Branching zone of the root tissue type and structure. flowering plants

The roots are constantly growing due to the division of the cells of the apical educational tissue. The root sheath facilitates the movement of the root in the soil and protects the educational tissue. The epidermis protects the root and ensures the absorption of water and minerals from the soil with the help of root hairs. Wood conducts substances absorbed from the soil into the stem. The bast provides the transport of organic matter from the leaves to the root cells. Mechanical fabrics give strength to the root.

All roots (main, lateral, adventitious) are arranged in the same way. They can branch, they never form leaves.

Root bast

Bast cells are located next to the wood, through which organic substances formed in leaves and stems enter the root.

mechanical root tissue

The strength and elasticity of the root is provided by mechanical tissue.

Root cambium (educational tissue)

With age, a lateral educational tissue, the cambium, appears between the wood and the bast. Thanks to the division of cambial cells, new elements of wood and bast, a mechanical tissue, are formed. This ensures the growth of the root in thickness. At the same time, the root acquires additional functions - support and storage

Such a unique part of the plant requires special attention. After all, the root performs important functions, which is reflected in its structure. But before considering the structure of the plant root, let's highlight its features:

• no leaves and chloroplasts;
• keeps the plant in the soil;
• stores nutrients;
• grows throughout the life of the organism;
• takes water and minerals from the soil, delivers them to the ground part of the plant;
• used for vegetative propagation.

Types of root systems

Depending on the structure, the taproot and fibrous root systems are divided. The main root of the tap root was formed from the germinal root, and the rest grow from it. In the fibrous system, the main root is formed in exactly the same way, only later it dies off, leaving only adventitious roots.

Lengthwise cut

If you make a preparation and cut it along, you can study the structure of the plant root in more detail and see the following zones:

1. root cap - protects the tip from damage, if it is removed, the root will die;
2. division zone - located immediately behind the cap, its width is small (3 mm), consists of cells of the educational tissue that are continuously dividing;
3. growth zone (stretching) - here the cells elongate, taking on the final shape, it is due to them that the root grows in length;
4. suction zone - covered with root hairs;
5. root hairs - absorb water and minerals from the soil.

cross section

In this figure, you can distinguish between different types of tissues, the structure of cells of which depends on their functions. Distinguish:

• epidermis - integumentary tissue, performs a protective function;
• primary cortex - conducts water and minerals dissolved in it from the hairs to the center of the root, its cells are surrounded by a large amount of intercellular substance;
• primary bark (endoderm) - performs the function of a rod, the cells are stiff, most of them are dead;
• central axial cylinder - conducts water up to ground organs;
• pericycle - its cells are constantly dividing, new root hairs grow from them;
• conductive bundles - consist of sieve tubes and vessels, it is through them that water passes from the ground up - to the stem and leaves.

Root modifications

Root systems are not as diverse as leaves or flowers, since the soil is a homogeneous structure. But some of them are subject to modification, depending on the habitat and additional functions assigned to them. The most common metamorphosis:

• root crop - a thickening of the main root and the lower part of the stem, in which nutrients accumulate;
• board-like roots - the uppermost roots, passing at the border of soil and air, form triangular outgrowths, adjoin the trunk, are characteristic of tropical forest trees;
• aerial roots - grow in the aerial part of the plant, absorb rain moisture and oxygen from the air, are formed due to a lack of mineral salts in the soil;
• respiratory roots - help the plant perform the function of respiration.

The structure of the plant root can also change due to the occurrence of symbiosis with bacteria or fungi. The result is:

• mycorrhiza - the fungus grows on the root system, receiving organic substances, and the benefit of the plant is to obtain water from the symbiont with minerals dissolved in it;
• bacterial nodules - lateral roots are modified in such a way as to cooperate with nitrogen-fixing bacteria, which fix nitrogen from the air and convert it into a mineral form available to the plant; bacteria, on the other hand, obtain a safe place to live and nutrients from the roots.

Root sections - video

Phylogenetically, the root arose later than the stem and leaf - in connection with the transition of plants to life on land and probably originated from root-like underground branches. The root has neither leaves nor buds arranged in a certain order. It is characterized by apical growth in length, its lateral branches arise from internal tissues, the growth point is covered with a root cap. The root system is formed throughout the life of the plant organism. Sometimes the root can serve as a place of deposition in the supply of nutrients. In this case, it is modified.

Root types

The main root is formed from the germinal root during seed germination. It has lateral roots.

Adventitious roots develop on stems and leaves.

Lateral roots are branches of any roots.

