Different organs of higher plants perform different functions. So the roots absorb water and minerals, and photosynthesis occurs in the leaves, as a result of which organic substances are formed. However, all plant cells need both water and organic matter. Therefore, a transport system is needed to ensure the delivery of necessary substances to one organ from another. In plants (mainly angiosperms) this function is performed conductive fabrics.
In woody plants, conductive tissues are part of wood And bast. For wood it is carried out rising current: Water and minerals rise from the roots. On bast it is 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 always go down. Substances can move horizontally and sometimes in the opposite direction. For example, organic matter goes up to growing shoots that are above storage tissue or photosynthetic leaves.
So, in plants, the movement of aqueous solution and organic substances are separated. The composition of the wood includes vessels, and in the composition of the bast - sieve tubes.
The vessels are a chain 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). U gymnosperms and there are ferns instead of vessels tracheids, along which water moves more slowly. It follows that the vessels have a more perfect structure. The vessels are called differently trachea.
The reason why water moves faster in vessels than in tracheids is due to 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. The vessels are essentially a hollow tube; their cells have large holes (perforations) at the points of connection 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 horizontally. It enters the parenchyma cells and neighboring vessels (vessels are usually arranged in bundles).
Sieve tubes are formed by living elongated cells. Organic substances move through them. At the top and bottom, the vascular cells are connected to each other due to numerous pores. This connection is similar to a sieve, hence the name. It turns out to be 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 life. The satellites and tubes are connected to each other through special pores.
Wood and bast consist of more than just conductive tissues. They also include parenchyma and mechanical tissues. Conductive tissues together with mechanical ones form vascular-fibrous bundles. Parenchyma often plays the role of storage tissue (especially in wood).
Wood has another name xylem, and bast - phloem.
This type belongs to complex tissues and consists of differently differentiated cells. In addition to the conductive elements themselves, the tissue contains mechanical, excretory and storage elements. Conductive tissues unite all plant organs into a single system. There are two types of conducting tissues: xylem and phloem (Greek xylon - tree; phloios - bark, bast). They have both structural and functional differences.
The conducting elements of xylem are formed by dead cells. They carry out long-distance transport of water and substances dissolved in it from the root to the leaves. The conducting elements of the phloem preserve the living protoplast. They carry out long-distance transport from photosynthetic leaves to the root.
Typically, xylem and phloem are located in the plant body in a certain order, forming layers or vascular bundles. Depending on the structure, there are several types of vascular bundles, which are characteristic of certain groups of plants. The collateral open bundle between the xylem and phloem contains the cambium, which provides secondary growth. In a bicollateral open bundle, the phloem is located relative to the xylem on both sides. Closed bundles do not contain cambium, and hence are not capable of secondary thickening. You can find two more types of concentric bundles, where either phloem surrounds xylem, or xylem surrounds phloem.
Xylem (wood). The development of xylem in higher plants is associated with ensuring water exchange. Since water is constantly removed through the epidermis, the same amount of moisture must be absorbed by the plant and added to the organs that carry out transpiration. It should be taken into account that the presence of a living protoplast in water-conducting cells would greatly slow down transport; dead cells here turn out to be more functional. However, a dead cell does not have turgidity; therefore, the membrane must have mechanical properties. Note: turgescence is a state of plant cells, tissues and organs in which? they become elastic due to the pressure of the cell contents on their elastic membranes. Indeed, the conducting elements of the xylem consist of them elongated along the axis of the organ dead cells with thick lignified shells.
Initially, xylem is formed from the primary meristem - procambium, located at the tops of the axial organs. First, protoxylem is differentiated, then metaxylem. Three types of xylem formation are known. In the exarch type, protoxylem elements first appear at the periphery of the procambium bundle, then metaxylem elements appear in the center. If the process goes in the opposite direction (i.e. from the center to the periphery), then this is an endarchic type. In the mesarchic type, xylem is formed in the center of the procambial bundle, after which it is deposited both towards the center and towards the periphery.
The root is characterized by an exarchal type of xylem formation, while the stems are characterized by an endarchic type. In low-organized plants, the methods of xylem formation are very diverse and can serve as systematic characteristics.
Some? In plants (for example, monocots), all procambium cells differentiate into conducting tissues that are not capable of secondary thickening. In other forms (for example, woody ones), lateral meristems (cambium) remain between the xylem and phloem. These cells are able to divide, renewing the xylem and phloem. This process is called secondary growth. In many, growing in relatively stable climatic conditions, plants, growth is constant. In forms adapted to seasonal climate changes - periodically.
The main stages of differentiation of procambium cells. Its cells have thin membranes that do not prevent them from stretching during the growth of the organ. The protoplast then begins to lay down a secondary shell. But this process has distinct features. The secondary shell is not deposited in a continuous layer, which would not allow the cell to stretch, but in the form of rings or in a spiral. Elongation of the cell is not difficult. In young cells, the rings or turns of the helix are located close to each other. In mature cells, cells diverge as a result of cell elongation. The ringed and spiral thickenings of the shell do not interfere with growth, but mechanically they are inferior to the shells, where the secondary thickening forms a continuous layer. In this regard, after growth ceases, elements with a continuous lignified shell (metaxylem) are formed in the xylem. It should be noted that the secondary thickening here is not ringed or spiral, but dotted, scalariform, mesh-shaped. Its cells are not capable of stretching and die within a few hours. This process occurs in a coordinated manner in nearby cells. A large number of lysosomes appear in the cytoplasm. Then they disintegrate, and the enzymes contained in them destroy the protoplast. When the transverse walls are destroyed, the cells located in a chain above each other form a hollow vessel. Most angiosperms and some? pteridophytes have blood vessels.
A conducting cell that does not form through perforations in its wall is called a tracheid. The movement of water through tracheids occurs at a lower speed than through vessels. The fact is that in tracheids the primary shell is not interrupted anywhere. The tracheids communicate with each other through pores. It should be clarified that in plants the pore is only a depression in the secondary shell up to the primary shell and there are no through perforations between the tracheids.
The most common are bordered pores. In them, a channel facing the cell cavity forms an extension - a pore chamber. Most pores coniferous plants on the primary shell they have a thickening - a torus, which is a kind of valve and is able to regulate the intensity of water transport. By shifting, the torus blocks the flow of water through the pore, but after that it can no longer return to its previous position, performing a one-time action.
The pores are more or less round, elongated perpendicular to the elongated axis (a group of these pores resembles a ladder; therefore, such porosity is called staircase). Through the pores, transport occurs both in the longitudinal and transverse directions. Pores are present not only in tracheids, but also in individual vascular cells that form the vessel.
From the point of view of evolutionary theory, tracheids represent the first and main structure that conducts water in the body of higher plants. It is believed that the vessels arose from tracheids due to lysis of the transverse walls between them. Most pteridophytes and gymnosperms do not have vessels. Their movement of water occurs through tracheids.
In the process of evolutionary development, vessels arose in different groups plants repeatedly, but they acquired the most important functional significance in angiosperms, in which? they are present along with tracheids. It is believed that the possession of a more advanced transport mechanism helped them not only survive, but also achieve a significant variety of forms.
Xylem is a complex tissue; in addition to water-conducting elements, it also contains others. Mechanical functions are performed by libriform fibers (Latin liber - bast, forma - form). The presence of additional mechanical structures is important because, despite the thickening, the walls of the water-conducting elements are still too thin. They are not able to support large masses on their own. perennial plant. The fibers developed from tracheids. They are characterized by smaller sizes, lignified (lignified) shells and narrow cavities. Pores without borders can be found on the wall. These fibers cannot conduct water; their main function is support.
The xylem also contains living cells. Their mass can reach 25% of the total volume of wood. Since these cells have rounded shape, then they are called wood parenchyma. In the plant body, parenchyma is located in two ways. In the first case, the cells are arranged in the form of vertical strands - this is strand parenchyma. In another case, the parenchyma forms horizontal rays. They are called pith rays because they connect the pith and cortex. The core performs a number of functions, including storing substances.
Phloem (bast). This is a complex tissue, as it is formed by different types of cells. The main conducting cells are called sieve elements. The conducting elements of the xylem are formed by dead cells, while in the phloem they retain a living, albeit highly modified, protoplast during the period of functioning. The phloem carries out the outflow of plastic substances from photosynthetic organs. All living plant cells have the ability to conduct organic substances. And hence, if xylem can be found only in higher plants, then the transport of organic substances between cells is also carried out in lower plants.
Xylem and phloem develop from apical meristems. At the first stage, protophloem is formed in the procambial cord. As surrounding tissues grow, it stretches, and when growth is complete, metaphloem is formed instead of protophloem.
In different groups of higher plants two types can be found sieve elements. In pteridophytes and gymnosperms it is represented by sieve cells. Sieve fields in the cells are scattered along the side walls. The protoplast retains a somewhat destructed nucleus.
In angiosperms, sieve elements are called sieve tubes. They communicate with each other through sieve plates. Mature cells lack nuclei. However, next to the sieve tube there is a companion cell, formed together with the sieve tube as a result of mitotic division of the common mother cell (Fig. 38). The companion cell has a denser cytoplasm with a large number of active mitochondria, as well as a fully functioning nucleus, a huge number of plasmodesmata (ten times more than in other cells). Companion cells influence the functional activity of anucleate tube sieve cells.
