Fungi are heterotrophic, spore bearing eukaryotes. Fungi may be single-celled or multicellular. Multicellular fungi are composed of networks of long filamentous, branched structures called hyphae. The hyphae often aggregate in a dense network known as a mycelium. The hyphae may be without cross walls (as in the case of lower fungi) or divided into compartments by formation of septa (in the higher fungi). 

However, even in the septate hyphae, the cytoplasm of the cells is continuous via a central pore in the septum. In certain stages, often during transition to the sexual or asexual reproduction phase, the mycelium forms tissue like aggregates, called plectenchyma. A typical plectenchyma is the flesh of the mushroom. Although fungi lack true organs, the mycelia of ascomycetes and basidiomycetes may become organised into more complex reproductive structures called fruiting bodies, or sporocarps.

Nature and Origin

Several large species of fungi like mushrooms, morels, smuts, ts and rusts, were known to man since the time immemorial, but nature, origin and development were unknown at that time. The Alternaria, Cercoil record shows their presence in the Devonian and Pre-cambrian periods. There is evidence in the ancient literature of human culture that these plants were used as food by human-beings.

At present, about 5,100 genera and more than 50,000 species of fungi are known. They grow in a variety of habitats and show much diversity in their form, structure, physiological processes and methods of reproduction.

Characteristics of Kingdom Fungi

The distinctive characters of Fungi are:

(a) They are heterotrophic due to the absence of chlorophyll. 

(b) They become either parasites or saprophytes (heterotrophs).

(c) Thallus is made up of a colourless filamentous structure called hyphae. The hyphae with branches and sub-branches form a complex network called mycelium.

(d) Food material is stored in the form of glycogen and oil globules.

(e) Cell wall is made up of fungus cellulose.

(f) The nuclei are eukaryotic in nature.

 (g) Gradual and progressive reduction of sexuality is present.

Basis of Classification of Kingdom Fungi

Fungi has been variously classified by different authors depending upon the criteria they have used for the purpose. However, there is agreement to classify true fungi into four different classes as given above:

Because in fungi structure of mycelium does not exhibit much variation, following criteria are considered mainly for classification :

• Morphology of reproductive structures.
• Physiology and biochemistry of various fungal

Classification of Kingdom Fungi

The classification of fungi is not yet settled and many systems of classification have been proposed from time to time by various mycologists. With the advancement of our knowledge, newer systems of fungal classification are being proposed and the older ones are discarded. Most of the earlier systems of fungal classification were based on the following criteria: 

• Thallus organisation and cell wall composition. 
• Kinds of asexual spores-their colour, shape, septation and flagellation.
• Presence or absence of sexual reproduction.
• Nature of the life cycle. However, in more recent systems of classification, besides morphological criteria, host specialisation, physiological, cytological, genetical, serological and biochemical characters have also been taken into consideration.

Ainsworth Classified Fungi into two divisions. They were Mycota and Eumycota.

Mycota are slime mould which was further divided into different classes such as Acrasiomycetes, Hydromyxomycetes, Myxomycetes and Basidiomycetes.

Division Eumcota is further divided into four subdivisions. They are:

• Mastigomycotina
• Zygomycotina
• Ascomycotina
• Basidiomycotina

• Mastigomycotina is a former polyphyletic taxonomic grouping, a subdivision, of fungi that was similar to Phycomycetes and included the zoosporic classes Chytridiomycetes, Hyphochytriomycetes, Plasmodiophoromycetes, and Oomycetes. It is now classified as a subclass of fungi.
• Zygomycota, also known as zygote fungi, is a former division or phylum of the kingdom Fungi that includes zygote fungi. The Mucoromycotina and Zoopagomycota are the two phyla in which the members are now classified. There are approximately 1060 species known to exist. Its habitat is primarily terrestrial, with most of its inhabitants living in soil or on decaying plant or animal matter.
• Ascomycotina is a large subdivision of Eumycota that includes Hemiascomycetes and Plectomycetes, as well as Pyrenomycetes and Discomycetes; sac fungi; and is considered a division of the kingdom Fungi in some classification systems. Hemiascomycetes and Plectomycetes are two of the most common species of Ascomycota. Ascomycota, subdivision Ascomycota, subdivision Ascomycotina; Ascomycota, subdivision Ascomycota, subdivision Ascomycotina.
• Basidiomycotina (puffballs), Tiliomycetes (which includes the orders Ustilaginales (smuts) and Uredinales (rusts), and Hymenomycetes are all members of the Basidiomycotina. Gasteromycetes are fungi that produce spores on their basidium (mushrooms, toadstools, agarics and bracket fungi).

Economic Importance of Fungi

Fungi play a significant role in our daily life. They are our greatest friends as well as foes. Some fungi are prized for their usefulness to mankind, while others are highly injurious as they are responsible for several diseases in plants, animals and human-beings. Some important useful and harmful activities of fungi are discussed below.

Fungi are beneficial to us, directly or indirectly, in many ways. They keep our environment clean by removing organic debris; they provide food, medicines and several other useful products. Fungi also play an important role in many industries.

Mushrooms are the fruiting bodies of higher fungi – Ascomycetes Basidiomycetes. They have been used as food by human beings for e time immemorial. Their use, as recorded in the ancient Indian rature, dates back to 3000 B.C. Mushrooms were also known to be ancient Greek and Roman civilizations. Egyptians considered them God’s food and used them only on certain festive occasions. In recent times, mushrooms were utilised only for taste and flavour, and were collected from their natural habitats. Later, the techniques for the cultivation of mushrooms were developed at the beginning of the 18th century in France.

There are about 7,000 species of mushrooms, but a little over 100 species are suitable for human consumption. Only a few species of mushrooms are commercially cultivated.

Conclusion

Along with some heterotrophic bacteria, fungi are major decomposers. They are required for recycling of inorganic resources in the biosphere. Like some of the bacteria, some fungi also produce diseases. Especially important are the fungal diseases of plants, for fungi are more harmful to plants than bacteria are to human beings. Branch of biology which deals with the study of fungi is called mycology. The study of diseases caused in plants are included in plant pathology.