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Meristematic cells have some distinctive features. They are usually isodiametric and compactly arranged without leaving intercellular spaces. Cells have dense cytoplasm, very small vacuoles and quite prominent nuclei.
Plastids are in the pro-plastid stage and ergastic matters are usually absent. Walls are thin, homogeneous and made of cellulose (Fig. 135). These are the characters of truly meristematic cells; departures are of course noticed in some cases, as in cambium of the vascular bundles.
Meristems may be classified on the basis of origin and position. According to origin, they may be pro-meristem, primary meristem and secondary meristem. Those which occur at the very embryonic stage, what, in fact, constitutes the foundations of the organs, are called pro-meristems.
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Primary meristems build up the fundamental parts of the plants. They usually derive their origin from pro-meristem and retain the meristematic nature. Some permanent cells regain the power of cell division; they are called secondary meristems, e.g. cork cambium and inter-fascicular cambium.
According to position, meristems may be apical, intercalary and lateral. Apical meristems are located at the apices of the axis, i.e. stem-tip and root-tip, referred to as growing point. Apical meristems are responsible for growth in length of the axis.
In plants like grasses, Equisetum, some meristematic cells are left behind during the growth of the apical meristem, and are intercalated between regions of permanent cells.
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They are called intercalary meristems, which, of course, gradually lose meristematic nature and become permanent. Lateral meristems, as the name implies, are strips of meristematic cells placed sideways or laterally on the axis, i.e. parallel with the sides of the organ in which they occur, e.g. cambium and cork-cambium.
Growth in thickness, as in dicotyledonous stems, is entirely due to the activities of lateral meristems.
The primary meristem at the apical region is usually differentiated into three zones, viz. dermatogen, periblem and plerome. The dermatogen is the outermost one-layered zone passing right over the apex. It forms the epidermis of the adult region.
The periblem is one-layered at the tip, but many-layered below. It gives rise to the cortex. The plerome is the innermost zone which produces the central cylinder or stele. Some cells of the plerome show tendency of elongation. They are called pro-cambial strands, which form the vascular bundles.