The structure of mature sieve cells has some peculiarities. There is no vacuole, and therefore the cytoplasm is greatly liquefied. The nucleus may be absent (in angiosperms) or in a wrinkled, functionally inactive state. Ribosomes and the Golgi complex are also absent, but the endoplasmic reticulum is well developed, which not only penetrates the cytoplasm, but also passes into neighboring cells through the pores of the sieve fields. Well-developed mitochondria and plastids are found in abundance.
Transport of substances between cells occurs through holes located on the cell membranes. Such openings are called pores, but unlike the pores of tracheids, they are through. It is assumed that they represent greatly expanded plasmodesmata, on the walls, which? callose polysaccharide is deposited. The pores are arranged in groups, forming sieve fields. In primitive forms, sieve fields are randomly scattered over the entire surface of the shell; in more advanced angiosperms, they are located at the ends of adjacent cells adjacent to each other, forming a sieve plate. If there is one sieve field on it, it is called simple, if there are several, it is called complex.
The speed of movement of solutions through sieve elements is up to 150 cm per hour. This is a thousand times faster than the speed of free diffusion. Active transport probably takes place, and numerous mitochondria of sieve elements and companion cells supply the necessary ATP for this.
The duration of activity of the phloem sieve elements depends on the presence of lateral meristems. If they are present, then the sieve elements work throughout the life of the plant.
In addition to sieve elements and companion cells, the phloem contains bast fibers, sclereids and parenchyma.
6.1. The importance and variety of conductive tissues
Conductive tissues are the most important component of most higher plants. They are an obligatory structural component of the vegetative and reproductive organs of spore and seed plants. Conducting tissues, together with cell walls and intercellular spaces, some cells of the main parenchyma and specialized transmitting cells form a conducting system that ensures long-distance and radial transport of substances. Due to the special design of cells and their location in the plant body, the conducting system performs numerous but interrelated functions:
1) the movement of water and minerals absorbed by the roots from the soil, as well as organic substances formed in the roots, into the stem, leaves, and reproductive organs;
2) movement of photosynthesis products from the green parts of the plant to places of their use and storage: roots, stems, fruits and seeds;
3) the movement of phytohormones throughout the plant, which creates a certain balance of them, which determines the rate of growth and development of the vegetative and reproductive organs of plants;
4) radial transport of substances from conducting tissues to nearby living cells of other tissues, for example, to assimilating leaf mesophyll cells and dividing meristem cells. Parenchyma cells of the medullary rays of wood and bark may also take part in it. Transmissive cells with numerous protrusions of the cell membrane, located between the conducting and parenchymal tissues, are of great importance in radial transport;
5) conductive tissues increase the resistance of plant organs to deforming loads;
6) conducting tissues form a continuous branched system that connects plant organs into a single whole;
The emergence of conductive tissues is the result of evolutionary structural transformations associated with the emergence of plants onto land and the separation of their air and soil nutrition. The most ancient conducting tissues, tracheids, were found in fossil rhinophytes. They reached their highest development in modern angiosperms.
During the process of individual development, primary conducting tissues are formed from the procambium at the growth points of the seed embryo and renewal buds. Secondary conducting tissues, characteristic of dicotyledonous angiosperms, are generated by the cambium.
Depending on the functions performed, conducting tissues are divided into tissues of ascending current and tissues of descending current. The main purpose of ascending tissue is to transport water and minerals dissolved in it from the root to the higher above-ground organs. In addition, organic substances formed in the root and stem, such as organic acids, carbohydrates and phytohormones, move through them. However, the term “upward current” should not be taken unambiguously as movement from bottom to top. Ascending tissues ensure the flow of substances in the direction from the suction zone to the shoot apex. In this case, the transported substances are used both by the root itself and by the stem, branches, leaves, reproductive organs, regardless of whether they are located above or below the level of the roots. For example, in potatoes, water and mineral nutrition elements enter through ascending tissues into stolons and tubers formed in the soil, as well as into above-ground organs.
Downstream tissues ensure the outflow of photosynthetic products into the growing parts of plants and storage organs. In this case, the spatial position of photosynthetic organs does not matter. For example, in wheat, organic substances enter the developing grains from leaves of different tiers. Therefore, the names “ascending” and “descending” fabrics should be treated as nothing more than an established tradition.
6.2. Conductive tissues of ascending current
The ascending tissues include tracheids and vessels (tracheas), which are located in the woody (xylem) part of plant organs. In these tissues, the movement of water and substances dissolved in it occurs passively under the influence of root pressure and evaporation of water from the surface of the plant.
Tracheids are of more ancient origin. They are found in higher spore plants, gymnosperms and, less frequently, in angiosperms. In angiosperms they are typical of the smallest branching of leaf veins. Tracheid cells are dead. They have an elongated, often spindle-shaped shape. Their length is 1 – 4 mm. However, in gymnosperms, for example in Araucaria, it reaches 10 mm. The cell walls are thick, cellulose, and often impregnated with lignin. The cell membranes have numerous bordered pores.
Vessels formed at later stages of evolution. They are characteristic of angiosperms, although they are also found in some modern representatives of the departments Mosses (genus Sellaginella), Horsetails, Ferns and Gymnosperms (genus Gnetum).
The vessels consist of elongated dead cells located one above the other and called vessel segments. In the end walls of the vessel segments there are large through holes - perforations, through which long-distance transport of substances occurs. Perforations arose during evolution from the bordered pores of tracheids. As part of the vessels they are ladder and simple. Numerous scalariform perforations are formed on the end walls of the vessel segments when they are laid obliquely. The openings of such perforations have an elongated shape, and the partitions separating them are located parallel to each other, resembling the steps of a staircase. Vessels with scalariform perforation are characteristic of plants of the Ranunculaceae, Limonaceae, Birch, Palm, and Chastukhova families.
Simple perforations are known in evolutionarily younger families, such as Solanaceae, Cucurbitaceae, Asteraceae, and Poaceae. They represent one large hole in the end wall of the joint, located perpendicular to the axis of the vessel. In a number of families, for example, Magnoliaceae, Roseaceae, Irisaceae, Asteraceae, both simple and scalariform perforations are found in vessels.
The side walls have uneven cellulose thickenings that protect the vessels from excess pressure created by nearby living cells of other tissues. There may be numerous pores in the side walls, allowing water to escape outside the vessel.
Depending on the nature of the thickenings, the types and nature of the location of the pores, the vessels are divided into annular, spiral, bispiral, reticular, scalariform and point-pore. In annular and spiral vessels, cellulose thickenings are arranged in the form of rings or spirals. Through non-thickened areas, the transported solutions diffuse into the surrounding tissues. The diameter of these vessels is relatively small. In reticulate, scalariform, and punctate-pore vessels, the entire lateral wall, with the exception of the locations of simple pores, is thickened and often impregnated with lignin. Therefore, their radial transport of substances occurs through numerous elongated and pinpoint pores.
Vessels have a limited lifespan. They can be destroyed as a result of blockage by tills - outgrowths of neighboring parenchyma cells, as well as under the influence of centripetal pressure forces of new wood cells formed by the cambium. During evolution, blood vessels undergo changes. The vessel segments become shorter and thicker, oblique transverse septa are replaced by straight ones, and scalariform perforations become simple.
6.3. Conductive tissues of descending current
Descending tissues include sieve cells and sieve tubes with companion cells. Sieve cells have a more ancient origin. They are found in higher spore plants and gymnosperms. These are living, elongated cells with pointed ends. In the mature state, they contain nuclei as part of the protoplast. In their side walls, in the places of contact of adjacent cells, there are small through perforations, which are collected in groups and form sieve fields through which substances move.
Sieve tubes consist of a vertical row of elongated cells separated from each other by transverse walls called sieve plates, in which sieve fields are located. If a sieve plate has one sieve field, it is considered simple, and if it has several, it is considered complex. Sieve fields are formed by numerous through holes - sieve perforations of small diameter. Plasmodesmata pass through these perforations from one cell to another. Callose polysaccharide is placed on the walls of the perforations, which reduces the lumen of the perforations. As the sieve tube ages, callose completely plugs the perforations and the tube stops working.
When a sieve tube is formed, a special phloem protein (F-protein) is synthesized in the cells that form them and a large vacuole develops. It pushes the cytoplasm and nucleus towards the cell wall. Then the vacuole membrane is destroyed and the internal space of the cell is filled with a mixture of cytoplasm and cell sap. The F protein bodies lose their distinct outlines and merge, forming strands near the sieve plates. Their fibrils pass through perforations from one segment of the sieve tube to another. One or two companion cells, which have an elongated shape, thin walls and living cytoplasm with a nucleus and numerous mitochondria, are tightly adjacent to the segments of the sieve tube. Mitochondria synthesize ATP, which is necessary for the transport of substances through sieve tubes. In the walls of the companion cells there are a large number of pores with plasmadesmata, which is almost 10 times greater than their number in other mesophyll cells of the leaf. The surface of the protoplast of these cells is significantly increased due to numerous folds formed by the plasmalemma.
The speed of movement of assimilates through sieve tubes significantly exceeds the speed of free diffusion of substances and reaches 50–150 cm/hour, which indicates the active transport of substances using ATP energy.
The duration of operation of sieve tubes in perennial dicotyledons is 1–2 years. To replace them, the cambium constantly forms new conducting elements. In monocots lacking a cambium, sieve tubes last much longer.
6.4. Conductive bundles
Conductive tissues are located in plant organs in the form of longitudinal strands, forming conductive bundles. There are four types of vascular bundles: simple, general, complex and fibrovascular.
