Definition of the root and its functions. Classification of root systems by origin and structure. The main functions of plant roots The main function of the root is to

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The main functions of plant roots

Introduction

3. Root functions

5. Root anatomy

Literature

Introduction

Komren is an axial, (usually) underground vegetative organ of higher plants, which has unlimited growth in length and positive geotropism. The root fixes the plant in the soil and ensures the absorption and conduction of water with dissolved minerals to the stem and leaves.

There are no leaves on the root, and there are no chloroplasts in the root cells.

In addition to the main root, many plants have lateral and adventitious roots. The totality of all the roots of a plant is called the root system. In the case when the main root is slightly expressed, and the adventitious roots are expressed significantly, the root system is called fibrous. If the main root is expressed significantly, the root system is called pivotal.

Some plants lay down reserve nutrients in the root, such formations are called root crops.

Main Functions of the Root

Fixing the plant in the substrate;

Absorption, conduction of water and minerals;

supply of nutrients;

Interaction with the roots of other plants (symbiosis), fungi, microorganisms living in the soil (mycorrhiza, legume nodules).

Vegetative reproduction

Synthesis of biologically active substances

In many plants, the roots perform special functions (aerial roots, sucker roots).

Root origin.

The body of the first plants that landed on land had not yet been dissected into shoots and roots. It consisted of branches, some of which rose vertically, while others pressed against the soil and absorbed water and nutrients. Despite the primitive structure, these plants were provided with water and nutrients, as they were small in size and lived near water.

In the course of further evolution, some branches began to go deeper into the soil and gave rise to roots adapted to more perfect soil nutrition. This was accompanied by a deep restructuring of their structure and the appearance of specialized tissues. Rooting was a major evolutionary achievement that allowed plants to take up drier soils and produce large shoots that rose up into the light. For example, bryophytes do not have real roots, their vegetative body is small in size - up to 30 cm, mosses live in humid places. In ferns, real roots appear, this leads to an increase in the size of the vegetative body and to the flowering of this group in the Carboniferous period.

1. Modifications and specialization of roots

The roots of some buildings are prone to metamorphosis.

Root changes:

The root crop is a thickened adventitious root. The main root and the lower part of the stem are involved in the formation of the root crop. Most root plants are biennial. Root crops consist mainly of storage basic tissue (turnips, carrots, parsley).

Root tubers (root cones) are formed as a result of thickening of the lateral and adventitious roots.

Hook roots are a kind of adventitious roots. With the help of these roots, the plant "sticks" to any support.

Stilted roots - act as a support.

Aerial roots - lateral roots, grow in the aerial part. They absorb rainwater and oxygen from the air. Formed in many tropical plants in conditions of high humidity.

Mycorrhiza is the cohabitation of the roots of higher plants with fungal hyphae. With such a mutually beneficial cohabitation, called symbiosis, the plant receives water from the fungus with nutrients dissolved in it, and the fungus receives organic substances. Mycorrhiza is characteristic of the roots of many higher plants, especially woody ones. Fungal hyphae, braiding thick lignified roots of trees and shrubs, act as root hairs.

Bacterial nodules on the roots of higher plants - the cohabitation of higher plants with nitrogen-fixing bacteria - are modified lateral roots adapted to symbiosis with bacteria. Bacteria penetrate the root hairs into young roots and cause them to form nodules. In this symbiotic cohabitation, bacteria convert the nitrogen in the air into a mineral form available to plants. And plants, in turn, provide bacteria with a special habitat in which there is no competition with other types of soil bacteria. Bacteria also use substances found in the roots of higher plants. Most often, bacterial nodules are formed on the roots of plants of the legume family. In connection with this feature, legume seeds are rich in protein, and members of the family are widely used in crop rotation to enrich the soil with nitrogen.

Respiratory roots - in tropical plants - perform the function of additional respiration.

2. Features of the structure of the roots

The set of roots of one plant is called the root system.

