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PLANT ANATOMY AND MORPHOLOGY

Overview of Plant Cells and Nutrition

  • Plants are multicellular organisms composed of eukaryotic cells with cellulose cell walls.
  • They exhibit photoautotrophic type of nutrition, meaning they produce their food using light energy.
  • Key organelles present in plant cells include:
    • Vacuole: Large storage organelles.
    • Plastids: Organelles that perform photosynthesis and store products (e.g., starch).
  • Storage carbohydrate primarily consists of starch.

Plant Tissues

  • Plant tissues are defined as groups of plant cells sharing the same origin, structure, and function.
  • Comparison:
    • Plant Tissue:
    • Cell wall composed of cellulose derived from glucose; basic building unit of the cell.
    • Types:
      1. Xylem: Vascular bundles responsible for water transport.
      2. Phloem: Vascular bundles responsible for nutrient transport.
    • Animal Tissue: Composed of various cell types without cellulose walls.

Histology - Definition and Overview

  • Histology is the biological science that studies the microscopic structure of tissues.

  • Plant tissues can be categorized according to several criteria:

    1. According to Origin:
    • True Tissues:
      • Originate from cells that repeatedly divide and stay interconnected.
    • False Tissues:
      • Originate from separate cells that later aggregate.
    1. According to Cell Wall Thickness and Shape:
    • Parenchyma:
      • Characteristics: Oval-shaped cells with thin walls; abundant intercellular spaces; least specialized type.
      • Types of Parenchyma:
      • Palisade Parenchyma: Elongated cells, primarily found in leaves, containing chlorophyll.
      • Mesenchyme (Spongy Mesophyll): Globular cells with intercellular spaces, found in soft and young parts of plants.
      • Aerenchyma: Cells with oval shapes and large intercellular spaces, typically in aquatic plants.
    • Prosenchyma:
      • Elongated cells without intercellular spaces; walls can be thickened and found in vascular bundles.
    • Collenchyma:
      • Cells with unevenly thickened walls, providing flexibility and support.
      • Types of Collenchyma:
      • Angular Collenchyma: Thickening at cell angles (e.g., leaf petiole).
      • Tangential (Lamellar) Collenchyma: Thickened along the face of cell walls (e.g., stem edges).
      • Lacunar Collenchyma: Thickenings located where cells are in contact, leaving intercellular spaces.
    • Sclerenchyma:
      • Composed of cells with evenly thickened walls; mature sclerenchyma cells are dead.
      • Types:
      • Fibres: Elongated cells providing maximum support found throughout the plant body.
      • Sclereids: Various shapes, found in leaves and fruits (e.g., nut shells, seeds).

Developmental Levels of Plant Tissues

  1. Meristematic Tissue:

    • Composed mainly of parenchymatic cells; have thin walls, abundant cytoplasm, and large nuclei.
    • Types of Meristematic Tissue:
    • Protomeristem: The initial terminal group of cells at the root or stem tip.
    • Primary Meristem (Histogen): Forms from protomeristem and includes:
      • Procambium: Develops into primary xylem and phloem.
      • Protoderm: Gives rise to the epidermis.
      • Ground Meristem: Includes the periblem (basis for cortex) and plerum (ground tissue).
    • Secondary Meristem: Permanent tissue reverting back to meristematic state; includes cambium (produces secondary xylem and phloem) and phellogen (produces cork).
    • Latent Meristem: Meristematic cells among permanent tissue, activated under special conditions (e.g., root pericycle).

  2. Permanent Tissue:

    • Composed of cells that have lost the ability to divide; specialized for unique functions.
    • Dermal Tissues:
      • Function: Cover and protect organisms, facilitate substance exchange.
      • Epidermis: One cell layer without intercellular spaces; forms from primary meristem tissues.
      • Features of Epidermis:
      • Cuticle: Waxy layer preventing water loss.
      • Stomata: Pores for gas exchange guarded by guard cells.
      • Hydathodes: Similar to stomata; involved in guttation.
      • Trichomes: Outgrowths serving protective functions, can be unicellular or multicellular, including glandular types.
    • Vascular Tissues:
      • Composed of vascular bundles; essential for nutrient and water transport.
      • Xylem: Transports water; composed of tracheids (narrow, dead cells) and tracheae (hollow tubes).
      • Phloem: Transports nutrients; includes sieve tubes which are living cells with perforated walls.

Ground Tissues in Plants

  • Ground tissue types and functions:
    1. Assimilation Tissue: Parenchyma that contains chloroplasts; primarily found in leaves as chlorenchyma.
    2. Storage Tissue: Stores metabolites like starch, lipids, and water in specific organs such as cacti.
    3. Excretory Tissue:
    • Idioblasts: Single cells that excrete important substances.
    • Nectary: Excretes nectar for pollinator attraction.
    • Latex Cells: Produce latex containing rubber and other compounds.
    1. Mechanical Tissues: Includes sclerenchyma and collenchyma; provide physical support.

Practice Questions

  1. How does collenchyma differ from sclerenchyma?
  2. By what processes does gaseous exchange occur in large plants?
  3. Why don’t all forest trees reach the height of redwoods, despite the height advantage?

Plant Organs

  • Define a plant organ as a set of tissues functioning collectively towards a specific purpose.
  • Only vascular plants possess true organs; spore plants have simpler structures referred to as pseud-organs.
  • Types of pseud-organs include cauloid (stem-like), rhizoid (root-like), and phyloid (leaf-like).