The branch of biology dealing with fungi or diseases caused by fungus is known as mycology. In fact, mycology is among the earliest branch of knowledge pertaining to microbiology. At the same time, it may be mentioned that fungi usually comprise the most common microorganism that is studied in microbiology. Everyone is familiar with fungi for they have seen it grow in hair-like structures on decaying fruits and stale bread. And since the ancient periods, fungi have been used by man to produce fermented bread as well as alcoholic infusions. That fungus is responsible for the decay of fruits as well as animal substance, has been identified by people since time immemorial. In fact, fungi have been so closely related to the moldering organic substances that they have almost turned out to be identical with moldiness, disintegration as well as decomposition.
Fungi are present in an assortment of remarkable sizes - ranging from comparatively big and compressed organizations like puff-balls, mushrooms, toadstools and bracket fungi that found to be assailing decomposing trees by means of their various systems of strands in the soil, often related to the roots of plants, as far as the miniscule single-celled yeasts. Nevertheless, all fungi are basically eukaryotic (single-celled organism whose cells contain a distinct membrane-bound nucleus) beings. As mentioned, the fungi cells have a nucleus and nucleoli bound by a membrane. While their cellular respiration takes place in the mitochondria in the cytoplasm, the fungal cells also posses a complicated array of internal membrane structure.
Although the fungi demonstrate an amazing assortment of size and shape, they may be generally categorized in two groups - the yeasts and the moulds. In fact, the moulds are also known as filamentous or mycelia fungi and they are made up of an arrangement of strands or filaments that are known as hyphae (singularly known as the hypha). The hyphae are actually intertwined into a formation known as a mycelium (plural called mycelia). Actually, 'huphe' is a Greek term denoting a web, while the term mycelium has been drawn from the word 'mukes' denoting mushroom. Moulds are capable of replicating themselves both by asexual as well as sexual procedures, while the fruiting and mycelium of any mould are together known as the fungal thallus, which translated into Greek means 'green shoot'. The germ cells or spores of the fungi are very significant in recognizing the fungi. Often the mycelia tissue is also known as an anastomosis (inosculation), as it encompasses a mesh of cross-connecting hyphae. In fact, the tips of the hyphae grow to develop into the mycelial formations. In addition, these hyphae comprise filaments branching out now and then. The cytoplasm present in the infantile hyphae fills up the hollow space in the filaments. However, further back from the developing tips of the hyphae, the cytoplasm has more and more vacuoles. In other words, they turn out to be more vacuolated at the tips. In fact, the oldest hyphae are vacant formations that may even be detached from the remaining part of the mycelium.
When closely scrutinized, it is seen that in most of the moulds, the hyphae are separated into segments by the normal presence of cross-walls or septa (singularly known as septum). These formations provide firmness to the strands or filaments as well as help to organize the passage of nutrients all the way through the complex arrangement of the mycelia. Incidentally, the intricacy of the septa also differs - while a plain septa possesses a solitary central pore, a number of septa found in advanced fungi possess a dolipore (single double-layered membrane) arrangement wherein a slender central pore is bordered by pierced membranes called parenthesomes that resemble a cap and is composed of shapeless substances. The separate sections present in the septate hypha may enclose a solitary nucleus and they are considered to be uninucleate or they may even possess multiple nuclei and are, hence, also known as multi-nucleate. Phycomycetes, which are considered to be primitive fungi, do not possess any septa to segregate the hyphae into different segments. On the other hand, the aseptate hypha is expressed as coenocytic - a term derived from the Greek word 'kinos' or 'kutos' commonly denoting a vessel. Infrequently, the septa grows into Phycomycetes, however, their role is limited to detach the reproductive parts from the vegetative body of the fungus or to isolate the older segments of the thallus. Very much different from the septa of other fungi, the formations detailed by the Phycomycetes are firm salvers and they never possess any central pore.
