Green Algae and Land Plants - Summary

Why Study Green Algae and Plants?

• The Viridiplantae consist of green algae and land plants.
• Green algae: Photosynthetic organisms in freshwater habitats.
• Land plants: Key photosynthesizers in terrestrial environments.
• Plants are studied because we could not live without them.

Ecosystem Services

• Green algae and land plants provide:
  • Oxygen production via photosynthesis.
  • Soil building by providing food for decomposers.
  • Soil retention and prevention of nutrient loss.
  • Water retention in soil.
  • Climate moderation via shade and wind reduction.

Plants as Primary Producers

• Land plants are dominant primary producers in terrestrial ecosystems.
• Convert sunlight into chemical energy.
• Key to the carbon cycle.
  • Take $CO<em>2$ from the atmosphere to make sugars then fix much more $CO</em>2$ than they release.

Human Uses of Plants

• Food: Artificial selection has dramatically changed plant characteristics
• Fiber and Building Materials: Raw material clothing, rope, household articles, lumber for houses and furniture and fibers for paper.
• Medicines: About 25% of prescriptions include plant-derived molecules (synthesized to repel herbivores).

Studying Green Algae and Land Plants

• To understand diversification:
  • Compare morphological traits.
  • Analyze the fossil record.
  • Estimate phylogenetic trees.

Similarities Between Green Algae and Land Plants

• Chloroplast structure is the same.
• Thylakoid arrangements are similar.
• Cell walls, sperm, and peroxisomes are similar.
• Chloroplasts synthesize starch.
• Three groups most similar to land plants: Zygnematophyceae, Coleochaetophyceae, Charophyceae.

Major Morphological Differences among Land Plants

• Nonvascular plants:
  • Lack vascular tissue.
  • Include mosses.
  • Use spores for reproduction.
• Seedless vascular plants:
  • Have vascular tissue.
  • Use spores for reproduction.
  • Include ferns.
• Seed plants:
  • Have vascular tissue.
  • Make seeds (embryo + nutritive tissue + protective layer).
  • Include angiosperms and gymnosperms.

Fossil Record

• Five major events in land plant diversification: stomata, vascular tissue, roots, leaves.
• Origin of Land Plants: fossils include cuticle and sporopollenin.
• Silurian-Devonian Explosion: Fossils from major plant lineages with adaptations for terrestrial habitats (416 to 359 mya).
• Carboniferous Period: Extensive coal deposits from seedless vascular plants (359 to 299 mya).
• Diversification of Gymnosperms: Prominent in the fossil record from 299 mya to 145 mya.
• Diversification of Angiosperms: Appear about 150 mya with wind/insect transported pollen.

Molecular Phylogenies

• Green plants are monophyletic.
• Zygnematophyceae is the closest living relative to land plants.
• Land plants are monophyletic.
• Nonvascular plants are the earliest-branching, paraphyletic groups.
• Seedless vascular plants are paraphyletic, vascular plants are monophyletic.
• Seed plants (gymnosperms + angiosperms) are monophyletic.

Adaptations to Dry Conditions

• Natural selection favored:
  • Preventing water loss.
  • Protection from UV radiation.
  • Moving water to tissues without direct access.
• Cuticle: Waxy layer preventing water loss.
• Stomata: Pores for gas exchange.
• Flavonoids: UV-absorbing compounds.

Importance of Upright Growth

• Better access to sunlight.

Origin of Vascular Tissue

• Elongated cells organized into tissues.
• Cellulose-containing cell walls.
• Lignin: Strong polymer for structural support.

Reproduction in Dry Conditions

• Spores encased in sporopollenin.
• Gametes produced in multicellular structures.
• Embryos retained on and nourished by the parent plant.

Alternation of Generations

• Multicellular haploid (gametophyte) alternates with multicellular diploid (sporophyte).
• Zygotes and spores are both single cells that divide by mitosis to form a multicellular individual. Zygotes develop into sporophytes; spores develop into gametophytes.
• Zygotes are diploid and spores and gametes are haploid.
• Spores are produced by meiosis inside sporangia; gametes are produced by haploid inside gametangia
• Nonvascular plants: Gametophyte-dominant.
• Vascular plants: Sporophyte-dominant.
• Gymnosperms/Angiosperms: Microscopic gametophytes.
• Pollen: Tiny male gametophytes with sporopollenin coat.
• Seed: Embryo + nutrients + protective coat.