The composition of the root systems includes roots of various nature.

Distinguish:

main root,

side roots,

adventitious roots.

The main root develops from the germinal root. Lateral roots occur on any root as a lateral branch. Adventitious roots are formed by the shoot and its parts.

An organ is a body part of an organism that has a certain structure and performs certain functions.

The body of higher plants is differentiated into vegetative and generative (reproductive) organs.

Vegetative organs form the body of a higher plant and support its life for a long time. Due to the close structural and functional interaction of vegetative organs - root, stem and leaf - all manifestations of plant life as an integral organism are carried out: the absorption of water and minerals from the soil, phototrophic nutrition, respiration, growth and development, vegetative reproduction.

3. Root functions

The root is the axial organ of the plant, which serves to strengthen the plant in the substrate and absorb water and dissolved minerals from it. In addition, various organic substances (growth hormones, alkaloids, etc.) are synthesized in the root, which then move through the xylem vessels to other plant organs or remain in the root itself. Often it is a storage place for spare nutrients.

In root plants (aspen, poplar, willow, raspberry, cherry, lilac, field thistle, etc.), the root performs the function of vegetative reproduction: adnexal buds are formed on their roots, from which above-ground shoots develop - root offspring.

The formation of roots was a significant evolutionary achievement, thanks to which the plants adapted to a more perfect soil nutrition and were able to form large shoots that rise up to the sunlight.

4. Types of roots and types of root systems

The root that develops from the germinal root of the seed is called the main root. Lateral roots capable of branching depart from it. Roots can also form on the aerial parts of plants - stems or leaves; such roots are called adventitious. The totality of all the roots of a plant makes up the root system.

There are two main types of root systems: taproot, which has a well-developed main root, which is longer and thicker than others, and fibrous, in which the main root is absent or does not stand out among numerous adventitious roots. The tap root system is characteristic mainly for dicotyledonous plants, fibrous - for most monocots.

The root grows in length due to cell division of the apical (apical) meristem. The tip of the root is covered in the form of a thimble with a root cap, which protects the delicate cells of the apical meristem from mechanical damage and promotes root movement in the soil. The root cap, consisting of living thin-walled cells, is continuously renewed: produces new young cells Cap cells produce a rich mucus that coats the root, making it easier to glide between soil particles Mucus also creates favorable conditions for beneficial bacteria to settle in. It can also affect the availability of soil ions and provide short-term protection of the root from drying out, The lifespan of the cells of the root cap A is 9 days, depending on the length of the cap and the type of plant.

5. Root anatomy

On a longitudinal section of the root tip, several zones can be distinguished: division, growth, absorption and conduction (Fig. 8.6).

Rice. 8.6. Young root zones

(a - general view; b - longitudinal section of the root tip): I - root cap; II - growth zone; III -- zone of root hairs (zone of absorption); IV - the area of conduct; I - laying lateral root; 2 - root hairs on the epibleme; 3 --- epiblema; 4 - exoderm; 5 - primary bark; b - endoderm; 7 - pericycle; 8 -- axial cylinder; 9 - cells of the root cap; 10 - apical meristem.

The division zone is located under the cap and is represented by cells of the apical meristem. Its length is about 1 mm. Behind the division zone is a stretch zone (growth zone) only a few millimeters long. Cell growth in this zone provides the main elongation of the root. The suction zone (root hair zone) up to several centimeters long begins above the stretch zone; The function of this zone is clear from its name.

It should be noted that the transition from one zone to another occurs gradually, without sharp boundaries. Some cells begin to elongate and differentiate while still in the dividing zone, while others reach maturity in the elongated zone.

