Meristems Or Meristematic Tissue
A meristematic tissue consist of a group of cells which remain in a continuous state of division or they retain their power of division.
1. They are composed of immature cells which are in a state of division and growth.
2. Usually the intercellular spaces are not found among those cells.
3. The cells may be rounded, oval or polygonal in shape; they are always living and thin walled.
4. Each cell of meristematic tissue possesses abudent cytoplasm and one or more nuclei in it.
5. The vacoule in the cells may be quite small or altogether absent.
Classification of Meristems:
Various system of classifying meristems have been prosposed 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.
1.Meristems Based on Stage of Development
Promeristem Or Primordial Meristem:
Promeristem is the region of new growth in a plant body where the foundation of new organs or parts of organs is initiated. Sometimes it is also called primordial meristem, urmeristem and embryonic meristem. From the viewpoint of it's structure, this region consists of the initials and their immediate derivatives. The cells of this region are isodiametric, 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 cells of this region begin 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.
2. Meristems Based on Origin of Initiating Cells
Primary and Secondary Meristems:
The meristems are classified as primary and secondary, on the bases of types of tissue in which origin occurs.
The primary meristems are those that build up the primary part of the 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 Or cork cambium. It is formed from mature cells-cortical, plepidermal Or phloem cells.
3. Meristems Based on Position in Pant Body
Regarding their position in plant body, the meristems may be classified into three groups apical meristem, intercalary meristem and lateral meristem.
Apical 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 origins increase in length. The initiation of growth takes place by one or 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 in higher vascular plants they occur in groups which may be terminal and sub-terminal in position.
Intercalary Meristems:
The intercalary meristems are merely portions 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 meristems are internodal in their position. In early stages the internode is wholly or partially meriatematic, but later on some of it's 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 masses 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 basal meristematic region. These meristematic region are short living and ultimately disappear, to become permanent tissues.
Lateral Meristems:
The lateral meristems are composed of such initials which divide mainly in one plane(periclinally) and increase the diameter of an organ. They add to the bulk of existing tissues or give rise to new tissues. These tissues are responsible for growth in thickness of plant body. The cambium and the cork cambium are examples of this type.
4. Meristems Based on Function
Regarding their function, a system of classification of meristems has been proposed. The primary meristem at the apex of the stem and root is distinguished into three tissues- protoderm, procambium and ground or fundmental meristem. The protoderm comprises the outermost tissues which develop 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 cells in the ground or fundamental meristem, but in longitudinal section the cells appear to be long and pointed. The ground or fundmental meristem develops 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.