People have been wondering about why shoots grow up and roots grow down for centuries. People were doing experiments back in the 1800 to try to find answers. They wondered whether roots just grew toward water and shoots grew toward light. That would make sense, since roots take up water and leaves collect light. To test these hypotheses (science guesses) they put water above and light below, but plants still sent roots down and shoots up. One scientist did an experiment like your rolling Petri dish experiment, where plants were attached to a water wheel that turned slowly. He found the roots growing in all directions. (In case youre wondering, he had to use a water wheel because this was long before people had figured out how to use electric current to do work. Imagine how they might have done an experiment where the light was below the plant before electric lights.)
Scientists decided that gravity itself was what caused the plants to grow shoots up/roots down. Tropisms are ways that plants move or grow toward or away from certain things. We call these things cues or stimuli.
Geotropism literally means growth that is affected by the earth (you can see that geo is a word fragment that means earth: geology, geography, etc.).
Seeds usually start growing in the soil where theres no light and where water may be above or below them. A seed that had genes that made its roots grow down and its shoots grow up based on gravity has a better chance of surviving and reproducing than one that grows in all directions until it finds light and water.
More recently, plants were grown in space to see what would happen. At first, they grew in all directions, then the shoots started to go toward light, while roots started to grow toward water. Remember that these were guesses that the first plant scientists had 200 years ago! It turns out that they were sort of right. So whats going on? Plants also have genes that let shoots and leaves grow toward the light (phototropism) and roots grow toward water (hydrotropism). Maybe they just become active later or are not as strong as the geotropism genes. How would you test that? Thank you for asking.