Which root system is better than taproot? The structure of the root system of a plant: types of root systems, water for plants. "Taproot" in books

In such plants, the root, which develops as a continuation of the stem, is called the main one; the lateral ones extend from it. The top of the root, together with the lower thickened part of the stem, forms a caudex - single-headed if there is one stem, or multi-headed if there are several of them. Renewal buds are formed on the caudex. Well-known plants have a taproot system aquilegia, armeria maritima, gypsophila paniculata, mullein, lupine, poppies, spurge, many umbrellas (including feverweed), lumbago, ash tree. The taproot may be thick (spindle-shaped), like acanthus, broad-leaved bells, nettle-leaved, milky-flowered, codonopsis, lunaria, mallow, baptisia.

Taproot plants do not like transplants - it is better to plant them immediately in a permanent place. In the flower garden they consistently occupy their assigned niche, which is why they are valuable. If replanting is still required, you can cut off the main root at a depth with a shovel in the spring, then by autumn root system will become more branched and compact, and the transplant will be more successful.

How do plants with tap root systems reproduce?

Taproot plants are often propagated by seeds. Seedlings and young plants can bulge in spring on clay soils, and after the snow melts they need to be buried. However, if the plant does not set seeds or is a varietal plant, root and green cuttings or root divisions can be used.

What are root suckers?

Some plants with tap roots are able to form buds randomly on shallow, horizontally growing roots. As an example we can give anemone (forest, Japanese and its hybrids), bells (rapunzel, speckled and its hybrids, Takeshima), thermopsis, horseradish. They form a constantly growing clump above the root zone and are most often aggressive, like many rhizomatous plants; care and reproduction are the same. Autumn anemones, which do not like transplanting, are divided in the spring by cutting through the soil between the stems with a knife or shovel. A year later, divisions are planted in the spring.

Plant propagation by green cuttings

With green cuttings(using green stems and leaves) it is useful to use rooting agents, for example "Kornevin". It is better to root such cuttings in a greenhouse located in a shaded, cool place. At small quantities material can be covered with plantings plastic bottle. Plantings are regularly sprayed and ventilated. After rooting (from 1 to 1.5 months), the greenhouse is opened. For the winter, plants are covered with spruce branches or leaves. Planted in spring.

Stem cuttings can be propagated aquilegia, rocking, lupine, oriental poppy, peony. They are taken during active growth, that is, in some ( lupine, poppy) throughout the growing season, in others ( aquilegia, peony) - before flowering. Usually used top part shoot, which is cut into pieces of 2-3 internodes. In some cases, side rosettes or small shoots with a heel - a piece of caudex - are torn off (not cut off). The cuttings are planted at an angle in holes made with a stick, 1-1.5 cm deep.

Cuttings with leaves incarvillea(July), lupine(July), fraxinella(June). Well-formed leaves are selected and pulled off the stem, which is called “heeled.” Planted with an inclination to a depth of 1-1.5 cm. Rooting time is from 1 month for incarvillea to 2.5 months for ash.

Master class on plant propagation by root cuttings

Cuttings are taken from plants that are capable of forming buds on the roots: acanthus, varietal mullein, oriental poppy and its varieties, eryngium, kermek, lumbago. A more successful result can be obtained by planting the cuttings in pots with loose, light soil, as for sowing. It is necessary to maintain constant soil moisture, but without stagnant water. Stimulants are not used.

The timing of cuttings is individual. For example, oriental poppy is cut after the foliage dies before frost. Cuttings are cut up to 5 cm long. Rooting occurs after 1-2 months. Mullein cuttings are taken until the beginning of June.

1 step. Dig up the mother bush of the oriental poppy and cut off one or two large roots with a knife. Cut them into pieces 5-8 cm long, making the lower cuts oblique.

Step 2. Stick the root cuttings strictly vertically flush with the soil with the top up, sprinkle with a layer of soil or coarse sand about 1.5 cm and water.

Step 3. Cover the pots with cuttings with film or glass and place in the shade. After the leaves appear, remove the film.

Master class on plant propagation by dividing roots

Divide thick roots with a formed caudex and renewal buds (aquilegia, umbelliferae, lupine, spurge, ash). This is done before active growth begins, in the spring. True, this method is rarely used - it is quite unreliable.

1 step. Dig up the mother plant, cut the main root lengthwise, so that on each half there remains a piece of stem with two or three buds.

Step 2. Dry the cut and sprinkle with ash.

Step 3. Plant the division in a pot or in a permanent place.

Root system all the roots of a plant are called. It is formed by the main root, lateral roots and adventitious roots. main root plants develop from an embryonic root. Adventitious roots usually grow from the lower parts of the plant stem. Lateral roots develop on the main and adventitious roots.

