Root functions. The root is the main organ of a higher plant. The functions of the roots are as follows:

They absorb water and mineral salts dissolved in it from the soil, transport them up the stem, leaves and reproductive organs. The suction function is performed by root hairs (or mycorrhiza) located in the suction zone.

Due to its high strength, the plant is fixed in the soil.

When water, ions of mineral salts and photosynthesis products interact, products of primary and secondary metabolism are synthesized.
Under the action of root pressure and transpiration, ions of aqueous solutions of mineral substances and organic substances move along the vessels of the root xylem along the ascending current into the stem and leaves.
In the roots are deposited in reserve nutrients(starch, inulin, etc.).
Biosynthesis of secondary metabolites (alkaloids, hormones and other biologically active substances) is carried out in the roots.
The growth substances synthesized in the meristematic zones of the roots (gibberellins, etc.) are necessary for the growth and development of the aerial parts of the plant.
Due to the roots, symbiosis is carried out with soil microorganisms - bacteria and fungi.
With the help of roots, vegetative propagation of many plants occurs.

10. Some roots perform the function of a respiratory organ (monstera, philodendron, etc.).

11. The roots of a number of plants perform the function of "stilted" roots (ficus banyan, pandanus, etc.).

12. The root is capable of metamorphoses (thickenings of the main root form “root crops” in carrots, parsley, etc.; thickenings of lateral or adventitious roots form root tubers in dahlias, peanuts, chistyak, etc., shortening of roots in bulbous plants).

Root - an axial organ, usually cylindrical in shape, with radial symmetry, possessing geotropism. It grows as long as the apical meristem is preserved, covered with a root cap. On the root, unlike the shoot, leaves never form, but, like the shoot, the root branches, forming root system.

The root system is the totality of the roots of a single plant. The nature of the root system depends on the ratio of growth of the main, lateral and adventitious roots.

^ Types of roots and root systems. In the embryo of the seed, all organs of the plant are in their infancy. The main, or first, root develops from embryonic root. The main root is located in the center of the entire root system, the stem serves as a continuation of the root, and together they form a first-order axis. The area on the border between the main root and the stem is called root collar. This transition from stem to root is noticeable by the different thicknesses of the stem and root: the stem is thicker than the root. The section of the stem from the root neck to the first germinal leaves - cotyledons is called hypocotyl knee or hypocotyl. From the main root to the sides depart lateral roots successive orders. This root system is called pivotal, in many dicotyledonous plants it is capable of branching. A branched root system is a type of tap root system. Lateral branching of the root is characterized by the fact that new roots are laid at some distance from the top and are formed endogenously - in the internal tissues of the maternal root of the previous order due to the activity of the pericycle. The more lateral roots depart from the main root, the larger the area of \u200b\u200bplant nutrition, so there are special agricultural practices, enhancing the ability of the main root to form lateral, for example, pinching or dive main root by l/3 of its length. After diving for some time, the main root stops growing in length, while the lateral roots grow intensively.

In dicotyledonous plants, the main root, as a rule, persists throughout life, in monocots, the germinal root quickly dies off, the main root does not develop, and shoots form from the base adnexal roots, which also have branches of the first, second, etc. orders. This root system is called fibrous. Adventitious roots, like lateral roots, are laid endogenously. They can form on stems and leaves. The ability of plants to develop adventitious roots is widely used in plant growing during vegetative propagation of plants (propagation by stem and leaf cuttings). Overground stem cuttings propagate willow, poplar, maple, blackcurrant, etc.; leafy cuttings - uzambar violet, or saintpaulia, some types of begonias. Underground cuttings of modified shoots (rhizomes) are propagated by many medicinal plants, for example, May lily of the valley, officinalis, etc. Some plants form many adventitious roots when hilling the lower part of the stem (potato, cabbage, corn, etc.), thereby creating additional nutrition.

In higher spore plants (mosses, horsetails, ferns), the main root does not appear at all, they form only adventitious roots extending from the rhizome. In many dicotyledonous herbaceous rhizomatous plants, the main root often dies off and the system of adventitious roots extending from the rhizomes (snotweed, nettle, creeping ranunculus, etc.) predominates.

In terms of depth of penetration into the soil, the first place belongs to the taproot system: the record depth of penetration of the roots, according to some reports, reaches 120 m! However, the fibrous root system, having a mostly superficial location of the roots, contributes to the creation of a sod cover and prevents soil erosion.

The total length of the roots in the root system is different, some roots reach several tens or even hundreds of kilometers. For example, in wheat, the length of all root hairs reaches 20 km, and in winter rye, the total length of the roots of the first, second, and third orders is over 180 km, and with the addition of roots of the fourth order, 623 km. Although the root grows throughout its life, its growth is limited by the influence of the roots of other plants.

The degree of development of root systems on different soils in different natural areas is not the same. Yes, in sandy deserts where they lie deep ground water, the roots of some plants go to a depth of 40 m or more (grass Selin, prosopis earring from the legume family, etc.). Ephemeral plants of semi-deserts have superficial root system, which is adapted to the rapid absorption of early spring moisture, which is quite sufficient for the rapid passage of all phases of plant vegetation. On clay, poorly aerated podzols of the taiga forest zone, the root system of plants is 90% concentrated in the surface layer of the soil (10-15 cm), the plants have "nourishing roots" (European spruce). For example, saxaul has roots in different time years use moisture from different horizons.

A very important factor in the distribution of the root system is moisture. The direction of the roots goes in the direction of greater humidity, however, in water and in waterlogged soil, the roots branch much weaker.

The degree of development of root systems, the depth of root penetration, and other plastic characteristics of the root depend on external conditions and, at the same time, are hereditarily assigned to each plant species.

^ Young root zones. In a young root, there are: 1) a division zone covered with a root cap; 2) cell elongation zone, or growth zone; 3) the zone of absorption, or the zone of root hairs; 4) conductive zone.

^ Division zone represents the tip of the root, covered on the outside root cap, protecting the apical, or apical, meristem. To the touch, the young tip of the root is slippery due to the mucus secreted by the cells. As the root grows in length, the mucus reduces the friction of the root tip against the soil. According to Academician V.L. Komarov, the root cap "digs the ground", it protects the dividing cells of the meristem from mechanical damage and also controls positive geotropism the root itself, i.e., promotes the growth of the root and its penetration into the depths of the soil. The root cap consists of living parenchymal cells containing starch grains. Under the cover there is a division zone, or root cone, represented by the primary educational tissue (meristem). As a result of active division of the apical meristem of the root, all other zones and tissues of the root are formed. The division zone of a young root is only 1 mm long. Outwardly, it differs from other zones in yellow.

^ Stretch zone, or growth zone, a few millimeters long, it is externally transparent, consists of practically non-dividing, but longitudinally stretching cells. Cells increase in size, vacuoles appear in them. Cells are characterized by high turgor. In the stretch zone, differentiation of the primary conductive tissues occurs and begins to form permanent tissues root.

Above the stretch zone is suction zone. Its length is 5 - 20 mm. The absorption zone is represented by root hairs - outgrowths of epidermal cells. With the help of root hairs, water and salt solutions are absorbed from the soil. The more numerous the root hairs, the greater the absorptive surface of the root. About 400 root hairs can be located per 1 mm at the root surface. Root hairs are short-lived, live 10 - 20 days, after which they die. The length of the root hairs different plants from 0.5 - 1.0 cm. Young root hairs form above the extension zone, and die off above the absorption zone, so the root hair zone constantly moves as the root grows and the plant is able to absorb water and nutrients dissolved in it from different soil horizons .

