There are several places where plants store food, the most common being in the roots or in their fruit. One way of knowing where the food stores are is by observing which parts of the plant are eaten by animals.
Root vegetables such as carrots and turnips are really the taproot of the carrot or turnip plant. Green plants use the energy of sunlight to produce carbohydrates to provide fuel for the plant to grow and survive. These carbohydrates are often transported down to the root system where they are stored. Some plants use some of their carbohydrates to produce fruit. While the carbohydrates in the fruit are no longer accessible to the plant, they do induce animals to eat the fruit and distribute the seeds, increasing the parent plant's chances for successful reproduction.