plants.G. Animals.A.2 Autotrophic organisms are: A. Viruses.B. Pisces.V. Animals.G. Plants containing chlorophyll.A.3 Bacterial cell: A. Neuron.B. Axon.V. Dendrite.G. Vibrio cholerae.A.4 A distinctive feature of plant cells is the presence of: A. Nucleus.B. Cytoplasm.V. Membrane.G. Cell wall made of cellulose.A.5 As a result of mitosis, the following occurs: A. Isolation.B. Regeneration of tissues and organs of the body..V. Digestion.G. Breathing.A.6 Indicate one of the provisions of the cell theory: A. One drop of pure nicotine (0.05 g) is enough to kill a person.B. All new cells are formed by division of the original cells.B. Viruses and bacteriophages are representatives of the animal kingdom.G. Viruses and bacteriophages are representatives of the Subkingdom Multicellular. A.7 Reproduction is: A. Obtaining nutrients from the environment. B. Release of unnecessary substances.B. Reproduction of one's own kind.G. The entry of oxygen into the body.A.8 The process of formation of female reproductive gametes is called: A. OogenesisB. SpermatogenesisB. CrushingG. DivisionA.9 Internal fertilization occurs in: A. Shark.B. Pike.V.Obezyan.G. Frogs.A.10 For a developing human embryo, the following is harmful: A. Walking in the fresh air.B. Compliance by the expectant mother with the diet.V. Drug addiction of a woman.G. Compliance by the expectant mother with the work and rest regime. A.11 Indirect type of development - in: A. Homo sapiens. B. Apes.V. Narrow-nosed monkeys.G. Cabbage butterflies.A.12 Genopite is the totality of all: A. Signs of the organism.B. Genes of organisms.V. Bad habits.G. Useful habits.A.13 In dihybrid crossing, the inheritance of: A. Many characters is studied.B. Three signs.B. Two signs.G. One characteristic. TASK B. Short answer tasks B.1 Find a match..1. A dominant trait in a person. A. Gray eyes.2. A recessive trait in humans. B. Brown eyes. B. Blonde hair.G. Black hair.1 2B. 2 Compare the characteristics of asexual and sexual reproduction. Enter the answer number in the correct column.Sexual reproduction. Asexual reproduction1. One individual participates in the reproduction process.2. The process of reproduction involves two individuals of different sexes.3. The beginning of a new organism is given by the zygote, which arises as a result of the fusion of male and female reproductive cells.4. The beginning of a new organism (organisms) is given by a somatic cell.5. Dysentery bacillus.6. Male and female pond frog.Q.3 Choose the correct answer. Write down the numbers of the correct statements. No___________1. Sperm is the female reproductive gamete.2. Sperm is the male reproductive gamete3. The egg is the male reproductive gamete4. The egg is the female reproductive gamete5. Oogenesis is the process of development of the eggs.6. Oogenesis is the process of sperm development.7. Spermatogenesis is the process of egg development.8. Spermatogenesis is the process of sperm development9. Fertilization is the process of fusion of sex gametes: two spermatozoa.10. Fertilization is the process of fusion of sex gametes: two eggs.11. Fertilization is the process of fusion of sex gametes: sperm and egg. Q.4 Establish the correct sequence of complication of organisms according to plan: non-cellular life forms - prokaryotes - eukaryotes. 1. Influenza virus H7N92. Freshwater amoeba.3. Vibrio cholerae.B.5 A heterozygous (Aa) black rabbit is crossed with a heterozygous (Aa) black rabbit. 1. What kind of phenotypic cleavage should be expected with such a crossing? A. 3:1; B. 1:1; V. 1:2:12. What percentage is the probability of having white rabbits (homozygous for two recessive genes - aa)? Answer:_________________B.6 Read the text carefully, think and answer the question: “The study of the internal structure of the cell forced scientists to remember the possible evolutionary role of symbiosis - in the middle of the last century, after the advent of the electron microscope, discoveries in this area rained down one after another. It turned out, in particular , that not only plant chloroplasts, but also mitochondria - the “energy plants” of any real cells - are actually similar to bacteria, and not only in appearance: they have their own DNA and they reproduce independently of the host cell." (Based on materials from the journal " Around the world"). Which organelles have their own DNA?

