The set of all roots. root systems. See what the "root system" is in other dictionaries

variety of roots. Usually plants have numerous and highly branched roots. The totality of all the roots of one individual forms a single morphological and physiological relation root system .

The composition of the root systems includes morphologically different roots - the main, lateral and adventitious.

main root develops from the germinal root.

Lateral roots arise on the roots (main, lateral, subordinate), which in relation to them are designated as maternal. They form at some distance from the apex, usually in the absorption zone or somewhat higher, acropetally, i.e. in the direction from the base of the root to its apex.

The initiation of the lateral root begins with cell division of the pericycle and the formation of a meristematic tubercle on the surface of the stele. After a series of divisions, a root appears with its own apical meristem and cap. The growing germ makes its way through the primary bark of the maternal root and moves outward.

Lateral roots are laid in a certain position to the conductive tissues of the maternal root. Most often (but not always) they arise against xylem groups and therefore are arranged in regular longitudinal rows along the maternal root.

The endogenous formation of lateral roots (i.e., their initiation in the internal tissues of the maternal root) has a clear adaptive significance. If branching occurred at the very apex of the maternal root, then this would make it difficult for it to move in the soil (compare with the appearance of root hairs).

Scheme of the growth of the lateral root and its extension from the maternal root:

Acropetal initiation of lateral roots in the pericycle of the maternal root of susak (butomus):

Pts- pericycle; En- endoderm

Not all plants have roots branching in the manner described. In ferns, lateral roots are laid in the endodermis of the maternal root. In club mosses and some related plants, the roots branch dichotomously (forked) at the top. With such branching, one cannot talk about lateral roots - they distinguish between roots of the first, second and subsequent orders. Dichotomous branching of roots is a very ancient, primitive type of branching. The roots of club mosses preserved it, apparently, because they lived in loose and water-saturated soil and did not penetrate deeply into it. Other plants switched to a more advanced method of branching - the formation of lateral roots endogenously, above the stretch zone, and this helped them to settle on dense and dry soils.

adventitious roots very diverse, and, perhaps, their common feature only that these roots cannot be attributed to either the main or the lateral ones. They can also appear on stems. (stem-like adventitious roots), and on the leaves, and on the roots (root relative clauses roots). But in the latter case, they differ from lateral roots in that they do not show a strictly acropetal order of initiation near the apex of the maternal root and may appear in old root areas.

The variety of adventitious roots is manifested in the fact that in some cases the place and time of their establishment are strictly constant, while in other cases they are formed only when organs are damaged (for example, during cuttings) and with additional treatment with growth substances. There are many intermediate cases between these extremes.

The tissues in which adventitious roots arise are also diverse. Most often, these are meristems or tissues that have retained the ability for neoplasms (apical meristems, cambium, core rays, phellogen, etc.).

Origin Classification

Among the variety of adventitious roots, however, there are roots that deserve special attention. These are stem roots of club mosses, horsetails, ferns and other higher spores. They are laid on the shoot very early, in the apical meristem, and cannot be laid on the older parts of the shoots. Since in higher spores the seed and the embryo with the germinal root are absent, then the entire root system formed by adventitious roots. It is this root system that is considered the most primitive. She received the name primarily homorhizous (Greek homoios - the same and rhiza - root).

The emergence of a seed with an embryo and a main root in seed plants gave them a certain biological advantage, since it facilitated the rapid formation of a root system for the seedling during seed germination.

The adaptive possibilities of seed plants expanded even more after they acquired the ability to form adventitious roots in various tissues and various organs. The role of these roots is very great. Arising repeatedly on shoots and roots, they enrich and rejuvenate the root system, make it more viable and stable after damage, and greatly facilitate vegetative propagation.