Pollination

• Flowers: Stamens and carpels enclosed by sepals and petals.
• Directed-pollination hypothesis: Natural selection favors structures that reward animals for carrying pollen.
• Coevolution: Angiosperm diversity from coevolution with animal pollinators.
• Fruits: Derived from the ovary, enclosing seeds.

Green Algae

• Paraphyletic group of ~8000 species.
• Primary producers in freshwater habitats and unusual environments (snowfields, ice floes).
• Live in association with other organisms (endosymbionts in planktonic protists, lichens).

Key Lineages of Green Algae

• Ulvophyceae (Ulvophytes): Marine/freshwater, sexual/asexual reproduction.
• Charophyceae (Stoneworts): Freshwater, multicellular, sexual/asexual reproduction, closely related to land plants.
• Coleochaetophyceae (Coleochaetes): Freshwater, multicellular, sexual/asexual reproduction, closely related to land plants.
• Zygnematophyceae (Conjugating algae): Freshwater, unicellular/multicellular, sexual/asexual reproduction (conjugation), closely related to land plants.

Nonvascular Plants

• First lineages to branch off the land plant phylogeny.
• Gametophyte-dominant.
• Anchored by rhizoids.
• Lack vascular tissue.
• Flagellated sperm.
• Spores dispersed by wind.

Key Lineages of Nonvascular Plants

• Hepaticophyta (Liverworts): Liver-shaped leaves, pores similar to stomata, sexual/asexual reproduction.
• Bryophyta (Mosses): Can withstand extreme drying, some have conducting tissues, sexual/asexual reproduction, form peat.
• Anthocerophyta (Hornworts): Sporophytes have stomata, sexual/asexual reproduction, some have symbiotic cyanobacteria.

Seedless Vascular Plants

• Paraphyletic group.
• Vascular tissue with lignin.
• Sporophyte-dominant.
• Gametophyte independent.
• Eggs retained, flagellated sperm.

Key Lineages of Seedless Vascular Plants

• Lycophyta (Lycophytes): Most ancient lineage with roots, sexual/asexual reproduction, some show heterospory.
• Psilophyta (Whisk ferns): Branching stems only, sexual/asexual reproduction, symbiotic fungi.
• Pteridophyta (Ferns): Common in moist habitats, large leaves (fronds), sexual reproduction.
• Equisetophyta (Horsetails): Hollow stems, reproductive stems with sporangia, sexual/asexual reproduction.

Seed Plants: Gymnosperms and Angiosperms

• Monophyletic group.
• Produce seeds and pollen grains.
• Angiosperms: Seeds in ovaries.
• Gymnosperms: Seeds not in ovaries.

Key Lineages of Gymnosperms

• Ginkgophyta (Ginkgoes): Deciduous, separate male and female plants, wind-carried pollen.
• Cycadophyta (Cycads): Resemble palms, separate male and female plants, wind-carried pollen, roots with symbiotic cyanobacteria.
• Cupressophyta (Redwoods, junipers, yews): Small leaf surface area, separate male and female cones, seeds dispersed by wind, birds, or mammals.
• Pinophyta (Pines, spruces, firs): Needle-like leaves, separate male and female cones, wind pollinated.
• Gnetophyta (Gnetophytes): Vines, trees, or shrubs, closely related to angiosperms, wind or insect pollen transfer, double fertilization.

Key Lineages of Angiosperms

• Basal Angiosperms: Oldest living lineages, share features with both monocots and eudicots.
• Monocotyledons (Monocots): Single seed leaf, flower parts in multiples of three, parallel leaf veins.
• Magnoliids: Large trees, shrubs, or vines, large flowers, net-veined leaves.
• Eudicotyledons (Eudicots): Two seed leaves, pollen grains with three grooves.