Centuries ago when farmers planted and harvested their crops, they knew little about the science involved. Nor did they have a large seed stock. But today, science is helping farmers improve their seed selection. As a result, consumers have access to a wide variety of safe, plentiful, and nutritious foods. In the future, new biotechnology tools like genetic engineering can help plant breeders continue this trend.   Scientists like ARS plant physiologist Autar K. Mattoo know the powerful contributions that science and technology can make to the world's food supply. 'Our goal is to develop plants for improved nutrition, longer shelf life, and resistance to harmful pathogens,' says Mattoo, who heads the ARS Vegetable Laboratory in Beltsville, Maryland.