ROOT SYSTEM ROOT SYSTEM

the totality of the roots of one plant, the general shape and character of the cut are determined by the ratio of the growth of the main, lateral and adventitious roots. With the predominant growth of ch. the root forms a core K. s. (lupine, cotton, etc.), with weak growth or death of hl. root and development of a large number of adventitious roots - fibrous K. s. (buttercup, plantain, all monocots). Degree of development of K. s. depends on the habitat: in the forest zone on podzolic, poorly aerated soils K. s. 90% concentrated in the surface layer (10-15 cm), in the zone of semi-deserts and deserts in some plants it is superficial, using early spring precipitation (ephemera) or condensation. moisture that settles at night (cacti), in others it reaches groundwater (at a depth of 18-20 m, camel thorn), in others it is universal, using moisture from different horizons at different times (juzgun, saxaul, etc.).

.(Source: “Biological Encyclopedic Dictionary.” Editor-in-chief M. S. Gilyarov; Editorial Board: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected . - M.: Sov.

The totality of all the underground roots of a plant formed during their growth and branching. There are tap root systems, where the main root predominates (for example, in species of the legume family), fibrous, formed from numerous roots of similar size (in cereals), and branched, in which several roots of the same degree of development are distinguished (in many trees). The total surface area of ​​the root system can be very significant. It is estimated that the rye plant has approx. 14 million roots, the total surface area of ​​which is 232 m².

.(Source: “Biology. Modern illustrated encyclopedia.” Chief editor A. P. Gorkin; M.: Rosman, 2006.)

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Lecture No. 5. Root and root system.

Questions:

Growing root zones.

Apical meristem of the root.

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.

The root (lat. radix) 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, like a thimble, is covered with a root cap. Branching and formation of adventitious buds in root shoot plants occurs endogenously (intragenously) as a result of the activity of the pericycle (primary lateral meristem).

Functions of the root.

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

2. plays an anchor role, securing 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 shoot plants it performs the function of vegetative propagation.

Classification of roots:

I. By origin roots are divided into main, subordinate clauses And lateral.

main root develops from the embryonic root of the seed.

Adventitious roots or adventitious roots(from the Latin adventicius - newcomer) are formed on other plant organs (stem, leaf, flower) . The role of adventitious roots in the life of herbaceous angiosperms is enormous, since in adult plants (both monocots and many 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 possible to easily propagate plants artificially - by dividing them into individual shoots or groups of shoots with adventitious roots.

Lateral 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 goes deep into the soil vertically downwards; large lateral roots are characterized by transverse geotropism, that is, under the influence of the same force they grow almost horizontally or at an angle to the soil surface; thin (suction) roots are not geotropic 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 root growing in the same direction (in the form of its continuation).

III. By shape the roots are also very diverse. The form of an individual root is called cylindrical, if it has the same diameter over almost its entire length. Moreover, it can be thick (peony, poppy); ishurous, or string-shaped (bow, tulip), and threadlike(wheat). In addition, they highlight knotty roots - with uneven thickenings in the form of nodes (meadowsweet) and claret-shaped - with evenly alternating thickenings and thin sections (hare cabbage). Storage roots can be conical, turnip-shaped, spherical, spindle-shaped and etc.

Root system.

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

Classification of root systems by origin:

tap root system develops from the embryonic root and is represented by the main root (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 dicotyledons;

adventitious root system develops on stems, leaves, and sometimes on flowers. The adventitious origin of roots is considered 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 protocorm (embryo tuber) develops, and subsequently adventitious roots develop on it;

mixed root system widespread among both dicotyledons and monocotyledons. In a plant grown from a seed, the main root system 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 consistently develops on the hypocotyl, epicotyl and subsequent metameres of the main shoot, and subsequently on the basal part of the lateral shoots. Depending on the type of plant, they are initiated and developed in certain parts of metameres (in nodes, under and above nodes, on internodes) or along their entire length.