Above the suction zone begins conduction zone, or zone of lateral roots. The water absorbed by the root and salt solutions are transported through the vessels of the wood upwards to the aerial parts of the plant.

There are no sharp boundaries between the root zones, but a gradual transition is observed.

6. Metamorphoses of the root. their biological significance. Mycorrhiza. Most plants in the same root system have distinctly different growth and sucking endings. Growth endings are usually more powerful, quickly elongate and move deep into the soil. Their elongation zone is well defined, and the apical meristems work vigorously. Sucking endings arising in 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.

At woody plants distinguish thick skeletal and semi-skeletal roots on which short-lived root lobes. The composition of the root lobes, continuously replacing each other, includes growth and sucking endings.

If the roots perform special functions, their structure changes. A sharp, hereditarily fixed modification of an organ, caused by a change in functions, is called metamorphosis. Root modifications are very diverse.

The roots of many plants form a symbiosis with hyphae of soil fungi, called mycorrhiza("mushroom root"). Mycorrhiza is formed on sucking roots in the absorption zone. The fungal component makes it easier for the roots to obtain water and mineral elements from the soil; fungal hyphae often replace root hairs. In turn, the fungus receives carbohydrates and other nutrients from the plant. There are two main types of mycorrhiza. gifs ectotrophic mycorrhiza form a sheath that envelops the root from the outside. Ectomycorrhiza is widespread in trees and shrubs. Endotrophic Mycorrhiza occurs mainly in herbaceous plants. Endomycorrhiza is located inside the root, hyphae are introduced into the cells of the bovine parenchyma. Mycotrophic nutrition is very widespread. Some plants, such as orchids, cannot exist at all without symbiosis with fungi.

On the roots of legumes, special formations appear - nodules in which bacteria from the genus Rhizobium settle. These microorganisms are able to assimilate atmospheric molecular nitrogen, converting it into a bound state. Part of the substances synthesized in the nodules are absorbed by plants, bacteria, in turn, use the substances found in the roots. This symbiosis is of great importance for agriculture. leguminous plants thanks to an additional source of nitrogen, they are rich in proteins. They provide valuable food and fodder products and enrich the soil with nitrogenous substances.

Very widespread hoarding roots. They are usually thickened and strongly parenchymatized. Strongly thickened adventitious roots are called root cones, or root tubers(dahlia, some orchids). Many, more often biennial, plants with a tap root system develop a formation called root crop. Both the main root and the lower part of the stem take part in the formation of the root crop. In carrots, almost the entire root crop is made up of roots; in turnips, the root forms only the most lower part root crop ( rice. 4.12).

Roots cultivated plants arose as a result of long-term selection. In root crops, the storage parenchyma is highly developed and mechanical tissues have disappeared. In carrots, parsley, and other umbellifers, the parenchyma is strongly developed in the phloem; in turnips, radishes and other cruciferous plants - in xylem. In beets, reserve substances are deposited in the parenchyma formed by the activity of several additional layers of cambium ( rice. 4.12).

Many bulbous and rhizomatous plants form retractors, or contractile roots ( rice. 4.13, 1). They can shorten and pull the shoot into the soil to the optimum depth during a summer drought or winter frosts. Retracting roots have thickened bases with transverse wrinkling.

Respiratory roots, or pneumatophores (rice. 4.13, 2) are formed in some tropical woody plants living in conditions of lack of oxygen (taxodium, or swamp cypress; mangrove plants that live along the swampy shores of ocean coasts). Pneumatophores grow vertically upwards and protrude above the soil surface. Through a system of holes in these roots, connected with the aerenchyma, air enters the underwater organs.

In some plants, to maintain shoots in the air, additional support roots. They depart from the horizontal branches of the crown and, having reached the soil surface, branch intensively, turning into columnar formations that support the crown of the tree ( columnar banyan roots) ( rice. 4.15, 2). stilted roots extend from the lower parts of the stem, giving the stem stability. They form in mangrove plants, plant communities that develop on tropical ocean shores flooded at high tide ( rice. 4.15, 3), as well as in corn ( rice. 4.15, 1). Ficus rubbery are formed plank-shaped roots. Unlike columnar and stilted, they are by origin not adventitious, but lateral roots.

Rice. 4.15. ^ Support roots: 1 - stilted corn roots; 2 - columnar banyan roots; 3 - stilted roots of rhizophora ( etc- tidal zone; from- ebb zone; silt- the surface of the muddy bottom).

Escape concept. Morphological division of the shoot. Knots and internodes. Apical shoot growth. The structure and activity of the cone of growth. A shoot is a stem with leaves and buds located on it.

The parts of the stem where leaves develop are called nodes.
Stem sections between two nearest nodes called internodes.
The angle between the leaf and the internode above called the leaf axil.
An axillary bud forms in the leaf axil. The escape consists of repeating sections - metamers.
One metamere includes an internode, a node, a leaf, and an axillary bud. A shoot is a complex consisting of a stem and leaves. The primary shoot is laid in the embryo, where it is represented by a kidney. The kidney consists of an embryonic stalk - epicotyl, apical meristem and one or more leaf primordia (leaf rudiments). As the seed germinates, the stem lengthens. New leaf primordia develop from the apical meristem. leaf primordia leaves develop and leaf axils kidney primordia are formed. This algorithm of development during the formation of the shoot system of a plant can be repeated many times.

In the formed shoot, nodes are distinguished - the part of the shoot where the leaf is connected to the stem; internodes - part of the shoot between nodes, usually part of the stem; leaf sinuses - the angle between the leaf and the ascending part of the stem.

Part of the escape are the kidneys. This is, first of all, the apical bud, representing the growth cone of the shoot. AT leaf axils in seed plants, axillary, or lateral buds are formed. If they develop one above the other (honeysuckle, Walnut, Robinia, etc.), they are called serial. If the buds develop in the axils of the leaves next to each other (plums, cereals, etc.), then they are called collateral. Kidneys can form endogenously in the region of internodes. These kidneys are called accessory.

In trees and shrubs of a cold and temperate climate, wintering or resting buds are formed, which are often called eyes. New shoots develop from these buds the next year. The outer leaves of these buds usually turn into bud scales that protect the inner parts of the bud from damage.

Wintering or dormant buds are also formed in perennial grasses, on those organs that do not die off for the winter, i.e. on rhizomes, at the base of stems, etc. These kidneys are called renewal kidneys. Of these, above-ground shoots develop in the spring.

All of the above kidneys are called vegetative. Such kidneys consist of an apex, rudimentary nodes, rudimentary internodes, leaf primordia, above which bud primordia can develop, and rudimentary leaves.

From a kidney that does not have renal primordia, a simple or unbranched the escape. A branched branch develops from a kidney with renal primordia. the escape.

In addition, seed plants also have generative buds. These are flower buds and buds that give rise to gymnosperm cones. They differ from vegetative appearance. In addition to the apex, rudimentary internodes, and rudimentary nodes, such buds have primordia that give rise to parts of the flower or parts of cones. At the buds that give rise to inflorescences, flower primordia are formed.

