Plants. Plant roots. Root system types. Root functions. root zones. Modification of roots Do all plants have roots briefly
Questions:
1.Root functions
2. Types of roots
3. Types of root system
4. Root zones
5. Modification of the roots
6. Life processes at the root
1. Root functions
Root is the underground organ of the plant.
The main functions of the root:
- supporting: the roots fix the plant in the soil and hold it throughout its life;
- nutritious: through the roots the plant receives water with dissolved mineral and organic substances;
- storage: in some roots can accumulate nutrients.
2. Types of roots
There are main, adventitious and lateral roots. When the seed germinates, the germinal root appears first, which turns into the main one. Adventitious roots may appear on the stems. Lateral roots extend from the main and adventitious roots. Adventitious roots provide the plant with additional nutrition and perform a mechanical function. Develop when hilling, for example, tomatoes and potatoes.
3. Types of root system
The roots of one plant are root system. The root system is rod and fibrous. In the tap root system, the main root is well developed. It has most dicotyledonous plants (beets, carrots). At perennials the main root may die off, and nutrition occurs at the expense of lateral roots, so the main root can only be traced in young plants.
The fibrous root system is formed only by adventitious and lateral roots. It has no main root. Monocotyledonous plants, for example, cereals, onions, have such a system.
Root systems take up a lot of space in the soil. For example, in rye, the roots spread in breadth by 1-1.5 m and penetrate deep into 2 m.
4. Root zones
In a young root, the following zones can be distinguished: root cap, division zone, growth zone, absorption zone.
root cap has more dark color, this is the very tip of the root. Root cap cells protect the root tip from damage by soil solids. The cells of the cap are formed by the integumentary tissue and are constantly updated.
Suction zone has many root hairs, which are elongated cells no more than 10 mm long. This zone looks like a cannon, because. root hairs are very small. Root hair cells, like other cells, have a cytoplasm, a nucleus, and vacuoles with cell sap. These cells are short-lived, quickly die off, and in their place new ones are formed from younger superficial cells located closer to the root tip. The task of the root hairs is the absorption of water with dissolved nutrients. The absorption zone is constantly moving due to cell renewal. It is delicate and easily damaged during transplantation. Here are the cells of the main tissue.
Venue . It is located above suction, does not have root hairs, the surface is covered with integumentary tissue, and conductive tissue is located in the thickness. The cells of the conduction zone are vessels through which water with dissolved substances moves into the stem and leaves. There are also vascular cells, through which organic substances from the leaves enter the root.
The entire root is covered with cells of mechanical tissue, which ensures the strength and elasticity of the root. The cells are elongated, covered with a thick shell and filled with air.
5. Modification of the rootsThe depth of penetration of the roots into the soil depends on the conditions in which the plants are located. The length of the roots is affected by humidity, soil composition, permafrost.
Long roots are formed in plants in dry places. This is especially true for desert plants. So, in camel thorn, the root system reaches 15-25 m in length. In wheat on non-irrigated fields, the roots reach a length of up to 2.5 m, and on irrigated fields - 50 cm, and their density increases.
Permafrost limits root growth in depth. For example, in the tundra, the roots of a dwarf birch are only 20 cm. The roots are superficial, branched.
In the process of adaptation to environmental conditions, the roots of plants have changed and began to perform additional functions.
1. Root tubers act as nutrient storage instead of fruits. Such tubers arise as a result of thickening of the lateral or adventitious roots. For example, dahlias.
2. Root crops - modifications of the main root in plants such as carrots, turnips, beets. Root crops are formed by the lower part of the stem and the upper part of the main root. Unlike fruits, they do not have seeds. Root crops have 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.
3. Attachment roots (suckers) - adnexal measles that develop in plants of tropical places. They allow you to attach to vertical supports (to a wall, rock, tree trunk), bringing the foliage to the light. An example would be ivy and clematis.
4. 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.
5. aerial roots are formed in plants growing in humid equatorial and tropical forests. Such roots hang down and absorb rainwater from the air - they are found in orchids, bromeliads, some ferns, monstera.
Aerial prop roots are adventitious roots that form on the branches of trees and reach the ground. Occur in banyan, ficus.
6. Stilted roots. Plants growing in the intertidal zone develop stilted roots. High above the water, they hold large leafy shoots on unsteady muddy ground.
