plant root structure
The main functions of the root: it ensures the fixation of the plant in the soil, the absorption of the soil aqueous solution of salts and its transport to the aerial parts of the plant.
Additional functions: storage of nutrients, photosynthesis, respiration, vegetative reproduction, excretion, symbiosis with microorganisms, fungi. The first true roots appeared in ferns.
The root embryo is called the germinal root and is laid simultaneously with the bud in the seed embryo.
Plants are distinguished:
main root. It is formed from the germline and persists throughout life. Always alone.
Lateral roots. They branch off from the roots (main, additional, lateral). When branching, they form roots of the 2nd, 3rd, etc. order.
adventitious roots. Formed in any part of the plant (stem, leaves).
The totality of all the roots of a plant forms the root system. The root system is formed throughout the life of the plant. Its formation is provided mainly by lateral roots. There are two types of root systems: taproot and fibrous.
The growth of the root, its branching continues throughout the life of the plant organism, that is, it is practically unlimited. Meristems - educational tissues - are located at the top of each root. The proportion of meristematic cells is relatively high (10% by weight versus 1% for the stem).
Determining the size of root systems requires special methods. It turned out that the total surface of the roots usually exceeds the surface of the above-ground organs by 104-150 times. When growing a single rye plant, it was found that the total length of its roots reaches 600 km, while 15 billion root hairs are formed on them. These data speak of the enormous potential for growth of root systems. However, this ability is not always shown. With the growth of plants in phytocenoses, with a sufficiently large density of their structure, the size of the root systems noticeably decreases.
From a physiological point of view, the root system is not homogeneous. Rather, not the entire surface of the root is involved in the absorption of water. In each root, several zones are distinguished (Fig. 1). True, not always all zones are expressed equally clearly.
The end of the root is protected from the outside by a root cap resembling a rounded cap melting from living thin-walled oblong cells. The root cap serves as protection for the growing point. The cells of the root cap are sloughed off, which reduces friction and facilitates the penetration of the root into the depth of the soil. Under the root cap is the meristematic zone. The meristem consists of numerous small, rapidly dividing, densely packed cells, almost entirely filled with protoplasm. The next zone is the stretch zone. Here the cells increase in volume (stretch). At the same time, differentiated sieve tubes appear in this zone, followed by a zone of root hairs. With a further increase in the age of the cells, as well as the distance from the root tip, the root hairs disappear, cutinization and corking of the cell membranes begin. Water absorption occurs mainly in the cells of the elongation zone and the zone of root hairs.
Rice. 1. Scheme of the structure of the root:
A - longitudinal section: 1-root cap; 2- meristem; 3-stretch zone; 4- zone of root hairs; 5- branching zone;
B - cross section (according to M.F. Danilova): 1 - rhizoderm; 2 - root hair; 3 - parenchyma; 4 - endoderm; 5- Caspari belts; 6 - pericycle; 7 - phloem; 8 - xylem. Dashed arrows are the paths of movement of substances absorbed from the external solution. Solid arrows are the path of solutions along the symplast; intermittent - the path along the apoplast.
The root surface in the zone of root hairs is covered with rhizoderma. It is a single-layer tissue with two types of cells that form and do not form root hairs. It has now been shown that the cells that form root hairs are distinguished by a special type of metabolism. In most plants, rhizodermal cells have thin walls. Following the rhizoderm to the pericycle, there are cells of the cortex. The cortex consists of several layers of parenchymal cells. An important feature of the cortex is the development of systemic large intercellular spaces. On the border of the cortex and the central cylinder, one layer of cells closely adjacent to each other develops - the endoderm, which is characterized by the presence of Casparian bands. The cytoplasm in the cells of the endoderm adheres tightly to the cell membranes. With aging, the entire inner surface of endoderm cells, with the exception of through cells, becomes covered with suberin. With further aging, more layers may be superimposed on top. Apparently, it is the cells of the endoderm that serve as the main physiological barrier to the movement of both water and nutrients. Conductive tissues of the root are located in the central cylinder. When considering the structure of the root in the longitudinal direction, it is important to note that the beginning of the growth of root hairs, the appearance of Casparian hairs in the walls of the endoderm, and the differentiation of xylem vessels occur at the same distance from the apical meristem. It is this zone that is the main zone for supplying plants with nutrients. Typically, the absorption zone is 5-10 cm long. Its value depends on the rate of root growth as a whole. The slower the root grows, the shorter the absorption zone.