Simple bundles consist of one type of conductive tissue. For example, in the marginal parts of the leaf blades of many plants there are small-diameter bundles of vessels and tracheids, and in the flowering shoots of lilies - from only sieve tubes.
Common bundles are formed by tracheids, vessels and sieve tubes. Sometimes the term is used to refer to the metamer bundles that run through the internodes and are leaf trails. The complex bundles include conductive and parenchymal tissues. The most perfect, diverse in structure and location are the vascular-fibrous bundles.
Vascular-fibrous bundles are characteristic of many higher spore plants and gymnosperms. However, they are most typical of angiosperms. In such bundles, functionally different parts are distinguished - phloem and xylem. Phloem ensures the outflow of assimilates from the leaf and their movement to places of use or storage. The xylem transports water and substances dissolved in it from the root system to the leaf and other organs. The volume of the xylem part is several times greater than the volume of the phloem part, since the volume of water entering the plant exceeds the volume of assimilates formed, since a significant part of the water is evaporated by the plant.
The diversity of vascular-fibrous bundles is determined by their origin, histological composition and location in the plant. If the bundles are formed from the procambium and complete their development as the supply of educational tissue cells is used up, as in monocots, they are called closed for growth. In contrast, in dicotyledons, open tufts are not limited in growth, since they are formed by the cambium and increase in diameter throughout the life of the plant. In addition to conductive tissues, vascular-fibrous bundles may include basic and mechanical tissues. For example, in dicotyledons, phloem is formed by sieve tubes (ascending tissue), bast parenchyma (ground tissue), and bast fibers (mechanical tissue). The xylem consists of vessels and tracheids (conductive tissue of descending current), wood parenchyma (ground tissue) and wood fibers (mechanical tissue). The histological composition of xylem and phloem is genetically determined and can be used in plant taxonomy to diagnose different taxa. In addition, the degree of development of the component parts of the bunches can change under the influence of plant growth conditions.
Several types of vascular-fibrous bundles are known.
Closed collateral vascular bundles are characteristic of the leaves and stems of monocot angiosperms. They lack cambium. Phloem and xylem are located side-by-side. They are characterized by certain design features. Thus, in wheat, which differs in the C 3 -pathway of photosynthesis, bundles are formed from procambium and have primary phloem and primary xylem. In the phloem, there is an earlier protophloem and a later in time formation, but larger cell metaphloem. The phloem part lacks bast parenchyma and bast fibers. In the xylem, smaller protoxylem vessels are initially formed, located in one line perpendicular to the internal border of the phloem. The metaxylem is represented by two large vessels located next to the metaphloem perpendicular to the chain of protoxylem vessels. In this case, the vessels are arranged in a T-shape. The V-, Y- and È-shaped arrangement of vessels is also known. Between the metaxylem vessels, in 1–2 rows, there is small-celled sclerenchyma with thickened walls, which become saturated with lignin as the stem develops. This sclerenchyma separates the xylem zone from the phloem. On both sides of the protoxylem vessels there are wood parenchyma cells, which probably play a transfusion role, since during the transition of the bundle from the internode to the leaf pad of the stem node, they participate in the formation of transfer cells. Around the vascular bundle of the wheat stem there is a sclerenchyma sheath, better developed on the protoxylem and protophloem side; near the lateral sides of the bundle, sheath cells are arranged in one row.
In plants with the C 4 type of photosynthesis (corn, millet, etc.), in the leaves around the closed vascular bundles there is a lining of large chlorenchyma cells.
Open collateral bundles are characteristic of dicotyledonous stems. The presence of a cambium layer between the phloem and xylem, as well as the absence of a sclerenchyma sheath around the bundles, ensures their long-term growth in thickness. In the xylem and phloem parts of such bundles there are cells of the main and mechanical tissues.
Open collateral bundles can be formed in two ways. Firstly, these are bundles primarily formed by the procambium. Then the cambium develops from the cells of the main parenchyma, producing secondary elements of phloem and xylem. As a result, the bundles will combine histological elements of primary and secondary origin. Such bunches are characteristic of many herbaceous flowering plants of the Dicotyledonous class, which have a bunched type of stem structure (legumes, Rosaceae, etc.).
Secondly, open collateral bundles can be formed only by the cambium and consist of xylem and phloem of secondary origin. They are typical for herbaceous dicotyledons with a transitional type of anatomical structure of the stem (asteraceae, etc.), as well as for root crops such as beets.
In the stems of plants of a number of families (Pumpkin, Solanaceae, Campanaceae, etc.) there are open bicollateral bundles, where the xylem is surrounded on both sides by phloem. In this case, the outer section of the phloem, facing the surface of the stem, is better developed than the inner one, and the cambium strip, as a rule, is located between the xylem and the outer section of the phloem.
Concentric beams come in two types. In amphicribral bundles, characteristic of fern rhizomes, the phloem surrounds the xylem; in amphivasal bundles, the xylem is located in a ring around the phloem (rhizomes of iris, lily of the valley, etc.). Concentric bundles are less common in dicotyledons (castor beans).
Closed radial vascular bundles are formed in areas of the roots that have a primary anatomical structure. The radial bundle is part of the central cylinder and passes through the middle of the root. Its xylem has the appearance of a multi-rayed star. Phloem cells are located between the xylem rays. The number of xylem rays largely depends on the genetic nature of the plants. For example, in carrots, beets, cabbage and other dicotyledons, the xylem of the radial bundle has only two rays. Apple and pear trees can have 3–5 of them, pumpkins and beans have four-rayed xylem, and monocots have multi-rayed xylem. The radial arrangement of xylem rays has adaptive significance. It shortens the path of water from the suction surface of the root to the vessels of the central cylinder.
In perennial woody plants and some herbaceous annuals, such as flax, vascular tissues are located in the stem without forming clearly defined vascular bundles. Then they talk about the non-tufted type of stem structure.
6.5. Tissues regulating radial transport of substances
Specific tissues that regulate the radial transport of substances include exoderm and endoderm.
The exoderm is the outer layer of the primary root cortex. It is formed directly under the primary integumentary epiblema tissue in the zone of root hairs and consists of one or several layers of tightly packed cells with thickened cellulose membranes. In the exodermis, water entering the root through the root hairs experiences resistance from the viscous cytoplasm and moves into the cellulose membranes of the exodermal cells, and then leaves them into the intercellular spaces of the middle layer of the primary cortex, or mesoderm. This ensures efficient flow of water into the deeper layers of the root.
In the zone of conduction in the root of monocots, where epiblema cells die and slough off, the exodermis appears on the surface of the root. Its cell walls are saturated with suberin and prevent the flow of water from the soil into the root. In dicotyledons, the exoderm in the primary cortex is sloughed off during root molting and replaced by periderm.
The endoderm, or inner layer of the primary root cortex, is located around a central cylinder. It is formed by one layer of tightly closed cells of unequal structure. Some of them, called permeable, have thin shells and are easily permeable to water. Through them, water from the primary cortex enters the radial vascular bundle of the root. Other cells have specific cellulose thickenings of the radial and internal tangential walls. These thickenings, saturated with suberin, are called Casparian belts. They are impermeable to water. Therefore, water enters the central cylinder only through passage cells. And since the absorbing surface of the root significantly exceeds the total cross-sectional area of the endodermal passage cells, root pressure arises, which is one of the mechanisms for the flow of water into the stem, leaf and reproductive organs.
Endoderm is also part of the bark of the young stem. In some herbaceous angiosperms, it, like the root, may have Casparian belts. In addition, in young stems the endodermis can be represented by a starch-bearing sheath. In this way, the endodermis can regulate water transport in the plant and store nutrients.
6.6. The concept of the stele and its evolution
Much attention is paid to the emergence, development in ontogenesis and evolutionary structural transformations of the conducting system, since it ensures the interconnection of plant organs and the evolution of large taxa is associated with it.
At the suggestion of the French botanists F. Van Tieghem and A. Dulio (1886), the set of primary conducting tissues, together with other tissues located between them and the pericycle adjacent to the bark, was called a stele. The stele may also include a core and a cavity formed in its place, as, for example, in bluegrass. The concept of “stele” corresponds to the concept of “central cylinder”. The stele of the root and stem is functionally unified. The study of stele in representatives of different divisions of higher plants led to the formation of the stele theory.
There are two main types of stele: protostele and eustele. The most ancient is the protostele. Its conducting tissues are located in the middle of the axial organs, with xylem in the center, surrounded by a continuous layer of phloem. There is no pith or cavity in the stem.
There are several evolutionarily related types of protostele: haplostele, actinostele and plectostele.
The original, primitive type is the haplostele. Its xylem has a rounded cross-section and is surrounded by an even, continuous layer of phloem. The pericycle is located around the conductive tissues in one or two layers [K. Esau, 1969]. The haplostele was known from fossil rhyniophytes and is preserved in some psilotophytes (tmesipterus).
A more developed type of protostele is the actinostele, in which the xylem in cross section takes the shape of a multi-rayed star. It is found in fossil Asteroxylon and some primitive lycophytes.
Further separation of the xylem into separate sections located radially or parallel to each other led to the formation of a plectostele, characteristic of lycophyte stems. In actinostele and plectostele, phloem still surrounds the xylem on all sides.
During evolution, the siphonostele arose from the protostele, distinctive feature which is a tubular structure. In the center of such a stele there is a core or cavity. In the conducting part of the siphonostele, leaf slits appear, due to which a continuous connection between the core and the bark occurs. Depending on the method of mutual arrangement of xylem and phloem, the siphonostele is ectophloic and amphiphloic. In the first case, the phloem surrounds the xylem on one, outer side. In the second, phloem surrounds the xylem on both sides, external and internal.