7. Respiratory roots form in plants that lack oxygen to breathe. Plants grow in excessively moist places - in marshy swamps, backwaters, sea estuaries. The roots grow vertically upwards and come to the surface, absorbing air. An example would be brittle willow, swamp cypress, mangrove forests.
6. Life processes at the root
1 - Absorption of water by roots
The absorption of water by root hairs from the soil nutrient solution and its conduction through the cells of the primary cortex occurs due to the difference in pressure and osmosis. The osmotic pressure in the cells causes minerals to penetrate into the cells, because. their salt content is less than in the soil. The intensity of water absorption by the root hairs is called the suction force. 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 immoderate application. mineral fertilizers. Root pressure can be confirmed by a series of experiments.
A plant with roots falls into a glass of water. Pour a thin layer over the water to protect it from evaporation. vegetable oil and note the level. After a day or two, the water in the tank dropped below the mark. Consequently, the roots sucked in water and brought it up to the leaves.
Purpose: to find out the main function of the root.
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. The water in the glass tube rises and flows out. This proves that the root absorbs water from the soil into the stem.
Objective: To find out how temperature affects the operation of the root.
One glass should be warm water(+17-18ºС), and the other with cold (+1-2ºС). In the first case, water is released abundantly, in the second - little, or completely stops. 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.
2 - Mineral nutrition
The physiological role of minerals is very great. They are the basis for the synthesis organic compounds and directly affect the metabolism; act as catalysts for biochemical reactions; affect the turgor of the cell and the permeability of the protoplasm; are the centers of electrical and radioactive phenomena in plant organisms. With the help of the root, the mineral nutrition of the plant is carried out.
3 - Breath of the roots
For normal growth and development of a plant, it is necessary that the root receive Fresh air.
Purpose: to check the presence of respiration at the roots.
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. After a while, the plant in the second vessel will stop growing, wither, and eventually die. The death of the plant occurs due to the lack of air necessary for the respiration of the root.
It has been established that the normal development of plants is possible only in the presence of three substances in the nutrient solution - nitrogen, phosphorus and sulfur 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.
The root of plants performs various mechanical and physiological functions. The most important of them are: the absorption of water, organic and mineral substances from the soil and their transfer to the roots and leaves. In addition, the roots help the plant to gain a foothold in the soil, it is less sensitive to the effects of atmospheric phenomena (strong wind, rain, etc.). They practically grow together with, therefore, quite often, when pulling a plant out of tiny hairs, particles of soil remain.
With the help of roots, the plant is connected with the organisms that inhabit the layer (mycorrhiza). This obligatory part of the plant organism helps in the synthesis and accumulates useful substances necessary for the growth of the plant. In addition, the root is responsible for vegetative propagation- the formation of a new plant, which appears by the decay of tubers or rhizomes in the mother individual.
But not all plants have the same roots. A fairly common structure is the taproot. Such an underground structure of a plant organism has one large rod, from which a large number of small hairs extend. There is a bundle, in which there are several large rod hairs (for example, many types of herbs). Such plants are extremely useful for the soil, because their dense structure protects it from erosion.
Everyone is well aware of plants that, as they grow, accumulate a lot of useful substances. sweet potatoes and bright to that example. In addition, there are plants that do not need soil. So, some types of orchids are on trees, and they get all the necessary substances and moisture from the air, but, for example, poison ivy attached to trees with aerial roots.
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The root is the axial organ of higher plants, usually located underground, providing the absorption and transportation of water and minerals, and also serving to fix the plant in the soil. Depending on the structure, three types of root systems are distinguished: taproot, fibrous, and also mixed.
The root system of a plant is formed by roots of various nature. Allocate the main root, which develops from the germinal root, as well as lateral and adventitious. The lateral ones are a branch from the main one and can form on any of its sections, while the adventitious roots most often begin their growth 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 indefinitely, and the main root in plants with a tap root system reaches an impressive size. This is necessary to optimize the extraction of water and nutrients from soils where groundwater occurs at a considerable depth. Many species have a tap root system - trees, shrubs, as well as herbaceous plants: birch, oak, dandelion, sunflower, .
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 the same length. Often, in plants, the main root grows first, from which the lateral ones begin to depart, but in the process further development plants it dies. A fibrous root system is characteristic of plants that reproduce vegetatively. It is usually found in - coconut tree, orchids, ferns, grasses.