Each root (main, lateral, adventitious) has the ability to branch, which significantly increases the surface of the root system, and this contributes to a better strengthening of the plant in the soil and improves its nutrition.

Types of root systems

There are two main types of root systems: taproot, which has a well-developed main root, and fibrous. The fibrous root system consists of a large number of adventitious roots, the same in size. The entire mass of roots consists of lateral or adventitious roots and looks like a lobe.

A highly branched root system forms a huge absorbing surface. For example,

• the total length of winter rye roots reaches 600 km;
• length of root hairs - 10,000 km;
• the total surface of the roots is 200 m 2.

This is many times greater than the area of ​​the above-ground mass.

If the plant has a well-defined main root and adventitious roots develop, then a mixed-type root system (cabbage, tomato) is formed.

External structure of the root. The internal structure of the root

Root zones

root cap

The root grows in length with its tip, where the young cells of the educational tissue are located. The growing part is covered with a root cap that protects the tip of the root from damage and facilitates the movement of the root in the soil during growth. The latter function is carried out due to the property of the outer walls of the root cap to be covered with mucus, which reduces friction between the root and soil particles. They can even push apart soil particles. The cells of the root cap are living, often containing grains of starch. The cells of the cap are constantly updated due to division. Participates in positive geotropical reactions (direction of root growth towards the center of the Earth).

The cells of the division zone are actively dividing, the length of this zone varies in different species and in different roots of the same plant.

Behind the division zone there is an extension zone (growth zone). The length of this zone does not exceed a few millimeters.

As linear growth is completed, the third stage of root formation begins - its differentiation, a zone of differentiation and specialization of cells (or a zone of root hairs and absorption) is formed. In this zone, the outer layer of the epiblema (rhizoderm) with root hairs, the layer of the primary cortex and the central cylinder are already distinguished.

The structure of the root hair

Root hairs are highly elongated outgrowths of the outer cells covering the root. The number of root hairs is very high (from 200 to 300 hairs per 1 mm2). Their length reaches 10 mm. Hairs are formed very quickly (in young seedlings of an apple tree in 30-40 hours). Root hairs are short-lived. They die off in 10-20 days, and new ones grow on the young part of the root. This ensures the development of new soil horizons by the root. The root continuously grows, forming more and more new areas of root hairs. Hairs can not only absorb ready-made solutions of substances, but also contribute to the dissolution of certain soil substances, and then absorb them. The area of ​​the root, where the root hairs have died, is able to absorb water for some time, but then becomes covered with cork and loses this ability.

The sheath of the hair is very thin, which facilitates the absorption of nutrients. Almost the entire hair cell is occupied by a vacuole surrounded by a thin layer of cytoplasm. The nucleus is at the top of the cell. A mucous sheath is formed around the cell, which promotes gluing of root hairs with soil particles, which improves their contact and increases the hydrophilicity of the system. Absorption is facilitated by the secretion of acids (carbonic, malic, citric) by root hairs, which dissolve mineral salts.

Root hairs also play a mechanical role - they serve as a support for the top of the root, which passes between the soil particles.

Under a microscope on a cross section of the root in the absorption zone, its structure is visible at the cellular and tissue levels. On the surface of the root is the rhizoderm, below it is the bark. The outer layer of the cortex is the exoderm, inward from it is the main parenchyma. Its thin-walled living cells perform a storage function, conduct nutrient solutions in the radial direction - from the absorbing tissue to the vessels of the wood. They also synthesize a number of vital organic substances for the plant. The inner layer of the cortex is the endoderm. Nutrient solutions coming from the cortex to the central cylinder through the cells of the endoderm pass only through the protoplast of the cells.

The bark surrounds the central cylinder of the root. It borders on a layer of cells that retain the ability to divide for a long time. This is the pericycle. Pericycle cells give rise to lateral roots, adnexal buds, and secondary educational tissues. Inward from the pericycle, in the center of the root, there are conductive tissues: bast and wood. Together they form a radial conducting beam.

The conducting system of the root conducts water and minerals from the root to the stem (upward current) and organic matter from the stem to the root (downward current). It consists of vascular fibrous bundles. The main components of the bundle are the sections of the phloem (through which substances move to the root) and xylem (through which substances move from the root). The main conducting elements of the phloem are sieve tubes, xylems are tracheas (vessels) and tracheids.