When the amphiphloic siphonostele is divided into a network or rows of longitudinal strands, a dissected stele, or dictyostele, appears, characteristic of many fern-like plants. Its conducting part is represented by numerous concentric conducting bundles.
In horsetails, an arthrostele arose from the ectophloic siphonostele, which has a segmented structure. It is distinguished by the presence of one large central cavity and separate vascular bundles with protoxylem cavities (carinal canals).
In flowering plants, on the basis of the ectophloic siphonostele, an eustele, characteristic of dicotyledons, and an ataxostele, typical of monocotyledons, were formed. In the eustela, the conducting part consists of separate collateral bundles that have a circular arrangement. In the center of the stele in the stem there is a core, which is connected to the bark with the help of medullary rays. In the ataxostele, the vascular bundles have a scattered arrangement; between them there are parenchyma cells of the central cylinder. This arrangement of the bundles hides the tubular structure of the siphonostele.
Emergence various options siphonosteles are an important adaptation of higher plants to increase the diameter of the axial organs - root and stem.
25. 8.1. Excretory system of plants and its significance
Plant life is a genetically determined set of biochemical reactions, the speed and intensity of which are significantly modified by the conditions of the growing environment. These reactions produce a wide variety of by-products that are not used by the plant to build the body or to regulate the exchange of substances, energy and information with the environment. Such products may be removed from the plant in different ways: with the death and separation of branches and sections of rhizomes, with the falling of leaves and peeling of the outer layers of the crust, as a result of the activity of specialized structures of external and internal secretion. Together, these devices form the excretory system of plants.
Unlike animals, the excretory system in plants is not aimed at removing nitrogen compounds, which can be reutilized in the process of life.
The excretory system of plants is multifunctional. In its structures the following are carried out: synthesis, accumulation, conduction and release of metabolic products. For example, in the secretory cells of the resin ducts in the leaves of coniferous trees, resin is formed, which is released through the resin ducts. In the nectaries of linden flowers, sweet nectar juice is formed and secreted. In special containers in the fruit shell of citrus fruits, they accumulate essential oils.
The formation and release of metabolic by-products has a variety of adaptive significance:
Attracting pollinating insects. The flowers of apple, cucumber and other entomophilous cross-pollinators produce nectar that attracts bees, and the foul-smelling secretions of the rafflesia flower attract flies;
Repelling herbivores (cumin, nettle, etc.);
Protection against bacteria and fungi that destroy wood (pine, spruce, etc.);
The release of volatile compounds into the atmosphere, which helps purify the air from pathogenic bacteria;
Extracellular digestion of prey in insectivorous plants due to the release of proteolytic enzymes (sundew, aldrovanda, etc.);
Mineralization of organic residues in the soil due to the release of special soil enzymes;
Regulation of the water regime through water stomata - hydathodes, located along the edge of the leaf blade (strawberries, cabbage, crassula, etc.);
Regulation of water evaporation as a result of the release of volatile ether compounds, which reduce the transparency and thermal conductivity of air near the surface of the sheet ( conifers);
Regulation of the salt regime of cells (pigweed, quinoa, etc.);
Changes in the chemical and physical properties of the soil, as well as regulation of the species composition of soil microflora under the influence of root exudates;
Regulation of the interaction of plants in a phytocenosis through root, stem and leaf secretions, called allelopathy (onions, garlic, etc.).
The substances released by plants are very diverse. Their nature depends on the genotype of the plants.
Many species produce water (strawberries, cabbage), salts (pigweed, quinoa), monosaccharides and organic acids (dandelion, chicory), nectar (linden, buckwheat), amino acids and proteins (poplar, willow), essential oils (mint, rose) , balsam (fir), resins (pine, spruce), rubber (hevea, kok-sagyz), mucus (root cap cells, swelling seeds different plants), digestive juices (sundew, butterwort), toxic liquids (nettle, hogweed) and other compounds.
26. 1.1. Polarity
Polarity is the presence of biochemical, functional and structural differences in diametrically opposed parts of the organs of an entire plant organism. Polarity affects the intensity of biochemical processes in the cell and the functional activity of organelles, and determines the design of anatomical structures. At the level of an integral plant organism, polarity is associated with the direction of growth and development under the influence of gravitational forces.
The phenomenon of polarity is observed at different levels of plant organization. At the molecular level, it manifests itself in the structure of molecules of organic substances, primarily nucleic acids and proteins. Thus, the polarity of DNA chains is determined by the special order in which its nucleotides are connected. The polarity of polypeptides is associated with the presence of an amino group –NH 2 and a carboxyl group –COOH in the amino acid composition. The polarity of chlorophyll molecules is due to the presence of a tetrapyrrole porphyrin core and alcohol residues - methanol and phytol.
Cells and their organelles can have polarity. For example, at the regeneration pole of the Golgi complex, new dictyosomes are formed, and at the secretory pole, vesicles are formed, which are associated with the removal of metabolic products from the cell.
Cell polarity arises during their ontogenesis. Cell polarization occurs as a result of the emergence of pH gradients, electrical charges and osmotic potential, concentrations of O 2 and CO 2, calcium cations, physiologically active substances and mineral nutrition elements. It can also occur under the influence of mechanical pressure, forces surface tension, influence of neighboring cells.
Cell polarity can be symmetrical or asymmetrical. Symmetrical polarity is an indispensable condition for the division of the original cells and the formation of equivalent daughter cells. In particular, microtubules of the cytoskeleton move to the equatorial plane and participate in the formation of the phragmoplast; chromosomes divide and their chromatids, with the help of the pulling threads of the achromatin spindle, diverge to opposite poles of the cell. This is where ribosomes and mitochondria begin to concentrate. Golgi complexes move to the center and are specifically oriented in space. Their secretory poles are directed towards the equator of the dividing cell, which ensures the active participation of this organelle in the formation of the middle plate and the primary cell membrane.
Asymmetrical polarity is characteristic of specialized cells. Thus, in a mature egg, the nucleus is shifted to the pole oriented towards the chalaza of the ovule, and a large vacuole is located at the micropillar pole.
The asymmetrical polarity is even more pronounced in the fertilized egg - the zygote. In it, the nucleus, surrounded by tubules of the endoplasmic reticulum, is located on the chalazal side of the cell. Here, the elements of the cytoskeleton are more densely located, the optical density of the cytoplasm increases, and an increased content of enzymes, phytohormones and other physiologically active substances is noted. Its functionally more active part becomes apical, and the opposite part becomes basal.
From the apical cell during the development of the seed embryo, an embryonic stalk with a growth cone and primordial leaves is formed. The basal cell forms the pendulum, and later the spinal root.
Due to the polarity of the cells of the developing embryo, the mature seed also becomes polarized. For example, in a wheat seed there is an endosperm on one side and an embryo on the other, in which the apical meristem of the stem and the root tip are at different poles.
Unlike cells, the polarity of plant organs is more universal. It manifests itself in the structure of the shoot and root system, as well as their components. For example, the basal and apical parts of the shoot differ morphologically, anatomically, histologically, biochemically and functionally. In many flowering plants, at the top of the shoot there is an apical bud, in which the most important morphogenetic center of the plant is located - the growth cone. Thanks to its activity, the primordia of leaves, lateral axillary buds, nodes and internodes are formed. Differentiation of the cells of the primary meristem leads to the appearance of primary integumentary tissues, histological elements of the primary cortex and the central cylinder.
The tip of the shoot is a powerful attracting center. The main flow of water and mineral nutrition elements dissolved in it and organic substances synthesized in the root are sent here. The preferential supply of cytokinin to the apical bud leads to apical dominance. The cells of the apex of the apical bud actively divide, ensuring the growth of the stem in length and the formation of new leaves and axillary buds on the main axis. In this case, a correlative inhibition of the development of axillary buds is observed.
A good example The morphological polarity of the shoot is the structure of the wheat stem. As you move from bottom to top, from the basal part of the shoot to the apical part, the internodes become longer, their thickness in the middle part first increases and then gradually decreases. In the leaves of the upper tier, the ratio of the width of the leaf blade to its length is significantly greater than in the leaves of the lower tier. The position of the leaves in space also changes. In wheat, the leaves of the lower tier droop down, the leaves of the middle tier are located almost plagiotropically, i.e. parallel to the surface of the earth, and the flag (upper) leaves tend to an orthotropic - almost vertical - position.
The polarity of the shoot is clearly visible at the anatomical level. In wheat species, the subspike internode, compared to lower internodes, is characterized by a smaller diameter, fewer vascular bundles, and better developed assimilation parenchyma.
In dicotyledonous plants, the anatomical polarity of the shoot is enhanced due to the emergence of secondary lateral educational tissues - cambium and phellogen. Phellogen gives rise to secondary integumentary tissue phellem, which, saturated with suberin, turns into a cork. In tree species, the activity of phellogen leads to the formation of a tertiary integumentary complex - a crust. The cambium provides the transition to the secondary anatomical structure of the stem below the level of the apical bud.
