Where is the conductive tissue located in plants? Types of plant tissues and their functions. Types of tissues in higher plants

Conductive tissue consists of living or dead elongated cells that look like tubes.

In the stem and leaves of plants there are bundles of conductive tissue. In the conductive tissue, vessels and sieve tubes are isolated.

Vessels- series-connected dead hollow cells, the transverse partitions between which disappear. Through the vessels, water and minerals dissolved in it from the roots enter the stem and leaves.

sieve tubes - elongated non-nuclear living cells, connected in series with each other. Through them, organic substances from the leaves (where they were formed) move to other organs of the plant.

The conductive fabric provides transportation of water with minerals dissolved in it.

This tissue forms two transport systems:

ascending(from roots to leaves);
descending(from leaves to all other parts of plants).

The ascending transport system consists of tracheids and vessels (xylem or wood), and the vessels are more perfect conducting means than tracheids.

In descending systems, the flow of water with photosynthesis products passes through sieve tubes (phloem or bast).

Xylem and phloem form vascular fibrous bundles - the "circulatory system" of the plant, which penetrates it completely, uniting it into one.

Scientists believe that the emergence of tissues is associated in the history of the Earth with the release of plants on land. When part of the plant was in the air, and the other part (root) - in the soil, it became necessary to deliver water and mineral salts from the roots to the leaves, and organic matter - from the leaves to the roots. So in the course of evolution flora two types of conductive fabrics arose - wood and bast.

Through wood (through tracheids and vessels), water with dissolved minerals rises from the roots to the leaves - this is a water-conducting, or ascending, current. Through the bast (through sieve tubes), the organic substances formed in green leaves enter the roots and other organs of the plant - this is a downward current.

educational fabric

Educational tissue is found in all growing parts of the plant. Educational tissue consists of cells that are able to divide throughout the life of the plant. The cells here lie very quickly towards each other. Thanks to division, they form many new cells, thereby ensuring the growth of the plant in length and thickness. The cells that appeared during the division of educational tissues are then transformed into cells of other plant tissues.

It is the primary tissue from which all other plant tissues are formed. It consists of special cells capable of multiple division. It is from these cells that the embryo of any plant consists.

This tissue is preserved in an adult plant. It is located:

at the bottom of the root system and at the tops of the stems (ensures the growth of the plant in height and the development of the root system) - apical educational tissue;
inside the stem (ensures the growth of the plant in width, its thickening) - lateral educational tissue.

Unlike other tissues, the cytoplasm of the educational tissue is thicker and denser. The cell has well-developed organelles that provide protein synthesis. The core is characterized large sizes. The mass of the nucleus and cytoplasm are maintained in a constant ratio. The enlargement of the nucleus signals the beginning of the process of cell division, which occurs through mitosis for the vegetative parts of plants and meiosis for sporogenous meristems.

In the process of evolution is one of the reasons that made possible way out plants on dry land. In our article, we will consider the features of the structure and functioning of its elements - sieve tubes and vessels.

Conductive Fabric Features

When serious changes in climatic conditions occurred on the planet, plants had to adapt to them. Before that, they all lived exclusively in the water. In the ground-air environment, it became necessary to extract water from the soil and transport it to all plant organs.

There are two types of conductive tissue, the elements of which are vessels and sieve tubes:

Bast, or phloem - is located closer to the surface of the stem. Along it, organic substances formed in the leaf during photosynthesis move towards the root.
The second type of conductive tissue is called wood, or xylem. It provides an upward current: from the root to the leaves.

plant sieve tubes

These are the conductive cells of the bast. Between themselves they are separated by numerous partitions. Outwardly, their structure resembles a sieve. That's where the name comes from. The sieve tubes of plants are alive. This is due to the weak pressure of the downward current.

Their transverse walls are permeated with a dense network of holes. And the cells contain many through holes. All of them are prokaryotes. This means that they do not have a formalized core.