Finally, there are the so-called mixed buds, from which leafy shoots with flowers are formed.

The morphological characteristic of the shoot implies a description of the structure of nodes, internodes, and buds. The type of leaf arrangement must be indicated. In most plants, it is alternate - there is one leaf per node, but it can be opposite or whorled. A certain type of leaf arrangement forms a leaf mosaic, which allows the best way use the space to ensure uniform illumination of the sheet.

The division of leaves into three categories is also associated with the process of growth and development of the shoot: lower leaves, middle leaves, apical, or upper leaves. In the morphological description of leaves, median leaves are usually described, but complete morphological description requires a separate description of all categories of leaves, because even middle leaves on one shoot they have differences. This phenomenon is called heterophylly or diversity.

Apical shoot growth - the growth of the shoot in length due to the modification of the growth cone, the initiation and growth of rudimentary leaves at its base. In the process of modification, the growth cone increases in length, becomes more complex and changes its shape.

Bud. This is a rudimentary escape. It consists of a meristematic axis ending in a cone of growth (rudimentary stem) and leaf primordia (rudimentary leaves), that is, from a series of rudimentary metameres. The differentiated leaves located below cover the cone of growth and primordia. This is how the vegetative bud is arranged. In a vegetative-reproductive bud, the cone of growth is turned into a rudimentary flower or rudimentary inflorescence. Reproductive (flower) buds consist only of a rudimentary flower or inflorescence and do not have rudiments of photosynthetic leaves.

13. Metamorphosed shoots.

Their occurrence is often associated with the performance of the functions of a receptacle for spare products, transferring adverse conditions of the year, vegetative propagation.

Rhizome- this is a perennial underground shoot with a horizontal, ascending or vertical direction of growth, which performs the functions of accumulating spare products, renewal, vegetative reproduction. The rhizome has reduced leaves in the form of scales, buds, adventitious roots. Spare products accumulate in the stem part. Growth and branching occurs in the same way as in a regular shoot. The rhizome is distinguished from the root by the presence of leaves and the absence of a root cap at the top. The rhizome can be long and thin (wheatgrass) or short and thick. Annually, above-ground annual shoots are formed from the apical and axillary buds. The old parts of the rhizome gradually die off. Plants with horizontal long rhizomes that form many above-ground shoots quickly occupy a large area, and if these are weeds (wheatgrass), then the fight against them is rather difficult. Such plants are used to fix the sands (grass, aristida). In grassland, cereals with long horizontal rhizomes are called rhizomatous (bent grass, bluegrass), and with short ones - bushy (timothy grass, belous). Rhizomes are found mainly in perennial herbaceous plants, but sometimes in shrubs (euonymus) and shrubs (lingonberries, blueberries).

Tuber- this is a thickened part of the shoot, a container of spare products. Tubers are above ground and underground.

elevated tuber is a thickening of the main (kohlrabi) or side (tropical orchids) shoot and bears normal leaves.

underground tuber- thickening of the hypocotyl (cyclamen) or short-lived underground shoot - stolon (potato). The leaves on the underground tuber are reduced, in their axils there are buds called eyes.

Elevated stolon- this is a short-lived creeping shoot that serves to spread (capture the territory) and vegetative propagation. It has long internodes and green leaves. Adventitious roots are formed on the nodes, and a shortened shoot (rosette) is formed from the apical bud, which, after the death of the stolon, continues to exist independently. The aboveground stolon sympodial grows. Aboveground stolons that have lost the function of photosynthesis and perform mainly the function of vegetative reproduction are sometimes called mustaches (strawberries).

Bulb- this is a shortened stem (bottom), bearing numerous, closely spaced leaves and adventitious roots. At the top of the donut is a kidney. In many plants (onion, tulip, hyacinth, etc.), an above-ground shoot is formed from this bud, and a new bulb is formed from the lateral axillary bud. The outer scales are in most cases dry, membranous and protective function, internal - fleshy, filled with spare products. The shape of the bulbs are spherical, ovoid, flattened, etc.

Corm it looks like an onion, but all its leaf scales are dry, and spare products are deposited in the stem part (saffron, gladiolus).

spines have various origins- from the shoot (apple, pear, blackthorn, hawthorn, honey locust, citrus fruits), leaf (barberry) or its parts: rachis (astragalus), stipules (white acacia), portion of the plate (composite). Spines are characteristic of plants in hot, dry habitats.

tendrils are formed from a shoot (grapes), a leaf or its parts: rachis and several leaves (peas), plates (rank.), Stipules (sarsaparilla). Used to attach to a support.

Phyllocladia- These are flat leaf-shaped shoots located in the axils of reduced leaves. Flowers form on them. They are found in plants of predominantly arid habitats (butcher's needle, phyllanthus). trapping devices- modified leaves characteristic of insectivorous plants (dew, flycatcher). They have the form of jugs, urns, bubbles, or slamming and wrapping plates. Small insects, falling into them, die, dissolve with the help of enzymes and are consumed by plants as an additional source of minerals.

Root. Functions. Types of roots and root systems. Anatomical structure of the root. Mechanism of soil solution entry into the root and its movement into the stem. Root modifications. The role of mineral salts. The concept of hydroponics and aeroponics.

Higher plants, unlike lower plants, are characterized by the division of the body into organs that perform various functions. Distinguish between vegetative and generative organs higher plants.

Vegetative organs - parts of the body of plants that perform the functions of nutrition and metabolism. Evolutionarily, they arose as a result of the complication of the body of plants when they landed and the development of air and soil environments. The vegetative organs include the root, stem and leaf.

1. Root and root systems

The root is an axial organ of plants with radial symmetry, growing due to the apical meristem and not bearing leaves. The root growth cone is protected by a root cap.

The root system is the totality of the roots of a single plant. The shape and nature of the root system are determined by the ratio of growth and development of the main, lateral and adventitious roots. The main root develops from the germinal root and has positive geotropism. Lateral roots arise on the main or adventitious roots as offshoots. They are characterized by transversal geotropism (diageotropism). Adventitious roots occur on stems, roots, and rarely on leaves. In the case when the main and lateral roots are well developed in the plant, a tap root system is formed, which may contain adventitious roots. If the adventitious roots are predominant in the plant, and the main root is invisible or absent, then a fibrous root system is formed.

Root functions:

Absorption from the soil of water with mineral salts dissolved in it. The function of absorption is performed by root hairs (or mycorrhiza) located in the absorption zone.

Anchoring the plant in the soil.

Synthesis of products of primary and secondary metabolism.

Biosynthesis of secondary metabolites (alkaloids, hormones and other biologically active substances) is carried out.

Root pressure and transpiration ensure the transport of aqueous solutions of mineral substances through the vessels of the root xylem (upward current), to the leaves and reproductive organs.

Reserve nutrients (starch, inulin) are deposited in the roots.

Synthesize in the meristematic zones the growth substances necessary for the growth and development of the aerial parts of the plant.

Carry out symbiosis with soil microorganisms - bacteria and fungi.

Provide vegetative reproduction.

Some plants (monstera, philodendron) act as a respiratory organ.

Root modifications. Very often the roots perform special functions, and in connection with this they undergo changes or metamorphoses. Root metamorphoses are fixed hereditarily.

Retractors (contractile) The roots of bulbous plants serve to immerse the bulb in the soil.

