The word fungus usually invokes images of mushrooms and toadstools or some sort of spongy, creeping growth. Although mushrooms are fungi, the forms which a fungus may take are varied. There are over 100,000 species of described fungi and probably over 200,000 undescribed. Fungi are nonmotile, filamentous eukaryotes which lack plastids and photosynthetic pigments. Other fungi survive as parasitic decomposers, absorbing their food, in solution, through their cell walls.
Most fungi live on the substrate upon which they feed.
Numerous hyphae penetrate the wood, cheese, soil, or flesh in which they are growing. The hyphae secrete digestive enzymes which break down the substrate, enabling the fungus to absorb the nutrients contained within the substrate.