I. MOLDS, MILDEWS, MUSHROOMS & MAN     TOP

 To us, we may associate fungi with ringworm, athlete's foot (Fig. 1-2), growth on our culture dishes in the laboratory, or to the blackish “crud” growing in our shower stall or around the kitchen sink. After water leaks or flooding, they can cause considerable damage to untreated wood.

Fig. 1-2. Athlete's foot (ringworm) caused by a fungus.

 Whereas, outside we see fungi as the mysterious mushrooms (Fig. 1-3), puffballs, and other fleshy fungi (Fig. 1-4) that appear and soon fade away. These are all fungi, but what links them together?

A scientific definition for fungi is as follows: Fungi are eukaryotic, heterotrophic organisms that reproduce by uniflagellate or sessile spores and whose haploid filamentous tubular body called mycelium (Fig. 1-5) is surrounded by walls of chitin, glucan, rarely other compounds, and who derive their nutrition by  absorption.

Fig. 1-5. Fungal mycelium.

 Included in what we call fungi are molds found in water, soil, on foodstuff, textiles, and building materials; causing diseases of all kinds of plants and animals; jelly fungi, bracket fungi, mushrooms, puffballs, stinkhorns, and birdsnest fungi.

These are some confusing terms to be bouncing off your cranium this early in the course. Let us examine each term individually, and in that way get a better understanding about fungi. Eukaryotic (eu=true; karyote=nucleus) organisms have true nuclei (Fig. 1-6) and other organelles  that are membrane-bound. 

Fig. 1-6. Transmission electron micrograph of a nucleus (large membrane bound  organelle) in a fungal cell.

Like other eukaryotic organisms, fungi have nuclei with chromosomes and nucleoli. Fungal nuclei are haploid, i.e. having a single set of chromosomes (N), or reduced number of chromosomes in the same nucleus. Mitochondria are membrane bound bodies that play a role in fungal metabolism (Fig. 1-7). 

Fig. 1-7. Mitochondria in a fungal cell.

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Fungi lack chlorophyll (achlorophyllous) and, therefore, cannot manufacture their own food through the process of photosynthesis. They are heterotrophic (hetero=another; trophic=food) and must derive their nutrients from organic matter formed by other organisms. Spores are reproductive units usually enclosed by walls that function in dissemination and survival. Motile spores have hair-like structures,  flagella that propel them, while most fungal spores are  sessile, i.e. they lack flagella Spores can be of all sizes, shapes, and colors (Fig. 1-8). 

Fig. 1-8. Fungal spores of various shapes and sizes.

Spores are to the fungus as seeds are to plants. In most cases spores germinate by a germ tube and form microscopic threads we call  mycelium. A single thread is called a  hypha (pl. hyphae) (Fig. 1-9) but in most fungi the hyphae will branch extensively to form a  spawn or mold-like growth. 

Fig. 1-9. Germinated spore with hypha.

The mycelium is really the body of a fungus. When a mushroom is stimulated to reproduce, the mycelium will differentiate and intertwine to form the button or primordial stage of a mushroom. In real life they look like small mushrooms  (Fig. 1-10) or bracket fungi.

Fig. 1-10. Primordial (or 'button') stage.

A common mushroom will serve to illustrate how and where spores are formed in some of our larger fungi (Fig. 1-11). 

Fig. 1-11. Mushrooms with gills.

The under surface of a mushroom bears blade-like structures we call  gills. A cross section of a gill will reveal spores formed in abundance on the gill surface (Fig. 1-12). 

Fig. 1-12. Fungal gill with spores on the surface.

A closer look with the scanning electron microscope reveals that spores are borne on club-shaped structures called  basidia (Fig. 1-13). 

Fig. 1-13. Four spores borne on a basidium.

At maturity, mushrooms release spores by the millions (Fig. 1-14) and many will land on substrates that have adequate moisture, nutrient, and temperature to support germination.  

Fig. 1-14. Mushrooms releasing millions of mature spores.

The following image from Stamets and Chilton’s book (The Mushroom Cultivator, Agarikon Press, Olympia, WA, 1983) depicts the typical life cycle of a mushroom  (Fig. 1-15).  

Fig. 1-15. Life cycle of a typical mushroom.

Many fungi such as the yeasts remain cellular and their colonies consist of hundreds of individual cells. Fungal cells are surrounded by walls containing chitin, glucan, or rarely other compounds and are able to absorb nutrient in liquid form. Fungal cells do not have a mouth or gullet and therefore cannot ingest food for internal digestion. There is external digestion and the absorption of their solubilized nutrients.