Reservers the roots are thickened and strongly parenchymatized. In connection with the accumulation of reserve substances, they acquire onion, conical, tuberous, and other forms. Storage roots include 1) roots in biennial plants. Not only the root, but also the stem (carrots, turnips, beets) takes part in their formation. 2) root tubers - thickening of adventitious roots. They are also called root cones(dahlia, sweet potato, chistyak). Necessary for the early appearance of large flowers.

Roots - trailers have climbing plants (ivy).

aerial roots characteristic of epiphytes (orchids). They provide the plant with absorption of water and minerals from the moist air.

Respiratory plants growing on waterlogged soils have roots. These roots rise above the soil surface and supply the underground parts of the plant with air.

stilted roots are formed in trees growing in the littoral of tropical seas (mangroves). Strengthens plants in loose soil.

Mycorrhiza- symbiosis of the roots of higher plants with soil fungi.

Nodules - tumor-like growths of the root bark as a result of symbiosis with nodule bacteria.

Columnar roots (roots - props) are laid as adventitious on the horizontal branches of the tree, reaching the soil, grow, supporting the crown. Indian banyan.

Some perennials adventitious buds are laid in the root tissues, which later develop into ground shoots. These escapes are called root suckers, and the plants root offspring(aspen - Populustremula, raspberry - Rubusidaeus, sow thistle - Sonchusarvensis, etc.).

Anatomical structure of the root.

In a young root, 4 zones are usually distinguished in the longitudinal direction:

division zone 1 - 2 mm. Represented by the tip of the growth cone, where active cell division occurs. It consists of cells of the apical meristem, and is covered by a root cap. It performs a protective function. When in contact with the soil, the cells of the root cap are destroyed with the formation of a mucous membrane. It (the root cap) is restored due to the primary meristem, and in cereals - due to a special meristem - calyptrogen.

Stretch zone is several mm. Cell divisions are practically absent. Cells are maximally stretched due to the formation of vacuoles.

Suction zone is several centimeters. This is where cell differentiation and specialization takes place. Distinguish integumentary tissue - epiblema with root hairs. Epiblema (rhizoderma) cells are living, with a thin cellulose wall. Some cells form long outgrowths - root hairs. Their function is the absorption of aqueous solutions by the entire surface of the outer walls. Therefore, the length of the hair is 0.15 - 8 mm. On average, from 100 to 300 root hairs are formed per 1 mm 2 of the root surface. They die in 10-20 days. play a mechanical (supporting) role - they serve as a support for the tip of the root.

Venue stretches up to the root neck and makes up most of the length of the root. In this zone, there is an intensive branching of the main root and the appearance of lateral roots.

Transverse structure of the root.

On a transverse section in the absorption zone in dicotyledonous plants, and in monocotyledonous plants, in the conduction zone, three main parts are distinguished: integumentary-absorption tissue, primary cortex, and the central axial cylinder.

Integumentary-absorption tissue - rhizoderm performs integumentary, suction, and also, partially, support functions. Represented by a single layer of epiblema cells.

The primary cortex of the root is most powerfully developed. It consists of exoderm, mesoderm = parenchyma of the primary cortex and endoderm. Exoderm cells are polygonal, tightly adjacent to each other, arranged in several rows. Their cell walls are impregnated with suberin (corking) and lignin (lignification). Suberin provides impermeability of cells to water and gases. Lignin gives it strength. The water and mineral salts absorbed by the rhizodermis pass through the thin-walled cells of the exoderm = passage cells. They are located under the root hairs. As the rhizodermal cells die off, the ectoderm can also perform an integumentary function.

The mesoderm is located under the ectoderm and consists of living parenchymal cells. They perform a storage function, as well as the function of conducting water and salts dissolved in it from the root hairs to the central axial cylinder.

The inner single-row layer of the primary cortex is represented by the endoderm. There are endoderm with Casparian bands and endoderm with horseshoe-shaped thickenings.

Endoderm with Casparian bands is the initial stage of endoderm formation, in which only the radial walls of its cells are thickened due to their impregnation with lignin and suberin.

In monocotyledonous plants in the cells of the endodermis, further impregnation of the cell walls with suberin occurs. As a result, only the outer cell wall remains unthickened. Among these cells, cells with thin cellulose membranes are observed. These are checkpoints. They are usually located opposite the rays of the xylem bundle of the radial type.

It is believed that the endoderm is a hydraulic barrier, facilitating the movement of minerals and water from the primary cortex into the central axial cylinder, and preventing their reverse flow.

The central axial cylinder consists of a single-row pericycle and a radial vascular fibrous bundle. The pericycle is capable of meristematic activity. It forms lateral roots. The vascular fibrous bundle is the conducting system of the root. In the root of dicotyledonous plants, the radial bundle consists of 1–5 xylem rays. Monocots have 6 or more xylem rays. Roots have no core.

In monocotyledonous plants, the structure of the root does not undergo significant changes during the life of the plant.

For dicot plants on the border of the suction zone and the zone of strengthening (conduction), there is a transition from the primary to secondary structure root. The process of secondary changes begins with the appearance of layers of cambium under the areas of the primary phloem, inward from it. The cambium arises from the poorly differentiated parenchyma of the central cylinder (stele).

Between the rays of the primary xylem from the cells of the procambium (lateral meristem), arcs of the cambium are formed, closing on the pericycle. The pericycle partially forms the cambium and phellogen. The cambial regions arising from the pericycle form only the parenchymal cells of the medullary rays. Cambium cells lay secondary xylem toward the center, and secondary phloem outward. As a result of the activity of the cambium, open collateral vascular-fibrous bundles are formed between the rays of the primary xylem, the number of which is equal to the number of rays of the primary xylem.

At the site of the pericycle, a cork cambium (phellogen) is laid, giving rise to periderm, the secondary integumentary tissue. The cork isolates the primary cortex from the central axial cylinder. The bark dies and is shed. The periderm becomes the integumentary tissue. And the root is actually represented by the central axial cylinder. In the very center of the axial cylinder, the rays of the primary xylem are preserved, between them there are vascular-fibrous bundles. The complex of tissues outside the cambium is called the secondary cortex. That. the root of the secondary structure consists of xylem, cambium, secondary cortex and cork.

Absorption and transport of water and minerals by the root.

Absorption of water from the soil and delivery to ground organs is one of the most important functions of the root, which arose in connection with the emergence of land.

Water enters the plants through the rhizoderm, in the absorption zone, the surface of which is increased due to the presence of root hairs. Xylem is formed in this zone of the root, providing an upward flow of water and minerals.

The plant absorbs water and minerals independently of each other, because. these processes are based on different mechanisms of action. Water passes into the root cells passively due to osmosis. In the root hair there is a huge vacuole with cell sap. Its osmotic potential ensures the flow of water from the soil solution into the root hair.

Mineral substances enter the root cells mainly as a result of active transport. Their absorption is facilitated by the release of various organic acids by the root, which convert inorganic compounds into a form available for absorption.

In the root, the horizontal movement of water and minerals occurs in the following sequence: root hair, cortical parenchyma cells, endoderm, pericycle, parenchyma of the axial cylinder, root vessels. Horizontal transport of water and minerals occurs in three ways:

The path through the apoplast (a system consisting of intercellular spaces and cell walls). Primary for the transport of water and ions of inorganic substances.

