Meristem- Meaning, Classification, Types
Meristems or Meristematic Tissue
A meristematic tissue consists of a group of cells which remain in continuous state of division or they retain their power of division.
Characteristics of meristematic tissue
- They are composed of immature cells which are in a state of division and growth.
- Usually the intercellular spaces are not found among these cells.
- The cells may be bounded, oval or polygonal in shape: they are always living and thin walled.
- Each cell of meristematic tissue possesses abundant cytoplasm and one or more nuclei in it.
- The vacuoles in the cells may be quite small or altogether absent.
Meristems and Growth of Plant Body
Beginning with the division of the oospore, the vascular plant generally produces new cells and forms new organs until dies. In the beginning of the development of the plant embryo cell division occurs throughout the young organism. But as soon as the embryo develops and converts into an independent plant the addition of new cells is gradually restricted to certain parts of the plant body, while the other parts of the plant remain concerned with activities other than growth. This shows that the portion of embryonic tissue persists in the plant throughout its life and the mature plant is a composite of adult juvenile tissues. These juvenile tissues are known as the meristems. The presence of meristems remarkably differentiates the plant from the animal. In the plant growth resulting from meristematic activity is possible throughout the life of the organisms, whereas in the animal body the multiplication of the cells mostly ceases when the organism attains adult size and the number of organs is fixed.
The term meristem (Greek word ‘meristos’, meaning ‘divisible’) emphasizes the cell-division activity characteristic of the tissue which bears this name. It is obvious that the synthesis of new living substance is a fundamental part of the process of formation of new cells by division The living tissues other than the meristems may also produce new cells, but the meristems carry on such activity indefinitely, because they not only add cells to plant body, but also perpetuate themselves there is, some of the products o division in the meristem do not develop into adult cells but remain meristematic.
The meristems usually occur at the apices of all main and lateral shoots and roots and thus then number in a single plant becomes quite large. In addition, plants bearing secondary tissue increase in thickness possess extensive meristems, the vascular and cork cambia, responsible for the secondary growth. The combined activities of all these meristems give rise to a complex and large plant. The primary growth, initiated in the apical meristems expands the plant body and produces the reproductive parts. On the other hand, the cambia, aid in maintenance of the expanding body is increasing the volume of the conducting system and forming supporting protecting cells.
Classification of Meristems
Various systems of classifying meristems have been proposed by many eminent workers which are based on the characteristics such as, stage of development, position in plant body, origin function and topography. No system is exclusive and rigid. A few important types have been discussed here:
Meristems Based on Stage Development
Promeristem or primordial meristem- Promeristem is the region of new growth in a plant body where the foundation new organs or parts of organs is initiated. Sometimes it is also called primordial meristem, urmeristem and embryonic meristem . From the viewpoint of its structure this region consists of the initial and their immediate derivatives. The cells of this region are isodiametic thin-walled, vacuolate or non-vacuolate, with active cytoplasm and early stages of pits. Prominent nuclei and inconspicuous intercellular spaces may be seen. As soon as the cell of this region beginning to change in size shape, and character of wall and cytoplasm, setting off the beginning of tissue differentiation, they are no longer a part of typical meristem; they have passed beyond that earlier stage.
Meristems Based on Origin of Initiating Cells
The meristems are classified as primary and secondary on the basis type of tissue in which origin occurs.
The primary meristems are those that build up the primary part of plant and consist in parts of promeristem. In primary meristems, promeristem is always the earliest stage. The possession of promeristem continuously from an early embryonic origin is characteristic of primary meristems. The main primary stems are the apices of roots, stems, leaves and similar appendages.
The secondary meristem appears later at a stage of development of an organ of a plant body. Secondary meristems always arise in permanent tissues and they are always found lying lateral along the side of the stem and root. Sometimes some of the primary permanent tissues acquire the power of division and become meristematic. These tissues build up the secondary meristem. Secondary meristems are so called because they arise as new meristems in tissue which is not meristematic. The most striking example of secondary meristem is phellogen is cork cambium. It is formed from mature cells-cortical, epidermal or phloem cells.
Meristems Based on Position in Plant Body
As regards their position in plant body, the meristems may be classified into there group apical meristem, intercalary meristem and lateral meristem.
The apical meristem lies at the apex of the stem and root of vascular plants. Very often they are also found at the apices of the leaves . Due to the activity of these meristems, the organs increase in length. The initiation of growth takes place by one more cells situated at the tip of the organ. These cells always maintain their individuality and position and are called ‘apical cells’ or ‘apical initials’ Solitary apical cells occur in pteridophytes, whereas is higher vascular plants they occur in groups which may be terminal and sub-terminal in position.
The intercalary meristems are merely portion of apical meristems that have become separated from the apex during development by layers or more mature or permanent tissues and left behind as the apical meristem moves on in growth. The intercalary mentioned are intermodal in their position. In early stages the internode is wholly or partially meristematic, but later on some of its part, becomes mature more rapidly than the rest and in the internode a definite continuous sequence of development is maintained. The intercalary meristems are found lying in between mass of permanent tissues either at the leaf base or at the base of internode. Such meristems are commonly found in the stems of grasses and other monocotyledonous plants and horsetails, where they are basal. Leaves of many monocotyledons (grasses) and some other plants, such as Pinus, have meristematic region. These meristematic regions are short living and ultimately disappear to become permanent tissues.
The lateral meristems are composed of such initial which divide mainly in one plane (periclinally) and increase the diameter of an organ .They add to the bulk of existing tissues to give rise to new tissues. These tissues are responsible of growth in thickness of plant body. The cambium and the cork cambium are examples for this type.
Meristems Based on Function
As regards their function a systems of classification of meristems was proposed by Haberlandt in the end of nineteenth century. He suggested that the primary meristem at the apex of the stem and root is distinguished into three tissues- Protoderm, procambium and ground or fundamental meristem. The protoderm is the outermost tissue which develops into epidermis. The procambium develops into primary vascular tissues. It forms isolated strands of elongated cells very near to the central region; in cross-section each procambium appears as a small group of cell in the ground or fundamental meristem, but in longitudinal section the cells appear to be long and pointed. The ground or fundamental meristem develop into ground tissue and pith; the cells of this region are large, thin walled, living and isodiametric. In later stages, they become differentiated into hypodermis, cortex, endodermis, pericycle, pith rays and pith.
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