Kingdom Fungi

NCERT grounding

NCERT Class 11 Biology, Chapter 2 (Biological Classification), §2.3, defines Kingdom Fungi as a unique kingdom of heterotrophic organisms showing great diversity in morphology and habitat. The text fixes three identity features: a body of thread-like hyphae forming a mycelium, a cell wall of chitin and polysaccharides, and absorptive heterotrophic nutrition. It then divides the kingdom into four classes — Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes — on the basis of mycelial morphology, mode of spore formation and fruiting body.

"The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for the division of the kingdom into various classes." — NCERT Class 11 Biology, §2.3

The fungal body & the four classes

Fungi sit in their own kingdom precisely because they fail the tests for both plants and animals. They are eukaryotic like both, but their nutrition is heterotrophic — they cannot photosynthesise — and yet they possess a rigid cell wall, which animals never have. The wall is the decisive character: it is built of chitin (a nitrogen-containing polysaccharide) rather than the cellulose of plants or the peptidoglycan of bacteria. Whittaker's table places them as eukaryotic, chitin-walled, with a multicellular or loose-tissue body organisation and saprophytic or parasitic nutrition.

Hyphae and mycelium — septate vs coenocytic

With the sole exception of unicellular yeasts, the fungal body is filamentous: long, slender, thread-like tubes called hyphae, whose interwoven network is the mycelium. How the hyphae are partitioned splits the kingdom into two structural camps. Some hyphae are continuous tubes filled with multinucleate cytoplasm and no cross walls — these are coenocytic (aseptate) hyphae, characteristic of Phycomycetes. Others are divided by cross walls called septae into distinct cells — septate hyphae, the rule in Ascomycetes, Basidiomycetes and Deuteromycetes.

Nutrition — saprophytic, parasitic, symbiotic

All fungi absorb soluble organic matter through their walls; they never ingest. Those that draw nutrition from dead organic substrates are saprophytes, the principal decomposers of nature. Those that live on living plants or animals are parasites — the wheat-rust fungus Puccinia and the mustard parasite Albugo are NCERT examples. A third mode is symbiotic: fungi partner with algae to form lichens, and with the roots of higher plants to form mycorrhiza. These four terms — saprophyte, parasite, lichen, mycorrhiza — are a recurring NEET matching set.

Reproduction — spores and the sexual cycle

Fungi reproduce by three routes. Vegetative reproduction is by fragmentation, fission and budding. Asexual reproduction is by spores — conidia (produced exogenously on conidiophores), sporangiospores (produced endogenously inside a sporangium) and motile zoospores. Sexual reproduction yields oospores, ascospores or basidiospores. The various spores are borne in distinct structures called fruiting bodies.

The sexual cycle runs through three NCERT-fixed steps, and the order is a favourite target. First plasmogamy — fusion of the protoplasms of two gametes. Then karyogamy — fusion of the two nuclei. Finally meiosis in the zygote, producing haploid spores. In some fungi the two haploid cells fuse straight into a diploid (2n) cell. But in Ascomycetes and Basidiomycetes an intervening stage appears in which each cell carries two nuclei (n + n) of different parental origin — the dikaryon, and the phase is the dikaryophase. Only later do the parental nuclei fuse.

The four classes

The classification basis — mycelial morphology, spore formation and fruiting body — sorts the kingdom into four classes. The first three are defined by their sexual spore; the fourth is a holding pen for fungi whose sexual stage is unknown.

Phycomycetes — the algal fungi

Phycomycetes live in aquatic habitats, on decaying wood in damp places, or as obligate plant parasites. Their mycelium is aseptate and coenocytic. Asexual reproduction is by motile zoospores or non-motile aplanospores, produced endogenously in a sporangium. Sexual fusion of two gametes — isogamous, anisogamous or oogamous — gives a zygospore. NCERT examples: Mucor, Rhizopus (the bread mould) and Albugo (mustard parasite).

Ascomycetes — the sac fungi

Ascomycetes are mostly multicellular (Penicillium) or rarely unicellular (yeast, Saccharomyces). They may be saprophytic, decomposers, parasitic or coprophilous. Mycelium is branched and septate. Asexual conidia are produced exogenously on conidiophores. Sexual ascospores are produced endogenously inside sac-like asci (singular ascus), themselves arranged in fruiting bodies called ascocarps. Examples: Aspergillus, Claviceps, Neurospora (a genetics workhorse); morels and truffles are edible delicacies.

Basidiomycetes — mushrooms, rusts, smuts

Basidiomycetes are the mushrooms, bracket fungi and puffballs, plus the parasitic rusts and smuts. Mycelium is branched and septate. Asexual spores are generally absent; vegetative reproduction is by fragmentation. Sex organs are absent — plasmogamy occurs by fusion of two somatic cells of different strains, producing a dikaryotic mycelium that gives rise to a basidium. Karyogamy and meiosis in the basidium yield four basidiospores borne exogenously; the basidia sit in fruiting bodies called basidiocarps. Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust).

Deuteromycetes — the imperfect fungi

Deuteromycetes — Fungi Imperfecti — are so named because only their asexual or vegetative phases are known. They reproduce only by asexual conidia; the mycelium is septate and branched. The class is a taxonomic waiting room: once the sexual (perfect) stage of a member is discovered, it is moved to Ascomycetes or Basidiomycetes. Many are decomposers of litter aiding mineral cycling. Examples: Alternaria, Colletotrichum, Trichoderma.

Worked examples

Common confusion & NEET traps