Cell structure and functions of the phloem. What is xylem and phloem in biology Classification of phloem by time of occurrence
Phloem is a complex conductive tissue through which photosynthesis products are transported from leaves to places of their use or deposition (to growth cones, underground organs, ripening seeds and fruits, etc.).
Primary phloem differentiates from procambium, secondary phloem (bast) is derived from cambium. In stems, the phloem is usually outside the xylem, while in leaves it faces the underside of the lamina. Primary and secondary phloem, in addition to the different power of sieve elements, differ in that the first one lacks core rays.
The composition of the phloem includes sieve elements, parenchymal cells, elements of the core rays and mechanical elements (Fig. 47). Most of the cells of a normally functioning phloem are alive. Only a part of the mechanical elements dies. Actually conducting function is carried out by sieve elements. There are two types: sieve cells and sieve tubes. The terminal walls of the sieve elements contain numerous small through tubules, collected in groups in the so-called sieve fields. In sieve cells, elongated and having pointed ends, sieve fields are located mainly on the side walls. Sieve cells are the main conducting element of the phloem in all groups of higher plants, excluding angiosperms. Sieve cells do not have satellite cells.
The sieve tubes of angiosperms are more perfect. They consist of individual cells - segments, located one above the other. The length of the individual segments of the sieve tubes ranges from 150-300 microns. The diameter of the sieve tubes is 20-30 microns. Evolutionarily, their segments arose from sieve cells.
The sieve fields of these segments are located mainly at their ends. The sieve fields of two segments located one above the other form a sieve plate. Sieve tube segments are formed from elongated cells of the procambium or cambium. In this case, the mother cell of the meristem divides in the longitudinal direction and produces two cells. One of them turns into a segment, the other into a satellite cell. A transverse division of the satellite cell is also observed, followed by the formation of two or three similar cells located longitudinally one above the other next to the segment (Fig. 47). It is assumed that satellite cells, together with segments of sieve tubes, constitute a single physiological system and, possibly, contribute to the promotion of the current of assimilants. During its formation, the segment has a parietal cytoplasm, a nucleus and a vacuole. With the onset of functional activity, it noticeably stretches. Many small holes-perforations appear on the transverse walls, forming tubules several micrometers in diameter, through which cytoplasmic cords pass from segment to segment. A special polysaccharide is deposited on the walls of the tubules - callose, narrowing their lumen, but not interrupting the cytoplasmic strands.
As the sieve tube segment develops, mucus bodies form in the protoplast. The nucleus and leukoplasts, as a rule, dissolve, the border between the cytoplasm and the vacuole - the tonoplast - disappears and all living contents merge into a single mass. In this case, the cytoplasm loses its semi-permeability and becomes completely permeable to solutions of organic and inorganic substances. Mucus corpuscles also lose their shape, merge, forming a mucus cord and clusters near the sieve plates. This completes the formation of the sieve tube segment.
The duration of the functioning of sieve tubes is small. In shrubs and trees, it lasts no more than 3-4 years. As we age, the sieve tubes become clogged with callose (forming the so-called corpus callosum) and then die off. Dead sieve tubes are usually flattened by neighboring living cells pressing on them.
The parenchymal elements of the phloem (bast parenchyma) consist of thin-walled cells. Reserve nutrients are deposited in them and, in part, short-range transport of assimilants is carried out along them. Companion cells are absent in gymnosperms and their role is played by the few cells of the bast parenchyma adjacent to the sieve cells.
The core rays, continuing in the secondary phloem, also consist of thin-walled parenchymal cells. They are intended for the implementation of short-range transport of assimilates.
Bast (phloem) is a complex conductive tissue through which photosynthesis products (organic substances) are transported from leaves to all plant organs (to rhizomes, fruits, seeds, etc.). The phloem is formed by cell division of the procambium (primary) and cambium (secondary). The bast is located in the stem outside of the cambium under the bark, and in the leaves - closer to the underside of the plate. Under the cambium in the trunk is wood.
