Fungal Morphology and Forms

You are probably familiar with yeast, bread mold, rust, smut, and mushrooms. They all are members of the fungal Kingdom. Fungi exhibit a range of structures: unicellular, plasmodium-like filamentous, and pseudoparenchy-matous.

However, the different forms show common cellular, physiological and biochemical characteristics.

Baker’s yeast is a unicellular fungus. It is very minute in size and looks like a pinhead under the light microscope. Most fungi are microscopic but several grow very large. For example, mushrooms, motels, and puffballs can be seen with unaided eyes.

Under the microscope, a slime mold looks like a protozoan with a naked amoeboid mass of protoplasm. Bread mould (Mucor), pink mould (Neurospora) and green mould (Penicillium) show branched filaments.

Whereas mushrooms, morels, and puffballs are the fruiting bodies formed by the close packing of several interwoven filaments.

When conditions are suitable the fruiting bodies develop from the mycelium which otherwise grows beneath the surface of the ground. A mushroom consists of an umbrella-like cap and a stalk or stipe.

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Fungal Morphology and Forms

The reproductive structures in fungi are formed from vegetative structures and exhibit a variety of forms on the basis of which fungi are classified. A few members of these divisions are listed below.

1. Unicellular Forms

a. Yeast

The fungi are unicellular, often multicellular or acellular eukaryotic organisms. The most common unicellular fungi are yeasts. Which are of wide occurrence. Yeast is found on the sticky sugary surface of ripe fruit and grows in any sugar solution.

The individual cells adhere to one another forming a chain. Single Cells are hyaline but the colonies appear greenish or brownish in colour.

The fine structure of a yeast cell is of the eukaryotic type. It has a well-defined nucleus, mitochondria, endoplasmic reticulum and other organelles.

Close to the nucleus, a large area of cytoplasm is occupied by vacuole. The cell wall of yeast has 2-3 layers made of chitin and polysaccharides – glucan and mannans,

Depending upon the stage of development variable amounts of proteins, lipids and other substances are found accumulated in the cell.

Yeasts are distributed well over the surface of earth. They are abundant on substrates that contain sugars, like the nectar of flowers and surface of fruits. They are also found in soil, animal excreta, milk and on the vegetative parts of plants and also in some other habitats.

Yeasts are noted particularly for their ability to utilize carbohydrates, hence the name Saccharomycetes is applied to this group. Another unicellular fungus is Olpidium, the simplest chytrid, which is a simple globular cell without branches.

b. Slime Moulds

Unicellular forms are also seen in slime during a certain stage of their life cycle. You must remember that slime moulds are not considered true fungi. Their characteristics resemble both protozoa and fungi. That is why it has been difficult to classify them.

These curious organism show unicellular (multinucleate) protozoan-like or multicellular fungus-like stages during the course of their life cycle. Slime moulds are further classified as cellular slime moulds and plasmodial slime moulds.

c. Cellular type

In the vegetative stage Dictyostelium discoideum, a cellular slime mould is small independent, uninucleate haploid cell called myxamoeba. Like amoeba, it feeds on bacterial by phagocytosis and multiplies by binary fission.

Phagocytosis is the process in which a cell flows around particles in its surroundings and takes them into the cytoplasm.

At a later stage the individual myxamoebae come together and form a single multinucleate slug but the individual myxamoebae retain their intact cell membranes. This structure is called pseudoplasmodium.

In the reproductive stage, sporangia-bearing spores are formed like in true fungi, each spore germinates to form an amoeba like structure.

d. Plasmodial Type

In plasmodial slime moulds, for example Echinosteliurn minutum, in the vegetative stage, a large mass of multinucleate amoeboid cytoplasm with characteristic diploid nuclei is formed. But unlike cellular slime moulds, the individual cells are not delimited by cell membrane.

The cell wall is absent. It feeds on encysted myxamoebae and bacteria and may spread over a large area. The plasmodium does not have a definite size or shape. It may be globose, flat and sheet-like spreading over a large area in the form of a very thin network.

When the plasmodium creeps over the surface of the substratum, it changes its shape accordingly and engulfs particles of food on its way. Finally, it matures and changes into the fructification typical of the species.

The entire plasmodium takes part in the formation of fructifications, which bear spores resulting from meiosis. The spores germinate to produce flagellated cells which develop into plasmodium (myxamoeba).

Slime mould plasmodia are often brilliantly coloured ranging from colourless to shiny grey, black, violet, blue, green, yellow, orange and red. The yellow and the white plasmodia are probably the most commonly encountered.

