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Fungus
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Most fungi have vegetative bodies[?] composed of one-cell-thick filaments called hyphae (singular: hypha). These generally do not coalesce into a visible object, but instead form a microscopic network within the substrate, called the mycelium, through which food is absorbed. The more conspicuous parts of fungi like mushrooms are fruiting bodies[?], which are reproductive structures that produce spores.
Division of hyphae into cells is either incomplete, in which case they are called septate and the dividing walls are called septa (singular: septum), or absent, in which case they are called coenocytic. Fungi generally have cell walls made from chitin and other materials. The hyphae may be modified to produce highly specialized cellular-scale structures. For instance, fungi that parasitize plants grow haustoria[?] that pierce their cell walls and digest them from the inside, while some soil-dwelling fungi trap roundworms and other small animals.
Most chytrids, which are generally considered the most primitive group of fungi, do not form hyphae and instead grow directly from spores into multinucleate sporangia. A few other fungi have reverted from a mycelial to a unicellular organization. These are the yeasts, which belong among the Ascomycetes, and the Microsporidia[?], a group of reduced parasites whose relationships to the other fungi are uncertain.
Fungi are heterotrophic, that is they get their energy by breaking down organic molecules, and cannot synthesize organic molecules from inorganic substances like plants. They feed by secreting exoenzymes[?] into the surrounding substrate, which act like digestive enzymes breaking apart large organic molecules, but function outside the organism. The smaller organic molecules are then absorbed[?].
In the Zygomycota, the heterokaryon produces multiple fruiting bodies, in the form of minuscule stalks with sporangia at the end. Most Ascomycota produce fruiting bodies called ascocarps, composed entirely of hyphae. These are usually bowl- or cup-shaped, but some have sponge-like structures. On the inside of the cup, each hypha terminates in an ascus, which produces eight spores.
In the Basidiomycota, the heterokaryon produces a new mycelium which may live for years without producing a fruiting body. The familiar mushrooms are examples of these. They usually have a stalk, composed mainly of hyphae, and a cap, under which there are sheetlike structures called gills. On the surface of each gill there are numerous hyphal cells called basidia, with several spores on the end of each.
Except among the chytrids, where spores are propelled by a posterior flagellum, all fungal spores are non-motile. They develop into new mycelia, which invade some substrate and repeat the life cycle. These may become very large, often several square metres in size; fairy circles[?] are an example.
Two main ecological niches occupied by fungi are that of the decomposer[?] (detritivore[?]) and that of the parasite. The only difference between detritivorous and parasitic fungi is that the latter grow on living organisms, while the former grow on dead organisms. Many decomposer fungi live as mycorrhizae, in mutualistic relationships with plants.
Some fungi, usually Ascomycetes, live as lichens. A lichen is a very close mutualistic relationship between a fungus and a photosynthetic microorganism, usually a cyanobacterium or green alga[?]. A lichen behaves in so many ways like a single organism that lichens are actually given genus and species names.
Fungal diseases include:
Fungal infections may also be a factor in causing specific replant disease.
In addition to being a direct cause of a disease, fungus can also worsen the condition of one whose immunity is weak, for example AIDS patients, to a point of death. In humans, a fungal infection is called a mycosis[?].
The water molds, of which potato blight is the best known example, show a hyphal organization and were once considered fungi. However they, and the closely related hypochytrids[?], are not actually related to the true fungi and instead belong among a group called the stramenopiles, together with the golden algae and their allies. The chytrids have also been excluded from the fungi due to the presence of flagellated spores, but are of definite relation to the others and so are now usually treated with them.
Slime molds were also originally placed here, because they produce fruiting bodies, but are now recognized to be several distinct groups of amoeboids.
Fungi are generally believed to have evolved from the same group of flagellates that gave rise to animals and choanoflagellates. Similarities include the structure of motile cells, when present, and the common presence of chitin in some groups.
Many orchids require fungus to germinate.
Fungi of the genus Penicillium produce penicillin, the first antibiotic known to modern science. Many bacteria have since become resistant to penicillin, but it is still used against Streptococcus and other very dangerous germs.
Other fungi include:
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