Roots, like other plant organs, are also characterized by structural and functional polarity. Due to positive geotropism, the basal part of the root is located at the soil surface. It is directly connected to the root collar - the place where the root passes into the stem. The apical part of the root is usually buried in the soil. It undergoes interconnected processes of growth and development. Cell division of the apex meristem ensures linear growth of the root. And as a result of cell differentiation, qualitatively new structures are formed. Root hairs appear on epiblema cells. The elements of the primary cortex are formed from the cells of the periblema, and the pericycle and the conducting elements of the central cylinder are formed from the pleroma. This design of young root sections ensures active absorption of water and minerals, as well as their supply to higher located sections of the root. The functions of the basal part of the root are somewhat different. Thus, in perennial dicotyledons, the basal part of the root performs transport, supporting and storage functions. The anatomical structure corresponds to the performance of these functions. Conductive tissues are better developed here, the bark is formed due to the activity of the cambium, cover tissue represented by a cork. The polarity of the root structure ensures the diversity of its functions.
Polarity is also characteristic of the reproductive organs of plants. Thus, the flower, being a modified shortened shoot, retains the signs of shoot polarity. The parts of the flower are located on it in a natural sequence: calyx, corolla, androecium and gynoecium. This arrangement contributes to better capture of pollen by the stigma of the pistil, and also provides protection for the generative parts of the flower by the vegetative parts. The polarity of the inflorescences is very indicative. In indeterminate inflorescences, the flowers of the basal part are formed first. They reach large sizes and form fruits with well-developed seeds. In particular, larger seeds in sunflower are formed in the peripheral part of the basket, and in the complex ear of wheat best seeds are formed in the first flowers of the spikelets. The flowers of the apical part of the inflorescences form later. The fruits and seeds obtained from them are smaller, and their sowing qualities are lower.
Thus, polarity is an important structural, functional and biochemical feature of plants, which has adaptive significance and which must be taken into account in agronomic practice.
1.2. Symmetry
The world around us is characterized by integrity and harmonious order. In nearby outer space, the position, mass, shape and trajectory of objects in the Solar System are harmonious. On Earth, seasonal and daily changes in the most important physical parameters of the conditions of existence of living organisms are harmonious, which are also characterized by a harmonious coordination of structure and functions. The sages of the school of Pythagoras believed that harmony is “a way of harmonizing many parts, with the help of which they are united into a whole.” Symmetry is a reflection of harmony in nature. According to the definition of Yu.A. Urmantsev symmetry is a category that denotes the preservation of features of objects relative to their changes. In utilitarian terms, symmetry represents the uniformity of structure and mutual arrangement of similar components of a single whole. Symmetry is inherent in both minerals and living things. However, the forms of symmetry and the degree of their manifestation in different objects vary significantly.
An essential feature of the symmetry of objects of different origin is its correlative nature. Comparison of structures is carried out at several characteristic points. Points of a figure that give the same picture when viewing the figure from different sides are called equal. These can be equidistant points on a straight line, points of intersection of the sides of an isosceles triangle and a square, faces of a polyhedron, points on a circle or on the surface of a ball, etc.
If in a figure for some point X there is no other equal point, then it is called singular. Figures that have one special point and several equal points are called rosettes. Shapes that do not contain equal points are considered asymmetrical.
Axes and planes of symmetry pass through singular points. In an isosceles triangle there can be three, in a square - four, in an equilateral pentagon - five planes of symmetry. Accordingly, two rays of symmetry are formed on a straight line, a triangle is characterized by three-ray symmetry, a square is characterized by four-ray symmetry, and a circle is characterized by multi-ray symmetry.
Movements of a figure, as a result of which each point is replaced by an equal point, and each singular point remains in place, are called symmetry transformations, and figures for which a symmetry transformation is permissible are considered symmetrical.
The most common forms of symmetry transformation are:
1. Reflection - the movement of each point located at a certain distance from a fixed plane along a straight line perpendicular to this plane to the same distance on the other side of it (for example, the mirror symmetry of a zygomorphic pea flower);
2. Rotate – moving all points at a certain angle around a fixed axis (for example, multi-ray symmetry of an actinomorphic cherry flower);
3. Parallel transfer, for example, the location of metamers on the shoot.
The forms of symmetry transformation are not identical and cannot be reduced to one another. Thus, in the case of reflection, the plane remains motionless; when turning – straight (axis); and with parallel transfer, not a single point remains in place.
Symmetrical figures with several equal points can have a different number of singular points. So, the socket has one special point. The main form of symmetry transformation for it is rotation. Figures that do not have special points are characterized by parallel translation, or shift. Such figures are conventionally called infinite.
2.2.1. Features of the manifestation of symmetry in plants. The symmetry of plants differs from the symmetry of crystals in a number of features.
1) The symmetry of plants is determined not only by the symmetry of the molecules that form their cells, but also by the symmetry of conditions environment, in which plant development takes place.
Special cases of symmetry of the habitat can be considered the uniformity of the composition of the root layer of the soil, the uniform distribution of water and mineral nutrition elements in the soil, and the equal distance of plants growing nearby. These circumstances must be taken into account when growing cultivated plants.
2) Plants, like other living organisms, do not have absolute identity of the elements of their constituent parts. This is primarily determined by the fact that the conditions for the formation of these parts are not absolutely identical. On the one hand, this is due to the different times in which the growth cone isolates different metameres, including the primordia of leaves, buds, nodes and internodes. The same applies to generative organs. Thus, the marginal tubular flowers in the sunflower basket are always larger than the flowers in the central part of the inflorescence. On the other hand, during the growing season, both regular and random changes in development conditions are observed. For example, the air and soil temperature, illumination and spectral composition of light change statistically during the growing season. Local changes in the supply of nutrients in the soil and the effects of diseases and pests can be random.
The absence of absolute identity of constituent parts is of great importance in the life of plants. The emerging heterogeneity is one of the mechanisms of ontogeny reliability.
3) Plants are characterized by substantial, spatial and temporal symmetry.
A. Substantial symmetry consists in the exact repetition of the shape and linear parameters of structures, as well as in the exact change of these parameters. It is characteristic of crystals of inorganic substances, molecules of organic compounds, cell organelles, anatomical structures and organs of plants. High level DNA molecules differ in their substance symmetry. When the humidity of the preparations is close to physiological, the DNA molecule is in the B-form and is characterized by a clear repetition of the parameters of its constituent elements. Each turn of the DNA molecule contains 10 nucleotides; the projection of the turn onto the axis of the molecule is 34.6 Å (1 Å = 1·10–10 m); the projection distance between neighboring nucleotides is 3.4 Å, and the linear distance is 7 Å; the diameter of the molecule when oriented along phosphorus atoms is close to 20 Å; The diameter of the major groove is approximately 17 Å, and that of the minor groove is 11 Å.
The substantial symmetry of plants is characterized not only by the exact repetition of structural parameters, but also by their regular change. For example, in spruce, the decrease in trunk diameter per 1 m of length when moving from its basal part to the top is relatively constant. The violet canna (Canna violacea) has a flower that is traditionally considered asymmetrical. Its sepals, petals and staminodes have different sizes. However, the change in the linear dimensions of these flower members remains relatively constant, which is a sign of symmetry. Only here, instead of mirror symmetry, other forms of symmetry develop.
B. Spatial symmetry consists in the naturally repeating spatial arrangement of similar components in plants. Spatial symmetry is widespread in the plant world. It is characteristic of the arrangement of buds on a shoot, flowers in an inflorescence, flower members on a receptacle, pollen in an anther, scales in gymnosperm cones, and many other cases. A special case of spatial symmetry is longitudinal, radial and mixed symmetry of the stem.
Longitudinal symmetry occurs during parallel transfer, i.e. spatial repetition of metamers in the shoot structure. It is determined by the structural similarity of the constituent parts of the shoot - metamers, as well as by the compliance of the length of internodes with the rule of the “golden section”.
A more complex case of spatial symmetry is the arrangement of leaves on the shoot, which can be whorled, opposite, or alternate (spiral).
Radial symmetry occurs when the axis of a structure is combined with planes of symmetry passing through it. Radial symmetry is widespread in nature. In particular, it is characteristic of many species of diatoms, a cross section of the stem of higher spore plants, gymnosperms and angiosperms.
The radial symmetry of the shoot is multi-element. It characterizes the location of the component parts and can be expressed by different dimensional indicators: the distance of vascular bundles and other anatomical structures from a special point through which one or several planes of symmetry pass, the rhythmic alternation of anatomical structures, the divergence angle, which shows the displacement of the axis of one structure relative to axis is different. For example, in a cherry flower, five corolla petals are spatially symmetrically arranged. The place of attachment of each of them to the receptacle is at the same distance from the center of the flower, and the divergence angle will be equal to 72º (360º: 5 = 72º). In a tulip flower, the divergence angle of each of the six petals is 60º (360º: 6 = 60º).
C. Temporal symmetry of plants is expressed in the rhythmic repetition in time of the processes of morphogenesis and other physiological functions. For example, the cone of growth of leaf primordia is isolated at more or less equal intervals of time, called a plastochron. Seasonal changes in vital processes in perennial polycarpic plants are repeated very rhythmically. Temporal symmetry reflects the adaptation of plants to daily and seasonal changes in environmental conditions.
4) Plant symmetry develops dynamically during ontogenesis and reaches its maximum expression during sexual reproduction. In plants, an indicative example in this regard is the formation of radial symmetry of the shoot. Initially, the cells of the growth cone are more or less homogeneous and not differentiated. Therefore, it is morphologically difficult to identify a cell or group of cells through which a plane of symmetry could be drawn.