The root system of plants performs two main functions. Firstly, it holds the plant in the soil. Secondly, the roots absorb from the soil the water and minerals dissolved in it that the plant needs.

If a plant develops a powerful main root, it forms taproot system. If the main root remains undeveloped or dies, and adventitious roots develop, then the plant develops fibrous root system.

Taproot type of root system

The taproot system is characterized by a well-developed main root. By appearance it looks like a rod. The main root grows from the embryonic root.

The taproot system is formed not only by the main root, but also by small lateral roots extending from it.

The tap root system is characteristic of many dicotyledonous plants. Beans, clover, sunflower, carrots, and dandelion have a well-developed main root.

However, many perennial plants with the original tap root system, sooner or later the main root dies. Instead, numerous adventitious roots grow from the stem.

There is a subtype of tap root system - branched root system. In this case, several lateral roots receive strong development. While the main root remains shortened. The type of branched root system is characteristic of many trees. This root system allows you to firmly hold the powerful trunk and crown of the tree.

The tap root system penetrates deeper into the soil than the fibrous root system.

Fibrous type of root system

A fibrous root system is characterized by the presence of many approximately identical adventitious roots, which form a kind of bundle. Adventitious roots grow from aboveground and underground parts of the stem, less often from leaves.

Plants with fibrous root systems may also have a living main root. However, if it is preserved, it does not differ in size from the other roots.

A fibrous root system is characteristic of many monocots. Among them are wheat, rye, onions, garlic, corn, potatoes.

Although the fibrous root system does not penetrate as deeply into the soil as the tap root system, it occupies large area near the soil surface and entwines soil particles more tightly, which improves the absorption of the aqueous solution.

The root system of a plant is formed by roots of various natures. There is a main root, which develops from the embryonic root, as well as lateral and adventitious ones. Lateral roots are a branch from the main one and can form on any part of it, while adventitious roots most often begin to grow from the lower part of the plant stem, but can even form on the leaves.

Tap root system

The tap root system is characterized by a developed main root. It has the shape of a rod, and it is because of this similarity that this type got its name. The lateral roots of such plants are extremely weakly expressed. The root has the ability to grow unlimitedly, and the main root of plants with a taproot system reaches impressive sizes. This is necessary to optimize the extraction of water and nutrients from soils where groundwater lie at considerable depth. Many species have a tap root system - trees, shrubs, as well as herbaceous plants: birch, oak, dandelion, sunflower, etc.

Fibrous root system

In plants with a fibrous root system, the main root is practically not developed. Instead, they are characterized by numerous branching adventitious or lateral roots of approximately equal length. Often, plants first grow a main root, from which lateral roots begin to emerge, but in the process further development plants it dies. A fibrous root system is characteristic of plants that reproduce vegetatively. It usually occurs in - coconut tree, orchids, ferns, cereals.

Mixed root system

Often a mixed or combined root system is also distinguished. Plants belonging to this type have a well-differentiated main root and multiple lateral and adventitious roots. This structure of the root system can be observed, for example, in strawberries and wild strawberries.

Root modifications

The roots of some plants are so modified that it is difficult at first glance to attribute them to any type. These modifications include roots - thickening of the main root and lower part of the stem, which can be seen in turnips and carrots, as well as root tubers - thickening of lateral and adventitious roots, which can be seen in sweet potatoes. Also, some roots may not serve to absorb water with salts dissolved in it, but for respiration (respiratory roots) or additional support (stilted roots).

Imagine grasses, shrubs and trees without roots. Huge oaks and small herbaceous plants, rootless, will find themselves lying helplessly on the ground. The roots of the plant strengthen themselves in the soil. With the help of roots, plants are firmly held in one place throughout their life.

Growing from the small root of the seed embryo, the root of adult plants, especially trees and shrubs, penetrates deeply into the soil, reaches large sizes and powerfully holds the heaviest trunk and branches with leaves. To imagine the strength with which roots hold trees, open an umbrella during a strong wind and try to hold it in your hands. The wind will violently tear the umbrella from your hands, making it very difficult to hold it.

A heavy tree trunk with all its branches and leaves can be compared to a giant umbrella. A hurricane wind can pick up such an “umbrella” and tear a tree out of the ground. However, that's not what happensvery often. The roots that hold the tree in the soil are very strong.Of course, not all roots are as powerful as tree roots. In annuals herbaceous plants The roots are often small and shallowly penetrated into the soil. Let's get acquainted with the roots of various plants.Low grass with a thin panicle of inconspicuous flowers grows almost everywhere. It's bluegrass. Find bluegrass and dig it up by the roots. Also dig up the dandelion, trying to damage its root as little as possible.