The path through the symplast (a system of cell protoplasts connected by plasmodesmata). Carries out the transport of mineral and organic substances.

The vacuolar pathway is the movement from vacuole to vacuole through other components of adjacent cells (plasma membranes, cytoplasm, vacuole tonoplast). Applicable exclusively for the transport of water. For the root is insignificant.

In the root, water moves along the apoplast to the endoderm. Here, its further advance is hindered by the Caspari bands, so further water enters the stele along the symplast through the passage cells of the endoderm. This switching of pathways regulates the movement of water and minerals from the soil into the xylem. In the stele, water encounters no resistance and enters the conducting vessels of the xylem.

Vertical transport water is coming along dead cells, so the movement of water is provided by the activity of the root and leaves. The root supplies water to the vessels of the stem under pressure, called the root. It occurs as a result of the fact that the osmotic pressure in the root vessels exceeds the osmotic pressure of the soil solution due to the active release of mineral and organic substances into the vessels by the root cells. Its value is 1 - 3 atm.

Evidence of root pressure is the "weeping of the plant" and guttation.

"Crying of a plant" - the release of liquid from a cut stem.

Guttation is the release of water from an intact plant through the tips of the leaves when it is in a humid atmosphere or intensively absorbs water and minerals from the soil.

The upper force of water movement is the suction force of the leaves, provided by transpiration. Transpiration is the evaporation of water from the surface of leaves. The sucking force of leaves in trees can reach 15 - 20 atm.

In the vessels of the xylem, water moves in the form of continuous water threads. Between water molecules there are forces of adhesion (cohesion), which causes them to move one after another. The adhesion of water molecules to the walls of vessels (adhesion) provides an upward capillary flow of water. The main driving force is transpiration.

For the normal development of the plant, the roots must be provided with moisture, access to fresh air and the necessary mineral salts. All this plants are obtained from the soil, which is the top fertile layer of the earth.

To increase the fertility of the soil, various fertilizers are applied to it. Fertilizing during plant growth is called top dressing.

There are two main groups of fertilizers:

Mineral fertilizers: nitrogen (nitrate, urea, ammonium sulfate), phosphate (superphosphate), potash (potassium chloride, ash). Complete fertilizers contain nitrogen, phosphorus and potassium.

Organic fertilizers - substances of organic origin (manure, bird droppings, peat, humus).

Nitrogen fertilizers dissolve well in water, promote plant growth. They are applied to the soil before sowing. For the ripening of fruits, the growth of roots, bulbs and tubers, phosphorus and potash fertilizers are needed. Phosphate fertilizers are poorly soluble in water. They are brought in in the fall, along with manure. Phosphorus and potassium increase the cold resistance of plants.

Plants in greenhouses can be grown without soil, in an aquatic environment that contains all the elements necessary for the plant. This method is called hydroponics.

There is also an aeroponics method - air culture - when the root system is in the air and is periodically irrigated with a nutrient solution.

The root is an axial organ that has radial symmetry and grows 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 by a root cap. 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 certain organic substances;

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

Root classification. By origin, the roots are divided into main, adventitious and lateral. The root that develops from the germinal root of the seed is called main; roots that arise on other plant organs (stem, leaf, flower) are called adnexal. 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.

In young stems, the rudiments of adventitious roots are formed endogenously - from the cells of the interfascicular parenchyma, in old ones - from the parenchyma near the cambium of the core rays, that is, the cambium of parenchymal origin. Adventitious roots of monocot stems arise in the parenchyma surrounding the bundles.

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

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. This 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 formed from the seed, in which the protokorm (embryonic tuber) develops, and subsequently - adventitious roots 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.

Classification of root systems in form.

The main root system is called pivotal if the main root noticeably exceeds the lateral ones in length and thickness;

With a similar size of the main and lateral roots, the root system fibrous. 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).

The root is the underground axial element of plants, which is their most important part, their main vegetative organ. Thanks to the root, the plant is fixed in the soil and is held there throughout the whole life cycle, and is also provided with water, minerals and nutrients contained in it. Exist different types and types of roots. Each of them has its own distinctive characteristics. In this article, we will look at existing species roots, types of root systems. We will also get acquainted with their characteristic features.

What are the types of roots?

The standard root is characterized by a filiform or narrow-cylindrical shape. In many plants, in addition to the main (main) root, other types of roots are also developed - lateral and adventitious. Let's take a closer look at what they are.

main root

This plant organ develops from the germinal root of the seed. The main root is always one (other types of plant roots are usually present during plural). It remains in the plant throughout the life cycle.

The root is characterized by positive geotropism, that is, due to gravity, it deepens into the substrate vertically down.

adventitious roots

Adventitious called the types of plant roots that are formed on their other organs. These organs can be stems, leaves, shoots, etc. For example, cereals have so-called primary adventitious roots, which are laid down in the stalk of the seed germ. They develop in the process of seed germination almost simultaneously with the main root.

There are also leaf adventitious types of roots (formed as a result of rooting of leaves), stem or nodal (formed from rhizomes, aboveground or underground stem nodes), etc. lower nodes powerful roots are formed, which are called aerial (or supporting).

The appearance of adventitious roots determines the ability of the plant to vegetative propagation.

Lateral roots

Lateral are called roots that arise as a lateral branch. They can form both on the main and adventitious roots. In addition, they can branch off from lateral roots, as a result of which lateral roots of higher orders (first, second and third) are formed.

Large lateral organs are characterized by transverse geotropism, that is, their growth occurs in an almost horizontal position or at an angle to the soil surface.

What is the root system?

The root system is called all types and types of roots that one plant has (that is, their totality). Depending on the ratio of growth of the main, lateral and adventitious roots, its type and character is determined.

Types of root systems

If the main root is very well developed and noticeable among the roots of another species, this means that the plant has a rod system. It is found mainly in dicotyledonous plants.

The root system of this type is characterized by deep germination into the soil. So, for example, the roots of some grasses can penetrate to a depth of 10-12 meters (thistle, alfalfa). The depth of penetration of tree roots in some cases can reach 20 m.

If adventitious roots are more pronounced, developing in large numbers, and the main one is characterized by slow growth, then a root system is formed, which is called fibrous.

As a rule, some of the herbaceous plants are also characterized by such a system. Despite the fact that the roots of the fibrous system do not penetrate as deeply as those of the rod system, they better braid the soil particles adjacent to them. Many loose-shrub and rhizomatous grasses, which form abundant fibrous thin roots, are widely used for fixing ravines, soils on slopes, etc. The best turf grasses include awnless bonfire, fescue, and others.

modified roots

In addition to the typical ones described above, there are other types of roots and root systems. They are called modified.

storage roots

The stocks include root crops and root tubers.

A root crop is a thickening of the main root due to the deposition of nutrients in it. Also, the lower part of the stem is involved in the formation of the root crop. Consists mostly of storage base tissue. Examples of root crops are parsley, radishes, carrots, beets, etc.

If the thickened storage roots are lateral and adventitious roots, then they are called root tubers (cones). They are developed in potatoes, sweet potatoes, dahlias, etc.

aerial roots

These are lateral roots growing in the aerial part. Found in a number of tropical plants. Water and oxygen are absorbed from the air. Available in tropical plants growing in conditions of lack of minerals.

respiratory roots

This is a kind of lateral roots that grow upward, rising above the surface of the substrate, water. Such types of roots are formed in plants growing on too moist soils, in swamp conditions. With the help of such roots, vegetation receives the missing oxygen from the air.

