Meristematic Tissue

Meristematic tissues in plants contain cells that can divide repeatedly to form new cells. These tissues are found in the growing regions of plants, such as the tips of roots and stems, and their main function is to promote plant growth. Meristematic tissue consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells continue to divide until they become differentiated and lose the ability to divide.

1.0Meristematic tissue

Meristematic tissue, a term coined by Carl Wilhelm von Nägeli, is derived from the Greek word "meristos," meaning divided or divisible. This specialized plant tissue is characterized by actively dividing cells, retaining the remarkable ability to differentiate into various specialized cell types. Found in regions of plants where growth occurs, such as the tips of roots and shoots. 

2.0Characteristics of Meristematic Tissue

Meristematic tissue possesses distinctive characteristics that contribute to its role in the growth and development of plants:

• Undifferentiated Nature: Meristematic tissue remains undifferentiated, lacking specialized structures or functions.
• Continuous Cell Division: The meristem cells in plants are in a perpetual state of division, comprising predominantly immature cells.
• Primary Cell Wall Composition: Meristematic cells exhibit primary cell walls, thin, flexible, and primarily composed of cellulose with abundant plasmodesmatal connections.
• Absence of Secondary Cell Wall: Secondary cell walls are notably absent in meristematic cells.
• Cell Size and Shape: Cells within meristematic tissue are generally small and isodiametric in shape.
• Dense Cytoplasm and Prominent Nucleus: Meristematic cells feature dense cytoplasm and possess a prominent, large nucleus.
• Vacuole Presence: Normally absent or present in small sizes if found in meristematic cells.
• High Metabolic Activity: Meristematic cells are metabolically highly active, lacking reserve food storage.
• Plastid Absence or Proplastid Stage: Plastids are typically absent, with any presence limited to the proplastid stage.
• Compact Tissue Structure: Intercellular spaces are nonexistent, and cells are closely packed together, creating a compact tissue arrangement.
• Ergastic Substances Absence: Ergastic (non-living) substances are not present in meristematic tissue.

3.0Types of Meristematic Tissue

Meristematic tissue based on origin and development:

1. Promeristem/Primordial Meristem/Urmeristem

• Originating in the early stages of embryonic development, promeristem gives rise to primary meristem.
• Examples include embryonic meristem.

2. Primary Meristem

• Cells derived from promeristem constitute primary meristem, actively contributing to the formation of the primary plant body.
• Appearing early in the plant's life cycle, primary meristem includes cells constantly undergoing division and giving rise to primary permanent tissues through the process of differentiation.
• Examples comprise apical meristem, intercalary meristem, and intrafascicular cambium (Fascicular vascular cambium).

3. Secondary Meristem

• Developing from primary permanent tissue through dedifferentiation, secondary meristem emerges later in the plant's growth.
• The activity of secondary meristems is associated with secondary growth in plants.
• Examples involve interfascicular cambium and cork cambium in dicot stems, as well as vascular cambium and cork cambium in dicot roots.

Meristematic tissue based on their position:

1. Apical Meristem

• Example of primary meristem, apical meristems are located at the tips of roots and shoots, contributing to primary growth by producing primary tissues.
• Responsible for the increase in the length of plant organs, such as root apex and shoot apex.

2. Intercalary Meristem

• Another example of primary meristem, intercalary meristems are situated at the base of internodes in monocot stems (e.g., grasses, bamboo, sugarcane) and at the base of leaves.
• Operates between mature tissues, leading to an increase in the length of leaves and other plant organs.
• Common in grasses and aids in regenerating parts removed by grazing herbivores.

Note: Both apical and intercalary meristems are considered primary meristems because they appear early in a plant's life and contribute to the formation of the primary plant body.

3. Lateral Meristem

• Lateral meristems are positioned on the lateral sides of plant organs, enhancing the circumference, girth, and thickness of plant organs.
• All secondary meristems fall under the category of lateral meristems. Lateral meristems are of both primary and secondary origin

a. Primary Lateral Meristems

• Marginal Meristem: Located at the margin of young leaves, it increases the width (not thickness) of the leaf. It is responsible for the total growth of the leaf.
• Intrafascicular Cambium/Fascicular Cambium: Found inside the vascular bundles of dicot stems and gymnosperm stems.

b. Secondary Lateral Meristems

• Include interfascicular cambium and cork cambium (phellogen) in dicot stems and gymnosperm stems, as well as cork cambium and vascular cambium in dicot roots.
• Generally, lateral meristems have a cylindrical shape.
• The term "secondary meristem" refers to meristems occurring in mature regions of roots and shoots, particularly in plants producing woody axes.

Meristematic tissue on the basis of function:

1. Protoderm

• The protoderm produces the epidermis, a dermal tissue. 
• It protects the plants from any mechanical shocks.

2. Procambium

• Procambium is a meristematic tissue in plants that produces primary vascular tissues, such as xylem and phloem.
• It helps in the conduction of water and nutrients to different parts of the plant.

3. Ground Meristem

• Ground meristem is a plant tissue that produces the ground tissues of a plant, including the cortex and pith.
• It  includes parenchyma cells, collenchyma cells, and sclerenchyma cells.