Colour changes have been observed to occur within a plasmodium under laboratory conditions. Most slime moulds live in cool, shady, moist places in the woods, on decaying logs. Dead leaves or other organic matter which holds abundant moisture.

2. Filamentous Forms

Most fungi are filamentous. You may have noticed on a piece of stale bread a web of very fine and delicate threads.

These are formed when a fugal spore lands on the bread and germinates into a small tube-like outgrowth, which further grows as transparent, tubular filaments in all directions.

Each of these filaments is called hypha, the basic unit of fungal body. The mass of interwoven hyphae constituting the body of a fungus is called mycelium.

It may consist of highly dispersed hyphae, or it may be a cottony mass of hyphae. The aerial hyphae that bear reproductive structures are called reproductive hyphae.

The fugal mycelium has an enormous surface to volume ratio and is close to the food source. This large surface-to- volume ratio is a marvelous adaptation for absorptive mode of nutrition.

The mycelium of fungi is covered with a cell wall made of chitin, a polysaccharide that is also found in the exoskeleton of insects and crustaceans However, in some fungi the cell wall contains cellulose and lignin- like substances.

The protoplasm of mycelium may be continuous throughout the mycelium so there will be several nuclei scattered throughout the cytoplasm. This condition is termed as coenocytic, such non- septate hyphae are observed in the members of the Division Zygomycetes e. g. Mucor and Rhizopus.

The septa or cross walls in the non-septate mycelia are formed only to cut off reproductive structuies or to seal off a damaged portion. Such septa are solid plates without any pores.

The members of other classes of fungi like ‘Ascomycetes and Basidiomycetes e.g. Aspergillus and Penicilliun develop internal cross walls i.e., septa, which divide the hyphae into segments. The septa appear at regular intervals. The segments may be uninucleate or multinucleate.

The septa, in these cases have perforations through which cytoplasmic strands including nuclei can migrate from one cell to the other. The presence of septa gives mechanical support to the hyphae.

The reproductive structures are also separated from vegetative structures h septa but these are not perforated.

In some groups of fungi the mycelium formed on germination of spores consists of uninucleate segments (monokaryotic) initially.

This is called primary mycelium. Later when fusion occurs either between hyphal segments of the same mycelium or different mycelium, the segments contain two nuclei (dikaryotic).

This conversion is called dikaryotisation and the mycelium is called secondary mycelium. This stage may last for a long period. When this mycelium gets organized into a specialized structure, it is termed tertiary mycelium.

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3. Pseudoparenchymatous Forms

The fungus mycelium normally, as mentioned above, is a mass of loosely interwoven hyphae that form a network. In some fungi, the entire mycelium or its parts undergo various modifications.

The walls of the hyphae in the mass get fused and they lose their individuality. As a result the hyphal mass, in cross-section appears to be a continuous structure. It resembles the parenchymatous tissue of higher plants, but it is not a true parenchyna as found in higher plants. In fungi such a tissue is called plectenchyma.

Plectenchyma can further be differentiated into two types. The plectenchyma with rounded fungal cells is called pseudoparenchyma and with less compacted elongated cells is called prosenchyma.

Often, the hyphae in many fungi aggregate and get organised into various structures that may be vegetative or reproductive in nature. Some examples of such structures are stroma, sclerotium and rhizomorph.

Stroma is an indefinte body formed in Daldinia. It commonly develops reproductive structures

Selerotia are tough and resting bodies. These are formed in Clavicepssp. The interior cells in the sclerotium are hyaline and stored with food and the outer cells are thick walled, black and crust-like.

In some fungi, hyphae lose individuality and form thick, dark brown, hard strands. These are called rhizomorphs because they appear like roots. In parasitic fungi the hyphae may enter the cell wall of the host and form haustoria for obtaining nourishment.

In summary, fungi exhibit a range of structures from unicellular, plasmodium-like filamentous to pseudoparenchymatous. Most fungi are multicellular and branched filaments. The mycelium is the main part of fungal body and it may aggregate to produce stoma, sclerotia and rhizomorphs.

Fungi grow on variety of substrates that contain traces of organic compounds. Some of the members can grow under extreme conditions of temperature and osmotic concentration of the solute.

Fungi show a range of morphological forms. Unicellular fungi like yeast are rare. Slime moulds are either unicellular or plasmodium-like at a certain stage of the life cycle.

Most fungi are multicellular, branched filaments. The mycelium is the main part of the fungal body. The reproductive structures are born on the reproductive hyphae.

Various kinds of structures arise when the entire mycelium or its part aggregate and give rise to special structures such as stroma, sclerotia, rhizomorphs arid others.

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