Mixed root system
Often, a mixed or combined root system is also distinguished. Plants belonging to this type have both a well-differentiated main root and multiple lateral and adventitious roots. Such a structure of the root system can be observed, for example, in strawberries and 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 root crops - 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 observed in sweet potato. Also, some roots may not serve to absorb water with salts dissolved in it, but for respiration (respiratory roots) or additional support (stilted roots).
The roots fix the plant in the soil, provide soil water and mineral nutrition, and sometimes serve as a place for deposition of reserve nutrients. In the process of adaptation to environmental conditions, the roots of some plants acquire additional functions and are modified.
What are the types of roots
Plants are divided into main, adventitious and lateral roots. When a seed germinates, it first develops into an embryonic root, which later becomes the main root. Adventitious roots grow on the stems and leaves of some plants. Lateral roots can also depart from the main and adventitious roots.
Root systems
All the roots of the plant are folded into the root system, which is tap and fibrous. In the rod system, the main root is more developed than the others and resembles a rod, while in the fibrous system it is underdeveloped or dies off early. The first is most typical for, the second - for monocots. However, the main root is usually well expressed only in young dicotyledonous plants, and in old ones it gradually dies off, giving way to adventitious roots growing from the stem.
How deep are the roots
The depth of the roots in the soil depends on the growing conditions of the plant. Wheat roots, for example, grow 2.5 m in dry fields, and no more than half a meter in irrigated fields. However, in last case the root system is denser.
Tundra plants themselves are stunted, and their roots are concentrated near the surface due to permafrost. In dwarf birch, for example, they are at a depth of about 20 cm maximum. Roots desert plants, on the contrary, are very long - this is necessary to achieve ground water. For example, the leafless barnyard is rooted 15 m into the soil.
Root modifications
To adapt to conditions environment the roots of some plants have changed and acquired additional functions. So, the root crops of radishes, beets, turnips, turnips and rutabaga, formed by the main root and the lower parts of the stem, store nutrients. The thickenings of the lateral and adventitious roots of the chistyak and dahlia became root tubers. Ivy roots help the plant to attach to a support (wall, tree) and bring the leaves to the light.
The root is one of the main organs of the plant. It performs the function of absorption from the soil with elements of mineral nutrition dissolved in it. The root anchors and holds the plant in the soil. In addition, the roots are of metabolic importance. As a result of the primary synthesis, amino acids, hormones, etc. are formed in them, which are quickly included in the subsequent biosynthesis that occurs in the stem and leaves of the plant. Reserve nutrients can be deposited in the roots.
The root is an axial organ with a radially symmetrical anatomical structure. The root grows in length indefinitely due to the activity of the apical meristem, the delicate cells of which are almost always covered by the root cap. Unlike the shoot, the root is characterized by the absence of leaves and, therefore, dismemberment into nodes and internodes, as well as the presence of a cap. The entire growing part of the root does not exceed 1 cm.
The root cap, about 1 mm long, consists of loose thin-walled cells, which are constantly replaced by new ones. At the growing root, the cap is practically updated every day. The exfoliating cells form a slime that facilitates the movement of the root tip in the soil. The functions of the root cap are to protect the growing point and provide the roots with positive geotropism, which is especially pronounced at the main root.
A dividing zone about 1 mm in size, composed of meristem cells, adjoins the cap. The meristem in the process of mitotic divisions forms a mass of cells, providing root growth and replenishing the cells of the root cap.
The division zone is followed by the stretch zone. Here, the length of the root increases as a result of cell growth and the acquisition of a normal shape and size by them. The extension of the stretch zone is several millimeters.
Behind the stretch zone is the suction or absorption zone. In this zone, the cells of the primary integumentary root - the epiblema - form numerous root hairs that absorb the soil solution of minerals. The absorption zone is several centimeters long, it is here that the roots absorb the bulk of the water and salts dissolved in it. This zone, like the two previous ones, gradually moves, changing its place in the soil with the growth of the root. As the root grows, the root hairs die, the absorption zone appears on the newly growing root area, and the absorption of nutrients occurs from the new soil volume. In place of the former absorption zone, a conduction zone is formed.