Root life processes

Water transport at the root

Absorption of water by root hairs from the soil nutrient solution and its conduction in the radial direction along the cells of the primary cortex through the passage cells in the endodermis to the xylem of the radial vascular bundle. The intensity of water absorption by the root hairs is called the suction force (S), it is equal to the difference between the osmotic (P) and turgor (T) pressure: S=P-T.

When the osmotic pressure is equal to the turgor pressure (P=T), then S=0, water stops flowing into the root hair cell. If the concentration of substances in the soil nutrient solution is higher than inside the cell, then water will leave the cells and plasmolysis will occur - the plants will wither. This phenomenon is observed in conditions of dry soil, as well as with excessive application of mineral fertilizers. Inside the root cells, the sucking force of the root increases from the rhizoderm towards the central cylinder, so water moves along the concentration gradient (i.e., from a place with a higher concentration to a place with a lower concentration) and creates a root pressure that raises a column of water along the xylem vessels , forming an upward current. It can be found on spring leafless trunks when "sap" is harvested, or on cut stumps. The outflow of water from wood, fresh stumps, leaves, is called the "weeping" of plants. When the leaves bloom, they also create a sucking force and attract water to themselves - a continuous column of water is formed in each vessel - capillary tension. Root pressure is the lower motor of the water current, and the sucking power of the leaves is the upper one. You can confirm this with the help of simple experiments.

Absorption of water by roots

Target: find out the main function of the root.

What we do: a plant grown on wet sawdust, shake off its root system and lower its roots into a glass of water. On top of the water to protect it from evaporation, pour a thin layer of vegetable oil and mark the level.

What we observe: after a day or two, the water in the tank dropped below the mark.

Result: therefore, the roots sucked in the water and brought it up to the leaves.

One more experiment can be done, proving the absorption of nutrients by the root.

What we do: we cut off the stem of the plant, leaving a stump 2-3 cm high. We put a rubber tube 3 cm long on the stump, and put a curved glass tube 20-25 cm high on the upper end.

What we observe: the water in the glass tube rises and flows out.

Result: this proves that the root absorbs water from the soil into the stem.

Does the temperature of the water affect the rate of absorption of water by the root?

Target: find out how temperature affects root operation.

What we do: one glass should be with warm water (+17-18ºС), and the other with cold water (+1-2ºС).

What we observe: in the first case, water is released abundantly, in the second - little, or completely stops.

Result: this is proof that temperature has a strong effect on root performance.

Warm water is actively absorbed by the roots. Root pressure rises.

Cold water is poorly absorbed by the roots. In this case, the root pressure drops.

mineral nutrition

The physiological role of minerals is very great. They are the basis for the synthesis of organic compounds, as well as factors that change the physical state of colloids, i.e. directly affect the metabolism and structure of the protoplast; act as catalysts for biochemical reactions; affect the turgor of the cell and the permeability of the protoplasm; are centers of electrical and radioactive phenomena in plant organisms.

It has been established that the normal development of plants is possible only in the presence of three non-metals in the nutrient solution - nitrogen, phosphorus and sulfur and - and four metals - potassium, magnesium, calcium and iron. Each of these elements has an individual value and cannot be replaced by another. These are macronutrients, their concentration in the plant is 10 -2 -10%. For the normal development of plants, microelements are needed, the concentration of which in the cell is 10 -5 -10 -3%. These are boron, cobalt, copper, zinc, manganese, molybdenum, etc. All these elements are found in the soil, but sometimes in insufficient quantities. Therefore, mineral and organic fertilizers are applied to the soil.

The plant grows and develops normally if the environment surrounding the roots contains all the necessary nutrients. Soil is such an environment for most plants.

Root breath

For normal growth and development of the plant, it is necessary that fresh air enter the root. Let's check if it is?

Target: do roots need air?

What we do: Let's take two identical vessels with water. We place developing seedlings in each vessel. We saturate the water in one of the vessels every day with air using a spray bottle. On the surface of the water in the second vessel, pour a thin layer of vegetable oil, as it delays the flow of air into the water.

What we observe: after a while, the plant in the second vessel will stop growing, wither, and eventually die.

Result: the death of the plant occurs due to the lack of air necessary for the respiration of the root.

Root modifications

In some plants, reserve nutrients are deposited in the roots. They accumulate carbohydrates, mineral salts, vitamins and other substances. Such roots grow strongly in thickness and acquire an unusual appearance. Both the root and the stem are involved in the formation of root crops.

