Molds are the common name for a group of fungi often characterized by the presence of threadlike filaments, called hyphae, that mass together to form mycelia, vegetative bodies that resemble cotton. Some molds, however, such as the SLIME MOLDS, are more amoeba-like and form multinucleate masses of protoplasm called plasmodia. 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. The rhizoids absorb the food, and water as well.

Molds can reproduce both asexually and sexually. To produce asexually, some molds develop special reproductive hyphae (sporangiophores), which extend into the air. Black knobs, or spore cases (sporangia), appear at the ends of these hyphae after a few days. When mature, the sporangia break open and release their spores, which will germinate if they reach a suitable environment. Sexual reproduction is accomplished through a form of conjugation. Two different mating types of hyphae, termed plus and minus, form short, specialized side branches. If the tip of a plus branch meets the tip of a minus branch, conjugation occurs. Each tip becomes a gamete cell, with two fusing to form a zygote. The zygote matures into a zygospore, which may germinate after one to several months if conditions are favorable.
Some molds, like water molds, live on the bodies of dead insects and other animals in the water. Others are parasites, invading the tissue of living animals. Molds of the genera Aspergillus and Penicillium, often referred to as MILDEW, appear on food and form characteristic growths. Several species of Penicillium have economic value. Some are used in cheesemaking, in which enzymes secreted by the mold give the cheese its flavor. Another species produces the antibiotic PENICILLIN.

Mold is growing on bread that was left in a warm, dark, damp place. Molds are the common name for a group of fungi which often have threadlike filaments, called hyphae.

Bibliography: Christensen, Clyde M., Molds and Men: An Introduction to the Fungi, 3d ed. (1965), and Molds, Mushrooms, and Mycotoxins (1975); Dube, H. C., An Introduction to Fungi (1983); Moore-Landecker, Elizabeth, Fundamentals of the Fungi, 3d ed. (1990).