Unit I: Fungi

Fungi (Unit - I) Introduction: The word ‘fungus’ has been taken from Latin, which means “mushroom.” The scientific study of fungi is known as “Mycology” (Greek: mykes = mushroom; logos = discourse). Definition: Fungi are a special group of plants that are achlorophyllous and heterotrophic thalloid eukaryotic organisms of variable forms, behavior, and life cycles. Occurrence: They are cosmopolitan in distribution and can occur in any habitat where life is possible. Some fungi exist as free-living or as saprophytes, others are parasitic and still others are air-borne. Many species are parasitic, infecting plants, animals, and human beings. General Characteristics of Fungi: The plant body of fungi typically consists of branched and filamentous hyphae, which form a net-like structure known as mycelium. The hyphae are aseptate and coenocytic or septate and uninucleate. Multinucleate: Except in the exception of slime molds, the protoplast remains surrounded by a distinct cell wall. The main component of the cell wall is the fungal element known as chitin. In some, the loosening of chitin is replaced by cellulose and glucan. The hyphae contain cellulosic and vacuolated cytoplasm. They possess membranous cell organelles and many granules, such as glycogen, starch, etc. Fungi are entirely devoid of chlorophyll, but carotenoids are normally present. The higher fungi are immotile, i.e., motile cells do not occur in the life cycle at any stage, but in lower fungi, the reproductive cells are motile. They show acrozoospores/planospores. Flagella are of two types: (i) whiplash (acronematic): flagella are smooth with 9+2 organization and insert (penultimate). (ii) tinsel (pantonymematic): flagella with numerous hair-like fibrils and an anterior surface. They are heterotrophic, i.e., they do not manufacture their own food material. They are usually grouped into the following three categories on the basis of their modes of nutrition: (i) Parasitic (obtain food from living plants and animals). (ii) Saprophytic (obtain food from dead decaying organic matter). (iii) Symbiont (grow on living organisms and both are mutually benefited). Most of the fungi, only a part of the vegetative mycelium forms reproductive units and the rest remains vegetative. Such fungi are known as “Eucarpic.” However, in unicellular fungi, if the whole vegetative cell is transformed into a reproductive unit, they are called as “Holocarpic.” They reproduce by vegetative, asexual, and sexual means. The vegetative reproduction may take place by fragmentation, fission, or budding. Asexual reproduction occurs by asexual spores such as conidia. Some spores such as conidia, oidia, sporangiospores, zoospores, and aplanospores. Except for the class Deuteromycetes, sexual reproduction occurs in all the groups of fungi. Thallus Organization in Fungi: The following points highlight the two main types of thallus found in fungi: Unicellular Thallus Multicellular or Filamentous Thallus Unicellular Thallus: In some of the lower fungi, such as the chytrids, the thallus is monocentric or a spherical, single-celled structure. At the time of reproduction, it becomes reproductive uniflagellate. The latter produces the asexual or sexual cells. Such fungi are called as holocarpic. In others, both vegetative and reproductive stages do not occur together in the same thallus. Plasmodiophora have a vegetative phase consisting of a naked, multi-nucleate, amoeboid mass of protoplasm, termed as plasmodium. The protoplast of the diploid haustorium cleaves to form the resting spores. The yeasts, which are related to the filamentous forms, also have a unicellular thallus. Filamentous Thallus: The vast majority of the fungi have a filamentous thallus that originates when the germination of spores. The spore germinates and extends by elongation, forming other ramifications of life. In some, the germination produces only a short thallus structure of limited growth. Constitute the thallus and is technically called a hypha. The spores of most of the fungi, however, give rise to a fluffy thallus consisting of a cottony mass of fine branched filaments called hyphae. Some of the hyphae, at a certain stage of maturity, extend and form aerial hyphae and bear the reproductive body. The rest spread over or within the substrate and continue the normal activity. Such fungi are called mycelium. The hypha is thus a structural unit of the mycelium. A single hypha is a tube filled with protoplasm, bounded by a membrane, and covered by a rigid cell wall. The medium upon which mycelium spreads is known as the substratum. Mycelium carries out general activities like providing cell nutrition, reproduction, respiration, excretion, and growth. The hyphae are usually colorless, particularly those embedded in the substratum. In some fungi, they become colored: black, orange, yellow, red, blue, and brown are the usual tints. The color is usually confined to the hyphal wall. Even when the pigments are present in the protoplasm, they do not form an integral part of the living matter. The pigments play no role in the physiology of the fungus. The growth in length takes place at the tip of the hyphae and is thus termed as apical.