Lecture_20_-_Living_on_land_-_plant_evolution_2200_Sp25_TO_POST

Living on Land: The Evolution of Land Plants

Learning Objectives

• Understand the impact of plants on ecosystems, species interactions, and human life.

• Explore adaptations required for terrestrial life in plants.

• Examine the transition from non-vascular to vascular plants.

• Analyze the evolution of seeds, flowers, and fruit and their role in plant diversity.

Diversity of Plant Species

• Estimated between 250,000-390,000 described plant species.

• Plants are categorized as a monophyletic clade of multicellular taxa, depending on species definitions.

  • They’ve descended from a single common ancestor and all are multicellular autotrophic eukaryote

• Most Aquatic plants are protists

  • Plants generally refer to Land Plants

Relationship of Plants with Charophytes

• Closest living relatives of land plants are Charophytes (green algae).

  • Both are photosynthetic, not a trait

  • Contain chloroplasts with chlorophyll a and b. not unique to this relationship

  • Possess protein rings for cellulose synthesis.

    • Other algae have linear protein

  • Unique flagellated sperm structure.

  • Unique cell division structure called phragmoplasts.

    • Structure is connected with cytokinesis

Reasons for Colonizing Land

• Plants first colonized land for several reasons:

  • Increased CO2 availability.

  • More sunlight for photosynthesis.

  • Abundant nutrients in soil.

  • Availability of habitat with low competition.

Key Adaptations for Terrestrial Life

• Plants develop various adaptations for survival on land, including:

  • Waxy cuticle to minimize water loss.

  • Stomata to facilitate gas exchange, aids in control with evaporative water loss

  • Flavonoids provide UV protection.

  • Mycorrhizae roots for efficient nutrient access.

  • Maternal tissue protects embryos and provides nutrients.

  • Spore protection supports dispersal in early land plants.

• Alternation of generations:

  • Life cycle includes multicellular diploid and haploid stages.

    • This evolved in all land plants independently although it’s seen in some prortists

Evolutionary Overview of Land Plants

• Major groups include:

  • Streptophyta (green algae)

  • Land plants split into: Bryophytes (non-vascular) and Tracheophytes (vascular).

  • Tracheophytes further split into Euphyllophytes (seed plants).

Characteristics of Early Land Plants

• Bryophytes (moss,liverworts) (non-vascular plants):

  • 470 MYA

    • Lack vascular tissues, meaning they do not efficiently transport water/nutrients.

    • Possess rhizoids for anchoring, instead of true roots.

    • Reliance on water for sperm to reach eggs, indicating damp habitat preference.

      • Sperm have to swim!!

    • Dominant life stage is haploid.

      • No variance or genetic variation, make sit harder for them to adapt

    • Spores for dispersal by wind

      • Turn into multicellular haploid structures

Advances in Plant Evolution

Seedless Vascular Plants

• Tracheophytes include vascular tissues for resource transport.

  • 425 MYA

• Development of roots and leaves due to vascularization leads to increased height.

• Life cycle dominated by diploid stage.

Vascular System Functionality

• Xylem: specialized for water conduction and structural support via lignin in the walls of cells.

• Phloem: distributes sugars, amino acids, and other synthesized products.

• Roots absorb essential nutrients and water; leaves enhance photosynthesis, increased surface area

Seed Plants (360 MYA)

• Unlike seedless vascular plants:

  • Seeds protect embryos and enhance survival through difficult conditions and disperse long distances

    • Seeds are embryos with a protective coat made of maternal tissue and food supplies

  • Pollen protects male gametes

    • Gametophyte makes gametes, it’s surrounded by pollen wall protecting it for dessication and harsh environment

    • They don’t require water to travel and fertilize the egg until they land on a stigma and travel down to an ovary

      • Increase in genetic diversity

    • All plants have sperm, but not all have pollen

  • Spores are enclosed in reproductive structures

    • All plants have sperm and spores but not all have seeds

Diversity of Angiosperms

• Angiosperms (flowering plants) vs. gymnosperms:

  • Development of flowers attracts pollinators.

    • Flowers are specialized shoots with modified leaves

  • Fruits aid in seed dispersal.

    • Fruits develop from ovaries

    • Ovules in ovaries, after fertilization ovary walls thicken

  • Mutualistic relationships with animals increase reproductive fitness.

    • Flowers and fruits are under selection by animal mutualists

Plant Dominance Through Eras

• Carboniferous Period (360-300 MYA): Seedless vascular plants dominated.

• Permian Period (300-250 MYA): Gymnosperms became predominant.

• Mid-Cretaceous Period (145-65 MYA): Angiosperms emerged as dominant flora in temperate/milder climates.