Comprehensive Notes on Plant Tissues: Meristematic and Permanent

Classification of Plant Tissue

• Plant tissue is categorized based on the power of cell division into two main types:

  • Meristematic Tissue: Cells that are capable of dividing throughout their life.
  • Permanent Tissue: Cells that have lost the ability to divide and represent the modification of meristematic tissue.

• Plants can be further classified as:

  • Dicotyledon (Dicot): Plants with two cotyledons.
  • Monocotyledon (Monocot): Plants with one cotyledon.
  • Gymnosperm: Plants that do not produce fruits or flowers and have seeds that lack a cover (naked seeds).
  • Angiosperm: Plants that produce fruits and flowers and have seeds protected by a cover.

Meristematic Tissue: Characteristics and Functions

• Definition: Tissues composed of cells that are actively dividing.
• Cellular Characteristics:
  • Cell Wall: Very thin to facilitate frequent division.
  • Nucleus: Large and prominent to manage the genetic information and control fast division.
  • Cytoplasm: Dense and granular.
  • Vacuoles: Generally absent. Why? Because the cells divide so fast that a large vacuole would occupy space and hinder the physical process of cell division.
  • Chloroplasts: Generally absent because the focus of the cell is division rather than photosynthesis, and their presence would also hinder division.
  • Mitochondria: Present in higher numbers to provide the significant amount of energy required for rapid cell division.
  • Metabolic Activity: Highly active metabolism compared to permanent tissues.

Types of Meristematic Tissue based on Position

• Apical Meristem:

  • Position: Found at the tips of roots and stems (shoot apex and root apex).
  • Function: Responsible for increasing the height (primary growth) of the plant.
  • Cell Wall: Primarily composed of cellulose.
  • Occurrence: Found in all plants.

• Lateral Meristem:

  • Position: Found on the sides of the stem or root (axial alignment).
  • Function: Responsible for increasing the girth or diameter (secondary growth) of the plant.
  • Cell Wall: Thickening involves lignin.
  • Example: Cambium.
  • Occurrence: Mostly found in Gymnosperms and Dicots.

• Intercalary Meristem:

  • Position: Located at the nodes or the base of leaves/internodes.
  • Function: Responsible for the growth of branches and the elongation of internodes.
  • Occurrence: Found especially in grasses and certain Monocots.

Permanent Tissue

• Definition: These are tissues that are no longer able to divide. They are derived from meristematic tissues through a process of differentiation.
• General Characteristics:
  • Cell Wall: Can be thin or thick.
  • Cytoplasm: Relatively dilute or less dense.
  • Nucleus: Small compared to meristematic cells.
  • Vacuoles: Large vacuoles are present.
  • Metabolic Activity: Comparatively passive/less active.
  • Intercellular Space: Often present.

Simple Permanent Tissues

Simple permanent tissues are made up of only one type of cell. There are three main types:

• Parenchyma:

  • Nature: Living cells.
  • Cell Wall: Thin and made of cellulose.
  • Intercellular Space: Present (large gaps between cells).
  • Location: Known as the "Universal Tissue" because it is found in every plant and every plant organ. It is also called "Ground Tissue" or "Filling/Packaging Tissue."
  • Functions:
    • Packaging of organs.
    • Food storage.
    • Protection.
    • Excretion.
  • Specialized Types:
    • Aerenchyma: Parenchyma with large air cavities found in aquatic plants to provide buoyancy/floatation (sponginess).
    • Chlorenchyma: Parenchyma containing chlorophyll; its primary function is photosynthesis.

• Collenchyma:

  • Etymology: "Collen" means glue.
  • Nature: Living cells.
  • Cell Wall: Thickenings at corners made of hemi-cellulose and pectin.
  • Intercellular Space: Absent or very little.
  • Location: Found in the hypodermis of dicot plant stems. It is notably absent in dicot roots and monocot plants.
  • Functions: Provides flexibility, elasticity, and mechanical support to the plant.

• Sclerenchyma:

  • Nature: Dead cells.
  • Cell Wall: Very thick due to the deposition of Lignin.
  • Intercellular Space: Absent.
  • Shape: Often isodiametric (same diameter).
  • Types:
    • Sclerenchyma Fibers: Thin, long, fiber-like structures. Examples include Coconut husk, Jute, and Flax.
    • Sclereids: Provide grittiness or roughness. Found in the pulp or skin of Pear, Apple, Guava, and Betelnut (Supari).
  • Functions: Provides extreme hardness and mechanical strength.

Complex Permanent Tissues

Complex tissues are made up of more than one type of cell working together as a unit. The two types are Xylem and Phloem.

Xylem (The Wood)

• Function: Conducts water and minerals from roots to leaves.
• Direction of Flow: Upward (Unidirectional).
• Components:
  • Tracheids: Long, narrow, unicellular, dead cells. Found in all plants.
  • Vessels: Short, wide, multicellular, dead cells. Primarily found only in Angiosperms.
  • Xylem Fiber (Sclerenchyma): Dead cells provide structural and mechanical strength.
  • Xylem Parenchyma: The only living cell in the xylem. Responsible for sideway/lateral transport of water and food storage/energy.

Phloem (The Bast)

• Function: Conducts food (photosynthates/sucrose) from leaves to other parts of the plant.
• Direction of Flow: Bidirectional (upward and downward).
• Components:
  • Sieve Tubes: Living cells of the phloem but uniquely do not have a nucleus at maturity. They are the main conductive part and have pores at the end plates.
  • Companion Cells: Very important living cells. Known as "Sister Cells" to sieve tubes because they both originate from the same mother cell. The companion cell nucleus controls the activity of the sieve tube. They are non-perforated.
  • Phloem Parenchyma: Living cells used for food storage.
  • Phloem Fiber (Sclerenchyma): The only dead cell in phloem. Provides mechanical strength and hardness.

Summary Comparison: Xylem vs. Phloem

Summary Comparison: Simple Permanent Tissues

Quotes and Discussion

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• Notes include a handwritten reference to "Mary Hug Lines" and "Narendra Modi."