Plant Reproduction

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Plant Reproduction
AP Biology Unit 5
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Alternation of Generations

• All land plants can exist in two different
  multicellular forms
• Sporophyte (2n)
• Gametophyte (n)
• In a life cycle, plant generations alternate
  between sporophyte and gametophyte
• Each form gives rise to the other form

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Sporophyte vs. Gametophyte

• Sporophyte (2n)
• Diploid, multicellular form of a plant
• Forms haploid spores through meiosis
• Gametophyte (n)
• Haploid, multicellular form of a plant
• Forms gametes through mitosis

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Alternation of Generation
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How is this different from other organisms?

• In other organisms (like humans) the haploids are
  unicellular (sperm and egg)
• In plants both forms (sporophyte and gametophyte)
  can be multicellular

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Evolution of Plant Reproduction

• Each type of plant has a dominant form
  (sporophyte or gametophyte)
• Dominant form the plant we think of
• As plants have evolved, there have been changes
  to relationship between sporophyte and
  gametophyte

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Gametophyte/Sporophyte Relationships

• 3 different possibilities
• Larger gametophyte, sporophyte is dependent on it
• Larger sporophyte, gametophyte is independent
• Larger sporophyte, gametophyte is dependent

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Question

• What trend do you see between the evolution of
  plants (from nonvascular ? vascular nonseed ?
  seed plants) and the alternation of generations?
• As plants have evolved, the gametophyte has
  become more reduced, sporophyte has become more
  dominant

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Reproductive Organs

• Archegonium
• Female sex organ ? produces egg
• Antheridium
• Male sex organ ? produces sperm

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Vascular Plants Homosporous vs. Heterosporous

• Vascular plants have sporophylls
• modified leaves that produce sporangia (which
  become spores)
• Homosporous plants
• Most ferns
• Heterosporous plants
• All seed plants (angiosperms and gymnosperms) and
  some seedless plants

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Water vs. Wind

• Mosses and Ferns require water to help the sperm
  reach the egg.
• Seed plants no longer require water to reproduce
• Pollen (male gametophyte) can be carried by wind,
  animals, etc.

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Gymnosperm Life Cycle

• Sporophyte is the dominant form
• Heterosporous
• Have naked seeds nothing around the seed
• Cones are either male (contain pollen) or female
  (contain ovule)

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Flower Anatomy

• The flower is the reproductive organ of an
  angiosperm
• Female part Carpel
• Consists of the stigma, style, and ovary
• Male part Stamen
• Consists of anther and filament

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Angiosperms

• Dominant form is the sporophyte
• Heterosporous
• Male gametophyte pollen grain
• Female gametophyte embryo sac

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Female Gametophyte

• The megaspore will go through MITOSIS to create a
  series of different cells in the female
  gametophyte
• Cells involved in fertilization are
• Egg (n) haploid
• Polar nuclei (2n) 2 nuclei in one cell

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Double Fertilization

• Angiosperm fertilization actually consists of 2
  separate fertilization events double
  fertilization
• Both sperm in the pollen tube are used
• One sperm fertilizes the egg ? diploid zygote
  (2n) forms
• The other sperm fertilizes the polar nuclei (2n)
  ? forms triploid endosperm (3n)

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Double Fertilization

• Double fertilization is unique to angiosperms
• Through this process, resources are used most
  efficiently since endosperm can only develop in a
  fertilized ovule.
• Endosperm starchy material that serves as
  nutrition for the growing embryo

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Other Post-Fertilization events

• Embryo develops (diploid)
• Seed coat forms
• Ovary develops into fruit around the seed

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Germination and Seedling Formation

• Seeds are dormant until they begin germination
• When conditions are right, seeds will begin to
  germinate to form a seedling.
• Germination starts with imbibitionwhen water
  enters the seed and triggers enzymes that are
  needed for growth.

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Seed Benefits

• What are the evolutionary benefits of seeds?
• Dispersal- embryo can travel far from parent
• Protection- seed coat protects embryo
• Dormancy- can wait until conditions are favorable
  to germinate
• Storage- food for the embryo can be stored inside
  seed (ex. endosperm)

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Monocots vs. Eudicots

• Most angiosperms can be classified as either
  Monocots or Eudicots