1. Warm Up #
Why is it sometimes hard to see a plant’s response to a stimulus?

2. Plant Responses

3. Essential Questions How do plants respond to environmental stimuli?
How do plants respond to chemical stimuli?

4. How do plants respond to stimuli? ACTIVITY
Take a pot of young radish seedlings and place toothpicks parallel to a few of them
Place pot near a light source so that it’s to one side of the pot
Check position of the seedlings in relation to the toothpicks after 30 mins.
What did you notice?
What did it look like after a day?

5. Stimuli and Plant Responses
Stimuli – any changes in an organism’s environment that cause a response
can be slow (plant growing toward light) or quick (Venus flytrap’s response to touch)
Responds to external (light, touch, and gravity)and internal (hormones) stimuli
BrainPOP – Plant Growth

6. Environmental Stimuli Growth Responses
Tropism – a response that
results in plant growth toward
or away from a stimulus
Positive – towards the stimulus
Plant bending toward light
Negative – away from the stimulus
Plant stem growing upward against gravity
Phototropism – plant growing toward or away from light
Maximizes the amount of light the plant gets

7. Environmental Stimuli Growth Responses
Thigmotropism – plant’s response to
touch
Gravitropism – plant’s
response to gravity
Stems grow up, roots
grow down

8. Environmental Stimuli Flowering Responses
Photoperiodism – plant’s response to the number of hours of darkness in its environment
Long-day plants: flower when exposed to less than hours of darkness; flower in summer
Short-day plants: require 12 or more hours of darkness; flower late summer/early fall
Day-neutral plants: flower when reach maturity and conditions are right

9. Chemical Stimuli
Plant hormones – substances that act as chemical messengers within plants
Auxin – cause increased plant growth toward the light
Ethylene – helps stimulate the ripening of fruit that’s produced by fruits, seeds, flowers, and leaves

10. Chemical Stimuli
Gibberellins – increases the rate of cell division and cell elongation; increases growth of stems and leaves
Cytokinins – increases the rate of cell division and in some plants slow the aging process of flowers and fruits; found in roots

11. Humans and Plant Responses