Supporting (board-shaped) roots

These types of tree roots are characteristic of large species (beech, elm, poplar, tropical, etc.). They are triangular vertical outgrowths formed by lateral roots and passing near or above the soil surface. They are also called board-shaped, because they resemble boards that are leaning against a tree.

Sucker roots (haustoria)

This is a type of additional adventitious roots developing on the stem. climbing plants. With their help, plants have the ability to attach to a certain support and climb (weave) up. Such roots are available, for example, in tenacious ficus, ivy, etc.

Retractable (contractile) roots

Characteristic of plants, the root of which is sharply reduced in the longitudinal direction at the base. An example would be plants that have bulbs. Retractable roots provide bulbs and root crops with some recess in the soil. In addition, their presence determines the tight fit of rosettes (for example, in a dandelion) to the ground, as well as the underground position of the vertical rhizome and root collar.

Mycorrhiza (fungus root)

Mycorrhiza is a symbiosis (mutually beneficial cohabitation) of the roots of higher plants with fungal hyphae, which braid them, acting as root hairs. Fungi provide plants with water and nutrients dissolved in it. Plants, in turn, provide fungi with organic substances necessary for their vital activity.

Mycorrhiza is inherent in the roots of many higher plants, especially woody ones.

bacterial nodules

These are modified lateral roots that are adapted for symbiotic cohabitation with nitrogen-fixing bacteria. The formation of nodules occurs due to the penetration of young roots into the interior. Such mutually beneficial cohabitation allows plants to receive nitrogen, which bacteria transfer from the air into a form accessible to them. Bacteria, on the other hand, are given a special habitat where they can function without competing with other types of bacteria. In addition, they use substances present in the roots of vegetation.

Bacterial nodules are typical for plants of the legume family, which are widely used as ameliorants in crop rotations in order to enrich soils with nitrogen. Taproot legumes, such as blue and yellow alfalfa, red and sainfoin, horned locust, etc., are considered the best nitrogen-fixing plants.

In addition to the above metamorphoses, there are other types of roots, such as prop roots (help strengthen the stem), stilted roots (help plants not to sink in liquid mud) and root suckers (have adventitious buds and provide vegetative propagation).

Root and root systems

Root- the main vegetative organ of a plant, which in a typical case performs the function of soil nutrition. The root is an axial organ that has radial symmetry and grows in length indefinitely due to the activity of the apical meristem. It differs morphologically from the shoot in that leaves never form on it, and the apical meristem is always covered by a root cap.

In addition to the main function of absorbing substances from the soil, the roots also perform other functions:

1) the roots strengthen (“anchor”) the plants in the soil, make it possible to grow vertically and shoot up;

2) various substances are synthesized in the roots, which then move to other organs of the plant;

3) reserve substances can be deposited in the roots;

4) roots interact with the roots of other plants, microorganisms, fungi that live in the soil.

The totality of the roots of one individual forms a single morphological and physiological relation root system.

The composition of root systems includes roots of various morphological nature - main root, lateral and adnexal roots.

main root develops from the germinal root. Lateral roots are formed on the root (main, lateral, subordinate), which in relation to them is designated as maternal. They arise at some distance from the apex, in the direction from the base of the root to its top. Lateral roots are laid endogenously, i.e. in the internal tissues of the maternal root. If branching occurred at the apex itself, it would make it difficult for the root to move through the soil. adventitious roots can occur on stems, and on leaves, and on roots. In the latter case, they differ from lateral roots in that they do not show a strict order of initiation near the apex of the maternal root and may appear in old root areas.

By origin, the following types of root systems are distinguished ( rice. 4.1):

1) main root system represented by the main root (first order) with lateral roots of the second and subsequent orders (in many shrubs and trees, most dicotyledonous plants);

2) adventitious root system develops on stems, leaves; found in most monocotyledonous plants and many dicotyledons that reproduce vegetatively;

3) mixed root system formed by the main and adventitious roots with their lateral branches (many herbaceous dicots).

Rice. 4.1. Types of root systems: A - main root system; B - system of adventitious roots; C - mixed root system (A and C - tap root systems; B - fibrous root system).

Distinguished by shape rod and fibrous root systems.

AT pivotal In the root system, the main root is strongly developed and is clearly visible among the other roots. AT fibrous root system, the main root is invisible or absent, and the root system is composed of numerous adventitious roots ( rice. 4.1).

The root has potentially unlimited growth. However, under natural conditions, the growth and branching of roots is limited by the influence of other roots and soil conditions. environmental factors. The bulk of the roots is located in the upper soil layer (15 cm), the richest in organic matter. The roots of trees deepen on average by 10-15 m, and in width they usually spread beyond the radius of the crowns. The root system of corn goes to a depth of about 1.5 m and about 1 m in all directions from the plant. The record depth of root penetration into the soil was noted in the desert mesquite shrub - more than 53 m.

In one rye bush grown in a greenhouse, the total length of all roots was 623 km. The total growth of all roots in one day was approximately 5 km. General surface of all roots in this plant was 237 m 2 and was 130 times larger than the surface of above-ground organs.

Zones of the young root ending - these are parts of a young root that are different in length, perform different functions and are characterized by certain morphological and anatomical features ( rice. 4.2).

The tip of the root is always covered from the outside root cap protecting the apical meristem. The sheath consists of living cells and is constantly updated: as old cells are shed from its surface, the apical meristem forms new young cells to replace them from the inside. The outer cells of the root cap flake off while still alive, producing a copious mucus which facilitates the root to move through the hard soil particles. The cells of the central part of the cap contain many starch grains. Apparently, these grains serve statoliths, i.e., they are able to move in the cell when the position of the root tip in space changes, due to which the root always grows in the direction of gravity ( positive geotropism).

Under the cover is dividing zone, represented by the apical meristem, as a result of which all other zones and tissues of the root are formed. The division zone has dimensions of about 1 mm. The cells of the apical meristem are relatively small, multifaceted, with a dense cytoplasm and a large nucleus.

Following the division zone is located stretch zone, or growth zone. In this zone, cells almost do not divide, but strongly stretch (grow) in the longitudinal direction, along the axis of the root. The volume of cells increases due to the absorption of water and the formation of large vacuoles, while high turgor pressure pushes the growing root between the soil particles. The stretch zone is usually small and does not exceed a few millimeters.

Rice. 4.2. General view (A) and longitudinal section (B) of the root end (scheme): I - root cap; II - zones of division and stretching; III - suction zone; IV - beginning of the conduction zone: 1 - growing lateral root; 2 - root hairs; 3 - rhizoderma; 3a - exoderm; 4 - primary bark; 5 - endoderm; 6 - pericycle; 7 - axial cylinder.

Next comes absorption zone, or suction zone. In this zone, the integumentary tissue is rhizoderma(epiblema), the cells of which bear numerous root hairs. The stretching of the root stops, the root hairs tightly cover the soil particles and, as it were, grow together with them, absorbing water and mineral salts dissolved in it. The absorption zone extends up to several centimeters. This area is also called zone of differentiation, since it is here that the formation of permanent primary tissues occurs.

