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Gravity & Plants Experiments

When a seedling germinates, it must know which direction to grow its photosynthesizing leaves and where to extend moisture-catching roots. Plants grow in response to gravity in a process known as gravitropism. Although the exact mechanism of this orientation is not known, most science books suggest that plants sense the direction of gravity and release hormones that stimulate growth in different locations. You can perform experiments on plant gravitropism at home using simple materials.

Basic Observations
- A simple experiment with germinating seedlings or beans will verify the phenomenon of gravitropism and offer an opportunity for you to qualitatively observe directional growth. Germinate seeds or beans in a wet paper towel held in a clear chamber, such as an empty film canister or plastic bag. Affix the paper towel with tape and place the entire apparatus in a safe location where it will remain in the same orientation.
  
  After germination, wait for the seedlings to grow a few centimeters of root and leaf in a particular direction. Turn the chamber 90 degrees and allow the seeds to continue growing. Observe the direction of growth and any bends in the stems and roots.
Light Deprivation
- To perform the chemical reactions involved in gravitropism, plants use energy that they have stored in the form of starch. Deplete a seedling's starch reserves by keeping it in the dark for 24 hours. Expose this seedling to a directional change by laying it horizontally and replace the plant in the dark. Match it with a control plant that was not previously light-deprived.
  
  Observe the plants for the next several hours, checking periodically for bent growth. Record when the plants bend to observe whether starch reserves are necessary for gravitropic bending.
Gravity Sensing in Roots
- Roots exhibit positive gravitropism, meaning they grow in the direction of the gravitational field they sense. The directional sensing occurs in the tip of the root, a phenomenon which you can explore in an experiment. Begin with germinating seeds that have grown several centimeters of root downward. Snip off the tip of some seedlings, while you leave others alone as a control group.
  
  Change the orientation of the germinating seedlings. In a normal plant, the roots will turn and continue growing downward as a response to gravity. Check the snipped plants to see if they have lost their gravitropic abilities.
Light vs. Gravity
- Plants react to light with phototropism, as well as gravitropism. Pit these forces against each other to see which has a greater sway over the direction of growth. Germinate seeds, then place them in an environment where gravity pulls from below but light comes from the side. Use grow lights or standard fluorescent bulbs to focus light from a particular direction. Observe whether the direction of growth is more strongly influenced by light or gravity. The results may be different for stems and roots.

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