In plants with a mixed root system, usually already in the autumn 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 shoots of the second, third and subsequent orders, and the main root system dies off after two or three years, and only the system of adventitious roots remains in the plant. Thus, during life, the type of root system changes: main root system - mixed root system - adventitious root system.

Classification of root systems by shape.

Taproot system – This is a root system in which the main root is well developed, noticeably longer and thicker than the lateral ones.

Fibrous root system called when the main and lateral roots are of similar size. It is usually represented by thin roots, although in some species they are relatively thick.

A mixed root system can also be a taproot if the main root is significantly 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 roots (thin, short-lived, intensively branching).

2. Young root zones

Young root zones- these are different parts of the root along the length, performing different 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 length of the stretch zone is several millimeters. Within this zone, differentiation of primary conducting tissues begins.

The area of ​​the root that bears root hairs is called suction zone. The name reflects its function. In the older part, root hairs constantly die off, and in the younger part they constantly form again. This zone extends from several millimeters to several centimeters.

Above the suction zone, where the root hairs disappear, begins venue 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 is different 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 it is always covered with a cover, like a thimble. The apical meristem of the root is always covered with a sheath, like a thimble. The volume of the meristem is closely related to the thickness of the root: in thick roots it is larger than in thin ones, but the meristem is not subject to seasonal changes. In the formation of lateral organ primordia, the apical meristem of the root not participating, therefore, its only function is the formation of new cells (histogenic function), subsequently differentiating into cells of permanent tissues. Thus, if the shoot apical meristem plays both histogenic and organogenic roles, then the root apical meristem plays only histogenic roles. The cap 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 hair-bearing layer - the rhizoderm.

In the roots of horsetails and ferns, the only initial cell, as in the apex of their shoots, has the shape of a trihedral pyramid, the convex base of which faces 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 which, dividing, give rise to the root cap. From the remaining cells subsequently develop: protoderm differentiating into rhizoderm, primary cortex zone, central cylinder.

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

In grasses, sedges, whose initials also make up 3 floors, the cells of the lower floor produce only the cells of the root cap, so this layer is called calyptrogene. The protoderm is separated from the primary cortex - a derivative of the middle floor of the initials - peribles. The central cylinder develops from the cells of the upper floor - pleroma, as in dicotyledons.

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

A very important feature of the root apical meristem is also that the initial cells themselves divide very rarely under normal conditions, making up resting center. The volume of the meristem increases due to their derivatives. However, in case of damage to the root tip caused by irradiation, exposure to mutagenic factors and other reasons, the resting center is activated, its cells rapidly divide, promoting the regeneration of damaged tissues.

Primary root structure

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

In the primary structure, a fundamental distinction is made between:

1. integumentary tissue consisting of one layer of cells with root hairs - epiblema or rhizoderm

2. primary cortex,

3. central cylinder.

Cells rhizoderm 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 a cuticle. 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 are formed in large numbers. In the absence of hairs, water penetrates into the root through the thin outer cell walls.

Root hairs appear as small outgrowths of trichoblasts. Hair growth occurs at its tip. Due to the formation of hairs, the total surface of the suction zone increases ten times or more. Their length is 1...2 mm, and in cereals and sedges it reaches 3 mm. Root hairs are short-lived. Their lifespan does not exceed 10...20 days. After they die, the rhizoderm is gradually shed. By this time, the underlying layer of cells of the primary cortex differentiates into a protective layer - exodermis. Its cells are tightly closed; after the rhizoderm falls off, their walls become suberized. The adjacent cells of the primary cortex are often suberized. The exoderm is functionally similar to cork, but differs from it in the arrangement of cells: the tabular cells of the cork, formed during tangential divisions of the cells of the cork cambium (phellogen), are located in cross sections in regular rows, and the cells of the multilayer exoderm, having polygonal outlines, are in a checkerboard pattern. In the powerfully developed exodermis, passage cells with unsuberized walls are often found.