Drawing. Tree trunk and its layers
Structure
Phloem tissues and its cellular composition are divided into three types depending on the functions performed: sieve tubes with cells; mechanical tissues (sclereids and fibers); bast parenchyma with parenchymal cells. Basically, the bast is made up of sieve tubes that allow dissolved nutrients to move down the stem. The tubes are formed by sieve cells that fit tightly and connect to each other.
Cells
The cells are living, thin-walled and have an elongated shape. They do not have a nucleus, and the cytoplasm is contained in the central part. The transverse walls of the cells have small through holes through which the strands of the cytoplasm pass into neighboring cells.
Sieve tubes run along the entire length of the plant. In deciduous plants, satellite cells adjoin and connect to the segments of the sieve tubes, which also take part in the transport of substances. Sieve tubes do not function for long, only one growing season, they are gradually clogged with callose, and then die off. Only some perennial plants have a lifespan of more than 2 years.
Functions
Mechanical fabrics - thick-walled bast fibers serve for strength, and also perform a supporting function. The bast parenchyma contains thin-walled parenchyma cells, which serve for the deposition of reserve nutrients, as well as for their short-range transportation.
If in the xylem the movement of dissolved mineral substances is carried out only upwards to the leaves from the roots, then in the phloem the movement of organic substances (sucrose, carbohydrates, amino acids, phytohormones) from the leaves occurs to those plant organs that consume or store them. The highest intensity of consumption of substances is observed in the tops of the shoots, emerging leaves, roots. Many plants have storage organs: tubers, bulbs, etc. The speed of transport is quite high and amounts to tens of centimeters per hour. Experiments have shown that leaf donors most often feed nearby plant organs. For example, shoot leaves provide fruits, lower leaves provide roots. In addition, phloem transport is two-way, depending on the vegetative phase, for example, storage organs can transport carbohydrates to opening leaves.
If the bark on a tree is cut in a circle to wood, then organic matter will no longer flow to the roots, and the tree will dry out over time.
Related content:
Composition and structure of phloem elements. Phloem, like xylem, is a complex tissue and consists of conductive (sieve) elements, several types of parenchymal cells and phloem (bast) fibers. Let us first consider the conducting elements of the phloem. The conducting elements of the phloem are called sieve because on their walls there are groups of small through holes (perforations), similar to strainers. These areas of the cell wall are surrounded by thickened ridges and are called sieve fields. Sieve elements, unlike tracheal elements, are living cells. Strands of cytoplasm pass through the perforations of the sieve fields, along which solutions of organic substances move.
Sieve elements, like tracheal elements, are of two types: sieve cells and sieve tubes. sieve cells - long prosenchymal, with sieve fields on longitudinal walls. sieve tubes formed by a vertical row located one above the other segment cells, transverse partitions between which are turned into sieve plates with wider perforations than sieve fields. Sieve fields are preserved on the longitudinal walls. On the sieve plates are "strainers" (sieve fields). If there is one "sieve" on the sieve plate, it is called simple, and if several complex.
Sieve cells are more primitive and are found in ferns and gymnosperms. Sieve tubes are functionally more perfect than sieve cells and are characteristic exclusively of angiosperms. The segments of the sieve tubes are physiologically dependent on their neighbors. companion cells and have a common origin with them, since they are formed from the same initial cells.
In the evolution of sieve elements, there is a clear parallelism with the evolution of tracheal elements. Sieve cells gave rise to segments of sieve tubes, which shortened and expanded in the course of evolution, their transverse walls first occupied an oblique and then horizontal position, complex perforated plates were replaced by simple ones.
Histogenesis of the sieve tube. The sieve tube has a number of remarkable features that are more convenient to consider in ontogenetic development.
