Chapter 26: The Colonization of Land

Introduction

This chapter explores the evolutionary transition of life from aquatic to terrestrial environments, focusing on the colonization of land by plants, fungi, and animals. It highlights the major adaptations and evolutionary milestones that enabled life to thrive on land, as well as the ecological impacts of these transitions.

The Greening of Earth

Early Life on Land

• Initial Conditions: For more than 2 billion years, Earth's surface was lifeless. Only cyanobacteria and protists existed on land about 1.2 billion years ago.

• Colonization: Within the last 500 million years, plants, fungi, and animals began to colonize land.

• First Forests: The first forests formed approximately 385 million years ago, dramatically altering Earth's ecosystems.

Fungi and Their Role in Colonization

Fungal Structure and Nutrition

• Multicellular Fungi: Fungi are composed of networks of filaments called hyphae, which form a mass known as a mycelium. This structure maximizes surface area for nutrient absorption.

• Fungal Reproduction: Fungi reproduce both sexually and asexually, often involving the fusion of hyphae from different individuals.

Fungi-Plant Symbiosis

• Mycorrhizae: A symbiotic association between fungal hyphae and plant roots, where fungi transfer nutrients to the plant partner. This relationship was crucial for early plant colonization of land.

• Arbuscules: Branched fungal structures that penetrate plant root cells, facilitating nutrient exchange.

Derived Traits in Plants

Adaptations for Life on Land

• Cuticle: A waxy coating on plant surfaces that prevents water loss and protects against microbial attack.

• Stomata: Specialized pores that allow for gas exchange (CO2 and O2) between the plant and the environment.

• Alternation of Generations: A reproductive cycle in which plants alternate between multicellular haploid (gametophyte) and diploid (sporophyte) generations.

Alternation of Generations (Life Cycle)

• Gametophyte: Produces haploid gametes by mitosis.

• Sporophyte: Formed by the fusion of gametes (zygote), produces haploid spores by meiosis.

Major Plant Groups and Evolution

Plant Phylogeny

• Charophyte Green Algae: Closest relatives of land plants.

• Bryophytes: Nonvascular plants such as mosses.

• Seedless Vascular Plants: Include ferns and their relatives.

• Gymnosperms: Seed plants that do not form flowers (e.g., conifers).

• Angiosperms: Flowering plants, the most diverse and widespread group.

Highlights of Plant Evolution

• Nonvascular Plants (Bryophytes): Lack vascular tissue; gametophyte is the dominant generation.

• Seedless Vascular Plants: Possess vascular tissue; sporophyte is dominant.

• Seed Plants: Include gymnosperms and angiosperms; seeds protect and nourish the embryo.

Gametophyte-Sporophyte Relationships

Seed Plants: Gymnosperms and Angiosperms

Gymnosperms

• Ovule to Seed: Gymnosperms produce seeds that are not enclosed in ovaries ("naked seeds").

• Examples: Conifers such as pine, spruce, and fir.

Angiosperms

• Flowers and Fruits: Angiosperms produce flowers for reproduction and fruits to protect seeds and aid in their dispersal.

• Monocots vs. Dicots: Angiosperms are divided into monocots (one seed leaf) and dicots (two seed leaves).

• Examples of Monocots: Grains (wheat, rice, maize), orchids, grasses, lilies, onions.

• Examples of Dicots: Most flowering plants, including beans, oaks, maples, and squashes (e.g., Cucurbita pepo).

Physical and Chemical Environment

Lichens and Soil Formation

• Lichens: Symbiotic associations between a photosynthetic microorganism (usually algae or cyanobacteria) and a fungus.

• Ecological Role: Lichens are pioneer species on bare rock or soil, breaking down surfaces and promoting soil formation, which enables plants to grow.

Biotic Interactions

Types of Interactions

• Mutualism: Both species benefit from the interaction (e.g., mycorrhizal fungi and plants).

• Parasitism: One species benefits at the expense of the other.

• Impact: The colonization of land by plants and fungi increased the available energy and nutrients on land, profoundly affecting biotic interactions and ecosystem development.