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

Cortical cells supply the rhizoderm with plastic substances and themselves participate in the absorption and conduction of substances that move through the protoplast system ( simplast), and along cell walls ( apoplast).

The innermost layer of the bark is endoderm, which acts as a barrier that controls the movement of substances from the cortex to the central cylinder and back. The endoderm consists of tightly packed cells, slightly elongated in the tangential direction and almost square in cross section. In young roots, its cells have Casparian belts - sections of the walls characterized by the presence of substances chemically similar to suberin and lignin. Casparian belts encircle the transverse and longitudinal radial walls of the cells in the middle. Substances deposited in the Casparian belts close the openings of the plasmodesmal 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 remains. In many dicotyledonous and gymnosperm plants, differentiation of the endodermis usually ends with the formation of Casparian belts.

In monocotyledonous plants that do not have secondary thickening, the endodermis changes over time. The suberization process extends to the surface of all walls; before this, the radial and internal tangential walls become greatly thickened, while the outer ones almost do not thicken. In these cases they talk about horseshoe-shaped thickenings. The thickened cell walls subsequently become lignified, and the protoplasts die. Some cells remain alive, thin-walled, only with Casparian belts; they are called pass-through cells. They provide a physiological connection between the primary cortex and the central cylinder. Typically, passage cells are located against 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 made up of mechanical tissue, or parenchyma, but this “core” is not homologous to the core of the stem, since the tissues composing it are of procambial origin.

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

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

If one strand of xylem and, accordingly, one strand of phloem are formed, the bundle is called monarch (such bundles are found in some ferns), if there are two strands - diarchic, as in many dicotyledons, which can also have tri-, tetra- and pentarchy bundles, and In the same plant, the lateral roots may differ from the main one in the structure of the vascular bundles. The roots of monocots are characterized by polyarchal bundles.

In each radial strand of xylem, more wide-lumen metaxylem elements are differentiated inward from the protoxylem elements.

The formed strand of xylem can be quite short (iris); the inner part of the procambium in this case differentiates into mechanical tissue. In other plants (onions, pumpkins), the xylem on cross sections of the roots has a star-shaped outline; in the very center of the root there is the widest lumen metaxylem vessel, from which rays of xylem strands extend, consisting of elements whose diameters gradually decrease from the center to the periphery. In many plants with polyarchal bundles (cereals, sedges, palms), individual elements of metaxylem can be scattered throughout the cross section of the central cylinder between parenchyma cells or elements of mechanical tissue.

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 pteridophytes, the primary structure of the root is preserved throughout life (the secondary structure is not formed in them). As the age of monocot plants increases, changes in primary tissues occur at the root. So, after desquamation of the epiblema, the exoderm becomes the covering tissue, and then, after its destruction, successively layers of cells of the mesoderm, endoderm and sometimes the pericycle, the cell walls of which become suberized and lignified. Due to these changes, old monocot roots have a smaller diameter than young ones.

There is no fundamental difference between gymnosperms, dicotyledons and monocotyledons in the primary structure of the roots, but in the roots of dicotyledons and gymnosperms the cambium and phellogen are formed early and secondary thickening occurs, leading to a significant change in their structure. Individual sections of the cambium in the form of arcs arise from the procambium or thin-walled parenchyma cells on the inner side of the phloem strands between the rays of the primary xylem. The number of such sections is equal to the number of primary xylem rays. The cells of the pericycle, located opposite the strands of primary xylem, dividing in the tangential plane, give rise to sections of the cambium that close its arches.

Usually, even before the appearance of the cambium of pericyclic origin, the cambium arcs begin to lay cells inward that differentiate into elements of secondary xylem, primarily wide-lumen vessels, and outward - elements of 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 the primary xylem, collateral bundles arise between the ends of its radial cords, which differ from typical collateral bundles of stems in the absence of primary xylem in them. The cambium of pericyclic origin produces parenchyma cells, the totality of which makes up rather wide rays that continue the strands of primary xylem - the primary medullary rays.