In some advanced variety of fungi, the neighboring hyphae are able to blend passively to produce a three-dimensional complex formation. In fact, such three-dimensional formations give rise to reproductive fruiting bodies of the fungi. In addition, the hyphae also have the ability to develop other specific structures that display a great extent of internal organization due to synchronized development.
The rhizomorphs (a dense mass of hyphae forming a root-like structure characteristic of many fungi) are basically filaments resembling ropes and possess an extremely distinguished formation. These arrangements seem to grow in response to pressure and in nature they grow in comparatively arid surroundings like the conditions present in sandy soils. Fungi also possess a substance called sclerotia (singularly known as sclerotium) that are basically solidified formations that enable the moulds to remain alive even in an inactive or dormant condition. By nature, sclerotia are colored and are large enough to be seen by the naked eye. Usually, they have a spherical shape, but may also appear in various asymmetrical forms. The cells of the exterior walls of a sclerotium have broad walls providing the structure a substantial and defensive covering. This thick, protective coat surrounds a central cortex of hyphae that enclose the food preserves needed during the period when it lies dormant. Usually, the food reserves or nutrients are preserved as glycogen or oil globules. Nevertheless, the most common formations produced by fungi are toadstools and mushrooms. These fungi formations are extremely complicated structures that participate in reproduction of the microorganism and display an amazing degree of internal segregation as well as organization. Usually, a stalk called stipe holds up the cap known as pileus and the gills of the fungi grow beneath this cap. It needs to be mentioned here that these gills are responsible for releasing the spores or germs that cause fungal diseases. In fact, the growth of these gills is an extremely harmonized procedure that reacts to ecological stimulus. It is essential for the gills to grow vertically for them to be able to release the spores effectively. Such vertical growth of the gills is possible owing to geotropism (development of an organism in response to gravity). Close observation of the development process of the gills have shown that if the developing formation is slanted, then the gills would develop vertically but in a lying position.
Usually, the fungal thallus in yeasts comprises of a solitary cell. In fact, yeasts are primarily single-celled fungi that vary in shape from round to oval or elongated. Even the size of the yeast may differ from two to approximately 10 micrometers. It may be noted that only a restricted number of yeasts possess intricate additional cellular casings. An instance of this type of yeast is the Cryptococcus neoformans - a pathogen that results in a chronic type of meningitis (inflammation of the meninges of the brain and the spinal cord) mostly found in patients affected by acquired immune deficiency syndrome (AIDS). In such cases, the encasing or capsule mucopolysaccharide assists the yeast to avoid the body's defense mechanisms, and thereby, enables it to cause and spread the diseases.
In general, the reproduction of the yeasts involves an asexual budding process. In the first stage, the parental yeast cell enlarges to form a swelling or protuberance that expands into a fungal spore (blastospore) that ultimately detaches from its parent cell. The term 'blastospore' has been derived from the Greek words 'blastos' and 'sprout' denoting a spore developed from a sprout. However, in the fission (cell division to reproduce) yeasts like Schizosaccharomyces pombe, the parental yeast cell divides into two offspring in such as way that is slightly similar to the transverse or intersecting binary fission as in the case of reproduction of bacteria. In fact, the yeasts hardly ever have a genuine multi-cellular arrangement. Some yeasts have an appearance of a series or chains of drawn out cells that are known as pseudomycelia (singularly know as pseudomycelium) or pseudohyphae (singularly called pseudohypha). In fact, pseudomycelia are basically extended yeast cells that emerge from the buds sticking on together in branching successions. Each separate cell present in a pseudomycelium is free from each other and, dissimilar to the units present in the septate hyphae of moulds, these cells are not connected with one another by means of pores. Yeast cells possessing a normal unicellular structure may bunch at the terminations or alternately the length of the side of a pseudomycelium. Such developments are known as secondary blastospores. At the same time, there are a number of yeasts that are able to fabricate septate - genuine mycelia under specific situations of growth.