Dichotomous branching in the root system of club moss (Lycopodium clavatum):

1 - part of the root system; 2 - the first isotomous (equilateral) branching; 3 - anisotomous (unequal) branching; 4 - isotomic branching of the thinnest roots; I am the escape PT - conductive tissues; H - case

The emergence of adventitious roots on the roots of the lollipop (Lotus corniculatus):

1 - cross section of a three-year-old root; 2 - bundles of roots of the 2nd order in the scars of adventitious temporary roots; 3 - formation of adventitious roots on the basis of a two-year-old root; BC - lateral root; PC - adventitious root

The root system, composed of the main and adventitious roots (with their lateral branches), is called alloric (Greek alios - another) .

In many angiosperms, the main root of the seedling dies off very soon or does not develop at all, and then the entire root system (secondarysea) composed only of systems of adventitious roots. In addition to monocots, many dicots have such systems, especially those that reproduce vegetatively (strawberries, potatoes, coltsfoot, etc.).

Morphological classification

Morphological types root systems are also established on other grounds. AT pivotal root system, the main root is highly developed and clearly visible among the other roots . In the tap system, additional stem-like adventitious roots may appear, as well as adventitious roots on the roots. Often such roots are short-lived, ephemeral.

AT fibrous In the root system, the main root is invisible or absent, and the root system is composed of numerous adventitious roots. Cereals have a typical fibrous system. If stem-like adventitious roots are formed on a shortened vertical rhizome, then a racemose root system arises. Adventitious roots that arose on a long horizontal rhizome make up a fringed root system. . Sometimes (in some clovers, cinquefoils), adventitious roots that have arisen on a horizontal shoot become very thick, branch and form secondary rod root system.

Root systems:

1 - primary sea, surface; 2 - allorizny, core, deep; 3 - alloiznaya, rod, superficial; 4 - alloriznaya, fringed; 5 - secondary seaweed, fibrous, universal. The main root is in black.

Secondary rod root systems:

M- maternal individual; D- offspring

Root systems are also classified according to the distribution of the mass of roots over soil horizons. The formation of surface, deep and universal root systems reflects the adaptation of plants to the conditions of soil water supply.

However, all the listed morphological features give the most initial idea of ​​the diversity of root systems. In any root system, changes are constantly taking place, balancing it with the shoot system in accordance with the age of the plant, relations with the roots of surrounding plants, the change of seasons, etc. Without knowledge of these processes, it is impossible to understand how the plants of forests, meadows, swamps live and interact.

Root differentiation in root systems. As described above, sections of the root located at different distances from its apex perform different functions. However, differentiation does not stop there. In the same root system, there are roots that perform different functions, and this differentiation is so deep that it is expressed morphologically.

Most plants have distinct growth and sucking endings. Growth endings are usually more powerful than sucking endings, quickly elongate and move deeper into the soil. The elongation zone in them is well expressed, and the apical meristems work vigorously. Sucking endings arising in large numbers on growth roots, elongate slowly, and their apical meristems almost stop working. The sucking endings, as it were, stop in the soil and intensively “suck” it.

Sucking roots are usually short-lived. Growth roots can turn into long-lived ones, or they die off after a few years along with sucking branches.

In fruit and other trees, thick skeletal and semi-skeletal roots on which short-lived fouling root lobes. The composition of the root lobes, continuously replacing each other, includes growth and sucking endings.

Root lobe:

RO - growth ending; CO - sucking ending

The roots that have penetrated into the depths have different functions and, consequently, a different structure than the roots in the surface layers of the soil. Deep roots that have reached groundwater provide the plant with moisture if it is lacking in the upper soil horizons. Surface roots growing in the humus horizon of the soil supply the plant with mineral salts.

Root differentiation is manifested in the fact that in some roots the cambium builds up a large number of secondary tissues, while other roots remain thin, even cambial .

In monocotyledons, the cambium is completely absent in all roots, and the differences in roots, often very sharp, are determined when they are laid on the mother organ. The thinnest roots may have a diameter of less than 0.1 mm, and then their structure is simplified: the xylem in the cross section consists of 2–4 elements, and even roots are described in which the phloem is completely reduced.

