Image Courtesy of: Dazzo, Frank; Mel YokoyamaCellulose is the strong material in plant cell walls that makes plant stems stringy and tough so that they can stand up. Cellulose is difficult to break down, so animals that eat lots of plants (like cows) have a hard time digesting their food. Some microbes living in the stomachs of many plant-eating animals can easily break down cellulose, and in so doing provide the animal with food that it can use.