Chapter 17

Symbiotic Relationships

• Definition: A symbiotic relationship involves different organisms living closely together, often to mutual benefit.

• Plants and Fungi: Over 90% of plants engage in a symbiotic relationship with fungi, particularly through structures called mycorrhizae, where plant roots and fungal hyphae intertwine.

• Historical Significance: This relationship is believed to have begun over 500 million years ago when plants began to occupy terrestrial environments.

Plant Evolution and Diversity

• Ancestry: The aquatic ancestors of land plants are from over 500 million years ago.

• Habitat: Early plants occupied nearshore habitats.

• Adaptation to Drought: The experience of periodic drying led these plants to develop drought tolerance, which was crucial for survival on land.

Key Adaptations (by 470 million years ago)

• Moisture Retention: Mechanisms to maintain moisture in plant bodies.

• Structural Support: Development of strong structures to support plants against gravity.

• Seed Dispersal: Adaptations for effective seed dispersal mechanisms to ensure propagation.

Timeline of Plant Evolution

• Transition from algal ancestors to land plants around 470 mya.

  • Vascular Plants: Emerged around 425 mya.

  • Seed Plants: First appeared approximately 360 mya.

Major Groups of Plants (by 360 mya)

• Nonvascular Plants: Examples include mosses and liverworts.

• Vascular Plants: Include lycophytes (e.g., club mosses) and monilophytes (e.g., ferns).

• Gymnosperms: Naked seed plants such as conifers.

• Angiosperms: Flowering plants that include grasses, shrubs, and trees.

Dominance of Seedless Vascular Plants

• Era: 359-299 million years ago (Coal Forests).

• Characteristics: Dominated by mosses and ferns; significant levels of photosynthesis altered atmospheric CO2 levels leading to climate cooling.

• Fossil Fuels: Remains of these plants form current fossil fuels.

Seed Plants Overview

• Current Diversity: Seed plants account for over 90% of modern plant species.

  • Gymnosperms: Include conifers known for producing naked seeds.

  • Angiosperms: Known as flowering plants, produce seeds enclosed within fruits, crucial for various ecosystems.

Flower and Fruit Adaptations

• Flower Production: Is energy-intensive but results in significant reproductive advantages.

  • Pollinator Attraction: Adaptation of flowers to attract pollinators is common; 90% of angiosperms rely on animals for pollen transfer.

• Fruit: Represents a ripened ovary of a flower aiding in seed dispersal (e.g., dandelions, edible fruits).

Mycorrhizal Fungi

• Role: Mycorrhizae are critical in assisting plants with nutrient absorption from the soil, enhancing water and mineral uptake significantly.

• Types: Various types of fungi form these beneficial relationships with plants, important for soil health and ecosystem stability.

Fungal Structure and Reproduction

• Hyphae: The basic structural units of fungi that make up the mycelium.

• Reproductive Structures: Fungi produce spores via specialized fruiting bodies.

• Life Cycle: Involves alternation of generations, with both haploid (n) and diploid (2n) phases, and includes heterokaryotic stages with unfused nuclei.

Types of Fungi

• Chytrids: Primitive fungi known for their flagellated spores.

• Zygomycetes: Known for producing zygospores.

• Ascomycetes: Sac fungi, responsible for producing spores in sac-like structures.

• Basidiomycetes: Club fungi, include mushrooms and shelf fungi.

Plant Structures and Functions

• Leaves: Where photosynthesis occurs; important adaptations such as the cuticle to minimize water loss.

• STOMATA: Structures allowing gas exchange; critical for respiration and photosynthesis.

• Support Systems: Stems and vascular tissues provide necessary physical support and nutrient transportation.

• Mycorrhizal Associations: Enhance nutrient and water uptake through roots' interaction with fungal networks, leading to improved plant health and growth.