INTRODUCING THE FUNGI
Fungi are a diverse group of eukaryotic organisms that share several distinguishing features. In contrast to plants and some protistans, fungi lack chloroplasts and are unable to perfom photosynthesis to produce their own organic molecules. Thus, fungi must obtain preformed organic molecules from the environment. Fungal cell walls contain chitin, a complex carbohydrate that is very resistant to degradation by other microorganisms, whereas plant cell walls contain cellulose. In addition, fungi secrete digestive enzymes into the environment to break down organic molecules, and then the fungi absorb these products. This method of obtaining nutrients is quite different from the internal digestion that is employed by animals.

Some fungi are unicellular organisms (for example, baker's yeast); however, most fungi consist of threadlike filaments referred to as vegetative hyphae. These hyphae elongate into food sources and absorb nutrients from the environment. A mass of vegetative hyphae is referred to as a mycelium; this is what you see on a piece of molded bread. Because hyphae are able to extend and branch throughout the environment, a fungus is able to increase its surface area for absorption of nutrients. One fungus in northern Michigan has been found to extend 150,000 square meters through a forest!