The cells of a meristematic tissue are immature and are in an active state of division. The cells are usually closely packed with no intercellular spaces. The shape of the meristematic cells varies. They may be oval, polygonal rounded or rectangular in shape. The cell inclusions will be in their precursor stage. The nuclei are very prominent. The plane of division in the cells varies. It may be periclinal (parallel to the surface) or anticlinal (perpendicular to the surface).
Classification of meristems:
There have been different systems of classification based on different criteria, such as origin, location, function etc. A few important types of classification are given below:
A. Meristems based of stage of method of development:
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Based on their stage of development meristems may be classified into promeristems and marginal meristems.
Promeristems constitute the region of new growth in a plant body, where the foundation of new organ is initiated. Promeristem is also known as embryonic meristem, primordial meristem or Urmeristem. Structurally the promeristem region consists of initials and their derivatives. The cells have thin cell walls, prominent nucleus, dense vocuolate or non vacuolate cytpolasm. Pits are in their early stages of development. When the cells of the promeristem begin to differentiate they may be said to have lost the characters of a promeristem.
Marginal meristems generally give rise to tissues of cortex, epidermis etc. Based on the plane of division they may be further classified into mass meristems, plate meristems and rib meristems.
B. Meristems based on their origin:
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Based on the history of the initiating cells meristems are classified into primary and secondary meristems. Primary meristems make up the fundamental parts. They consist of a part of the promeristem. Mainly primary meristems are located at the apices of plant organs.
Secondary meristems always arise late in the ontogeny of the plant. They take their origin in permanent tissues. In position, they are always lateral.
C. Meristems based on their location in the plant body:
According to the location, there are three types of meristem, viz., apical intercalary and lateral.
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Apical Meristems:
As the very name suggests are located at the apices of shoot, root and other organs. They constitute the growing points of plants. The growth in length of the plant and plant organs is due to apical meristems. Intercalary Meristems represent the detached portions of the apical meristems f separated due to the growth of an organ. Intercalary meristems are always internodal. In the early stages of development of the plant, whole or part of the internode is meristematic.
Subsequently a portion of it matures, as a result, patches of meristems are found embedded in permanent tissues. Lateral Meristems are present along the side of the stem. The cells divide in only one plane and add to the girth of the tissues. Typical example of a lateral meristem is the vascular cambium Vascular cambium produces secondary Xylem and Phloem thus bringing about secondary growth.
D. Meristems based on their function:
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According to the function performed by the derivatives of the meristems, they may be classified into protoderm, procambium and ground meristem. This classification was proposed by Haberlandt at the end of the 19th century. Protoderm develops into epidermis. Procambium gives rise to the primary vascular tissues and the ground meristem are thin and isodiametric. They give rise to hypodermis, endoder and pericycle.