ROOT SYSTEM ROOT SYSTEM

a set of roots of one plant, general shape and the nature 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 large number 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." Ch. ed. M. S. Gilyarov; Editorial team: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected. - M.: Sov. Encyclopedia, 1986.)

root system

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

The root is divided into main, lateral and subordinate ones. The main root grows from the seed, it is most powerfully developed and grows vertically downwards (1st order root). Lateral roots extend from the main one (2nd order roots) and branch repeatedly. Adventitious roots (third-order roots) extend from the lateral roots, which never depart from the main one, have a varied structure and can form on stems and leaves.

The totality of all the roots of a plant is called - root system. There are two types of root systems - taproot and fibrous. IN core the root system has a strongly pronounced main root, and fibrous consists only of adventitious and lateral roots, the main root is not expressed. The roots in the root system differ in appearance, age and functions performed. The thinnest and youngest roots perform mainly the functions of growth, water absorption and nutrient absorption. Older and thicker roots are anchored in the soil and conduct moisture and nutrients to the above-ground organs of the plant. In addition to typical roots, some plants have modified roots, for example, thickened storage, aerial, respiratory or supporting roots. Common storage roots are root vegetables (carrots, beets, parsley); if adventitious roots become storage roots, they are called root tubers.

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

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

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

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

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

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 matter;

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 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 - with evenly alternating thickenings and thin sections (rabbit cabbage). Storage roots there may be conical, turnip-shaped, spherical, spindle-shaped 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 to be 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 the shoots of the second, third and subsequent orders, and the main root system dies off after two to three years, and only the system of adventitious roots remains in the plant. Thus, during life, the type of root system changes: the main root system - a mixed root system - a system of adventitious roots.

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(rice.).

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), which subsequently differentiate 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 one - 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 calyptrogen. 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 apical meristem of the root is also that the actual initial cells in normal conditions are divided very rarely, 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 depending on different types plants. 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 total surface 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: tabular cells of cork, formed during tangential divisions of cells of the cork cambium (phellogen), are located on transverse sections in the right rows, and the cells of the multilayer exodermis, 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 monarchic (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 most wide-open 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 metaxylem elements can be scattered throughout cross section a central cylinder between parenchyma cells or mechanical tissue elements.

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 strands, 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. Impermeability of the stopper to liquids and gaseous substances due to suberinization of the walls of its cells and is the cause of the death of the primary cortex, losing 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.

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