The root grows in length using the apical growth point. A growth point (meristem) is a group of cells capable of active cell division. It is located not at the very end of the root, but under the root cap, which protects it 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 being covered with mucus, which reduces friction between the root and soil particles. In some plants, the cells of the root cap are very strong and are capable of pushing apart soil particles. The root cap is present on the roots of all land plants and is absent in many hydrophytes.
The cells of the root cap are living and often contain starch grains. The cells of the outer layer of the cap slough off. As they die, the cap is restored from the inside due to the cells formed by the growth point (in dicotyledons) or their own separate meristem (in monocotyledons).
Root growth point cells ( apical meristem) are actively dividing, the length of this zone is different types and different roots of the same plant are not the same. Thus, the meristematic zone of the growth roots of pedunculate oak is 250-2000 microns, and in sucking roots it is 45-65 microns; in Norway maple, respectively, 200-300 and 50-150 microns.
Behind the division zone is an extension zone (called the growth zone). The length of this zone does not exceed a few millimeters (for tobacco - 320 microns; for corn - 7; for beans - 5 mm).
As linear growth completes, 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 (Fig. 1).
The epiblema consists of a single layer of thin-walled and tightly packed cells capable of forming root hairs. The length of root hairs is 0.15-8 mm (in orchids - up to 3, in cereals - 1.2-1.5, in sedums - 2.4 mm); especially large hairs on the roots of cuttings, larger in herbaceous plants than in woody ones. The hair shell is very thin (0.6-1 microns), 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 cover is formed around the cell (due to the pectin substances of the shells), which promotes the gluing of root hairs with soil particles, which improves their contact and increases the hydrophilicity of the system. Absorption is also facilitated by the secretion of acids by root hairs (carbonic, malic, citric, oxalic), which dissolve mineral salts.
Root hairs also play a mechanical role - they serve as support for the root tip, which passes between the soil particles. Root hairs form very quickly (in young apple tree seedlings within 30-40 hours). Their number per 1 mm 2 of root surface in a humid chamber is 425 for corn, about 300 for apple trees, and 230 for peas, which in terms of the entire plant amounts to billions. Root hairs do not function for long. As the root grows, dead hairs are replaced by new ones.
The primary cortex consists of many layers of living cells. The cell membranes of the outer layers (exoderm) can thicken. After the death of the epiblema, these layers of bark are filled protective function. The inner layer of the cortex (endoderm) also has thickened shells. The endoderm consists of a single row of cells forming a dense ring without intercellular spaces around the central part of the root. Sometimes biseriate endoderm (schisandra chinensis) is found. Thickenings on the thin radial walls of its cells (Casparian's belts) form a continuous system of thickenings around the central cylinder.
Central cylinder consists of a conducting system externally surrounded by a ring of living cells that are capable of dividing and are called the pericycle. Due to the division of pericycle cells, lateral roots are formed. Additional buds can also be formed in the pericycle, which then develop into above-ground shoots - root sprouts (such sprouts form poplar, aspen, willow, lilac, raspberry, blackberry and other plants).
The root vascular system conducts water and minerals from the root to the stem (upward flow) and organic matter from the stem to the root (downward flow). It consists of vascular-fibrous bundles. The main components of the bundle are 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 phloem are sieve tubes, xylem - tracheae (vessels) and tracheids. Parenchyma cells and cells are associated with the conducting elements of phloem and xylem mechanical fabric sclerenchyma.
The roots vary in shape ( types of root systems):
- Rod, which are a direct continuation of the stem into the ground and are characteristic of most dicotyledonous plants. By origin, this will be the main root. The tap root can be: threadlike, fusiform, onion, branchy(Fig. 57, 1 —4 ).
- fibrous roots. IN large number they extend from the base of the stem and have more or less the same thickness. Fibrous roots are found in monocots(for example, in cereals). By origin these will be adventitious roots (Fig. 57, 5 ).
When developing from a seed, the root usually initially grows much faster than the above-ground part of the plant. The depth of penetration of roots into the soil is very significant. total length the roots are much longer than the above-ground parts of the plant. U thistle, For example ( Cirsiumarvense), malicious weed of our fields, the root penetrates to a depth of over 6 m. At the camel thorn (Alhagicamelorum), growing in deserts Central Asia, the roots go deep into the ground up to 20 meters or more, ensuring the delivery of water to the plant from deep subsoil layers. This gives her the opportunity to keep the leaves fresh during the hottest period, when other plants burn out. In ordinary onion roots penetrate to a depth of 1 m. The diameter of its root system is approximately 60 cm.
