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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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