Overview
This lecture covers the evolutionary history, major adaptations, and classification of land plants, emphasizing their transition from aquatic ancestors and key features like alternation of generations and nonvascular plant life cycles.
Evolution and Major Adaptations of Land Plants
- Land plants (embryophytes) evolved from green algae about 475 million years ago.
- Moving to land required adaptations to prevent desiccation and support reproduction outside water.
- Key adaptations include roots for water/mineral absorption, stems for support, and stomata for gas exchange.
- Chemical adaptations like waxy cuticles (secondary products) reduce dehydration.
- Lignin strengthened cell walls for upright growth; sporopollenin protects spores from drying out.
- Gametangia (antheridia: male, archegonia: female) evolved to keep gametes and embryos moist.
- Embryos develop within female gametangia, which is why land plants are called embryophytes.
Alternation of Generations
- All plants display alternation of generations: haploid gametophyte (n) and diploid sporophyte (2n).
- Gametophytes produce gametes by mitosis; sporophytes produce spores by meiosis.
- Unlike animals, plant gametes form by mitosis, not meiosis.
- Spores can develop directly into new organisms, while gametes must fuse during fertilization.
Evolutionary Milestones in Land Plants
- Four major periods: origin from aquatic ancestors, evolution of vascular tissue, diversification of seedless vascular plants, and evolution of seeds (gymnosperms and angiosperms).
- Seeds protect embryos and allow dormancy until favorable conditions arise.
- Angiosperms (flowering plants) have seeds enclosed within ovaries; gymnosperms have "naked" seeds on cones.
- Early land plants likely adapted to shallow, seasonally dry habitats before fully colonizing land.
Classification and Features of Nonvascular Plants (Bryophytes)
- Nonvascular plants (bryophytes) include liverworts (hepatophyta), mosses (bryophyta), and hornworts (anthocerophyta).
- Bryophytes lack true vascular tissue and rely on diffusion and osmosis for transport.
- Bryophyte life cycles are dominated by the haploid gametophyte generation.
- Mosses have rhizoids for anchoring and reproduce in moist environments using flagellated sperm.
- The sporophyte generation is dependent on the gametophyte for nutrition.
- Spore dispersal occurs through specialized capsule structures with peristome teeth and operculum lids.
- Protonema is the haploid, filamentous spore germination stage resembling green algae.
Key Terms & Definitions
- Embryophytes — land plants whose embryos develop within female gametangia.
- Gametangia — organs that protect gametes; antheridia (male), archegonia (female).
- Sporopollenin — tough polymer protecting spores/pollen from desiccation.
- Alternation of Generations — plant lifecycle alternating between haploid gametophyte and diploid sporophyte.
- Bryophytes — nonvascular plants: mosses, liverworts, hornworts.
- Protonema — the haploid, filamentous, early stage of moss gametophyte development.
Action Items / Next Steps
- Learn the bryophyte lineages and their scientific names.
- Be able to distinguish dominant generations in plant life cycles.
- Review alternation of generations and differences between spores and gametes.