Fungi (singular- fungus) are a group of organisms with nuclei and rigid cell walls, but without chlorophyll. They may be unicellular or in multicellular filaments. The filaments are called hyphae. A fungus may produce a system of branching filaments, called the mycelium. The filamentous fungi are sometimes called molds. Unicellular fungi are often called yeasts. Some fungi may produce both yeast and mycelial mold phases. Mildew, in layperson's terms, describes the staining, and likely the degradation of the materials, caused by fungi or molds. Mildew is also used by plant pathologists to identify plant diseases, such as "powdery mildew," caused by fungi.

What defines a fungus has changed a number of times since the 1950s. Currently, many mycologists define fungi as organisms that are nucleated, achlorophyllous, typically they reproduce sexually and asexually by spores, and whose somatic structure is composed of filamentous branched or yeast, which are surrounded by cells walls composed of chitin

Fungi are included with some of the earth’s most important organisms, both in terms of their ecological and economic roles because of their role in continuing the cycle of nutrients through ecosystems by breaking down dead organic material. In addition, most vascular plants could not grow without the symbiotic fungi, or mycorrhizae, that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs (such as penicillin and other antibiotics), foods like mushrooms, truffles and morels, and the yeasts provide bubbles for, champagne, beer and bread.

Fungi also cause a number of plant and animal diseases: in humans, fungi cause ringworm, athlete’s foot, and several more serious diseases. Additionally, fungal diseases are very difficult to treat because fungi are more chemically and genetically similar to animals than other organisms. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and can cause severe damage to crops. However, a number of fungi are important "model organisms" for studying problems in genetics and molecular biology in particular the yeasts.

Classifications of Fungi

Fungi are usually classified in four divisions:

Chytridiomycota (chytrids)
Zygomycota (bread molds)
Ascomycota (yeasts and sac fungi)
Basidiomycota (club fungi)

Placement into a division is based on the way in which the fungus reproduces sexually. The shape and internal structure of the sporangia, which produce the spores, are the most useful character for identifying these various major groups.

There are also two conventional groups, which are not recognized as formal taxonomic groups these are:

Deuteromycota (fungi imperfecti)
The Deuteromycota includes all fungi, which have lost the ability to reproduce sexually. As a result, it is not known for certain into which group they should be placed, and thus the Deuteromycota becomes a convenient place to dump them until someone gets around to working out their biology.

Unlike other fungi, the lichens are not a single organism, but rather a symbiotic association between a fungus and an alga. The fungal member of the lichen is usually an ascomycete or basidiomycete, and the alga is usually a cyanobacterium or a chlorophyte (green alga). Often the fungal partner is unable to grow without the algal symbiont, making it difficult to classify these organisms. Here they will be treated as a separate group, but it should be realized that they are neither single organisms, nor a monophyletic group.

Some organisms carry the name of mold or fungus, but are not classified in the Kingdom Fungi. These include the slime molds and water molds (Oomycota). Slime molds are now known to be a mixture of three or four unrelated groups, and the oomycetes are now classified in the Chromista, with the diatoms and brown algae.

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