Meristematic Tissues and its Types - Practice Questions & MCQ

Meristematic Tissues or Meristems

• All the cells of the embryo of the plant are capable of dividing and giving rise to new cells. 
• But as the plant grows, this feature becomes restricted to only a few regions. 
• Meristem is one such localized region in which actual cell division occurs. 
• Therefore, the growth in plants is referred to as localized growth.
• Meristematic cells divide to form permanent cells as well add to the volume of meristematic cells. 
   

Features of meristematic cells:

1. Meristematic cells are oval, polygonal, and spherical in shape.
2. These are compactly arranged and lack intercellular spaces.
3. These are thin, homogenous, and have a cellulosic cell wall.
4. They have dense cytoplasm and prominent nuclei.
5. The central vacuole is absent.
6. These are undifferentiated cells. They contain proplastids.
7. They have an enormous capacity to divide. 
8. They are metabolically very active but they do not store food material and further no plastids in them.
    

Classification of Meristematic Tissues

Meristematic tissues can be classified on the basis of:

1. Origin and development
2. Location
3. Function
4. Plane of division

Meristematic tissues on the basis of origin and development:

1. Promeristem/Primordial Meristem: This meristem originates from the embryo. It is present in the regions where an organ or a part of the plant body is initiated. It occupies a small area at the tips of the stem and root. The promeristem gives rise to all other meristems including the primary meristem.

2. Primary Meristem: It originates from the promeristem. It is located in the apices of roots, stems, and leaf primordia. Primary meristem gives rise to the primary permanent tissue. It is responsible for the primary growth of the plant.

3. Secondary Meristem: It does not have any promeristem present priorly. It arises in permanent tissue through the process of dedifferentiation of permanent cells. They perform the secondary growth of the plant.
 

Meristematic tissues on the basis of location:

1. Apical Meristem: It is present in the apices of the main and lateral shoot and root. It is the primary meristem that adds to the length of the plant. Solitary apical cells occur in ferns and other Pteridophytes while apical initials are found in other vascular plants.

Root apical meristem is present behind the root cap. It is sub-apical in position. 
Shoot apical meristem is present at the apex. During the formation of the leaves and elongation, some left behind cells of the shoot apical meristem constitutes the axillary bud. It is also called the marginal meristem.

2. Intercalary Meristem: This meristem is that portion of apical meristem which is left behind during the growth of the axis and formation of permanent cells. It is present mostly at the base of the node (e.g., Mint), the base of internode (e.g., the stem of many monocots viz., Wheat, Grasses, Pteridophytes like Equisetum), or at the base of the leaf (e.g., Pinus). The intercalary meristems ultimately disappear and give rise to permanent tissues.

3. Lateral Meristem: This meristem occurs laterally in the axis parallel to the sides of the stem and root. These are cylindrical meristem and result in secondary growth that increases the girth of the plant. The interfascicular cambium and cork cambium are the lateral meristem.
 

Meristematic tissues on the basis of function:

1. Protoderm: It is the outermost layer of apical meristematic cells that produce the epidermal tissue system.

2. Procambium: It is present below protoderm. They produce primary vascular bundles.

3. Ground Meristem: It produces the rest of the permanent tissues including hypodermis, cortex, endodermis, pericycle, pith, and medullary rays.

Meristematic tissues on the basis of the plane of division:

1. Mass Meristem: These meristematic cells divide into all planes through anticlinal divisions (perpendicular to the outer surface). They cause an increase in volume. For example, the formation of the cortex.

2. Plate Meristem: These meristematic cells divide into two planes through anticlinal divisions. They cause an increase in area. For example, the formation of the lamina of the leaf and epidermis.

3. Rib or File Meristem: These meristematic cells divide only in one plane and cause an increase in length. For example, the formation of the lateral root.