Roots

If reserve substances accumulate in the main root and at the base of the stem of the main shoot, root crops (carrots) are formed. Root-forming plants are mostly biennials. In the first year of life, they do not bloom and accumulate a lot of nutrients in root crops. On the second - they quickly bloom, using the accumulated nutrients and form fruits and seeds.

root tubers

In dahlia, reserve substances accumulate in adventitious roots, forming root tubers.

bacterial nodules

The lateral roots of clover, lupine, alfalfa are peculiarly changed. Bacteria settle in young lateral roots, which contributes to the absorption of gaseous nitrogen from the soil air. Such roots take the form of nodules. Thanks to these bacteria, these plants are able to live on nitrogen-poor soils and make them more fertile.

stilted

A ramp growing in the intertidal zone develops stilted roots. High above the water, they hold large leafy shoots on unsteady muddy ground.

Air

Tropical plants that live on tree branches develop aerial roots. They are often found in orchids, bromeliads, and some ferns. Aerial roots hang freely in the air, not reaching the ground and absorbing moisture from rain or dew that falls on them.

Retractors

In bulbous and corm plants, for example, crocuses, among the numerous thread-like roots, there are several thicker, so-called retracting roots. Reducing, such roots draw the corm deeper into the soil.

Pillar-shaped

Ficus develop columnar above-ground roots, or support roots.

Soil as a habitat for roots

The soil for plants is the environment from which it receives water and nutrients. The amount of minerals in the soil depends on the specific features of the parent rock, the activity of organisms, the vital activity of the plants themselves, and the type of soil.

Soil particles compete with roots for moisture, holding it on their surface. This is the so-called bound water, which is divided into hygroscopic and film. It is held by the forces of molecular attraction. The moisture available to the plant is represented by capillary water, which is concentrated in the small pores of the soil.

Antagonistic relations develop between the moisture and the air phase of the soil. The more large pores in the soil, the better the gas regime of these soils, the less moisture the soil retains. The most favorable water-air regime is maintained in structural soils, where water and air are located simultaneously and do not interfere with each other - water fills the capillaries inside the structural aggregates, and air fills the large pores between them.

The nature of the interaction between the plant and the soil is largely related to the absorptive capacity of the soil - the ability to retain or bind chemical compounds.

Soil microflora decomposes organic matter into simpler compounds, participates in the formation of soil structure. The nature of these processes depends on the type of soil, the chemical composition of plant residues, the physiological properties of microorganisms, and other factors. Soil animals take part in the formation of the soil structure: annelids, insect larvae, etc.

As a result of a combination of biological and chemical processes in the soil, a complex complex of organic substances is formed, which is combined by the term "humus".

Water culture method

What salts a plant needs, and what effect they have on its growth and development, was established by experiment with aquatic cultures. The aquatic culture method is the cultivation of plants not in soil, but in an aqueous solution of mineral salts. Depending on the goal in the experiment, you can exclude a separate salt from the solution, reduce or increase its content. It was found that fertilizers containing nitrogen promote the growth of plants, those containing phosphorus - the earliest ripening of fruits, and those containing potassium - the fastest outflow of organic matter from leaves to roots. In this regard, fertilizers containing nitrogen are recommended to be applied before sowing or in the first half of summer, containing phosphorus and potassium - in the second half of summer.

Using the method of water cultures, it was possible to establish not only the need of a plant for macroelements, but also to find out the role of various microelements.

Currently, there are cases when plants are grown using hydroponics and aeroponics methods.

Hydroponics is the cultivation of plants in pots filled with gravel. The nutrient solution containing the necessary elements is fed into the vessels from below.

Aeroponics is the air culture of plants. With this method, the root system is in the air and automatically (several times within an hour) is sprayed with a weak solution of nutrient salts.

The root serves to fix the plant in the soil and absorb water and minerals from it. The root that develops from the germinal root of the seed embryo is calledmain . depart from the main root lateral, which can branch to formadnexal roots.

The totality of all the roots of a plant is called the root system. If the main root is clearly distinguished in the root system, then such a system is calledpivotal . A root system consisting of several equally developed roots is calledfibrous . The tap root system is characteristic mainly for dicotyledons, fibrous - for most monocots.

Root zones. The top of the root is covered with cells that protect it from damage by soil particles - this is root cap. Its cells are constantly exfoliating from the outside, and from the inside, to replace the dead ones, new ones are continuously formed due to cell division of the educational tissue of the root tip.

Under the root sheath there is a zone dividing cells educational fabric. The cells formed here grow intensively and stretch along the root axis. The area of ​​the root formed by such cells is called stretch zone. Above it begins root hair zone(or suction zone). Here, individual cells of the root skin are elongated, forming root hairs that absorb water and minerals from the soil. Root hairs are small in size (their length is not more than 10 mm) and short-lived. Dead root hairs are replaced by new ones as the root grows. Between the suction zone and the base of the stem is the longest zone holding.

