Bast (phloem) is a complex conductive tissue through which photosynthetic products (organic substances) are transported from leaves to all plant organs (rhizomes, fruits, seeds, etc.). Phloem is formed by the division of procambium (primary) and cambium (secondary) cells. The bast is located in the stem outside the cambium under the bark, and in the leaves - closer to the underside of the blade. Under the cambium in the trunk is wood.
Drawing. Tree trunk and its layers
Structure
Phloem tissue and its cellular composition are divided into three types depending on the functions performed: sieve tubes with cells; mechanical tissues (sclereids and fibers); bast parenchyma with parenchyma cells. Basically, bast consists of sieve tubes that allow dissolved nutrients to move down the stem. The tubes are formed by sieve cells that fit tightly and connect to each other.
Cells
The cells are living, thin-walled and elongated. They lack a nucleus, and the central part contains cytoplasm. The transverse walls of the cells have small through holes through which strands of cytoplasm pass into neighboring cells.
Sieve tubes extend along the entire length of the plant. In deciduous plants, satellite cells, which also take part in the transport of substances, adjoin and connect with the segments of the sieve tubes. Sieve tubes do not function for long, only one growing season; they gradually become clogged with callose and then die. Only some perennial plants have a lifespan of more than 2 years.
Functions
Mechanical fabrics - thick-walled bast fibers serve for strength and also perform a supporting function. Bast parenchyma contains thin-walled parenchyma cells, which serve for the deposition of reserve nutrients, as well as their short-range transportation.
If in the xylem the movement of dissolved minerals occurs only upward to the leaves from the roots, then in the phloem the movement of organic substances (sucrose, carbohydrates, amino acids, phytohormones) from the leaves occurs to those plant organs that consume or store them. The highest intensity of substance consumption is observed in the tips of shoots, developing leaves, and roots. Many plants have storage organs: tubers, bulbs, etc. The transport speed is quite high and amounts to tens of centimeters per hour. Experiments have established that leaf donors most often feed nearby plant organs. For example, the leaves of the shoot provide fruits, the lower leaves provide roots. In addition, phloem transport is two-way, depending on the vegetative phase; for example, storage organs can transport carbohydrates to blossoming leaves.
If the bark on a tree is cut in a circle down to the wood, then organic substances will stop flowing to the roots, and the tree will dry out over time.
Related materials:
Phloem is a complex conductive tissue through which the products of photosynthesis are transported from leaves to places of their use or deposition (to growth cones, underground organs, ripening seeds and fruits, etc.).
Primary phloem differentiates from procambium, secondary phloem (phloem) is a derivative of cambium. In stems, phloem is usually located outside the xylem, and in leaves it faces the underside of the blade. The primary and secondary phloem, in addition to the different thickness of the sieve elements, differ in that the first lacks medullary rays.
The phloem consists of sieve elements, parenchyma cells, medullary ray elements and mechanical elements (Fig. 47). Most cells in a normally functioning phloem are living. Only part of the mechanical elements dies. The actual conducting function is carried out by sieve elements. There are two types: sieve cells and sieve tubes. The terminal walls of the sieve elements contain numerous small through tubules, collected in groups into the so-called sieve fields. In sieve cells, which are elongated and have pointed ends, the sieve fields are located mainly on the side walls. Sieve cells are the main conducting element of the phloem in all groups of higher plants, excluding angiosperms. Sieve cells do not have companion cells.
The sieve tubes of angiosperms are more advanced. They consist of individual cells - segments, located one above the other. The length of individual segments of sieve tubes ranges from 150-300 microns. The diameter of the sieve tubes is 20-30 microns. Evolutionarily, their segments arose from sieve cells.