Very often, special-purpose roots are differentiated in root systems (reserving, retracting, mycorrhizal, etc.).

plants.G. Animals.A.2 Autotrophic organisms are: A. Viruses.B. Pisces.V. Animals.G. Plants containing chlorophyll.A.3 Bacterial cell: A. Neuron.B. Axon.V. Dendrite.G. Vibrio cholerae.A.4 Distinctive feature plant cells is the presence of: A. Nuclei.B. Cytoplasms.B. Membrane.G. A cell wall made of cellulose. A.5 Mitosis results in: A. Isolation. B. Regeneration of tissues and organs of the body..V. Digestion.G. Breathing. A.6 Specify one of the positions cell theory: A. One drop of pure nicotine (0.05 g) is enough to kill a person. B. All new cells are formed during the division of the original cells.B. Viruses and bacteriophages are representatives of the animal kingdom.G. Viruses and bacteriophages are representatives of the Subkingdom Multicellular. A.7 Reproduction is: A. Obtaining nutrients from the environment.B. Isolation of unnecessary substances.B. Reproduction of their own kind.G. Entry of oxygen into the body. A.8 The process of formation of female gametes is called: A. Ovogenesis B. Spermatogenesis CrushingG. DivisionA.9 Internal fertilization occurs in: A. Akul.B. Pike.V.Monkey.G. Frogs. A.10 For a developing human embryo, the following are detrimental: Compliance future mother power mode.B. Drug addiction of a woman.G. Observance by the expectant mother of the regime of work and rest. A.11 Indirect type of development - in: A. Homo sapiens. B. Great apes.V. Narrow-nosed monkeys.G. Cabbage butterflies. A.12 Genopyt is the totality of all: Genes of organisms.B. Bad habits.G. Useful habits. A.13 In dihybrid crossing, inheritance is studied: A. Many traits. B. Three signs.B. Two signs.G. One sign. TASK B. Tasks with a short answer B.1 Find a match .. 1. Dominant trait in humans. A. Gray eyes.2. recessive trait in humans. B. Brown eyes. Blond hair.G. Black hair.1 2B. 2 Compare the characteristics of asexual and sexual reproduction. Write the answer number in the appropriate column. Sexual reproduction. Asexual reproduction 1. One individual participates in the process of reproduction.2. Two individuals of different sex participate in the process of reproduction.3. The beginning of a new organism is given by a zygote, resulting from the fusion of male and female germ cells.4. The beginning of a new organism (organisms) is given by a somatic cell.5. Dysentery bacillus.6. Male and female pond frog. B.3 Choose the correct answer. Write down the numbers of the correct statements. No. ___________ 1. Sperm is a female sex gamete. 2. The spermatozoon is the male sex gamete. The ovum is the male reproductive gamete. Ovum - female sex gamete5. Ovogenesis - the process of development of eggs.6. Ovogenesis is the process of development of spermatozoa.7. Spermatogenesis is the process of development of eggs.8. Spermatogenesis is the process of development of spermatozoa9. Fertilization is the process of fusion of sex gametes: two spermatozoa.10. Fertilization is the process of fusion of sex gametes: two eggs.11. Fertilization is the process of fusion of the sex gametes: sperm and egg. B.4 Set the correct sequence of the complication of organisms according to the plan: non-cellular life forms-prokaryotes-eukaryotes. 1. Influenza virus H7N92. Amoeba freshwater.3. Vibrio cholerae. B.5 A heterozygous (Aa) black rabbit is crossed with a heterozygous (Aa) black rabbit. 1. What kind of phenotypic splitting should be expected in such a crossing? A. 3:1; B. 1:1; V. 1:2:12. What percentage is the probability of the birth of white rabbits - (homozygous for two recessive genes- aa)? Answer:_________________В.6 Carefully read the text, think and answer the question: "Recall the possible evolutionary role of symbiosis, scientists were forced to study the internal structure of the cell - in the middle of the last century, after the appearance of the electron microscope, discoveries in this area fell one after another. It turned out, in particular that not only plant chloroplasts, but also mitochondria - " power plants"Any real cells really look like bacteria, and not only externally: they have their own DNA and they reproduce independently of the host cell." (According to the materials of the magazine "Around the World"). Which organelles have their own DNA?

