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THE TEMPEST
Plant Responses
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Unit 5 – Lesson 4 Notes
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•Plant
Your Responses
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• Changes resulting from internal and
external stimuli.
– Tropism
– Hormones
– Photoperiodism
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•Tropism
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• A response to a stimulus resulting in growth
change; can be positive (toward stimulus) or
negative (away from stimulus).
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•Tropism
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– Some plants grow around objects they touch.
• Thigmotropism
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•Tropism
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– Plants may grow toward a light source.
• Phototropism
– Gravity – downward growth of roots is a positive
response; upward growth of stem is a negative
response.
• Gravitropism
Life
THE TEMPEST
Watch 10 minutes of Discovery’s LIFE. Can
you identify different types of tropism?
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•Plant
Your Hormones
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• They control growth changes triggered by
tropisms.
– Ethylene – gas produced by many plants and
released into the air.
• Can promote cell growth between leaf and stem
resulting in leaf drop.
• Stimulates fruit ripening process.
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•Plant
Your Hormones
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– Auxin – helps with stem elongation and root formation
• Auxin causes plants to bend toward the light.
– Gibberellins – assists with plant germination and development
– Cytokinins – also cause fast growth through rapid cell division.
– Abscisic acid – slows or stops plant growth.
 Look on page 141 in your textbook for a visualization of these hormones.
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•Photoperiodism
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• Plants sensitivity to daily hours of light and darkness for flower
production
– Long-day plants – need less than 10-12 hours of darkness to
produce flowers.
Carnation
Pea Plant
Wheat
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•Photoperiodism
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– Short-day plants – want 12 or more hours of darkness to
produce flowers.
Coffee Plant
Strawberry
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•Photoperiodism
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– Plants that can flower within any range of darkness hours
are called day-neutral plants.
Cucumber
Roses
Tomato