The sieve fields of these segments are located mainly at their ends. The sieve fields of two segments located one above the other form a sieve plate. Sieve tube segments are formed from elongated procambium or cambium cells. In this case, the mother cell of the meristem divides longitudinally and produces two cells. One of them turns into a segment, the other into a companion cell. Transverse division of the companion cell is also observed, followed by the formation of two or three similar cells, located longitudinally one above the other next to the segment (Fig. 47). It is assumed that the companion cells, together with the segments of the sieve tubes, constitute a single physiological system and, possibly, contribute to the promotion of the current of assimilates. During its formation, the segment has wall cytoplasm, a nucleus and a vacuole. With the onset of functional activity, it noticeably stretches out. Many small perforations appear on the transverse walls, forming tubules with a diameter of several micrometers, through which cytoplasmic cords pass from segment to segment. A special polysaccharide, callose, is deposited on the walls of the tubules, narrowing their lumen, but not interrupting the cytoplasmic cords.
As the sieve tube segment develops, mucus bodies are formed in the protoplast. The nucleus and leukoplasts, as a rule, dissolve, the border between the cytoplasm and the vacuole - the tonoplast - disappears and all living contents merge into a single mass. In this case, the cytoplasm loses semi-permeability and becomes completely permeable to solutions of organic and inorganic substances. The mucus bodies also lose their outlines and merge, forming a mucus cord and accumulations near the sieve plates. This completes the formation of the sieve tube segment.
The duration of operation of sieve tubes is short. In shrubs and trees it lasts no more than 3-4 years. As we age, the sieve tubes become clogged with callose (forming the so-called corpus callosum) and then die. Dead sieve tubes are usually flattened by neighboring living cells pressing on them.
The parenchymal elements of the phloem (phloem parenchyma) consist of thin-walled cells. Spare nutrients are deposited in them and partly through them short-range transport of assimilates is carried out. In gymnosperms there are no companion cells and their role is played by the few cells of the bast parenchyma adjacent to the sieve cells.
The medullary rays, which continue in the secondary phloem, also consist of thin-walled parenchyma cells. They are intended for short-range transport of assimilates.
Topic: Plant tissues, their structure and functions
1. The concept of plant tissues. Classification of fabrics.
2. Educational tissues (meristems).
3. Integumentary tissues.
4. Basic fabrics.
5. Mechanical fabrics.
6. Conductive fabrics. Phloem and xylem. Conductive bundles.
7. Excretory tissues (secretory structures).
The concept of plant tissues. Classification of fabrics.
Integumentary tissues.
These are tissues located on the outside of all plant organs. Their function is to protect against drying out and damage to internal, lower tissues. In addition, they perform an excretory function and participate in gas exchange with the environment.
There are 3 types of integumentary tissue:
1. Primary - epidermis (skin),
2. Secondary - periderm (cork)
3. Tertiary - rhytide (crust).
Epidermis– primary integumentary tissue of leaves and stems. Cells e. live, with cellulose shells. Because this fabric performs a protective function, this fabric is dense; In plan, the cells have a sinuous outline, due to which they are firmly closed. Cells of organs that are elongated in length (linear leaves) have an elongated shape.
The surface of the skin is covered with a film - cuticle(cuticle), consisting of cutin. The cuticle protects cells from drying out; drops of water roll off its smooth surface (cabbage, ficus). Epidermal cells do not have colored plastids. These cells are transparent (they cover the leaf - the organ of photosynthesis) (compare with a window) and freely transmit light into the internal tissues.
Communication with the external environment of internal tissues is carried out through stomata. Gas exchange and transpiration (evaporation of water) occur through stomata.
Stoma consists of 2 guard cells and a gap between them (stomatal fissure). Guard cells are most often bean-shaped. The shells on the side of the slit are thickened. The stomatal fissure is represented by the front courtyard, the central gap and the back courtyard.
The opening and closing of stomata is determined by turgor phenomena.
This may be withering (cells in a state of plasmolysis) - closing. Filling with water - turgor - opening. Due to uneven thickening of the cell membrane.
The change in turgor in guard cells is determined by the process of photosynthesis and the conversion of starch into sugar and vice versa - sugar into starch. When starch is saccharified, sugar passes into cell sap, the concentration of cell sap increases, the suction force increases, water from neighboring cells enters the guard cells. The volume of vacuoles increases, the gap opens. Since photosynthesis is higher in the morning, when there are more ultraviolet rays, the stomata are open in the morning (mow the grass). Mow the scythe while there is dew, away with the dew and we'll go home.
