There are millions of species on this planet.The largest phylum Arthropoda alone comprises of millions of species. To study all of them properly we need some system for categorizing them, based on some common characteristics.
Apart from that think of the differences in language and you will find different names for the same species in different languages.In the absence of a system of nomenclature it would have been difficult to share scientific knowledge among the scientists of different nations.
To make things easier and systematic the need for classification was felt.Earlier attempts by people like Aristotle segregated living beings into plants and animals.This was good point to start with, but there are certain living beings which do not fit in either of plants or animals.So a three kingdom classification was devised.The three kingdom classification was used for some time.But as none of the scientific theory is permanent so further discoveries led to creation of a five kingdom classification.The five kingdom classification is the most accepted system of classification.

Reproduction in Fungi. Reproduction in fungi can be vegetative, asexual or sexual.
(a) Vegetative Reproduction. Reproduction in fungi can take place by vegetative means - fragmentation, fission and budding.
(b) Asexual Reproduction.Asexual reproduction is by spores called conidia or sporangiospores or zoospores.
(c) Sexual Reproduction. Sexual reproduction is by oospores, ascospores and basidiospores. The various spores are produced in distinct structures called fruiting bodies.
The sexual cycle involves the following three steps:
(i) Plasmogamy. Fusion of protoplasms between two motile or non-motile gametes is called plasmogamy.
(ii) Karyogamy. Fusion of two nuclei is called karyogamy.
(iii) Meiosis in zygote resulting in haploid spores.When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse. In some fungi the fusion of two haploid cells immediately results in diploid cells (2n).However, in other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n i.e. two nuclei per cell) occurs; such a condition is called a dikaryon and the phase is called dikaryophase of fungus.Later, the parental nuclei fuse and the cells become diploid.The fungi form fruiting bodies in which reduction division occurs, leading to formation of haploid spores.

Two kingdom classification was unable to accommodate certain living beings as they neither had properties of animals nor of plants.Through two kingdom classification it was not possible to include every set of characters in a particular kingdom.
Five kingdom classification could accommodate more set of characters.As a result it was easier to categorize all living beings.
For example bacteria can be autotrophic as well as heterotrophic.It was a difficult question to categorize them either under plant or animal.A separate kingdom was the most logical choice.
Similarly, viruses are a borderline case between living and non-living.When they don,t get host then they behave like a non-living.Once, inside a host they spring to life. Although the five kingdom classification does not have room for virus but it opened the way for a better and more inclusive system of classification.

(i) Heterocysts are present in some cyanobacteria. It is a large-sized, pale coloured, thick-walled cell which occurs in terminal, intercalary or lateral position in filamentous cyanobacteria,.e.g., Nostoc.
(ii) Features of cyanobacteria are
(a) Mucilage sheath is present.
(b) Thylakoids lack phycobilins. Therefore, photosynthesis is absent, but cyclic photophosphorylation occurs.
(c) Heterocyst is dependent for its nourishment on adjacent vegetative cells.
The thick wall is impermeable to oxygen, but permeable to nitrogen. It has enzyme nitrogenase which is specialised to perform nitrogen-fixation

Viruses are intermediate between living and non - living objects.
They resemble non - living objects in
(i) Lacking protoplast.
(ii) Ability to get crystallised.
(iii) High specific gravity which is found only in non - living objects.
(iv) Absence of respiration and energy storing system.
(v) Absence of growth and division.
(vi) Cannot live independent of a living cell.
They resemble living objects in
(i) Presence of genetic material (DNA or RNA).
(ii) Capable of mutation.
(iii) Irritability.
(iv) Can grow and multiply inside the host cell.

Viruses are intermediate between living and non - living objects.
They resemble non - living objects in
(i) Lacking protoplast.
(ii) Ability to get crystallised.
(iii) High specific gravity which is found only in non - living objects.
(iv) Absence of respiration and energy storing system.
(v) Absence of growth and division.
(vi) Cannot live independent of a living cell.
They resemble living objects in
(i) Presence of genetic material (DNA or RNA).
(ii) Capable of mutation.
(iii) Irritability.
(iv) Can grow and multiply inside the host cell.

Diseases caused by different classes of fungi are given below

Difference between ascocarp and basidiocarp are given below

Biological classification keeps with our understanding of life forms, Initially, new characters were taken by Aristotle, Theophrastus, John Ray and Linnaeus. Linnaeus divided plants on the basis of sex organs only.
As more and more knowledge gathered for delimitation of taxa, the ways for natural system of classification became clear.
This can be understood by following examples :
(i) Two kingdom classification was replaced by three kingdom classification when Haeckel (1866) proposed the kingdom of Protista.
(ii) It was converted into four-kingdom classification by Copeland when he proposed the kingdom-Monera.
(iii) Whittaker in 1969 proposed five kingdom classification
(iv) As the information about biochemistry of archaebacteria gathered, Carl Woese in 1990 raised another kingdom - Archaea.

Methanogens are a type of bacteria which live in the gut of ruminating animals.They assist those animals in digestion and the byproduct of that digestive process is methane.More number of livestock population results in increased methane level in the environment leading to global warming.So, indirectly methanogens can be responsible for global warming.