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

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

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

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

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

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

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

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

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 propagation.

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 (phellem) 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.

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