Protective formations of various kinds are found on the epidermis hairs (trichomes). They can be unicellular and multicellular, branched and stellate. Some of them serve as protection against plants being eaten by animals. Others, white in color, reflect the sun's rays and protect the plant from burns.
Burning hairs nettles perform a protective function. These are cells with a vesicular base; the cell sap contains burning substances. The hair has a cell membrane impregnated with lime and silica, making it hard and brittle. When pierced into the body, it breaks and cell sap is injected, as if from a syringe.
Skin cells cover young, growing plant organs. The epidermis lasts from several weeks to several months. By the end of summer, the epidermis on the stems of woody plants begins to be replaced by secondary integumentary tissue - cork.
Cork.
From the cells of the epidermis or from the cells of subepidermal tissue, meristematic tissue is formed, which is called cork cambium or phellogen. Phellogen cells divide only in the tangential direction. In this case, the internal daughter cell can become a cell of living tissue - phelloderms, then the outer one remains meristematic (phellogen cells), it divides again and the outer one becomes a cell plugs (phellems). More of these cells are deposited. Their shells cork, the cells die, their cavities fill with air. In this way, a covering cork tissue is formed. The entire complex (phellogen, phelloderm and phellem) is called periderm. The cover here is only a cork. Because phellogen cells divide only in the tangential direction, they deposit phellem and phelloderm cells only above and below themselves, and all periderm cells are located strictly one above the other. According to this feature, under a microscope, the periderm differs from other tissues.
Crust.
The longevity of the cork cambium varies from plant to plant. It usually dies off after a few months. A new cork cambium is formed from the deeper layers of the stem bark, it produces a new periderm, and the phelloderm of the first periderm, isolated by the new cork, has not received nutrition, and dies. This gradually forms a complex of dead periderms called the crust. The crust is the tertiary covering tissue. The stem of the tree grows in thickness, and under its pressure the bark cracks. It is separated either by rings or scales. Therefore, a distinction is made between ringed bark (cherry, eucalyptus) and scaly bark (pine).
Cork is a water- and gas-permeable tissue, the cells underneath rapidly divide, a tubercle is formed, tearing the outer tissues - the cork and the epidermis - and a lentil is formed. It is through the lentils that the internal tissues communicate with the external environment.
Lentils are easily recognized by the tubercles on the branches of trees and shrubs. In birch they look like dark transverse stripes, clearly visible on the white surface of the periderm.
Mechanical fabrics.
The first plants originated in water. Because water is a dense medium (denser than air. That is itself) served as a support for the algae body. Even the giant algae Macrocystis has several thallus. tens of m, and “branches” reaching 180 m can stay near the surface of the water, where they sway in ocean currents.
When plants reach land, they need to move their stems up to the light and support them without the help of water, which was no longer around them. The branches of terrestrial plants must counteract the gravity of leaves, flowers, and fruits.
The necessary strength of the plant is provided by the turgor of living tissue cells and the strength of the integumentary tissues. In addition, the plant has a system of mechanical tissues, which are the reinforcement, the skeleton of the plant body.
There are 3 types of mechanical tissues: collenchyma, sclerenchyma and sclereids (stony cells).
Collenchyma occurs most often in dicotyledonous plants at the periphery of stems and leaf petioles. This is a living tissue; its cells have cellulose membranes. The cell membranes are thickened, but not over the entire surface, but in certain areas. Depending on this they distinguish:
- corner cell - the membranes thicken due to the layering of cellulose in the corners of the cells.
- Lamellar cells - the tangential walls of the cells thicken.
- Loose cells - in the early stages, cells are separated in the corners and intercellular spaces are formed, the cell walls thicken in those places where they adjoin the intercellular spaces.
Since collenchyma is a living tissue, it does not prevent the organ from growing in thickness, although it is located on the periphery of the organ. Therefore, it is characteristic of dicotyledons.
