1. Plants and External Stimuli
Big Idea 14: Organization and Development of Living Organisms
Topic V: Plant Response to Stimuli

2. Plants have various needs- food, air, water, and space to grow
Plants have various needs- food, air, water, and space to grow. Plants receive most of these things from the outside world- externally. Plants get some of these things from making them inside- internally. Plants get sunlight from the sun (external) and then use it to make glucose in their leaves (internal).

3. If an animal needs something, it can simply go get it unlike plants
If an animal needs something, it can simply go get it unlike plants. Because most plants are often rooted in soil, they must find different ways to get their needs in order to survive.
One of the main ways is called tropism. Tropism is how a plant responds to stimuli in the environment which can be moving away from (negative) or towards (positive) the stimulus.

4. There are various stimuli that can cause a plant to react
There are various stimuli that can cause a plant to react. Some of the stimuli are heat, light, gravity, water, touch, and chemicals. These are the stimuli we’ll be examining in class today. Let’s look at each closely.

5. Phototropism
Phototropism is simply growing or bending in response to light. Simply put, a plant will bend towards the light. How will this affect the plant?

6. Hydrotropism
Hydrotropism is the way a plant grows or bends in response to water. Why would it be important for some parts of a plant to be pulled toward water?

7. Geotropism
Gravity is an invisible force which tries to pull two objects towards each other- like pulling me to the Earth! Geotropism is a response in plants to gravity. Which parts of the plant shows positive response to gravity (towards the ground)? Which part of the plant shows negative tropism (away from the ground)?

8. Thigmotropism
When plants bend or grow because of touch, it is called thigmotropism. One example would be when vines wrap around an arbor frame. What are some other ways a plant can be touched?

9. Thermotropism is the movement of a plant because of temperature.
One example is the curling of the Rhododendron leaves in cold temperature.
Another example is the wilting of a daisy that received too much heat.
What are some negative effects of too much heat?

10. A movement of a plant caused by a chemical stimulus.
Chemotropism
A movement of a plant caused by a chemical stimulus.
Some chemicals are good like the ones that make the pistil make seeds from the eggs and the pollen.
Others are harmful like
oil or acids mixed
with water and soil.
What are some other chemicals that could affect a plant?

11. Vocabulary Review PHOTOTROPISM: response to light
CHEMOTROPISM: response to chemicals
GEOTROPISM:
response to gravity
THERMOTROPISM: response to temperature
HYDROTROPISM: response to water
THIGMOTROPISM: response to touch

12. Discussion Questions
How does the response of a root to gravity help a plant?
What do you think would happen if you place a light source below the plant? How would the stem grow?
Which do you think has a stronger influence on a plant- geotropism or phototropism? Why do do you think that?
What do you think would happen to a plant that grows in the dark?
What would happen if a plant received more of a chemical fertilizer than another?

13. WHAT I THINK WILL HAPPEN CLUES I READ OR OBSERVED
Inference Chart
EVENT
WHAT I THINK WILL HAPPEN
CLUES I READ OR OBSERVED
WHAT REALLY HAPPENED