The soil solution enters the root mainly through the suction zone, so the larger the surface of this root area, the better it performs its main suction function. It is in connection with this function that part of the skin cells is extended into root hairs 0.1–8 mm long (see Fig. 8.6). Almost the entire root hair cell is occupied by a vacuole surrounded by a thin layer of cytoplasm. The nucleus is located in the cytoplasm near the top of the hair. Root hairs are able to cover soil particles, as if growing together with them, which facilitates the absorption of water and minerals from the soil. Absorption is also facilitated by the secretion of various acids (carbonic, malic, citric, oxalic) by root hairs, which dissolve soil particles.

Root hairs are formed very quickly (for young apple seedlings in 30-40 hours). One individual of a four-month-old rye plant has about 14 billion root hairs with an absorption area of about 400 m2 and a total length of more than 10 thousand km; the surface of the entire root system, including root hairs, is approximately 640 m2, i.e., 130 times greater than that of the shoot. Root hairs function for a short time - usually 10-20 days. Replace dead root hairs in the lower part of the root with new ones. Thus, the most active, suction zone of the roots is constantly moving deeper and to the sides, following the growing tips of the branchings of the root system. At the same time, the total suction surface of the roots increases all the time.

root system plant anatomical

Rice. 8.7. Cross section of the root

(a - monocotyledonous, 6 - dicotyledonous plants): I - central (axial) cylinder; 2 - remnants of epible-we; 3 - exoderm; 4 - parenchyma of the primary cortex; 5 - endoderm; 6 - pericycle; 7 - phloem; 8 - xylem; 9 - passage cells of the endoderm; 10 - root hair.

On the transverse section, the bark and the central cylinder are distinguished at the root (Fig. 8.6 and 8.7).

The primary cortex is covered with a kind of epidermis, the cells of which are involved in the formation of root hairs. In this regard, the epidermis of the root is called rhizodermis or epiblema.

The primary cortex consists of exoderm, parenchyma, and endoderm. The exoderm consists of one or more layers of cells, the walls of which are capable of thickening. After the death of the epidermis, these layers of the cortex perform the function of the integumentary tissue. Thickening of the shell also has an inner layer of the cortex - the endoderm.

The axial or central cylinder consists of a conducting system (xylem and phloem) surrounded by a ring of living pericycle cells capable of meristematic activity.

Due to cell division of the pericycle, lateral roots are formed. The inner part of the central cylinder in most roots is occupied by a complex vascular bundle of a radial structure: radially located sections of the primary xylem alternate with sections of the primary phloem. In monocots and ferns, the primary root structure is preserved throughout life. In dicotyledonous and gymnosperms, due to the activity of the cambium, the secondary structure of the root is formed: changes occur in the central cylinder (the cambium forms secondary conductive tissues), causing the growth of the root in thickness.

6. Mineral nutrition of plants

Mineral nutrition is a set of processes of absorption from the soil, movement and assimilation of macro- and microelements (N, S, P, K, Ca, Mg, Mn, Zn, Fe, Cu, etc.) necessary for the life of a plant organism. Together with photosynthesis, mineral nutrition constitutes a single process of plant nutrition.

The entry of water and dissolved substances into root cells through biological membranes is carried out due to such processes as osmosis, diffusion, facilitated diffusion, and active transport (see Chap. 1).

The main driving forces that ensure the movement of the soil solution through the vessels of the root and stem to the nights, leaves and flowers are the suction force of transpiration and root pressure.

Almost all the minerals and water necessary for growth and development, plants receive from the soil - the upper fertile layer of the earth's crust, changed under the influence of natural factors and human activity.

7. Importance of tillage and fertilization in the life of cultivated plants

The amount of water and minerals in the soil is determined by its physical and chemical properties, the vital activity of microorganisms and plants, the type of soil, etc. The combination of all these factors determines the fertility of the soil, on which the productivity of agricultural plants largely depends. Therefore, scientifically substantiated tillage (peeling, plowing, cultivation, rolling, harrowing, etc.) plays a primary role in increasing its fertility. As a result, plants receive the best conditions for growth and development throughout the growing season.