Sclerenchyma– the most common reinforcing tissue in plant organs. It consists of dead prosenchymal cells with pointed ends. The cell membranes are lignified, very thickened. The material of sclerenchyma cell walls is not inferior to construction steel in terms of strength and elasticity. Sclerenchyma is found in the vegetative organs of almost all higher plants. Sclerenchyma cells are called fibers.
Sclereids- these are mechanical cells that have a parenchymal shape and highly thickened lignified shells. The most common are the so-called Stony cells that form the shells of nuts (hazel, acorn, walnut), fruit seeds (cherries, plums, apricots), they are found in the pulp of quince and pear fruits. In the leaves of tea and camellia there are sclereids with branches (asterosclereids).
Fur is common to all. tissues thickening of cell membranes, due to which the strength of these tissues is achieved.
Conductive fabrics.
Plant life is inextricably linked with the conduction and distribution of nutrients. Water with minerals dissolved in it constantly moves from the roots along the stem to the leaves, flowers and fruits. This flow of substances is called an upward current.
Organic substances are produced in the leaves of plants. They are carried to all plant organs, where they are used to build plant cells and are stored in reserve. This flow of substances is called downward.
The movement of matter in both directions is accomplished with the help of conductive tissues. The cells of all conducting tissues are elongated and have the shape of tubes.
The ascending current moves through the tracheids and vessels.
Tracheids are more ancient in origin. These are narrow, long prosenchymal cells, pointed at the ends. Their shells are lignified, thickened, and the cells themselves are dead, the thickening of the shells is uneven. It can be spiral, ladder, ringed.
The tracheids are adjacent to each other and communication between them is carried out through pores, spaces between thickenings and perforations. These holes are usually not solid, so they provide a slow current.
Vessels– these are connections of tubular cells (like a pipeline welded from relatively short pipes). The vascular cells are dead, the partitions between them are broken. The cell membranes of blood vessels are lignified. The length of the vessels can be from several. cm to 1-1.5 m. In the process of ontogenesis, the vessel is formed as follows:
There are different types of vessels:
1. ringed
2. spiral
3. stairs
4. mesh
5. porous
Tracheids are more ancient in origin and more primitive conductive elements. They are characteristic of gymnosperms; they are also found in flowering plants. And tracheas are more progressive and are characteristic only of flowering plants.
The descending current is carried out through sieve tubes with companion cells. Unlike tracheids and vessels, sieve tubes (ethmoid tubes) are living and have cytoplasm. Sieve tubes consist of elongated tubular cells. The partitions between them are perforated like a sieve. These partitions are called sieve plates.
The length of the sieve tubes ranges from fractions of a mm to 2 mm. During ontogenesis, sieve tubes are formed in this way:
It is assumed that the cells of the sieve tubes do not have a nucleus, but they are alive. Obviously, the nuclei of companion cells somehow regulate the functions of the sieve tubes.
Conducting tissues in plant organs are arranged in a certain order; they form various types of bundles, which are also called vascular-fibrous bundles.
The main parts of the vascular bundle are xylem (it provides upward current) and phloem (it provides downward current). Xylem and phloem are complexes of tissues.
Let's consider what fabrics are included in their composition. Let's fill out the table:
Histological composition of phloem and xylem
Conductive elements and parenchyma are required in vascular bundles, but fibers are not always present. Therefore, the term “vascular-fibrous bundles” does not apply to all bundles.
Phloem and xylem are formed as a result of the activity of the cambium, which usually deposits phloem to the periphery of the stem or root, and xylem to the center.
If in this case all the educational tissue (i.e., the cambium) is spent on the formation of Cs and Fl, then the bundle is called closed. If the cambium layer is preserved in the bundle, then the bundle is called open. In an open conductive bundle, the formation of elements Fl and Kc continues, the bundle can expand and grow.
Depending on the relative position of Kc and Fl, vascular-fibrous bundles are of different types.
Most common collateral(side-sided) bundles. Such a bundle is called collateral. Fl. And Kc are located on the same radius of the organ. Collateral bundles are either closed or open.
Collateral
closed open
Closed collateral bundles are characteristic of monocot stems and leaves. Open collateral bundles are characteristic of the roots and stems of dicotyledonous plants; they are very rarely found in leaves.
