Certain vegetables -- potatoes, onions, turnips, beets, and carrots are some of them -- grow both above and below the ground. Their green leaves, stems, and vines grow on top of the soil; the parts we eat, called tubers, bulbs, or roots, grow underneath. Those serve as the plants' personal food supply, storing energy for the leaves above. And they take water and minerals from the soil they grow in.
When you place these vegetables in jars or dishes of water instead of the soil in a garden, it's like having X-ray vision. You get to see the entire plant: leaves and stems, the main root, and the thin roots, called feeders, that the plant sends out into the water searching for nourishment.