Meristos equals to divided. These are composed of cells that are in a state of division or retain the power of dividing. These cells are essentially alike, being either spherical, oval or polygonal in shape without any intercellular spaces, their walls thin and homogenous, the protoplasm abundant and active with large nuclei and the vacuoles small or absent. Meristematic tissues may be apical and lateral. The apical meristem lies at the apex of the stem and the root and gives rise to primary permanent tissues.
The meristems which are present from the embryomic stage and persist throughout the life of a plant are known as Primary Meristem. Secondary meristems are those which arise as new meristems in non meristematic or permanent tissue, later at a certain stage of development. On the basis of plane of cell division are three type of meristems called mass, plate and riibmeristem. On the basis of position in the plant body meristems may be apical, intercalary and lateral. Apical meristems are present at the growing points of stem and roots. These organs increase in length due to the activity of cells of apical meristems. Generally it consists of many cells but in Pteridophytes only one cell forms the apical meristem.
APICAL MERISTEM:
(i) Stem Apex
A median longitudinal section through the apex of a stem when examined under the microscope shows that the apical meristem or growing region is composed of a mass of small, rounded of polygonal cells whice are all essentially alike and are in the state of division. These meristematic cells constitute the promeristem. The cells of promeristem soon differentiate into three regions i.e. dermatoges, periblem and plerone. The cells of these three regions grow and give rise to primary permanent tissues in the nature portion of the stem. The section further shoes on either side a number of outgrowth which arch over the growing apex. These are the Young leaves of the bud which cover and protect the tender growing apex of the setm.
1. Dermatogen: Derma = skin, gen = producing. This is single outermost layer of cells. It passes right over the apex and continues downwards as a single layer. The cells divided by radial walls only i.e. at right angels to the surface of the stem and increase in circumference thus keeping pace with the increasing growth in volume underlying tissues. Dermatogen gives rise to the skin layer or epidermis of stem.
2. Periblem: Peri = around, blema = covering. This lies internal to the dermatogen and is the middle region of the apical meristem. At the apex it is single layered but on lower side it becomes multilayered. It forms the cortex of the stem which is often in dicoteledons differentiated into hypodermis, general cortex and endodermis.
3. Plerone: Pleres = full. This lies inter to the periblem and is the central region of stem apex. At a little distance behind the apex certain groups or strands of cells show a tendency to elongate. These groups or strands of elongated cells are said to form the procambium. In T. S of the stem each procambium appears as a small group of cells which soon become differentiated into elements of xylem on the inner side and of phloem on the outer, together forming into a vascular bundle. A portion may remain undifferentiated and it forms cambium of the vascular bundle lying in between xylem and Phloem. Plerome as a whole gives rise to the central cylinder or stele which in dicot stem bears pericycle, medullary rays, pith and vascular bundles.
(ii) Root Apex
A median longitudinal section through the apex of the root shoes that it is covered over and protected by many layered tissue which constitutes root cap. The apical meristem or growing region lies within and behind the root cap.
The promeristem as in the stem early differentiates into (1) Dermatogen (2) Periblem and (3) Plerome.
In many roots these three regions are not clearly marked.
(1). Dermatoges: As in the stem, this is single layered but at the apex it merges into the periblem outside this the dermatogens cuts off many new cells, forming a small called tissue known as calyptogen. The calyptrogen is also meristematic and by repeated divisions of its cells gives rise to root cap. As the root passes through the hard soil, the root cap often wears away but then it is renerved by underlying calyptrogen. The walls of outer cells of root cap may be modified into mucilage which helps the root to push forward in the soil more easily. At the little distance from root tip outermost layer bears large number of unicellular root hairs. Dermatogen continues upwards as a single outermost layer called epiblema.
(2). Periblem: As in the stem this also is single layered at the apex and many layered higher up. Periblem forms middle region or cortex of the root.
(3). Plerome: Its structure and function are practically the same as those of stem. But here some procambial strands give rise to bundles of vessels (xylem) and others to bundles of siene ubes (phloem) in an alternating manner.
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