Later, the tunica cells form the protodermis, from which the epidermis is formed. From the main meristem, assimilating and storage tissues develop, as well as primary mechanical tissues and the core. In the peripheral zone, cords of narrow and long procambium cells are formed, from which conducting tissues will develop. When the procambium is completely laid down, continuous layers of phloem and xylem are formed from it. If the procambium is laid down in the form of cords, then separate conducting bundles are formed from it. In woody plants, the perennial, seasonally changing functional activity of the cambium will lead to the formation of secondary bark and annual wood rings, which will enhance the radial symmetry of the stem.
5) The symmetry of plants changes during evolution. Evolutionary transformations of symmetry are of great importance in the development of the organic world. Thus, the emergence of bilateral symmetry was a major morphophysiological adaptation (aromorphosis), which significantly increased the level of organization of animals. In the plant kingdom, a change in the number of basic types of symmetry and their derivatives is associated with the emergence of multicellularity and the emergence of plants on land. In unicellular algae, in particular in diatoms, radial symmetry is widespread. In multicellular organisms, various forms of parallel transfer appear, as well as mixed forms of symmetry. For example, in charophyte algae, the radial symmetry of the cross section of the thallus is combined with the presence of an axis of metamer transfer.
The evolution of the conduction system was of great importance in the formation of the symmetry of higher plants. The radially symmetrical protostele of the stem of primitive forms was replaced by a complex of bilaterally symmetrical (monosymmetric) collateral bundles that form the eustele and provide radial symmetry of the stem in angiosperms.
The general trend in the evolution of symmetry in living organisms, including plants, is a decrease in the level of symmetry. This is due to a decrease in the number of basic types of symmetry and their derivatives. Thus, evolutionarily earlier flowering plants (the Magnoliaceae family, the Ranunculaceae family, etc.) are characterized by polynomial, free, spirally arranged flower parts. The flower turns out to be actinomorphic, i.e. polysymmetrical. The formulas of such flowers tend to be: Å Ca ¥ Co ¥ A ¥ G ¥ . During evolution, a reduction in the number of flower members, as well as their fusion, is observed, which invariably leads to a decrease in the number of planes of symmetry.
In evolutionarily young families, for example, in the Lamiaceae or Poagrass, the flowers become zygomorphic (monosymmetric). Dissymmetrization of flowers clearly has an adaptive significance associated with the improvement of pollination methods. This was often facilitated by the conjugate evolution of the flower and pollinators - insects and birds.
The specialization of the symmetry of flowers in an inflorescence has acquired great importance. Thus, in the sunflower basket, the sterile marginal zygomorphic flowers have a large false tongue yellow, formed by three fused petals. The fertile flowers in the central part of the inflorescence are actinomorphic; they are formed by five small equal-sized petals fused into a tube. Another example is representatives of the Celery family. In their complex umbel, the marginal flowers are weakly zygomorphic, while the other flowers remain typically actinomorphic.
The evolution of plant structure and symmetry is not straightforward. The predominant role of dissymmetrization is replaced and supplemented at certain stages of evolution by symmetrization.
Thus, the symmetry of plants and their components is very multifaceted. It is associated with the symmetry of its constituent elements at the molecular, cellular, histological-anatomical and morphological level. Symmetry develops dynamically during ontogenesis and phylogeny and ensures the connection of plants with the environment.
1.3. Metamerism
An important morphophysiological adaptation of plants is metamerism, which is the presence of repeating elementary finite structures, or metamers, in the system of a whole organism. The metameric structure ensures multiple repetitions of the constituent parts of the shoot and, therefore, is one of the mechanisms of ontogeny reliability. The metameric structure is characteristic of different systematic groups of plants. It is known from charophyte algae, horsetails and other higher spore plants, gymnosperms, terrestrial and aquatic angiosperms. Functionally different parts of plants are metameric - vegetative and generative. With the participation of metamerically arranged organs, a metameric system is formed in plants. A special case of the formation of metameric systems is branching.
The metamere of the vegetative zone of an angiosperm shoot includes a leaf, a node, an internode and a lateral axillary bud, which is located at the base of the internode on the side opposite to the place of leaf attachment. This bud is covered by the leaf of the previous metamer. The metameres of the generative zone are very diverse. For example, in wheat, the metameres of a complex spike consist of a segment of the spike shaft and a spikelet attached to it. Sometimes plants have a shoot transition zone. In some species and varieties of wheat, it can be represented by scales of underdeveloped spikelets.
Metamerism of plants is a morphological expression of the specificity of their growth and morphogenesis, which proceed rhythmically, in the form of repeating subordinate cycles localized in the foci of the meristem. The rhythm of the formation of metamers is inextricably linked with the periodicity of growth processes characteristic of plants. The formation of metamers is the primary morphogenetic process in plant development. It forms the basis for the complication of organization in ontogenesis and reflects the process of polymerization, which is one of the mechanisms of evolution of higher plants.
The formation and development of metameres is ensured primarily by the function of the apical and intercalary meristems.
In the vegetative bud, as well as in the embryo of the germinating seed, as a result of mitotic division, the volume of the growth cone of the embryonic stalk increases. Subsequent active division of cells in the peripheral zone of the growth cone leads to the formation of a leaf primordium - leaf primordium and insertion disk. At this time, the meristematic activity of the tunica and the central meristematic zone decreases somewhat, but the cells of the insertion disk are actively dividing. The upper part of the disc is the site of attachment of the leaf primordium and, as it grows in thickness, a node is formed from it. An internode develops from the lower part of the insertion disk. Here the rudimentary tubercle of the lateral axillary bud is formed on the side opposite the midrib of the primordial leaf. Collectively, the leaf primordium, insertion disk, and bud primordium constitute a rudimentary metamer.
As the formation of the embryonic metamer is completed, the activity of cell division in the tunica and central meristematic zone increases again. The volume of the smooth part of the growth cone increases again, reaching a maximum before the initiation of the next leaf primordium. Thus, a new one begins to form on the apical part of the previous rudimentary metamere. This process is genetically determined and rhythmically repeats itself many times. In this case, the metamer formed first will be located in the basal part of the shoot, and the ontogenetically youngest one will be located in the apical part. Accumulation of the number of metamer primordia in the kidney T.I. Serebryakova called it maturation. To denote the maximum number of metamers deposited in the kidney, she proposed the term “kidney capacity.”
The growth and development of internodes, as components of a metamer, are largely determined by the activity of the intercalary meristem. The division of cells of this meristem and the elongation of their derivatives leads to the elongation of internodes.
Metamers have a number characteristic features, which allow them to ensure the structural and functional integrity of the plant organism.
1) Polarity of metamers. Each metamer has a basal and an apical part. The term “apical part” suggests that the upper part of the metamere either contains an apical meristem or is oriented towards the apex.
The basal and apical parts differ in morphological, histological-anatomical and physiological-biochemical characteristics. For example, in wheat, when moving from the basal to the apical part of the internode, the thickness of the stem, the diameter of the medullary lacuna and the thickness of the peripheral sclerenchyma ring first increase and then gradually decrease; the number of cells in the strands of assimilation parenchyma increases significantly; The radial diameter of vascular bundles decreases, as does the number of vessels in the xylem.
2) Symmetry of metamers. The symmetry of plant organs is ensured by the symmetry of the metamers that form them, which arises as a result of a specific sequence of cell division in the growth cone.
3) Heterochronicity of the formation of metamers. Metamers are isolated at different times, alternately, by a growth cone. Therefore, the first metameres of the basal part of the shoot are ontogenetically older, and the last metameres of the apical part are younger. Morphologically, they differ in leaf parameters, length and thickness of internodes. For example, in cereals, the leaves of the upper tier are wider than the leaves of the lower tier, and the subspike internodes are longer and thinner than the lower ones.
4) Variability of characteristics of metamers. Ontogenetically younger metameres have a smaller amplitude of trait variability. Therefore, the anatomical characteristics of the subspike internode can be used with greater accuracy to identify varieties cereal crops and drawing up breeding programs.
5) Optimal design of metamers. The optimal biological structure is one that requires a minimum amount of organic matter to build and maintain. Since the structure of plants is inextricably linked with their function, the criterion of optimality can be the adaptability of plants to growing conditions and the ratio of their seed productivity to the mass of vegetative organs. The optimal design of the entire plant is ensured by the optimal design of its constituent metamers.
6) A complete plant is a polymer system formed by a set of metamers. Polymerity manifests itself at all levels of organization of the plant organism. Thus, in the DNA structure there are many repetitions of genes; The karyotype of many plants is represented by a diploid or polyploid set of chromosomes; plastids, mitochondria, ribosomes and other organelles are found in large quantities in the cell. An adult plant is a subordinate set of shoots, roots and reproductive organs that have a metameric structure. The metameric system ensures high productivity of plants and significantly increases the reliability of their ontogenesis.
27. 4.1. Primary anatomical structure of the root
The features of the primary structure are clearly manifested in longitudinal and transverse sections of the root tip.
On longitudinal section four zones can be distinguished at the root tip
The root cap zone covers the apical meristems of the root. It consists of living cells. Their surface layer constantly peels off and lines the passage along which the root moves. The sloughing cells also produce mucus, which facilitates the movement of the root tip in the soil. The cells of the central part of the cap, or columella, contain starch grains, which contributes to the geotropic growth of the root. The cells of the cap are constantly renewed due to the division of cells of a special educational tissue - calyptrogen, characteristic of monocots.