Now look at the roots of the dug up plants.

Dandelion has a well-developedmain root. It develops from the embryonic root of the seed. Small branches extend from the main root lateral roots.

Bluegrass has many roots, almost equal in length and thickness, and they grow in a bunch. These roots grow from the stem and are called subordinate clauses. The main root is not noticeable among the adventitious roots of bluegrass.

If you look at the roots of a wide variety of plants, you will find that some of them are similar to dandelion roots, while others are similar to bluegrass roots.

All the roots of a plant taken together constitute itroot system.

The main roots develop from the radicle of the seed embryo and usually look like rods. Therefore, plants with gooda developed main root, the root system is called core. If the main root is invisible among all the others growing in a bunch, then the root system is called fibrous.

Thus, no matter how diverse the flowering plants are, the root system of some will be fibrous, while others will be taprooted.

It has been noted that most dicotyledonous plants have taproot systems developing from the embryonic root of the seed. For example, sorrel, beans, sunflowers, carrots, all trees, shrubs and many other plants have a clearly visible main root.

Monocots often have a fibrous root system. All our cereals, onions, garlic and relatively few other plants have a fibrous root system.

It is interesting to watch how the fibrous root system develops. The main root, developing from the radicle of the seed embryo, soon stops growing. It becomes invisible among the many adventitious roots growing from the underground part of the stem. The adventitious roots are almost equal in thickness, grow in a bunch and hide the main root that has stopped growing.

So, roots can form in different ways. First, roots develop from the radicle of the seed embryo. This main roots. Secondly, the roots grow from the stem. Thisadventitious roots.Thirdly, roots grow from both the main and adventitious roots. This lateral roots. It is interesting to note that adventitious roots develop not only from the underground part of the stem, but also from above-ground shoots.

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

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

1) roots strengthen (“anchor”) plants in the soil, making vertical growth and shoots upward possible;

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

3) reserve substances can be deposited in the roots;

4) roots interact with the roots of other plants, microorganisms, and fungi living in the soil.

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

Root systems include roots of different morphological nature - main root, lateral And subordinate clauses roots.

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

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

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

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

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

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

They are distinguished by shape core And fibrous root systems.

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

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

One rye bush grown in a greenhouse had a total length of all roots of 623 km. The total growth of all roots in one day was approximately 5 km. The total surface of all roots of this plant was 237 m2 and was 130 times larger than the surface of the above-ground organs.

Young root ending zones - these are parts of a young root of different lengths, performing different functions and characterized by certain morphological and anatomical features ( rice. 4.2).

The root tip is always covered from the outside root cap, protecting the apical meristem. The cap consists of living cells and is constantly renewed: as old cells are exfoliated from its surface, the apical meristem forms new young cells to replace them from the inside. The outer cells of the root cap exfoliate while still alive; they produce abundant mucus, which facilitates the movement of the root among solid soil particles. The cells of the central part of the cap contain many starch grains. Apparently, these grains serve statolites, i.e., they are able to move in the cell when the position of the root tip in space changes, due to which the root always grows in the direction of gravity ( positive geotropism).

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

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

Rice. 4.2. General view (A) and longitudinal section (B) of the root ending (diagram): I – root cap; II – division and extension zones; III – suction zone; IV – beginning of the conduction zone: 1 – growing lateral root; 2 – root hairs; 3 – rhizoderm; 3a – exodermis; 4 – primary cortex; 5 – endoderm; 6 – pericycle; 7 – axial cylinder.

Next comes absorption zone, or suction zone. In this zone the covering tissue is rhizoderm(epiblema), the cells of which carry numerous root hairs. The extension of the root stops, the root hairs tightly cover the soil particles and seem to grow together with them, absorbing water and mineral salts dissolved in it. The absorption zone extends up to several centimeters. This zone is also called zone of differentiation, since this is where the formation of permanent primary tissues occurs.

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

The cells of the absorption and conduction zones occupy a fixed position and cannot move relative to the soil areas. However, the zones themselves, due to constant apical growth, continuously move along the root as the root end grows. The absorption zone constantly includes young cells from the side of the stretch zone and at the same time excludes aging cells that become part of the conduction zone. Thus, the root suction apparatus is a mobile formation that continuously moves in the soil.

Internal tissues also appear consistently and naturally in the root ending.

Primary structure of the root. The primary structure of the root is formed as a result of the activity of the apical meristem. The root differs from the shoot in that its apical meristem deposits cells not only inside, but also outside, replenishing the cap. The number and location of initial cells in the root apices vary significantly in plants belonging to different systematic groups. Derivatives of initials are already differentiated into primary meristems – 1) protodermis, 2) main meristem and 3) procambium(rice. 4.3). From these primary meristems in the absorption zone, three tissue systems are formed: 1) rhizoderm, 2) primary cortex and 3) axial (central) cylinder, or stele.