Lecture number 5. Root and root system.

Questions:

Root zones.

Apical meristem of the root.

The primary structure of the root.

Secondary structure of the root.

Definition of the root and its functions. Classification of root systems by origin and structure.

Root (lat. radix) - an axial organ with radial symmetry and growing in length as long as the apical meristem is preserved. The root differs morphologically from the stem in that leaves never appear on it, and the apical meristem is covered with a root cap like a thimble. Branching and initiation of adventitious buds in root offspring plants occurs endogenously (internally) as a result of the activity of the pericycle (primary lateral meristem).

Root functions.

1. The root absorbs water from the soil with minerals dissolved in it;

2. performs an anchor role, fixing the plant in the soil;

3. serves as a receptacle for nutrients;

4. takes part in the primary synthesis of some organic matter;

5. in root plants, it performs the function of vegetative reproduction.

Root classification:

I. By origin roots are divided into main, adnexal and lateral.

main root develops from the germinal root of the seed.

adventitious roots or adventitious roots(from lat adventicius - alien) are formed on other plant organs (stem, leaf, flower) . The role of adventitious roots in the life of herbaceous angiosperms is enormous, since in adult plants (both monocots and many dicots) the root system mainly (or only) consists of adventitious roots. The presence of adventitious roots on the basal part of the shoots makes it easy to propagate plants artificially by dividing them into separate shoots or groups of shoots with adventitious roots.

Side roots are formed on the main and adventitious roots. As a result of their further branching, lateral roots of higher orders appear. Most often, branching occurs up to the fourth or fifth orders.

The main root has positive geotropism; under the influence of gravity, it deepens into the soil vertically down; large lateral roots are characterized by transverse geotropism, i.e., under the action of the same force, they grow almost horizontally or at an angle to the soil surface; thin (suction) roots do not possess geotropicity and grow in all directions. Root growth in length occurs periodically - usually in spring and autumn, in thickness - begins in spring and ends in autumn.

The death of the apex of the main, lateral or adventitious root sometimes causes the development of a lateral one growing in the same direction (as its continuation).

III. By shape roots are also very diverse. The form of a single root is called cylindrical, if for almost the entire length it has the same diameter. At the same time, it can be thick (peony, poppy); ischiform, or string-shaped (bow, tulip), and filiform(wheat). In addition, allocate knotty roots - with uneven thickenings in the form of knots (meadowsweet) and beaded - with evenly alternating thickenings and thin areas (hare cabbage). storage roots can be conical, turnip-shaped, spherical, fusiform and etc.

root system.

The totality of all the roots of one plant is called the root system.

Classification of root systems by origin:

main root system develops from the germinal root and is represented by the main root (of the first order) with lateral roots of the second and subsequent orders. Only the main root system develops in many trees and shrubs and in annual and some perennial herbaceous dicots;

adventitious root system develops on stems, leaves, sometimes on flowers. The adventitious origin of roots is regarded as more primitive, since it is characteristic of higher spores, which have only a system of adventitious roots. The system of adventitious roots in angiosperms is apparently formed in orchids, from the seed of which a protokorm (embryonic tuber) develops, and subsequently adventitious roots develop on it;

mixed root system widely distributed among both dicots and monocots. In a plant grown from a seed, the system of the main root first develops, but its growth does not last long - it often stops by the autumn of the first growing season. By this time, a system of adventitious roots develops consistently on the hypocotyl, epicotyl, and subsequent metameres of the main shoot, and subsequently on the basal part of the side shoots. Depending on the plant species, they are initiated and developed in certain parts of metameres (at nodes, under and above nodes, at internodes) or along their entire length.