The living elements of the cytoplasm of sieve tubes remain only for a certain time. The duration of this period varies according to wide range- from 2 to 15 years. This indicator depends on the type of plant and the conditions of its growth. Sieve tubes transport water and organic substances synthesized during photosynthesis from leaves to roots.

Vessels

Unlike sieve tubes, these elements of conductive tissue are dead cells. Visually, they resemble tubes. Vessels have dense shells. With inside they form thickenings that look like rings or spirals.

Thanks to this structure, the vessels are able to perform their function. It consists in the movement of soil solutions of minerals from the root to the leaves.

The mechanism of soil nutrition

Thus, the movement of substances in opposite directions is simultaneously carried out in the plant. In botany, this process is called the ascending and descending current.

But what forces cause water from the soil to move upwards? It turns out that this happens 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 in the soil are minerals, without which the development of tissues and organs will be impossible. So, 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.

Rising, the water molecules tightly adhere 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 causes soil solutions to penetrate through the root hairs into the bark, and further to the xylem vessels. On them, water rises to the stem. Further, under the action of transpiration, water enters the leaves.

In the veins next to the vessels are sieve tubes. These elements carry downward current. Under influence sunlight Glucose polysaccharide is synthesized in leaf chloroplasts. The plant uses this organic matter for growth and life processes.

So, the conductive tissue of the plant ensures the movement of aqueous solutions of organic and mineral substances throughout the plant. Her building blocks are vessels and sieve tubes.

Conductive tissue is one of the plant tissues that is essential for the movement of nutrients throughout the body. It is an important structural component of generative and vegetative organs of reproduction.

The conducting system is a collection of cells with intercellular pores, as well as parenchymal and transmission 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 - in the soil. This is how the problem of the transfer of plastic and mineral compounds appeared. Thanks to the appearance of conductive tissues, the circulation of fluid, minerals, ATP throughout the body became possible.

Features of the structure of the conductive tissue of plants

The structure of the conducting tissue of plants is quite complex, since they contain different structural and functional elements. It includes xylem (wood) and phloem (bast), through which water moves in two directions.

Xylem (wood)

To xylem include the following fabrics:

Actually conductive (tracheids and tracheas);
mechanical (wood fibers);
parenchymal.

Vessels (tracheas) and tracheids can be dead elements of the conducting tissue of plants, 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 shells of the cells become lignified and their uneven thickening occurs, and the transverse walls are destroyed, forming through openings. Tracheae are on average 10 cm long, but in some plants - up to 2 (oak) or 3-5 m (tropical vines).

tracheids- unicellular spindle-shaped elements with points at the 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. Thickenings have the form of rings, spirals, mesh. Tracheids differ from tracheas in the absence of holes, so the movement of fluid here goes 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 the phloem are sieve tubes and cells, which are combined into single system through special fields and intercellular contacts.

sieve tubes- oblong, living cells, their sizes range from 0.1 mm to 2 mm. Like the vessels, they are the longest in vines. Their longitudinal walls are also thickened, but remain cellulose and do not lignify. The transverse membranes perforate like a sieve and are called sieve plates.

Organic synthesis products (ATP energy) move from the leaves to the lower parts, along dissociated protoplasts (a mixture of vacuolar juice with cytoplasm).

The cytoplasm of cells is preserved, and the nucleus is destroyed at the very beginning of tube formation. Even in the absence of a nucleus, 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 cells in the direction of the sieve tube. Companion cells are a kind of pantry of enzymes that are secreted through the pores into the segment of the sieve tube and stimulate the movement of organic substances through them.

Companion cells and sieve tubes are closely related and cannot function separately.

Sieve cells do not have special satellite cells and do not lose nuclei; sieve fields are randomly scattered on the side walls.

The conducting tissues of plants, their structure and functions are 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 tree, which is closer to the axial part, at herbal plants- more in the root system, stem.	The upward movement of water and minerals from the soil into the roots, leaves, inflorescences.
Phloem has companion cells and sieve tubes, which are built from living cells.	Bast (phloem) is located under the bark, is formed due to the division of cambial cells.	Descending movement of organic compounds from green, photosynthetic parts into the stem, root.