Tillage is accompanied by a decrease in the size of soil particles. This leads to an increase in the absorption and water-holding capacity of the soil. The fragmentation of soil particles contributes to an increase in their surface, which allows the soil to retain solutions of mineral substances for a long time, bind them into less soluble compounds, and thereby slow down their leaching.

Loose soil is characterized by good water permeability and increased moisture capacity. With low water permeability, rain and especially melt water does not have time to be absorbed into the soil, flowing down the slopes, carrying small soil particles with it, causing its erosion. In the absence of runoff, water stagnates on the surface of the field, blocking air access to the soil. This leads to inhibition and even death of plants (for example, the soaking of winter crops in spring). Loose soil contains a significant amount of capillary moisture, which fills the capillary gaps between soil particles. Under the influence of capillary forces, this moisture can rise to the upper horizons of the soil, creating an upward current. This is especially important in summer, when the rate of water evaporation from the soil surface increases and plants experience difficulties in water supply.

The thermal regime of soils is associated with water and air regimes. For example, an increase in soil temperature increases the movement of water in it, as well as the decomposition of organic compounds and the formation of minerals. Therefore, the sooner the soil is processed in the spring, the sooner and deeper it will warm up, especially if there are large pores in the soil.

Thus, mechanical tillage creates a moderately loose arable layer, optimal water, air and thermal conditions of the soil, activates the vital activity of microorganisms that convert organic humus substances into mineral salts, which are absorbed by plant roots in the form of aqueous solutions. When tilling the soil, weeds, pests and pathogens of plants are destroyed, plant residues and fertilizers are embedded in the soil.

Usually, fertile soil contains a sufficient amount of such important mineral nutrients as nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, etc. Their amount carried out with one crop is relatively small. However, when one crop after another is removed from the field and the necessary elements are withdrawn from the cycle, the content of some of them (most often potassium) may decrease so much that it becomes necessary to apply fertilizers containing deficient elements. Nutrient deficiencies cannot be replaced by any other agricultural practices.

Fertilizers are substances necessary for the mineral nutrition of plants and increasing soil fertility. According to the chemical composition, fertilizers are usually divided into organic and mineral.

Organic fertilizers (manure, peat, slurry, compost, sapropels, bird droppings, etc.) contain nutrients in the form of organic compounds of plant and animal origin. They decompose very slowly and can provide plants with both macro- and microelements for a long time. In addition, organic fertilizers improve the physical properties of the soil: increase its structure, water-holding capacity, improve the thermal regime, and activate the activity of soil microorganisms.

Doses of manure depend on soil and climatic conditions, biological characteristics of the crop and the quality of fertilizers. For example, the following are considered the optimal doses of bedding manure for the main crops: for winter cereals - 20--30 t / ha, for corn and potatoes - 50--70, for root crops and vegetables - 70--80 t / ha. In this case, it is necessary to additionally apply mineral fertilizers.

Mineral fertilizers contain all nutrients necessary for plants. Their classification is based on the chemical composition of fertilizers - nitrogen, phosphorus, potash, complex, lime, microfertilizers. All of them are easier and faster than organic ones, they decompose in the soil. Mineral fertilizers are applied in autumn or spring simultaneously with sowing seeds, often in the form of top dressing during various periods of plant vegetation.

Bacterial fertilizers (nitragin, azotobacterin, phosphorobacterin) are preparations containing soil microorganisms useful for agricultural plants that can improve the root nutrition of plants.

Fertilizers can significantly increase crop yields. It is believed that in the world every fourth inhabitant eats products obtained as a result of the use of fertilizers.

The importance of fertilizers also lies in the fact that they not only increase yields, but also, when used correctly, improve the quality of crop products. For example, nitrogen fertilization of winter wheat during heading (milky ripeness) increases the protein content in the grain by 1-3%, and the application of phosphorus and potassium fertilizers increases the starch content in potato tubers, sugar in root crops, and fiber flax yield.

