Fungi the Kingdom of Recycles – Approximately 100,000species of organisms called “fungi” are known and many more are estimated to be present. This group includes notorious pathogens such as disastrous rusts, smuts of wheat and corn, and molds found growing on important crops and foodstuff. Delicacies such as mushrooms, truffles and morels, and other organisms of commercial use such as penicillium – the source of antibiotic penicillin, and the yeasts – used in bakeries and breweries are also members of this group. Ecological role of fungi as decomposers is paralleled only by bacteria.

Taxonomic status of fungi has changed from that of ‘a group of Plant kingdom’ to a separate kingdom “Fungi”. They resemble plants in some respects – have cell wall, lack centrioles and are non-motile. But fungi resemble more animals than plants. Unlike plants and like animals, fungi are heterotrophs, lack cellulose in their cell wall and contain chitin – the chemical found in external skeleton of arthropods. For this reason, some mycologists (scientists who study fungi) think that fungi and animals probably arose from a common ancestor.

But fungi are different from animals in having cell wall, being absorptive heterotrophs and non-motile. So fungi are neither plants nor animals. Their DNA studies also confirm that they are different from all other organisms. They show a characteristic type mitosis, called ‘nuclear mitosis’. During nuclear mitosis, nuclear envelope does not break; instead the mitotic spindle forms within the nucleus and the nuclear membrane constricts between the two clusters of daughter chromosomes. (in some fungi nuclear envelope dismantles late). Because fungi are distinct from plants, animals and protists in many ways, they are assigned to a separate kingdom ‘Fungi’.

The Body of Fungus

The body of a fungus, called mycelium, consists of long, slender, branched tubular thread like filaments called the hyphae (singular hypha). Hyphae spread extensively over the surface of substratum. Chitin in their wall is more resistant to decay than are cellulose and lignin which make up plant cell wall. Hyphae may be septate or non-septate. Septate hyphae are divided by cross-walls called septa (singular septum) into individual cells containing one or more nuclei.

Non-septate hyphae lack septa and are not divided into individual cells; instead these are in the form of an elongated multinucleated large cell. Such hyphae are called coenocytic hyphae, in which cytoplasm moves effectively, distributing the materials throughout. Septa of many septate fungi have a pore through which cytoplasm flows from cell to cell, carrying the materials to growing tips and enabling the hyphae to grow rapidly when food and water are abundant and temperature is favourable. All parts of fungus growing through the substrate are metabolically active. Extensive spreading system of hypae provides enormous surface area for absorption.

the_fungus_body_planFig 8.1 The fungus body plan:(a) Fungus mycelium growing on agar plate (b) Hyphae of mycelium (c) A coenocytic hypha (d) a septate hypha with porous septa and monokaryotic cells (e) A septate hypha with dikaryotic cell.

Hyphae may be packed together and organized to form complex reproductive structures such as mushrooms, puff balls, morels etc. which can expand rapidly. Yeast are non-hyphal unicellular fungi.

All fungal nuclei are haploid except for transient diploid zygote that forms during sexual reproduction.

A single mycelium may produce upto a kilometer of new hyphae in only one day. A circular clone of armillaria,a pathogenic fungus afflicting conifers, growing out from a central focus, has been measured upto 15 hectares (1 hectare = 10000 m2). Could it be the world’s largest organism?