Facilitator/Educator Guide: How Does Gravity Affect Root Growth?
How do plants respond to gravity? See how gravity affects plant root growth using plant seeds, plastic ziplock bags, paper towels, and a large cardboard box or dark closet.
Plant roots grow down in response to the pull of gravity. Gravitropism (also called geotropism) is the directional growth of an organism in response to gravity. Plant roots display positive gravitropism, which means they grow in the direction of gravity.
Plant roots perceive gravity using special cells called statocytes. These cells contain small bodies called statoliths that sink to the bottom of the cells in response to gravity. The process plants employ to determine the direction of gravity using statoliths is similar to dropping a rock to determine which way is down. The statocyte cell senses where the statoliths touch the inside of the cell and in this way the cell "knows" which way is down. The statocyte communicates this information to other parts of the root through a process called signal transduction. Transduction means that information is converted from one form to another, and the signal being sent is chemical. All biological systems use signal transduction constantly. The information is sent from the statocytes to cells in the root tip, and in this way the root tip "learns" which way is down and grows in that direction.
In this science activity, students will use plant seeds, plastic ziplock bags, paper towels, tape, and a large cardboard box or dark closet to explore how gravity affects the direction of seedling root growth.
This science activity can serve as a starting point for a variety of science and plant biology discussions. Here are a few examples of questions that can be used to start a discussion:
- When a seedling is rotated 90 degrees clockwise, how would this affect the direction the roots grow in?
- If a seedling is growing in a horizontal container and cannot grow downward, what direction would the roots grow in?
- What is phototropism? How does it affect how a plant grows?
- How quickly do you think a plant can respond to a change in the direction of gravity? How could you design an experiment to test this?
- If you could somehow increase the strength of gravity, how do you think this would affect plant root growth? How do you think it would affect root growth if you could get rid of gravity, such as by growing plants in space?
Needed for preparing ahead:
- Plastic ziplock sandwich bags (3)
- Paper towels (3)
- Permanent pen (1) or a pen and tape
- Water (0.25 cups)
- Large cardboard box (1) and non-transparent tape, or a dark closet
Needed for each demo or small group at the time of the science activity:
- Plastic ziplock sandwich bags with damp paper towels inside (3)
- Radish seeds (recommended) or tomato, basil, or thyme seeds (15)
- Push pins (3) or strong tape
- Permanent pen (1) or a pen and tape
- Large lightproof cardboard box (1), or a dark closet
|Figure 1. You need only a few simple household materials to do this fun science activity. You can use a dark closet instead of a large cardboard box, and a pen and tape instead of a permanent marker.|
What to Do
Prepare Ahead (<10 minutes)
- On the three plastic ziplock sandwich bags, use a permanent marker (or a pen and tape) to label the four edges on one side of each bag: "Up," "Down," "Left," and "Right."
- Fold the paper towels in half. Fold them in half again to make three roughly square-shaped folded paper towels.
- Put the folded paper towels in the labeled ziplock sandwich bags, arranging the paper towels so that the last folded edge is at the bottom of the bag. Make sure the bags can zip closed. If the paper towels are too big, cut off excess from the top and sides.
|Figure 2. Fold the paper towels so that each fits in a labeled ziplock bag, with the last folded edge at the bottom of the bag.|
- Moisten the paper towels in each bag with water so that it is not dry or dripping wet. Pour a little water in the bag (about one tablespoon), close it, and swish the water around. If some paper towel is still dry, add a little more water. If there is excess water in the bag, pour the excess water out.
- Zip the bags closed until they are needed for the activity. For each classroom demo or small group, prepare a total of three bags this way.
- If you do not have a lightproof closet to use for this activity, use a large cardboard box and make sure that it is lightproof. If the box has any cracks where light can penetrate, seal them with non-transparent tape, such as duct tape. Do not completely seal the box yet so that you can easily put the ziplock bags inside later.
Science Activity (30-45 minutes)Day 1
- Each classroom demo or small group should have 15 radish seeds and three labeled plastic ziplock sandwich bags with damp, folded paper towels in them.
- Have the students carefully open each bag and then open the damp paper towels within the bags so that two layers of paper towel are on either side of the bag. Open the bags and damp paper towels a little more than halfway.
- Tell students to place five seeds in the middle of the bags, roughly evenly spaced from each other. None of the seeds should be close to the sides of the bag.
|Figure 3. Have students place five seeds in the middle of each bag, evenly spaced from each other and not close to the bag's sides.|
- Carefully zip the bags closed, leaving some air in each. You can blow a little air into each bag, but make sure you do the same to every bag (with about the same amount of air in each). Holding each bag up to a light, look at the middle of the bags to make sure the seeds are still in the middle.
