AP Biology 2005-2006
Chapter 4.
Plant Reproduction
KEY CONCEPT All plants alternate between two phases in their life cycles.
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Plant life cycles alternate between producing spores and gametes. A two-phase life cycle is called alternation of
generations. haploid phase diploid phase alternates between
the two fertilizationfertilization
meiosismeiosis
SPOROPHYTE PHASE
GAMETOPHYTE PHASE
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Animal vs. Plant life cycle
multicellular2n
multicellularsporophyte
2n
multicellulargametophyte
1n
unicellulargametes
1n
spores2n
gametes1n
Animal Plant
alternation of generations
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diploid
Alternation of generations
haploid
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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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Gametophytes of seed plants
male gametophyte pollen in male cone
female gametophyte develops in female
cone
seed naked in cone
male gametophyte pollen in anthers of
flower
female gametophyte develops in ovaries
of flower
seed protected in ovary ovary wall can
develop into fruit
Gymnosperm Angiosperm
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The sporophyte is the dominant phase for seed plants.
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Gymnosperm life cyclefemale
gametophytein cone
malegametophyte
in pollen
sporophytein seed
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Angiosperm life cycle
femalegametophyte
in ovary
malegametophyte
in pollen
sporophytein seed
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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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Flowers
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Flower Modified shoot
with 4 rings of modified leaves sepals petals stamens
male carpals
female
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Male & female parts of flower
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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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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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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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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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stigma
pollen tube
ovary
ovule micropyle
zygote (2n)
endosperm (3n)
embryo sac
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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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Pollination “syndromes”Generalist HummingbirdButterfly
Hawk mothBee
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Self-pollinationWhy?
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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Preventing self-pollination
Various mechanismsstamens & carpels may mature at different times arranged so that animal pollinator won’t transfer
pollen from anthers to stigma of same flowerbiochemical self-incompatibility = block pollen
tube growth
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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 twohaploid cells
– two cells form asingle pollen grain
pollen grain
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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
One female gametophyte can form in each ovule of a flower’s ovary.
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Pollination occurs when a pollen grain lands on a stigma.
pollen tubepollen tube
spermsperm
stigmastigma
– one cell from pollen grain forms pollen tube– other cell forms two sperm that travel down tube
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Flowering plants go through the process of double fertilization.
femalegametophyte
ovule
egg
sperm
polar nuclei
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endosperm
seed coat
embryo
– one sperm fertilizes the egg
– other sperm unites with polar nuclei, forming endosperm
– endosperm provides food supply for embryo
Flowering plants go through the process of double fertilization.
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Each ovule becomes a seed.
The surrounding ovary grows into a fruit.
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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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Fertilization in flowering plants
Development of the new sporophyte
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Plant embryo
endosperm
cotyledons
embryo
seed coat
ovary wall
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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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Seed dispersal
• Why disperse?
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Seed dispersal
• Why disperse?
• sample more (better?) sites for germination/growth
• avoid predation or disease
• avoid competition
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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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Seeds and Seed Germination
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Seed structure
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Seed Dormancy
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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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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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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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Germination: The breaking of dormancy
The growth of the embryo and its penetration of the seed coat
Break down of barriersAbrasion of seed coat
(soil particles)Decomposition of seed coat (soil microbes, gut
enzymes)Cracking of seed coat
(fire)Change in physical state - rehydration
Destruction and dilution of inhibitors
Light, temperature, waterProduction of growth
promoters
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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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Seed Germination
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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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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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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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Once photosynthesis begins, the plant is called a seedling.
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GerminationSTAGE 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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Any Questions??
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Aaaaah…Structure-Function
yet again!
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