Where is the conductive tissue located in plants?

If you make a cross section of a tree, you can see several layers. Substances move along two of them: along the wood and in the bast.

The bast (responsible for the downward movement) is located under the bark, and when the initial cells divide, the elements that are outside go to the bast.

Wood is formed from cambial cells that have moved to the central part of the tree and provide an upward current.

The role of conductive tissue in plant life

The 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 the cells of an educational tissue, where there is an intensive division. Transfer cells with multiple protrusions in the membrane are also required for this type of transport.
Conductive tissues make plants more flexible and resistant to external influences.
Vascular tissue is a single system that unites all plant organs.

Almost all multicellular living organisms are composed of various types fabrics. This is a collection of cells that are similar in structure, united common functions. For plants and animals they are not the same.

Diversity of tissues of living organisms

First of all, all tissues can be divided into animal and vegetable. They are different. Let's take a look at them.

What are animal tissues?

Animal tissues are of the following types:

nervous;
muscular;
epithelial;
connecting.

All of them, except for the first, are divided into smooth, striated and cardiac. Epithelial is divided into single-layer, multilayer - depending on the number of layers, as well as cubic, cylindrical and flat - depending on the shape of the cells. Connective tissue combines such types as loose fibrous, dense fibrous, reticular, blood and lymph, fatty, bone and cartilaginous.

Variety of plant tissues

Plant tissues are of the following types:

main;
integumentary;
mechanical;
educational.

All types of plant tissues combine several types. So, the main ones include assimilation, storage, aquifer and air. combine species such as bark, cork and epidermis. Conductive tissues include phloem and xylem. Mechanical is divided into collenchyma and sclerenchyma. Educational include lateral, apical and intercalary.

All fabrics perform certain functions, and their structure corresponds to the role they perform. This article will discuss in more detail the conductive tissue, the structural features of its cells. We will also talk about its functions.

Conductive tissue: structural features

These tissues are divided into two types: phloem and xylem. Since they are both formed from the same meristem, they are located next to each other in the plant. However, the structure of the conductive tissues of the two types is different. Let's talk more about the two types of conductive tissues.

Functions of conductive tissues

Their main role is the transport of substances. However, the functions of conductive tissues belonging to more than one species differ.

The role of xylem - holding solutions chemical substances from the root up to all other organs of the plant.

And the function of the phloem is to conduct solutions in the opposite direction - from certain organs of the plant along the stem down to the root.

What is xylem?

It is also called wood. The conductive tissue of this type consists of two different conductive elements: tracheids and vessels. It also includes mechanical elements - wood fibers, and the main elements - wood parenchyma.

How are xylem cells arranged?

Conductive tissue cells are divided into two types: tracheids and vascular segments. A tracheid is a very long cell with intact walls, in which there are pores for the transport of substances.

The second conducting element of the cell - the vessel - consists of several cells, which are called vascular segments. These cells are located one above the other. Through holes are located at the junctions of the segments of the same vessel. They are called perforations. These holes are necessary for the transport of substances through the vessels. The movement of various solutions through the vessels occurs much faster than through the tracheids.

The cells of both conductive elements are dead and do not contain protoplasts (protoplasts are the contents of the cell, with the exception of that is, the nucleus, organelles and cell membrane). There are no protoplasts, since if they were in the cell, the transport of substances through it would be very difficult.

Through vessels and tracheids, solutions can be transported not only vertically, but also horizontally - to living cells or neighboring conductive elements.

The walls of the conductive elements have thickenings that give the cell strength. Depending on the type of these thickenings, the conductive elements are divided into spiral, ringed, ladder, mesh and point-pore.

Functions of mechanical and basic elements of xylem

Wood fibers are also called librioform. These are elongated cells that have thickened lignified walls. They perform a supporting function that ensures the strength of the xylem.