Bicollateral bundle(double-sided).
On the same radius of the organ there are 2 sections Fl (outside and inside) and 1 section Ks. Between the outer Fl and Kc there is a layer of cambium. Such bunches are found in the stems of nightshade and pumpkin plants (Table Art. Pumpkin).
Radial beam- Fl and Kc are located alternating at different radii. Characteristic of roots with a primary structure (iris).
Concentric(either Fl surrounds Kc, or vice versa). Amphivasal (Ks surrounds Fl) (lily of the valley rhizome). Amphicribral (Fl surrounds Ks) (fern rhizome).
Phloem similar to xylem in that it also contains tubular structures modified in accordance with their conducting function. However, these tubes are composed of living cells that have cytoplasm; they have no mechanical function. There are five types of cells in the phloem: sieve tube segments, companion cells, parenchyma cells, fibers and sclereids.
Sieve tubes and companion cells
Sieve tubes- these are long tubular structures through which solutions of organic substances, mainly sucrose solutions, move in the plant. They are formed by end-to-end joining of cells called sieve tube segments. In the apical meristem, where the primary phloem and primary xylem (vascular bundles) are formed, one can observe the development of rows of these cells from procambial cords.
First phloem to emerge, called protophloem, appears, like protoxylem, in the zone of growth and elongation of the root or stem. As the tissues surrounding it grow, the protophloem stretches and a significant part of it dies, that is, it ceases to function. At the same time, however, new phloem is formed. This phloem, which matures after elongation is completed, is called metaphloem.
The sieve tube segments are very characteristic structure. They have thin cell walls consisting of cellulose and pectin substances, and in this way they resemble parenchyma cells, however, their nuclei die off upon maturation, and only a thin layer of cytoplasm remains, pressed against the cell wall. Despite the absence of a nucleus, the segments of the sieve tubes remain alive, but their existence depends on the companion cells adjacent to them, developing from the same meristematic cell. The sieve tube segment and its companion cell together constitute one functional unit; the companion cell's cytoplasm is very dense and highly active. The structure of these cells, revealed using an electron microscope, is described in detail in our article.
Characteristic feature sieve tubes is the presence of sieve plates. This feature immediately catches the eye when viewed under a light microscope. The sieve plate arises at the junction of the end walls of two adjacent segments of the sieve tubes. Initially, plasmodesmata pass through the cell walls, but then their channels expand, turning into pores, so that the end walls take on the appearance of a sieve through which the solution flows from one segment to another. In a sieve tube, sieve plates are located at certain intervals, corresponding to the individual segments of this coarse. The structure of sieve tubes, companion cells and bast parenchyma, revealed using an electron microscope, is shown in the figure.
Secondary phloem, developing, like the secondary xylem, from the bundle cambium, its structure is similar to the primary phloem, differing from it only in that strands of lignified fibers and medullary rays of the parenchyma are visible in it (Chapter 22). However, secondary phloem is not as strongly expressed as secondary xylem, and moreover, it is constantly renewed.
Bast parenchyma, bast fibers and sclereids
Bast parenchyma and bast fibers are present only in dicotyledons; they are absent in monocotyledons. In its structure, phloem parenchyma is similar to any other, but its cells are usually elongated. In secondary phloem, parenchyma is present in the form of medullary rays and vertical rows, just like the woody parenchyma described above. The functions of bast and wood parenchyma are the same.
Bast fibers are no different from the sclerenchyma fibers described above. Sometimes they are found in the primary phloem, but more often they can be found in the secondary phloem of dicotyledons. Here these cells form vertical strands. As is known, secondary phloem experiences stretching during growth; it is possible that sclerenchyma helps it resist this effect.
Sclereids in phloem, especially in older ones, are quite common.
