Biological Classification

Kingdom Fungi

The fungi are filamentous; except yeast (unicellular). The body consists of long, slender thread-like structures; called hyphae. The network of hyphae is called mycelium. Some hyphae are continuous tubes which are filled with multinucleated cytoplasm. Such hyphae are called coenocytic hyphae. The other type of hyphae has septae or cross-walls. The cell wall of fungi is composed of chitin and polysachharides.

Most of the fungi are heterotrophic and are saprophytes. Some are parasites. Some of the fungi also live as symbionts. Some of the symbiont fungi live as lichens; in association with algae. Some of the symbiont fungi live as mycorrhiza; in association with roots of higher plants.


Reproduction in Fungi:

Reproduction by vegetative means takes place by fragmentation, fission and budding. Some fungi reproduce asexually by forming spores which are called conidia or sporangiospores or zoospores. Sexual reproduction is by oospores, ascospores and basidiospores. The spores are produced in distinct structures called fruiting bodies.

The sexual cycle involves three steps which are as follows:

During sexual reproduction, two haploid hyphae of compatible mating types come together and fuse. In some fungi, the fusion of two haploid cells immediately results in a diploid cell (2n). In other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage occurs. In this stage, two nuclei are present in each cell. This condition is called dikayron. The parental nuclei fuse at a later stage and the cells become diploid. Reduction division in the fruiting bodies leads to the formation of haploid spores.


On the basis of morphology of mycelium, mode of spore formation and fruiting bodies; Kingdom Fungi is divided into following classes:

Phycomycetes:

The members of phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places. They can also be found as obligate parasites on plants.

Mycelium: Aseptate and coenocytic.

Reproduction:

Examples: Mucor, Rhizopus and Albugo (the parasitic fungi on mustard).

Ascomycetes:

They are commonly known as sac-fungi. They are unicellular or multicellular. They are saprophytic, decomposers, parasitic or coprophilous. Those growing on dung are called coprophilous.

Mycelium: Branched and septate.

Reproduction:

Asexual spores are exogenously produced on the special mycelium called conidiophores. Sexual spores are called ascospores. They are produced endogenously in sac like asci. These asci are arranged in various kinds of fruiting bodies called ascocarps.

Examples: Aspergillus, Claviceps, Neurospora, yeast, penicillium, morels, baffles, etc.


Basidiomycetes:

They grow in soil, on logs and tree stumps. Some of them also grow in living plant bodies as parasites. Mushrooms, bracket fungi or puffballs are the commonly known forms.

Mycelium: Branched and septate.

Reproduction:

Asexual spores are usually absent. Vegetative reproduction by fragmentation is common. Sex organs are absent. But plasmogamy is brought about by fusion of two vegetative cells of different strains. The resultant structure is dikaryotic which finally forms the basidium. Karyogamy and meiosis are responsible for formation of four basidiospores in a basidium. The basiodiospores are exogenously produced. Basidia are arranged in fruiting bodies called basidiocarps.

Examples: Agaricus (mushroom), Ustilago (smut) and Puccinia (rust).

Deuteromycetes:

They are usually known as imperfect fungi because only the asexual or vegetative phase of them is known. A large number of deuteromycetes are decomposers, while some members are parasites.

Mycelium: Branched and septate.

Examples: Alternaria, Colletotrichum and Trichoderma.



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