Scheme of histogenesis of the segment of the sieve tube and accompanying cells:
1 - original cell with vacuole and tonoplast; 2 - formation of a sieve tube segment with F-protein and an accompanying cell; 3 - disintegration of the nucleus, tonoplast and endoplasmic reticulum, formation of sieve perforations; 4 - sieve perforations are finally formed; 5, 6 - clogging of sieve perforations with callose; IN- vacuole; Ka- callose; Pl- plastids; Etc- perforations; SC- accompanying cells; T- tonoplast; I- core
The meristem cell, which gives rise to the segment of the sieve tube, divides longitudinally . The two sister cells subsequently maintain numerous plasma connections with each other. One of the cells (larger) turns into a segment of the sieve tube, the other into an accompanying cell (or into two or three cells in the case of additional division). The resulting element is stretched, taking the final dimensions. The shell thickens somewhat, but remains non-lignified. At the ends, sieve plates are formed with perforations at the site of plasmodesmata. On the walls of these holes is deposited callose - a special polysaccharide, chemically close to cellulose. In a functioning sieve tube, the callose layer only narrows the lumen of the holes, but does not interrupt the plasma bonds in them. Only with the end of the activity of the callose tubes clogs the perforations.
The protoplast of the sieve tube exhibits a number of remarkable changes that are unique to these elements. At first, it occupies a parietal position, surrounding a central vacuole with a well-defined tonoplast. Round bodies appear in the cytoplasm phloem protein(P-protein), especially numerous in dicotyledonous plants. As the sieve element develops, the F-protein bodies lose their distinct outlines, blur and merge together, forming clusters near the sieve plates. Through the perforations, the F-protein fibrils pass through the perforations from segment to segment.
In the protoplast, the tonoplast is destroyed, the central vacuole loses its definition, and the center of the cell is filled with a mixture of vacuolar juice with the contents of the protoplast.
Most notably, as an element matures, its core is destroyed. However, the element remains alive and actively conducts substances.
An important role in the passage of assimilates through sieve tubes belongs to accompanying cells (companion cells), which retain nuclei and numerous active mitochondria. In small leaf veins, mitochondria can take the form of a mitochondrial reticulum. There are numerous plasma connections between the sieve tubes and the accompanying cells adjacent to them. The rate of linear movement of assimilates along the phloem (50-150 cm/h) is higher than the rate that could be provided only by free diffusion in solutions. It remains to be assumed that the living contents of the sieve elements and especially the accompanying cells are active; with the expenditure of energy, participates in the movement of assimilates. This assumption is consistent with the fact that the movement of assimilates requires intensive respiration of phloem cells: if breathing is difficult, then movement stops.
In dicotyledonous plants, sieve tubes usually work for one to two years. Then the sieve plates are covered with a continuous layer of callose, the thin-walled elements of the phloem are crushed, and the cambium forms new elements.
In plants devoid of annual cambial growth, sieve elements are much more durable. So, in some ferns, the work of sieve elements up to 5-10 years is noted, in some monocots (palm trees) even up to 50-100 years, although the last dates are being questioned.
34. Complex tissues - phloem (bast) and xylem (wood): formation, histological composition, topography in organs.
Conductive tissues in plant organs combine with other elements to form complex tissues - xylem And phloem.
C y l e m a , or wood, consists of primary (procambial) and secondary (cambial) elements that perform certain functions: conductive tissues - vessels and tracheids, mechanical - wood fibers, storage tissues - wood parenchyma and replacement fibers.
Phloem a, or lub, also includes elements of primary (procambial) and secondary (cambial) origin for various purposes: conductive tissues - sieve cells or sieve tubes with satellite cells, mechanical tissue - bast fibers, storage tissue - bast parenchyma. Sometimes mechanical fibers are absent. Often lactic or other secretory structures are formed in the phloem.
35. Conductive bundles: formation, composition, types, patterns of placement in organs, taxonomic and diagnostic significance.
Xylem and phloem usually accompany each other, forming conductive, or vascular fibrous, bundles.
Conductive bundles formed by the procambium that do not have a cambium are called closed, and bundles with cambium - open, since they can grow in size for a long time . Depending on the location of the xylem and phloem, bundles are distinguished: collateral, bicollateral, concentric and radial.