In plants with a mixed root system, usually already in the fall of the first year of life, the main root system constitutes an insignificant part of the entire root system. Subsequently (in the second and subsequent years), adventitious roots appear on the basal part of the shoots of the second, third and subsequent orders, and the main root system dies off after two or three years, and only the adventitious root system remains in the plant. Thus, during life, the type of the root system changes: the system of the main root - the mixed root system - the system of adventitious roots.

Classification of root systems by shape.

Tap root system - this is a root system in which the main root is well developed, noticeably exceeding the lateral ones in length and thickness.

Fibrous root system is called with a similar size of the main and lateral roots. Usually it is represented by thin roots, although in some species they are relatively thick.

A mixed root system can also be pivotal if the main root is much larger than the others, fibrous, if all roots are relatively equal in size. The same terms apply to the system of adventitious roots. Within the same root system, roots often perform different functions. There are skeletal roots (supporting, strong, with developed mechanical tissues), growth roots (fast-growing, but little branching), sucking (thin, short-lived, intensively branching).

2. Young root zones

Young root zones- these are different parts of the root along the length, performing unequal functions and characterized by certain morphological features (Fig.).

Above is located stretch zone, or growth. In it, the cells almost do not divide, but strongly stretch (grow) along the axis of the root, pushing its tip deep into the soil. The extension of the stretch zone is several millimeters. Within this zone, the differentiation of the primary conductive tissues begins.

The zone of the root that bears the root hairs is called suction zone. The name reflects its function. In the older part, root hairs constantly die off, and in the young part they are constantly re-formed. This zone has a length from several millimeters to several centimeters.

Above the suction zone, where the root hairs disappear, begins holding area, which extends along the rest of the root. Through it, water and salt solutions absorbed by the root are transported to the overlying organs of the plant. The structure of this zone varies in different parts of it.

3. Apical meristem of the root.

In contrast to the shoot apical meristem, which occupies the terminal, i.e. terminal position, root apical meristem subterminal, because she is always covered with a cap, like a thimble. The apical meristem of the root is always covered with a cap, like a thimble. The volume of the meristem is closely related to the thickness of the root: it is larger in thick roots than in thin ones, but the meristem is not subject to seasonal changes. In the formation of the buds of the lateral organs, the apical meristem of the root does not participate, therefore, its only function is the neoplasm of cells (histogenic function), subsequently differentiating into cells of permanent tissues. Thus, if the apical meristem of the shoot plays both a histogenic and organogenic role, then the apical meristem of the root plays only a histogenic role. Chekhlik is also a derivative of this meristem.

Higher plants are characterized by several types of structure of the root apical meristem, differing mainly in the presence and location of the initial cells and the origin of the hairy layer - the rhizoderm.

In the roots of horsetails and ferns, the only initial cell, as in the apex of their shoots, has the form of a trihedral pyramid, the convex base of which is turned downwards, towards the cap. The divisions of this cell occur in four planes parallel to the three sides and the base. In the latter case, cells are formed that, dividing, give rise to the root cap. From the rest of the cells subsequently develop: protoderm, differentiating into rhizoderm, zone of primary cortex, central cylinder.

In most dicotyledonous angiosperms, the initial cells are arranged in 3 floors. From the cells of the upper floor, called pleroma in the future, a central cylinder is formed, the cells of the middle floor - periblema give rise to the primary cortex, and the lower - to the cells of the cap and protodermis. This layer is called dermacalyptrogen.

In grasses, sedges, whose initials are also 3 floors, the cells of the lower floor produce only root cap cells, so this layer is called calyptrogen. The protodermis separates from the primary cortex - a derivative of the middle floor of the initials - problems. The central cylinder develops from the cells of the upper floor - pleroma, as in dicots.

