Science Fair Project: Biology
How do plants know to grow upward?
In this experiment, you'll start by testing the reaction of a potted plant to being tipped on its side. Once you've seen how a mature plant behaves, you'll grow beans and test them to see if they know which way is up.
Find a hill in your town with trees growing on it, the steeper the better. Look closely at the direction the trunks are growing and you'll see that they grow straight up despite the fact that the hill slopes below them. Plants have a knack for sensing gravity and growing their roots down while the stems and leaves grow vertically upward. This is because of a chemical they have called auxin. Auxin makes plants grow longer and when gravity acts, the auxin tends to fall to the low side of a plant and its leaves. This makes the lower sides of the stem and leaves grow slightly longer, which turns the plant upward. Roots act differently, as they are a different part of the plant. On the root, the auxin causes slower growth. So, when auxin gathers on the lower side of the root, the upper side grows a little longer and the root turns downward. You'll get to observe both cases in this experiment.
3 small, mature potted plants
Several pinto beans
Place all three potted plants in a sunny location, but tip one on its side toward the sun, tip another on its side away from the sun, and leave the third upright.
Water each plant as you would normally (you can turn them right-side-up to water them) and record their growth. This part of the experiment may take longer with some plants, so be patient. However, it shouldn't take too long for the tipped plants to try to right themselves.
Soak the beans in the glass filled with water overnight before starting.
Pour out the water and place the beans to one side of a paper towel, folded in half.
Carefully, roll the beans up in the paper towel and wet it just so it's moist, but not dripping.
Fold a piece of aluminum foil around the paper towel so that the entire towel is covered and sealed.
Place the aluminum foil containing the beans into the glass with one end up and let the beans sit for one week.
After a week has passed, open the foil and carefully unroll the paper towel. Do not touch the beans as you will be using them again. You will be reusing both the foil and paper towel as well, so take care not to rip either.
Record the direction of growth of both the stem parts of the beans and the roots. They should be just beginning to grow and show that they have found “up” despite their orientation when you put them in the glass.
Take a picture of the beans to document their growth.
Moisten the beans as before, repack them in the paper towel and aluminum foil, and return them to the glass. But this time point the end that was originally up toward the bottom (turn the foil upside down).
After another week passes, open the foil and record the new growth. You should notice that after the original direction of growth, the beans adjusted to being placed upside down and continued their growth in the “right” direction. Take another picture for documentation.
QUESTIONS FOR THE SCIENTIST
What did you observe about the growth of the potted plants?
Was there a difference between the growth of the plant tipped toward the sun and the plant tipped away from the sun?
How do you know that it was gravity and not the sun or another factor that made the plants grow the way they did?
Did the beans grow in the direction you expected during the first week?
After the second week, did the direction of the stems and roots change?
Why do you think this change occurred?
Can you tell that the beans started in one direction and then changed? How can you tell?
From the time they are just seeds, plants have the ability to know which way is up and immediately begin growing in that direction. Mature plants already have their root system in place, but will continually adjust the direction of stem and leaf growth in order to find up. Some plants that grow very tall will actually wind their way through and around obstacles in order to find their way up.
As a follow-up, you can plant your beans in soil by placing them on their sides and watch them adjust once more to the direction of gravity. Soon enough, the stems will pop through the soil and the roots will find their way to the bottom.