Modifications (metamorphoses) of roots. In the process of historical development, the roots of many plant species acquired, in addition to the main ones, some additional functions. One of these functions is storage. The main root thickened as a result of deposition of nutrients is called a root crop. Root crops are formed in a number of biennial plants (turnips, carrots, beets, rutabaga, etc.). Thickenings of lateral or adventitious roots (orchid, lyubka, chistyak, dahlia, etc.) are called root tubers or root cones. Reserve nutrients of root crops and root tubers are spent on the formation and growth of vegetative and generative organs of plants.

Many plants develop contractile, aerial, stilted and other types of roots.

Contractile, or retracting, roots are able to significantly contract in the longitudinal direction. At the same time, they draw the lower part of the stem with renewal buds, tubers, bulbs deep into the soil and thus ensure the transfer of an unfavorable cold winter period. Tulips, narcissus, gladiolus, etc. have such roots.

In tropical plants, adventitious aerial roots are able to capture atmospheric moisture, and powerful branched stilted roots on mangrove trunks provide plant resistance to breaking waves. At low tide, the trees rise on their roots, as if on stilts.

Plants growing in a swamp or soils poor in oxygen form respiratory roots. These are processes of lateral roots growing vertically upwards and towering above the water or soil. They are rich in air-bearing tissue - aerenchyma - with large intercellular spaces through which atmospheric air enters the underground parts of the roots.

Literature

1. Fedorov Al. A., Kirpichnikov M.E. and Artyushenko Z.T. Atlas on the descriptive morphology of higher plants.

2. Stem and root / Academy of Sciences of the USSR. Botanical Institute. V.L. Komarov. Under total ed. corresponding member Academy of Sciences of the USSR P.A. Baranov.

3. Photographs by M. B. Zhurmanov - M.--L .: Publishing house of the Academy of Sciences of the USSR, 1962. - 352 p. -- 3,000 copies.

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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 substances;

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 monocotyledons and many dicotyledons) 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 plant species. Roots that develop in water usually do not have root hairs, but if these roots then penetrate the soil, hairs form 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 cross 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 Caspari 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 inside and outside 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 pro-cambial 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. Differentiation of procambial cells into elements of protophloem, and then protoxylem, begins at the periphery, i.e., xylem and phloem are laid exarchically, in the future 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 a stellate outline, in the very center of the root there is the widest vessel of the metaxylem, from which xylem strands extend in rays, consisting of elements whose diameters 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 desquamation of the epiblema, 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.

There is no fundamental difference between gymnosperms, dicots, and monocots in the primary structure of the roots, but cambium and phellogen are laid early in the roots of dicots and gymnosperms, 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 stem bundles in 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 (phellema) outward during tangential division, and phelloderm cells inward. The impermeability of the cork to liquid and gaseous substances due to the suberinization of its cell walls 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.

The root is the axial organ of the plant, which serves to strengthen the plant in the substrate and absorb water and dissolved minerals from it. In addition, various organic substances (growth hormones, alkaloids, etc.) are synthesized in the root, which then move through the vessels xylem into other plant organs or remain in the root itself. Often it is a storage place for reserve nutrients (rhizome, tuber...).

The formation of roots was a significant evolutionary achievement, thanks to which the plants adapted to a more perfect soil nutrition and were able to form large shoots that rise up to the sunlight.

Types of roots and types of root systems

The root grows in length due to cell division of the apical (apical) meristem. The tip of the root is covered in the form of a thimble with a root cap, which protects the delicate cells of the apical meristem from mechanical damage and promotes root movement in the soil. The root cap, consisting of living thin-walled cells, is continuously renewed: produces new young cells Cap cells produce a rich mucus that coats the root, making it easier to glide between soil particles Mucus also creates favorable conditions for beneficial bacteria to settle in. It can also affect the availability of soil ions and provide short-term protection of the root from drying out, The lifespan of the cells of the root cap is A-9 days, depending on the length of the cap and the type of plant.

root anatomy

Root anatomy. On a longitudinal section of the root tip, several zones can be distinguished: division, growth, absorption, and conduction (Fig. 1).