The root along the length can be divided into several sections that have a different structure and perform different functions. These areas are called root zones. The root cap and the following zones are distinguished: division, stretching, suction and conduction.
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 cone. Therefore, the microscopic structure of the root in the suction zone is called primary. In monocotyledonous plants, the primary structure is also preserved in the conduction zone. Here, only the most superficial layer with root hairs, the rhizoderm (epiblema), is missing. The protective function is performed by the underlying tissue - the exoderm.
In the primary structure of the root, three parts are distinguished: the rhizoderm, the primary cortex, and the axial (central) cylinder.
The primary cortex accounts for the bulk of the primary tissues of the root. Its cells accumulate starch and other substances. This tissue contains numerous intercellular spaces that are important for the aeration of root cells. The outermost cells of the primary cortex, which lie directly below the rhizoderm, are called the exoderm. The bulk of the cortex (mesoderm) is formed by parenchymal cells. The innermost layer is called the endoderm. This is a series of tightly closed cells (without intercellular spaces).
The central or axial cylinder (stele) consists of conductive tissues surrounded by one or more layers of cells - the pericycle.
The inner part of the central cylinder in most plants is occupied by a continuous strand of primary xylem, which gives protrusions in the form of ribs to the pericycle. Between them are strands of primary phloem.
In dicotyledonous and gymnosperms, the cambium appears at an early age in the central cylinder of the root between the xylem and phloem, the activity of which leads to secondary changes and, ultimately, the secondary structure of the root is formed. To the center, the cambium deposits cells of the secondary xylem, and to the periphery, cells of the secondary phloem. As a result of the activity of the cambium, the primary phloem is pushed outward, while the primary xylem remains in the center of the root.
Following changes in the central cylinder of the root, changes occur in the cortical part. Pericycle cells begin to divide around the entire circumference, resulting in a layer of cells of the secondary meristem - phellogen (cork cambium). The phellogen, in turn, dividing, lays the phellema outward, and the phelloderm inward. A periderm is formed, the cork layer of which isolates the primary cortex from the central cylinder. As a result, the entire primary crust dies off and is gradually shed; The outer layer of the root becomes the periderm. The phelloderm cells and the remnants of the pericycle further grow and form the parenchymal zone, which is called the secondary root cortex (Fig. 2).
With the development of the storage parenchyma of the main root, the formation of storage roots or root crops occurs. There are root crops:
1. Monocambial (radish, carrot) - only one layer of cambium is laid, and reserve substances can accumulate either in the xylem parenchyma (xylem type - radish) or in the phloem parenchyma (phloem type - carrot);
2. Polycambial - at certain intervals, a new layer of cambium (beets) is laid.
Rice. 2. Transition from the primary structure of the root to the secondary:
1 - primary phloem, 2 - primary xylem, 3 - cambium, 4 - pericycle, 5 - endoderm, 6 - mesoderm, 7 - rhizoderm, 8 - exoderm, 9 - secondary xylem, 10 - secondary phloem, 11 - secondary cortex, 12 - fellogen, 13 - fellema.
It should be noted that, in general, root systems are much less diverse compared to aboveground organisms, due to the fact that their habitat is more homogeneous. This does not exclude the possibility that root systems change under the influence of certain conditions. The influence of temperature on the formation of root systems is well shown. As a rule, the optimal temperature for the growth of root systems is somewhat lower compared to the growth of aboveground organs of the same plant. Nevertheless, a strong decrease in temperature noticeably inhibits the growth of roots and promotes the formation of thick, fleshy, little branching root systems.