I – root cap zone; II – growth zone; III – zone of root hairs; IV – holding area. 1 – epiblema, 2 – pericycle, 3 – endoderm, 4 – primary cortex, 5 – exodermis, 6 – central cylinder, 7 – root hair; 8 – formation of a lateral root.
The growth zone consists of two subzones. In the division subzone, root growth occurs due to active mitotic cell division. For example, in wheat, the proportion of dividing cells (mitotic index) is 100–200 ppm. The division subzone is a valuable material for cytogenetic studies. Here it is convenient to study the number, macro- and microstructure of chromosomes. In the elongation subzone, the meristematic activity of cells decreases, but due to a certain balance of phytohormones, primarily auxins and cytokinins, root growth occurs due to axial elongation of young cells.
The absorption zone can rightfully be called the root hair zone, as well as the differentiation zone, since the epiblema, primary cortex and central cylinder are formed here.
Epiblema is a special, constantly renewed integumentary tissue consisting of two types of cells. Thin-walled root hairs 1–3 mm long develop from trichoblasts, thanks to which water and substances dissolved in it are absorbed. Root hairs are short-lived. They live for 2 - 3 weeks and then desquamate. Atrichoblasts do not form root hairs and perform an integumentary function.
The primary cortex regulates the flow of water into the conducting tissues of the central cylinder. When the central cylinder is formed, protophloem tissues are the first to form, through which organic substances necessary for its growth enter the root tip. Higher up the root, at the level of the first root hairs, vascular elements of the protoxylem appear. The initial vascular tissues develop into the primary phloem and primary xylem of the radial vascular bundle.
The conduction zone occupies the largest part of the root. It performs numerous functions: transport of water and mineral and organic substances dissolved in it, synthesis of organic compounds, storage of nutrients, etc. The conduction zone ends with the root collar, i.e. the transition point between the root and the stem. In dicotyledonous angiosperms, at the beginning of this zone there is a transition from the primary to the secondary anatomical structure of the root.
A cross section in the area of root hairs reveals structural and topographical features of the primary anatomical structure of the root (Fig. 2). On the surface of the root there is an epiblema with root hairs, with the help of which water and substances dissolved in it are absorbed. Unlike the epidermis, the epiblema is not covered with a cuticle and does not have stomata. Below it is the primary cortex, consisting of exoderm, mesoderm and endoderm.
1 – root tip zones; 2 – 6 – transverse sections at different levels; Vx - secondary xylem; Vf – secondary phloem; K – cambium; Mx – metaxylem; Mf – metaphloem; Pd – protoderm; Px – protoxylem; Prd – periderm; Pf – protophloem; Pc – pericycle; Rd – rhizoderm; Ex – exodermis; En – endoderm; C – cover.
Exodermal cells are densely folded and have thickened membranes. The exoderm provides the apoplastic supply of water to the deeper layers of the cortex, and also gives the root strength from the surface.
The middle layer of the cortex, mesoderm, consists of thin-walled parenchyma cells and has a loose structure due to the presence of numerous intercellular spaces through which water moves to the exoderm.
The endoderm, or inner layer of the primary cortex, consists of a single row of cells. It consists of cages with Casparian belts and passage cages. Cells with Casparian belts have thickened lateral (radial) and tangential (end) membranes facing the central cylinder. These cellulose thickenings are impregnated with lignin and therefore cannot pass water. In contrast, passage cells are thin-walled and located opposite the xylem rays. It is through the passage cells that water enters the radial conductive bundle of the central cylinder.
The central cylinder of the root, or stele, consists of several layers of cells. Immediately below the endodermis there are one or more rows of pericycle cells. Of these, during the transition to the secondary structure of the root in dicotyledonous angiosperms and gymnosperms, an interfascicular cambium and phellogen (cork cambium) are formed. In addition, pericycle cells participate in the formation of lateral roots. Behind the pericycle there are cells of the radial vascular bundle.
The vascular bundle is formed from procambium. First, protophloem cells are formed, and then at the level of the first root hairs - protoxylem cells. Cells of conducting tissues arise exarchically, i.e. from the surface of the beam, and subsequently develop in the centripetal direction. In this case, the earliest, larger vessels are located in the center of the xylem, and the younger ones, of smaller diameter, are on the periphery of the xylem ray. Phloem cells are located between the xylem rays.
The number of xylem rays depends on the systematic position of the plants. For example, some ferns may have only one xylem ray and one section of phloem. Then the bun is called monarch. Many dicotyledonous angiosperms are characterized by diarchic bundles with two xylem rays. In addition, they have plants with tri-, tetra- and pentarchy bundles. In monocotyledonous angiosperms, polyarchal vascular bundles with multiradiate xylem are typical.
The structure of the radial vascular bundle affects the way lateral roots are formed. In a diarchic root they form between the phloem and xylem, in a triarchic and tetrarchic root they form opposite the xylem, and in a polyarchal root they form opposite the phloem.
The cellular structure of the epiblema and primary cortex ensures the occurrence of root water pressure. Water is absorbed by root hairs, leaves them in the cell membranes of the exoderm, then enters the intercellular spaces of the mesoderm, and from them through thin-walled passage cells into the vessels of the radial vascular bundle. Since root hairs have a larger total surface area than passage cells, the speed of water movement, and therefore its pressure, increases as it approaches the vessels. The difference in pressure that occurs constitutes root pressure, which is one of the mechanisms for the flow of water into the stem and other plant organs. The second important mechanism for moving water is transpiration.
28. 4.2. Secondary anatomical structure of the root
In dicotyledonous angiosperms and gymnosperms, the primary anatomical structure of the root in the conduction zone is supplemented by structures of secondary origin, which are formed due to the emergence and meristematic activity of secondary lateral educational tissues - cambium and phellogen (cork cambium) (Fig. 3). When moving to a secondary structure, the following significant changes occur fundamentally.
A. The appearance in the central cylinder of the cambium root and the secondary xylem and secondary phloem generated by it, which bandwidth are much superior to the elements of the original radial conductive beam.
Rice. Cellular structure of an annual linden stem. Longitudinal and transverse sections: 1 - system of integumentary tissues (from the outside to the inside; one layer of the epidermis, cork, primary cortex); 2-5 - bast : 2 - bast fibers, 3 - sieve tubes, 4 - satellite cells, 5 - bast parenchyma cells; 6 - cambium cells, stretched and differentiated in the outer layers; 7-9 cellular elements of wood: 7 - vascular cells, 8 - wood fibers, 9 - wood parenchyma cells ( 7 , 8 And 9 shown also large); 10 - core cells.
Water and minerals supplied through the root must reach all parts of the plant, while the substances produced in the leaves during photosynthesis are also intended for all cells. Thus, a special system must exist in the plant body to ensure the transport and redistribution of all substances. This function is performed in plants conductive fabrics. There are two types of conductive fabrics: xylem (wood) And phloem (bast). Along the xylem it is carried out rising current: movement of water with mineral salts from the root to all organs of the plant. It goes along the phloem downward current: transport of organic substances coming from leaves. Conducting tissues are complex tissues because they consist of several types of differently differentiated cells.
Xylem (wood). Xylem consists of conducting elements: vessels, or trachea, And tracheid, as well as from cells that perform mechanical and storage functions.
Tracheids. These are dead elongated cells with obliquely cut pointed ends (Fig. 12).
Their lignified walls are greatly thickened. Typically, the length of tracheids is 1-4 mm. Arranged in a chain one after another, tracheids form the water-conducting system in ferns and gymnosperms. Communication between neighboring tracheids occurs through pores. By filtration through the pore membrane, both vertical and horizontal transport of water with dissolved minerals. The movement of water through the tracheids occurs at a slow speed.
Vessels (trachea). The vessels form the most perfect conducting system, characteristic of angiosperms. They are a long hollow tube consisting of a chain of dead cells - vessel segments, in the transverse walls of which there are large holes - perforations. These holes allow rapid flow of water. Vessels are rarely single; they are usually located in groups. The diameter of the vessel is 0.1 - 0.2 mm. At an early stage of development, cellulose thickenings are formed from the xylem procambium on the inner walls of the vessels, which subsequently become lignified. These thickenings prevent the vessels from collapsing under the pressure of neighboring growing cells. First are formed ringed And spiral thickenings that do not prevent further cell elongation. Later, wider vessels appear with staircases thickenings and then porous vessels that are characterized by the largest thickening area (Fig. 13).
Through non-thickened areas of vessels (pores), horizontal transport of water occurs into neighboring vessels and parenchyma cells. The appearance of vessels in the process of evolution provided angiosperms with high adaptability to life on land and, as a result, their dominance in the modern vegetation cover of the Earth.
Other xylem elements. In addition to conducting elements, xylem also includes wood parenchyma and mechanical elements - wood fibers, or libriform. Fibers, like vessels, arose in the process of evolution from tracheids. However, unlike vessels, the number of pores in the fibers decreased and an even thicker secondary shell formed.
Phloem (bast). Phloem carries out a downward flow of organic substances - products of photosynthesis. Phloem contains sieve tubes, satellite cells, mechanical (bast) fibers and bast parenchyma.
Sieve tubes. Unlike the conducting elements of xylem, sieve tubes are a chain of living cells (Fig. 14).
The transverse walls of two adjacent cells that make up the sieve tube are penetrated a large number through holes forming a structure resembling a sieve. This is where the name sieve tubes come from. The walls that support these holes are called sieve plates. Through these openings the transport of organic substances from one segment to another occurs.
