Mushrooms, molds, yeasts, truffles; these are all belong to the group of organisms known as fungi. They are not plants or animals or bacteria (although many are microscopic), they are – fungi. There are about 70,000 described species of fungi.  This is however only the tip of the iceberg; most fungi are still undiscovered and undescribed and the actual number probably exceeds 1.5 million! (Hawksworth, 1991; Hawksworth et al., 1995). This makes them the second most diverse group of eukaryotic organisms on earth after the animals. Linnaeus, the founder of modern classification, considered  fungi to be plants (of a peculiar type) and included them in them plant kingdom. However, based on various lines of evidence including DNA, fungi are thought to have evolved at least 900 million years ago from an ancestral organism that also gave rise to animals.  So fungi are more closely related to animals than they are to plants, and have been very successful in an evolutionary sense. (To get an idea of the diversity of fungi and their relationship to other organisms, check out the Tree of Life website http://tolweb.org/tree/. The study of fungi is a discipline in itself known as mycology, but fungi are the subject of study by many other types of scientists, including plant pathologists, microbiologists, biotechnologists, medical mycologists, soil scientists and ecologists. 

What makes fungi distinctive?

The main feature that separates fungi from plants and links them to animals is that they cannot make their own food by photosynthesis as plants do (technically fungi and animals are heterotrophic, where as plants are autotrophic). They depend on other living organisms or the dead remains of other organisms for sustenance just as we animals do. (see “How do Fungi make a Living?” below)

There are two other features that make a fungus recognisable as a fungus: (a) the way they grow and consequently the way their “bodies” are organized;  (b) the way they reproduce.

Most large animals and plants have bodies composed of different tissues (nervous system, stomach, leaves, roots etc.) that carry out different functions.  The tissues in turn are composed of tiny membrane-bound units known as cells.  These can be very variable in shape depending on the tissue. Tissues grow by controlled cell division.

Fungi in general have a much simpler body-plant than this.  Instead of a body of more or less rigid tissues, the fungal “body” consists of a network of microscopic tubular filaments known as hyphae (Fig. 1); the whole network is called a mycelium.  Individual hyphae have a rigid tubular wall composed of chitin (the main ingredient of insect exoskeletons).  Hyphae grow at the tip and can extend for quite long distances through soil for example in the case of soil fungi.  Hyphae can also branch, thus giving rise to a three-dimensional network or mycelium. As they extend, the hyphae release enzymes that attack and breakdown any potential food molecules in the immediate vicinity to smaller compounds such as sugars, and these are absorbed through the hyphal wall and into the cytoplasm.  This method of acquiring food is similar to what animals do, except that animal secrete digestive internally into a stomach, contained large food items that the animal has eaten.

As the mycelium grows, it radiates outward digesting all potential food materials in its path.  Occasionally it is possible to observe this phenomenon in your own garden lawn.  “Fairy Rings” (Fig. 2) often appear in summer on lawns, fields and golf courses They are a cause of great aggravation to green-keepers because the grass along the margin of the ring grows much more lushly than the grass outside the ring, while the grass in the interior of the ring often dies back or is stunted. Fairy rings are caused most often by the “fairy ring mushroom” (Marasmius oreades).  This fungus decomposes organic matter in the soil. Life for this fungus begins as a spore in the soil; the spore germinates producing a hypha that begins to extend and branch, forming a circular colony of mycelium.  The colony derives its nutrition by breaking down organic matter in the soil and it expands outwards as it grows.   Nitrogen is released from the organic matter breakdown along the leading edge of the colony and some of this is exploited by the grass, which grow more strongly along the edge.  However, nitrogen is scarce in the centre of the ring because it has been absorbed by the fungus and the grass is stunted. In autumn, as growth slows down, the fungus begins to reproduce (see below) by producing sporocarps above ground; they appear as mushroom-like structure comprising a cap, with gills underneath (where the spores are formed), on a stalk.

Most fungi grow by means of a mycelium but the scale of growth can be variable.  Although it starts microscopically small, a mycelial colony of the fairy ring mushroom can expand up to 10 m in diameter although usually much smaller.  Some fungal colonies can be much bigger the current record is held by Armillaria ostoyae, the bootlace fungus, a colony of which in Malheur Forest in eastern Oregon was found to cover 2,200 acres or almost 9 square kilometres or 1600 football pitches! The age of this individual could be in excess of 8500 years!  If you walk through this forest you will not be able to see this fungus because its mycelium is hidden in the soil.  However in autumn it produces its sporocarps and the effects on the fungus can be seen in the forest because it is a parasite-it kills trees.