PHLOEM PHLOEM
(from the Greek phloios - bark), plant tissue that transports photosynthetic products from leaves to places of consumption and storage (underground organs, growth points, ripening fruits and seeds, etc.). Primary F., which is divided into protophloem and metaphloem, is differentiated from procambium, secondary (phloem) is a derivative of cambium. In stems, F. is located outside (in some plants and on the inside) of the xylem. In leaves, F. faces downwards. on the side of the plate, in roots with a radial vascular bundle, F. strands alternate with xylem strands. F. is also involved in the deposition of reserve substances, the release of final metabolic products, and the creation of the plant’s support system. F. Consists of conducting elements, phloem parenchyma cells, fibers and sclereids.
.Plants with active secondary thickening have radial layers of parenchyma cells - bast rays. In archegonial plants, the conducting elements are represented by prosenchymal sieve cells, on the side walls of which there are areas with thin tubules - sieve fields. Flowering plants are characterized by sieve tubes - single-row strands of elongated cells (segments), the terminal walls of which carry sieve fields, called. sieve plates. Mature sieve elements are usually anucleate, so contact with living parenchyma cells is important for their normal functioning. In gymnosperms, these are Strasburger cells located in the cord parenchyma or rays adjacent to the sieve cells; in flowering plants, these are accompanying cells developing from the same mother cell as the sieve tube segment. The remaining cells of the phloem parenchyma can be starch-bearing, crystal-bearing, some of them participate in the formation of secretion containers (for example, resin) or are sclerified, turning into sclereids. The composition of plant elements, the features of their structure and arrangement are specific to each plant species. (see ROOT, STEM) fig. at Art.
(Source: “Biological Encyclopedic Dictionary.” Editor-in-chief M. S. Gilyarov; Editorial Board: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected . - M.: Sov.phloem Conductive tissue of higher plants that transports photosynthetic products (assimilates) from leaves to places of their consumption or storage - roots, growing points, fruits, etc. Primary phloem is formed by the apical meristem, secondary phloem, or phloem, is formed by the apical meristem.. The main element of phloem is sieve tubes, through which assimilates are transported. The speed of their movement through the phloem is 50-150 cm/h, which is higher than the speed that could be the result of free diffusion. In different systematic groups of plants (even in different species of the same genus), the composition and structure of the phloem have differences.
.(Source: “Biology. Modern illustrated encyclopedia.” Chief editor A. P. Gorkin; M.: Rosman, 2006.)
Synonyms:
See what "PHLOEM" is in other dictionaries:
PHLOEM, plant tissue equipped with vessels that transports photosynthetic products from leaves to places of consumption. Phloem includes several types of CELLS. The most important of them are elongated hollow cells called sieve cells... ... Scientific and technical encyclopedic dictionary
- (from the Greek phloios bark), tissue of higher plants that serves to carry organic substances that are synthesized in the leaves (sucrose, etc.) to the roots. The main elements of phloem are sieve tubes, companion cells, parenchyma cells and... ... Big Encyclopedic Dictionary
Lub Dictionary of Russian synonyms. phloem noun, number of synonyms: 2 bast (4) fabric (474) ... Synonym dictionary
- (from the Greek phloios bark, bast), tissue of higher plants that transports photosynthetic products from leaves to other organs (ripening fruits, seeds, roots) ... Modern encyclopedia
Part of the vascular bundle of plants. Both the elements that conduct water through the plant and the elements that conduct organic substances are collected in special vascular bundles and, moreover, in such a way that part of the bundle is occupied by elements that conduct water, and the rest... ... Encyclopedia of Brockhaus and Efron
Syn. term bast Geological Dictionary: in 2 volumes. M.: Nedra. Edited by K. N. Paffengoltz et al. 1978 ... Geological encyclopedia
Phloem- (from the Greek phloios bark, bast), tissue of higher plants that transports photosynthetic products from leaves to other organs (ripening fruits, seeds, roots). ... Illustrated Encyclopedic Dictionary
Cross section of a flax stem: 1. loose pith, 2. protoxylem, 3. xylem, 4. phloem, 5 ... Wikipedia
- (from the Greek phloiós bark, bast), tissue of higher plants that serves to carry organic substances that are synthesized in the leaves (sucrose, etc.) to the roots. The main elements of phloem are sieve tubes, companion cells, parenchyma cells and... ... encyclopedic Dictionary
