1. Plant Reproduction
AP Biology
Unit 5

2. 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

3. 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

4. Alternation of Generation

5. 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

6. 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

7. Gametophyte/Sporophyte Relationships
3 different possibilities
Larger gametophyte, sporophyte is dependent on it
Larger sporophyte, gametophyte is independent
Larger sporophyte, gametophyte is dependent

8. 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

10. Reproductive Organs Archegonium Antheridium
Female sex organ  produces egg
Antheridium
Male sex organ  produces sperm

11. 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

12. 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.

13. 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)

14. 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

15. Angiosperms Dominant form is the sporophyte Heterosporous
Male gametophyte = pollen grain
Female gametophyte = embryo sac

16. 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

17. When the pollen grain lands on the stigma, it will extend a pollen tube down the style to get to the egg
Specific cells in the pollen tube go through mitosis to form 2 sperm
Sperm will enter

18. 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)

19. 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

20. Other Post-Fertilization events
Embryo develops (diploid)
Seed coat forms
Ovary develops into fruit around the seed

21. 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 imbibition—when water enters the seed and triggers enzymes that are needed for growth.

22. 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)

23. Monocots vs. Eudicots
Most angiosperms can be classified as either Monocots or Eudicots