Complex metabolic reactions in plants are explained by the special structure of their body parts: root, stem, leaves, called vegetative organs. They are responsible for the processes of photosynthesis, transpiration, osmosis. In this paper, we study the structure and functions of plant elements such as root hairs. These are important structures that determine the absorption of water and mineral salts from the soil.
Root - the vegetative organ of seed plants
The underground part of gymnosperms and flowering plants is represented by two types of root system: stem and fibrous. They consist of the main, lateral, subordinate (in monocotyledonous plants) roots and a large number of small structures called root hairs.
These are outgrowths represented by single epiblema cells (rhizoderma). They are called "trichoblasts." Being a support and performing the functions of storing organic substances, absorption and reproduction (the so-called root offspring of cherries, willows), the root indirectly participates in metabolic processes such as transpiration, respiration, and photosynthesis.
Suction zone
The main root has a complex anatomical structure, and its various sections perform diverse functions. In this regard, they are usually called zones. Based on the function of the root hairs (above the stretch area included in the growth zone), an accumulation of outgrowths of the integumentary tissue is located. This area is called the suction zone. It is from one to three centimeters. In this area, from 200 to 1500 or more elongated epiblema cells can be located. They do not live long: from several hours to 20 days, and then die. At the same time, new structures form from the rhizoderm. A root hair cell, in contact with the soil, is capable of absorbing water molecules and dissolved salts from it in the form of sodium, chlorine, magnesium, acid residues of nitrate, nitrite and phosphate acids.
Epiblema and features of its structure
This plant tissue belongs to the group of primary meristems. By participating in the division, its cells provide the formation of elements such as root hairs. This occurs in the outer layer of the root educational tissue - the phallogen. The rhizoderm formed during the growing season dies. In its place, young peridermal cells are formed - the secondary integumentary tissue, incapable of absorbing soil solutions. A new root hair, the function of which is the absorption of water and mineral salts, is formed from the overlying area of the epiblem.
Root hair function
These structures are formed from protrusions of the rhizoderm and are single cells of the primary meristem, capable of absorbing the soil solution. Over time, they are extended, and the cell membrane becomes able to pass in both hypotonic and highly concentrated salt solutions. When mineral fertilizers, for example, nitrogen and potassium, are applied, the content of ammonium, potassium, and nitrate ions in the soil increases. This happens in the spring, as it is the best time to apply this type of fertilizer. A soil solution containing the above types of ions penetrates the Trichoblast cytoplasm by passive diffusion.
Autumn application of phosphorus fertilizers, which require more time to dissolve, causes the root hairs to absorb ions of acidic residues of phosphate and metaphosphoric acids. With the beginning of sap flow in late February - early March, almost the entire volume of rhizoderm cells is filled with vacuoles, the nucleus is displaced to the top of the root hair. The cell itself is capable of secreting molecules of organic acids: oxalic, malic. They dissolve humus particles, enhancing the absorption process. The formation of root hairs occurs quite quickly. Despite the short life span, they are able to absorb large volumes of soil solution. For example, in a woody plant, the absorption area is about 120 to 640 m 2 .
What are trichoblasts
Earlier, we studied the structural features and functions of the primary tissue of plant covers. It consists of one layer of cells and is called the epiblema, located on young lateral roots growing from the main or adnexal roots. Root hairs are outgrowths of integumentary tissue, which are strongly elongated structures. It must be remembered that all epiblema cells are polypotent, that is, capable of forming root hairs. But they are formed only from trichoblasts - protrusions of the epiblema, which have the appearance of microscopic tubercles.
The integumentary tissue responsible for the formation of trichoblasts has cytological structural features: for example, the cuticle and thick cellulose wall are absent in its cells. In the cytoplasm there are a large number of organelles synthesizing ATP molecules - mitochondria. They are necessary, since the absorption of water and mineral salts requires energy. Trichoblasts also do not have stomata - elements of integumentary tissue responsible for the processes of plant respiration and transpiration - evaporation of water.
How salt solutions penetrate plant roots
Trichoblast and the root hair formed from it, whose function is the absorption of water and mineral salts from the soil, can be considered an osmotic system. The absence of a rigid cell wall and the elasticity of the membrane promotes the transport of molecules from the external environment to the cytoplasm. In special organelles of the root hair - vacuoles, hypertonic solutions of glucose, fructose, malic, citric and oxalic acids accumulate.
Both the membrane and the tonoplast of the cell have selective semipermeability. Therefore, the soil solution, being less concentrated than cell sap, penetrates the root hair according to the laws of osmosis. The water potential of solutions coming from the soil is higher than this indicator in the tonoplast, and the osmotic potential is lower. Water and mineral salts are transported from the root hair cell to the xylem. This is a conductive tissue that forms the vessels of plants - trachea or tracheids. According to them, the soil solution moves up the stem to the leaves and other parts of the plant.
The pick and its meaning
To increase the absorption area of the root system, it is necessary to achieve an increase in the number of lateral roots. Their epiblema containing trichoblasts will form additional root hairs. To do this, use the mechanical method of pinching the root apex, destroying the division zone located above the root cover. It is called a pick. This technique stimulates the growth of the lateral parts, on which a large number of root hairs develop. In this case, the growth of the main root in length stops. A large area of the absorption zone has a positive effect on the growth and development of the plant, increasing its yield and vitality.
In this article, we studied the structural features of the absorption zone of the root of angiosperms, and also found out what function the root hair developing from the epiblema performs.