Chapter 39

Reproduction in plants

• Most flowering plants display sexual reproduction
  • Two gametes fuse to produce offspring with a unique combination of genes
• They undergo Alternation of Generations 
  • Two multicellular life cycle stages
  • diploid
    • Spore producing sporophyte
      • produces spores by meiosis
        • a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
  • haploid
    • Gamete producing gametophyte
      • produces gametes by mitosis
        • a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
• Egg is Female
• Sperm is Male

Evolutionary Trends in the Plant Kingdom

• Sporophyte has become larger, more complex
  • Flowering plants
    • Sporophyte independent 
    • Dependent gametophyte is only a few cells contained within flowers
• Gametophyte has become smaller, less complex
  • Moss
    • Sporophytes small and dependent on gametohyte (Dominant form)
• Female
  • 7 cells
• Male
  • 2-3 cells

Flower and Sexual Cycle

• Flowers
  • ONLY in angiosperms
  • All sizes, shapes, colors, and aromas
• Essential process of Sexual reproduction occurs within flowers
  • Meiosis/cytokenesis
    • reduces chromosome number
  • Syngamy (fertilization)
    • restores chromosome number

"Ideal" Flower

• Uses highly modified leaves arranged in whorls (circular) at the tip of a highly modified stem
• A flower is a highly modified determinate (short term) shoot system

• Pedical, receptical, 4 sets of highly modified leaves are all 2N and part of the sporophyte generation
• Pollen (sperm) and eggs of embryo sac are part of the 1N generation
• Pedical
  • flower stalk
• Recepticle
  • tip of modified stem with 4 whorls attached

Sexual Cycle

Male

• Pollen formation
  • occurs within the anther of stamen
• Anther
  • Bilobed with 2 pollen chambers per lobe

1. 2N microspore mother cell
2. meiosis/cytokenesis
3. 4 1N microspores
4. Each: mitosis/cytokenesis
   unequal and incomplete
5. 1N Generating cell
   1N Tubecell

Male Gametophyte

Pollination

• Transfer of pollen from the anther to the stigma
• Self-pollination
  • Transfer with the same flower or between flowers on the same plant
    
    • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •  
• Cross-Pollination
  • Transfer between  flowers of other plants

Pollinating Agents

Mechanisms utilized for transfer of pollen

1. Wind
   • small/lightweight pollen
2. Water
   • Transfer with a few aquatic plants
3. Animals
   • Majority of plants
   • Utilized as a "trick and reward" system
   • nectar, colors, and aromas to attract animals

Female

Ovule Development

• Ovule
  • future seed
• Enclosed within the ovary of pistol  (carpel)
• One to many ovules per ovary
  • ovary will become fruit
• Ovule attached to central axis or to wall of hollow fruit
  • always enclosed
  • angiosperms
• within ovule is 1 large 2N cell
  • megaspore mother cell

1. 2N megaspore mother cell
2. meisos/sytokenesis
3. 4 1N Megaspores
4. 3 degrade
5. 2N Functional megaspore
6. Series of 3 mitosis/cytokenesis cycles
   Incomplete and unqueal
7. 7-celled embryo sac
   8 nuclei

Female gametophyte

• 1N Functional megaspore
  • 3 mitosis/cytokenesis divisions
• One cell with 1 nucleus becomes 8 nuclei but only 7 cells

Embryo sac

• 8 nuclei, 7 cell structure
• female gametophyte
• 3 antipodal cells (1N)
  • opposite end from micropyle
• 1 central cell with 2 large 1N polar nuclei
• 2 Synergids (1N)
  • Micropyle end on outside
• 1 egg (1N)
  • Middle at micropyle end

Syngamy (fused gametes)

• 1N egg + 1N sperm = 2N zygote (single fertilized egg)
• Pollen grain germination
  • tube cells form pollen tube (delivers sperm)
  • generative cell divides by mitosis/cytokenesis to produce 2 sperm

• Pollen tube enters micropyle
  • digests tube cell nucleus
• Pollen tube enters one synergid
  • releases it's content (sperm)
  • synergid ruptures
  • mycropyle closes

• "Double fertilization" (double fusion)
  • 1N egg +1N sperm = 2N zygote
  • 1N sperm +2 1N polar nuclei = 3N primary endosperm cell

• Post fertilization with ovule
• 2N zygote grows by mitosis/cytokenesis into 2N multicellular embryo
• 3N primary endosperm cell grows by mitosis/cytokenesis into 3N multicellular endosperm
  • nutrient tissue for embryo

• Ovule/ovary with 2N zygote  mature/enlarges with sugars/H2O into a fruit (mature ovary) with enclosed seeds (mature ovules)
• Seed dispersal (seeds enclosed withing a fruit)
• agents
  • wind
  • water
  • animals - majority

Seed germination

• Seed with 2N embryo enters period of dormancy
• dormancy broken by a combination of internal (hormones) and external factors (environmental)
• radical (first root) emerges and grows down
• shoot emerges and grows up