Meristematic Tissue

A meristem is a simple tissue composed of a group of similar and immature cells that repeatedly undergo mitotic divisions to form new cells. Meristematic tissue is a mass of undifferentiated cells that undergo repeated cell division. Have isodiametric, rounded, oval or polygonal shaped immature cells with no intercellular spaces. Have thin, elastic walls, made of cellulose and inconspicuous nuclei. Based on their origin, meristems are further divided into primary and secondary meristems. Primary meristematic tissue is derived directly from the meristem of the embryo. Secondary meristems are formed secondarily from permanent tissue. Some of the permanent tissues acquire the power of division, which is called dedifferentiation.

Characteristics of Meristematic Cells:-

• Possess the ability to divide indefinitely.
• Have small, polygonal-shaped cells with no intercellular spaces.
• Have thin, elastic, cellulosic walls.
• Conspicuous nucleus and dense cytoplasm.
• Proplastids present instead of plastids 
• The rate of respiration is very high, with large scale synthetic activity.
• A little amount of reserve food.

Meristematic cells are parent cells from which all other types of cells are formed.

Types of meristematic tissue based on their origin:-

Primary meristem:- 

Primary meristem develops directly from the meristem of the embryo and can undergo repeated cell division. Depending upon their position,  meristems are of three types:-

Apical meristem:-

• present at the tips of stem, roots and branches. They produce growth in length. These are primary meristems.

 Shoot apical meristem or shoot apex 

• Shoot apical meristem develops from the plumule of the embryo.
• Shoot apexes are conical or dome-shaped in outline. 
• Present at the tip of the stem and its branches as terminal buds. 
• It is truly terminal. 
• Derivatives of apical meristem produce the primary plant body.

Root apex or root apical meristem 

• Root apex develops from the radicle of the embryo. Found at the tip of the main root and its branches.

• In the case of adventitious roots, the root apical meristem is produced from derivatives of the shoot apex. 
• It is a sub-terminal because it is covered by a root cap. 
• It doesn’t produce lateral appendages and the root branches develop much behind the apex.
• In many cases, a quiescent centre is found at the centre of the root apex, where the frequency of cell division is very low due to little synthesis of new protein, RNAs and DNA. Due to the presence of a quiescent centre, the root apical meristem appears cup-shaped or hemispherical.  

Intercalary meristem:- 

• These are primary meristems.

• These are derived from the apical meristem and have been separated from them by the formation of permanent tissues in between. 
• Intercalary meristem helps in the elongation of the organs and also allows the fallen stems of cereal to become erect. 
• These are commonly found on the bases of leaves, above the nodes( eg:- grasses) or below the nodes (eg:- mint).

Lateral meristem:-

• Occurs on both sides.
• Increases the girth of the plant.
• It is a type of secondary meristem.

Secondary meristem:-

• Formed secondarily from permanent tissue.
• Some of the permanent cells regain the capability to undergo division, called dedifferentiation.
• Also termed as cylindrical meristem.
• Common examples of secondary meristem are interfascicular cambium, vascular cambium and cork cambium.

Vascular Cambium:-

• Present in between the xylem and phloem of vascular bundles.
• These are made of parenchymatous cells.

Interfascicular cambium :-

• Formed from the adjoining cells of medullary rays that become meristematic.
• Together, interfascicular cambium and intrafascicular cambium form the cambial ring. The cambial ring divides and forms new cells on both the inner and peripheral sides. The cells that cut off towards the pith mature into the secondary xylem and the cells that cut off towards the periphery mature into the secondary phloem. The continuous division of vascular cambium increases the girth or diameter of the stem and as a result, the outer cortical and epidermal layers get destroyed.

Cork Cambium:-

• Also called phellogen.
• These are present in the cortex region and have narrow, thin-walled and rectangular cells.
• They are responsible for the replenishment of the outer cortical and epidermal layers that get destroyed during the formation of secondary phloem and xylem.
• The phellogen or cork cambium cuts off cells on both sides.
• The outer cells mature into Cork or phellem. The cork has suberin deposition and is impervious to water.
• The inner cells mature to form a secondary cortex or phelloderm. The secondary cortex is made of parenchymatous cells.
• Phellogen, phellem and phelloderm are collectively called periderm.

CONCLUSION:-

Meristematic tissues can undergo cell division an indefinite number of times. These are composed of simple cells i.e. made of one type of cell. Based on their origin, it is divided into primary and secondary meristems. Primary meristems are further divided into intercalary, and apical meristems based on their location. Apical meristem is responsible for increasing the vertical length of the plant. The intercalary meristem is responsible for the elongation of organs. Lateral meristem is involved in increasing the girth of the plant. Secondary meristems consist of permanent cells that regain the capability to divide and are responsible for the secondary growth of plants. Secondary meristem includes vascular cambium of the roots, interfascicular vascular cambium of the stem, cork cambium or phellogen.