The elements in the xylem are wood parenchyma. These are cells with lignified shells, in which simple pores are located. However, at the junction of the parenchyma cell with the vessel, there is a fringed pore that connects to its simple pore. Wood parenchyma cells, unlike vascular cells, are not empty. They have protoplasts. The xylem parenchyma performs a reserve function - nutrients are stored in it.

How does the xylem of different plants differ?

Since tracheids in the process of evolution arose much earlier than vessels, these conductive elements are also present in lower land plants. These are spores (ferns, mosses, club mosses, horsetails). Most gymnosperms also possess only tracheids. However, some gymnosperms also have vessels (they are present in the gneatids). Also, as an exception, these elements are also present in some ferns and horsetails.

But angiosperms (flowering) plants all have both tracheids and vessels.

What is phloem?

The conductive tissue of this type is also called bast.

The main part of the phloem - sieve conductive elements. Also in the structure of the bast there are mechanical elements (phloem fibers) and elements of the main tissue (phloem parenchyma).

Features of the conductive tissue of this type are that the cells of the sieve elements, in contrast to the conductive elements of the xylem, remain alive.

The structure of sieve elements

There are two types of them: sieve cells and the First are elongated and have pointed ends. They are permeated with through holes through which the transport of substances occurs. Sieve cells are more primitive than multicellular sieve elements. They are characteristic of such plants as spores and gymnosperms.

In angiosperms, the conducting elements are represented by sieve tubes, consisting of many cells - segments of sieve elements. through holes two neighboring cells form sieve plates.

Unlike sieve cells, there are no nuclei in the mentioned structural units of multicellular conducting elements, but they still remain alive. An important role in the structure of the phloem of angiosperms is also played by satellite cells located next to each cell-segment of sieve elements. Companions contain both organelles and nuclei. They are metabolized.

Given that the cells of the phloem are alive, this conductive tissue cannot function for a long time. At perennials the period of her life is three to four years, after which the cells of this conductive tissue die off.

Additional phloem elements

In addition to sieve cells or tubes, elements of the main tissue and mechanical elements are also present in this conductive tissue. The latter are represented by bast (phloem) fibers. They perform a supporting function. Not all plants have phloem fibers.

The elements of the main tissue are represented by the phloem parenchyma. It, like the xylem parenchyma, performs a reserve role. It stores such substances as tannins, resins, etc. These phloem elements are especially developed in gymnosperms.

Phloem of various plant species

In lower plants, such as ferns and mosses, it is represented by sieve cells. The same phloem is characteristic of most gymnosperms.

Angiosperms have multicellular conducting elements: sieve tubes.

The structure of the conducting system of a plant

Xylem and phloem are always located side by side and form bundles. Depending on how the two types of conductive tissue are located relative to each other, several types of bundles are distinguished. The most common are collaterals. They are arranged in such a way that the phloem lies on one side of the xylem.

There are also concentric bundles. In them, one conductive tissue surrounds another. They are divided into two types: centrophloem and centroxylem.

The conductive tissue of the root usually has radial bundles. In them, the xylem rays depart from the center, and the phloem is located between the xylem rays.

Collateral bundles are more characteristic of angiosperms, and concentric bundles are more characteristic of spore and gymnosperms.

Conclusion: A Comparison of Two Types of Conductive Fabrics

As a conclusion, we present a table that summarizes the main data on two types of conductive plant tissues.

Conductive tissues of plants

	Xylem	Phloem
Structure	It consists of conductive elements (trachea and vessels), wood fibers and wood parenchyma.	Consists of conducting elements (sieve cells or sieve tubes), phloem fibers and phloem parenchyma.
Features of conducting cells	Dead cells lacking plasma membranes, organelles and nuclei. They have an elongated shape. They are located one above the other and do not have horizontal partitions.	Living in the walls of which there are a large number of through holes.
Additional elements	Wood parenchyma and wood fibers.	Phloem parenchyma and phloem fibers.
Functions	Carrying substances dissolved in water up: from the root to the organs of plants.	Transport of chemical solutions down: from the ground organs of plants to the root.