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

Phelloderm cells can divide repeatedly, forming a parenchyma zone to the periphery of the conducting tissues, in the cells of which reserve substances are usually deposited. The tissues located outward from the cambium (phloem, ground parenchyma, phelloderm and cork cambium) are called secondary cortex. On the outside, the roots of dicotyledonous plants, which have a secondary structure, are covered with cork, and a crust forms on old tree roots.

Related information.

Diversity of roots. Plants usually have numerous and highly branched roots. The totality of all the roots of one individual forms a single morphological and physiological root system .

Root systems include morphologically different roots - main, lateral and adventitious.

main root develops from the embryonic root.

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

The initiation of a lateral root begins with the division of pericycle cells and the formation of a meristematic tubercle on the surface of the stele. After a series of divisions, a root appears with its own apical meristem and cap. The growing rudiment makes its way through the primary cortex of the mother root and moves out.

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

The endogenous formation of lateral roots (i.e., their formation in the internal tissues of the mother root) has a clear adaptive significance. If branching occurred at the very apex of the mother root, this would complicate its advancement in the soil (compare with the appearance of root hairs).

Scheme of growth of a lateral root and its extension from the mother root:

Acropetal formation of lateral roots in the pericycle of the mother root of Susak (Butomus):

Pc- pericycle; En – endoderm

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

Adventitious roots are very diverse, and, perhaps, their only common feature is that these roots cannot be classified as either main or lateral. They can also appear on stems (stem clauses roots), both on the leaves and on the roots (root clauses roots). But in the latter case, they differ from lateral roots in that they do not exhibit a strictly acropetal order of origin near the apex of the mother root and can arise in old sections of the roots.

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

The tissues in which adventitious roots arise are also varied. Most often, these are meristems or tissues that have retained the ability to form new cells (apical meristems, cambium, medullary rays, phellogen, etc.).

Classification by origin

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

The emergence of a seed with an embryo and a main root in seed plants gave them a certain biological advantage, since it made it easier for the seedling to quickly form a root system during seed germination.

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

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

1 - part of the root system; 2 - first isotomic (equally forked) branching; 3 - anisotomous (unequally forked) branching; 4 - isotomic branching of the thinnest roots; I am the escape; PT - conductive tissue; H - cover

The appearance of adventitious roots on the roots of commonweed (Lotus corniculatus):

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

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

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

Classification by morphology

Morphological types of root systems have also been established based on other characteristics. IN core in the root system, the main root is highly developed and clearly visible among the other roots . Additional stem-like adventitious roots, as well as adventitious roots on roots, may appear in the taproot system. Often such roots are short-lived and ephemeral.

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

Root systems:

1 - primary-morizal, superficial; 2 - allorizal, core, deep; 3 - allois, core, superficial; 4 - allorizal, fringed; 5 - secondary rhizome, fibrous, universal. The main root is blackened.

Secondary root systems:

M- maternal individual; D- daughter individuals

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

However, all of the listed morphological features give the most initial idea of ​​the diversity of root systems. Changes continuously occur in any root system, balancing it with the shoot system in accordance with the age of the plant, relationships with the roots of surrounding plants, changing seasons, etc. Without knowledge of these processes, it is impossible to understand how plants in forests, meadows, and swamps live and interact.

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

Most plants have distinct height And sucking graduation. The growth ends are usually more powerful than the sucking ends, quickly elongate and move deeper into the soil. The stretch zone in them is well defined, and the apical meristems work vigorously. The sucking endings, which appear in large numbers on the growing roots, lengthen slowly, and their apical meristems almost stop working. The sucking endings seem to stop in the soil and intensively “suck” it.

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

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

Root lobe:

RO - growth end; CO - sucking ending

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

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

In monocots, all roots have no cambium at all, and the differences in the roots, often very sharp, are determined when they are formed on the maternal organ. The thinnest roots can have a diameter of less than 0.1 mm, and then their structure is simplified: the xylem in a cross section consists of 2 - 4 elements, and even roots are described in which the phloem is completely reduced.

Very often, roots for special purposes (storing, retracting, mycorrhizal, etc.) are differentiated in root systems.