Conventionally Fungi have been included in plant kingdom. But in pursuance of Whittaker's five kingdom classification Fungiand Plants (Algae, Bryophytes, Pteridophytes Gymmosperms and Angiosperms) are described here as two separate kingdoms. Angiosperms are not described in detail here.
Fungi are non chlorophyllous, eukaryotic, organisms. They are a large and successful group. They are universal in their distribution. They resemble plants in that they have cell walls. But they lack chlorophyll which is the most important attribute of plants. They are ubiquitous in habitat which ranges from aquatic to terrestrial. They grow in dark and moist habitat and on the substratum containing dead organic matter. Mushrooms, moulds and yeasts are the common fungi. They are of major importance for the essential role they play in the biosphere and for the way in which they have been exploited by man for economic and medical purposes. The study of fungi is known as Mycology. It constitutes a branch of microbiology because many of the handling techniques used, such as sterilizing and culturing procedures are the same as those used with bacteria.
They have definite cell wall made up of chitin - a biopolymer made up of n- acetyl glucosamine units.
They are without chlorophyll, hence they exhibit heterotrophic mode of nutrition. They may be saprotrophic in their mode of nutrition or parasitic or symbiotic.
They are usually non - motile except the subdivision Mastigomycotina.
Their storage product is not starch but glycogen and oil.
They reproduce mostly by spore formation. However sexual reproduction also takes place.
The body structure of fungi is unique. The somatic body of the fungus is unicelllular or multi-cellular or coenocytic. When multi-cellular it is composed of profusely branched interwoven, delicate, thread like structures called hyphae, the whole mass collectively called mycelium. The hyphae are not divided into true cells. Instead the protoplasm is either continuous or is interrupted at intervals by cross walls called septa which divide the hyphae into
compartments similar to cells. Thus hyphae may be aseptate(hyphae without cross walls) or septate (hyphae with cross walls).When aseptate they are coenocytic containing many nuclei. Each hypha has a thin rigid wall, whose chief component is chitin, a nitrogen containing polysaccharide also found in the exoskeleton of arthropods. Within the cytoplasm the usual eukaryotic organelles are found such as mitochondria, golgi-apparatus, endoplasmic reticulum, ribosomes and vacuoles. In the older parts, vacuoles are large and cytoplasm is confined to a thin peripheral layer.
Fungi are heterotrophic in their mode of nutrition that is they require an organic source of carbon. In addition they require a source of nitrogen, usually organic substances such as amino acids. The nutrition of fungi can be described as absorptive because they absorb nutrients directly from outside their bodies. This is in contrast to animals which normally ingest food and then digest it within their bodies before absorption takes place. With fungi, digestion is external using extra-cellular enzymes. Fungi obtain their nutrients as saprotrophs,parasites or symbionts.
A saprotroph is an organism that obtains its food from dead and decaying matter. It secretes enzymes on to the organic matter so that digestion is outside the organism. Soluble products of digestion are absorbed and assimilated within the body of the saprotroph.
Saprotrophic fungi and bacteria constitute the decomposers and are essential in bringing about decay and recycling of nutrients. They produce humus from animal and plant remains. Humus, a part of soil, is a layer of decayed organic matter containing many nutrients. Some important fungi are the few species that secrete the enzyme cellulase which breaks down cellulose. Cellulose being an important structural component of plant cell walls, rotting of wood and other plant remains is achieved by these decomposers secreting cellulases.