BIG IDEA 14 : ORGANIZATION AND DEVELOPMENT OF LIVING ORGANISMS

TOPIC V: PLANT RESPONSE TO STIMULI

Plants and External Stimuli

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).

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.

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.

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?

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?

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)?

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?

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.

Thermotropism

What are some negative effects of too much heat?

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.

Chemotropism

Others are harmful like

oil or acids mixed with water and soil.

What are some other chemicals that could affect a plant?

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

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?

Inference Chart

EVENT WHAT I THINK WILL

HAPPEN

CLUES I READ OR OBSERVED

WHAT REALLY HAPPENED