Geotropism: Definition, Examples & Experiments

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  • 0:00 What is Geotropism?
  • 1:13 Early Experiments
  • 1:53 Examples of Geotropism
  • 2:23 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this lesson, you'll learn about geotropism, which is why some parts of plants grow upward, while other parts grow toward the ground. After completing the lesson, test your knowledge with a quiz.

What is Geotropism?

If you were to put a plant on its side, you'd notice that, after awhile, the roots would start growing downward while the stem would grow upward. This response to gravity is called geotropism, or gravitropism.

The roots are exhibiting positive geotropism, or growth with gravity, while the stem is exhibiting negative geotropism, or growth against gravity.

Different parts of a plant grow in opposite directions because they have different functions. The roots need to grow into the ground to provide stability for the plant, as well as to access water and nutrients. The stem needs to grow upward toward the sunlight so the plant can photosynthesize, or make food from sunlight.

So, how do different parts of a plant know which way to grow? Plants have organelles called statoliths, which settle at the bottom part of their cells and allow plants to sense gravity. They also have a hormone called auxin, which stimulates elongation and growth in plant cells. For root cells, the statoliths and auxin trigger downward growth, while cells above ground are signaled to grow upward.

Early Experiments

Charles Darwin was the first to document both positive and negative geotropism in plants. He was also instrumental in correctly describing phototropism, which is plant growth toward a light source. Darwin's work eventually led to the discovery of auxin and its relationship to cell growth in plants.

Scientists are now discovering that there might be other factors that trigger geotropism in plants. Some mutant varieties of plants exhibit geotropism even without statoliths, though their growth is much slower. There also might be other organelles besides statoliths in plant cells that contribute to geotropism.

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