The life span of the root hair does not exceed 10-20 days. Above the suction zone, where the root hairs disappear, begins holding area. Through this part of the root, water and salt solutions absorbed by the root hairs are transported to the overlying organs of the plant. Lateral roots are formed in the conduction zone (Fig. 4.2).

The cells of the suction and conduction zones occupy a fixed position and cannot move relative to the soil areas. However, the zones themselves, due to constant apical growth, continuously move along the root as the root ending grows. Young cells are constantly included in the absorption zone from the side of the stretching zone and at the same time aging cells are excluded, passing into the composition of the conduction zone. Thus, the suction apparatus of the root is a mobile formation that continuously moves in the soil.

In the same way, internal tissues appear consistently and naturally in the root ending.

The primary structure of the root. The primary structure of the root is formed as a result of the activity of the apical meristem. The root differs from the shoot in that its apical meristem deposits cells not only inward, but also outward, replenishing the cap. The number and location of initial cells in the root apexes vary considerably in plants belonging to different systematic groups. Derivatives of initials already near the apical meristem differentiate into primary meristems - 1) protodermis, 2) main meristem and 3) procambium(rice. 4.3). From these primary meristems, three tissue systems are formed in the suction zone: 1) rhizoderma, 2) primary cortex and 3) axial (central) cylinder, or stele.

Rice. 4.3. Longitudinal section of the tip of an onion root.

rhizoderma (epiblema, root epidermis) - absorbent tissue formed from protoderms, the outer layer of the primary root meristem. In functional terms, the rhizoderm is one of the most important plant tissues. Through it, water and mineral salts are absorbed, it interacts with the living population of the soil, and through the rhizoderm, substances that help soil nutrition are released from the root into the soil. The absorbing surface of the rhizodermis is greatly enlarged due to the presence of tubular outgrowths in some of the cells - root hairs(Fig. 4.4). The hairs are 1-2 mm long (up to 3 mm). In one four-month-old rye plant, approximately 14 billion root hairs were found with a absorption area of 401 m 2 and a total length of more than 10,000 km. In aquatic plants, root hairs may be absent.

The wall of the hair is very thin and consists of cellulose and pectin. Its outer layers contain mucus, which helps to establish closer contact with soil particles. Slime creates favorable conditions for the settlement of beneficial bacteria, affects the availability of soil ions and protects the root from drying out. Physiologically, the rhizoderm is highly active. It absorbs mineral ions with the expenditure of energy. The hyaloplasm contains a large number of ribosomes and mitochondria, which is typical for cells with high level metabolism.

Rice. 4.4. Cross section of the root in the suction zone: 1 - rhizoderma; 2 - exoderm; 3 - mesoderm; 4 - endoderm; 5 - xylem; 6 - phloem; 7 - pericycle.

From main meristem formed primary cortex. The primary cortex of the root is differentiated into: 1) exoderm- the outer part, lying directly behind the rhizoderm, 2) the middle part - mesoderm and 3) the innermost layer - endoderm (rice. 4.4). The bulk of the primary cortex is mesoderm, formed by living parenchymal cells with thin walls. The cells of the mesoderm are located loosely, the gases necessary for cell respiration circulate along the system of intercellular spaces along the axis of the root. In marsh and aquatic plants, the roots of which lack oxygen, the mesoderm is often represented by aerenchyma. Mechanical and excretory tissues may also be present in the mesoderm. The parenchyma of the primary cortex performs a number of important functions: it participates in the absorption and conduction of substances, synthesizes various compounds, reserve nutrients, such as starch, are often deposited in the cells of the cortex.

The outer layers of the primary cortex, underlying the rhizoderm, form exoderm. The exoderm arises as a tissue that regulates the passage of substances from the rhizoderm to the cortex, but after the death of the rhizoderm above the absorption zone, it appears on the root surface and turns into a protective integumentary tissue. The exoderm is formed as a single layer (rarely several layers) and consists of living parenchymal cells tightly closed together. As the root hairs die, the walls of the exoderm cells are covered on the inside with a layer of suberin. In this respect, the exoderm is similar to the cork, but unlike it, it is primary in origin, and the cells of the exoderm remain alive. Sometimes in the exoderm, pass cells with thin, non-corked walls are preserved, through which selective absorption of substances occurs.

The innermost layer of the primary cortex is endoderm. It surrounds the stele in the form of a continuous cylinder. Endoderm in its development can go through three stages. At the first stage, its cells fit tightly to each other and have thin primary walls. Thickenings in the form of frames are formed on their radial and transverse walls - Caspari belts (rice. 4.5). The belts of neighboring cells are closely connected with each other, so that a continuous system of them is created around the stele. Suberin and lignin are deposited in Caspari bands, which makes them impermeable to solutions. Therefore, substances from the cortex to the stele and from the stele to the cortex can pass only along the symplast, i.e., through the living protoplasts of endoderm cells and under their control.

Rice. 4.5. Endoderm at the first stage of development (scheme).

At the second stage of development, suberin is deposited over the entire inner surface of endoderm cells. However, some cells retain their original structure. it check cells, they remain alive, and through them the connection between the primary cortex and the central cylinder is carried out. As a rule, they are located opposite the rays of the primary xylem. In roots that do not have secondary thickening, the endoderm can acquire a tertiary structure. It is characterized by a strong thickening and lignification of all walls, or more often the walls facing outward remain relatively thin ( rice. 4.7). Passage cells are also preserved in the tertiary endoderm.

Central(axial) cylinder, or stele formed in the center of the root. Already close to the division zone, the outermost layer of the stele forms pericycle, the cells of which retain the character of the meristem and the ability for neoplasms for a long time. In a young root, the pericycle consists of a single row of thin-walled living parenchymal cells ( rice. 4.4). The pericycle performs several important functions. In most seed plants, lateral roots are laid in it. In species with secondary growth, it participates in the formation of the cambium and gives rise to the first layer of the phellogen. In the pericycle, the formation of new cells often occurs, which are then included in its composition. In some plants, the adventitious buds also appear in the pericycle. In old roots of monocots, the cells of the pericycle are often sclerified.

Cells behind the pericycle procambia, which differentiate into primary conductive tissues. Phloem and xylem elements are laid in a circle, alternating with each other, and develop centripetally. However, the xylem in its development usually overtakes the phloem and occupies the center of the root. On a transverse section, the primary xylem forms a star, between the rays of which there are sections of phloem ( rice. 4.4). This structure is called radial conducting beam.

A xylem star can have a different number of rays - from two to many. If there are two, the root is called diarchic if three - triarchal, four - tetrarch, and if a lot - polyarchal (rice. 4.6). The number of xylem rays usually depends on the thickness of the root. In the thick roots of monocot plants, it can reach 20-30 ( rice. 4.7). In the roots of the same plant, the number of xylem rays can be different; in thinner branches, it is reduced to two.

Rice. 4.6. Types of structure of the axial cylinder of the root (scheme): A - diarch; B - triarch; B - tetrarch; G - polyarchy: 1 - xylem; 2 - phloem.

Spatial separation of strands of primary phloem and xylem, located at different radii, and their centripetal laying are characteristic features of the structure of the central cylinder of the root and are of great biological importance. The elements of the xylem are as close as possible to the surface of the stele, and it is easier for them, bypassing the phloem, to penetrate the solutions coming from the bark.

