3.7

Plant Structure Overview

  • Plants consist of two primary systems:

    • Shoot System: Comprising the vegetative and reproductive portions.

    • Vegetative Portion: Includes everything that is not reproductive, such as leaves and stems.

    • Reproductive Portion: Contains flowers and, in fruit-producing plants, the fruits as well.

    • Root System: Supports the plant and absorbs water and nutrients from the soil.

Plant Tissues

  • Two main types of plant tissue:

    • Meristematic Tissue:

    • Characteristics:

      • Undifferentiated and actively dividing cells.

      • Positioned in various locations within the plant.

    • Locations of Meristematic Tissue:

      • Apical Meristems: Found at tips of roots and shoots, essential for length growth.

      • Lateral Meristems: Allow vascular plants to grow in girth (thickness).

      • Present in gymnosperms and dicots (true woods).

      • Monocots do not undergo secondary growth nor have lateral meristems.

      • Intercalary Meristematic Tissue:

      • Specific to monocots (e.g., grasses).

      • Allows growth from the base of the leaf blade where it attaches to the stem, compensating for loss when the tops are mowed.

  • Permanent Tissues:

    • Definition: Tissues that have differentiated and are no longer actively dividing.

    • Types of Permanent Tissues:

    • Dermal Tissue:

      • Function: Covers the outer surface of the plant, similar to human skin (epidermis).

      • Structures within Dermal Tissue:

      • Cuticle: Protective waxy layer secreted by the epidermis to prevent desiccation.

      • Stomata: Openings for gas exchange (CO₂ intake and H₂O release) regulated by guard cells which open and close stoma.

      • Trichomes: Hair-like projections that provide a fuzzy feel, help reduce water loss, reflect solar energy, and may contain chemicals that deter herbivores.

      • Root Hairs: Increase root surface area for efficient water absorption, extensions of dermal tissue.

    • Vascular Tissue:

      • Comprised of xylem and phloem, vital for plant transport systems.

      • Xylem:

      • Function: Transports water and minerals.

      • Characteristics: Xylem cells are dead at maturity, forming hollow tubes.

      • Types of Xylem Cells:

        • Tracheids: Present in all vascular plants, simpler structure.

        • Vessel Elements: More complex, found only in angiosperms, enhance water conduction.

      • Importance of Lignin: A structural molecule that provides support to xylem.

      • Phloem:

      • Function: Conveys sugars, hormones, and nutrients.

      • Characteristics: Cells called sieve-tube elements, reliant on companion cells for nutrients as they lack mitochondria and organelles.

      • Companion cells support sieve tube elements with energy and nutritional needs.

    • Ground Tissue:

      • Function: Fills space between other tissues, involved in photosynthesis and storage of starch (excess sugars).

      • Present notably in roots and leaves where photosynthesis occurs.

Growth in Plants

  • Types of Growth:

    • Primary Growth: Occurs in all vascular plants at apical meristems, allowing vertical growth.

    • Example: Tomato plant elongating.

    • Secondary Growth: Involves lateral meristems, allowing for increased girth (thickness).

    • Only occurs in certain plants like gymnosperms (softwood) and dicots (hardwood).

    • Monocots lack this type of growth and remain herbaceous.

Cambium and Tree Structure

  • Cambium: Meristematic tissue located just below the bark, crucial for secondary growth by producing secondary xylem.

  • Tree Cross Sections:

    • Sapwood: Lighter layer that still conducts water despite being composed of dead xylem cells.

    • Heartwood: Older, darker central part that no longer conducts water but provides structural support.

    • Growth Rings: Form based on seasonal growth changes in temperate climates.

    • Spring Growth: Large diameter, thin-walled xylem cells, lighter colored.

    • Fall Growth: Smaller diameter, thick-walled xylem cells, darker colored.

Conclusion

  • The study of plant structure and tissue types is critical for understanding plant growth, adaptation, and physiology. The intricate relationships between meristematic and permanent tissues and their roles in plant life cycle highlight the complexity of plant biology. Students should prepare for practical assessments with lab observations, particularly regarding stomata and cross-sectional tree anatomy.