Stems-Leaves

PH BIO SCI: STEMS & LEAVES

1. Stem

• Definition: Organ that supports leaves, flowers, and fruits

  • Functions:

    • Holds leaves to maximize sunlight for photosynthesis.

    • Transports water and minerals from roots to leaves.

    • Distributes food from leaves to other plant parts.

• Structure:

  • Nodes: Points on stems where leaves attach.

  • Internodes: Sections of stem between nodes.

2. External Morphology

• Basic Structure: Consists of stem and attached leaves.

• Parts:

  1. Node: Where leaves are attached.

  2. Internode: Between two nodes.

  3. Bud:

     • Axillary Bud: Between petiole and stem, potentially grows into branches or flowers.

     • Terminal Bud: Located at twig tips, aids in length growth.

  4. Stipules: Leaf-like appendages at petiole base.

  5. Leaf Scar: Mark left from dropped leaves.

  6. Bundle Scars: Mark positions of vascular tissues in leaf scars.

  7. Lenticels: Allow gas exchange in the stem.

3. Internal Morphology

• Young Dicot Stems:

  • Primary Growth: Initiates elongation and cellular patterns.

  • Three Main Regions:

    1. Epidermal Region: Protects the stem; often covered in cutin.

    2. Cortex: Between epidermis and vascular cylinder, made of parenchyma; may contain collenchyma and sclerenchyma.

    3. Vascular Cylinder: Contains xylem and phloem, responsible for transport.

• Old Woody Dicot Stem:

  • Secondary Growth: Increases stem thickness; occurs in perennial dicots.

  • Cambium Activity: Forms additional vascular tissues; secondary xylem (wood) and secondary phloem.

  • Annual Rings: Created from variations in cambium activity; indicate tree age.

4. Heartwood and Sapwood

• Heartwood: Dark, inactive core providing support, filled with tannins.

• Sapwood: Lighter, outer zone actively transporting water and nutrients.

5. Periderm

• Definition: Protective tissue replacing the epidermis upon maturity.

• Components:

  • Phellogen: Cork cambium, producing cork.

  • Phellem: Cork, dead and water-resistant.

  • Phelloderm: Secondary cortex.

6. Monocot Stems

• Lack secondary growth, consist of primary tissues only.

• Structure:

  • Epidermis: Single layer, may have a cuticle.

  • Cortex: Large, thin-walled cells.

  • Vascular Cylinder: Scattered vascular bundles, termed closed bundles (one xylem, one phloem).

7. Specialized Stems

• Modify for specific functions:

  • For Asexual Reproduction:

    • Runner: Creeping stems rooting at nodes (e.g., strawberry).

    • Stolon: Lateral, underground stems (e.g., araru, gabi).

    • Offset: Short, thickened branches that produce leaves and roots (e.g., water lettuce).

    • Sucker: Lateral branches growing upward (e.g., chrysanthemum).

  • Modified for Other Functions:

    • Tendrils: Leafless branches for climbing (e.g., squash).

    • Spines and Thorns: Defense structures from axillary buds or leaf modifications (e.g., cacti).

    • Cladophylls: Flattened, leaf-like stems (e.g., asparagus).

    • Phylloclade: Fleshy, modified stems for storage (e.g., Opuntia).

8. Transport of Nutrients in Plants

• Transport Mechanisms:

  1. Passive Transport: Movement without energy, following water potential gradient.

  2. Active Transport: Energy-requiring movement for specific ions (e.g., sodium-potassium pumps).

  3. Short-Distance Transport:

     • Cell-to-cell: Through cell walls.

     • Symplast: Via cytoplasmic connections (plasmodesmata).

     • Apoplast: Between cell walls and intercellular spaces.

  4. Long-Distance Transport:

     • Involves translocation of sugars via phloem.

     • Pressure flow hypothesis explains movement of sap from sources to sinks.

9. Leaves

• Definition: Main site for photosynthesis, lateral outgrowth from stems.

• Morphology: Varies significantly among plant species.

10. External Morphology of Leaves

• Parts:

  1. Stipules: Outgrowths at the base.

  2. Petiole: Leaf stalk; absent in sessile leaves.

  3. Leaf Base: Area near petiole, can be a leaf sheath in monocots.

  4. Leaf Blade: Main photosynthetic area with veins.

• Phyllotaxy: Leaf arrangements:

  • Alternate: One leaf per node.

  • Opposite: Two leaves per node.

  • Whorled: Multiple leaves per node.

11. Leaf Types

• Simple Leaf: Single blade.

• Compound Leaf: Blade divided into leaflets.

  • Types:

    • Pinnately Compound: Leaflets along a central rachis.

    • Palmately Compound: Leaflets radiating from a single point.

12. Leaf Venation

• Types:

  • Netted/Reticulate: Dicot characteristic, branching veins.

  • Parallel: Monocot characteristic, non-branching veins.

13. Internal Morphology of a Dicot Leaf

• Structure:

  1. Upper Epidermis: Seals moisture with waxy cuticle; few stomata.

  2. Mesophyll: Contains palisade and spongy parenchyma, where most photosynthesis occurs.

  3. Veins: Contain xylem (upper) and phloem (lower).

  4. Lower Epidermis: Has more stomata for gas exchange.

14. The Stomata

• Function: Regulate water loss and gas exchange.

• Mechanism: Guard cells control stomatal opening/closing.

• Factors: Internal CO2 levels, temperature, humidity, and the hormone abscisic acid affect stomatal behavior.

15. Specialized Leaves

• Adaptations for various functions:

  1. Reproduction: Growth of plantlets.

  2. Aeration: Enlarged structures for buoyancy.

  3. Support: Tendrils or modified leaf bases.

  4. Protection: Bud scales, spines, and bracts.

  5. Storage: Suction in desert plants.

  6. Attraction: Colorful bracts.

  7. Absorption: Uncutinized surfaces for water absorption.

16. Transpiration

• Definition: Water vapor loss from plants, primarily through leaves.

• Epidermis Role: Waxy cuticle prevents excessive water loss.

• Types:

  1. Stomatal Transpiration: Major pathway via stomata.

  2. Cuticular Transpiration: Minor pathway through cuticle.

• Factors Influencing Rate:

  • Humidity, temperature, and wind velocity.

• Guttation: Water droplets exuded through leaf margins, occurring during moist conditions.