Fungi are a large and successful group of organisms and they range in size unicellular yeasts to large toadstools, puffballs and stinkhorns, and occur in a very wide range of habitats both aquatic and terrestrial. All fungi are eukaryotic and heterotrophic, and they obtain their food by absorption rather than by ingestion. Fungi include numerous molds growing on damp organic matter such as bread, leather, decaying vegetation, etc., unicellular yeasts which grow on the sugary surfaces of ripe fruits and many parasitic foil causing diseases of crops like mildews, smuts and rusts. Some fungi are parasitic on animals. Mycology is the brunch of botany dealing with the study of fungi. Fungi are eukaryotic, spore-bearing, achlorophyllous organisms that generally reproduce sexually and asexually.
GENERAL FEATURES OF FUNGI
Fungi are eukaryotes that lack chlorophyll, and are, therefore, heterotrophic. The body structure of fungi is unique. It typically consists of branched and filamentous hyphae, which form a net-like structure, known as mycelium. The hyphae are aseptale and coenocytic or septate. In most fungi the septa are porous and permit cytoplasmic flow from one cell to another. In some the nuclei are scattered through a continuous mass of cytoplasm—acoenocytic structure.
The hyphae have a thin, rigid wall made up of chiefly chitin, a nitrogen-containing polysaccharide. Within the cytoplasm usual eukaryote organelles such as mitochondria, Golgi apparatus, endoplasmic reticulum, ribosomes and vacuoles are present. n the older hyphae, vacuoles are large and cytoplasm is confined to a thin peripheral layer.
Fungi have a vegetative (assimilative) and a reproductive phase. During the assimilative phase, the hyphal mass grows in the substratum, whereas in reproductive phase the mycelium grows aerially and produces fruiting structures and reproductive cells.
Nutrition of Fungi
Fungi require an organic source of carbon; nitrogen, usually in the form of amino acids; inorganic ions like K+ and Mg++; trace elements such as Fe, Zn and Cu; and organic growth factors such as vitamins. Some of the fungi, particularly obligate parasites, require a wide range of ready-made components; others can synthesize their own requirements given only a source of carbohydrate and mineral salts. Some of the fungi may synthesize most of their requirements but need some particular amino acids or vitamins. The fungi’s nutrition can be described as absorptive since they absorb nutrients straight from outside their bodies. This is in dissimilarity to animals which normally ingest food, and then digest it within their bodies before of absorption takes place.
Fungi get their nutrients as saprotrophs, parasites, or symbionts. Saprophytic fungi ate those that obtain their nutrition from the dead rotting organic matter. They produce a variety of digestive enzymes to utilize a wide range of substrates. Parasitic fungi obtain their nutrition from other living organisms. Fungal parasites may be facultative or obligate, and more commonly attack plants than animals. Obligate parasites are usually restricted to a narrow range of hosts from which they require specific nutrients, whereas facultative parasites are less specialized and may grow on a variety of hosts or substrates. Obligate parasites possess specialized penetration and absorption devices called haustoria.
The obligate parasites secrete some specific enzymes which hydrolyze the proteins and carbohydrates of the host cell. The success of the parasite depends on the continued life of the host. Haustoria are rarely produced by facultative parasites. Symbiont fungi grow on other living organisms and both are mutually benefited. Lichens and mycorrhiza are common examples of symbioses. Lichens are symbiotic associations between algae and fungi; the algal component contributes organic food from photosynthesis, and the fungal component which is protected from high light intensity absorbs water and salt minerals. The plant is helped by the association because the fungal hyphae help in engrossing water, nitrogen and other minerals from the soil.