Topic 9: Plant Structure Part II

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Topic 9 Plant Structure Part II

• Biology HL

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Structure and Function of Flowers

• Flowers are typically the site of external sexual
  organs in plants for reproduction
• Most flowers have both male/female anatomy
• Female gametes contained in ovules located in
  the ovaries
• Male gametes contained in pollen grains
  produced by the anthers
• A fertilized zygote formed by pollen fusing with
  the ovules

http//www.webquest.hawaii.edu/kahihi/sciencedicti
onary/P/perfectflower.php
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Pollination

• Transfer of pollen from an anther to a stigma
• Since plants themselves do not move, an outside
  agent is typically required for successful cross
  pollination to occur
• Wind dispersal
• Insect/Bee dispersal
• Animal dispersal

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Pollination

• Flower structure typically adapted to method for
  pollination
• Bees flowers are typically equipped with
  landing platforms and has strategically placed
  anthers to catch pollen
• Birds sweet nectars for attraction

http//www.mrac.ca/story/protecting-against-pests
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Wasp Flower

• Tricking nature to do its bidding!

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After Pollination

• Pollen grains germinate on the stigma of the
  flower
• Pollen tube with the male gametes travels down
  the style to the ovary
• Fusion of male and female gametes
• Ovaries with fertilized ovules develop into
  fruits
• The fertilized ovules develop into seeds
• Ultimate job of a fruit is seed dispersal

http//www.motherearthnews.com/organic-gardening/s
elf-pollinating-apples.aspx
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Structure of a seed

• Testa -Seed Coat protective layer
• Micropyle opening for the pollen
• Plumule (embryo shoot)
• Radicle embryo root
• Cotyledon - food storing seed leaf

http//www.cmg.colostate.edu/gardennotes/137.html
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Structure of a seedling
Cotyledons provide energy and nutrients for
germination! First foliage leaves are about to
open! Bend in the stem protects the leaves as
the shoot pushes up through the soil Stem
between the cotyledons and the first foliage
leaves has grown Branches of the main root
increase the surface area for absorption
Main root growing downward into the soil
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Factors Needed For Seed Germination

• Must have adequate water, oxygen and temperatures
• Water
• Dry tissues must be rehydrated
• Oxygen
• Needed for aerobic cellular respiration
• Temperature
• Germination involves enzymes!
• Extreme high temps and low temps are not
  conducive to germination

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Metabolic Events of Germination

• Stage One
• Absorption of water leads to the rehydration of
  living tissues inside the seed
• Stage Two
• Gibberellin is a plant hormone that is produced
  in the cotyledon of the seed. Production begins
  after rehydration.
• Stage Three
• Gibberellin stimulates the production of amylase
• Amylase catalyses the digestion of starches into
  maltose

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Metabolic Events of Germination

• Stage Four
• Maltose is transported from food storage to the
  places that are growing like the embryo root and
  embryo shoot
• Stage Five
• Maltose is finally converted into glucose
• Glucose is used in aerobic cell respiration OR it
  is used to synthesize cellulose
• Stage Six
• The seedling leaves will finally reach light and
  begin photosynthesis

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Auxin and Phototropism

• Phototropism plants grow towards light and will
  literally bend and contort towards it
• Auxin a plant hormone that acts as a growth
  promoter
• Research has shown that auxin redistributes
  itself in the shoot tips from the lighter side to
  the darker.
• Additional growth on the shadier side causes the
  plant to bend towards the light

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Auxin Mechanism

• Pumps in plasma membrane auxin efflux carriers
• Pumps distributed unevenly
• Auxin is able to be redistributed as needed in
  tissues
• Auxin works by binding to auxin receptors in
  plant cells
• Once auxin binds to the receptor, transcription
  of specific genes is promoted
• This affects the growth of the cells

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Photoperiodic Control of Flowering

• Why do certain plants only bloom at specific
  times of the year?
• Studies show that its not the length of day but
  the length of night that controls flowering
• Short-day plants
• Long-day plants

http//www.flowersgrowing.com/chrysanthemum/
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Phytochrome and Photoperiodism

• Plants have the ability to sense day and night
  light/dark with extreme accuracy
• Phytochrome is a pigment in the leaves that
  exists in two interconvertible forms
• Pr absorbs red light with a wavelength of 600nm
• Inactive form of phytochrome
• Once the red light is absorbed it is rapidly
  converted into Pfr
• Active form of phytochrome
• As soon as light disappears Pfr is gradually
  converted back into Pr

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Pfr and Flowering

• The reversion of Pfr is most likely what is used
  as a darkness timer
• Pfr in short day plants acts an inhibitor to
  flowering. Once all of the Pfr has been
  converted to Pr then flowering will begin
• In long day plants, Pfr binds to proteins which
  then trigger genes involved in flowering to
  switch on