Molds grow over many surfaces, such as wood and food, and thrive best in warm and moist conditions.
Many, however, do well at freezing temperatures, presenting problems for refrigerated foods. Perhaps the most widely studied mold is the familiar bread mold, Rhizopus nigricans, which appears on bread that has been moistened, exposed to air, and placed in a warm, dark place. Bread molds have different types of hyphae. Those that spread along the surface are called stolons. At intervals along the stolons, clusters of shorter hyphae, called rhizoids, extend down into the food supply and secrete enzymes that break down sugar and starch into digestible food.