Soil moisture is of great importance for the formation of root systems. The distribution of roots across soil horizons is often determined by the distribution of water in the soil. Usually, during the first period of the life of a plant organism, the root system grows extremely intensively and, as a result, reaches the wetter soil layers more quickly. Some plants develop shallow root systems. Located close to the surface, strongly branching roots are intercepted by precipitation. In dry areas, often deep and shallow rooting plant species grow side by side. The first provide themselves with moisture due to the deep layers of the soil, the second due to the assimilation of precipitation.
Important for the development of root systems is aeration. It is the lack of oxygen that is the reason for the poor development of root systems on waterlogged soils. Plants adapted to grow on poorly aerated soils have a system of intercellular spaces in their roots, which, together with intercellular spaces in stems and leaves, form a single ventilation system.
Of great importance are nutritional conditions. It is shown that the application of phosphorus fertilizers contributes to the deepening of root systems, and the application of nitrogen fertilizers to their enhanced branching.
Root modification
The root of the plant performs many functions. The most important of them for the life of the plant are the retention of the plant in the soil and the absorption of water with dissolved minerals. In addition to the above roots, it is also characteristic to perform a number of other functions, which is why their structure is modified. Due to metamorphosis, the modified root loses its similarity with the usual root.
Root crop
In some biennial plants (turnips, parsley, carrots), the root is transformed into a root crop, which is a thickened adventitious root. The main root and the lower section of the stem take part in the formation of the root crop. In the structure of the root crop, the main place belongs to the storage main tissue.
Depending on the structure of the root, three types of root crops are distinguished: carrot, beet and rare.
1. Carrot-type root crops- vegetables with an elongated root, which can be cylindrical, conical, elongated - conical, fusiform and blunt or sharp end. Root crops of this type have a clearly differentiated bark (phloem) and core (xylem). Between them is a cork cambium. From above, the root crop is covered with a natural periderm. In terms of composition and amount of nutrients, the bark is more valuable than the core. Root crops of this type include carrots, parsley, celery, parsnips.
2. Beet roots- vegetables with rounded, round-flat, oval or elongated roots. Represented by table and sugar beets. As a vegetable crop, only table beets are used. The root crop has dark red flesh with rings of lighter toga, which is due to the alternation of xylem (light rings) and phloem (dark rings) tissues. The smaller the specific gravity is occupied by xylems, the higher the nutritional value of beets.
3.
Rare root crops- vegetables with rounded, turnip-shaped, elongated-conical roots. A feature of their internal structure is the radial arrangement of the secondary xylem, phloem and parenchymal tissue. The cambial layer is located directly below the periderm. Root crops of this type include radishes, radishes, rutabaga and turnips.
Root crops of all types are characterized by common morphological features: the head in the upper part with leaf petioles and buds at the base, the root body (the main edible part) and the tip of the root (the main one), and beet-type root crops have side roots. In other root crops, thin lateral roots are easily torn off during harvesting and, as a rule, are absent.
A feature of all root crops is their ability to heal mechanical damage by cell suberization, as well as their easy digestibility. The most easily fading are carrot-type root crops, radishes, the least - beets, radishes, turnips and rutabaga.
Root tubers (root cones)
Many angiosperms have root tubers (or root cones) on their roots. Their origin is associated with a thickening of adventitious and lateral roots. Root tubers are a reservoir of nutrients. They are also used for vegetative propagation of plants.
The most famous of the plants with a typical root tuber is the dahlia. Swollen root cones extend from one point - the base of the old stem. These modified roots provide the plant with nutrients. During the growth period, their own thin roots grow from them, which extract water and nutrients from the soil. Plants with root cones are propagated by individual cones with a bud (eye) at the end (dahlia, eremurus, clivia, buttercup).
Hook roots, stilted roots, aerial roots, plank roots, sucker roots
Roots-hooks- a kind of adventitious roots that allow the plant to easily attach to any support. Such roots are found in ivy and other climbing plants.
An amazing phenomenon in nature is the stilted roots, which serve as a support for the plant. Such roots have the ability to resist strong loads, due to the presence of mechanical tissues in all departments. They are found in plants of the genus Pandaus, growing on oceanic islands in the tropics, where strong hurricane winds prevail.