In the center of the root there is a conductive tissue, and between it and the skin of the root, the main tissue is developed, consisting of large colorless living cells. Water with mineral salts dissolved in it moves from bottom to top, and solutions of organic substances necessary for root growth move from top to bottom along sieve tubes.

Bibliography:

1. M.M. Musienko, P.S. Slavniy, P.G. Balan. A handyman for the 7th class of the main lighting of the initial mortgages. - K.: "Geneza", 2007

2. Sha Banov D.A., Shabanova G.V. Biology. A handyman for the 7th class of the main lighting of the initial mortgages. - H.: "Osvita", 2003

Obtaining organic matter and increasing the size occurs in different zones of the root. Each zone differs in structure, length, function.

How are the zones

The main root develops from the embryo and grows exclusively deep into the soil. It is divided into five zones. The root zones are described below in order from tip to stem.

• root cap . This is a denser and darker formation at the very end of the root. The case can be seen without a magnifying glass. It does not change in size and always, throughout life, covers the top (tip) of the root.
• Divisions . It is located immediately behind the case and is only 1 mm in length. Here the cells of the entire root are formed.
• growth or stretching . This is a smooth segment of the root, the length of which is 6-9 mm. Cells here practically do not divide.
• Suction . The most important part of the root. The length is several centimeters. Fine hairs form a "fluff" around the root. Hair grows up to 1 cm.
• Conduction or zone of lateral roots . The rest of the root is from hairs to a green stem. It has a dense cover and a wide diameter. At this point, the root branches out to the sides.

Rice. 1. Scheme of root zones.

The place where the root meets the stem is called the root collar. Usually this part is dark and resembles bark in density.

Root zones and their functions

The cells of each zone differ in morphology and functions. The table "Zones of the root and their functions" describes the main processes occurring in different parts of the root.

Zone	Cells	Function
	Compacted, quickly die off, secrete mucus	Protection of young cells from soil damage
divisions	Small, rapidly dividing	There is a continuous increase in the length of the root due to cell division
Stretching	They have a cylindrical, elongated shape, small vacuoles merge into one large	The cells stretch and help the root move deeper into the soil
Suction	They have a thin membrane and suction hairs	Minerals are absorbed from the soil with the help of hairs with water.
lateral roots	Dead and living, conductive fluids	Nutrients obtained from the soil are carried up the stems to the leaves, and organic matter is carried down to feed the root cells.

The root of the plant has positive geotropism, i.e. constantly growing downward towards the center of the earth. The cells of the root cap are responsible for this ability.

Fabric properties

The internal structure of the root is represented by three types of tissue:

• epiblema (rhizoderma) - outer skin
• primary cortex - includes exoderm, mesoderm, endoderm;
• central, axial cylinder or stele -consists of pericycle and procambium.

Rice. 2. The internal structure of the root.

Let's take a closer look at the features of each fabric.

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• epiblema . Each root hair 8-10 mm long is part of a rhizodermal cell. Fine hairs are densely arranged to each other. There are up to 300 hairs per square millimeter. Most of the cell is occupied by a vacuole, which is surrounded by a thin layer of cytoplasm. Due to the pressure created, water and mineral salts enter the cell through the membrane. The hairs die off in 10-12 days. During this time, new processes grow in the lower part of the zone. Such a change of cells allows the hairs to remain at the same distance from the tip and constantly, as the root grows, go deeper into the soil.

Rice. 3. The structure of the root hair.

• primary bark. The exoderm contains larger cells than the inner layers. When the epiblema dies, the exoderm takes its place. The mesoderm stores nutrients. The endoderm is formed by a single cell layer encircling the axial cylinder.
• Stele. pericycle - the top layer of the axial cylinder. The procambium includes two types of tissues - xylem or wood and phloem or bast. The denser xylem includes vessels that deliver nutrients from the hairs to the stems and leaves. Through a thin bast containing sieve tubes, organic substances come to each cell of the root.

Due to the constantly growing tip, the cells gradually replace each other in different zones. The upper dividing cells, as the root moves into the soil, stretch and become the cells of the elongation zone. Elongated cells absorb and conduct organic substances.

What have we learned?

Each section of the root performs a specific function due to special cells that form tissues. Zones allow you to grow inside the earth, absorb substances from the soil and conduct them to all other plant parts.

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