The segments of the sieve tube are connected by peculiar pores to companion cells (see below). The tubes communicate with parenchyma cells through simple pores. Mature sieve cells lack a nucleus, ribosomes and the Golgi complex, and their functional activity and vital activity are supported by satellite cells.
Companion cells (accompanying cells). They are located along the longitudinal walls of the sieve tube segment. Companion cells and sieve tube segments are formed from common mother cells. The mother cell is divided by a longitudinal septum, and from the two resulting cells, one turns into a segment of the sieve tube, and from the other one or more companion cells develop. Companion cells have a nucleus, cytoplasm with numerous mitochondria, active metabolism occurs in them, which is associated with their function: to ensure the vital activity of nuclear-free sieve cells.
Other elements of phloem. The composition of phloem, along with conducting elements, includes mechanical bast (phloem) fibers And phloem parenchyma.
Conductive bundles. In a plant, conducting tissues (xylem and phloem) form special structures - conducting bundles. If the bundles are partially or completely surrounded by strands of mechanical tissue, they are called vascular-fibrous bundles. These bundles penetrate the entire body of the plant, forming a single conducting system.
Initially, conducting tissues are formed from cells of the primary meristem - procambia. If, during the formation of a bundle, the procambium is completely spent on the formation of primary conducting tissues, then such a bundle is called closed(Fig. 15).
It is not capable of further (secondary) thickening because it does not contain cambial cells. Such bunches are characteristic of monocotyledonous plants.
In dicotyledons and gymnosperms, a part of the procambium remains between the primary xylem and phloem, which later becomes fascicular cambium. Its cells are capable of dividing, forming new conductive and mechanical elements, which ensures secondary thickening of the bundle and, as a consequence, growth of the stem in thickness. The vascular bundle containing the cambium is called open(see Fig. 15).
Depending on the relative position of xylem and phloem, several types of vascular bundles are distinguished (Fig. 16)
Collateral bundles. Xylem and phloem are adjacent to each other side by side. Such bunches are characteristic of the stems and leaves of most modern seed plants. Typically, in such bundles, the xylem occupies a position closer to the center of the axial organ, and the phloem faces the periphery.
Bicollateral bundles. Two strands of phloem adjoin the xylem side by side: one - with inside, the other - from the periphery. The peripheral strand of phloem mainly consists of secondary phloem, the internal strand consists of primary phloem, as it develops from procambium.
Concentric beams. One conducting tissue surrounds another conducting tissue: xylem - phloem or phloem - xylem.
Radial beams. Characteristic of plant roots. The xylem is located along the radii of the organ, between which there are strands of phloem.
CONDUCTIVE FABRICS
Conductive tissues transport nutrients in two directions. Ascending (transpiration) current liquids (aqueous solutions and salts) goes through vessels And tracheids xylem (Fig. 32) from the roots up the stem to the leaves and other plant organs. Descending current (assimilation) organic matter is carried from the leaves along the stem to the underground organs of the plant through
special sieve tubes phloem (Fig. 33). The conducting tissue of the plant is somewhat reminiscent of the human circulatory system, since it has an axial and radial highly branched network; nutrients enter every cell of a living plant. In each plant organ, xylem and phloem are located side by side and are presented in the form of strands - conducting bundles.
There are primary and secondary conducting tissues. Primary ones differentiate from procambium and are formed in young plant organs; secondary conducting tissues are more powerful and are formed from cambium.
Xylem (wood) presented tracheids And trachea, or vessels.
Tracheids- elongated closed cells with obliquely cut jagged ends, in a mature state they are represented by dead prosenchymal cells. The length of the cells is on average 1 - 4 mm. Communication with neighboring tracheids occurs through simple or bordered pores. The walls are unevenly thickened; according to the nature of the thickening of the walls, tracheids are distinguished as annular, spiral, scalariform, reticulated and porous (Fig. 34). Porous tracheids always have bordered pores (Fig. 35). Sporophytes of all higher plants have tracheids, and in most horsetails, lycophytes, pteridophytes and gymnosperms they serve as the only conducting elements of the xylem. Tracheids
perform two main functions: conduction of water and mechanical strengthening of the organ.
Trachea, or vessels, are the main water-conducting elements of the xylem of angiosperms. Tracheas are hollow tubes consisting of individual segments; in the partitions between the segments there are holes - perforation, thanks to which the fluid flow is carried out. Tracheas, like tracheids, are a closed system: the ends of each trachea have beveled transverse walls with bordered pores. The tracheal segments are larger than the tracheids: in diameter they are about different types plants from 0.1 - 0.15 to 0.3 - 0.7 mm. The length of the trachea ranges from several meters to several tens of meters (for lianas). The trachea consists of dead cells, although in the initial stages of formation they are alive. It is believed that tracheae arose from tracheids in the process of evolution.
In addition to the primary shell, most vessels and tracheids have secondary thickenings in the form of rings, spirals, ladders, etc. Secondary thickenings form on the inner wall of the vessels (see Fig. 34). Thus, in an annular vessel, the internal thickenings of the walls are in the form of rings located at a distance from each other. The rings are located across the vessel and slightly obliquely. In a spiral vessel, the secondary membrane is layered from the inside of the cell in the form of a spiral; in a mesh vessel, the non-thickened areas of the shell look like slits, reminiscent of mesh cells; in the scalene vessel, thickened places alternate with non-thickened ones, forming something like a ladder.
Tracheids and vessels - tracheal elements - are distributed in the xylem in different ways: in a cross section in continuous rings, forming ring-vascular wood, or scattered more or less evenly throughout the xylem, forming scattered vascular wood. The secondary shell is usually impregnated with lignin, giving the plant additional strength, but at the same time limiting its growth in length.
In addition to vessels and tracheids, xylem includes beam elements, consisting of cells forming the medullary rays. The medullary rays consist of thin-walled living parenchyma cells through which nutrients flow horizontally. The xylem also contains living wood parenchyma cells, which function as short-range transport and serve as a storage site for reserve substances. All xylem elements come from the cambium.
Phloem- conductive tissue through which glucose and other organic substances are transported - products of photosynthesis from leaves to places of their use and deposition (to growth cones, tubers, bulbs, rhizomes, roots, fruits, seeds, etc.). Phloem is also primary and secondary.
Primary phloem is formed from procambium, secondary (phloem) - from cambium. Primary phloem lacks medullary rays and a less powerful system of sieve elements than tracheids. During the formation of the sieve tube, mucus bodies appear in the protoplast of the cells - segments of the sieve tube, which take part in the formation of the mucus cord near the sieve plates (Fig. 36). This completes the formation of the sieve tube segment. Sieve tubes function in most herbaceous plants one growing season and up to 3-4 years for trees and shrubs. Sieve tubes consist of a number of elongated cells communicating with each other through perforated septa - strainer. The shells of functioning sieve tubes do not become lignified and remain alive. Old cells become clogged with the so-called corpus callosum, and then die and are flattened under the pressure of younger functioning cells on them.
Refers to phloem bast parenchyma, consisting of thin-walled cells in which reserve nutrients are deposited. By medullary rays Secondary phloem also carries out short-range transportation of organic nutrients - products of photosynthesis.
Conductive bundles- strands formed, as a rule, by xylem and phloem. If the vascular bundles are adjacent to strands
mechanical tissue (usually sclerenchyma), then such bundles are called vascular-fibrous. Other tissues may also be included in the vascular bundles - living parenchyma, laticifers, etc. The vascular bundles can be complete, when both xylem and phloem are present, and incomplete, consisting only of xylem (xylem, or woody, vascular bundle) or phloem (phloem , or bast, conductive bundle).
The vascular bundles were originally formed from procambium. There are several types of conductive bundles (Fig. 37). Part of the procambium can be preserved and then turn into cambium, then the bundle is capable of secondary thickening. This open bundles (Fig. 38). Such vascular bundles predominate in most dicotyledonous and gymnosperm plants. Plants with open bunches are able to grow in thickness due to the activity of the cambium, and the woody areas (Fig. 39, 5) are approximately three times larger than the bast areas (Fig. 39, 5). 2) . If, during differentiation of the vascular bundle from the procambial cord, all the educational tissue is completely spent on the formation of permanent tissues, then the bundle is called closed(Fig. 40). Closed
vascular bundles are found in the stems of monocots. Wood and bast in bundles may have different relative position. In this regard, several types of vascular bundles are distinguished: collateral, bicollateral (Fig. 41), concentric and radial. Collateral, or side-by-side, - bundles in which xylem and phloem are adjacent to each other. Bicollateral, or double-sided, - bundles in which two strands of phloem adjoin the xylem side by side. IN concentric in bundles, xylem tissue completely surrounds phloem tissue or vice versa (Fig. 42). In the first case, such a bundle is called centrifloem. Centrophloem bundles are present in the stems and rhizomes of some dicotyledonous and monocotyledonous plants (begonia, sorrel, iris, many sedges and lilies). Ferns have them. There are also
intermediate vascular bundles between closed collateral and centrifloem ones. Found in the roots radial bundles in which the central part and rays along the radii are left by wood, and each ray of wood consists of central larger vessels, gradually decreasing along the radii (Fig. 43). The number of rays varies from plant to plant. Between the wood rays there are bast areas. The types of conductive bundles are shown schematically in Fig. 37. Vascular bundles stretch along the entire plant in the form of cords, which begin in the roots and run along the entire plant along the stem to the leaves and other organs. In leaves they are called veins. Their main function is to conduct descending and ascending currents of water and nutrients.