The primary structure of the root
The primary structure of the root arises as a result of differentiation of the meristem of the apex. In the primary structure of the root near its tip, three layers are distinguished: the outer one is the epiblem, the middle one is the primary cortex, and the central axial cylinder is the stele.
Internal tissues naturally and in a certain sequence arise in the division zone in the apical meristem. Here there is clear division into two departments. The outer section, originating from the middle layer of initial cells, is called Periblem. The inner section comes from the upper layer of initial cells and is called the Pleroma.
The pleroma gives rise to a stele, while some cells turn into vessels and tracheids, others into sieve tubes, others into core cells, etc. Periblema cells turn into the primary root cortex, consisting of parenchymal cells of the main tissue.
From the outer layer of cells - dermatogen - the primary integumentary tissue - epiblema, or rhizoderm - is isolated on the root surface. It is a single-layer tissue that reaches its full development in the absorption zone. The formed rhizoderm forms the thinnest numerous outgrowths - root hairs. The root hair is short-lived and only in the growing state actively absorbs water and substances dissolved in it. The formation of hairs contributes to an increase in the total surface of the suction zone by 10 or more times. The length of the hair is not more than 1 mm. Its shell is very thin and consists of cellulose and pectin.
The primary cortex that emerged from the periblem consists of living thin-walled parenchymal cells and is represented by three distinct layers: endoderm, mesoderm, and exoderm.
Directly to the central cylinder (stele) adjoins the inner layer of the primary cortex - the endoderm. It consists of one row of cells with thickenings on the radial walls, the so-called Casparian bands, which are interspersed with thin-walled cells - through cells. Endoderm controls the flow of substances from the cortex to the central cylinder and vice versa.
Outward from the endoderm is the mesoderm - the middle layer of the primary cortex. It consists of loosely arranged cells with a system of intercellular spaces through which intensive gas exchange takes place. In the mesoderm, plastic substances are synthesized and moved to other tissues, reserve substances accumulate, and mycorrhiza is located.
The outer part of the primary cortex is called the exoderm. It is located directly under the rhizoderm, and as the root hairs die off, it appears on the root surface. In this case, the exoderm can perform the function of an integumentary tissue: thickening and corking of the cell membranes and the death of the cell contents occur. Among the corked cells, there remain non-corked cells through which substances pass.
The outer layer of the stele adjacent to the endoderm is called the pericycle. Its cells retain the ability to divide for a long time. In this layer, the lateral roots are laid, therefore the pericycle is called the root layer.
The roots are characterized by alternation of xylem and phloem sections in the stele. The xylem forms a star (with a different number of rays in different groups of plants), and between its rays is the phloem. In the very center of the root there may be xylem, sclerenchyma, or thin-walled parenchyma. Alternation of xylem and phloem along the periphery of the stele - salient feature root, which distinguishes it sharply from the stem.
The primary root structure described above is characteristic of young roots in all groups of higher plants. In club mosses, horsetails, ferns and representatives of the class Monocotyledons of the Department of Flowering Plants, the primary structure of the root is preserved throughout its life.
Secondary structure of the root
In the roots of gymnosperms and dicots angiosperms the primary structure of the root is preserved only until the beginning of its thickening as a result of the activity of secondary lateral meristems - cambium and phellogen (cork cambium). The process of secondary changes begins with the appearance of layers of cambium under the areas of the primary phloem, inward from it. The cambium arises from the poorly differentiated parenchyma of the central cylinder. Inside, it deposits elements of the secondary xylem (wood), outside - elements of the secondary phloem (bast). At first, the cambium layers are separated, but then they close and form a continuous layer. This is due to the division of pericycle cells against xylem rays. The cambial regions arising from the pericycle are formed only by the parenchymal cells of the medullary rays, the remaining cells of the cambium form the conducting elements - xylem and phloem. This process can continue for a long time, and the roots reach a considerable thickness. In the perennial root, in its central part, there remains a distinctly expressed primary ray xylem.
The cork cambium (phellogen) also appears in the pericycle. It lays out layers of cells of the secondary integumentary tissue - corks. The primary cortex (endoderm, mesoderm and exoderm), isolated by a cork layer from the internal living tissues, dies.
Root systems
The totality of all the roots of a plant is called the root system. Its composition involves the main root, lateral and adventitious roots.
