BIO

• Ancestors of land plants (Algae) occupied aquatic habitats
• Plants appeared in terrestrial environment

  400 million years ago
• Plants have colonized and become very successful on land
• Terrestrial environments are much more difficult than aquatic (i.e., harsh)
• Aquatic environment


  • Water available everywhere / plentiful; organisms bathed in fluid
  • Water could be used for support against gravity
  • Water was crucial in effecting fertilisation; gametes released into water
• Terrestrial environment


  • Water is hard to find and retain; plants faced with potential desiccation
  • Support system needed or plants had to sprawl unsupported on the ground
  • Plants needed mechanisms to effect fertilisation in absence of water
• Advantages of a terrestrial habitat

  • Light for photosynthesis is available for longer periods, not blocked by turbulent water
  • Carbon dioxide is plentiful in atmosphere and circulates freely, but does not circulate freely in water (dissolved)
  • Competition was minimal at the time of colonisation of land
  • Nutrients are more in soil
• Major problems faced by plants on land

  • Desiccation (drying up)- water loss
  • Obtaining and transporting water and nutrients
  • Support against gravity
  • UV radiation
  • Reproduction
• Adaptations of plants to life on land

  1. Developed cuticle, waxy covering to restrict water loss in dry environment

  2. Developed stomata to allow and regulate gas exchange between the atmosphere and interior of the plant

  3. Developed vascular tissue to transport water and food from source to other parts of plants

  4. Developed roots to acquire water & nutrients and anchor plant

  5. Developed mutualistic association with fungi to promote nutrient uptake

  6. Developed xylem with lignin for support

  7. Developed stems for mechanical strength/support, light-catching organs, and seed/spore dispersal

  8. Developed pectin to hold cells together as plants got taller

  9. Developed pigments to protect against UV light

  10. Developed gametangia to enclose gametes and prevent drying out

  11. Developed protected embryos

  12. Developed thick spore walls containing sporopollenin to prevent drying and resist decay

  13. Developed means of reproducing and dispersing on land
• Alternation of generations


  A type of life cycle that occurs in all plants and some algae, involving two genetically different generations - gametophyte (n) and sporophyte (2n)
• Alternation of generations

  1. Fertilization

  2. Meiosis
• Alternation of generations


  • A multicellular diploid phase alternates with a multicellular haploid phase
  • Consists of two distinct organisms - a gametophyte plant/body (genetically haploid) and a sporophyte plant/body (genetically diploid)
• Gametophyte generation


  1. Produces gametes by mitosis

  2. Two gametes (from different organisms or same organism) fuse to produce a zygote

  3. Zygote develops into a diploid plant of the sporophyte generation
• Sporophyte generation


  1. Produces spores by meiosis

  2. Spores germinate and develop into a haploid gametophyte of the next generation
• Definition of alternation of generations


  A life cycle in plants where a diploid, spore-producing sporophyte generation alternates with a haploid, gamete-producing gametophyte generation
• To proceed from sporophyte (2n) to gametophyte (n) phase


  Meiosis and fertilization must take place
• Fertilization


  Fusion of haploid gametes (n) to double the number of chromosomes (2n)
• Meiosis


  Division process where two sets of chromosomes in diploid parent is reduced to a single set of chromosomes (n)
• Gametes (n) are produced by mitosis, not meiosis
• Spores (n) are produced by meiosis
• Isomorphic alternation of generations


  • Sporophyte and gametophyte generations look exactly the same although they differ genetically (e.g. algae)
• Heteromorphic alternation of generations


  • Sporophyte and gametophyte look different (e.g. ferns)
• Plants have the best of both worlds with alternation of generations
• Meiosis during spore production in sporophyte


  Variations occur through reshuffling of genes, producing many spores with different genetic makeup
• Spores germinating into gametophytes


  Produce gametophytes with different genetic makeup
• Gamete formation in gametophytes by mitosis


  Maintains the genetic identity of gametophytes
• Gametes fertilizing to produce a zygote


  Results in offspring that vary from both parents and from one another
• Variations in the zygote

  Passed on to the new sporophyte, allowing populations to become increasingly better adapted to their environment
• Gradual increase in dominance of sporophyte generation with evolutionary advancement
• In higher plants, the plant is represented by the sporophyte, which is larger, longer living and self-sufficient
• In lower (non-vascular) plants, the plant is represented by the gametophyte, which is larger, longer living and more self-sufficient
• Gymnosperms have seeds that develop outside of an ovary, which is different from angiosperms where seeds develop inside an ovary.
• The nucleus is the control center of the cell, containing genetic material (DNA) that determines an organism's characteristics.
• Genes are segments of DNA that contain instructions for making specific proteins or controlling other genes.
• The seed coat protects the embryo during germination.
• Chromosomes are thread-like structures made up of DNA and proteins found inside the nucleus.
• The term "gymnosperm" comes from Greek words meaning "naked seed".
• Chloroplasts are responsible for photosynthesis, converting sunlight into energy used by cells.
• Conifers are gymnosperms that include pine trees, fir trees, cedars, redwoods, cypresses, junipers, larches, hemlocks, Douglas Fir, and spruces.
• Cycads are gymnosperms that resemble palms or ferns and can be found in tropical rainforests around the world.