7.13: Tropisms

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Why are these plants turning sideways?

Plants respond to their environment in how they grow. In this picture, the light source is probably off to the left side. As a result, the plants grow in this direction to get more light.

Tropisms

Plants may not be able to move to another location, but they are able to change how they grow in response to their environment. Growth toward or away from a stimulus is known as a tropism (Table below). Auxins, a class of plant hormones, allow plants to curve in specific directions as they grow. The auxin moves to one side of the stem, where it starts a chain of events that cause rapid cell growth on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend.

Name	Stimulus
Phototropism	Light
Gravitropism	Gravity
Thigmotropism	Touch

Phototropism

You might have noticed that plants bend toward the light. This is an example of a tropism where light is the stimulus, known as phototropism (Figure below). To obtain more light for photosynthesis, leaves and stems grow toward the light. On the other hand, roots grow away from light. This is beneficial for the roots, because they need to obtain water and nutrients from deep within the ground.

Gravitropism

So, how do the roots of seeds underground know to grow downward? How do the roots deep in the soil know which way is up? Gravitropism is a growth toward or away from the pull of gravity (Figure below). Shoots, the new growth of a plant, also show a gravitropism, but in the opposite direction. If you place a plant on its side, the stem and new leaves will curve upward.

Thigmotropism

Plants also have a touch response called thigmotropism. If you have ever seen a morning glory or the tendrils of a pea plant twist around a pole, then you know that plants must be able to sense the pole. Thigmotropism works much like the other tropisms. The plant grows straight until it comes in contact with the pole. Then, the side of the stem that is in contact with the pole grows slower than the opposite side of the stem. This causes the stem to bend around the pole.

Summary

Tropisms are growth toward or away from a stimulus.
Types of tropisms include gravitropism (gravity), phototropism (light), and thigmotropism (touch).

Explore More

Use the resource below to answer the questions that follow.

Phototropism and Auxin at http://www.youtube.com/watch?v=4-2DZo2ppAY (2:13)

Explain how scientists determined that the signal for phototropism was occurring in the growing tip of a plant?
Explain how scientists determined the signal for phototropism migrated up and down a plant shoot but did not move across the plant shoot?
How did an agar block help scientists determine that some substance moving through the plant was responsible for the phototropic response?
What hormone causes the phototropic response to light?

Review

What is a tropism?
If you tip a plant on its side, what will happen? Why?
The tendril of a bean meets a metal pole. What will happen to the tendril? Why?
Why do leaves and stems grow towards light?