What Do Fungi Look Like?

Many of us are familiar with the appearance of mushrooms and toadstools. But these structures are simply the large, macroscopic fruiting bodies produced by some groups of fungi. The actively growing and reproductive structures of most species are microscopic, and although most fungi are mycelial (filamentous), there are some exceptions to this growth form.

Mycelial (filamentous):

Labelled diagram of a branching hypha.

Most fungi are composed of microscopic filaments called HYPHAE, which branch to eventually form a network of hyphae, called a MYCELIUM (colony). The mycelium extends over or through whatever substrate the fungus is using as a source of food. [CLICK TO VIEW A LIGHT MICROGRAPH OF BRANCHING HYPHAE - 72KB]

Each hypha is essentially a tube, containing PROTOPLASM surrounded by a RIGID WALL. Depending upon the species, the protoplasm may form a continuous, uninterrupted mass running the length of the branching hyphae, or the protoplasm may be interrupted at intervals by cross-walls called SEPTA. Septa divide up hyphae into individual discrete cells or interconnected HYPHAL COMPARTMENTS.

Hyphae exhibit APICAL GROWTH (i.e. they elongate at their tips) and, at least in theory, are capable of growing indefinately, provided that environmental conditions remain favourable for growth. In reality, of course, their environment eventually limits or restricts their growth.

Labelled diagram of a germinating spore.

Hyphae may initially develop from a GERM-TUBE (a short, immature hypha) that emerges from a germinating spore. Spores are the microscopic dispersal or survival propagules produced by many species of fungi.

Although most fungi are mycelial (filamentous), the following represent exceptions to this growth form:

Unicellular and primitively branched Chytrids (Chytridiomycota):

Labelled diagram of a unicellular chytrid. Diagram of a primitively branched chain of cells.

Fungi belonging to the Chytridiomycota exist as either single round cells (unicellular species) or primitively branched chains of cells. In either case, the fungus may be anchored to its substrate by structures called RHIZOIDS.

Yeasts (unicellular):

Yeasts, which are used in a variety of commercially important fermentation processes (e.g. bread-making, brewing beers and wines), are capable of reproducing asexually and sexually.

Labelled diagram illustrating a yeast budding.

Yeasts reproduce asexually by either:

1. BUDDING (e.g. Saccharomyces cerevisiae), or

2. BINARY FISSION (splitting into two equal halves; e.g. Schizosaccharomyces pombe).

Labelled diagram illustrating a yeast cell undergoing binary fission.

Dimorphism (i.e. existing in two forms):

Some fungi are capable of alternating between a mycelial growth form and a unicellular yeast phase. This change in growth form is often in response to some change in environmental conditions. This phenomenon is exhibited by several species of fungi that are pathogenic in humans, e.g. Paracoccidioides brasiliensis.

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