Thus, different groups of plants differ in the origin of the protoderm, which subsequently differentiates into the rhizoderm. Only in spore archegonial and dicotyledons does it develop from a special initial layer; in gymnosperms and monocots, the rhizoderm turns out to be formed by the primary cortex.

A very important feature of the root apical meristem is also that the initial cells proper under normal conditions divide very rarely, amounting to resting center. The volume of the meristem increases due to their derivatives. However, when the root tip is damaged due to irradiation, exposure to mutagenic factors and other causes, the resting center is activated, its cells divide intensively, contributing to the regeneration of damaged tissues.

The primary structure of the root

Differentiation of root tissues occurs in the absorption zone. By origin, these are primary tissues, since they are formed from the primary meristem of the growth zone. Therefore, the microscopic structure of the root in the suction zone is called primary.

In the primary structure, the following are fundamentally distinguished:

1. integumentary tissue, consisting of a single layer of cells with root hairs - epiblem or rhizoderm

2. primary cortex,

3. central cylinder.

Cells rhizoderms elongated along the length of the root. When they divide in a plane perpendicular to the longitudinal axis, two types of cells are formed: trichoblasts developing root hairs, and atrichoblasts, performing the functions of integumentary cells. Unlike epidermal cells, they are thin-walled and do not have cuticles. Trichoblasts are located singly or in groups, their size and shape vary in different types plants. Roots that develop in water usually do not have root hairs, but if these roots then penetrate the soil, hairs form in in large numbers. In the absence of hairs, water enters the root through the thin outer cell walls.

Root hairs appear as small outgrowths of trichoblasts. Hair growth occurs at its top. Due to the formation of hairs, the total surface of the suction zone increases tenfold or more. Their length is 1 ... 2 mm, while in grasses and sedges it reaches 3 mm. Root hairs are short-lived. Their life expectancy does not exceed 10 ... 20 days. After their death, the rhizoderm is gradually shed. By this time, the underlying layer of cells of the primary cortex differentiates into a protective layer - exoderm. Its cells are tightly closed, after the fall of the rhizoderm, their walls cork. Quite often, the cells of the primary cortex adjoining it also cork. The exoderm is functionally similar to the cork, but differs from it in the arrangement of cells: the tabular cells of the cork, formed during tangential cell divisions of the cork cambium (phellogen), are arranged in transverse sections in regular rows, and the cells of the multilayer exoderm, which have polygonal outlines, are staggered. In a powerfully developed exoderm, passage cells with non-corked walls are often found.

The rest of the primary cortex - the mesoderm, with the exception of the innermost layer, which differentiates into the endoderm, consists of parenchymal cells, most densely located in the outer layers. In the middle and inner parts of the cortex, the cells of the mesoderm have a more or less rounded outline. Often the innermost cells form radial rows. Intercellular spaces appear between cells, and in some aquatic and marsh plants there are rather large air cavities. In the primary bark of some palm trees, lignified fibers, or sclereids, are found.

The cells of the cortex supply the rhizoderm with plastic substances and are themselves involved in the absorption and conduction of substances that move both through the protoplast system ( simplastu), and along the cell walls ( apoplast).

The innermost layer of the cortex endoderm, which acts as a barrier that controls the movement of substances from the crust to the central cylinder and vice versa. The endoderm consists of tightly closed cells, slightly elongated in the tangential direction and almost square in cross section. In young roots, its cells have Casparian belts - sections of the walls characterized by the presence of substances chemically similar to suberin and lignin. Casparian belts encircle the transverse and longitudinal radial walls of the cells in the middle. Substances deposited in the Casparian bands close the openings of the plasmodesmenal tubules located in these places, however, the symplastic connection between the cells of the endoderm at this stage of its development and the cells adjacent to it from the inner and outer sides is preserved. In many dicotyledons and gymnosperms, endoderm differentiation usually ends in the formation of Caspari bands.

