Plant Tissues

Definition of Tissue

A tissue is a group of similar cells that work together to perform a specific function.

Classification of Plant Tissues

Plant tissues are broadly classified into:

1. Meristematic Tissues: Tissues with actively dividing cells, responsible for growth.
2. Permanent Tissues: Tissues composed of cells that have lost the ability to divide and have taken on a permanent shape, size, and function. These are further divided into Simple and Complex permanent tissues.

Meristematic Tissues

Meristematic tissues are the foundation of plant growth, composed of actively dividing, undifferentiated cells that give rise to all other tissues and organs in a plant. They are the plant's "growth engine."

• Characteristics:

  • Actively Dividing: These cells are in a constant state of division.
  • Undifferentiated: They are not yet specialized for a particular function.
  • Small and Isodiametric: The cells are typically small and roughly spherical or cuboidal.
  • Dense Cytoplasm and Prominent Nucleus: They have a dense cytoplasm and a large nucleus to control cell division.
  • Thin Cell Walls: Their primary cell walls are thin and flexible, allowing for easy expansion and division.
  • Minimal or No Vacuoles: They have very small or no vacuoles.
  • Self-Renewal: Some cells remain meristematic, ensuring a continuous supply of new cells.

• Classification of Meristematic Tissues:

  • 1. Apical Meristems: For Primary Growth (Length)

    • Located at the tips (apices) of roots and shoots, apical meristems are responsible for the primary growth of a plant, which is the increase in its length.
    • Shoot Apical Meristem: Found at the tip of the stem, it gives rise to leaves, flowers, and branches.
    • Root Apical Meristem: Located at the tip of the root, it is responsible for the downward growth of the root system. It is protected by a root cap.
    • [Diagram of Apical Meristems]

  • 2. Lateral Meristems: For Secondary Growth (Girth)

    • Found along the sides of stems and roots, lateral meristems are responsible for secondary growth, which is the increase in the thickness or girth of the plant. This type of growth is most prominent in woody plants.
    • Vascular Cambium: Located between the xylem and phloem, it produces secondary xylem (wood) and secondary phloem.
    • Cork Cambium (Phellogen): Develops in the cortex and produces a protective outer layer of cork cells (bark).
    • [Diagram of Lateral Meristems]

  • 3. Intercalary Meristems: For Growth in Internodes

    • Located at the base of leaves or in the internodes of some monocots, like grasses. They are responsible for the rapid elongation of stems and leaves. This is why grass can continue to grow after being mowed.
    • [Diagram of Intercalary Meristems]

Simple Permanent Tissues

These tissues are made up of only one type of cell. They are primarily involved in providing support, protection, and storage.

• Parenchyma:

  • Structure: The most common plant tissue. Cells are isodiametric (roughly spherical), have thin cellulose walls, a large central vacuole, and may have intercellular spaces.
  • Location: Forms the bulk of non-woody structures.
  • Functions:
    • Storage: Stores food (starch), water, fats, and proteins.
    • Photosynthesis: Chlorenchyma (parenchyma with chloroplasts) performs photosynthesis.
    • Gas Exchange: Intercellular spaces facilitate gas exchange. Aerenchyma (parenchyma with large air cavities in aquatic plants) provides buoyancy.
    • Turgor and Support: Maintains the turgidity of the plant.
  • [Diagram of Parenchyma Tissue]

• Collenchyma:

  • Structure: Elongated, living cells with unevenly thickened primary cell walls (cellulose and pectin), especially at the corners.
  • Location: Found in young stems and leaf stalks.
  • Functions: Provides flexible mechanical support, allowing bending without breaking.
  • [Diagram of Collenchyma Tissue]

• Sclerenchyma:

  • Structure: Rigid, supportive tissue with thick, lignified secondary walls. Usually dead at maturity.
    • Fibers: Long, narrow cells with tapering ends.
    • Sclereids (Stone Cells): Shorter, irregularly shaped cells (e.g., gritty texture of pears).
  • Location: Found in stems, around vascular bundles, in leaf veins, and in hard coverings of seeds and nuts.
  • Functions: Provides rigid mechanical strength and protection.
  • [Diagram of Sclerenchyma Tissue]

Complex Permanent Tissues

These tissues are made up of more than one type of cell, working together as a unit.

• Xylem:

  • Function: The principal water-conducting tissue. Transports water and minerals from roots to other parts of the plant (unidirectional). Also provides mechanical support.
  • Structure: Composed of four cell types:
    • Tracheids: Elongated, dead cells with tapering ends and lignified walls.
    • Vessels: Shorter, wider, dead cells arranged end-to-end, forming a continuous tube.
    • Xylem Fibers: Sclerenchymatous fibers for support.
    • Xylem Parenchyma: The only living cells; for food storage and radial water conduction.
  • [Diagram of Xylem Tissue]

• Phloem:

  • Function: The main food-conducting tissue. Transports sugars from leaves to other parts of the plant (bidirectional). Also transports signaling molecules.
  • Structure: Composed of four cell types:
    • Sieve Tubes: Elongated, living cells arranged end-to-end with perforated end walls (sieve plates). Lack a nucleus at maturity.
    • Companion Cells: Specialized parenchyma cells that control the functions of sieve tubes.
    • Phloem Fibers (Bast Fibers): Sclerenchymatous fibers for support.
    • Phloem Parenchyma: Living parenchyma cells for food storage and radial transport.
  • [Diagram of Phloem Tissue]