Plant Structure and Function

Plant Cells and Tissues

Overview of Plant Structure and Function

  • Plants have developed structures and functions to adapt to various environments over time.
  • This chapter investigates roots, stems, and leaves, focusing on their structure and function.

Plant Cells

  • All living organisms are made up of cells, with plant cells exhibiting unique characteristics:
    • Central Vacuole: Large fluid-filled structure that helps maintain turgor pressure in the plant cells.
    • Plastids: Include chloroplasts for photosynthesis, chromoplasts for color, and leucoplasts for storage.
    • Cell Wall: Rigid outer layer providing structure and protection.
Types of Plant Cells
  • There are three basic types of plant cells:
    • Parenchyma Cells:
    • Structure: Usually cube-shaped or elongated, loosely packed, large central vacuole, thin walls.
    • Function: Involved in photosynthesis, storage of nutrients and water, and healing processes.
    • Examples: Bulk of nonwoody plants and fleshy parts of fruits.
    • Collenchyma Cells:
    • Structure: Thickened cell walls, usually grouped in strands.
    • Function: Provides support to growing regions, maintains flexibility.
    • Examples: Found in celery stalks and other young, still-growing parts of plants.
    • Sclerenchyma Cells:
    • Structure: Thick, rigid walls, often dead at maturity.
    • Function: Support and strength, primarily in mature regions of the plant.
    • Examples: The hard shells of nuts and seed coats.

Plant Tissue Systems

  • Tissues are organized into three systems, each performing distinct functions:
    • Dermal Tissue System:
    • Function: Protective outer covering, regulates gas exchange.
    • Components: Epidermis, cuticle, and stomata.
    • Ground Tissue System:
    • Function: Support, storage, and metabolism.
    • Components: Parenchyma, collenchyma, sclerenchyma cells.
    • Vascular Tissue System:
    • Function: Transport of water, nutrients, and organic compounds.
    • Components: Xylem and phloem.
Vascular System Details

  • Xylem: Conducts water and mineral nutrients from the roots upwards.

    • Composed of tracheids and vessel elements (both types are dead at maturity).
    • Water moves through xylem via cytoplasmic connections called pits.
    • Tracheids are long with tapering ends, while vessel elements have broader openings or no walls, forming continuous vessels for efficient water transport.

  • Phloem: Distributes sugars and some mineral nutrients.

    • Composed of sieve tube members and companion cells, where the former transport sugars through sieve plates, and the latter assist in transport.

Meristems and Growth

  • Plant growth originates from meristems:
    • Apical Meristems: Located at the tips of stems and roots, responsible for primary growth (increase in length).
    • Lateral Meristems: Responsible for secondary growth (increase in diameter).
    • Intercalary Meristems: Found in some monocots, allowing for regrowth after being damaged (e.g., grass).
Primary vs. Secondary Growth
  • Primary Growth: Involves cell division at the tips of roots and stems.
  • Secondary Growth: Involves lateral meristems, increasing stem and root width, often resulting in wood formation in dicots and gymnosperms.

Roots

  • Functions: Anchor plants, absorb water and nutrients, store organic compounds.
  • Type of Roots:
    • Taproot: Large primary root (e.g., carrots).
    • Fibrous Roots: Branching root system common in monocots (e.g., grasses).
    • Adventitious Roots: Roots originating from unusual places (e.g., prop roots in corn).
Root Structures
  • Root Cap: Covers root tip, protecting the apical meristem.
  • Root Hairs: Extension of epidermal cells that increases surface area for water absorption.
  • Layers of Roots:
    • Epidermis: Outermost protective layer.
    • Cortex: Located just inside the epidermis, primarily for storage and transport.
    • Endodermis: Inner boundary of the cortex that regulates water movement.
    • Pericycle: Layer that produces lateral roots.
Primary and Secondary Growth in Roots
  • Primary Growth in Roots: Controlled by apical meristem, leading to elongation of the root.
  • Secondary Growth in Roots: Involves the formation of vascular cambium, leading to the production of additional vascular tissues (secondary xylem and phloem).

Stems

  • Functions: Support leaves, transport materials, store nutrients.
  • Structure:
    • Node: Point where leaves attach.
    • Internode: Space between nodes.
    • Buds: Contain apical meristems and can develop into shoots.
Primary and Secondary Growth in Stems
  • Primary Growth: Similar to roots, apical meristems contribute to primary elongation.
  • Secondary Growth: Occurs via vascular cambium, contributing to thicker stems, resulting in wood; both secondary xylem (wood) and secondary phloem form.

Leaves

  • Functions: Main site of photosynthesis; adapted to capture sunlight and regulate gas exchange.
  • Types of Leaves: Simple leaves, compound leaves, and doubly compound leaves.
Leaf Structure
  • Blade: Broad flat part where photosynthesis occurs.
  • Petiole: Stalk attaching blade to stem.
  • Tissues:
    • Dermal Tissue: Contains epidermis.
    • Mesophyll: Contains chloroplast-rich parenchyma cells,
    • Palisade Mesophyll: Main site of photosynthesis, tightly packed.
    • Spongy Mesophyll: Irregularly shaped cells with air spaces, facilitating gas exchange.
    • Vascular Tissue: Arranged in veins, continuous with vascular tissues of stems.
Leaf Adaptations
  • Varied adaptations depending on environmental conditions (e.g., tendrils, spines, or tubular leaves in carnivorous plants).
  • Gas Exchange Regulation: Controlled by stomata, with guard cells regulating their opening and closing based on water availability.

Review and Critical Thinking Questions

  1. Describe the three basic types of plant cells.
  2. Explain how the structure of roots, stems, and leaves relates to their specific functions.
  3. Compare and contrast primary and secondary growth in plant structures.
  4. Discuss the significance of vascular tissue in plant functions.
  5. Analyze how leaf structure contributes to photosynthesis and gas exchange.

Essential Plant Nutrients

  • Macronutrients (required in large amounts): Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Sulfur.
  • Micronutrients (required in smaller amounts): Iron, Manganese, Boron, Chlorine, Zinc, Copper, Molybdenum.