Collateral bundles characterized by the arrangement of phloem and xylem side by side, on the same radius. At the same time, in the axial organs, the phloem occupies the outer part of the bundle, the xylem - the inner one, and in the leaves - vice versa. Collateral bundles can be closed (monocots) and open (dicots).
Bicollateral bundles always open, with two sections of phloem - inner and outer, between which is located xylem. The cambium is located between the outer phloem and xylem. Bicollateral vascular fibrous bundles are characteristic of representatives of this family. pumpkin, nightshade, kutrovye and some others.
Concentric bundles closed. They are centrophloem, if the xylem surrounds the phloem, and centroxylem, if the phloem surrounds the xylem. Centrophloem bundles are formed more often in monocotyledonous plants, centoxylem - in ferns.
Radial bundles closed. In them, phloem and xylem alternate along the radii. Radial bundles are characteristic of the root absorption zone, as well as the root passage zone of monocotyledonous plants.
36. Morphology as a branch of botany: purpose, methods, basic morphological concepts. General patterns of plant organisms (organ, polarity, symmetry, reduction, metamorphosis, similarity and homology, etc.).
Morphology of plants, phytomorphology, the science of the patterns of structure and processes of plant formation in their individual and evolutionary-historical development. One of the most important branches of botany. As M.'s development r. plant anatomy, which studies the tissue and cellular structure of their organs, plant embryology, which studies the development of the embryo, and cytology, the science of the structure and development of cells, emerged from it as independent sciences. Thus, M. r. in a narrow sense, studies the structure and shaping, mainly at the organismal level, however, its competence also includes consideration of the patterns of the population-species level, since it deals with the evolution of form.
The main problems of M. R.: the identification of the morphological diversity of plants in nature; studying the regularities of the structure and mutual arrangement of organs and their systems; studies of changes in the general structure and individual organs during the individual development of a plant (ontomorphogenesis); clarification of the origin of plant organs during the evolution of the plant world (phylomorphogenesis); study of the impact of various external and internal factors on shaping. Thus, without being limited to the description of certain types of a structure, M. r. seeks to elucidate the dynamics of structures and their origin. In the form of a plant organism and its parts, the laws of biological organization are externally manifested, that is, the internal interconnections of all processes and structures in the whole organism.
In theoretical M. r. distinguish between 2 interrelated and complementary approaches to the interpretation of morphological data: identifying the causes of the emergence of certain forms (in terms of factors directly affecting morphogenesis) and elucidating the biological significance of these structures for the life of organisms (in terms of fitness), which leads to preservation of certain forms in the process of natural selection.
The main methods of morphological research are descriptive, comparative and experimental. The first is to describe the forms of organs and their systems (organography). The second is in the classification of descriptive material; it is also used in the study of age-related changes in the organism and its organs (comparative ontogenetic method), in elucidating the evolution of organs by comparing them in plants of different systematic groups (comparative phylogenetic method), in studying the influence of the external environment (comparative ecological method). And, finally, with the help of the third - experimental - method, controlled complexes of external conditions are artificially created and the morphological reaction of plants to them is studied, and internal relationships between the organs of a living plant are studied through surgical intervention.
A number of general patterns include the presence of a certain type of symmetry, the properties of polarity, the ability to metamorphize, reduce and abort, and regenerate.
