1. Chapter 39: Plant Responses to Internal and External Signals Ms. Klinkhachorn April 26, 2011 AP Biology

2. Reminder! Saturday Session – 9:00 to 12:30ish New content (No review) Multiple choice practice exam 2 on Monday during 3 rd and 4 th periods

3. 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

5. 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

6. 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

7. 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)

9. 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

12. 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?