Leaf Structure and Function

Leaf Function

• Leaves are the plant organs specializing in photosynthesis and transpiration.
• Leaves are the primary photosynthetic structures, developing on stems.
• Photosynthesis converts light energy to chemical energy.
• Leaves are the main photosynthetic organs in most plants.

Leaf Functions

• Manufacture food through photosynthesis.
• Gas (air) exchange – respiration.
• Protect vegetative and floral buds.
• Transport water – transpiration.

Gross Structure of Leaf

• Lamina: The expanded leaf blade.
• Petiole: The stalk – a slender stalk.
• Stipules: Outgrowths at the base of the petiole.
• Apex: The tip of the leaf.
• Margin: The edge of the leaf.
• Veins: Network of vascular tissues arising from the midrib.
• Midrib: The central conducting and supporting structure of the blade.
• Leaflet: Distinct and separate segment of a compound leaf.
• Petiolule: The stalk of a leaflet.
• Rachis: The main axis of a pinnately compound leaf.
• Pulvinus: The swollen base of the petiole or petiolule.

Microscopic Structure of Leaf

• Cuticle: The outermost layer of both upper and lower surfaces; clear and waxy to prevent water loss.
• Epidermis: A thick layer of cells that protects the inner tissues.
• Mesophyll: Tissue between the epidermal layers where photosynthesis occurs.
• Stomata: Openings in the leaf surfaces and stems for gas exchange.
• Veins: Contains vascular tissue (xylem and phloem).
• Bundle Sheath: A compact layer of cells surrounding a vascular bundle.

Simple vs. Compound Leaves

• Simple Leaves: The blade is a single piece.
• Compound Leaves: The blade consists of two or more separate parts on a common petiole.

Compound Leaf Types

• Palmately Compound: Leaflets radiate from one central point.
• Pinnately Compound: Leaflets are arranged on both sides of a central axis.
• Bi-pinnately Compound: Leaflets of a compound leaf are arranged on smaller leaflets.

Phyllotaxy or Leaf Arrangement

• Alternate: Characterized by only one leaf at each node.
• Opposite: Characterized by two leaves at one node.
• Whorled: Characterized by three or more leaves per node.
• Distichous: One leaf per node, arranged in two rows when viewed from above.
• Secund: One leaf per node arranged in one direction only.
• Spiral: One leaf per node arranged in spiral form.
• Decussate: Two leaves per node, but arranged in four directions.
• Rosette: Leaves arranged tightly at the plant crown.

Leaf Venation

• Parallel-veined Leaves: Consists of slender ribs or veins running parallel to each other..
• Netted-veined or Reticulate Leaves: Veins branch and rebranch into finer veinlets that unite to form meshes.
• Types of Netted Venation:
  • Pinnate: One midrib
  • Palmate: Several large veins branching into the blade from the end of the petiole.

Leaf Margin

• Entire: Continuous even line margin without teeth or notches.
• Serrate: Sharp teeth pointing forward like the teeth of a saw.
• Dentate: Triangular or tooth-like edges.
• Crenate: Rounded teeth.
• Undulate: Wavy line margin.
• Sinuate: Slightly wavy margin.
• Incised: Irregular teeth.
• Lobed: Deep incisions do not reach much more than halfway to the blade.

Leaf Shape

• Linear: Narrow leaves with nearly parallel margins.
• Lanceolate: Widest below and tapering upward; shape like a lance.
• Oblong: Not conspicuously narrowed. Sides are parallel.
• Oval: Widest in the middle, tapering on both ends.
• Cordate: Heart-shaped.

Leaf Modification

• Bud Scales: Overlapping modified leaves that protect the growing tissues of plant buds.
• Spines: Protect the plant from grazing animals and other enemies (e.g., cactus leaves).
• Water Storage: Fleshy structures with a thick layer of cutin serving for water storage (e.g., cobweb houseleek).
• Tendrils: Used for climbing (e.g., bitter gourd).
• Stem Tendrils: Branch modifications that help the plant climb.
• Leaf Tendrils: Leaf or leaflet modifications emerging from leaf nodes; support the stem.

Leaf Physiological Process

• Photosynthesis: Plants use energy from sunlight to produce sugar.

• Stage 1 (Energy capture): Energy captured from sunlight.

• Stage 2 (Light conversion): Light energy converted to chemical energy.

• Stage 3 (Chemical formation): Chemical energy powers formation of organic compounds using CO2.

• Photosynthesis equation: 6CO2 + 6H2O → C6H12O6 + 6O2

• Respiration: Sugars produced by photosynthesis are used directly in cellular respiration.

• Aerobic Respiration: Occurs only in the presence of oxygen.

• Anaerobic Respiration: Occurs in the absence of oxygen.

• Transpiration: The process of water loss in the form of water vapor. Eliminates excess water from the plant body.

• Stomatal Mechanism: Stomata open during the day and close at night to regulate water loss and CO2 uptake.

This quiz covers the essential functions and gross structure of leaves in plants, including their role in photosynthesis, gas exchange, and water transport. Learn about the different parts of a leaf, such as the lamina, petiole, and stipules, and their respective functions. Enhance your understanding of plant biology through this focused assessment.