Now you know everything about the conductive tissues of plants: what they are, what functions they perform and how their cells are arranged.

Conductive tissues are complex, since they consist of several types of cells, their structures have an elongated (tubular) shape, and are penetrated by numerous pores. The presence of holes on the end (lower or upper) sections provide vertical transport, and the pores on the side surfaces contribute to the flow of water in the radial direction. Conductive tissues include xylem and phloem. They are found only in ferns and seed plants. Conductive tissue contains both dead and living cells.
Xylem (wood) is dead tissue. Includes basic structural components(tracheas and tracheids), wood parenchyma and wood fibers. It performs both a supporting and conductive function in the plant - water and mineral salts move up the plant along it.
tracheids - dead single cells of a spindle-shaped form. The walls are strongly thickened due to the deposition of lignin. A feature of tracheids is the presence of bordered pores in their walls. Their ends overlap, giving the plant the necessary strength. Water moves through the empty gaps of the tracheids, without encountering obstacles in the form of cellular contents on its way; from one tracheid to another, it is transmitted through the pores.
In angiosperms, tracheids have developed into blood vessels (trachea). These are very long tubes formed as a result of the “docking” of a number of cells; the remains of the end partitions are still preserved in the vessels in the form of perforated rims. Vessels vary in size from a few centimeters to several meters. In the first vessels of protoxylem formation, lignin accumulates in rings or in a spiral. This allows the vessel to continue to stretch during growth. In the vessels of the metaxylem, lignin is concentrated more densely - it is an ideal "water conduit" that acts over long distances.
?1. How are tracheas different from tracheids? (Answer at the end of the article)
?2 . How are tracheids different from fibers?
?3 . What do phloem and xylem have in common?
?4. How are sieve tubes different from tracheae?
Parenchymal xylem cells form peculiar rays connecting the core with the cortex. They conduct water in the radial direction, store nutrients. New xylem vessels develop from other parenchymal cells. Finally, wood fibers are similar to tracheids, but unlike it, they have a very small internal lumen, therefore, they do not conduct water, but give additional strength. And they also have simple pores, not bordered ones.
Phloem (bast)- this is a living tissue that is part of the bark of plants, a downward flow of water with assimilation products dissolved in it is carried out through it. The phloem is formed by five types of structures: sieve tubes, companion cells, bast parenchyma, bast fibers, and sclereids.
These structures are based on sieve tubes , formed as a result of the connection of a number of sieve cells. Their walls are thin, cellulose, the nuclei die off after maturation, and the cytoplasm is pressed against the walls, making way for organic substances. The end walls of the cells of the sieve tubes gradually become covered with pores and begin to resemble a sieve - these are sieve plates. To ensure their vital activity, satellite cells are located nearby, their cytoplasm is active, the nuclei are large.
?5 . Why do you think that when sieve cells mature, their nuclei die off?
ANSWERS
?1. Tracheas have multicellular structures and do not have end walls, while tracheids are unicellular, have end walls and bordered pores.
?2 . Tracheids have bordered pores and a well-defined lumen, while in fibers the lumen is very small and the pores are simple. They also differ in functions, tracheids perform a transport role (conductive), and mechanical fibers.
?3. Phloem and xylem are both conductive tissues, their structures are tubular in shape, they include cells of the parenchyma and mechanical tissues.
?4. Sieve tubes consist of living cells, their walls are cellulose, they carry out the downward transport of organic substances, and the trachea are formed by dead cells, their walls are strongly thickened with lignin, they provide an upward transport of water and minerals.
?5. Downward transport occurs along sieve cells, and the nuclei, carried away by the current of substances, would cover a significant part of the sieve field, which would lead to a decrease in the efficiency of the process.