Viridiplantae Overview and Ecological Importance

Introduction to Biology Chapter 28: Green Algae and Land Plants

• Green plants (Viridiplantae) include 500,000 species, vital in terrestrial and aquatic ecosystems
• Green algae: crucial photosynthetic organisms in freshwater habitats
• Land plants: key photosynthesizers in terrestrial environments

Why Biologists Study Green Algae and Land Plants

• Essential organisms: Provide food, fuel, fibers, and pharmaceuticals
• Crucial industries: Supports agriculture, forestry, and horticulture
• Issues: Weeds and invasive species are problems

Ecosystem Services

• Ecosystem: all organisms and their interactions with nonliving components in an area
• Ecosystem Services: Goods and services ecosystems provide to humans
  • Supporting: Nutrient, water cycles, photosynthesis, soil formation
  • Provisioning: Food, freshwater, fiber, biomass fuel, natural medicine
  • Regulating: Air quality, water runoff, erosion, pollination, climate, natural hazards
  • Cultural: Existence values, ethical values, recreation, ecotourism

Plants as Primary Producers

• Land plants are dominant primary producers, using sunlight to convert CO₂ into sugars
• Sugars support all other organisms in terrestrial habitats
• Key to the carbon cycle: Reduce CO₂, maintain clean air

Plants Providing Humans

• Food: Artificial selection has dramatically changed crops.
• Fuel: Historically relied on wood burning; fossil fuels replaced wood
• Fiber and Building Materials: Plants provide raw materials. Woody plants, like trees, provide lumber and paper fibers
• Medicines: About 25% of US prescriptions contain plant-derived molecules, mostly for repelling herbivores.

How Biologists Study Green Algae and Land Plants

• Analyzing morphological traits
  • Green algae: unicellular, multicellular, colonial, inhabit marine, freshwater, and moist terrestrial habitats.
  • Majority of green algae habitat is aquatic; whereas, majority of land plants are terrestrial.
• Similarities between green algae and land plants
  • Freshwater green algae are closely related to land plants
  • Key traits include chloroplast structure, thylakoid arrangements, cell walls, sperm, and peroxisomes
  • Three green algal groups are very similar to land plants based on DNA sequence analysis

Major Morphological Differences Among Land Plants

1. Nonvascular plants: Lack vascular tissue, use spores for reproduction (e.g., mosses)
2. Seedless vascular plants: Have well-developed vascular tissue, use spores for reproduction (e.g., ferns)
3. Seed plants: Vascular tissue, make seeds (e.g., gymnosperms, angiosperms.)

What Themes Occur in the Diversification of Land Plants?

• Evolution of land plants required adaptations for terrestrial life

Adaptation to Dry Conditions

• Natural selection favored early land plants with adaptations to dry conditions, preventing water loss, UV protection, and water movement
• Adaptations: Cuticle and stomata

Cuticle

• Cuticle: watertight sealant; covers above-ground plant parts, increasing survival and preventing loss of CO₂

Adaptations to Dry Conditions / Stomata

• Stomata: specialized pore surrounded by guard cells; allow gas exchange in photosynthetic tissue necessary for CO₂ uptake.

Protection from UV Irradiation

• UV light damages DNA causing thymine dimers
• Water absorbs UV light
• Plants had UV-absorbing compounds (acted as sunscreen)

Importance of Upright Growth

• First land plants kept tissues in contact with moist soil
• Intense competition for space and light

Origin of Vascular Tissue

• Fossils of upright plants contained elongated cells organized into water conducting tissues
• Biologists hypothesized the cells participated in water transport based on fossilized cell characteristics

Mapping Evolutionary Changes on the Phylogenetic Tree

• Cuticle, stomata, and vascular tissues were key adaptations for land colonization
• Fundamental adaptations to dry conditions evolved once
• Convergent evolution: vessels evolved independently in different groups of plants

Plant Reproduction

• Life cycles of sexually reproducing eukaryotes increase genetic variability due to meiosis and fertilization
• Increasing the number of individuals
• Dispersing individuals to new habitats
• Innovations for efficient plant reproduction
  • Spores encased in tough coat dispersed by wind
  • Gametes produced in multicellular structures
  • Embryos retained and nourished by parent plants (embryonic retention)
  • Gametangia; prevent drying and damage

Reproductive Organs

• Individuals produce distinctive male (antheridia) and female (archegonia) gametangia (analogous to animal testes and ovaries)

Alternation of Generations

• In coleochaetes, stoneworts, and conjugating algae:
  • Multicellular forms are haploid
  • Only the zygote is diploid
• In all land plants:
  • Sporophyte (diploid multicellular) produces spores (haploid) via meiosis
  • Spores germinate into gametophyte (haploid multicellular) producing haploid gametes (egg and sperm) via mitosis
  • Fertilization produces a diploid zygote that grows into a sporophyte, completing the cycle

Gametophyte-Dominant Life Cycles

• In nonvascular plants, gametophytes are larger; sporophytes are smaller and shorter-lived
• Sporophytes are largely nutritionally dependent on gametophytes

Sporophyte-Dominant Life Cycles

• In vascular plants, sporophytes are much larger; gametophytes are smaller and shorter-lived

Reduction of Gametophyte

• Sporophyte-dominated life cycles are advantageous for diploid cells to react to varying environmental conditions more efficiently than haploid cells

Heterospory

• Nonvascular and most seedless vascular plants are homosporous (produces one type of spore)
• Seed plants are heterosporous (produce megaspores and microspores that develop into female and male gametophytes, respectively)

Pollen and Seed Reproduction

• Sperm of nonvascular and seedless vascular plants swim to egg; water must be available
• Seeded plants rely on pollen to transport sperm through air in dry environments
• Seeds contain an embryo and a food supply with a protective coat, allowing for widespread dispersal

Angiosperms

• Most diverse land plants today (350,000+ species)
• Success revolves around flowers
  • Stamen contains anther where microsporangia develop
  • Carpel has ovary that has ovules which become megasporangia
• Flowers' shapes and colors attract different pollinators

Fruit

• Fruits are structures derived from ovaries; enclose seeds
• Tissues derived from ovaries are often nutritious and brightly colored
• Evolution of flowers made efficient pollination possible
• Evolution of fruit made efficient seed dispersal possible

Angiosperm Radiation

• Angiosperms divided into two major groups (monocots and eudicots) based on morphological traits (including cotyledons, vascular tissue, veins and flowers)