In monocot plants that do not have a secondary thickening, the endoderm changes over time. The process of corking extends to the surface of all walls, before which the radial and internal tangential walls thicken greatly, and the outer ones almost do not thicken. In these cases, they speak of horseshoe-shaped thickenings. Thickened cell walls subsequently become lignified, protoplasts die off. Some cells remain alive, thin-walled, only with Caspari bands, they are called checkpoints. They provide a physiological connection between the primary cortex and the central cylinder. Usually, the passage cells are located opposite the xylem strands.

Central root cylinder consists of two zones: pericyclic and conductive. In the roots of some plants, the inner part of the central cylinder is a mechanical tissue, or parenchyma, but this "core" is not homologous to the core of the stem, since the tissues that make it up are of procambial origin.

The pericycle can be homogeneous and heterogeneous, as in many conifers, and among dicots, in celery, in which schizogenic receptacles of secretions develop in the pericycle. It can be single-layer and multi-layer, like a walnut. The pericycle is a meristem, since it plays the role of a root layer - lateral roots are laid in it, and in root offspring plants - adventitious buds. In dicots and gymnosperms, it is involved in the secondary thickening of the root, forming phellogen and partially cambium. Its cells retain the ability to divide for a long time.

The primary conductive tissues of the root form a complex conductive bundle, in which radial strands of xylem alternate with groups of phloem elements. Its formation is preceded by the initiation of the procambium in the form of a central cord. The differentiation of procambial cells into elements of protophloem, and then protoxylem, begins at the periphery, i.e., xylem and phloem are laid exarchically, and later these tissues develop centripetally.

If one strand of xylem is laid and, accordingly, one strand of phloem, the bundle is called monarchical (such bundles are found in some ferns), if two strands are diarchic, like in many dicots, which may also have tri-, tetra- and pentarch bundles, moreover in the same plant, the lateral roots may differ in the structure of the vascular bundles from the main one. The roots of monocots are characterized by polyarchic bundles.

In each radial strand of the xylem, the wider elements of the metaxylem differentiate inward from the elements of the protoxylem.

The formed xylem strand can be quite short (iris); in this case, the inner part of the procambium differentiates into a mechanical tissue. In other plants (onions, pumpkins), the xylem on the transverse sections of the roots has stellate outlines, in the very center of the root there is the most wide-lumen vessel of the metaxylem, xylem strands extend from it, consisting of elements, the diameters of which gradually decrease from the center to the periphery. In many plants with polyarchic bundles (cereals, sedges, palms), individual elements of the metaxylem can be scattered over the entire cross section of the central cylinder between parenchymal cells or elements of mechanical tissue.

The primary phloem, as a rule, consists of thin-walled elements, only some plants (beans) develop protophloem fibers.

Secondary structure of the root.

In monocots and ferns, the primary structure of the root is preserved throughout life (the secondary structure is not formed in them). With the increase in the age of monocotyledonous plants, changes in primary tissues occur at the root. So, after the epiblem is desquamated, the exoderm becomes the integumentary tissue, and then, after its destruction, successively layers of cells of the mesoderm, endoderm and sometimes the pericycle, the cell walls of which cork and lignify. In connection with these changes, the old roots of monocots have a smaller diameter than the young ones.

Fundamental difference there are no roots between gymnosperms, dicotyledonous and monocotyledonous plants according to the primary structure, but cambium and phellogen are laid early in the roots of dicotyledonous and gymnospermous plants and secondary thickening occurs, leading to a significant change in their structure. Separate sections of the cambium in the form of arcs arise from the procambium or thin-walled parenchymal cells on the inner side of the phloem strands between the rays of the primary xylem. The number of such areas is equal to the number of rays of the primary xylem. Pericycle cells located opposite strands of primary xylem, dividing in the tangential plane, give rise to sections of the cambium that closes its arcs.

