Gravitropism: Definition & Examples

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  • 0:02 What Are Troposms?
  • 1:07 Positive & Negative…
  • 1:56 How a Plant Sense Gravity
  • 3:28 Shoots Grow Away from Gravity
  • 4:27 Lesson Summary
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Lesson Transcript
Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Have you ever wondered why a plant's shoots grow up, while its roots grow down? In this lesson, you'll learn about the mechanisms that control this peculiar plant behavior.

What are Tropisms?

One of the first scientific observations you probably made was how plants grow. Unlike us, plants can't get up and walk towards something they need or run away from something that threatens them. Instead, a plant responds to a stimulus , or something that causes a change, by changing how it grows. It may look like movement to us, but really the plant is just growing in one direction or another. The movements that plants make in response to stimuli are called tropisms.

Plants respond to a lot of different stimuli, including light (phototropism), touch (thigmotropism), and water in the soil (hydrotropism). In most cases, the different parts of the plant grow toward the source of the stimulus. Think about a plant sitting on a windowsill -- its shoot bends toward the light coming in from the window. However, there is one stimulus to which different parts of a plant respond differently. It's one of the most fundamental forces on earth: gravity.

Positive and Negative Gravitropism

Gravitropism is the movement or growth of plants in response to gravity. You see the effects of gravitropism on a daily basis, even if you don't realize it. If you've ever grown plants hydroponically, you probably noticed that the roots always grow down even though there's no soil for them to grow in. That's positive gravitropism, since the root is growing toward gravity.

However, even if you've never grown a plant in your life and haven't observed closely how roots grow, you've still observed gravitropism. The shoots or aerial parts of a plant grow away from gravity -- that is, they grow up. This is negative gravitropism, since the shoot of the plant is growing in the opposite direction of gravity.

How a Plant Senses Gravity

We know which way is down because we have specific organs in our body that can feel the force of gravity pulling us toward the earth. Even if we close our eyes we still know which way is down, since there are small cells in our ears that 'feel' gravity.

Plants are not much different than us -- they also have special cells at the very tips of their roots called statocytes (stato meaning 'balance' and cyte meaning 'cell') which can sense gravity. Within the statocytes are granules called statoliths (lith meaning 'stone') that are filled with starch. The starch makes the granules denser than the liquids inside the cell. This makes the granules sink toward the lowest side of the cell. When this happens, the statocytes release a hormone called auxin only on that side of the cell.

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