The primary structure of the root, the transition from the primary structure of the root to the secondary

The underground organ of most higher spore, gymnosperms and flowering plants is the root. For the first time it appears in the mice and performs not only the function of support, but also provides all other parts of the plant with water and dissolved in it with mineral salts. In gymnosperms and angiosperms, the main root develops from the embryonic root. In the future, a root system is formed, the structure of which differs from monocotyledonous and dicotyledonous plants. In our article we will study the primary and secondary anatomical structure of the root of flowering plants, the seeds of which have two cotyledons, and on specific examples we will show the role of plant tissues and structural elements of the underground part in providing vital activity to the plant organism.

The embryonic root and its development

In the process of germination of the seed, the first part of the embryo develops, called the embryonic rootlet. It consists of cells of the educational tissue - the primary meristem, the apical part of which is called apex. In the process of mitotic division of its constituent cells, the primary structure of the root is formed, consisting of an epilblem, a primary crust and an axial cylinder. Let us dwell on the morphological and physiological features of the primary educational tissue located at the apex of both the embryonic root and in the apical part of all young roots: the main, lateral and accessory ones. The species called the latter is found mainly in monocotyledonous plants. They develop from the bottom of the stem. So, the apex consists of the initial cells. In the process of development they form the primary meristem. Under its layer, the differentiation of cellular structures begins, leading to the emergence of a formed educational tissue, which determines the primary anatomical structure of the root. The plant retains it until the emergence of secondary meristems, called cambium and phellogen.

Episode: structure and meaning

The rhizoderm, or epilblem, is a layer of cells of the integumentary tissue located on the young central root and the lateral processes extending from it. The most important for the plant is a part of the cover cloth, which is located in the root zone, which absorbs water and mineral salts. In it, elongated epibblem cells form the root hairs. Their cytoplasm contains a large number of vacuoles, and the cell wall is very thin, without the cuticle. The rhizoderm is located on the root from the root canal to the zone of lateral roots, which is called conducting. It has been established that the position of the root hairs relative to the root cap, located on the top of the main root, practically does not change.

Root hairs and their role in the life of plants

Considering the primary structure of the root under the microscope, it can be found that the rhizoderm is a derivative of the uppermost layer, the dermatogen. It, in turn, is formed as a result of the division of the cells of the primary apex. The suction zone of the root is most sensitive to sudden changes in environmental conditions, so the measles can quickly die off. This is the main reason for the poor survival of seedlings and even her death. In the process of development of the sprout, the cells of the rhizoderm are dying off and sloughing off. Under them a layer of protective tissue is formed - the exoderm, partly taking part in the formation of the transmission elements. Thanks to them, water and solutions of mineral compounds from the root hairs enter the axial cylinder, which enters the primary structure of the root.

It contains conductive tissues, from which in the process of ontogenesis vessels develop - trachea and sieve-like tubes with companion cells. Not all plants form a developed system of root hairs. For example, in marsh and aquatic species, they are absent due to excess water in the environment.

Primary meristem - pericycle

This structure, which in the form of a ring covers the central cylinder and is located under the rhizoderm. It is represented by small, rapidly dividing cells of the educational tissue and is present in all tree and herbaceous plant forms, multiplying by seeds. All parts of the central cylinder develop precisely from the cells of the pericycle.

The primary structure of the root of the bipartite plant is a confirmation of the fact of the laying of the lateral and accessory roots in the outer layer of the educational tissue - the meristem. In representatives of dicotyledonous plants belonging to the families of Rosaceae, Legumes, Nightshade, it is then transformed into secondary species, for example, phellogen or cambium. The result of the mitotic division of the pericyclic cells is the emergence of homogeneous structures and functions of the embryonic zones of the future tissues - the periploma, from which the primary cortex is formed, and the dermatogen, which gives rise to the apical primary meristem.

Primary cortex

This part of the root is represented mainly by parenchyma cells. Part of the plant tissue adjacent to the epilblem is called the exoderm, the middle layer of the primary cortex is called the mesoderm. Considering the primary structure of the root under the microscope, a large number of intercellular spaces can be found in these areas. They serve as a place for circulation of oxygen and carbon dioxide, and therefore, participate in gas exchange. The internal site is represented by groups of cells arranged in the form of a dense strand.

After the destruction of the epilem, the areas of the exoderm are exposed, then they are tested in the zone of the lateral roots and subsequently perform a protective function. Through all three layers of the bark in the radial direction the molecules of water move and then enter the vessels of the central cylinder of the root. On them, thanks to root pressure and transpiration, water and solutions of mineral substances rise to the stem and leaves. In addition, organic compounds, for example, starch or inulin, can accumulate in parenchymal cells of the primary crust mesoderm.

Central cylinder

Considering under the microscope the primary structure of the root of a bipartite plant, one can find a structure such as a stela. This axial part contains several anatomical formations that perform the functions of carrying out the substances. They consist of primary tissue - xylem and form conductive elements, such as vessels (trachea). Solutions of glucose and other organic compounds move from the leaves and stems to the root along the sieve-like tubes located in the cortex, and water and mineral substances along the vessels (trachea) come from the axial root cylinder to the vegetative organs of the plant.

The role of cambium in the development of the root

The transition from the primary structure of the root to the secondary occurs at the seedling stage and is marked by the appearance of an educational tissue - cambium. One of its forms is formed from a proto-system of vascular bundles.

Then there are parts of the radiation cambium. Both these varieties of the secondary meristem merge into a common cambial ring lying between the bark and the central cylinder. Due to active mitotic division, cambium cells form two layers of secondary conducting tissues: internal, directed towards the stele - xylem and peripheral, facing the endoderm - phloem. As a result of the above described processes, the axial cylinder acquires a secondary structure, characteristic of all roots of dicotyledons.

What changes occur in the primary cortex

The appearance of secondary conducting tissues - phloem and xylem - causes transformation in the pericycles. Its cells, dividing by mitosis, form a layer of corky cambium - phallogen, which, in turn, forms a periderm. The constituent part of its cells begins to divide periclinally, which leads to the isolation of the primary cortex from the axial cylinder, and then to its death. Now the outer layer of the secondary root is the periderm with the remaining parts of the phelloderm and pericycle. As we can see, the primary and secondary structure of the root are radically different from each other. These differences apply to all of its departments, including the bark and the central cylinder. They are especially noticeable in the anatomical structure of the educational and integumentary tissues. The most important processes occurring at the root during the period of its growth can be considered the appearance of cambium and the laying of secondary conducting tissues. In the next subheading, we'll look at them in more detail.

Primary and secondary structure of the root

Differences in the morphology and physiological functions of the growing root of a bipartite plant can be represented in the form of a table:

In addition to the table, we note that the secondary thickening of the root of the roots in dicotyledonous plants is due to the mitotic activity of the cambium cells, and the root growth in length is associated with the renewal and movement of the cells of the apical meristem and the root cover into the soil layer. The tip of the central root overcomes the resistance of solid ground areas due to the high energy of growth, therefore, the roots of tree species of angiosperms can penetrate even asphalt during germination.