lecture 6

• Seed-free plants


  • Major roles in carbon source/sink dynamics both over Earth's history and in the present
• Plant reproductive anatomy evolution in the lineage leading to gymnosperms

  Innovations that took place
• Seeds and pollen


  Critical steps in plant evolution
• Geologic changes


  Favored gymnosperms over seed-free plants during the time of the dinosaurs
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• Embryophytes (Land Plants)


  • Nonvascular plants (bryophytes)
  • Vascular plants (seedless)
  • Seed plants
• After land plants took off, there was another bump in O2 levels by 1-2 orders of magnitude

  
• Rise in O2 levels due to photosynthesis


  CO2 is being fixed

  
• Plants in the Carboniferous Period


  • Lycophyte trees
  • Horsetail
  • Fern
• Plants in the Carboniferous Period


  • The first time plants get TALL!
• Carbon dioxide sinks expanded by plants in the Carboniferous Period


  1. Root acids weather continental rock, releasing calcium and magnesium that react with CO2 to form minerals

  2. Boggy, waterlogged, anoxic soils lead to slow, incomplete decay of dead trees to become peat, which is converted to coal over millions of years
• Lower CO2 levels


  Big drop in global temperature

  
• The Carboniferous Period led to a long, extended period with polar ice caps - an "Icehouse Earth" Period

  
• Azolla


  • Common names: fairy moss, mosquito fern, water fern
  • Aquatic and very reduced in form compared to other ferns
  • Forms a symbiotic association with nitrogen-fixing cyanobacteria that it makes a home for within pouches in its leaves
• Azolla use in organic rice farming in Southeast and South Asia


  • Ducks eat weeds and insects and Azolla
  • Azolla fixes nitrogen
  • Duck poop and dead Azolla fertilize soil
  • Higher rice yields with no chemical input
• Azolla


  • Doubles its biomass in 1.9 days, faster than some bacteria
  • About 50 mya in the middle Eocene, the continents were clumped such that the Arctic ocean was enclosed and cut off from broader currents, leading to a thin freshwater layer over this ocean that huge mats of Azolla covered
  • Dead Azolla mats accumulated as stagnant sediment for ~800,000 years, drawing 80% of CO2 out of atmosphere and sinking it to ocean floor, becoming Alaskan oil deposits

  
• "Seed-free" plants have been responsible for starting TWO "Icehouse Earth" periods by creating new mineral or organic carbon sinks, with lots of biomass and environmental conditions unfavorable for decay being critical to both events. We've been burning up those fossil fuel sinks as well as peat and rapidly heating our atmosphere today.
• Gymnosperms


  • gymnos = "naked", sperms = "seed"
  • Only ~1000 species today, but much more diverse in the past
  • Include gnetophytes, cycads, gingko, and (mostly) conifers
  • Seed plants do not rise initially, have to wait until favored ~250-280 mya when there was a big change in climate in their favor that "drained the swamp"
  • Heyday in the Jurassic, and still dominate certain biomes like boreal forests (taiga) and alpine forests
• Gymnosperm types


  • Gnetophytes
  • Cycads
  • Gingko
  • Conifers
• Gymnosperm trees


  • Tallest Trees: Coast Redwood
  • Biggest Trees: Giant Sequoia
  • Oldest Trees: Bristlecone Pine
• Rise of the seed plants

  1. After the Carboniferous, seed plants come to dominate the landscape

  2. Rise comes ~250-280 mya when there was a big change in climate in their favor that "drained the swamp" - formation of Pangea and uplift -> more arid land area
• Pollen


  Solution to Problem 4 of Adapting to Life on Land (Fertilization)
• Cones


  • Gymnosperm reproductive structures
  • Anatomically akin to compressed fern fronds or lycophyte strobili, with scales that are modified leaves with sporangia (sporophylls)
• Heterospory in gymnosperms


  1. Megasporangia make megaspores that germinate to become the female gametophyte

  2. Microsporangia make microspores that germinate to become the male gametophyte (pollen)

  3. Each sporangium type is produced by a different type of cone (either on same or different tree)

  4. Spores stay put in sporangia
• Foot


  • Embryo
  • Archegonium
  • 2 mm
  • Zygote
  • (2n)
  • (within archegonium)
• Archegonia


  • Female
  • gametophyte
  • (n)
  • Protonemata
  • (n)
  • Rhizoid
  • Sporangium
• Mature sporophytes


  • Capsule (LM)
  • Female gametophytes
  • Young sporophyte
  • (2n)
• Reminder: only one type of spore in seed-free plant life cycles
• Ovule


  • Mature sporophyte
  • (2n)
  • Ovulate cone
  • Integument
  • Pollen cone
  • Microsporocytes
  • (2n)
  • MEIOSIS
  • Microsporangia
  • Seedling
  • Megasporocyte (2n)
  • Megasporangium
  • (2n)
  • Germinating pollen grain
  • Microsporangium (2n)
  • Surviving megaspore (n)
  • Archegonium
  • Female gametophyte
  • Seeds
  • Food reserves
  • Seed coat (2n)
  • Sperm nucleus (n)
  • Pollen tube
  • Egg nucleus (n)
  • Haploid (n)
  • Diploid (2n)
  • Embryo (new sporophyte)
  • (2n)
  • Pollen grains (n)
  • MEIOSIS
  • FERTILIZATION
• What's in a cone? Depends which cone
• Pollen cone


  • Microsporocytes
  • (2n)
  • Pollen grains (n)
  • MEIOSIS
  • Microsporangia
  • Mature sporophyte
  • (2n)
  • Microsporangium (2n)
• Previously: A sporophyte (2n) makes sporophylls (2n) that hold sporangia (2n) that make spores (n) by meiosis, which disperse and germinate to develop into gametophytes (n)


  Adapted: The sporophyte (2n) makes pollen cones made up of sporophylls (scales, 2n) that hold microsporangia (2n) make microspores (n) by meiosis that germinate to develop into pollen grains (n)
• Key Innovation #1


  • Microspores not dispersed
• Key Innovation #2


  • Free-living male gametophyte moves by wind not water! (Solves Problem 4 of Life on Land - Fertilization)
• Pollen is encased in sporopollenin. Has undergone 2 cell divisions. No more motile sperm and no antheridia.
• Ovulate cone


  • Integument
  • Microsporocytes
  • (2n)
  • MEIOSIS
  • Microsporangia
  • Seedling
  • Megasporocyte (2n)
  • Megasporangium
  • (2n)
  • Germinating pollen grain
  • Microsporangium (2n)
  • Surviving megaspore (n)
  • Archegonium
  • Female gametophyte
  • Seeds
  • Food reserves
  • Seed coat (2n)
  • Sperm nucleus (n)
  • Pollen tube
  • Embryo
  • (2n)
  • Pollen grains (n)
  • MEIOSIS
  • FERTILIZATION
• Immature ovulate cone


  • Integument (2n)
  • Megaspore (n)
  • Spore wall
  • Megasporangium
  • (2n)
  • Pollen grain (n)
• Ovule = megasporangium surrounded by the protective structure that will become the seed coat (the integument)