The total length of the roots is also expressed as a significant figure. For most cultivated cereals it is 500-600 m, and in spring rye the total length of roots of the 1st, 2nd and 3rd orders turned out to be equal to 80 km. Total surface of the root system winter rye reaches 237 m 2 and exceeds the surface of the above-ground part by almost 130 times.
The root grows top. When the root is torn off, powerful development of lateral roots is observed. This feature of the root is used for picking plants (in gardening and horticulture). When picking, the plant is replanted and the main root is shortened by 1/2 - 1/3 of its length, which causes abundant branching of the remaining part, ensuring powerful growth of the root system.
The endings of the main, lateral and adventitious roots and their smallest branches are dressed externally with so-called root caps- a complex of cells that are constantly erased and exfoliated. New cells appear in their place. Thanks to the sliming of the membranes of the outer cells of the cap, root growth among solid soil particles is facilitated. The cap cells protect the root growth cone from damage (Fig. 58). All land plants have a root cap. In aquatic plants, when their roots grow in water, a cap does not form. When they are rooted in the soil, a cap develops at the ends of the roots. Material from the site
Somewhat above the root tip, in a narrow space of a few centimeters, the so-called root hairs, which are outgrowths of root epidermal cells. Through them, water and nutrient salts enter the plant. Root hairs cover this so-called absorption zone of the root. The total length of root hairs in one plant reaches an enormous size. According to calculations, the length of all root hairs of one wheat specimen reaches 20 km. As a result, the absorbing surface of the root increases 10-15-20 times.
The main root grows vertically downwards, perpendicular to the surface of the earth. This phenomenon is called positive geotropism. Lateral roots grow almost horizontally (they transversely geotropical). Smaller root branches are not geotropic. The roots never form or bear leaves or their rudiments.
The first to emerge from the seed coat is the root of the embryo. At first it is barely noticeable, but soon it stretches out and becomes large. Why is this happening?
The root of the embryo and the root of an adult plant, like all other organs, have cellular structure . The cells of the embryonic root absorb nutrient solutions from the seed, grow and divide.
If you apply transverse lines on the root of a pea sprout with ink at a distance of 2 mm from one another and place the sprout in a bottle with water poured to the bottom, after a day the distance between the marks located near the tip of the root will increase. This will happen because there is an area where young cells divide and grow. As a result of the division of these cells, new daughter cells are formed. The daughter cells in turn divide and grow. So a small root gradually turns into a big root.
The root grows from the top. It is not difficult to verify the validity of this conclusion.
If you break or cut off the tip root - its top, the growth of the root in length will stop. A root with a severed tip develops many lateral and adventitious roots. The root system becomes more powerful. Each of the lateral roots also grows at the top. This property of the root is used when transplanting seedlings of cabbage, tomatoes, asters and other cultivated plants that have a tap root system. Pinch the root while picking.
Picking- this is pinching off the tip of the root when planting young plants using a pointed peg resembling a pike.
This process got its name from the name of the peg - peaks, pickets. Picking seedlings causes the growth of lateral and adventitious roots and their proliferation in the upper, most fertile layer of soil 25 . So, in corn, the root system grows away from stem almost 2 m, and for onions - 60-70 cm.
Tree roots grow especially strongly. For example, in an adult apple tree they grow laterally at a distance of up to 15 m from the plant trunk, and to a depth of 3-4 m. The bulk of the roots develop at a depth of 15-18 cm from the soil surface. Therefore, you cannot grow vegetables, field crops or any other crops under the crowns of fruit trees.
The total length of all the roots of one plant is very large. For example, carrot roots are about 7 times longer than the above-ground part of the plant. Thanks to the growth of root systems, plants receive more nutrients from the soil. Root systems Different plants grow differently in the soil. In some they go far into the depths, in others they spread in breadth at a shallow depth 26 .
1. In what part of the root does it grow? How can this be proven?
2. What happens to the root if its top is cut off?
3. What is picking? For what purpose is it carried out?
4. How do the roots of different plants grow in the soil?
5. Why can’t you grow vegetables under the canopy of fruit trees?
>Grow some bean or pea sprouts and place them in bottles with a little water, attaching the sprouts to the caps that seal the bottles. Before this, pinch off a third of the root from half the seedlings. Watch how the undamaged and pinched roots of the seedlings grow. After a week, dry the seedlings, attach them to a sheet of thick paper and draw conclusions.
Korchagina V. A., Biology: Plants, bacteria, fungi, lichens: Textbook. for 6th grade. avg. school - 24th ed. - M.: Education, 2003. - 256 p.: ill.
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