Tropisms: Phototropic, Geotropic and Thigmotropic Plant Growth

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  • 0:05 Tropism
  • 1:17 Phototropism
  • 1:59 Geotropism
  • 2:53 Thigmotropism
  • 4:00 Lesson Summary
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Lesson Transcript
Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

Expert Contributor
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Animals aren't the only things that can respond to the environment. While plants may seem inanimate at times, they, too, can respond to the environment in order to better survive.


When you hear a loud noise, you may respond by jumping. External stimuli result in a response. We easily see this in animals, but it also happens in plants. We generally think of plants as passive. They simply - and much of the time, very slowly - grow. However, there is much more to plants than we perceive.

The way sunflowers turn in the direction of the sun throughout the day is an example of phototropism
Sunflower example of phototropism

When animals, including humans, respond to a stimulus (which is something that causes a response), we call it a reaction. For example, if someone touches your arm, you generally turn to react. In plants, the response to a stimulus is known as a tropism. This plant movement toward or away from a stimulus can come in many forms. Before we look at a few, let's better understand the word tropism. Like many words in science, tropism comes from a Greek word. Tropos means 'to turn'. Therefore, a tropism is a turn towards or away from a stimulus. When the movement is towards the stimulus, it is called positive tropism. Likewise, when the movement is away from the stimulus, it is called negative tropism. While there are several forms of tropism, we'll just focus on three key types: phototropism, geotropism and thigmatropism.


Phototropism is generally the tropism that makes the most sense. We know that plants grow towards the sun, so they can make food through photosynthesis. This movement in response to sunlight is called phototropism. Let's break this word apart. Tropos means 'to turn,' and photo means 'light.' Therefore, phototropism is a turn towards or away from light.

While most phototropism involves plants just growing toward the sun, some plants actually follow the sun throughout the day. For example, young sunflowers will orient their flowers to the sun. In the morning, they point east towards the rising sun. They then gradually follow the sun throughout the day, eventually pointing west towards the setting sun.


The downward growth of roots and the upward growth of shoots are examples of geotropism
Roots as example of geotropism

Let's now look at geotropism. We know that tropos means 'to turn,' so we already understand the second half of the word. Geo many sound familiar because of words such as 'geography' and 'geology.' Geo means earth; therefore, geotropism is movement towards or away from the earth. You may see geotropism used interchangeably with gravitropism, which is the movement towards or away from gravity.

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Additional Activities

Phototropism in Action

In this experiment, students will be observing phototropism in plants and recording their result. To do this experiment, you'll need a houseplant (such as a spider plant), a succulent, or an herb (like basil) and a bright light source. This experiment will ask students to take before and after pictures of their plants. However, if technology is not available, students can make detailed observations.

Student Instructions

Now that you're familiar with the different tropisms that plants exhibit, we're going to watch phototropism in action. For this experiment, you'll need access to a house plant (such as a spider plant), a succulent, or an herb as well as a strong light source and a camera. If you don't have access to a camera, you can make detailed observations about the location of the leaves and stems of the plant. Follow the instructions below, then answer the questions.

  1. Position your houseplant in front of the light source, with the light source directly to the right of the plant. Take a photo or carefully record observations about the location of the stem and leaves. Consider where they are in relationship to the lamp or other stationary objects.
  2. For one week, turn the light on during the day and off at night. Record observations or take a photo each day to record the location of the stem and leaves of the plant.
  3. After a week, take another photo of the final position of the plant.


  1. What happened to the position of the plant over time?
  2. How did this experiment represent a tropism? How did you know?
  3. How do you think we could test for other tropisms?

Expected Results

Students should observe that the plant changes the direction of growth towards the lamp. If this doesn't occur, it could be because other light sources were competing with the lamp. Try to allow the lamp to be the only light source. This experiment should demonstrate a tropism because the plant's position should change over time in response to the light. To test for other tropisms, students can try to position the plants differently, but still observe root growth with gravity and stem growth against gravity.

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