Air (respiratory) roots found in tropical trees that grow in oxygen-depleted soils, freshwater tropical swamps. These are lateral roots located above the ground. Thanks to aerial roots, oxygen and water are absorbed by the plant from the air in conditions of high humidity. So the respiratory roots provide the function of additional respiration.
plank roots- These are vertical above-ground roots characteristic of large trees growing in tropical rainforests. They develop at the base of the trunk, look like boards adjacent to the trunk, providing the plant with additional support.
Mycorrhiza
Mycorrhiza is a symbiosis of higher plant roots with fungal hyphae. This cohabitation is beneficial for both organisms, since the fungus receives ready-made organic substances from the plant, and the plant absorbs water with minerals from the fungus. Mycorrhiza is often found on the roots of higher plants, especially woody ones. One can imagine fungal hyphae intertwining with thick tree roots as root hairs, since their functions are similar.
Most of the perennials have mycorrhiza. It is assumed that mycorrhiza is one of the factors that contributed to the progress of flowering plants. Plants that feed on mycorrhizal fungi are called mycotrophic.
Distinguish two main types of mycorrhiza: ectotrophic and endotrophic. The hyphae of ectotrophic mycorrhiza cover the root only from the outside, sometimes penetrating into the intercellular spaces of the cow parenchyma. Ectomycorrhiza is common in many woody (pine, birch, oak, beech, etc.), as well as a number of herbaceous agricultural plants, especially in cereals. The fungus settles on the germinated seed root and in further development, especially in the tillering phase, provides abundant soil nutrition for the plant.
Endotrophic mycorrhiza is more common. It is characteristic of most flowering plants. Endomycorrhiza does not form a fungal sheath around the root, the root hairs do not die off, but the hyphae penetrate the root and invade the cells of the cow's parenchyma.
Epiphytic tropical orchids and some other plants have so-called aerial roots. They cover with a multi-layered epiblema called velamen. Velamen sometimes performs a photosynthetic function, and later it can participate in the absorption of atmospheric moisture, forming a dead spongy hygroscopic root cover.
Without mycorrhiza, the mycelium of symbiotic fungi can exist in the soil for a certain time, but will never form fruiting bodies. Therefore, under artificial conditions, it is impossible to obtain fruiting bodies of white fungus, russula mushrooms, fly agarics - they are all mycorrhiza-forming, and without a certain tree species they will not bear fruit. In turn, a plant without its fungal symbionts develops poorly, slowly, easily exposed to diseases and may even die.
Mycorrhiza is of great importance in tropical rainforests. Due to the strong leaching regime (daily rainfall), these forests are practically devoid of soil (all nutrients are washed out of the soil). Plants face an acute problem of nutrition. At the same time, there is a lot of fresh organic matter: fallen branches, leaves, fruits, seeds. But this organic matter is inaccessible to higher plants, and they come into close contact with saprotrophic fungi. Thus, the main source of minerals under these conditions is not soil, but soil fungi. Minerals enter the root directly from the hyphae of mycorrhizal fungi, which is why rainforest plants are characterized by a shallow root system. How effectively mycorrhiza works can be judged at least by the fact that tropical rainforests are the most productive communities on Earth, the maximum possible biomass develops here.
bacterial nodules
Also, plants can mutually cohabitate with nitrogen-fixing bacteria. So, bacterial nodules appear on the roots of higher plants - modified lateral roots that have adaptations for symbiosis with bacteria. Through the root hairs, bacteria enter the young roots and provoke the formation of nodules. The role of these bacteria is that they convert nitrogen from the air into a mineral form so that it becomes available for absorption by plants. Plants act as a defense for bacteria against competing soil bacteria. Bacteria also feed on substances from the roots of higher plants. The appearance of nodule bacteria was recorded, mainly on the roots of plants from the legume family. Therefore, legume seeds are enriched with protein, and in agriculture, representatives of this family are used in crop rotation to enrich the soil with nitrogen.
In tropical rain forests, there are plants that grow in the crowns of trees. They are called epiphytes(from the Greek "epi" - on and "phyton" - a plant). The roots of such plants are called aerial, as they usually hang from the branches and are able, like blotting paper, to absorb moisture and small particles in the air. Epiphytes include, for example, many orchids