Plant Tissues and Organs

Levels of Organization

  • The levels of organization in plants include atoms, molecules, cells, tissues, and organs.

Tissue Definition

  • A tissue is a group of cells with a similar structure performing the same function. Examples include muscle, bone, and blood in animals.

Types of Plant Tissues

  • Plant tissues are categorized into meristematic and permanent tissues.

Meristematic Tissue

  • Composed of cells capable of cell division, facilitating growth.
  • Cells are similar in structure, with thin cellulose cell walls.
  • Cells can be spherical, oval, polygonal, or rectangular.
  • Contain few vacuoles.
  • Found in regions of the plant that grow, mainly at the tips of roots and stems.
  • Continuously forms new cells.
  • Types include:
    • Apical meristem: Located at stem and root tips.
    • Lateral meristem.
    • Intercalary meristem: Found in monocot stems.

Permanent Tissue

  • Composed of mature cells incapable of cell division; develop specific structures for specific functions.
  • Divided into simple and complex tissues.
Simple Permanent Tissues

  • Composed of identical cells.

    • Parenchyma:

      • Cells are oval, round, and elongated with thin cell walls, dense cytoplasm, a small nucleus, and a large central vacuole.
      • Abundant intercellular spaces.
      • Found in soft parts of plants, cortex of roots, ground tissues in stems, and mesophyll of leaves; also in pith medullary rays and packing tissue in xylem and phloem.
      • Functions include packing tissue support to herbaceous stems, water and food storage, and transport of water and gases.

    • Collenchyma:

      • Characterized by extra cellulose deposition at the corners of cells.
      • Intercellular spaces are generally absent.
      • Mainly below the epidermis in dicotyledon stems and petioles and in midribs of dicot leaves.
      • Provides mechanical support and elasticity, allowing the plant to bend without breaking.

    • Sclerenchyma:

      • Dead cells with no protoplasm and greatly thickened, lignified cell walls.
      • Cells are closely packed, with lumen and cavity nearly absent and no intercellular spaces.
      • Composed of fibers and sclereids (stone cells).
      • Found in stems, roots, veins of leaves, and hard coverings of seeds and nuts.
      • Sclereids form the gritty part of ripe fruits and contribute hardness to seed coats and nutshells.
      • Functions mainly involve mechanical support and protection, providing strength, rigidity, flexibility, and elasticity to the plant body.
      • Fibers: Make stems and roots tough and rigid.
      • Sclereids: Sclereids/stone cells are found in the flesh of fruits like pears, pips of fruits, and shells of nuts.

    • Epidermis:

      • Transparent, one-cell thick layer usually covered with a cuticle, and includes guard cells with stomata.
      • Outermost layer of the plant body in leaves, stems, and roots.
      • Protects the plant from desiccation and infection, with guard cells and the cuticle reducing water loss.

    • Chlorenchyma:

      • Photosynthesis.
Complex Permanent Tissues
  • Composed of more than one different type of cell.
    • Xylem:
      • Transports water and ions, provides strengthening and support.
      • Composed of tracheids, vessels, xylem parenchyma, and xylem sclerenchyma.
      • Except for xylem parenchyma, all other xylem elements are dead and bounded by thick lignified walls.
      • Found in vascular bundles of roots, stems, and leaves.
      • Transports water and mineral salts upwards from the root to different parts of shoots.
      • Lignified walls of tracheids, vessels, and sclerenchyma provide mechanical strength to the plant body.
      • Components of Xylem:
        • Tracheids.
        • Vessels - vessel elements joined end to end.
        • Xylem fibres.
        • Xylem parenchyma.
    • Phloem:
      • Transports manufactured food (nutrients).
      • Composed of sieve tubes, companion cells, phloem parenchyma, and phloem fibers.
      • All phloem cells are living cells, except for fibers.
      • Found in vascular bundles of roots, stems, and leaves.
      • Transports food/nutrients from the leaves to storage organs and from storage organs to growing regions of the plant body (translocation).
      • Components of Phloem:
        • Sieve - Tube.
        • Companion Cells.

Plant Organs

  • The leaf

Leaf Tissue Anatomy

  • Dermal Tissue (Epidermis):
    • Upper Epidermis.
    • Lower Epidermis.
    • Cuticle.
    • Guard Cells.
    • Stoma.
  • Ground Tissue:
    • Palisade Parenchyma.
    • Spongy Parenchyma.
  • Vascular Tissue:
    • Xylem.
    • Phloem.
    • Vein.

Plant Parts and Tissue Location

  • Leaf:
    • Upper epidermis.
    • Lower epidermis.
    • Palisade mesophyll.
    • Spongy mesophyll.
    • Main vein phloem bundle.
    • Xylem vascular.
    • Side vein.
  • Stem:
    • Sclerenchyma.
    • Cambium.
    • Pith.
    • Cortex.
    • Xylem.
    • Phloem.
  • Root:
    • Epidermis.
    • Cortex.
    • Endodermis.
    • Pericycle.
    • Root hair.