Plant Structure and Function

Leaf Structure

• Main Parts of Leaves

  • Petiole: Connects leaf to stem.

  • Blade: Contains photosynthetic tissue.

  • Midrib: Central vein of the leaf.

• Internal Leaf Features

  • Epidermis: Outer layer of cells.

  • Waxy Cuticle: Prevents water loss; covers the epidermis.

  • Stomata: Pores, primarily on the underside, allow gas exchange (CO2 in, O2 out).

    • Importance for Photosynthesis: Leaves are primary photosynthetic organs, hence need for stomata.

    • Guard Cells: Cells that regulate the opening and closing of stomata to control water loss.

Stem Structure

• Key Features of Stems

  • Nodes: Points where leaves/branches emerge.

  • Internodes: Sections between nodes; do not grow new organs.

  • Meristematic Cells: Located in nodes, responsible for producing new organs (leaves, flowers).

Leaf Modifications

• Examples of Adaptations:

  • Cacti: Leaves modified into spines to reduce water loss and adapt to arid environments.

  • Identifying Leaves: Leaves grow from nodes; flat structures indicate they could be leaves.

Root Structure

• Main Parts of Roots

  • Root Apical Meristem (RAM): Area that produces new cells for root growth.

  • Root Cap: Protective covering that helps the ram grow through soil; secretes mucilage for ease of movement.

  • Root Hairs: Increase surface area for water and nutrient absorption.

Flower Structure

• Flowers: Occur only in angiosperms (flowering plants).

  • Components:

    • Whorls: Arrangements of flower parts—sepals, petals (non-reproductive), carpals, and stamens (reproductive).

Plant Evolution and Diversity

Overview of Plantae

• Chloroplasts: A synapomorphy of the Plantae group; derived from primary endosymbiosis between eukaryotic cells and cyanobacteria.

Transition to Land Plants

• Advantages of Life on Land:

  • Increased access to sunlight and CO2 for photosynthesis.

• Challenges of Life on Land:

  • Susceptibility to drying out.

  • Lack of structural support; need for adaptations like strong stems.

  • Reproductive challenges; gametes need specialized mechanisms to meet without aquatic environment.

Algae and Early Plant Lineages

• Glaucophytes: Unicellular freshwater algae retaining peptidoglycan in chloroplasts; strong evidence for bacterial origin of chloroplasts.

• Red Algae: Familiar in sushi (e.g., nori); contain unique pigments that give reddish color.

• Green Plants:

  • Chlorophyll Usage:

    • Use chlorophyll a, chlorophyll b; broader spectrum absorption due to carotenoids.

    • Starch storage in chloroplasts for energy management.

Conclusion

• The evolution of plants is characterized by adaptations for transitioning from aquatic environments to terrestrial habitats, facing various ecological challenges and developing unique structures for survival.