The soil solution enters the root mainly through the suction zone, so the larger the surface of this root area, the better it performs its main suction function. It is in connection with this function that part of the skin cells is elongated into root hairs 0.1–8 mm long (see Fig. 1). Almost the entire root hair cell is occupied by a vacuole surrounded by a thin layer of cytoplasm. The nucleus is located in the cytoplasm near the top of the hair. Root hairs are able to cover soil particles, as if growing together with them, which facilitates the absorption of water and minerals from the soil. Absorption is also facilitated by the secretion of various acids (carbonic, malic, citric, oxalic) by root hairs, which dissolve soil particles.

On the transverse section, the bark and the central cylinder are distinguished in the root (Fig. 1 and 4). The primary cortex is covered with a kind of epidermis, the cells of which are involved in the formation of root hairs. In this regard, the epidermis of the root is called rhizodermis or epiblema.

The axial or central cylinder consists of a conducting system (xylem and phloem) surrounded by a ring of living pericycle cells capable of meristematic activity.

Mineral plant nutrition

Mineral nutrition is a set of processes of absorption from the soil, movement and assimilation of macro- and microelements (N, P, K, Ca, Mg, Mn, Zn, Fe, Cu, etc.) necessary for the life of a plant organism. Together with photosynthesis, mineral nutrition constitutes a single process of plant nutrition.

The entry of water and dissolved substances into root cells through biological membranes is carried out due to processes such as osmosis, diffusion, facilitated diffusion, and active transport.

Root types

Many plants develop contractile, aerial, stilted and other types of roots.

Questions:
1.Root functions
2. Types of roots
3. Types of root system
4. Root zones
5. Modification of the roots
6. Life processes at the root

1. Root functions
Root is the underground organ of the plant.
The main functions of the root:
- supporting: the roots fix the plant in the soil and hold it throughout its life;
- nutritious: through the roots the plant receives water with dissolved mineral and organic substances;
- storage: some roots can accumulate nutrients.

2. Types of roots

There are main, adventitious and lateral roots. When the seed germinates, the germinal root appears first, which turns into the main one. Adventitious roots may appear on the stems. Lateral roots extend from the main and adventitious roots. Adventitious roots provide the plant with additional nutrition and perform a mechanical function. Develop when hilling, for example, tomatoes and potatoes.

3. Types of root system

The roots of one plant are the root system. The root system is rod and fibrous. In the tap root system, the main root is well developed. It has most dicotyledonous plants (beets, carrots). In perennial plants, the main root may die off, and nutrition occurs at the expense of lateral roots, so the main root can only be traced in young plants.

The fibrous root system is formed only by adventitious and lateral roots. It has no main root. Monocotyledonous plants, for example, cereals, onions, have such a system.

Root systems take up a lot of space in the soil. For example, in rye, the roots spread in breadth by 1-1.5 m and penetrate deep into 2 m.

4. Root zones
In a young root, the following zones can be distinguished: root cap, division zone, growth zone, absorption zone.

root cap has a darker color, this is the very tip of the root. Root cap cells protect the root tip from damage by soil solids. The cells of the cap are formed by the integumentary tissue and are constantly updated.

Suction zone has many root hairs, which are elongated cells no more than 10 mm long. This zone looks like a cannon, because. root hairs are very small. Root hair cells, like other cells, have a cytoplasm, a nucleus, and vacuoles with cell sap. These cells are short-lived, quickly die off, and in their place new ones are formed from younger superficial cells located closer to the root tip. The task of the root hairs is the absorption of water with dissolved nutrients. The absorption zone is constantly moving due to cell renewal. It is delicate and easily damaged during transplantation. Here are the cells of the main tissue.