Symmetry. In plant morphology, symmetry is understood as the ability to divide an organ into several mirror-like halves. The plane that divides the organ into symmetrical parts is called the plane, or axis, of symmetry. Vegetative organs can be monosymmetrical, bisymmetrical and polysymmetrical (radially symmetrical). Only one plane of symmetry can be drawn through a monosymmetrical organ; therefore, the organ can only be divided into two mirror-like halves. The leaves of a number of plants are monosymmetrical (common lilac - Syringa vulgaris, drooping birch - Betula pendula, European hoof - Asarum europaeum, etc.). Occasionally, monosymmetrical stems are found (the genus Lithops - Lithops from the Cactus family, the winged stem of the forest rank - Lathyrus sylvestris) and roots (board-shaped roots of ficuses). Flattened stems are bisymmetric, two planes of symmetry can be drawn through them (oblate bluegrass - Poa compressa, prickly pear - Opuntia polyacantha). If more than two planes of symmetry can be drawn through an organ, the organ is polysymmetrical. Polysymmetrical round stems (annual sunflower - Helianthus annuus), roots (common pumpkin - Cucurbita pepo), root crops (sowing radish - Raphanus sativus, common beet - Beta vulgaris), root cones of some plants (spring chistyak - Ficaria verna, dense-flowered asparagus "Sprenger "- Asparagus densiflorus "schprengeri"), unifacial leaves (sedum sedum - Sedum acre, onion - Allium cepa), stolons (potato - Solanum tuberosum). A special type of symmetry is asymmetry. Not a single plane of symmetry can be drawn through asymmetric organs. The leaves of elms are asymmetrical (smooth elm - Ulmus laevis, rough elm - Ulmus scabra), some begonias (royal begonia - Begonia rex, spotted begonia - Begonia maculata).
Polarity- one of the general patterns inherent not only in the entire plant organism, but also in its individual organs, as well as cells. Polarity is characterized by the presence of morphological and physiological differences at opposite ends of the plant body or its elements. Polarity inherent in roots and leaves, they have clear differences in tops and bases. Due to the property of polarity, plant organs are oriented in space in a certain way. The process of polarization is very complex and not fully understood.
All vegetative organs are capable of metamorphoses. The greatest variety of metamorphosed structures is typical both for shoots as a whole and for their components - leaves. Roots that are under relatively stable conditions of existence metamorphose less frequently, and root metamorphoses in autotrophic land plants are mainly associated with the performance of a storage function. In the process of morphological evolution, not only the morpho-physiological complication of various organs took place, but under the influence of the conditions of existence in some species, a reduction or even abortion of individual organs or their parts occurred.
At abortion body completely disappears. So, the root of the Salvinia floating fern (Salvinia natans) is aborted. Dodder leaves are aborted. The reduction and abortion of organs, like metamorphoses, are adaptive processes, the response of a plant to the conditions of existence. Often the terms "reduction" and "abortion" in the botanical literature are used as synonyms.
A common property of the vegetative organs of plants is the ability to regeneration, i.e., to restore the lost parts of the body. Regeneration underlies the vegetative propagation of plants. It can occur both in natural conditions and can be obtained under experimental conditions. The ability to regenerate in different taxa is different. The higher the degree of morphological and anatomical differentiation of a plant and its organs, the weaker their ability to regenerate. Regeneration occurs due to the restoration of the meristematic activity of parenchyma cells and their subsequent differentiation into the tissues of the vegetative organs.
Humanity has used wood for thousands of years. It was used for various purposes, mainly as a source of fuel. Also, wood is an excellent building material, it is used to create tools, weapons, furniture, containers, works of art, paper.
Due to the presence of growth rings, which during growth, and also as a result of seasonal fluctuations in temperature or humidity levels, most tree species form in their trunk, scientists can quite accurately determine the region in which the tree grew. Annual monitoring of changes in the width of tree rings and analysis of the content of certain isotopes of elements in them makes it possible to study in more detail the state of the climate and atmosphere in ancient times.
How is wood formed?
Wood is one of the components of the vascular fibrous bundle, it is opposed to another important part of the bundle, which is formed from the same procambium or cambium - bast, or phloem. In the process of formation of vascular fibrous bundles from procambium, two variants of events are possible:
- all procambial cells become elements of wood and bast with the formation of so-called closed bundles. This process is typical for higher spore, monocot and some dicot plants.
- on the border between the wood and the bast, there remains a layer of active tissue, which is called the cambium. In this case, open bundles are formed, which is typical for dicotyledonous and gymnosperms.
In the first scenario, the amount of wood does not change, and the plant cannot thicken. If development follows the second path, then due to the work of the cambium, the volume of wood increases annually, and the stem of the plant slowly becomes thicker. In tree species of the Russian region, wood is closer to the center (axis) of the tree, and bast is closer to the circle (periphery). A number of other plants have a somewhat different mutual arrangement of wood and bast.
