Plant reproduction involves an alternation of generations between haploid and diploid phases. In seed plants like angiosperms, the sporophyte phase is dominant. During reproduction, the male gametophyte (pollen grain) fertilizes the female gametophyte inside the ovule. This triggers double fertilization, where one sperm cell fuses with the egg to form a zygote embryo, and another sperm fuses with polar nuclei to form triploid endosperm tissue to nourish the developing seed.

1. Chapter 4.
Plant Reproduction
KEY CONCEPT
All plants alternate between two phases in their life
cycles.
AP Biology 2005-2006

2. Plant life cycles alternate between
producing spores and gametes.
 A two-phase life cycle is called alternation of
generations.
 haploid phase
 diploid phase
 alternates SPOROPHYTE
PHASE
between fertilization
fertilization
the two GAMETOPHYTE
meiosis
meiosis
PHASE
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3. Animal vs. Plant life cycle
Animal Plant
multicellular multicellular
sporophyte
2n
2n
gametes spores
1n 2n
unicellular multicellular
gametes gametophyte
1n 1n
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alternation of generations

4. Alternation of generations
diploid
haploid
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5. Evolutionary trends
 Alternation of generations
 dominant haploid plant
 bryophytes - mosses
 dominant diploid plant
 pteridophytes - ferns
 gymnosperm - conifers
 angiosperm - flowering plants
 Evolutionary advantage?
 reduction of gametophyte protects delicate
egg & embryo in protective sporophyte
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6. Gametophytes of seed plants
Gymnosperm Angiosperm
 male gametophyte  male gametophyte
 pollen in male cone  pollen in anthers of
flower
 female gametophyte  female gametophyte
 develops in female  develops in ovaries
cone of flower
 seed  seed
 naked in cone  protected in ovary
 ovary wall can
develop into fruit
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7.  The sporophyte is the dominant phase for
seed plants.
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8. Gymnosperm life cycle
female
gametophyte
in cone
male
gametophyte
in pollen
sporophyte
in seed
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9. Angiosperm life cycle
male
gametophyte
in pollen
female
gametophyte
in ovary
sporophyte
in seed
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10. Reproduction in angiosperm
 Sporophyte plant produces unique reproductive
structure = the flower
 male gametophyte = pollen grain
 develop within anthers of flower
 female gametophyte = embryo sac
 develop within ovaries of flower
 pollination by wind or animals brings pollen grain to
female gametophyte
 fertilization takes place within ovary
 double fertilization = embryo & endosperm
 seeds contain sporophyte embryo
 development of seeds in ovary
 ovary develops into fruit around the seed
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11. Flowers
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12. Flower
 Modified shoot
with 4 rings of
modified leaves
 sepals
 petals
 stamens
 male
 carpals
 female
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13. Male & female parts of flower
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14. Parts of flower
 Male
 stamens = male reproductive organs
 stamens have stalks (filament) &
terminal anthers which carry pollen
sacs
 pollen sacs produce pollen
 pollen grain = gametophyte
 sperm-producing structure
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15. Parts of flower
 Female
 carpels = female reproductive organs
 ovary at the base
 slender neck = style
 within the ovary are 1 or more ovules
 within ovules are embryo sacs
 female gametophyte = embryo sac
 egg-producing structure
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16. Fertilization in Angiosperms
 When pollen grain lands on stigma it
begins to grow pollen tube.
 Nucleus within pollen grain divides and
forms 2 sperm nuclei
 Pollen tube contains tube nucleus and
2 sperm nuclei
 Pollen tube grows into style and
eventually reaches ovary and enters
ovule
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17. Fertilization in Angiosperms
 Inside embryo sac, two fertilizations
occur
 One sperm nuclei fuses with egg nucleus
to produce diploid zygote – grows into
plant embryo
 Second sperm nucleus fuses with two
polar nuclei to form triploid (3N) cell –
food tissue known as endosperm.
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18. stigma
embryo
sac
pollen
tube
endosperm
(3n)
ovary zygote (2n)
ovule micropyle
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19. Fertilization (recap)
 Pollination
 pollen released from anthers is carried by wind
or animals to land on stigma
 pollen grain produces a pollen tube
 pollen tube grows down style into ovary & discharges
2 sperm into the embryo sac
 1 sperm fertilizes egg = zygote
 zygote develops into embryo
 ovule develops into a seed
 ovary develops into a fruit containing
1 or more seeds
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20. Pollination “syndromes”
Butterfly Hummingbird
Generalist
Hawk moth
Bee
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21. Self-pollination
Why?
Guarantees seeds (no need for mates or
pollinators)
May save resources if flowers are smaller
Maximizes transmission of own genes
Why not?
Inbreeding depression - short-term cost
Loss of genetic diversity - long-term cost
An evolutionary dead end? - very long-term
cost
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22. Preventing self-pollination
 Various mechanisms
 stamens & carpels may mature at different times
 arranged so that animal pollinator won’t transfer
pollen from anthers to stigma of same flower
 biochemical self-incompatibility = block pollen
tube growth
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23. Fertilization takes place within the
flower.
Male gametophytes, or pollen grains, are produced in
the anthers.
– male spores produced in
anthers by meiosis
– each spore divides by
mitosis to form two
haploid cells
– two cells form a
single pollen grain
pollen grain
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24. One female gametophyte can form in each ovule of a