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

Rhizoderma (epiblema, root epidermis) – absorbent tissue formed from protodermis, the outer layer of the primary root meristem. Functionally, rhizoderm is one of the most important plant tissues. Through it, water and mineral salts are absorbed, it interacts with the living population of the soil, and through the rhizoderm, substances that help soil nutrition are released from the root into the soil. The absorbing surface of the rhizoderm is greatly increased due to the presence of tubular outgrowths in some cells - root hairs(Fig. 4.4). The hairs are 1-2 mm long (up to 3 mm). One four-month-old rye plant has approximately 14 billion root hairs with an absorption area of ​​401 m2 and a total length of more than 10,000 km. U aquatic plants root hairs may be absent.

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

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

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

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

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

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

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

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

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

The xylem star can have a different number of rays - from two to many. If there are two of them, the root is called diarchical, if three – triarchic, four - tetrarchic, and if there is a lot - polyarchic (rice. 4.6). The number of xylem rays usually depends on the thickness of the root. In the thick roots of monocots it can reach 20-30 ( rice. 4.7). In the roots of the same plant, the number of xylem rays can be different; in thinner branches it is reduced to two.

Rice. 4.6. Types of structure of the axial cylinder of the root (diagram): A – diarchic; B – triarchic; B – tetrarchic; G – polyarchal: 1 – xylem; 2 – phloem.

The spatial separation of strands of primary phloem and xylem, located at different radii, and their centripetal location represent characteristics structures of the central cylinder of the root and are of great biological importance. The xylem elements are as close as possible to the surface of the stele, and solutions coming from the bark penetrate into them more easily, bypassing the phloem.

Rice. 4.7. Cross section of a monocot root: 1 – remains of rhizoderm; 2 – exodermis; 3 – mesoderm; 4 – endoderm; 5 – access cells; 6 – pericycle; 7 – xylem; 8 – phloem.

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

Rice. 4.8. Cross section of a corn root.

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

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

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

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

The cambium areas arising from the pericycle consist of parenchyma cells and are not capable of depositing elements of conducting tissues. They form primary medullary rays, which are wide areas of parenchyma between secondary conducting tissues ( rice. 4.10). Secondary core, or bark rays additionally arise with prolonged thickening of the root; they are usually narrower than the primary ones. The medullary rays provide a connection between the xylem and phloem of the root; radial transport of various compounds occurs along them.

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

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

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

Phelloderm cells and parenchyma, formed due to the division of pericycle cells, form parenchyma of the secondary cortex, surrounding conductive tissues (Fig. 4.10). On the outside, the roots of the secondary structure are covered with periderm. Crust is rarely formed, only on old tree roots.

Perennial roots of woody plants often become very thick as a result of prolonged activity of the cambium. The secondary xylem in such roots merges into a solid cylinder, surrounded externally by a ring of cambium and a continuous ring of secondary phloem ( rice. 4.11). Compared to the stem, the boundaries of the growth rings in the root wood are much less pronounced, the phloem is more developed, and the medullary rays are, as a rule, wider.

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

Specialization and metamorphosis of roots. Most plants in the same root system have distinctly different height And sucking graduation. The growth tips are usually more powerful, quickly lengthen and move deeper into the soil. Their elongation zone is well defined, and the apical meristems work energetically. 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.

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

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

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

Special formations appear on the roots of legumes - nodules, in which bacteria from the genus Rhizobium settle. These microorganisms are able to assimilate atmospheric molecular nitrogen, converting it into a bound state. Some of the substances synthesized in the nodules are absorbed by plants, and bacteria, in turn, use the substances found in the roots. This symbiosis has great importance For Agriculture. Legumes, thanks to an additional source of nitrogen, are rich in proteins. They provide valuable food and feed products and enrich the soil with nitrogenous substances.

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

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

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

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

Rice. 4.13. Root metamorphosis: 1 – gladiolus corm with retractor roots thickened at the base; 2 – respiratory roots with pneumatophores in Avicennia ( etc– high tide zone); 3 – aerial roots of an orchid.

Rice. 4.14. Part of a cross section of an orchid aerial root: 1 – velamen; 2 – exodermis; 3 – access cell.

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

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

Rice. 4.15. Support roots: 1 – stilted corn roots; 2 – columnar roots of banyan tree; 3 – stilted roots of rhizophora ( etc– high tide zone; from– low tide zone; silt– surface of the muddy bottom).