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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
Instructor: Danielle Weber

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

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.

Tropism

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

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, 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.

Geotropism

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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