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Lesson 13 Which way is up?

  Gravity as the Stimulus

Plants respond to gravity as a stimulus.


Gravity plays an important role in how plants grow. Just like how plants respond to light, different parts of the plant respond to gravity differently. The roots of a plant grow toward the source of gravity (the centre of Earth), while the stem grows away from that source. This response to gravity is called gravitropism. The roots show positive gravitropism while the stem shows negative gravitropism. This tropism is not as easily observed as phototropism, but it plays just as important of a role as phototropism in making sure the roots grow where they are supposed to and the leaves grow where they are supposed to. Gravitropism helps plants that are just germinating to know which way to send the roots to get water and minerals and which way to send the stem to break out of the soil into the light.

A13.8 Gravitropism
The current theory on gravitropism suggests specialized cells in the roots contain heavy starch particles. If a potted plant is knocked over, for example, these starch particles will settle in a new area of the cell. This signals the roots to grow in this new direction. The growth happens in the area of the roots where the starch particles settle.

  Did You Know?


A13.9 Reading from a lie detector

In the 1960s, Cleve Backster, an expert in lie detectors, decided to test a plant to see if it would have similar reactions as a human under stressful situations. He hooked a plant up to a lie detector and then injured the leaves to see how the plant would react. The plants reaction looked the same as when a human is lying. He concluded plants must have emotions just like people do. Other scientists were not able to repeat Backster’s results, leaving Backster’s work in question. If you are interested in watching a video on Backster’s results, please go to the following link.




  Read This

Please read pages 323 and 327 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on what gravitropism is and how it works. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What is the difference between positive and negative gravitropism? What part of the plant exhibits each of these?
Positive gravitropism is when the plant grows toward the source of gravity. This occurs in the roots. Negative gravitropism is when the plant grows away from the source of gravity. This occurs in the stem and leaves.
  1. In your own words, explain how scientists currently believe gravitropism works.

Your answer should be a variation of the following: Scientists currently believe there are large starch molecules in a plant’s roots. When the plant is tipped over, these starch molecules settle on the new bottom of the roots, where they are pulled by gravity, and promote growth in that direction.