The root system is rod or fibrous. The tap root system is characterized by the predominant development of the main root in length and thickness, and it stands out well from other roots. In the tap root system, in addition to the main and lateral roots, adventitious roots can also occur. Most dicotyledonous plants have a tap root system.
In all monocotyledonous plants and in some dicotyledons, especially those that reproduce vegetatively, the main root dies off early or develops poorly, and the root system is formed from adventitious roots that arise at the base of the stem. Such a root system is called fibrous.
For the development of the root system great importance have soil properties. The soil affects the structure of the root system, the growth of its roots, the depth of penetration and their spatial distribution in the soil.
The secretions of the roots create in the soil around it a zone teeming with bacteria, fungi and other microorganisms, which is called the rhizosphere. The formation of surface, deep and other root systems reflects the adaptation of plants to the conditions of soil water supply.
In addition, in any root system there are continuous changes associated with the age of plants, the change of seasons, etc.
Root specializations and metamorphoses
In addition to the main functions, the roots can perform some others, while the roots undergo modifications, their metamorphoses.
In nature, the phenomenon of symbiosis of the roots of higher plants with soil fungi is widespread. The ends of the roots, braided from the surface with hyphae of the fungus or containing them in the root bark, are called mycorrhiza (literally - "fungal root"). Mycorrhiza is external, or ectotrophic, internal, or endotrophic, and external-internal.
Ectotrophic mycorrhiza replaces the plant's root hairs, which usually do not develop. External and external-internal mycorrhiza was noted in woody and shrubby plants (for example, in oak, maple, birch, hazel, etc.).
Internal mycorrhiza develops in many species of herbaceous and woody plants(for example, in many types of cereals, onions, walnut, grapes, etc.). Species of such families as Heather, Wintergreen and Orchids cannot exist without mycorrhiza.
The symbiotic relationship between a fungus and an autotrophic plant is manifested in the following. Autotrophic plants provide the fungal symbiont with soluble carbohydrates available to it. In turn, the fungal symbiont supplies the plant with the most important mineral substances (the nitrogen-fixing fungal symbiont delivers nitrogen compounds to the plant, quickly ferments sparingly soluble reserve nutrients, bringing them to glucose, the excess of which increases the absorption activity of the roots.
In addition to mycorrhiza (mycosymbiotrophy), in nature there is a symbiosis of roots with bacteria (bacteriosymbiotrophy), which does not have such widespread like the first one. Sometimes growths called nodules form on the roots. Inside the nodules there are many nodule bacteria that have the ability to fix atmospheric nitrogen.
storage roots
Many plants are able to store reserve nutrients (starch, inulin, sugar, etc.) in their roots. Modified roots that perform the function of storage are called "root crops" (for example, in beets, carrots, etc.) or root cones (strongly thickened adventitious roots of dahlia, chistyak, lyubka, etc.). There are numerous transitions between root crops and root cones.
Retractor or contractile roots
In some plants, there is a sharp reduction in the root in the longitudinal direction at its base (for example, in bulbous plants). Retracting roots are widespread in angiosperms. These roots cause rosettes to fit tightly to the ground (for example, in plantain, dandelion, etc.), the underground position of the root collar and vertical rhizome, and provide some deepening of the tubers. Thus retracting roots help shoots to find the best depth in the soil. In the Arctic, retracting roots provide the experience of adverse winter period flower buds and renewal buds.
aerial roots
Aerial roots develop in many tropical epiphytes (from the families of Orchids, Aronnikovs and Bromeliads). They have aerenchyma and can absorb atmospheric moisture. On swampy soils in the tropics, trees form respiratory roots (pneumatophores), which rise up above the soil surface and supply underground organs with air through a system of holes.
Trees growing along the shores of tropical seas as part of mangroves in the tidal zone form stilted roots. Due to the strong branching of these roots, the trees remain stable on unsteady ground.
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 certain order located kidneys. 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 adventitious roots of the same 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 is different types and different roots of the same plant are not the same.
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. Hair can not only absorb ready solutions substances, but also to promote the dissolution of certain soil substances, and then absorb them. The area of the root where the root hairs have died off 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 for the plant organic matter. 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 the 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
At tropical plants living 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 to more simple connections, participates in the formation of soil structure. The nature of these processes depends on the type of soil, chemical composition plant residues, 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. Nutrient solution containing 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.
