Fungi are a separate kingdom of life that includes mushrooms, molds, mycorrhizae, yeasts, rusts, and other forms. Fungi are relatively unseen but important components of the environment. Together with the microscopic bacteria, protozoa, and microfauna they are key members of any terrestrial community whether it be a whole forest or a flower pot. Fungi are not animals or plants. Plants are able to produce their own food through photosynthesis which captures light energy and stores this energy in molecules such as carbohydrates (sugars, starch, cellulose). Fungi and animals digest other organisms to obtain energy and other nutrients. Animals typically "eat" food and digest it internally. Fungi have external digestion! They literally grow through their food secreting enzymes outside of their cells and absorbing the breakdown products. Fungi are able to do this because of their thread-like growth form. Fungal cells grow as extremely thin tubes, called hyphae (from the Greek word for web), that form a branched network, a mycelium. The primary component of fungal cell walls is chitin, a similar material to the outside covering (exoskeleton) of insects and other arthropods.
Mushroom-forming fungi have three main lifestyles or ways that they obtain food: saprobic, parasitic, and mycorrhizal. Saprobic fungi are decomposers, or recyclers, that live on dead organic material from plants, animals, and other fungi. They can be generalists, growing on a wide variety of substrates (food sources) or specialists, e.g., the mushroom Mycena luteopallens is found on walnut and hickory fruits. Parasitic fungi use live organisms as a food source, for example pathogens on trees. There are even fungi that attack other fungi, e.g., Hypomyces. Mycorrhizal fungi form beneficial partnerships with plants. The fungal hyphae interact with roots (mycorrhiza = fungus + root) and grow out into the soil, acting as extensions of the plant's root system. Some fungi have mixed lifestyles and can switch their eating habits! Wood provides a rich source of carbon for fungi but the low nitrogen supply can limit growth. Opportunistic fungi trap and digest other organisms, such as nematodes (microscopic worms), for nitrogen; they form various microscopic snares, lassoes, and toxic lollipops!
What is a mushroom?
Numerous fungi form visible kinds of fruitbodies which we call mushrooms or toadstools, brackets, puffballs, etc. Many mushroom fruitbodies are short-lived and seem to appear overnight. Collecting a mushroom is somewhat analogous to picking an apple off of an apple tree. The mushroom is a temporary reproductive structure or fruitbody of the much larger individual living in the soil or log. Unlike animals, fungi reproduce by spores. These microscopic cells, spread by air, water, or animals, allow the fungus to start growing in a new location or food source. Many fungi, such as yeasts, molds on food, powdery mildews on plants, athlete's foot fungus, and innumerous beneficial soil fungi, do not form visible fruitbodies. Their spores are produced on microscopic structures.
What is a lichen?
A lichen is a combined organism—a mutual symbiotic partnership between a fungus and an alga. Part of it is a fungus made up of filaments which forms the lichen structure. Inside of this lives an alga, many individual green cells wrapped by the fungus filaments. The fungus provides a protective home, water, and some nutrients for the alga. The alga is photosynthetic, utilizing solar energy to make its own food. Some of the food produced is transferred to the fungus. The fungus cannot live without the alga. Many species of fungi produce the wide variety of lichens that we find on rocks, trees, and on the ground. There are certain green algae and cyanobacteria (blue-green bacteria) that form these novel partnerships.