Green Algae and Land Plants: Evolution, Diversity, and Adaptations

Chapter 28: Green Algae and Land Plants

Introduction to Green Plants

Viridiplantae is a eukaryotic clade containing over 500,000 species, playing essential roles in both terrestrial and aquatic ecosystems. Green plants are divided into two main groups:

• Green algae: Important photosynthetic organisms in freshwater habitats.

• Land plants: Key photosynthesizers in terrestrial environments, enabling the colonization of land by multicellular life.

Before the evolution of land plants, terrestrial environments were dominated by bacteria, archaea, and single-celled protists.

28.1 Why Do Biologists Study Green Algae and Land Plants?

Importance of Green Algae and Land Plants

• Essential organisms: Provide food, fuel, fibers, and pharmaceuticals.

• Crucial industries: Agriculture, forestry, and horticulture depend on plants.

• Ecological issues: Weeds and invasive species can disrupt ecosystems and agriculture.

Plants Provide Ecosystem Services

• Ecosystem: All organisms and nonliving components in an area.

• Ecosystem service: Goods and services provided by ecosystems to humans, including supporting, provisioning, regulating, and cultural services.

Plants as Primary Producers

• Primary producers: Land plants convert sunlight into sugars, supporting all other terrestrial organisms.

• Carbon cycle: Plants reduce atmospheric CO2 and help maintain clean air.

Plants Provide Humans with Food, Fuel, Fiber, Building Materials, and Medicines

• Food: Artificial selection has led to dramatic changes in domesticated plants.

• Fuel: Historically, wood was the main fuel; fossil fuels have since replaced wood.

• Fiber and building materials: Plants provide raw materials such as lumber and paper fibers.

• Medicines: About 25% of US prescriptions contain plant-derived molecules, many of which are synthesized by plants to repel herbivores.

Source	Compound	Use
Cinchona	Quinine, Quinidine	Malaria prevention, Heart medication
Aspen, willow	Salicin	Pain relief (aspirin)
Wild yams	Steroids	Precursors for birth control pills, cortisone
Pacific yew	Taxol	Treating ovarian cancer
Curare vine	Tubocurarine	Muscle relaxant in surgery
Rosy periwinkle	Vinblastine, vincristine	Treating leukemia

28.2 How Do Biologists Study Green Algae and Land Plants?

Analyzing Morphological Traits

• Green algae can be unicellular, multicellular, or colonial, and inhabit marine, freshwater, or moist terrestrial environments.

• Most green algae are aquatic; most land plants are terrestrial.

Similarities Between Green Algae and Land Plants

• Freshwater green algae are hypothesized to be closely related to land plants.

• Key shared traits: chloroplast structure, thylakoid arrangement, cell walls, sperm, and peroxisomes.

• Three green algal groups (Zygnematophyceae, Coleochaetophyceae, Charophyceae) are most similar to land plants based on DNA analysis.

Major Morphological Differences Among Land Plants

• Nonvascular plants: Lack vascular tissue; reproduce via spores (e.g., mosses).

• Seedless vascular plants: Have vascular tissue; reproduce via spores (e.g., ferns).

• Seed plants: Have vascular tissue and produce seeds (e.g., angiosperms, gymnosperms).

28.3 What Themes Occur in the Diversification of Land Plants?

Adaptations for Terrestrial Life

The evolution of land plants required several key adaptations to survive and thrive on land:

• Controlling water loss

• Surviving intense sunlight

• Growing upright in air

• Reproducing without water

• Using animals to carry pollen and seeds (in some groups)

Adaptations to Dry Conditions

• Cuticle: A watertight sealant covering aboveground parts, preventing water loss but also limiting CO2 intake.

• Stomata: Pores surrounded by guard cells that allow gas exchange while minimizing water loss.

• UV-absorbing compounds: Protect DNA from damage caused by ultraviolet radiation.

• Upright growth: Competition for light led to the evolution of tissues that support vertical growth.

• Vascular tissue: Specialized cells for water and nutrient transport, enabling larger plant size and upright growth.

Mapping Evolutionary Changes

• Key adaptations (cuticle, stomata, vascular tissue) evolved once and enabled land colonization.

• Convergent evolution: Vessels evolved independently in angiosperms, gnetophytes, and some seedless vascular plants.

Reproduction in Dry Conditions

• Spores: Resistant to drying due to a tough coat of sporopollenin; dispersed by wind.

• Gametangia: Complex, multicellular structures that produce gametes and protect them from desiccation and damage.

• Embryonic retention: Embryos are retained and nourished by the parent plant.

Protective, Complex Reproductive Organs

• Antheridium (plural: antheridia): Sperm-producing structure.

• Archegonium (plural: archegonia): Egg-producing structure.

• These are analogous to testes and ovaries in animals.

Alternation of Generations

All land plants exhibit alternation of generations, a life cycle that alternates between multicellular haploid (gametophyte) and multicellular diploid (sporophyte) stages.

• In some algae, only the zygote is diploid; in land plants, both stages are multicellular.

Life cycle functions:

• Increase genetic variability (meiosis and fertilization)

• Increase the number of individuals

• Disperse individuals to new habitats

Summary Table: Major Plant Groups and Key Features

Group	Vascular Tissue	Seeds	Dominant Generation
Nonvascular plants	No	No	Gametophyte
Seedless vascular plants	Yes	No	Sporophyte
Seed plants	Yes	Yes	Sporophyte

Additional info: This summary includes expanded explanations and context for key terms and processes, as well as reconstructed tables for clarity and completeness.