Lecture 24


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Lecture 24

  1. 1. Lecture 24: Plant Reproduction & Development Covers Chapter 44
  2. 2. • AS mentioned in Lecture 22, what separates plants from their ancestors is the multicellular embryo. • Think of it as being very similar to humans….sperm meets egg, fertilization, development into a new organism AND, importantly, the new plant embryo develops ON the parent plant (attached to and dependent upon…just like us!)
  3. 3. Plant Sexual Life Cycle is complex • Called alternation of generations: sexual life cycle alternates between diploid and haploid stages that give rise to each other – The diploid form is called a sporophyte (this is the plant body that you see) – The haploid form is called a gametophyte (not a separate plant, but a part of the cycle that involves only the reproductive parts of the plant) Sporophyte/gametophyte are not names for the plants themselves, just the STAGE of the life cycle!
  4. 4. Plant Sexual Life Cycle
  5. 5. • Before we go over the process, let’s take a close look at the flower
  6. 6. Closeup on the flower* • Sexual reproductive structures of angiosperms (they are actually modified leaves): – Sepals: green leaves at base of flower – Petals: usually colorful (ATTRACT POLLINATORS) – Sporangia: sex organs • Stamen: male reproductive structure – Anther: area where pollen is produced – Filament: stalk that holds anther • Carpel: female reproductive structure – Stigma: sticky structure at top of carpel – Style: tube connecting stigma and ovary – Ovary: bulb-like structure at bottom of style that contains one or more eggs/ovules (Fruit is a mature ovary with seed inside!)
  7. 7. sepal ovules petal filament style stigma anther ovary carpel stamen (a) A representative dicot flower A Complete Flower Fig. 44-2
  8. 8. Male/female gametes* • Male gamete: SPERM/POLLEN/POLLEN GRAIN • Hundreds of thousands in each anther • Each pollen grain is TWO sperm and a tube cell surrounded by a tough, waterproof covering • One sperm will fertilize female egg, other will contribute to endosperm: food storage tissue that nourishes the embryo. • Female gamete: egg/ovule – Egg is the actual gamete – Egg ENCLOSED in a nutritive food layer (for future embryo) is called an OVULE – Each ovary can hold one or many ovules, depending on type of plant – Each ovule has numerous eggs in it.
  9. 9. How are gametes formed? • Each diploid cell in ovule & anther will do meiosis FIRST, producing haploid spores. – Male: Each spore will then do one round of mitosis, resulting in two gametes enclosed in a pollen grain – Female: Three of four eggs from meiotic division will degenerate, leaving one egg. This egg will do 3 rounds of mitosis, and result will be one egg that will be fertilized and all others will contribute to endosperm.
  10. 10. Female gamete formation
  11. 11. Male gamete formation
  12. 12. Pollen Grains Fig. 44-5
  13. 13. Pollinators & Plants • Animals and insects help plants to reproduce • Pollinators will transport pollen from one plant to another. • Flowers have evolved to attract pollinators and pollinators have co-evolved to be more effective at getting pollen! – Some flowers look like insects (to draw insects in) – Some flowers provide a place for insects to lay eggs – Some flowers have evolved structures to make it easier for pollinators to access pollen – Pollinators have evolved body structures that allow best access to pollen
  14. 14. “Pollinating” a Pollinator Fig. 44-14
  15. 15. Sexual Deception Promotes Pollination Fig. 44-16
  16. 16. The process!* • Sporophyte (diploid) plant grows and matures, developing male AND female sporangia ON THE SAME PLANT. • Cells in anther and ovary perform meiosis (making spores) and then spores do mitosis, resulting in pollen grains and ovule(s) with eggs inside. – THIS IS THE GAMETOPHYTE STAGE! • Pollen grain (from same plant or perhaps another) lands on stigma. • Tube cell in pollen grain burrows down the style (making a pollen tube) and merges with egg in the ovule. • Result is diploid zygote. • Zygote develops into an embryo: a seed!
  17. 17. Pollination and Fertilization of a Flower Fig. 44-8 pollen tube tube cell nucleus sperm sperm tube cell nucleus ovary central cell egg ovule pollen grain Pollination occurs when a pollen grain lands on the stigma of a carpel 1 A pollen tube grows down through the style of the carpel to the ovary; the tube cell nucleus travels at the tip of the tube, and the two sperm follow close behind 2 Double fertilization: One sperm fuses with the central cell One sperm fuses with the egg cell 3
  18. 18. The process-continued* • Seed contains embryo (diploid organism that is developing-but only so far) and endosperm- nutritive layer made of unfertilized eggs and extra sperm from pollen grain. • Seed will be dropped from plant, blown by the wind, picked up by an animal, or become encased in the ovary (fruit) depending on plant type. • Once seed “lands” in an area appropriate for growth, it will germinate. • We have a new sporophyte!!
  19. 19. Angiosperm Life Cycle Craig Savage You Tube
  20. 20. Seeds: Hooks & Spikes • You Tube: Global Zoo
  21. 21. Seeds • You tube: Global Zoo
  22. 22. Germination: details • Germination: embryo breaks out of seed and forms a seedling. This happens after a dormant period – Dormant period keeps seeds from breaking out of ripe fruit (dormant period ends after fruit rots/dries out) – Three criteria determine period of dormancy:* • *Drying: some seeds must be dried out before they will germinate • *Cold: some seeds require exposure to cold for a period of time, then warmer temps before germination • *Seed Coat Disruption: some seeds require the coat be disrupted before germination
  23. 23. Fruits • Fruit is a ripe ovary with seeds inside. • Packaging the embryo in fruit allows embryo to get far away from the mother plant…..better to spread DNA! Genetic diversity! • Only ripe fruit containing mature seeds are sweet and brightly colored • Some fruits: – Explode and disperse seeds – Float in water to other areas – Float in the wind (really lightweight fruit) – Have evolved to be edible and attract herbivores (eat fruit, travel, poop out seeds)
  24. 24. Some fruit have seeds on outside
  25. 25. Others have seeds on inside