Gravitropism

Materials:

• Clear plastic photo holder
• Small fast-growing seeds (radish or cherry tomato seeds work best)
• Paper towels
• Plastic tub
• Water
• Notebook
• Pencil

Procedure:

1. Fold a paper towel sheet in half.
2. Use water to dampen the paper towel.
3. Place six seeds on the paper towel.
4. Carefully slide the paper towel into the photo holder.
5. Set the photo holder upright in the tub.
6. Add water to the tub until it touches the paper towel.
7. Place the tub in a warm location. The seeds may require several days to germinate, or begin to sprout.
8. Think about this project's question: Do plant roots always grow down? Write down your guess, often called a hypothesis, in your notebook.
9. Check the experiment every day to make sure the paper towel is still damp. Splash some water on it when necessary.
10. As the seedling roots begin to grow, note the direction of growth.
11. When the roots are about a quarter or a half an inch long, turn the photo holder 90 degrees. The holder should now be sitting on its side in the tub, and the paper towel should still touch the water.
12. Continue to check the experiment daily. Draw a picture of the roots' progress in your notebook.
13. After the roots grow another quarter or half an inch, turn the photo holder upright again, and return it to the tub.
14. Continue to check the direction of root growth and sketch the results in your notebook. Was your hypothesis correct?

Results:

No matter how you turned the plant, the roots always grew down! By the end of this experiment, you should have plant roots that look like stair steps.

Why?

How could the roots see through your clever trick? Well, to understand roots' stubborn behavior, let's take a look at what a root's job is. Roots supply water and minerals to the rest of the plant. To do this, they have to go where the water and minerals are. Most plants are located in areas where all the yummy "plant food" is far below the surface. This means that roots need to always travel down into the ground to find and retrieve all the water and minerals that the rest of the plant needs to survive.

When you turned your plants, you  might have thought that the roots would have started growing sideways. However, special cells at the tips of each root direct growth towards gravity's pull. This type of growth is called positive gravitropsim. If you study that big word, you’ll see that gravi- comes from gravity, and tropism comes from a Greek word that means turn. In other words, turning towards gravity -- which is exactly what your roots did! Root cells are equipped with gravitropism to help them always be able to find water and minerals.

Now that you're well on your way to becoming a gravitropism expert, can you think of different ways to test this cool type of science? Do you think, for example, that only roots experience this sort of positive gravitropism? What about the stem, leaves or flowers on a plant? Keep guessing and testing new ways to expand your knowledge of the world around you -- just like a real scientist!