- Direct students to use the strong tape or push pins to carefully attach two of the seed bags to either an empty vertical wall in the dark closet, where they will not get disturbed, or on a vertical side in the large lightproof cardboard box. Have students secure the bags using the area above the zipper. Make sure the "Up" label is at the top. For the third bag, find a flat surface where it will not be disturbed, either in the closet or in the cardboard box, and lay it flat there. Use push pins or tape to secure it in place.
- Write a "V" on one of the vertical bags to stand for "vertical." On the other vertical bag, write an "R" for "rotate." This bag will be rotated 90 degrees clockwise every two days, starting in two days. On the bag lying flat, write an "H" for "horizontal."
- Close the box or closet and make sure that no light can get in.
|Figure 4. Carefully secure two bags on a vertical surface and the third bag on a horizontal surface, all either in the lightproof cardboard box or in a dark closet. Make sure "Up" is at the top of each bag. Label one vertical bag "V," one vertical bag "R" (for rotate), and label the horizontal bag "H."|
- Two days and again four days after putting the seeds in the bags, carefully remove each bag from the box or closet, one at a time. Do not leave any bag exposed to the light for more than 10 minutes. Do not try to rip tape off of the bags as it might tear them. Briefly hold the bags up to a light to look at the seeds.
- Tell students to write down their observations, such as how many seeds sprouted and the direction the roots are growing, for each bag.
- If water has collected in any bag, carefully open it and pour the water out. You can blow a little air into each bag, but make sure you do the same to every bag.
- Zip each bag sealed and put it back in its original location. Use new tape to secure the bags if needed. Rotate the bag marked with the "R" 90 degrees clockwise (so that "Left" is on top and "Right" on bottom at the end of day 3, and "Down" on top and "Up" on bottom at the end of day 5) before securing it in place.
- Six days after putting the seeds in the bags, carefully remove each bag from the box or closet and hold it up to a light to look at the seedlings. Ask students to write down their observations while the bags are still closed.
- Carefully open each bag and take out the paper towel, making sure to keep its orientation the same as it was inside the bag. Help students carefully open the paper towels to expose the sprouted seeds, trying not to break the fragile roots or stems.
- Have students write down their observations again, such as the direction the roots are growing in each bag. Did the roots of the seedlings in the rotated bag grow in the direction of gravity after the bag was rotated? Did the roots of the seedlings in the horizontal bag grow slower than the ones that could grow down in the vertical bag?
On day 3, if you are using radish seeds, about half of the seeds may have sprouted. On day 5, the seedling roots in the "V" bag should mostly point toward the "Down" label, in the "R" bag they should mostly point towards the "Right" label (which should be at the bottom), and in the "H" bag they should point in all different directions because they cannot grow down. On day 7, the roots in the "V" and "H" bags should look similar to how they did on day 5, except longer. In the "R" bag, the root near the root tip should be bent towards the "Up" label (which should now be at the bottom), while the root near the stem should be horizontal, running straight between the "Left" and "Right" labels. This is because the root tip guides the growth toward gravity, and as the direction of gravity changed, the root tip changed the direction of the new root growth.
|Figure 5. On day 7, the root tips of the seedlings in the "R" bag should be bent toward the "Up" label (which should now be at the bottom), and the area of the roots closer to the stems should be horizontal, running straight between the "Left" and "Right" labels.|
For Further Exploration
This science activity can be expanded or modified in a number of ways. Here are a few options:
- Do the roots grow faster when they're growing in the direction of gravity? You can have students repeat this activity, but use a ruler to measure the growth of the roots over time or at the end of the activity. Use a larger number of seeds to get more accurate results, which students could graph.
- How does light affect students' results? Repeat this activity, but prepare two sets of seeds (instead of one, as was used in this activity). Protect one set of seeds from light and expose the other set to light.
- In this activity students rotated seeds by 90 degrees, but how would other degrees of rotation affect root growth? Have students repeat this activity but make extra bags of seeds and rotate them using different degrees, such as 45 degrees and 180 degrees clockwise.
Downloads and Links
- How Does Gravity Affect Root Growth? Facilitator / Educator Guide PDF
- How Does Gravity Affect Root Growth? Student Guide web page or PDF
Teisha Rowland, PhD, Science Buddies