Potato Fruit on Red Tomato Plant
I need a little clarification about your question:
Are you wanting to know how to propagate potatoes next spring? If so, then you can click here to view the process of how to using certified seed potatoes for propagation next spring (See page 4). It is actually recommended to avoid using home-grown potatoes for planting, since they may be heavily infected with viruses that can reduce yield and quality of the new crop.
Or is your question concerning small, tomato-like fruit on the top of your potato plants? This is the actual fruit of the potato plant. Normally, we don't notice them because the flowers typically fall off the plants and never have the opportunity to go from flower to fruit. If you saved the ripe seeds inside the potato fruit, you would not get the same kind of potatoes as the parent plant. It would take several years to even grow these to a size that you could harvest. The only people saving and planting potato seeds are growers and breeders who are hoping to produce a new variety of potato. Furthermore, these potato fruit are not edible. More precisely, they are poisonous. They contain high amounts of solanine that can make the eater very ill. View this Michigan State U. Extension publication for more information.
I hope this answers your question.