Usually, even before the appearance of a cambium of pericyclic origin, cambial arcs begin to lay inward cells that differentiate into elements of the secondary xylem, primarily wide-lumen vessels, and outwards - elements of the secondary phloem, pushing the primary phloem to the periphery. Under the pressure of the formed secondary xylem, the cambial arches straighten, then become convex, parallel to the circumference of the root.

As a result of the activity of the cambium outside of the primary xylem, collateral bundles arise between the ends of its radial strands, which differ from typical collateral bundles of stems by the absence of primary xylem in them. Cambium of pericyclic origin produces parenchymal cells, the totality of which makes up rather wide rays that continue the strands of the primary xylem - the primary core rays.

In roots with a secondary structure, there is usually no primary bark. This is due to the laying of a cork cambium, a phellogen, in the pericycle along its entire circumference, separating cork cells (phellem) outward during tangential division, and phelloderm cells inward. The impermeability of the cork for liquid and gaseous substances due to the suberinization of the walls of its cells and is the cause of the death of the primary cortex, which loses its physiological connection with the central cylinder. Subsequently, gaps appear in it and it falls off - a root molt occurs.

Phelloderma cells can divide many times, forming a parenchymal zone to the periphery of the conductive tissues, in the cells of which reserve substances are usually deposited. The tissues located outward from the cambium (phloem, basic parenchyma, phelloderm and cork cambium) are called secondary cortex. Outside, the roots of dicotyledonous plants, which have a secondary structure, are covered with cork, and the crust is formed on old tree roots.

Similar information.

- these are the vegetative organs of higher plants that are underground and carry out water with dissolved minerals to the above-ground organs of plants (stem, leaves, flowers). The main function of the root is to anchor the plant in the soil.

The root is divided into main, lateral and adnexal. The main root grows from the seed, it is most powerfully developed and grows vertically downwards (root of the 1st order). Lateral roots depart from the main one (roots of the 2nd order) and branch many times. Adventitious roots (roots of the 3rd order) depart from the lateral roots, which never depart from the main one, have a diverse structure and can form on stems and leaves.

The totality of all the roots of a plant is called - root system. There are two types of root systems - rod and fibrous. AT pivotal the main root is strongly expressed in the root system, and fibrous consists only of adventitious and lateral roots, the main root is not expressed. Roots in the root system differ in appearance, age and function. The thinnest and youngest roots perform mainly the functions of growth, water absorption and nutrient absorption. Older and thicker roots are fixed in the soil, conduct moisture and nutrients to the ground organs of the plant. In addition to typical roots, some plants have modified roots, for example, thickened storage, aerial, respiratory, or supporting ones. Ordinary storage roots are root crops (carrots, beets, parsley), if adventitious roots become storage roots, they are called root tubers.

Along with the roots underground, there may also be modified shoots. Depending on the structure and functions performed, they are called rhizomes, stolons, tubers and bulbs.

rhizomes- these are underground shoots that grow mainly horizontally to the soil, less often vertically and perform the functions of storage and vegetative propagation. The rhizome looks like a root, but has a fundamental difference in internal structure. Adventitious roots are often formed on rhizomes at places called nodes. After a period of underground growth, the rhizomes may come to the surface and develop into a shoot with normal green leaves. Rhizomes live from several to 15-20 years.

stolons- these are underground shoots, at the end of which tubers, bulbs, rosette shoots develop. The stolon performs the function of vegetative reproduction and lives only one year.

Tuber- this is a thickened underground shoot that has the functions of storage and vegetative reproduction. The tuber has axillary buds.

Bulb- this is a modified underground shoot, less often a semi-aerial or shortened above-ground shoot, in which thickened fleshy leaves (scales) took over the storage function, and the stem is presented only in the lower part of the bulb in the form of a flat formation - the bottom, from which adventitious roots grow. The bulb provides the preservation of moisture and nutrients during the winter or summer dormant period of plants. After a dormant period, the plants usually bloom using the reserves accumulated in the bulb.