flower’s ovary.
 four female spores produced in ovule by
meiosis
 one spore develops into female
gametophyte
 female gametophyte contains seven
cells
 one cell has two nuclei, or polar nuclei
 one cell will develop into an egg
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25.  Pollination occurs when a pollen grain lands
on a stigma.
pollen tube
pollen tube
sperm
sperm
stigma
stigma
– one cell from pollen grain forms pollen tube
– other cell forms two sperm that travel down tube
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26.  Flowering plants go through the process of
double fertilization.
female
gametophyte
egg
sperm
polar nuclei
ovule
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27. Flowering plants go through the process of double
fertilization.
endosperm
– one sperm fertilizes
the egg seed coat
– other sperm unites
with polar nuclei,
forming endosperm
– endosperm provides
food supply for
embryo
embryo
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28.  Each ovule becomes a seed.
The surrounding ovary grows into a fruit.
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29. Fertilization in flowering
plants
 Double fertilization
 2 sperm from pollen
 1 sperm fertilizes
egg = diploid zygote
 1 sperm fuses with
2 polar nuclei to
form 3n endosperm
 endosperm = food
tissue in seed
 coconut milk
 grains
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30. Fertilization in flowering plants
 Development of the new sporophyte
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31. Plant embryo
endosperm
seed coat
cotyledons
ovary wall
embryo
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32. Fruit
 Fruit is a mature ovary
 seeds develop from ovules
 wall of ovary thickens to form fruit
 fruits protect dormant seeds &
aid in their dispersal
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33. Seed dispersal
• Why disperse?
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34. Seed dispersal
• Why disperse?
• sample more (better?) sites for
germination/growth
• avoid predation or disease
• avoid competition
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35. Seed dispersal
 Plants produce enormous numbers of seeds to
compensate for low survival rate
 a lot of genetic variation for natural selection to screen
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36. Seeds and Seed Germination
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37. Seed structure
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38. Seed
Dormancy
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39. Dormancy
 Metabolism falls
 Number of organelles per cell falls
 Dehydration – water content falls
 Vacuoles in cells deflate
 Food reserves become dense
crystalline bodies
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40. Maintaining dormancy
 Physical barriers
The seed coat (testa) is waxy =
waterproof and impermeable to oxygen
 Physical state – dehydrated
 Chemical inhibitors present e.g. salts,
mustard oils, organic acids, alkaloids
 Growth promoters absent
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41. Seed viability
 Viability: When a seed is capable of
germinating after all the necessary
environmental conditions are met.
 Average life span of a seed 10 to 15 years.
 Some are very short-lived e.g. willow (< 1
week)
 Some are very long-lived e.g. mimosa 221
years
 Conditions are very important for longevity
 Cold, dry, anaerobic conditions
 These are the conditions which are
maintained in seed banks
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42. Germination: The breaking of dormancy
The growth of the embryo and its penetration of the seed
coat
Break down of barriers
Abrasion of seed coat
(soil particles)
Decomposition of seed
coat (soil microbes, gut
enzymes)
Cracking of seed coat Change in physical
(fire) state - rehydration
Destruction and dilution
of inhibitors
Light, temperature, water
Production of growth
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43. Seeds begin to grow when
environmental conditions are favorable.
 Seed dormancy is a state in which the
embryo has stopped growing.
– Dormancy may end
when conditions
are favorable.
– While dormant,
embryo can
withstand extreme
conditions.
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44. Seed
Germination
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45.  Germination begins the growth of an embryo
into a seedling.
– water causes seed to swell and crack coat
– embryonic root, radicle, is first to emerge
– water activates enzymes that help send sugars to
embryo
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46.  Germination begins the growth of an
embryo into a seedling.
 water causes seed to swell and crack coat
 embryonic root, radicle, is first to emerge
 water activates enzymes that help send
sugars to embryo
– embryonic shoot, plumule, emerges next
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47.  Germination begins the growth of an
embryo into a seedling.
 water causes seed to swell and crack coat
 embryonic root, radicle, is first to emerge
 water activates enzymes that help send
sugars to embryo
 embryonic shoot, plumule, emerges next
– leaves emerge last
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48.  Once photosynthesis begins, the plant is called
a seedling.
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49. Germination
STAGE EVENTS
PREGERMINATION (a) Rehydration – imbibition of water.
(b) RNA & protein synthesis stimulated.
(c) Increased metabolism – increased respiration.
(d) Hydrolysis (digestion) of food reserves by
enzymes.
(e) Changes in cell ultra structure.
(f) Induction of cell division & cell growth.
GERMINATION (a) Rupture of seed coat.
(b) Emergence of seedling, usually radicle first.
POST GERMINATION (a) Controlled growth of root and shoot axis.
(b) Controlled transport of materials from food stores
to growing axis.
(c) Senescence (aging) of food storage tissues.
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50. Any Questions??
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51. Aaaaah…
Structure-Function
yet again!
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