A very simple geotropism experiment would be to set up seeds to sprout and then shown that the root tips respond to gravity. To do this at home you could use any packaged seeds, or you could use dried beans from the grocery store, which are inexpensive and work well for this. Take a paper towel and roll it into a cylinder. Stand the cylinder inside a drinking glass. Put more loosely wadded paper towels into the center of the cylinder so that the cylinder is pressed firmly to the glass from the inside. Add about a half cup of water to the paper towels in the center to start the wetting process; eventually you will want all the paper towels to be evenly damp.
Now carefully push several seeds down between the inner wall of the glass and the towels (you might want to use a pencil for this). Space them out so that they don't interfere with each other and so that you can see them well. Once you have the seeds in place, slowly add water a little at a time until the towels are wet. Then put a piece of plastic wrap over the mouth of the glass and put it in a dark place. (We do this so that phototropism isn't an issue.)
Once the seeds have sprouted roots, you should see that the roots are all growing toward the bottom of the glass. You can then prop the glass on its side and wait a day or two to show that the root tips change direction and head for the new "down".
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