood are vessels. They are long tubes formed by the walls of dead cells. At first, the cells were alive and had thin, expandable walls. Then the cell walls became lignified, the living contents died. The transverse partitions between the cells collapsed and long tubes formed. They consist of individual elements and look like bon ding kegs and lids. Water with substances dissolved in it freely passes through the vessels of the wood.

The conductive elements of the bast are living elongated cells. They are connected by their ends and form long rows of cells - tubes. There are small holes (pores) in the transverse walls of the bast cells. Such walls are similar to a sieve, therefore the tubes are called sieve. Solutions of organic substances move along them from the leaves to all organs of the plant.