It is the cell division of the cambium in the stem that ensures its growth in thickness. During the division of cambial cells? daughter cells formed is separated into wood, eh? - in the forehead. For this reason, the increase is very noticeable in wood. Cambium is not divided evenly, this process depends on the season. In the spring-summer period, division is active, as a result of which large cells are formed, by autumn the division slows down, and small cells form. In winter, the cambium does not divide. Thus, an annual growth of wood is provided, which is clearly visible in many trees, and is called an annual ring. By the number of annual rings, experts calculate the age of the shoot and the whole tree.
Wood contains already dead cellular elements with stiffened, mainly thick shells. The composition of the bast, on the contrary, is represented by elements of living cells, with living protoplasm, cell sap, and a thin non-wooden shell. At the same time, dead, thick-walled and stiffened elements can be found in the bast.
Both components of the vascular fibrous bundle have one more physiological difference. Raw juice moves through the wood from the ground to the leaves, which is water with useful substances dissolved in it. But plastic juice flows down the bast.
The process of lignification of cell membranes is characterized by the impregnation of the cellulose membrane with special substances, which are united under the general name lignin. The presence of lignin and, at the same time, the lignification of the shell can be easily determined using certain reactions. Due to lignification, plant shells grow in thickness and harden. At the same time, with light permeability to water, they lose their ability to absorb water and swell.
The structure of the bast
Phloem is the same as the bast. It is the conductive tissue of vascular plants. It is through it that the products of photosynthesis are transported to different parts of the plant, where they are used or accumulated.
In the stems of most plants, the woody bast is located outside the xylem, and in the leaves it faces the underside of the veins of the leaf blade. Conductive root bundles have alternating strands of phloem and xylem.
The bast of a tree by origin is divided into:
- primary, differentiating from procambium
- secondary, differentiating from the cambium.
The main difference between the primary and secondary phloem is the complete absence of core rays in the first. However, the cellular composition of both primary and secondary phloem is identical. They contain cells of various morphologies, and perform different functions:
- sieve elements (cells, tubes and companion cells). These elements provide the main transport
- sclerenchymal elements (sclereids and fibers), responsible for the support function
- parenchymal elements (parenchymal cells) are responsible for near radial transport.
Sieve tubes live quite a bit. Often the period of their life does not exceed 2-3 years, very rarely they live up to 10-15 years. Dead ones are regularly replaced by new ones. Sieve tubes take up little space in the bast and are most often connected in bundles. In addition to such bundles, the bast contains mechanical tissue cells - bast fibers, as well as cells of the main tissue.
Lub functions
One of the main functions performed by young bast is the phloem transport of juice. This juice is a solution of carbohydrates (in woody plants it is mainly sucrose). Carbohydrates are products of photosynthesis, in a fairly high concentration - 0.2-0.7 mol / liter (approximately from 7 to 25%). In addition to carbohydrates, the composition of the juice also includes other assimilates and metabolites (amino acids and phytohormones) in much smaller quantities. The transport speed reaches tens of centimeters per hour, which is significantly higher than the diffusion rate.
Phloem sap moves from the donor organs in which the process of photosynthesis is carried out to the acceptors - organs or areas in which these products of photosynthesis are used or deposited for later. Assimilates are consumed very intensively in the root system, shoot tips, growing leaves, and reproductive organs. Many plants have special storage organs - bulbs, tubers and rhizomes that act as acceptors.
Linden bast is the inner layer of the bark, which has a light yellow color. Its task is to ensure the strength of the stem. The bast layer is quite problematic to break in width, but along the stem it easily breaks up into thin fibers of great length.
The bast part of the stem is often used in the household, for example, the linden bast is famous for making matting and washcloths from it.
Note that if the bark on a tree is cut in a circle to a layer of wood, then organic substances will no longer be transported to the roots, and the tree will die after a while.
