Tropisms
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Printed: February 6, 2014
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C ONCEPT
Concept 1. Tropisms
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Tropisms
• Define tropism and explain examples of tropisms.
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 right
side. As a result, the plants grow in this direction to get more light.
Tropisms
Plants may not be able to move, 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 1.1). 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.
TABLE 1.1: Tropisms
Type of Tropism
Phototropism
Gravitropism
Thigmotropism
Stimulus
light
gravity
touch
Phototropism
You might have noticed that plants bend towards the light. This is an example of a tropism where light is the stimulus,
known as phototropism (Figure 1.1). 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.
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FIGURE 1.1
These seedlings bending toward the sun
are displaying phototropism.
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 1.2). 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.
FIGURE 1.2
These shoots are exhibiting gravitropism:
they are growing against the pull of gravity.
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.
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Concept 1. Tropisms
Vocabulary
•
•
•
•
•
gravitropism: Growth in response to gravity.
phototropism: Growth in response to light.
shoot: The new growth of a plant.
thigmotropism: Growth in response to touch.
tropism: Growth toward or away from a stimulus.
Summary
• Tropisms are growth toward or away from a stimulus.
• Types of tropisms include gravitropism (gravity), phototropism (light), and thigmotropism (touch).
Practice
Use the resource below to answer the questions that follow.
• Phototropism and Auxin at http://www.youtube.com/watch?v=4-2DZo2ppAY (2:13)
MEDIA
Click image to the left for more content.
1. Explain how scientists determined that the signal for phototropism was occurring in the growing tip of a plant?
2. Explain how scientists determined the signal for phototropism migrated up and down a plant shoot but did not
move across the plant shoot?
3. How did an agar block help scientists determine that some substance moving through the plant was responsible
for the phototropic response?
Review
1. If you tip a plant on its side, what will happen and why?
2. The tendril of a bean meets a metal pole. What will happen to the tendril and why?
References
1. Russell Neches (Flickr: r_neches). . CC-BY 2.0
2. Image copyright Marie C. Fields, 2012. . Used under license from Shutterstock.com
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