Venue . It is located above suction, does not have root hairs, the surface is covered with integumentary tissue, and conductive tissue is located in the thickness. The cells of the conduction zone are vessels through which water with dissolved substances moves into the stem and leaves. There are also vascular cells, through which organic substances from the leaves enter the root.

The entire root is covered with cells of mechanical tissue, which ensures the strength and elasticity of the root. The cells are elongated, covered with a thick shell and filled with air.

5. Modification of the roots

The depth of penetration of the roots into the soil depends on the conditions in which the plants are located. The length of the roots is affected by humidity, soil composition, permafrost.

Long roots are formed in plants in dry places. This is especially true for desert plants. So, in camel thorn, the root system reaches 15-25 m in length. In wheat on non-irrigated fields, the roots reach a length of up to 2.5 m, and on irrigated fields - 50 cm, and their density increases.

Permafrost limits root growth in depth. For example, in the tundra, the roots of a dwarf birch are only 20 cm. The roots are superficial, branched.

In the process of adaptation to environmental conditions, the roots of plants have changed and began to perform additional functions.

1. Root tubers act as nutrient storage instead of fruits. Such tubers arise as a result of thickening of the lateral or adventitious roots. For example, dahlias.

2. Root crops - modifications of the main root in plants such as carrots, turnips, beets. Root crops are formed by the lower part of the stem and the upper part of the main root. Unlike fruits, they do not have seeds. Root crops have biennial plants. In the first year of life, they do not bloom and accumulate a lot of nutrients in root crops. On the second - they quickly bloom, using the accumulated nutrients and form fruits and seeds.

3. Attachment roots (suckers) - adnexal measles that develop in plants of tropical places. They allow you to attach to vertical supports (to a wall, rock, tree trunk), bringing the foliage to the light. An example would be ivy and clematis.

4. Bacterial nodules. The lateral roots of clover, lupine, alfalfa are peculiarly changed. Bacteria settle in young lateral roots, which contributes to the absorption of gaseous nitrogen from the soil air. Such roots take the form of nodules. Thanks to these bacteria, these plants are able to live on nitrogen-poor soils and make them more fertile.

5. Aerial roots are formed in plants growing in humid equatorial and tropical forests. Such roots hang down and absorb rainwater from the air - they are found in orchids, bromeliads, some ferns, monstera.

Aerial prop roots are adventitious roots that form on the branches of trees and reach the ground. Occur in banyan, ficus.

6. Stilted roots. Plants growing in the intertidal zone develop stilted roots. High above the water, they hold large leafy shoots on unsteady muddy ground.

7. Respiratory roots form in plants that lack oxygen to breathe. Plants grow in excessively moist places - in marshy swamps, backwaters, sea estuaries. The roots grow vertically upwards and come to the surface, absorbing air. An example would be brittle willow, swamp cypress, mangrove forests.

6. Life processes at the root

1 - Absorption of water by roots

The absorption of water by root hairs from the soil nutrient solution and its conduction through the cells of the primary cortex occurs due to the difference in pressure and osmosis. The osmotic pressure in the cells causes minerals to penetrate into the cells, because. their salt content is less than in the soil. The intensity of water absorption by the root hairs is called the suction force. If the concentration of substances in the soil nutrient solution is higher than inside the cell, then water will leave the cells and plasmolysis will occur - the plants will wither. This phenomenon is observed in conditions of dry soil, as well as with excessive application of mineral fertilizers. Root pressure can be confirmed by a series of experiments.

A plant with roots falls into a glass of water. On top of the water to protect it from evaporation, pour a thin layer of vegetable oil and mark the level. After a day or two, the water in the tank dropped below the mark. Consequently, the roots sucked in water and brought it up to the leaves.

Purpose: to find out the main function of the root.

We cut off the stem of the plant, leaving a stump 2-3 cm high. We put a rubber tube 3 cm long on the stump, and put a curved glass tube 20-25 cm high on the upper end. The water in the glass tube rises and flows out. This proves that the root absorbs water from the soil into the stem.

