Luckily, we don't live in a world where that poses a risk. The chance of seeds germinating and growing in the stomach is zero. While there have been rare cases of plants growing in the lungs, the stomach is too harsh an environment for any seed to survive there. The Internet buzz with news from Ron Sveden, the 75-year-old Cape Cod man who discovered that a growth in his lung was not, as feared, a tumor but rather a pea plant.
A seed had somehow lodged in his lung, presumably after some food found its way through the wrong tube, and the seed then sprouted. Therefore, it seems very plausible to assume that seeds could sprout inside the human body and that, given a restricted subject, the plant could leave the body cavity and receive some sunlight and continue to grow. The seed has some energy stored in it to allow the plant to grow until it reaches the surface, although yes, there is usually a good amount of additional nutrients in the soil as well. Miraculously and confusingly, the growth turned out to be a pea sprout, sprouting from a seed that must have been accidentally inhaled during careless pea consumption.
But as we grew older, we gained a disappointing but improved degree of scientific knowledge and general skepticism, and eventually discovered that trunks and branches would not sprout from our torsos after eating seedless watermelon. And then there are the moments when plants and seeds end up inside your body by sheer luck and not by intentional insertion. But as for your question, apparently plants often germinate inside the human body, specifically inside the lungs, since the warm, humid environment is good for germination of a seed. Apparently, some seeds mixed with the snow, because a few days later, a plant began to sprout from his ear.