Meristematic tissues are generally known as meristems. The term ‘meristem’ was coined by Carl Wilhelm von Nägeli. They are undifferentiated plant tissues composed of immature cells that are not fully differentiated. Cells of the meristematic tissues can divide and remain young forever. Meristematic tissue acts as the building block of plants. It contains living cells that vary in shape and size. They can be oval, rounded or polygonal. They have a large nucleus, and the vacuole is absent. Cells do not possess intercellular space and are generally smaller in size.

Characteristics of Meristematic Tissues

• The cells are generally known as meristems.
• They are living cells.
• The cells have thin walls.
• The vacuoles are generally absent but can be present in smaller sizes.
• The cells are smaller in size and do not possess intercellular space.
• The cells have a very dense protoplasm.
• A single large nucleus is present in every cell.
• The cells are immature and young.
• The cells are capable of dividing.
• The metabolic rate of the cells is very high, and they do not store food.

Types of Meristem

• Based on the origin, meristematic tissues are divided into pro meristem, primary meristem, and secondary meristem. 

1. Primary meristem is associated with the embryonic cells and is responsible for cell division and growth of the plant. 
2. The secondary meristem develops from the primary meristem and is responsible for the secondary growth of the plant.

• Based on the position in the plant body, they can be of three types: apical meristem, intercalary meristem and lateral meristem.
• Based on their functions, meristem can be of three types: protoderm meristem, procambium meristem and ground meristem.
• Based on the plane of divisions of meristematic tissue, they can be of three types: mass meristem, rib meristem and plate meristem.

Lateral Meristem

• Lateral meristems are a type of meristematic tissue present in the plant’s lateral parts, hence responsible for the lateral growth of the plants.
• They are mostly present in the organs of dicotyledons and gymnosperms.
• They are associated with stems and roots on the lateral side.
• This meristem is responsible for increasing the thickness of the plant.
• When the lateral meristem divides, it gives rise to secondary permanent tissues, also known as secondary meristems.
• Usually, they divide preclinically or radially to form secondary permanent tissues.
• They contribute to the thickness of the plant by adding secondary tissues, thus responsible for the secondary growth in plants.
• Lateral or secondary meristems can be of two types: vascular cambium and cork cambium.

Vascular Cambium

• When the plant grows in diameter laterally by producing new tissues and forming a cylinder of tissue, it is called the vascular cambium.
• The vascular cambium forms the main path by which the roots and the stems of the plant grow.
• Xylem is present outside the vascular cambium, while secondary xylem and secondary phloem are inside.
• The vascular cambium is present from the roots’ tips to the shoots’ tips.
• It is present in all the plants which live for several years (perennial plants). It is also present in some annual plants.
• Tissues formed by the vascular cambium are known as secondary tissues.
• Vascular cambium is absent in herbaceous plants as they do not go through secondary growth.

Cork Cambium

• Cork cambium is also known as phellogens.
• They are mainly present in those parts of plants where they get chances to produce cork cells, which means the bark of the stem and roots of woody plants.
• They give rise to the periderm by replacing epidermal cells.
• The periderm comprises three layers: the outermost layer called the cork, the middle layer called the cork cambium and the innermost layer, the phelloderm.
• The cork is made up of dead cells, while the phelloderm is made up of living parenchyma cells.
• Different species show different cork cambium development.

Conclusion

Meristematic tissues are undifferentiated plant tissues that are composed of immature cells. The cells of the meristematic tissues can divide and remain young forever. Meristematic tissue acts as the building block of the plants. They can be of different types, one being lateral meristem.

Lateral meristems are a type of meristematic tissue present in the plant’s lateral parts, hence responsible for the lateral growth of the plants. They are mostly present in organs of dicotyledons and gymnosperms and are associated with the stems and roots on the lateral side. When the plant grows in diameter laterally by producing new tissues and forming a cylinder of tissue, it is called the vascular cambium.

The vascular cambium forms the main path by which the roots and the stems of the plant grow. Xylem is present outside the vascular cambium, while secondary xylem and secondary phloem are inside. Vascular cambium is absent in herbaceous plants as they do not go through secondary growth.