; Chapter 39 Plant Responses to Internal and External Signals
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Chapter 39 Plant Responses to Internal and External Signals

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									Chapter 39: Plant Responses to
 Internal and External Signals
        Ms. Klinkhachorn
         April 26, 2011
          AP Biology
                  Reminder!
• Saturday Session
  – 9:00 to 12:30ish
• New content (No review)

• Multiple choice practice exam 2 on Monday
  during 3rd and 4th periods
                    Tropism
• Tropism is a growth response from hormones
  – Results in plant growing toward or away from a
    stimulus
• Phototropism = growth in response to a light
  stimulus
  – Positive = toward the light
  – Negative = away from the light
                      Auxin
• Hormone that stimulates the elongation of
  cells
  – Lower the pH of the cell wall, which weakens it
    and results in growth
  – Can use these as herbicides
     Cytokinins and Gibberellins
• Cytokinins help with cell division and
  differentiation
  – Stimulate cytokinesis
• Gibberellins
  – Work with auxin to help with elongation
  – Help signal the seed to start sprouting
  – Dwarf plants don’t have these
      Abscisic Acid and Ethylene
• Abscisic acid slows growth
  – Works against the other hormones
  – Keeps seeds from sprouting when they aren’t
    ready
• Ethylene is a gas
  – Plays a roll in cell death (plants lose leaves and
    flowers)
  – Promotes the ripening of fruit (positive feedback)
               Photoperiodism
• Photoperiod = length of night and day
• Photoperiodism = physiological response to the
  photoperiod
  – Example: Flowering
• Kinds of photoperiodism
  – Short Day plants
     • Require period of continuous darkness (flower nearer to
       winter)
  – Long Day plants
  – Day Neutral plants
                Other Stimuli
• Response to gravity = gravitropism
  – Roots are positive gravitropism
  – Shoots (the stem, etc) are negative
     • Grow away from gravity
• Directional response to touch =
  thigmotropism
  – Example: vines grow around supports
• How would a plant respond to stress such as
  drought?

								
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