Fungus is a member of a large group of eukaryotic organisms which comprises microorganisms like moulds and yeasts and also the more well-known mushrooms. Such organisms are categorized as a kingdom Fungi that is separate from animals, plants, protists and bacteria. Fungi show a range of structures: unicellular, plasmodium similar to filamentous and pseudoparenchy-matous. Though, the various forms exhibit common cellular, physiological and biochemical features.
1) General: Fungi differ broadly in size and shape; from unicellular, microscopic organisms to multicellular forms simply view with the bare eye. Individual cells vary from 1 µ to 30 µ. Microscopic fungi present as either moulds or yeasts or both.
2) Moulds: The moulds form large multicellular collectives of long branching filaments, termed as hyphae. There are reproductive hyphae and vegetative hyphae. Spores are borne on the reproductive hyphae. (Fungal spores must not be perplexed by bacterial spores which are resistant bodies made for bacterial survival instead of reproductive purposes.) Spore shape, size and structure are employed in the categorization and recognition of fungi. The tube-like hyphae are accountable for the fluffy look of the macroscopic mould colony.
3) Yeasts: These are large (5 to 8 µ); single-celled organisms which seldom form filaments. Most yeasts reproduce through the asexual procedure of budding. Yeast colonies are generally characterized through a smooth surface identical to that of lots of bacteria.
The reproductive structures in fungi are made up from vegetative structures and represent a diversity of forms on the basis of which fungi are categorized. A few members of such divisions are listed as follows:
The most general unicellular fungi are yeasts that are of broad occurrence. Yeast is found on the sticky sugary surface of ripe fruit and develops in any sugar solution. The individual cells stick to one another making a chain. Single cells are hyaline however the colonies emerge greenish or brownish in color.
2) Slime Moulds:
Unicellular forms are as well seen in slime all through a certain phase of their life-cycle. Their features look like both protozoa and fungi. That is why it has been hard to categorize them. Such curious organism show unicellular (that is, multinucleate) protozoan-like or multicellular fungus-like phases throughout the course of their life-cycle. Slime moulds are further categorized as cellular slime moulds and plasmodial slime moulds.
3) Cellular type:
In the vegetative phase Dictyostelium discoideum, a cellular slime mould is small independent, uninucleate haploid cell termed as myxamoeba. Similar to amoeba, it feeds on bacterial by means of phagocytosis and multiplies through binary fission. Phagocytosis is the method in which a cell flows around particles in its surroundings and acquires them into the cytoplasm. At a later phase the individual myxamoebae come altogether and form a single multinucleate slug however the individual myxamoebae keep their intact cell membranes. This structure is termed as pseudo plasmodium.
4) Plasmodial type:
In plasmodial slime moulds, for illustration Echinosteliurn minutum, in the vegetative phase, a large mass of multinucleate amoeboid cytoplasm having features diploid nuclei is made. However unlike cellular slime moulds, the individual cells are not surrounded by the cell membrane. The cell-wall is absent. It nourishes on encysted myxamoebae and bacteria and might spread over a big area. The plasmodium doesn't encompass a definite size or shape.
Most of the fungi are filamentous. You might have observed on a piece of stale bread a web of very fine and delicate threads. These are made when a fugal spore lands on the bread and germinates into a small tube-like outgrowth that further grows as transparent, tubular filaments in all the directions. Each of such filaments is termed as hypha, the fundamental unit of fungal body. The mass of interwoven hyphae comprising the body of a fungus is termed as mycelium. It might comprise of highly dispersed hyphae, or it might be a cottony mass of hyphae. The aerial hyphae which bear reproductive structures are termed as reproductive hyphae. The fugal mycelium consists of an enormous surface to volume ratio and is close to the food source. This big surface-to-volume ratio is a wonderful adaptation for absorptive form of nutrition.
The mycelium of fungi is enclosed by a cell-wall build up of chitin, a polysaccharide that is as well found in the exoskeleton of insects and crustaceans. Though, in some fungi the cell-wall includes cellulose and lignin-like substances. The protoplasm of mycelium might be continuous all through the mycelium so that there will be some nuclei scattered all through the cytoplasm. This situation is known as coenocytic, such non-septate hyphae are noticed in the members of the Division Zygomycetes example: Rhizopus and Mucor. The septa or cross walls in the non-septate mycelia are made merely to cut off reproductive structures or to seal off the damaged part. Such septa are solid plates devoid of any pores.
The fungus mycelium in general, as illustrated above, is a mass of loosely interwoven hyphae that form a network. In a few fungi the whole mycelium or its portions experience different modifications. The walls of the hyphae in the mass get fused and they lose their uniqueness. As an outcome the hyphal mass, in cross section seems to be a continuous structure. It looks like the parenchymatous tissue of higher plants; however it is not a true parenchyma as found in the higher plants. In fungi like tissue is termed as plectenchyma.
Plectenchyma can further be distinguished into two types. The plectenchyma having rounded fungal cells is termed as pseudoparenchyma and having less compacted elongated cells is termed as prosenchyma.
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