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Up or Down?: Does Gravity Affect Plant Growth of Pinto Beans?

based on 12 ratings
Author: Janice VanCleave

Problem

How does gravity affect plant growth of pinto beans?

Materials

  • paper towel
  • tap water
  • 4 pinto beans
  • rubber band
  • masking tape
  • 8-inch (20-cm) -square piece of cardboard
  • permanent marking pen
  • 1-gallon (4-liter) resealable plastic bag

Procedure

  1. Fold the paper towel in half three times, then moisten the folded paper towel with water.
  2. Place the beans on the wet towel, evenly spaced as shown.
  3. Up or Down?

  4. Fold one end of the paper towel over the first bean. Continue folding the towel end-over-end to form a roll around the beans.
  5. Up or Down?

  6. Wrap the rubber band around the bean roll.
  7. Tape the bean roll to the center of the cardboard.
  8. Draw an arrow on the cardboard above one of the open ends of the bean roll.
  9. Place the cardboard inside the plastic bag so that the arrow points toward the open end of the bag. Seal the bag.
  10. Up or Down?

  11. Tape the bag in an upright position to any stationary vertical object that allows a good view of the bag, such as a window.
  12. Observe the bean roll as often as possible during each day for 7 days or until roots and stems extend from the end (s) of the bean roll.

Results

The stems of the plants in the bean roll grow upward and the roots grow downward.

Why?

Plants grow in certain directions because of the plant chemical auxin. Auxin makes plant cells elongate. The longer cells on one side then cause the plant to bend toward the shorter side. Different types of cells respond differently to the presence of this chemical. An increase in auxin increases the growth of stem cells, but inhibits the growth of root cells.

Gravity pulls auxin down toward the lowest part of the stems and roots of a plant. More growth occurs in the cells on the lower side of the stem, and less growth in the cells on the lower side of the root. The result is that the stem bends up and the roots bend down. The growth response of plants to gravity is called geotropism. Since stems grow in a direction opposite to the pull of gravity, they have a negative geotropism, while roots have a positive geotropism.

Let's Explore

Do stems and roots continue to be affected by gravity as they grow? Repeat the experiment, preparing 4 bean roll bags. Use a protractor to draw a 6-inch (15-cm) -diameter circle on the cardboard in each bag. Make a mark every 10° around each circle. When the stems are about 2 inches (5 cm) long, change the position of three of the bags so that the arrows on the bags point down, right, and left. Let the fourth bag, the control bag, remain with its arrow pointing up. Make daily observations for 7 days, using the circles to compare the change in the direction of each plant's stems and roots. Science Fair Hint: Take photographs to represent the procedure and results of the experiment.

Show Time!

  1.  
    1. Most plants grow on the side of a hill in the same up-and-down direction as plants on a flat surface. Demonstrate this by filling a shallow pan with about 2 inches (5 cm) of potting soil. Plant 6 to 8 pinto beans in the soil, and moisten the soil with water. Stand a toothpick in the soil vertically next to each bean. Prop up one end of the pan by placing a ball of clay at each corner. When the shoots emerge, use a protractor each day to measure the angle between the shoots and the toothpicks.
    2. Repeat the experiment, using seeds of different plants, such as squash, cucumber, lima bean, or corn. Use toothpicks of different colors to identify the different seeds used.
  2. Does gravity affect the growth of mature plants? With an adult's permission, lay a small houseplant on its side in a dark closet or under a cardboard box to prevent it from responding to light. Observe the position of the stems and leaves after 1 week and again 1 week later. Add water as needed to keep the soil moist. At the end of 3 weeks, carefully remove the soil from around the roots and observe their direction in relationship to the direction of the stems. Take a photograph of the plant at the beginning of the experiment and each time you observe the plant.

Up or Down?

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