Objective: To find out how temperature affects the operation of the root.

One glass should be with warm water (+17-18ºС), and the other with cold water (+1-2ºС). In the first case, water is released abundantly, in the second - little, or completely stops. This is proof that temperature has a strong effect on root performance.

Warm water is actively absorbed by the roots. Root pressure rises.

Cold water is poorly absorbed by the roots. In this case, the root pressure drops.

2 - Mineral nutrition

The physiological role of minerals is very great. They are the basis for the synthesis of organic compounds and directly affect the metabolism; act as catalysts for biochemical reactions; affect the turgor of the cell and the permeability of the protoplasm; are the centers of electrical and radioactive phenomena in plant organisms. With the help of the root, the mineral nutrition of the plant is carried out.

3 - Breath of the roots

For normal growth and development of the plant, it is necessary that fresh air enter the root.

Purpose: to check the presence of respiration at the roots.

Let's take two identical vessels with water. We place developing seedlings in each vessel. We saturate the water in one of the vessels every day with air using a spray bottle. On the surface of the water in the second vessel, pour a thin layer of vegetable oil, as it delays the flow of air into the water. After a while, the plant in the second vessel will stop growing, wither, and eventually die. The death of the plant occurs due to the lack of air necessary for the respiration of the root.

It has been established that the normal development of plants is possible only in the presence of three substances in the nutrient solution - nitrogen, phosphorus and sulfur and four metals - potassium, magnesium, calcium and iron. Each of these elements has an individual value and cannot be replaced by another. These are macronutrients, their concentration in the plant is 10-2-10%. For the normal development of plants, microelements are needed, the concentration of which in the cell is 10-5–10-3%. These are boron, cobalt, copper, zinc, manganese, molybdenum, etc. All these elements are found in the soil, but sometimes in insufficient quantities. Therefore, mineral and organic fertilizers are applied to the soil.

The plant grows and develops normally if the environment surrounding the roots contains all the necessary nutrients. Soil is such an environment for most plants.

Tests

660-01. A specialized organ of air nutrition of a plant is
A) green leaf
B) root crop
B) a flower
D) fruit

Answer

660-02. What role do roots play in plant life?
A) form organic compounds from inorganic compounds
B) cool plants
B) store organic matter
D) take in carbon dioxide and release oxygen

Answer

660-03. The main function of the root is
A) food storage
B) soil plant nutrition
C) absorption of organic matter from the soil
D) oxidation of organic substances

Answer

660-04. What is the most important role of the leaf in the life of the plant?
A) allows water to evaporate
B) performs a supporting function
B) is used as a protective organ
D) absorbs water and mineral salts

Answer

660-05. Under what conditions can water rise up in a plant?
A) in the absence of water evaporation
B) with constant evaporation of water
B) only during the day
D) only with closed stomata

Answer

660-06. The main role of leaves in plant life is
A) breathing
B) storage
B) photosynthetic
D) vegetative propagation

Answer

660-07. Evaporation of water from leaves contributes to
A) the movement of mineral salts in the plant
B) supplying leaves with organic matter
C) absorption of carbon dioxide by chloroplasts
D) increase the rate of formation of organic substances

Answer

660-08. The main function of the stem is
A) air nutrition of plants
B) storage of water and nutrients
B) carrying water and nutrients
D) evaporation of water

Answer

660-09. Which of the following is an adaptation to dry conditions?
A) broad leaves
B) many stomata
B) fleshy stems
D) creeping stems

Answer

660-10. Fungi that form mycorrhiza are obtained from the roots of plants
A) water
B) antibiotics
B) mineral salts
D) organic matter

Answer

660-11. The role of the stem in plant life is
A) strengthening the plant in the soil
B) the formation of organic substances
B) the movement of substances through the plant
D) absorption of water and mineral salts