Rice. 4.7. Cross section of the root of a monocot plant: 1 - remains of rhizoderm; 2 - exoderm; 3 - mesoderm; 4 - endoderm; 5 - checkpoints; 6 - pericycle; 7 - xylem; 8 - phloem.

The central part of the root is usually occupied by one or more large xylem vessels. The presence of a core is generally atypical for a root, however, in the roots of some monocots in the middle is small plot mechanical tissue ( rice. 4.7) or thin-walled cells arising from procambium (Fig. 4.8).

Rice. 4.8. Cross section of a corn root.

The primary root structure is characteristic of young roots of all plant groups. In spore and monocotyledonous plants, the primary structure of the root is preserved throughout life.

Secondary structure of the root. In gymnosperms and dicotyledonous plants, the primary structure does not last long and above the absorption zone is replaced by a secondary one. Secondary root thickening occurs due to the activity of secondary lateral meristems - cambium and phellogen.

Cambium arises in the roots from meristematic procambial cells in the form of a layer between the primary xylem and phloem ( rice. 4.9). Depending on the number of phloem cords, two or more zones of cambial activity are formed simultaneously. At first, the cambial layers are separated from each other, but soon the cells of the pericycle, lying opposite the rays of the xylem, divide tangentially and connect the cambium into a continuous layer surrounding the primary xylem. The cambium lays down layers secondary xylem (wood) and out secondary phloem (bast). If this process lasts a long time, then the roots reach a considerable thickness.

Rice. 4.9. The establishment and beginning of the activity of the cambium in the root of the pumpkin seedling: 1 - primary xylem; 2 - secondary xylem; 3 - cambium; 4 - secondary phloem; 5 - primary phloem; 6 - pericycle; 7 - endoderm.

The areas of the cambium that have arisen from the pericycle consist of parenchymal cells and are not capable of depositing elements of conducting tissues. They form primary core rays, which are wide areas of the parenchyma between the secondary conductive tissues ( rice. 4.10). Secondary core, or beams of wood appear additionally with prolonged thickening of the root, they are usually narrower than the primary ones. The core rays provide a link between the xylem and phloem of the root, and radial transport of various compounds occurs along them.

As a result of the activity of the cambium, the primary phloem is pushed outward and squeezed. The primary xylem star remains in the center of the root, its rays can persist for a long time ( rice. 4.10), but more often the center of the root is filled with secondary xylem, and the primary xylem becomes invisible.

Rice. 4.10. Cross section of pumpkin root (secondary structure): 1 - primary xylem; 2 - secondary xylem; 3 - cambium; 4 - secondary phloem; 5 - primary core beam; 6 - cork; 7 - parenchyma of the secondary cortex.

The tissues of the primary cortex cannot follow the secondary thickening and are doomed to death. They are replaced by secondary integumentary tissue - periderm, which can be stretched on the surface of a thickening root due to the work of phellogen. fellogen is laid in the pericycle and begins to lay out cork, and inside phelloderma. The primary bark, cut off by a cork from the internal living tissues, dies and is discarded ( rice. 4.11).

Phelloderm cells and parenchyma, formed by cell division of the pericycle, form parenchyma of the secondary cortex surrounding conductive tissues (Fig. 4.10). Outside, the roots of the secondary structure are covered with periderm. The crust is rarely formed, only on old tree roots.

Perennial roots of woody plants often thicken greatly as a result of prolonged activity of the cambium. The secondary xylem of such roots merges into a solid cylinder, surrounded on the outside by a cambium ring and a continuous ring of secondary phloem ( rice. 4.11). Compared with the stem, the boundaries of annual rings in the wood of the root are much less pronounced, the bast is more developed, and the medullary rays are, as a rule, wider.

Rice. 4.11. Cross section of a willow root at the end of the first growing season.

Specialization and metamorphoses of roots. Most plants in the same root system have distinctly different growth and sucking endings. Growth endings are usually more powerful, quickly elongate and move deep into the soil. Their elongation zone is well defined, and the apical meristems work vigorously. Sucking endings, which appear 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.

Woody plants have thick skeletal and semi-skeletal roots on which short-lived root lobes. The composition of the root lobes, continuously replacing each other, includes growth and sucking endings.

The roots of many plants form a symbiosis with hyphae of soil fungi, called mycorrhiza("mushroom root"). Mycorrhiza is formed on sucking roots in the absorption zone. The fungal component makes it easier for the roots to obtain water and mineral elements from the soil; fungal hyphae often replace root hairs. In turn, the fungus receives carbohydrates and other nutrients from the plant. There are two main types of mycorrhiza. gifs ectotrophic mycorrhiza form a sheath that envelops the root from the outside. Ectomycorrhiza is widespread in trees and shrubs. Endotrophic mycorrhiza is found mainly in herbaceous plants. Endomycorrhiza is located inside the root, hyphae are introduced into the cells of the bovine parenchyma. Mycotrophic nutrition is very widespread. Some plants, such as orchids, cannot exist at all without symbiosis with fungi.

On the roots of legumes, special formations appear - nodules in which bacteria from the genus Rhizobium settle. These microorganisms are able to assimilate atmospheric molecular nitrogen, converting it into a bound state. Part of the substances synthesized in the nodules are absorbed by plants, bacteria, in turn, use the substances found in the roots. This symbiosis is of great importance for agriculture. Legumes are rich in protein due to the additional source of nitrogen. They provide valuable food and fodder products and enrich the soil with nitrogenous substances.

Very widespread hoarding roots. They are usually thickened and strongly parenchymatized. Strongly thickened adventitious roots are called root cones, or root tubers(dahlia, some orchids). Many, more often biennial, plants with a tap root system develop a formation called root crop. Both the main root and the lower part of the stem take part in the formation of the root crop. In carrots, almost the entire root crop is composed of a root; in turnips, the root forms only the lowest part of the root crop ( rice. 4.12).

Fig.4.12. Root vegetables of carrots (1, 2), turnips (3, 4) and beets (5, 6, 7) ( xylem black on transverse sections; the horizontal dotted line shows the border of the stem and root).

Root crops of cultivated plants arose as a result of long-term selection. In root crops, the storage parenchyma is highly developed and mechanical tissues have disappeared. In carrots, parsley, and other umbellifers, the parenchyma is strongly developed in the phloem; in turnips, radishes and other cruciferous plants - in xylem. In beets, reserve substances are deposited in the parenchyma formed by the activity of several additional layers of cambium ( rice. 4.12).

Many bulbous and rhizomatous plants form retractors, or contractile roots ( rice. 4.13, 1). They can shorten and draw the shoot into the soil to the optimum depth during the summer drought or winter frosts. Retracting roots have thickened bases with transverse wrinkling.

Rice. 4.13. root metamorphoses: 1 - corm of gladiolus with retracting roots thickened at the base; 2 - respiratory roots with pneumatophores in Avicenna ( etc- tidal zone); 3 - aerial roots of an orchid.

Rice. 4.14. Part of a cross section of an aerial root of an orchid: 1 - velamen; 2 - exoderm; 3 - checkpoint.

Rice. 4.15. supporting roots: 1 - stilted corn roots; 2 - columnar banyan roots; 3 - stilted roots of rhizophora ( etc- tidal zone; from- low tide zone; silt- the surface of the muddy bottom).